Download Decomposition of trichloroethylene in a plasma

Decomposition of trichloroethylene in a plasma-catalytic
process
Jelle Reyniers
Supervisors: Prof. dr. Rino Morent, Prof. dr. ir. Nathalie De Geyter
Counsellors: Ir. Arne Vandenbroucke, Sharmin Sultana
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Chemical Engineering
Department of Applied Physics
Chairman: Prof. dr. ir. Christophe Leys
Faculty of Engineering and Architecture
Academic year 2014-2015
Decomposition of trichloroethylene in a plasma-catalytic
process
Jelle Reyniers
Supervisors: Prof. dr. Rino Morent, Prof. dr. ir. Nathalie De Geyter
Counsellors: Ir. Arne Vandenbroucke, Sharmin Sultana
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Chemical Engineering
Department of Applied Physics
Chairman: Prof. dr. ir. Christophe Leys
Faculty of Engineering and Architecture
Academic year 2014-2015
Acknowledgements
This master dissertation represents not only my experimental and written work during the last year,
it is also a milestone of my 6 years long engineering studies. I am sure that I will later nostalgically
and with a smile think back to this educational period.
This work could not have been successfully finished without the help of several people, which I would
first like to thank.
First and foremost I wish to thank my supervisors, Prof. dr. R. Morent and Prof. dr. ir. N. De Geyter,
for the chance they give me to perform my thesis at the faculty of applied physics. As chemical
engineer, the choice for plasma physics was a step into the unknown, but one I have never complained.
Lot of thanks to my counsellors, Ir. A. Vandenbroucke and S. Sultana, for the support on theoretical
and practical level. They were very motivated to guide me, and I could always count on quick
feedback when I had questions. Special thanks to J. Peelman for solving technical problems
concerning my experimental set-up.
Finally I want to thank my parents for the opportunity they give me to study at the university, and to
buy me a laptop to write my thesis.
Ghent, May 22, 2015
Jelle Reyniers
Permission for usage
The author gives permission to make this master dissertation available for consultation and to copy
parts of this master dissertation for use.
In case of any other use, the copyright terms have to be respected, in particular with regard to the
obligation to state expressly the source when quoting results from this master dissertation.
Ghent, May 22, 2015
The supervisors,
Prof. dr. Rino Morent
The author,
Prof. dr. ir. Nathalie De Geyter
Jelle Reyniers
Decomposition of trichloroethylene in a
plasma-catalytic process
Jelle Reyniers
Supervisors: Prof. dr. R. Morent and Prof. dr. ir. N. De Geyter
Counsellors: Ir. A. Vandenbroucke and S. Sultana
Abstract: In this work, the decomposition of trichloroethylene
(TCE) is investigated in a plasma-alone and post plasma-catalytic
system with CeO2, K-OMS-2 and Fe3O4/K-OMS-2. The influence
of several process parameters such as air humidity, gas flow rate
and TCE inlet concentration is investigated. In the absence of a
catalyst, the maximal TCE removal efficiency is obtained at an air
humidity of 15 %. In addition, humid air also enhanced the CO x
selectivity of the TCE removal process. A decrease of the TCE inlet
concentration had a positive effect on both TCE removal efficiency
and COx selectivity. However, the carbon mass balances during the
plasma-alone experiments are poor due to the formation of
polychlorinated
by-products
such
as
phosgene,
dichloroacetylchloride (DCAC) and trichloroacetaldehyde
(TCAA). Addition of a catalyst downstream of the plasma reactor
clearly improved the TCE removal efficiency and COx selectivity
compared to the plasma-alone process, even at low energy density
(40 – 80 J·L-1) and catalyst temperature (150 – 200 °C). The use of
K-OMS-2 led to a removal efficiency of 86 % and a COx selectivity
of 57 %, when operating at 40 J·L-1. Addition of Fe3O4
nanoparticles to K-OMS-2 further increased the COx selectivity
with 15 %, but led to a decrease of the removal efficiency with 24
%. The disadvantage of the cryptomelane type catalysts is their
high sensitivity to deactivation, possibly caused by chlorinated byproducts. CeO2 is less prone to deactivation but resulted in a lower
removal efficiency (66 %) and COx (23 %) selectivity, under the
same process conditions.
Keywords: non-thermal plasma, trichloroethylene, plasma
catalysis, multi-pin-to-plate reactor, FT-IR spectroscopy
I. INTRODUCTION
During the last decades, research revealed the existence of
several environmental problems caused by human activities and
stressed the negative impact of air pollution on health. In
addition, the media attention for these problems caused a
growing environmental awareness among the population, and
stimulated the research on developing methods to reduce the
emission of air pollutants [1].
Volatile organic compounds (VOCs) are a large group of
chemical compounds that strongly contribute to poor air quality.
Several VOCs are very harmful to human health due to their
carcinogenic and mutagenic effects. Hence, different techniques
have been developed to control VOC emissions such as
thermal/catalytic oxidation, adsorption and bioprocesses each
with their own advantages and limitations.
As a cost and energy efficient alternative, the use of nonthermal plasma (NTP) has been recognized to be relevant for the
removal of VOCs from dilute atmospheric pressure gas streams
[2, 3]. The use of NTP is attractive since the supplied energy is
used for the acceleration of electrons, instead of heating up the
total gas volume. The energetic electrons collide with
background molecules leading to the formation of reactive
radical species, which in turn react with VOCs. The major
drawback of the NTP removal technique is the formation of byproducts (e.g. ozone, NOx), which can be more harmful than the
target VOC itself. An attempt to overcome these limitations is
to combine NTP with heterogeneous catalysis [4-6].
Plasma-assisted catalysis is able to eliminate toxic pollutants
at low temperature due to a higher energy efficiency leading to
a low energy cost. Furthermore, the development of a suitable
catalyst will help to optimize the selectivity into
environmentally more friendly end products. By placing the
catalyst downstream of the discharge zone (so-called post
plasma-catalytic system), the catalyst is able to decompose
ozone formed in NTP into active oxygen species able to greatly
improve the oxidation of both the target VOC and hazardous byproducts.
In this work, the abatement of TCE in a post plasma-catalytic
(PPC) system is studied. NTP is generated in a DC multi-pin-toplate corona/glow discharge. Several catalysts are investigated
and evaluated based on the TCE removal efficiency, COx
selectivity and formation of by-products. The influence of
several parameters (gas flow rate, humidity, TCE inlet
concentration) on the performance of the plasma-alone and
plasma-catalytic system is investigated. Focus is on operating at
low energy density in combination with low catalyst
temperature to improve the energy efficiency of the TCE
abatement process.
II. EXPERIMENTAL SET-UP
Figure 1 shows a schematic diagram of the experimental set-
up. The inlet gas is supplied by a bottle of pressurized dry (< 3
ppm H2O) air (Alphagaz 1, Air Liquide) and is separated into
three flows by means of three mass flow controllers
(Bronkhorst®, El-Flow® select). The upper controller regulates
the TCE concentration by passing air through a bubbling bottle
containing liquid TCE (99.99% purity, Acros). The second
controller is needed to adjust the total gas flow rate of the inlet,
while the third controller regulates the gas flow through a H 2O
containing bubbling bottle. The latter is needed to adjust the
humidity level of the inlet gas. The TCE abatement and the
identification of the by-products were determined with a FT-IR
spectrometer (Bruker, Vertex 70) in combination with a
Quadrupole MS spectrometer (Hiden Analytical, HPR 20 QIC).
The optical length of the gas cell and the resolution of the FTIR spectrometer were set at 20 cm and 4 cm-1, respectively.
Spectra were taken after steady state condition and OPUS
(Bruker) software is used to collect and analyze the obtained
spectra. IR transparent by-products are determined via MS
analysis in the mass range 0 – 150 m/ z. The formation of ozone
is analyzed by an ozone monitor (Teledyne API Model 450 O 3
Monitor).
A multi-pin-to-plate configuration is applied as plasma
reactor, and is based on the concept of a negative DC
corona/glow discharge. The plasma source consists of ten
aligned sharp hollow crown-shaped cathode pins, connected in
parallel and positioned 28 mm from each other. The anode plate
is profiled with spherical surface segments centered on the tip of
each cathode pin. The discharge is generated by applying an
electric field between the cathode pins and the anode plate using
a DC power supply (Technix, SR40-R-1200) at atmospheric
pressure and room temperature. Uniform and stable glow
discharge operation is ensured by ballasting each cathode pin
with a 1.5 MΩ resistor.
The K-OMS-2 type catalysts were prepared in cooperation
with UCCS (Unité de Catalyse et de Chimie du Solide) from the
university of Lille. CeO2 was purchased from Panreac. Before
its use, the catalyst was calcinated for 4 h at 350 or 500 °C under
a stream of dry air with a flow of 200 ml·min-1. For all tests, 0.5
g of catalyst powder was diluted with 3 g of carborundum and
introduced in a cylindrical glass reactor located in a temperature
controlled tubular oven operating in the temperature range of
100 – 300 °C. The measurements were performed after thermal
balance was reached.
III. RESULTS AND DISCUSSION
A. Plasma-assisted TCE abatement
The experiments with NTP were evaluated based on the TCE
removal efficiency (η) and the selectivity towards CO and CO2,
calculated as follows:
Ƞ = (1 −
SCO =
SCO2 =
CTCE,out
) · 100
CTCE,in
CCO
η · CTCE,in · 2
(1)
(2)
· 100
CCO2
· 100
η · CTCE,in · 2
(3)
SCOx = SCO + SCO2
(4)
In addition to CO and CO2, the formation of hazardous
polychlorinated
by-products
such
as
phosgene,
trichloroacetaldehyde (TCAA) and dichloroacetylchloride
(DCAC) was detected via FT-IR.
Influence of gas flow rate
Figure 2 shows the evolution of the TCE removal efficiency as
function of the energy density for two different gas flow rates
(0.5 and 2 L·min-1). Until an energy density of 175 J∙L-1, the gas
flow rate has no visible effect on the removal efficiency. A
further increase of the energy density results in a slower rise of
the removal efficiency for a gas flow rate of 0.5 L∙min-1,
compared to 2 L∙min-1. If more than 80 % of TCE has to be
removed in polluted air, it is better to operate at higher gas flow
rates to minimize the energy to be supplied per unit of gas
volume. Moreover, more polluted air can be treated per unit of
time when operating at a higher gas flow rate.
0.5 L/min
2.0 L/min
Removal efficiency (%)
100
90
80
70
60
50
40
Figure 1: Schematic diagram of the experimental set-up
0
200
400
600
Energy density (J/L)
800
1000
Figure 2: TCE removal efficiency as function of the energy density, for
a gas flow rate of 0.5 and 2 L∙min-1 (dry air, CTCE = 500 ppm, T = 293
K, atmospheric pressure)
In contrast to the negative effect of decreasing gas flow rate on
TCE removal, the lowest gas flow rate resulted in the highest
selectivity towards CO and CO2. The gas flow rate had no
significant influence on the formation of by-products HCl,
ozone, phosgene and DCAC.
Influence of humidity
Water plays an important role for the removal of TCE due to
the formation of reactive hydroxyl radicals in NTP. A relative
humidity (RH) of 15 % resulted in the highest values for the
TCE removal efficiency for given energy density. This optimal
humidity level depends on the optimal balance between the
enhancement and inhibition effect of water on TCE removal.
The removal of TCE is enhanced with increasing humidity due
to the formation of strong oxidizing hydroxyl radicals via
H2 O + O → 2 OH
Influence of TCE inlet concentration
As shown in Figure 4, a decreasing TCE inlet concentration
enhanced the removal efficiency, and can be explained as
follows. Each TCE molecule shares fewer electrons and reactive
plasma species with increasing inlet concentration, thereby
reducing the probability of reaction between these species and a
certain TCE molecule.
A lower TCE concentration also enhanced the selectivity
towards CO and CO2 due to the higher amount of oxygen
radicals present in NTP.
The formation of phosgene and DCAC was strongly reduced
with decreasing TCE concentration, while the production of
ozone was increased.
(5)
OH + O → O2 + H
OH + ClO → HCl + O2
(6)
(7)
Since ClO and oxygen radicals significantly decompose TCE
[7], a reduced amount of these radicals by OH leads to a
suppression of the removal efficiency at high humidity levels.
The presence of water increased the COx selectivity of the
TCE removal process due to the strong oxidizing power of
hydroxyl radicals, shown in Figure 3. An increase of the humidity
from 5 to 80 % RH led to a shift in COx selectivity from 8.5 to
18.5 %, at an energy density of 75 J·L-1. However the COx
selectivity remains low, indicating that TCE is mainly
decomposed into chlorinated by-products.
250 ppm TCE
5% RH
10% RH
20% RH
50% RH
80% RH
80
70
60
50
40
30
15% RH
200
400
600
Energy density (J/L)
800
1000
Figure 4: TCE removal efficiency as a function of the energy density
for different TCE inlet concentrations. (dry air, Q = 0.5 L·min-1, T =
294 K, atmospheric pressure)
20
18
16
COx selectivity (%)
750 ppm TCE
90
0
2.8% RH
500 ppm TCE
100
Removal efficiency (%)
On the other hand, water has a negative effect on TCE abatement
since OH radicals strongly react with ClO and oxygen radicals
via
B. Plasma-catalytic TCE abatement
14
12
10
8
6
4
2
0
0
50
100
150
200
250
Energy density (J/L)
300
350
Figure 3: COx selectivity of the TCE abatement process as function of
the energy density for different humidity levels. (humid air, CTCE = 500
ppm, Q = 2 L·min-1, T = 294.5 K, atmospheric pressure)
Moreover, humidity enhanced the formation of oxidation
product HCl and suppressed the production of hazardous byproducts ozone and DCAC. Contrary, the presence of water led
to a small increase of the phosgene concentration in the outlet
gas.
To improve the TCE abatement process a catalyst was placed
downstream of the plasma source, in a so-called post plasmacatalytic (PPC) system. This enhanced the process performance
and induced a synergetic effect. Ozone plays an important role
in this since it can dissociate on the catalyst surface into active
oxygen species. These active species desorb from the surface
and oxidize chlorinated by-products and residual TCE, thereby
enhancing the removal efficiency and COx selectivity. To
evaluate the synergy in the PPC process, a synergy factor fTCE is
introduced as
ηplasma−catalysis
(8)
fTCE =
ηplasma−alone + ηcatalyst−alone
During the plasma-catalytic experiments three different
catalysts were investigated: CeO2, K-OMS-2 and Fe3O4/KOMS-2.
CeO2
In dry air, the highest TCE removal efficiency was achieved
for a catalyst temperature of 150 °C. Operating at 40 and 80 J·L1
resulted in a TCE removal of 66 and 81 %, respectively. This
Cl2 + O → Cl + ClO
C2 HCl3 + ClO → COCl2 + CHCl2
(9)
(10)
Cl by-products
90
80
PPC 100°C
PPC 250°C
100
PPC 150°C
PPC 300°C
PPC 200°C
90
80
70
60
50
40
30
CO
20
CO2
10
0
70
Selectivity (%)
the COx selectivity due to the strong oxidizing power of OH
radicals. The effect of humidity on the formation of oxygen
species on the catalyst surface is negligible since the catalyst is
only little activated at low temperature. An increase of the
catalyst temperature resulted in a maximal COx selectivity at a
humidity of 20 % RH. This humidity level led to the highest COx
selectivity due to the combination of strong oxidizing hydroxyl
radicals and a high amount of ozone decomposed on the catalyst
surface. A higher humidity inhibits the ozone formation and
production of active oxygen species, explaining the decreasing
COx selectivity.
COx selectivity (%)
corresponds to an enhancement of 15 % (for both energy
density) compared to the plasma-alone system. The synergetic
effect is clearly visible since the TCE removal efficiency in the
catalyst-alone system did not exceed 1 %. An increase of the
catalyst temperature from 150 to 300 °C slightly reduced the
TCE removal from 81 to 77 %, at 80 J·L-1. This is possibly
caused by little catalyst deactivation due to irreversible
adsorption of chlorinated by-products.
The PPC system also enhanced the COx selectivity compared
to the plasma-alone system, as shown in Figure 5. Operating at
40 J·L-1 and a catalyst temperature of 150 °C resulted in a COx
selectivity of 23 %, compared to 9.3 % in NTP. An increase from
150 to 300 °C led to a shift in COx selectivity from 23 to 60 %
due to activation of the ozone decomposition on the catalyst
surface. The enhanced oxidation with increasing temperature
resulted in an increased and decreased formation of HCl and
DCAC, respectively. Remarkable was the increased phosgene
production at higher catalyst temperature. A possible
explanation is the increased formation of ClO radicals with
temperature which further react with TCE towards phosgene via
0
60
10
20
30
40
50
60
Relative humidity (%)
70
80
Figure 6: The selectivity of the TCE removal process towards COx in a
plasma-catalytic (CeO2) system as a function of the relative humidity,
at an energy density of 280 J·L-1. (humid air, CTCE = 500 ppm, Q = 0.5
L·min-1, T = 294.5 K, atmospheric pressure)
50
40
30
Just as in the plasma-alone experiments, an increase of
humidity resulted in an increase and decrease of phosgene and
DCAC, respectively. An increase of the catalyst temperature
towards 300 °C completely removed DCAC.
20
10
0
NTP
PPC 100°C PPC 150°C PPC 200°C PPC 250°C PPC 300°C
Figure 5: The selectivity of the TCE removal process towards CO, CO2
and chlorinated by-products in a plasma-catalytic (CeO2) and a plasmaalone system for different temperatures, at an energy density of 40 J·L1. (dry air, C
-1
TCE = 500 ppm, Q = 0.5 L·min , atmospheric pressure)
The experiments with humid air were performed at an energy
density of 280 and 400 J·L-1. This high energy density was
needed to light up all cathode pins since the electronegativity
character of water resulted in a reduced electron density. An
increase of the humidity from 2.8 (dry) to 80 % resulted in a
small decrease of the removal efficiency from 95 to 87 %, when
operating at 280 J·L-1. This can be partly ascribed to a decrease
in catalytic ozone decomposition under humid conditions since
the amount of converted ozone is directly related to the
concentration of newly formed active oxygen species over the
catalyst surface. Another possible explanation is that the
competitive adsorption of water inhibits the adsorption of TCE,
thereby reducing the catalytic removal of TCE.
The influence of the humidity and catalyst temperature on the
COx selectivity of the PPC system is plotted in Figure 6. At low
catalyst temperature (< 200 °C) humidity positively influences
Cryptomelane (K-OMS-2)
The PPC experiments with K-OMS-2 were performed with
dry (2.8 % RH) and humid (15 % RH) air at a catalyst
temperature of 150 °C. Figure 7 shows the TCE removal
efficiency in NTP and PPC system as function of the energy
density. The experiments with dry air resulted in the highest
removal efficiency. At a catalyst temperature of 150 °C,
operating at 40 and 80 J·L-1 resulted in a removal efficiency of
86 and 94 %, respectively. This corresponds to a synergy factor
of 1.60 and 1.37 for 40 and 80 J·L-1, respectively and is clearly
higher than in the PPC system with CeO2. The synergetic effect
during the experiments with humid air was negligible.
PPC - dry
NTP - dry
PPC - 15% RH
NTP - 15% RH
100
Removal efficiency (%)
90
80
70
60
50
40
30
50
100
150
Energy density (J/L)
200
250
Figure 7: TCE removal efficiency in a plasma-alone and plasmacatalytic (K-OMS-2) system as function of the energy density, for two
different humidity levels. (humid air, CTCE = 500 ppm, Q = 0.5 L·min1, T = 295.5 K, atmospheric pressure)
The COx selectivity also reached higher values compared to
CeO2. At 150 °C and 40 J·L-1, a value of 57 % was reached. An
increase of the humidity from 2.8 to 15 % resulted in a further
increase to 71 % due to the strong oxidizing power of hydroxyl
radicals. For both dry and humid air, an increase of the energy
density in time from 40 to 250 J·L-1 resulted in a decrease of the
COx selectivity with 13 %. This was remarkable since the
opposite effect was observed during the plasma-alone and PPC
experiments with CeO2. The decreasing COx selectivity can be
explained by significant deactivation of the catalyst.
Figure 8: The selectivity of the TCE removal process towards COx in a
plasma-alone and plasma-catalytic system with K-OMS-2 and
Fe3O4/K-OMS-2 as function of the energy density. (dry air, CTCE = 500
ppm, Q = 0.5 L·min-1, T = 295.5 K, atmospheric pressure)
Analysis of the FT-IR outlet spectra of the PPC experiments
with K-OMS-2 and Fe3O4/K-OMS-2 revealed the presence of
two new bands at 794 and 772 cm-1 (Figure 9), corresponding to
CCl4 and CHCl3, respectively. These compounds were not
detected in the plasma-alone and PPC system with CeO2, and
may be likely formed through carbon-carbon cleavage of DCAC
and TCAA.
Fe3O4/K-OMS-2
K-OMS-2
CeO2
NTP
CCl4
0.14
0.12
CHCl3
0.1
Fe3O4/K-OMS-2
This cryptomelane composite is synthesized to improve the
TCE abatement to total oxidation since Fe3O4 is known for its
oxidizing properties. As presented in Figure 8, the use of
Fe3O4/K-OMS-2 resulted in an increase of the COx selectivity
with 15 % compared to K-OMS-2, at low energy density (40 –
80 J·L-1). However, an increase of the energy density in time
again resulted in a reduction of the COx selectivity due to
catalyst deactivation. In contrast to CO x selectivity, Fe3O4/KOMS-2 resulted in a significant decrease (23 %) of the TCE
removal efficiency compared to K-OMS-2. The synergetic
effect was negligible.
0.08
0.06
Absorbance (-)
0
0.04
0.02
0
810
800
790
780
770
Wavenumber (cm-1)
760
750
Figure 9: FT-IR spectra between 810 and 750 cm-1 of the outlet gas in
the plasma-alone and PPC system with CeO2, K-OMS-2 and Fe3O4/KOMS-2, at a catalyst temperature of 150 °C. The spectra were measured
at a TCE conversion between 85 – 87 %. (dry air, CTCE = 500 ppm, Q
= 0.5 L·min-1, T = 295.5 K, atmospheric pressure)
C. Plasma-catalytic TCE abatement scheme
The TCE abatement and selectivity results obtained in the
PPC system for different catalysts suggest a simplified reaction
scheme for the TCE abatement that is represented in Figure 10.
First, electron-molecule collisions in NTP convert N2 and O2
molecules to a mixture of ionized, excited, metastable and
radical species that are able to decompose TCE to
polychlorinated intermediates (phosgene, DCAC, TCAA) and
total oxidation products (CO, CO2, HCl, Cl2). Molecular oxygen
is also involved in a three body reaction leading to the formation
of O3. Downstream of the plasma source, O3 comes into contact
with the catalyst surface and dissociates towards molecular and
radical oxygen species. These surface species promote the
further oxidation of residual TCE and chlorinated by-products
towards CO, CO2, HCl and Cl2 via pathway 1. In case of
cryptomelane-type catalysts (K-OMS-2 and Fe3O4/K-OMS-2)
an additional reaction path (pathway 2) is present, leading to the
formation of CCl4 and CHCl3 through carbon-carbon cleavage
of DCAC and TCAA.
the removal efficiency with 24 %. The disadvantage of
cryptomelane type catalysts is the fast deactivation rate. CeO 2 is
less sensitive to deactivation but resulted in a lower removal
efficiency (66 %) and COx (23 %) selectivity, under the same
conditions.
ACKNOWLEDGEMENTS
The author would like to acknowledge the supervisors and
counsellors for their support.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
Figure 10: Simplified TCE degradation scheme in a plasma-catalytic
process
IV. CONCLUSION
The decomposition of TCE was investigated by NTP
generated in a DC multi-pin-to-plate corona/glow discharge and
post plasma-catalytic system using CeO2, K-OMS-2 and
Fe3O4/K-OMS-2 as catalyst. Regarding the plasma-alone
process:
- Higher gas flow rates positively influenced the TCE removal,
but led to a decrease of the COx selectivity.
- A maximal TCE removal efficiency is obtained for an air
humidity of 15 % RH.
- The COx selectivity is enhanced in humid air due to the
oxidizing power of OH radicals.
- A lower TCE inlet concentration has a positive effect on both
TCE removal efficiency and COx selectivity.
- The carbon mass balances are poor and hazardous by-products
such as phosgene, DCAC, TCAA and ozone are observed along
with oxidation products COx, HCl and Cl2.
The PPC experiments showed that the addition of a catalyst
downstream of the NTP reactor enhanced the TCE removal
efficiency and COx selectivity due to the ability of the catalyst
to decompose O3 towards active oxygen species able to oxidize
the polychlorinated by-products and residual TCE. A removal
efficiency and COx selectivity of 86 and 57 % were reached with
K-OMS-2, when operating at 40 J·L-1 and a catalyst temperature
of 150 °C. Addition of Fe3O4 nanoparticles to K-OMS-2 further
increased the COx selectivity with 15 %, but led to a decrease of
G. R. Parmar and N. N. Rao, "Emerging control technologies for volatile
organic compounds," Critical Reviews in Environmental Science and
Technology, 2008. 39(1): pp. 41-78.
J. Van Durme, J. Dewulf, C. Leys, H. Van Langenhove, "Combining nonthermal plasma with heterogeneous catalysis in waste gas treatment: A
review," Applied Catalysis B: Environmental, 2008. 78: pp. 324-333.
A. M. Vandenbroucke, R. Morent, N. De Geyter, C. Leys, "Non-thermal
plasmas for non-catalytic and catalytic VOC abatement," Journal of
Hazardous Materials, 2011. 195: pp. 30-54.
A. M. Vandenbroucke, M. Mora, C. Jiménez-Sanchidrián, F. J. RomeroSalguero, N. De Geyter, C. Leys, R. Morent, "TCE abatement with a
plasma-catalytic combined system using MnO2 as catalyst," Applied
Catalysis B: Environmental, 2014. 156: pp. 94-100.
R. Morent, J. Dewulf, N. Steenhaut, C. Leys, H. Van Langenhove, "
Hybrid plasma-catalyst system for the removal of trichloroethylene in
air," Journal of Advanced Oxidation Technologies, 2006. 9(1): pp. 53-58.
Y. Li, Z. Fan, J. Shi, Z. Liu, W. Shangguan, "Post plasma-catalysis for
VOCs degradation over different phase structure MnO2 catalysts,"
Chemical Engineering Journal, 2014. 241: pp. 251-258.
A. M. Vandenbroucke, R. Aerts, W. Van Gaens, N. De Geyter, C. Leys,
R. Morent, A. Bogaerts, "Modeling and Experimental Study of
Trichloroethylene Abatement with a Negative Direct Current Corona
Discharge," Plasma Chemistry and Plasma Processing, 2015. 35: pp.
217-230.
Table of contents
1
Preface ................................................................................................................................ 1
Problem statement ....................................................................................................... 1
Objectives .................................................................................................................... 2
PART 1: LITERATURE REVIEW
2
Volatile organic compounds ............................................................................................. 4
Definition ..................................................................................................................... 4
Trichloroethylene......................................................................................................... 6
Removal techniques for VOCs .................................................................................... 7
3
2.3.1
Thermal oxidation ................................................................................................ 8
2.3.2
Catalytic oxidation ............................................................................................... 9
2.3.3
Biological VOC removal ...................................................................................... 9
2.3.4
Adsorption ............................................................................................................ 9
2.3.5
Upcoming removal methods .............................................................................. 10
Plasma technology ........................................................................................................... 11
Definition ................................................................................................................... 11
Thermal and non-thermal plasma .............................................................................. 11
Characterization parameters of VOC abatement ....................................................... 13
3.3.1
Reduced electric field ......................................................................................... 13
3.3.2
Energy density .................................................................................................... 14
3.3.3
Removal efficiency ............................................................................................ 14
3.3.4
CO, CO2 and COx selectivity ............................................................................. 15
Discharge types in NTP ............................................................................................. 15
3.4.1
DC discharges .................................................................................................... 16
3.4.2
AC discharges .................................................................................................... 19
3.4.3
Pulsed corona discharge ..................................................................................... 22
TCE abatement in NTP.............................................................................................. 23
Ozone formation in NTP ........................................................................................... 25
4
Plasma catalysis ............................................................................................................... 26
Definition ................................................................................................................... 26
In-plasma vs. post-plasma catalysis........................................................................... 26
4.2.1
Plasma-catalyst interactions in IPC systems ...................................................... 26
4.2.2
Plasma-catalyst interactions in PPC systems ..................................................... 28
Common used catalysts ............................................................................................. 28
5
4.3.1
MnO2 based catalysts ......................................................................................... 28
4.3.2
Al2O3 based catalysts ......................................................................................... 29
4.3.3
TiO2 based catalysts ........................................................................................... 29
4.3.4
Zeolites ............................................................................................................... 29
Analytical techniques ...................................................................................................... 31
Infrared spectroscopy ................................................................................................ 31
Mass spectrometry ..................................................................................................... 33
PART 2: EXPERIMENTAL STUDY
6
Materials and methods ................................................................................................... 36
Experimental set-up ................................................................................................... 36
Gas inlet preparation .................................................................................................. 36
Plasma reactor............................................................................................................ 37
Catalytic unit.............................................................................................................. 39
6.4.1
Catalyst preparation............................................................................................ 40
6.4.2
Pressure drop over catalyst bed .......................................................................... 41
Gas outlet analysis ..................................................................................................... 41
7
6.5.1
FT-IR analysis .................................................................................................... 42
6.5.2
MS analysis ........................................................................................................ 44
6.5.3
Ozone monitor .................................................................................................... 45
Plasma characterization ................................................................................................. 46
Influence of gas flow rate .......................................................................................... 47
Influence of humidity ................................................................................................ 49
Influence of TCE ....................................................................................................... 52
8
Plasma-assisted TCE abatement ................................................................................... 55
Identification of TCE abatement products ................................................................. 55
Influence of gas flow rate .......................................................................................... 62
Influence of humidity ................................................................................................ 67
Influence of TCE ....................................................................................................... 73
9
Plasma-catalytic TCE abatement .................................................................................. 77
Cerium oxide (CeO2) ................................................................................................. 77
9.1.1
Dry air experiments ............................................................................................ 78
9.1.2
Influence of humidity ......................................................................................... 84
Cryptomelane (K-OMS-2) ......................................................................................... 89
9.2.1
Influence of humidity ......................................................................................... 89
9.2.2
Catalyst deactivation .......................................................................................... 92
Fe3O4/K-OMS-2 ........................................................................................................ 94
Plasma-catalytic TCE abatement scheme .................................................................. 97
10 Conclusion ....................................................................................................................... 99
Appendix ............................................................................................................................... 103
References ............................................................................................................................. 108
Abbreviations and symbols
𝑎
Molar absorption coefficient [L·mol-1·cm-1]
Ȧ
Angstrom
A
Absorbance
AC
Alternating current
c
Speed of light [cm·s-1]
C
Concentration [ppm]
CFC
Chlorofluorocarbons
d
Interelectrode distance [m]
D
Diameter [m]
DBD
Dielectric barrier discharge
DC
Direct current
DCAC
Dichloroacetylchloride
ɛ
Porosity
E
Electric field [V·m-1]
Ea
Activation energy [J·mol-1]
ED
Energy density [J·m-3]
Es
Energy supplied to the plasma [J]
f
Synergy factor
FT-IR
Fourier transform infrared spectroscopy
h
Planck’s constant [J·s]
I
Current [A]
IPC
In-plasma catalysis
λ
Wavelength [cm]
k
Reaction constant [depends on reaction order]
L
Length [m]
m/z
Mass to charge ratio [kg·C-1]
MS
Mass spectrometry
Ƞ
Removal efficiency [%]
N
The number of cathode pins
NMVOCs
Non-methane volatile organic compounds
NTP
Non-thermal plasma
OMS
Octahydral molecular sieve
𝜌
Density [kg·m3]
∆𝑃
Pressure drop [Pa]
Pel
Electric power supplied to the plasma [J·s-1]
PPC
Post plasma-catalytic
ppm
Parts per million
Q
Gas flow rate [m3·s-1]
R
Resistor [Ω]
RH
Relative humidity [%]
S
Selectivity [%]
SD
Surface discharge
SEM
Scanning electron microscope
SMF
Sintered metal fibers
T
Transmittance [%]
TCAA
Trichloroacetaldehyde
TCE
Trichloroethylene
µ
Viscosity [Pa·s]
U
Voltage [V]
UCCS
Unité de Catalyse et de Chimie du Solide
UV
Ultraviolet
v
Velocity [m·s-1]
𝜈̅
Wavenumber [cm-1]
V
Reactor volume [m3]
VOCs
Volatile organic compounds
Von
Onset voltage for the corona discharge [V]
1 Preface
Problem statement
The Industrial Revolution, which took place from the 18th to 19th centuries, led to a shift from
handcrafted products to powered, special-purpose machinery, factories and mass production.
This booming industrialization brought about a greater volume and variety of factory-produced
goods and raised the standard of living for many people. However, the negative side of the story
is the worrying impact of the industry on the environment. Environmental issues such as global
warming, depletion of ozone and the formation of smog are strongly linked with the increasing
industrialization. In addition, many industries emit waste gases that are harmful to human
health. After World War II, the air pollution is tackled through legislation by introduction of
emission standards. In addition, the media attention for these environmental problems caused a
growing environmental awareness among the population, and stimulated the research on
developing methods to reduce the emission of air pollutants.
An important group of harmful air pollutants are volatile organic compounds (VOCs). These
compounds are released from burning fuel (e.g. coal, natural gas, gasoline), solvents, paints,
glues, etc. Commonly emitted VOCs are benzene, toluene, styrene, formaldehyde and
trichloroethylene. Most of these compounds are toxic, even at low concentrations (< 200 ppm),
and some may be carcinogenic. Conventional techniques used for end-of-pipe treatment of
these VOCs are thermal oxidation, biological VOC removal, catalytic oxidation and adsorption.
However, these techniques are only cost effective for the removal of high concentrated VOC
gases. The increasingly strict legislation on the emission standards led to a growing demand for
removal techniques of low concentrated VOC gases.
A new innovative removal technique for pollutants can be found in the field of plasma physics:
atmospheric non-thermal plasma (NTP). These so-called cold plasmas have proven to be more
efficient than conventional techniques for the treatment of lightly contaminated waste gases,
due to the lower energy consumption and its flexibility. The main advantage of NTP is that the
supplied energy is used for the acceleration of electrons, instead of heating up the total gas
volume. The energetic electrons collide with background molecules leading to the formation of
reactive radical species, which in turn react with VOCs. The major drawback of the NTP
removal technique is the formation of undesired by-products (e.g. ozone, NOx), which can be
more harmful than the target VOC itself. However, the use of NTP in combination with a
heterogeneous catalyst can significantly reduce the formation of by-products by oxidation of
these compounds towards CO2, H2O, HX and X2 (with X being a halogen).
1
Objectives
During this thesis, the abatement of trichloroethylene (TCE) in a post plasma-catalytic process
is studied. Several parameters (gas flow rate, humidity, TCE concentration) are investigated
and the performance of the TCE removal process is evaluated based on the TCE removal
efficiency and the selectivity towards oxidation products (CO, CO2, HCl and Cl2).
In the first part, the plasma reactor is characterized based on the reduced current-voltage curves.
In addition, the production of ozone is monitored since this parameter determines the oxidative
power of the plasma. After characterization of the plasma, TCE decomposition experiments are
performed in a plasma-alone set-up. The degradation products of the abatement process are
identified with FT-IR and MS spectrometry.
The second part of this thesis focuses on the performance of the post plasma-catalytic process
by placing a catalyst downstream of the plasma reactor. Three different catalysts are tested,
namely CeO2, K-OMS-2 and Fe3O4/K-OMS-2. The K-OMS-2 type catalysts are prepared and
characterized in cooperation with UCCS (Unité de Catalyse et de Chimie du Solide) from the
university of Lille. The aim of the plasma-catalytic experiments is to increase the TCE removal
efficiency and the selectivity toward oxidation products, while operating at low energy input
conditions.
2
Part 1:
LITERATURE REVIEW
3
2 Volatile organic compounds
Definition
Volatile organic compounds (VOCs) are a large and diverse class of rapidly evaporating
compounds containing at least one carbon element and one or more other elements like
hydrogen, nitrogen, oxygen, phosphor, silicium, sulfur and halogens. Methane, ethane, CO,
CO2, organometallic compounds and organic acids are excluded from this definition. A
measure of the volatility of a compound is the vapor pressure. This is the pressure exerted by a
vapor in thermodynamic equilibrium with its condensed phase and depends on the temperature.
A higher temperature will result in a higher vapor pressure. Gaseous compounds at room
temperature have a vapor pressure which is higher than the atmospheric pressure (101.32 kPa).
According to the ‘Solvent Emissions Directive’ [1], VOCs are defined as organic compounds
having a vapor pressure of 0.01 kPa or more at 293.15 K, or a corresponding volatility under
particular conditions of use. This directive defines the consumption and emission thresholds for
industrial companies and describes existing VOC reduction technologies. Emissions of VOCs
have to be minimized due to their negative effects on the environment and on human health, as
illustrated in Figure 11. VOCs are toxic and have a negative impact on several phenomena such
as ozone depletion, the formation of tropospheric ozone and the greenhouse effect. The
reduction of the emissions of these pollutants is therefore very important and the emission
standards are becoming stricter, thereby increasing the demand for VOC removal techniques.
Figure 1. The impact of several air pollutants on environment and human health [2]
1
NMVOC = Non-Methane Volatile Organic Compounds
4
The largest source of VOCs is biogenic (1150 Tg C/year) and represents about 85 % of the total
VOC emission. The majority is produced by plants, the main compounds being isoprene and
terpene. The remaining VOC emission is due to anthropogenic activities (12 %, 161 Tg C/year)
and vegetation fires (3 %, 50 Tg C/year) [3]. Major sources of anthropogenic VOCs are paints,
protective coatings and solvents. Typical solvents are aliphatic hydrocarbons, glycol ethers
and acetone. Traffic also contributes significantly to emissions of VOCs because of their
presence in diesel and gasoline. Other examples of man-made VOC containing sources are
detergents, perfume, glue and building materials [4].
Depending on the chemical structure, VOCs can be divided into different classes: aliphatic and
olefinic compounds, chlorinated hydrocarbons, and aromatic hydrocarbons. Some common
VOCs and their sources are listed in Table 1.
Table 1. Overview of common VOCs and their sources [5]
Group
Compounds
Applications
Alkanes
Methane, ethane, propane,
butane, pentane
Fuel for industrial and
domestic heating, solvent
Alkenes and alkynes
Ethene, propene, butadiene,
ethyne, propyne, butyne
Basic compounds in the
chemical industry (e.g.
production of polymers )
Aldehydes
Formaldehyde
Adhesive, resin, insulation
material, disinfectant
Ketones
Acetone, butanone, pentanone
Solvent, detergent,
plasticizer
Alcohols
Methanol, ethanol, propanol
Solvent, fuel, disinfectant
Ethers
Diethyl ether
Solvent, detergent
Aromatic hydrocarbons
Benzene, toluene, xylene,
phenol
Solvent, fuel, use in paint
and ink
Chlorinated hydrocarbons
TCE, chloroform, vinyl
chloride, dichlorobenzene
Solvent, dry cleaning agent,
degreasing agent
Terpenes
Sabinene, carotene
Perfume, cosmetics, food
additive
A more commonly used classification divides VOCs in two groups: methane and non-methane
VOCs (NMVOCs). The major sources of methane emissions are energy production (natural gas
systems), agriculture (enteric fermentation), and waste management (decomposition of solid
waste) [6]. The concentration of methane in the air is hundreds of times higher than that of other
5
VOCs, but the reactivity, expressed as ozone-forming ability, is 20 to 100 times lower. For this
reason, a distinction is made between methane and NMVOCs.
Trichloroethylene
Trichloroethylene (TCE) is used as VOC during the experiments in this thesis to study the
performance of the non-thermal plasma assisted catalytic process. The removal of TCE has
been widely studied in literature [7-11]. The physical and chemical properties of TCE are
illustrated in Table 2.
Table 2. Physical and chemical properties of TCE [12]
Trichloroethylene
Molecular weight [g/mol]
131.4
Melting point [°C]
-87.1
Boiling point [°C]
86.7
Density at 20°C [g/cm3]
1.46
Vapor pressure at 20 °C [kPa]
7.6
Viscosity at 20 °C [mPa·s]
0.58
Upper explosive limit in air [vol. %]
41
Lower explosive limit in air [vol. %]
11
Solubility in water at 20 °C [g/100 g water]
0.107
TCE is primarily used as a solvent in combination with adhesives, lubricants, paints, varnishes,
paint strippers, pesticides and cold metal cleaners. It is used as an extraction solvent for greases,
oils, fats, waxes and tars. The textile industry uses TCE to scour cotton, wool and other fabrics.
It can also be used as a refrigerant for low temperature heat transfer [13].
The lifetime of TCE in the atmosphere is around four days. This is the time needed for the TCE
concentration to decay to 37 % of its original value. A lifetime of four days is relatively short.
However, TCE is continually released to the atmosphere. The major mechanism for TCE
destruction in the atmosphere is reaction with hydroxyl radicals resulting in degradation
products such as phosgene, dichloroacetylchloride, and formyl chloride [12].
6
Exposure to moderate amounts of TCE causes headaches, tremors, and loss of balance. Larger
exposures will cause dizziness or sleepiness, and may cause unconsciousness at very high
levels. Very large exposures may cause irreversible cardiac problems, nerve and liver damage,
and death. It is mildly irritating to the eyes, nose, and throat. Workers in TCE producing or
using industries are at risk of exposure. Consumers can be exposed to TCE via air from
production and processing facilities using TCE, or drinking water from contaminated water.
The primary sources of TCE emissions are the industries that manufacture it or use it in
production, such as the chemical industry, rubber manufacturers, the pharmaceutical industry,
the semiconductor industry, heavy equipment manufacturing, iron and steel manufacturing,
pulp and paper manufacture (for de-inking paper), the manufacturers of paints, inks, varnishes
and lacquers, and the manufacture of pens, pencils, art and office supplies [13]. The permissible
exposure limit for TCE is 50 ppm [14].
Removal techniques for VOCs
There are several techniques including physical, chemical and biological treatments available
to remove harmful VOCs by either recovery or destruction. Table 3 compares the various
available VOC removal technologies. Each technology has its own applicability depending
upon the type, concentration and gas velocity of the VOCs. The most important removal
techniques and applications will be briefly discussed.
7
Table 3. Current VOC removal techniques
2.3.1 Thermal oxidation
In a thermal oxidizer, VOCs are oxidized by combustion to CO2 and H2O. In the case of
chlorinated compounds and in the presence of impurities, a number of by-products will also be
formed such as HCl and Cl2 [15]. Thermal oxidation units consist of single chambers with
ceramic refractory walls, equipped with a propane or natural gas burner and a stack. The
oxidation occurs at temperatures between 800 and 1000 °C and a gas stream residence time up
to 2 seconds. A thermal oxidizer can handle streams with a VOC concentration between 10 to
10000 ppm while removal efficiencies between 95 – 99 % can be reached.
8
One of the limitations of thermal oxidizers is the large amount of fuel required to heat up the
gas stream to the temperature necessary for high-efficiency VOC destruction. However, the
newest thermal oxidizers make use of heat exchangers to recover part of the heat. These units
are called regenerative thermal oxidizers (Figure 2), because of their higher thermal efficiency.
Figure 2. Working principle of a recuperative thermal oxidation unit [16]
2.3.2 Catalytic oxidation
Catalysts are able to reduce the activation energy for oxidizing VOCs and thereby lowering the
required reaction temperature. By using a catalyst, only one third of the energy is needed
compared to thermal oxidation. For chlorinated compounds, catalysts such as chrome,
aluminum, cobalt oxide and copper oxide/manganese oxide are used. The main disadvantage
of the use of catalysts is their sensitivity towards impurities which deactivate the catalyst. Spent
catalyst which cannot be regenerated needs to be disposed. The removal efficiency of a catalytic
oxidizer is lower compared to a thermal oxidizer [17].
2.3.3 Biological VOC removal
Biological gas treatment techniques have been used as alternatives for the traditional physicalchemical techniques. VOCs diffuse through a membrane and come into contact with
microorganisms in a biofilm which is attached onto the membrane. Through oxidative and
reductive reactions, the VOCs are converted to carbon dioxide, water and organic biomass.
Biological VOC treatment is environmentally friendly and cost effective. Another advantage of
a membrane bioreactor is the possibility to separate the gas and liquid phases. In this way the
conditions of both phases can be optimized much more easily [18].
2.3.4 Adsorption
In air pollution control, adsorption can be used to remove low to medium VOC concentrated
streams. VOC molecules pass through a bed of solid particles and are held there by attractive
london dispersion forces. The adsorptive capacity of the solid increases with the gas phase
9
concentration, molecular weight, diffusivity, polarity, and boiling point [19]. Common used
solids in adsorption processes are zeolites, activated carbon and polymers.
2.3.5 Upcoming removal methods
The traditional methods of VOC removal have several technical and economic disadvantages,
especially for the removal of low concentrated VOC streams. In order to overcome these
disadvantages, new technologies such as photo-catalysis and plasma technology have been
developed. In this thesis, non-thermal plasma will be used to remove TCE in air and will be
discussed in more detail.
10
3 Plasma technology
Definition
The term plasma was first introduced in 1929 by Langmuir and Tonks [20] to describe a group
of charged particles in their studies about oscillations in the inner region of an electrical
discharge. Later, this definition was broadened to define a state of matter : ‘the fourth state of
matter’ [21]. When adding sufficient energy to a gas, the negatively charged electrons which
are strongly attracted to the positively charged nucleus of the gas atoms will overcome the pull
of the nucleus. The electrons are completely separated from the atoms and therefore have entire
freedom of movement. If atoms or molecules have lost one or more electrons they carry positive
charge outwardly and they become positive ions. Plasma is therefore considered as gas showing
collective behavior and consisting of particles which carry positive and negative charges, in the
extent that the overall charge comes to zero [22].
Plasma is widely used in practice and offer three major features that are attractive for
applications in chemistry and technology [23]:
1. The temperature of the plasma components and energy density can significantly exceed
those of conventional chemical technologies.
2. Very high concentrations of energetic and chemically active species (e.g. ions, radicals,
electrons, excited states, and photons) can be produced in plasmas.
3. Plasma systems can be far from thermodynamic equilibrium, providing very high
concentrations of active species while keeping bulk temperature as low as room
temperature.
Plasmas are used in several fields of application, such as extractive metallurgy, surface
treatment, etching in microelectronics, metal cutting and welding, and fluorescent/luminescent
lamps.
Thermal and non-thermal plasma
Depending on the conditions and amount of energy applied, a plasma discharge can be either
thermal or non-thermal. Thermal plasmas are characterized by the fact that all plasma
components are in thermal equilibrium. In non-thermal plasmas, there is a temperature
difference between electrons and the other plasma components.
(Non)-thermal plasmas can be generated by applying a voltage (DC, AC, pulsed) between two
electrodes located at a certain distance from each other. The power source generates an electric
field and donates energy to the free electrons present in the plasma volume. The accelerated
electrons collide with heavy particles, but only lose a small portion of their energy (electrons
11
are much lighter than heavy particles). That is the reason why the electron temperature is
initially higher than that of the heavy particles. Depending on the collision frequency and
transferred energy the temperature difference between electrons and heavy particles can
approach each other. This temperature difference is proportional to the square of the ratio of
the electric field (E) to the pressure (p). Only in the case of small E/p ratios electrons and heavy
particles reach thermal equilibrium. This kind of plasma is called a thermal plasma (TP) but
will not be discussed any further because this is beyond the scope of this study.
Numerous plasmas are far from the thermodynamic equilibrium and the plasma particles have
different temperatures. Due to insufficient energy transfer from electrons to heavy particles, the
electron temperature often significantly exceeds that of the heavy particles. In such nonequilibrium plasmas, ionization and chemical reactions are directly determined by electron
temperature and are not so sensitive to thermal processes and the gas temperature. This kind of
plasma is called non-thermal plasma (NTP) [23]. The generation of a NTP consists of many
elementary processes which can be divided into primary and secondary processes. These
processes and their timescales are summarized in Figure 3.
Figure 3. Timescale events of the elementary processes in a non-thermal plasma [24]
The primary process includes charge transfer, ionization, excitation, and dissociation with
typical timescales between 10-15 and 10-8 seconds. The main primary processes are electron
collisions with bulk gas molecules (N2, O2, H2O) resulting in the production of ionized and
excited molecules. The primary processes are shown below.
𝑒 − + 𝑁2 → 𝑒 − + 𝑁 + 𝑁
(1)
12
𝑒 − + 𝑁2 → 𝑒 − + 𝑁 + + 𝑁
(2)
𝑒 − + 𝑂2 → 𝑒 − + 𝑂(3 𝑃) + 𝑂(3 𝑃)
(3)
𝑒 − + 𝑂2 → 𝑒 − + 𝑂(3 𝑃) + 𝑂(1 𝐷)
(4)
𝑒 − + 𝑂2 → 2𝑒 − + 𝑂+ + 𝑂
(5)
𝑒 − + 𝐻2 𝑂 → 𝑒 − + 𝐻 + 𝑂𝐻
(6)
𝑒 − + 𝐻2 𝑂 → 2𝑒 − + 𝐻 + + 𝑂𝐻
(7)
The secondary process is the subsequent chemical reaction between the products of primary
processes (electrons, radicals, ions, and excited molecules). Additional radicals and reactive
molecules (O3, HO2, and H2O2) are also formed by radical and recombination reactions. The
timescale of the secondary processes is around 10-3 seconds. A more detailed description of the
occurring plasma reactions in TCE polluted air will be discussed later.
Non-thermal plasma techniques are economically attractive alternatives for conventional air
cleaning techniques due to the low energy consumption and high flexibility. In the non-thermal
plasma, the majority of the discharge energy goes into the production of energetic electrons,
rather than heating up the heavy particles (ions, radicals). Radicals produced during primary
and secondary processes are able to react with the pollutant molecules present in the air. The
aim is to convert these pollutants into less harmful oxidation products such as CO2 and H2O.
Characterization parameters of VOC abatement
There are several parameters which characterize the removal process of VOCs by using a NTP.
The performance of the VOC abatement can be determined based on the following parameters:
reduced electric field, energy density, removal efficiency, and COx selectivity.
3.3.1 Reduced electric field
The electric field E is the ratio of the voltage across the plasma to the distance between the
electrodes, and is given by
E=
with
Upl
d
E
Electric field [V·m-1]
Upl
Voltage over the plasma [V]
d
Interelectrode distance [m]
(8)
13
The reduced electric field strength is a measure of the electron energy in the plasma, and is the
ratio of the electric field E to the gas density N, given by
Upl
E
=
N N∙d
with
N
(9)
Gas density [molecules·m-3]
The physical unit of the reduced electric field is the Townsend: 1 Td = 10−21 V∙m-2
3.3.2 Energy density
The energy density (ED) expresses the amount of energy delivered to the gas to be treated per
unit of volume, and is defined as the applied electric power divided by the gas flow rate:
ℇ=
with
E𝑠 P𝑒𝑙
=
V
𝑄
Es
Energy supplied to the plasma [J]
ℇ
Energy density [J·m-3]
V
Reactor volume [m3]
Pel
Electric power supplied to the plasma [J·s-1]
Q
Gas flow rate [m3·s-1]
( 10 )
In this work, J·L-1 will be used as unit for the energy density. When the current and voltage
over the plasma are constant during the discharges, the electric power can be calculated as
Pel = Upl · Ipl
with
Upl
Voltage over the plasma [V]
Ipl
Current through the plasma [A]
( 11 )
3.3.3 Removal efficiency
The removal efficiency indicates the relative amount of VOC which is removed from the treated
gas stream, and can be calculated as follows:
Ƞ = (1 −
CVOC,out
) · 100
CVOC,in
( 12 )
14
with
η
Removal efficiency [%]
CVOC,in Inlet VOC concentration [ppm]
CVOC,out Outlet VOC concentration [ppm]
3.3.4 CO, CO2 and COx selectivity
The oxidizing power of a plasma can be determined by the CO and CO2 selectivity. The COx
selectivity is the sum of the CO and CO2 selectivity. The selectivities are defined as
SCO =
SCO2 =
CCO
η · CVOC,in · 2
· 100
CCO2
· 100
η · CVOC,in · 2
SCOx = SCO + SCO2
with
𝑆𝐶𝑂 , 𝑆𝐶𝑂2 , 𝑆𝐶𝑂𝑥
CO, CO2, COx selectivity [%]
𝐶𝐶𝑂 , 𝐶𝐶𝑂2
Outlet CO, CO2 concentration [ppm]
CVOC,in
VOC inlet concentration [ppm]
η
Removal efficiency [%]
( 13 )
( 14 )
( 15 )
One of the goals of the TCE removal experiments, performed in this thesis, is to maximize the
COx selectivity and to minimize the formation of hazardous by-products.
Discharge types in NTP
The classification of discharges that can occur in NTP is extensive and depends on the following
parameters [25]:
- Type of power supply: AC, DC, pulsed
- Electrode configuration
- Voltage level
- Pressure
An overview is given of the different discharge types based on the power source. Within this
classification the most relevant reactor technologies are described.
15
3.4.1 DC discharges
In this thesis, the TCE abatement experiments were performed with a DC power source.
Therefore, DC discharges will be described in detail. Three types of discharges can occur in
plasma generated by a DC power source: DC corona discharge, DC glow discharge and spark
discharge. The type of discharge depends on the applied voltage over the plasma and the current.
The different discharge regimes can be distinguished from each other when looking to the
reduced volt-ampere characteristic, which presents the relationship between plasma voltage (V)
and reduced current (I/V). A typical reduced volt-ampere characteristic is illustrated in Figure
4.
Figure 4. Reduced volt-ampere characteristic for a negative pin-to-plate discharge in ambient air [26]
DC corona discharge
Corona discharges are relatively low power electrical discharges that take place at or near
atmospheric pressure. The corona is invariably generated by strong electric fields associated
with small diameter needles, wires, or sharp edges on an electrode. The corona appears as a
faint filamentary discharge radiating outward from the discharge electrode [27]. This kind of
discharge is self-sustained and no external energy other than the electrical energy is needed to
sustain the gas ionization and to maintain the current flow [28]. In the corona regime the
discharge current I and the interelectrode voltage V are related through the following relation,
given by Townsend [29]:
16
I = C V (V − Von )
( 16 )
where Von is the onset voltage for the corona discharge and C is a factor which depends on the
electron mobility and inter-electrode distance. This linear relationship is visible in the reduced
volt-ampere characteristic (Figure 4).
Corona discharges can be operated both in positive as negative polarity and have certain
mechanisms in common. Both corona discharges are initiated by the acceleration of electrons
in the electric field generated by the DC power source. Accelerated electrons collide with gas
molecules resulting in electron/positive-ion pairs, which in turn undergo different processes
creating an electron avalanche which sustains the corona discharge. The ionization processes
occurring in positive and negative corona discharges are illustrated in Figure 5.
Figure 5. Scheme of the ionization processes in positive and negative DC corona discharges [24]
-
Positive DC corona discharge
Two regions can be distinguished in the interelectrode space of the positive DC corona: plasma
region and unipolar region. In the plasma region, electron-positive ion pairs (O2+, N2+) are
produced by collision with accelerated electrons. These ion pairs release new electrons, which
are accelerated again. This resulting electron avalanche sustains the corona discharge and is
called a Townsend avalanche, named after John Sealy Townsend, who discovered this
fundamental ionization mechanism. Next to ionization processes, energetic electrons also
trigger dissociation and excitation processes, producing other reactive species, such as radicals
and excited molecules. In the unipolar region, the electric field is too weak to produce secondary
electrons and the positive ions migrate towards the negative electrode. An increasing electric
17
field strength results in the production of more electrons and expands the plasma region. If the
number of electrons reaches a critical value, a thin weakly ionized plasma channel (streamers)
will be formed. These streamer discharges have a higher efficiency in the production of
chemical active species. Satoh et al. [30] investigated the decomposition of benzene in a
positive DC corona discharge between multi-pin and plane electrodes at atmospheric pressure,
and obtained CO2 as main degradation product. Zhang et al. [31] compared styrene removal in
air by positive and negative DC corona discharges, and reported that positive corona discharges
were more effective.
-
Negative DC corona discharge
When the sharp electrode is negatively polarized, this is called a negative corona discharge.
One of the differences in comparison to a positive corona discharge is the presence of three
regions in the interelectrode space of a negative corona discharge: inner plasma region,
intermediate plasma region and unipolar region. Just like in a positive corona discharge,
electron impact ionization processes produce positive ion pairs (O2+, N2+) and additional
electrons in the inner plasma region. The electron energy values decrease while electrons move
towards the intermediate plasma region. The positive ions move back towards the negative
polarized electrode and hit its surface. This results in a secondary electron emission. In the
intermediate region, electrons are combined with neutral molecules, producing negative ions.
The energy levels in this region are too low to cause further ionization reactions, but the
presence of positive and negative polarized species are able to trigger certain plasma reactions.
In the outer region, only a flow of negative ions and free electrons move towards the positive
electrode.
Positive and negative DC corona discharges possess a small active plasma volume and are
therefore not very suitable for the production of large quantities of chemical particles. The
applications of corona discharges are largely confined to electrostatic precipitation and
photocopy machines.
DC glow discharge
When increasing the plasma voltage in a corona discharge, the density of the electrons and the
electric field in the interelectrode space will both increase. Hence, when the electric field
becomes sufficiently high to produce intense gas ionization, the whole gap will be filled with
plasma and the corona discharge will go over to the regime of a glow discharge. When looking
at the glow discharge region in the reduced volt-ampere characteristic, it can be seen that the
current increases with voltage more sharply in comparison with the corona regime. This can be
explained by the increasing role of ionization which increases the conductivity in the
interelectrode space of the glow discharge [32].
It was thought for many years that a glow discharge could only exist at low pressures (1 to 104
Pa). With increasing plasma voltage at atmospheric pressure, a negative corona discharge will
quickly turn in a spark discharge due to local charge build-up on the anode which makes the
plasma becoming unstable. These sparks have to be avoided in order to protect the equipment.
18
Recently, several researchers have experimentally proven that glow discharges can also exist at
atmospheric pressure [33-36]. The glow discharges were generated by using stabilization
techniques to reduce the charge density on the anode which delays the spark discharge regime
[26, 37-39]. Callebaut et al. [26] intensively studied DC glow discharges and implemented the
following techniques to achieve a luminous glow discharge: (1) a fast gas flow. A gas flow with
a velocity higher than 10 meters per second is able to blow away local charge accumulations,
which delays the spark regime. (2) Optimization of the electrode geometry. By applying a
hollow cylindrical pin as cathode, the current will be better spread over the entire cathode, and
blocks the formation of sparks. A spherical crater anode instead of an anode plate leads to a
more uniform distribution of the current density over the spherical parts of the anode surface.
This geometry is able to reduce local charge accumulation.
Glow discharges are most suitable for the removal of VOCs within the DC discharges. The
entire space between anode and cathode is filled with plasma, which ensures that the entire
VOC-laden air stream comes into contact with the reactive plasma species. Furthermore, the
current density in a glow discharge is much higher than in a corona discharge providing a high
reactivity. Multi-pin-to-plate electrode systems for the generation of glow discharges were
successfully tested for air treatment applications [11, 37], and will also be used in this work for
the removal of TCE.
Spark discharge
At a critical voltage, a glow discharge will be transferred to a spark discharge. A spark is
accompanied by a sharp splash in current up to several amperes. This high current can cause
damage to the electrodes due to local thermal heating, which is the reason why spark discharges
cannot be used for the removal of VOCs.
3.4.2 AC discharges
Discharge generation using AC voltage is also possible, and covers a wide range of reactor
technologies. The most important AC discharges are described.
Dielectric barrier discharge (DBD)
Dielectric barrier discharges (DBD) are the most frequently used discharges to generate NTP
for VOC abatement experiments [40-44]. This kind of discharge is characterized by the
presence of one or more insulating layers (e.g. quartz, ceramic, glass), called the dielectric
barrier, which are located between the electrodes. DBDs are generally operated in a planar (onesided or two-sided) or cylindrical configuration, as shown in Figure 6. An interesting property
of DBDs is the formation of current filaments or microdischarges, occurring when the local
electron density in the discharge reaches a critical value. A microdischarge that reaches the
dielectric spreads into a surface discharge covering a region much larger than the original
channel diameter. Due to charge build up on the dielectric, the electric field at the location of a
microdischarge is reduced and thus terminating the current flow at this location. The use of a
dielectric barrier has two functions: (1) limiting the amount of charge deposited in a
microdischarge, thus preventing it from transitioning into a spark discharge, and (2) distributing
19
the microdischarges over the entire electrode surface which increases the probability of
collisions between bulk gas molecules and electrons [45]. Subrahmanyam et al. [44] developed
a novel DBD reactor in which a metallic catalyst serves as the inner electrode. The catalytic
electrode was prepared by deposition of MnOx and CoOx on sintered metal fibers (SMF) in the
form of a cylindrical tube. This reactor showed complete conversion and a selectivity of 80 %
to total oxidation products (H2O and CO2) at relative low energy input (235 J·L-1) during the
destruction of toluene (100 ppm) in air.
(a)
(b)
Figure 6: Planar (a) and cylindrical (b) DBD configurations [45]
Surface discharge
A surface discharge (SD) reactor is similar to a DBD reactor due to the presence of a dielectric
barrier. This type of reactor contains a series of strip electrodes attached on the surface of a high
purity layer (e.g. alumina, ceramic). A planar or cylindrical induction electrode is embedded in
its inside, facing to the discharge electrodes. A surface discharge reactor configuration is
presented in Figure 7. When applying a medium frequency (8-15 kHz), medium-high AC
voltage (4-6 kV peak), surface discharges are generated from the peripheral edges of each
discharge electrode and stretches out along the surface layer. The role of the induction electrode
is to enhance the length of these streamers by providing the streamer tips with the tangential
field of adequate intensity at any instant during its development. This means that long streamers
can be generated along the surface layer with a lower voltage. Masuda et al. developed a surface
discharge reactor composed of a high-purity alumina ceramic layer for the abatement of VOCs
[46] and CFC [47], and for the application as ozoniser [48]. More applications of surface
discharge reactors for the abatement of VOCs can be found in literature [49, 50].
20
Figure 7: Planar surface discharge configuration [25]
Ferroelectric pellet packed-bed reactor
When the discharge zone in a DBD reactor is filled with perovskite oxide pellets, the plasma
reactor is called a ferroelectric pellet packed-bed reactor (Figure 8). Application of an AC
voltage leads to polarization of the ferroelectric material and induces strong local electric fields
at the contact points between pellets. These strong electric fields result in the production of
partial discharges at the contact points between pellets and shift the electron distribution
towards higher energies [51]. The high energetic electrons tend to form active species through
electron-impact reactions (dissociation and ionization), rather than forming less useful species
through rotational and vibrational excitation. This leads to higher energy efficiency because
electron-impact reactions are mainly responsible for the decomposition of VOCs [25]. The main
disadvantage of these packed-bed reactors is the pressure drop over the reactor due to the
presence of the packing material.
Several ferroelectric materials are proposed in literature for the abatement of VOCs: PbZrO3PbTiO3 [52], NaNO2 [53], BaTiO3 [53-55], CaTiO3, SrTiO3, MgTiO4, PbTiO3 [56]. Among
these ferroelectric materials, BaTiO3 is widely most used due to its high dielectric constant
(2000 < ε < 10000) [57]. However, the performance of BaTiO3 was recently investigated and
compared with Ba0.8Sr0.2Zr0.1Ti0.9O3 as a special type of modified ferroelectric material for the
removal of toluene. The highest removal efficiency of toluene was 98 % with
Ba0.8Sr0.2Zr0.1Ti0.9O3 as packing material, and was 16 % higher than with BaTiO3 [58].
21
Figure 8: Geometries of packed bed discharges: (a) planar configuration and (b) coaxial configuration [59]
3.4.3 Pulsed corona discharge
A pulsed corona discharge applies a pulsed power supply, of which the typical waveforms of
the pulse voltage and discharge current are shown in Figure 9. The duration of a typical pulse
is less than 1 µs and the rise time is about tens of nanoseconds. There are two reasons to apply
such short pulse duration: (1) to prevent spark formation which can damage the reactor and
decreases the process efficiency, (2) to minimize the energy dissipation by ions which decreases
the energy efficiency.
Figure 9: Typical waveforms of pulse voltage and discharge current in pulsed corona discharges [60]
There are three main types of pulsed corona discharge reactors, depending on the used type of
electrode: point-to-plate, wire-to-plate and wire-to-cylinder [61]. Koh and Park [62] suggest to
use a wire-to-cylinder reactor due to the many advantages compared to the other electrode
configurations. The open structure of the wire-to-cylinder reactor results in low pressure drop
and the ability of space utilization that can be oriented in any position from horizontal to
22
vertical. A schematic representation of a wire-to-plate reactor containing a dielectric between
the electrodes is illustrated in Figure 10.
Figure 10: Schematic of a pulsed corona reactor (wire-to-plate configuration) [25]
TCE abatement in NTP
Before performing TCE abatement experiments, it is important to get insight in the underlying
mechanisms and reactions that enable the removal of TCE. A better understanding of the
removal process can yield measures to improve the removal efficiency and COx selectivity.
The destruction of TCE with NTP takes place through many possible pathways [63]. The first
possible pathway is the electron attachment of TCE, leading to the formation of C2HCl2 and a
chlorine anion:
C2 HCl3 + e− → C2 HCl2 + Cl−
k = 1.5 · 10−13 cm3 molecule−1 s−1
( 17 )
The contribution of the electron attachment of TCE depends on the electron density in the
plasma discharge. In corona discharges (low electron density), this contribution will be
significantly lower as in dielectric barrier discharges (high electron density).
Another pathway is the dissociation of TCE by reaction with atomic oxygen leading to the
formation of different by-products:
C2 HCl3 + O → CHOCl + CCl2
k = 5.7 · 10−13 cm3 molecules −1 s −1
( 18 )
C2 HCl3 + O → COCl + CHCl2
k = 8.7 · 10−14 cm3 molecules −1 s −1
( 19 )
C2 HCl3 + O → C2 Cl3 + OH
k = 6.3 · 10−15 cm3 molecules −1 s−1
( 20 )
The rate coefficients of the dissociation reactions with O are in the same order of magnitude as
for the electron attachment of TCE. However, atomic oxygen has a longer lifetime than the
electrons which increases the probability of reaction with atomic oxygen.
23
When the TCE removal process takes place in humid air, the TCE dissociation can also occur
by reaction with hydroxyl radicals:
C2 HCl3 + OH → CHCl2 + CHOCl
k = 3.1 · 10−13 cm3 molecules −1 s −1
( 21 )
C2 HCl3 + OH → C2 Cl3 + H2 O
k = 1.9 · 10−12 cm3 molecules −1 s−1
( 22 )
C2 HCl3 + OH → C2 HCl2 OH + Cl
k = 2.4 · 10−13 cm3 molecules −1 s−1
( 23 )
C2 HCl3 + OH → CHCl2 COCl + H
k = 2.4 · 10−14 cm3 molecules −1 s−1
( 24 )
The actual rates of the dissociation reactions again depend on the densities of the reactants, so
a higher humidity will result in a higher contribution of the dissociation reactions with hydroxyl
radicals.
There is also a possibility that TCE is decomposed by radicals produced by earlier occurred
decomposition reactions with TCE:
C2 HCl3 + ClO → CHCl2 + COCl2
k = 3.1 · 10−12 cm3 molecules −1 s−1
( 25 )
C2 HCl3 + Cl → C2 Cl3 + HCl
k = 7.3 · 10−16 cm3 molecules −1 s−1
( 26 )
The rate coefficient of reaction (25) is one order of magnitude higher than the previous
reactions. The density of ClO will be low at the beginning of the destruction process and the
reaction will not significantly contribute to the removal of TCE. However during the removal
process the Cl and ClO densities will rapidly increase, resulting in a high contribution of the
reactions (25) and (26) to the removal of TCE.
Another pathway is the dissociation by metastable nitrogen molecules. These molecules are
dominant dissociation species occurring in a NTP destruction process of an air polluted stream.
However, no reaction rate coefficients have been found in literature.
The reactions discussed above are not the only reactions that take place in the TCE abatement
process. Next to the electrons and radicals typically produced in air, a lot of by-products will
be formed during the removal process. Some of these by-products are able to further react with
TCE. An example of a by-product that will further react with TCE is CHCl2:
C2 HCl3 + CHCl2 → CHCl3 + C2 HCl2
( 27 )
Based on the TCE destruction processes discussed above, the presence of the most frequently
occurring degradation products such as trichloroacetaldehyde (TCAA), phosgene (COCl 2),
dichloroacetylchloride (DCAC), HCl and Cl2 can be explained.
24
Ozone formation in NTP
Another by-product formed during removal processes of air polluted streams in NTP is ozone.
Schönbein first identified the substance ozone during his electrolysis experiments with acidified
water in 1840. 17 years later, Werner designed the first ozone generator, in which ozone was
produced by the generation of electrical discharges in air at atmospheric pressure [64].
Ozone is toxic and has led to stringent regulations for ambient concentration standards for levels
in workplace, and limits for devices that produce ozone. Ozone is produced by devices like
laser printers, copiers, and electronic air cleaners. These applications rely on atmospheric
corona discharges. On the other hand, ozone is also used as disinfectant in water treatment
plants because of its strong oxidative character. In these applications, ozone is mostly produced
at high rates in dielectric barrier discharges in pure oxygen [65].
The ozone formation in gas discharges is initiated by the dissociation of O2 molecules by
electron impact reactions. The formed atomic oxygen undergoes a three-body recombination
reaction with O2, producing O3:
𝑂2 + 𝑒 − → 2O + 𝑒 −
( 28 )
𝑂 + 𝑂2 + 𝑀 → 𝑂3 + 𝑀
( 29 )
In reaction (29), M is a third collision partner needed in order to absorb excess energy. Next to
oxygen, nitrogen also plays an important role in the generation of ozone. A part of the ozone
formed in discharges results from processes with excited molecular states N2(A3∑) and
N2(B3∏) [66-68]. The production of oxygen atoms in reaction (28) results in the disappearance
of O3 by the following reaction:
𝑂 + 𝑂3 → 2𝑂2
( 30 )
Morent and Leys [69] studied ozone generation in a DC glow discharge of a multi-pin-to-plate
configuration and found out that the ozone concentration is directly proportional to current,
space averaged reduced electric field, and residence time.
The ozone production ability of a NTP is an important plasma parameter because it gives an
indication to which extent the plasma is able to break down pollutants. Because of the toxic
character of O3 and the formation of photochemical smog, the ozone concentration in the treated
gas at the outlet of the plasma reactor has to be as low as possible. Researchers [44, 59, 70, 71]
found out that ozone can be decomposed in a plasma reactor by applying a catalyst in
combination with the plasma. This principle is called plasma catalysis and will be further
discussed in the next chapter.
25
4 Plasma catalysis
Definition
The use of NTP as end-of-pipe technique for the removal of VOCs has not yet been industrially
implemented due to several drawbacks of the technology. The main drawbacks are the poor
energy efficiency, the formation of harmful by-products, and the low CO2 selectivity. To
overcome these disadvantages, researchers are combining the advantages of NTP and catalysis
in a technique called plasma catalysis [72-75]. By placing a catalyst inside or in close vicinity
of the discharge zone, the retention time will be increased due to adsorption of the VOC
molecules, favoring complete oxidation to CO2 and H2O. In many cases, a synergetic effect can
be observed which is caused by various mechanisms [10, 76, 77].
In-plasma vs. post-plasma catalysis
The combination of a NTP and a heterogeneous catalyst can be divided in two categories
depending on the location of the catalyst: in-plasma catalysis (IPC) and post-plasma catalysis
(PPC). IPC is a single stage process where the catalyst is placed inside of the active plasma,
while PPC is a two-stage process where the catalyst is located downstream of the plasma
reactor. The catalyst material can be introduced into the process in different ways: as a layer of
catalyst material (pellets, granulates, coated fibers, powders), as a packed bed (pellets,
granulates, coated fibers) or as a coating on the electrodes or reactor wall.
4.2.1 Plasma-catalyst interactions in IPC systems
Many researchers reported the synergetic effect between plasma and catalyst, improving the
VOC removal efficiency and leading to higher CO2 selectivity. Although the detailed
mechanism is still unclear, the synergetic effect can be explained by the activation of the
catalyst surface by plasma. Several mechanisms have been proposed in plasma-catalyst
systems: UV, ozone, changes in work function, plasma-induced adsorption/desorption, local
heating, activation of lattice oxygen, generation of electron-hole pairs and their subsequent
reactions, direct interaction of gas-phase radicals with the catalyst surface and the adsorbed
molecules etc. [78].
In an IPC configuration, both plasma and catalytic reactions take place simultaneously and
interact with each other. The introduction of a catalyst into the plasma discharge will have an
impact on the type of discharge. Microdischarges can be generated inside the catalyst pores,
resulting in more discharge per volume and increasing the mean energy density of the discharge
[79]. Moreover, insertion of ferroelectric pellets into the discharge zone leads to a shift in the
26
accelerated electron distribution. Ferroelectric materials are able to increase the electric field
with a factor between 10 and 250, leading to a more oxidative discharge [51].
The presence of a heterogeneous catalyst in the plasma discharge also increases the production
of reactive plasma species. Roland et al. [76] studied the oxidation mechanism of various
organic substances immobilized on non-porous and porous carriers and concluded that the
oxidizing species are formed in the pores of these porous carriers when exposed to NTP.
Insertion of TiO2 in the discharge zone, studied by Chavadej et al. [80], leads to an acceleration
of the formation of the superoxide radical anion O2- inhibiting recombination processes and
increasing the total catalytic activity. On the other hand, introduction of a catalyst in the plasma
can inhibit the formation of ionic species [81]. However, this effect did not impair the catalyst’s
role in reducing the emissions of ozone and carbon monoxide for this specific application:
indoor pollution control.
The reactive plasma species and electrons are able to trigger physical changes of the catalyst
material. NTP enhances the dispersion of the catalyst particles and influences the stability and
activity of the catalyst material. The oxidation state of the catalyst can also be influenced by
exposure to a plasma discharge. This was studied by Guo et al. [82], who exposed a Mn2O3
catalyst to a NTP for 40 hours. After the experiment, Mn3O4 was detected which is characterized
by its larger oxidation capability. Plasma exposure can also result in the enhancement of the
specific surface area or in a change of the catalytic structure. By comparing SEM images of
manganese oxide/alumina/nickel foam before and after discharge exposure (Figure 11), it can
be seen that the granularity of the grain on the catalyst surface becomes smaller and the
distribution becomes more uniform. Plasma exposure can even result in the formation of new
types of active sites, such as Al-O-O* which is observed in the pores of Al2O3 in IPC
experiments [83].
Figure 11. SEM images of manganese oxide/alumina/nickel foam (a) before and (b) after DBD [82]
The physical changes of a catalyst material when exposed to a NTP have an influence on the
adsorption of VOCs on the catalyst surface. Adsorption increases the retention time of the
VOCs and the interaction between short-lived plasma species (e.g. O(3P), O(1D), N(4S), OH*,
HO2*) with adsorbed VOC molecules leads to a more complete oxidation. Kim et al. [84]
27
reported that an IPC system shows zeroth-order kinetics, indicating the importance of surface
reactions in the decomposition of VOCs. The presence of water during decomposition processes
has to be avoided, because the adsorption of water results in a decrease of the reaction
probability of the VOC with the surface and therefore reduces the catalyst activity [85].
4.2.2 Plasma-catalyst interactions in PPC systems
In a PPC configuration, plasma does not directly interact with the catalyst because the reactive
species produced in the discharge zone disappear before they reach the catalyst surface due to
the short lifetime of the species. However, not only short-living unstable reactive species are
produced in plasma discharges, a fraction recombines to form more stable species such as
ozone. Because of the higher lifetime, ozone is able to reach the catalyst surface positioned
downstream of the plasma discharge. If a suitable catalyst is employed, ozone can dissociatively
adsorb onto the catalyst surface forming molecular and reactive atomic oxygen. The latter
species enhance the oxidation of remaining VOCs and unwanted by-products. This leads to an
improved energy efficiency and COx selectivity, and also reduces the emission of harmful ozone
into the air. The main advantage of a PPC configuration is that the plasma can convert the VOCs
into products which are easier to oxidize by the catalyst than the VOCs itself.
Common used catalysts
A lot of catalysts have been proposed in literature to enhance the removal of VOCs. By applying
a combination of several catalysts in the same system, the features of these catalysts can be
combined to enhance the removal of VOCs. The most studied catalysts are metal oxides and
zeolites, which will be discussed here.
4.3.1 MnO2 based catalysts
Manganese dioxide (MnO2) is a p-type oxide semiconductor and is common used in plasmacatalysis because of its ability to easily decompose ozone. This is due to some specific
properties of MnO2, such as the ease of synthesizing crystalline phases and the mobility of
oxygen in the crystal lattice which is able to create vacancies and promote the formation of
oxygen groups at the catalyst surface [86, 87]. Several researchers have investigated the
removal of VOCs in a NTP combined with MnO2. Einaga and Futamura [88] studied the
decomposition of benzene on MnO2. They reported that ozone, produced in the plasma
discharge, is decomposed on MnO2 to O2 and is able to oxidize benzene to form oxygencontaining by-products. These by-products are further oxidized to CO2 and CO. Futamura and
Gurusamy [89] observed synergetic effects for the decomposition of fluorinated hydrocarbons
with dielectric barrier discharge reactors filled with MnO2. Jarrige and Vervisch [90], who
coupled a fixed bed of MnO2/Al2O3 with a pulsed corona discharge for the removal of propane,
observed complete decomposition of ozone at ambient temperature. The produced oxygen
species further oxidize propane which leads to a greatly enhanced conversion and CO2
selectivity. Li et al. [91] investigated MnO2 catalysts with different phase structures, supported
28
on Al2O3 pellets, and applied in a post plasma-catalysis system for the removal of acetaldehyde.
An improved acetaldehyde removal efficiency and CO2 selectivity was observed, while the
formation of ozone and NOx in plasma was inhibited.
4.3.2 Al2O3 based catalysts
Aluminum dioxide (Al2O3), which is commonly called alumina, can exist in several phases of
which α-Al2O3 and ϒ-Al2O3 are most used in plasma catalysis. Vandenbroucke et al. [11]
observed synergetic effects by applying Pd(0.05 wt%)/ϒ-Al2O3 downstream of the plasma
reactor for the removal of TCE. Song et al. [92] compared the removal rates of VOCs (toluene
and propane) in a NTP and an IPC system with ϒ-Al2O3. The experimental results showed that
the removal rates of VOCs increase when exposing ϒ-Al2O3 to a NTP. This can be explained
by the longer residence time of the VOCs in the plasma reactor due to the adsorption on the
catalyst surface. Al2O3 is also able to suppress the formation of by-products during the VOC
abatement. This phenomenon was observed by Ogata et al. [93], who compared the abatement
of benzene in a conventional reactor packed with BaTiO3 pellets and an Al2O3 hybrid reactor
packed with a mixture of Al2O3 and BaTiO3 pellets. The Al2O3 hybrid reactor resulted in a
higher selectivity towards CO2 and the presence of the Al2O3 pellets suppressed the formation
of N2O.
4.3.3 TiO2 based catalysts
Titanium dioxide (TiO2) is a photocatalyst, which means that the catalyst can be activated by
irradiation with light of a particular wavelength. In the case of TiO2, UV light with a wavelength
of 388 nm is required to activate the catalyst. UV radiation is emitted by electronically excited
species produced by streamer discharges in air. Morent et al. [10] compared the removal
efficiency for TCE in a hybrid plasma-catalyst system consisting of a pin-to-mesh positive
corona discharge containing TiO2 pellets with the same system without catalyst. The presence
of TiO2 enhanced the TCE decomposition resulting in a maximum removal fraction of 85 % at
an energy density of 600 J·L-1 and 100 ppm TCE at the inlet. The oxygen partial pressuredependent behavior of catalysts TiO2, γ-Al2O3 and zeolites was evaluated by Kim et al. [78],
based on their enhancement factor and adsorption capability for the total oxidation of benzene.
The increase of the partial pressure of O2 enhanced both the removal efficiency of benzene as
well as the CO2 selectivity, regardless of the type of catalyst used. However, the TiO2 catalyst
showed the largest enhancement factor (100). Park et al. [94] investigated benzene
decomposition in a DBD-catalyst hybrid system. The following catalysts were studied and
attached on a glass barrier in the DBD plasma reactor: TiO2, Pt/TiO2 and V2O5/TiO2. The
V2O5/TiO2 catalyst hybrid system was most effective in decomposing benzene and controlling
by-products such as CO, CO2 and N2O.
4.3.4 Zeolites
Next to metal oxides, many studies have focused on the use of zeolites to improve the
performance of NTP systems. Ogata et al. [95] compared the performance of four types of
zeolites in a zeolite-hybrid plasma reactor for the decomposition of toluene. The major
29
synergetic effect in the zeolite-hybrid plasma reactor occurred from the decomposition of
toluene adsorbed in the internal area of the zeolite by active oxygen species. Kim et al. [96]
studied the use of various zeolites in a flow-type IPC reactor and proved that the support of Ag
nanoparticles enhanced both the catalytic activity and the expansion of plasma area on the
catalyst surface. Intriago et al. [97] showed that HY and H-ZSM-5 zeolites have important
catalytic activity for dichloromethane oxidation due to the high density of strong acid sites
which enhances the adsorption of hydrocarbons and catalytic activation of hydrocarbon halides.
30
5 Analytical techniques
Infrared spectroscopy
Infrared spectroscopy is the analysis of infrared light (IR) interacting with a molecule, and is
used both to obtain the chemical structure of a compound and as an analytical tool to determine
the concentration of a compound. IR refers to that part of the electromagnetic spectrum between
the visible and microwave region, and is characterized by two parameters: wavelength λ and
energy content E. Both parameters are related by the following formula:
𝐸=
ℎ𝑐
λ
( 31 )
with ℎ = 6.63 ∙ 10−34 𝐽 ∙ 𝑠 (Planck’s constant)
𝑐 = 300 ∙ 108 𝑐𝑚 ∙ 𝑠 −1 (speed of light)
IR spectra are mostly reported in cm-1, which refers to the unit of the wavenumber 𝜈̅ :
𝜈̅ =
1
λ
( 32 )
When the energy of the exposed IR radiation matches the energy of a specific molecular
vibration, absorption occurs. Figure 12 shows an IR spectrum of formaldehyde. The
wavenumber 𝜈̅ (cm-1) is plotted on the x-axis and is proportional to the energy. The
transmittance T (%) is plotted on the Y-axis and is the ratio of the transmitted intensities for
each wavelength with and without the sample.
Figure 12: The IR spectrum of formaldehyde [98]
31
The wavenumbers at which a molecule absorbs radiation give information about functional
groups present in the molecule. Certain groups of atoms absorb energy that give rise to bands
around a specific wavenumber. The correlation between wavenumbers and functional groups
are tabulated and can be used to analyze a certain spectrum.
The wavenumbers of the IR radiation, absorbed by a molecule, are determined by the
vibrational (and rotational) modes of that molecule. A molecule consisting of n atoms has a
total of 3n degrees of freedom, corresponding to the Cartesian coordinates of each atom in the
molecule. In a nonlinear molecule, 3 of these degrees are rotational, 3 are translational and the
remaining corresponds to fundamental vibrations. There are two types of molecular vibrations:
stretching and bending. H2O contains three fundamental vibrations, which are presented in
Figure 13.
Figure 13: Stretching and bending vibrational modes of water [99]
If the frequency of the IR radiation corresponds to the frequency of a vibrational mode of the
molecule, there is a net energy transfer which reduces the intensity of the radiation. Some
molecules such as O2 and N2 are not able to absorb IR radiation. In order to be IR active, a
vibration or rotation must cause a change in the dipole moment of the molecule. This change in
dipole moment allows the alternating electric field of the IR radiation to interact with the
molecule, and causes a change of the amplitude of one of these movements [99].
FT-IR spectrometer
Until 1960, dispersive infrared spectrometers were used as standard analytical technique for
organic compound characterization. However, this instrument showed a number of
disadvantages: slow scanning speed, frequent calibration with external reference needed, and
the occurrence of stray light. A new spectrometer has been developed to overcome the
drawbacks of the dispersive spectrometer: Fourier transform infrared (FT-IR) spectrometer.
Today, FT-IR spectroscopy is the standard for organic compound identification in modern
analytical laboratories.
A FT-IR spectrometer uses a Michelson interferometer to collect a spectrum, which is shown
in Figure 14. The interferometer consists of an IR light source, beamsplitter, two mirrors, a
laser and a detector. The IR beam coming from the source goes to the beamsplitter which splits
the beam into two parts. One part is transmitted to a movable mirror, while the other part is
reflected to a fixed mirror. The moving mirror moves back and forth at a constant velocity. The
two beams are reflected from the mirrors and recombine at the beamsplitter. This recombination
leads to an interference pattern, since the beam from the moving mirror has traveled a different
distance than the beam from the fixed mirror. This interference pattern is called an
32
interferogram and goes from the beamsplitter to the sample, where an amount of energy is
absorbed depending on the compounds in the sample. The transmitted beam reaches the detector
and reads information about every wavelength in the IR range simultaneously. This detector
signal is sent to a computer which performs a Fourier transform on the interferogram to convert
it into an IR spectrum [100, 101].
Figure 14: The Michelson interferometer [101]
Mass spectrometry
Mass spectrometry is a powerful analytical technique used to identify and quantify (un)known
compounds within a sample. The process involves the conversion of the sample into gaseous
ions, with or without fragmentation, which are then characterized by their mass to charge ratios
(m/z) and relative abundances. The different components of a mass spectrometer are illustrated
in Figure 15: a sample inlet, an ionization source, a mass analyzer and an ion detector.
Figure 15: Components of a Mass Spectrometer
The first step in the mass spectrometric analysis of compounds is the generation of gas phase
ions of the compound. Ions are generated by inducing either the loss or gain of a charge from a
neutral species by cationization, deprotonation, electron ejection, electron capture, or by
transferring a charged molecule from a condensed phase to the gas phase.
An extraction system removes the produced ions from the sample, which are then trajected
through a mass analyzer. The difference in mass of the fragments allows the mass analyzer to
33
sort the ions by their mass-to-charge ratio. Not all analyzers operate in the same way. Some
analyzers separate ions in space (quadrupole, quadrupole ion trap, magnetic sector), while
others separate ions by time (time-of-flight, time-of-flight reflectron, quad-time-of-flight).
Once the ions are separated by the mass analyzer, they are sent to the ion detector. The most
common used detector is the electron multiplier, which transfers the kinetic energy from the
incident ions to a surface that in turn generates secondary electrons. The produced electrical
signals are then transmitted to a computer that is able to construct the mass spectrum. A typical
mass spectrum of TCE is presented in Figure 16.
Figure 16: Mass spectrum of TCE [102]
All mass spectrometers operate at vacuum conditions (13 · 10−5 to 13 · 10−7 Pa) to allow ions
to reach the detector without colliding with other gaseous atoms and molecules. These collisions
have to be avoided due to their negative impact on the resolution and sensitivity. High pressures
may also cause high voltages to discharge that can damage the instrumentation, electronics, and
the computer system running the mass spectrometer [103, 104].
34
Part 2:
EXPERIMENTAL STUDY
35
6 Materials and methods
Experimental set-up
A schematic diagram of the experimental set-up is shown in Figure 17, and can be divided into
several sections. In the first section, the composition of the inlet gas is controlled by three mass
flow controllers. Two controllers regulate the inlet concentration of TCE and H2O, while the
third controller is used to adjust the flow rate of dry air. The three gas flows are mixed and sent
to the plasma reactor, in which TCE is treated. NTP is generated in the reactor by a DC highvoltage power source. For the execution of plasma-catalytic experiments, a catalyst bed was
placed downstream of the plasma reactor, in which the outlet gas of the plasma reactor is further
treated. This configuration is called a post plasma-catalytic process (PPC). The last section is
responsible for the analysis of the effluent by means of a FT-IR spectrometer and mass
spectrometer. The ozone concentration is determined by an ozone monitor.
Figure 17. Experimental set-up for the plasma-(catalytic) experiments
Gas inlet preparation
The inlet gas is supplied by a bottle of pressurized dry (< 3 ppm H2O) air (Alphagaz 1, Air
Liquide) and is separated into three flows by means of three mass flow controllers
(Bronkhorst®, El-Flow® select). The upper controller regulates the TCE concentration by
passing air through a bubbling bottle containing liquid TCE (99.99% purity, Acros). The
36
bubbling bottle was located in a thermostatic water bath maintained at –8°C by adding
antifreeze to the water. This low temperature is needed to decrease the volatility of TCE which
facilitates the generation of TCE/air mixtures with stable initial concentrations. The TCE gas
flow rate can be varied between 0 – 200 ml·min-1. The second controller is needed to adjust the
total gas flow rate of the inlet and has an operating range between 0 - 10000 ml·min-1. The
experiments will be performed at a constant volume flow rate of 500 and 2000 ml·min-1. The
humidity of the gas inlet can be adjusted by passing the gas through a H2O containing bubbling
bottle, which is regulated by the third mass flow controller (0 – 500 ml·min-1). A humidity
meter (Testo® 445) is located downstream of the H2O bubbler to measure the relative humidity
and the temperature of the gas inlet. The relative humidity is a good indication of the amount
of H2O present in the gas inlet, since the temperature variation (20 °C ± 2) in the lab can be
neglected. During the experiments, the relative humidity will be varied between 0 – 80 %. The
pure air, TCE containing and water containing gas flows are then mixed and sent to the plasma
reactor.
Plasma reactor
A multi-pin-to-plate configuration is applied as plasma reactor, and is based on the concept of
a negative DC corona/glow discharge. The cylindrical casing of the reactor is made of Teflon
and inserted in a glass tube which is closed by airtight fittings in order to prevent any leakages.
The feed gas flows through a rectangular duct with a cross section of 40 mm x 9 mm and a
length of 400 mm. The plasma source consists of ten aligned sharp hollow crown-shaped
cathode pins, connected in parallel and positioned 28 mm from each other. The anode plate is
profiled with spherical surface segments centered on the tip of each cathode pin. The spherical
surface segment has a radius of 17.5 mm and a depth of 5 mm. The distance between the 10
cathode pins and the single anode plate is 10 mm. A picture of the multi-pin-to-plate reactor is
illustrated in Figure 18.
Figure 18. Picture of the multi-pin-to-plate reactor
37
Uniform and stable glow discharge operation is ensured by ballasting each cathode pin with a
1.5 MΩ resistor (Rb). The fraction of the total electrical power that is dissipated in these resistors
amounts to 10 % at most. The total voltage (Utot) is distributed over the ballasting resistors and
the plasma (Upl). The electric circuit of the plasma reactor is illustrated in Figure 19. The
discharge is generated by applying an electric field between the cathode pins and the anode
plate using a DC power supply (Technix, SR40-R-1200), at atmospheric pressure and room
temperature. During experiments, the energy density of the plasma is adjusted by varying the
plasma voltage. A high-voltage probe (Fluke 80 K-40, division ratio 1/1000) measures the
voltage applied to the inner electrode. The discharge current is determined by recording the
voltage signal across a 100 Ω resistor (R) placed in series between the counter electrode and
ground. The current and plasma voltage is measured by a digital multimeter (Ohmeron MT
488B) with an accuracy of 0.8 and 0.5 %, respectively.
Figure 19. Electric circuit diagram of the multi-pin-to-plate reactor
with
Voltmeter
Utot
Total voltage [kV]
R
Resistor (100 Ω) needed to determine I
I
Discharge current [mA]
Ub
Voltage over ballasting resistor Rb [kV]
Upl
Plasma voltage [kV]
Rb
Ballasting resistor (1.5 MΩ) of a cathode pin
The plasma voltage Upl can be calculated via the voltage over resistor R and the voltage over
the ballasting resistors of the cathode pins:
𝑈𝑝𝑙 = 𝑈𝑡𝑜𝑡 − 𝐼 ∙ 𝑅 −
𝐼 ∙ 𝑅𝑏
𝑁
( 33 )
38
with
N
The number of cathode pins
Catalytic unit
During PPC experiments, the plasma treated gas is sent through a catalytic unit. This is a
cylindrical glass tube, in which the catalyst is disposed on a sintered glass plate. An illustration
of the catalyst bed is shown in Figure 20.
Figure 20. Catalytic unit consisting of a sintered glass plate, on which the catalyst is disposed
The catalytic bed is placed in an oven, which allows to vary the catalyst temperature. The
temperature of the catalyst is controlled by a temperature probe present in the glass wool of the
oven (Figure 21, left), and is connected to a temperature controller (Figure 21, right).
Figure 21. Illustration of the catalytic oven (left) and temperature controller (right)
39
6.4.1 Catalyst preparation
For each plasma-catalytic experiment, 0.5 g of new catalyst sample is diluted with 3 g finegrained inert carborundum (SiC) to obtain a uniform gas flow over the catalyst bed. Then the
catalyst is prepared by calcination with 200 ml·min-1 dry air at 350 °C (cryptomelane) and 500
°C (CeO2) for 4 hours. This step is necessary to remove impurities adsorbed on the active sites
of the catalyst. After calcination, the catalyst is cooled down to room temperature before being
maintained at the temperature of the experiments. A list of the catalysts studied during this
thesis can be found in Table 4.
Table 4. List of catalysts studied in the PPC process
Catalyst name
Structural formula
Cerium(IV)oxide
Cryptomelane
Iron oxide/Cryptomelane
CeO2
K-OMS-2
Fe3O4/K-OMS-2
Synthesis of the K-OMS-2 catalyst
Cryptomelane (K-OMS-2) was prepared by a reflux method. KMnO4 solution was added
dropwise to an aqueous solution of manganese acetate with a buffer solution
(KCH3COO/CH3COOH) at pH 4.5. The suspension was stirred vigorously under reflux for 24
h with a buffer solution (KCH3COO/CH3COOH) at pH 4.5. After filtration, the precipitate was
washed with deionized water and several times with distilled water until neutral pH. Then the
catalyst was dried in an oven at 100 °C.
Synthesis of the Fe3O4/K-OMS-2 catalyst
Fe3O4 nanoparticles were grafted onto the K-OMS-2 via a coprecipitation method. The key
factor in this synthesis is pH, controlling the morphology of K-OMS-2 nanowires and the
formation of Fe3O4 nanoparticles. First, (NH4)2Fe(SO4)2.6H2O and Fe(NO3)3.9H2O were
dissolved in water. KOH was added dropwise to decrease the acidity, which is needed to
precipitate Fe(OH)3 (pH 4.0) and Fe(OH)2 (pH 9.6). Above a pH of 9.6 the cryptomelane was
added. This high pH is needed so that the mixed valences (III/IV) of manganese in K-OMS-2
will not be destroyed by Fe2+ present in the solution. KOH was continuously added until the pH
reached 12, and Fe3O4 nanoparticles were gradually deposited onto the K-OMS-2 nanowires.
CeO2 catalyst
Cerium oxide was purchased from Panreac.
40
6.4.2 Pressure drop over catalyst bed
A gas flowing through a catalyst bed results in a pressure drop, which can be described by
Ergun’s equation
∆𝑃 150 · µ𝑔 · 𝑣𝑔 (1 − ɛ)2 1.75 · 𝜌𝑔 · 𝑣𝑔 2 1 − ɛ
=
+
𝐿
ɛ3
𝐷𝑝
ɛ3
𝐷𝑝 2
( 34 )
with
∆𝑃
Pressure drop over catalyst bed [Pa]
𝐿
Length of the catalyst bed [m]
µ𝑔
Gas viscosity [Pa·s]
𝑣𝑔
Gas velocity [m·s-1]
𝐷𝑝
Particle diameter [m]
ɛ
Bed porosity [-]
𝜌𝑔
Gas density [kg/m3]
The pressure drop resulted in a decreased TCE concentration in the gas passing through the
catalyst bed, in comparison with the gas flow through the by-pass. To avoid this effect, the same
pressure drop over the catalyst bed is manually set over the by-pass, by manipulating valves 1
and 2 in the by-pass (Figure 17). In this way, the same TCE inlet concentration in the by-pass
as over the catalyst bed is obtained, and can be measured. A pressure indicator (Druck DPI 705)
with a range between 0 – 700 bar is used to monitor the pressure drop over the bed. The plasmacatalytic experiments were performed at low gas flow rate (500 ml·min-1) in order to limit the
pressure drop over the catalyst bed. Temperature has an effect on the gas density and velocity,
but only slightly influenced the pressure drop.
Gas outlet analysis
TCE removal efficiency and the identification of the by-products were determined with a FTIR spectrometer. However, some of the by-products formed during TCE abatement are hardly
to distinguish from each other due to overlapping absorption bands of these compounds in the
IR spectrum. Moreover, compounds such as Cl2 only contain a symmetric bond that does not
change in dipole moment during vibration. These compounds are IR inactive and cannot be
determined via IR spectroscopy. For this reason, a mass spectrometer is used to determine these
products. Moreover, the combination of an IR-spectrum and MS-spectrum provides a more
reliable identification of the products formed during TCE abatement experiments. An ozone
monitor is used to measure the ozone concentration in the gas outlet.
41
6.5.1 FT-IR analysis
The FT-IR spectrometer (Bruker, Vertex 70) used during the experiments is illustrated in
Figure 22. The description of the working principle of the spectrometer can be found in 5.1.
Figure 22. Illustration of the FT-IR spectrometer (Bruker, Vertex 70)
The optical length of the gas cell and the resolution of the spectrometer were set at 20 cm and
4 cm-1, respectively. The mercury-cadmium-telluride (MCT) detector is nitrogen cooled and
OPUS (Bruker) software is used to collect and analyze the obtained spectra. Spectra were taken
after steady state condition and consisted of ten averaged measurements. Each spectrum is
recorded from 3045 to 600 cm-1, which is the region where TCE abatement products are
absorbed.
Quantitative analysis of TCE, CO and CO2
In order to study the TCE abatement in the plasma reactor, TCE must be quantified in the gas
flow. This can be done by constructing a calibration curve by choosing a characteristic IR band
of TCE with a high intensity, which does not interfere with absorption bands of by-products.
The absorbance is proportional to the TCE concentration via the Beer-Lambert law:
𝐴 = log
𝐼0
= 𝑎 · 𝐶𝑇𝐶𝐸 · 𝑑
𝐼
( 35 )
with
A
Absorbance [-]
I0
Intensity of the incident light [cd]
I
Transmitted light intensity [cd]
42
Molar absorption coefficient [L·mol-1·cm-1]
𝑎
TCE concentration [mol·L-1]
CTCE
d
Optical path length [cm]
The above formula can also be applied for the integrated area under a characteristic IR band,
since this is an averaging of the absorbance between two wavenumbers. The calibration curve
can be obtained from the integrated peak area by measuring a standard sample of known TCE
concentration. A TCE (500 ppm, 2 % uncertainty) containing calibration mixture is used for
the construction of the calibration curve, shown in Figure 23. The absorption band at 945 cm-1
is chosen as calibration peak, and is integrated between 916 – 966 cm-1. The calibration curve
results in a formula to calculate TCE concentrations based on the integrated surface area:
𝐶𝑇𝐶𝐸 =
500
·𝑆
1.3888 𝑖𝑛𝑡,𝑇𝐶𝐸
( 36 )
with
CTCE
TCE concentration [ppm]
Sint,TCE
Integrated surface area of the absorption band between
916 – 966 cm-1 [-]
600
TCE concentration (ppm)
500
400
300
200
100
0
0
0.2
0.4
0.6
0.8
1
1.2
Integrated surface area (-)
1.4
1.6
Figure 23. Calibration curve for the quantitative analysis of TCE
In addition to TCE, it is also interesting to quantify the amount of oxidation products CO and
CO2 present in the treated outlet gas. In this way, the selectivity of the TCE abatement reactions
towards COx can be determined and compared with the selectivity towards unwanted
chlorinated by-products. A calibration mixture containing CO and CO2 is used to construct the
corresponding calibration curves (Figure 24). The characteristic absorption bands for CO and
CO2 are integrated between 2144 – 2223 cm-1 and 2283 – 2393 cm-1, respectively. Based on the
calibration curves, the CO and CO2 concentration can be calculated as follows:
𝐶𝐶𝑂 =
250
·𝑆
0.2543 𝑖𝑛𝑡,𝐶𝑂
( 37 )
43
𝐶𝐶𝑂2 =
250
·𝑆
4.6948 𝑖𝑛𝑡,𝐶𝑂2
( 38 )
with
CCO,CO2
Sint,CO
Sint,CO2
COx concentration [ppm]
Integrated surface area of the CO absorption band between
2144 – 2223 cm-1 [-]
Integrated surface area of the CO2 absorption band between
2283 – 2393 cm-1 [-]
300
250
CO2 concentration (ppm)
CO concentration (ppm)
300
200
150
100
50
250
200
150
100
50
0
0
0
0.05
0.1
0.15
0.2
0.25
Integrated surface area (-)
0.3
0
1
2
3
4
Integrated surface area (-)
5
Figure 24. Calibration curve for the quantitative analysis of CO (left) and CO2 (right)
6.5.2 MS analysis
The mass spectrometer (MS) used during the experiments is a Quadrupole Triple Filter MS
(Hiden Analytical, HPR 20 QIC) equipped with a Faraday and SEM detector. A picture of the
mass spectrometer is shown in Figure 25.
Figure 25. Illustration of the Quadrupole Mass Spectrometer (Hiden Analytical, HPR 20 QIC)
44
MASsoft 7 software (Hiden Analytical) is applied for collecting and displaying data. Only the
maximum peak intensities and the corresponding m/z numbers are collected. The peaks are
represented as bar lines over the corresponding m/z. The Faraday (10-5 Torr) and SEM (10-7
Torr) detector are scanning in the mass range 1 – 44 and 45 – 150 m/ z, respectively. A resolution
of 50 with electron ionization energy of 70 V was adopted.
6.5.3 Ozone monitor
The ozone concentration in the gas outlet is measured with an ozone monitor (Teledyne API
Model 450 O3 Monitor), illustrated in Figure 26. The detection of ozone molecules is based on
absorption of 254 nm UV light due to an internal electronic resonance of the O3 molecule. The
monitor uses a mercury lamp and emits light through a hollow quartz tube that is alternately
filled with sample gas, then filled with gas scrubbed to remove ozone. The ratio of the intensity
of light passing through the scrubbed gas to that of the sample is determined and forms the basis
for the calculation of the ozone concentration via the Beer-Lambert law (equation (35)). The
specifications of the ozone monitor are listed in Table 5.
Table 5. Features of the ozone monitor (Teledyne API Model 450)
Measurement range
Lower detectable limit
Sample flow rate
Accuracy
Resolution
0 – 1000 ppm
0.003 ppm
1 – 2.5 L·min-1
± 0.1 %
0.001 ppm
Figure 26. Illustration of the ozone monitor (Teledyne API Model 450)
45
7 Plasma characterization
Before abatement experiments are performed, the multi-pin-to-plate discharge reactor is
characterized by its voltage-current characteristics and the ozone production during plasma
treatment.
Voltage-current curves
Voltage-current characteristics are obtained by plotting the relation between the plasma voltage
(Upl) and the current through the plasma (I). The product of these parameters expresses the
electrical power supplied to the gas. Mostly, the electrical power is divided by the volumetric
gas flow rate, which is called the energy density. This parameter expresses the amount of energy
supplied per unit of gas volume.
The voltage-current relation gives insight into the energy density at which transition from
corona to glow discharge occurs and the maximal achievable energy density. The latter is
limited by the occurrence of sparks at high voltage, and has to be avoided. In practice, reduced
current-voltage curves are applied to better visualize the different discharge regimes. In these
curves, the reduced current (Ired) is defined as the current divided by the plasma voltage.
The reduced current-voltage curves are obtained by manually increasing the plasma voltage
from 0 Volt to the voltage at which sparking occurs. The corona regime occurs at low voltage
(around 7 kV in dry air) and is visualized by weak purple light emitted at the sharp points of
the crown-shaped cathode pins. Plasma is only generated at the cathode pins due to significant
concentration of the electric field exclusively around these sharpened electrodes. The whole
inter-electrode gap is dark. In the corona regime, the reduced current increases proportional
with the plasma voltage. A further increase of the plasma voltage leads to a transition of the
corona to glow discharge. In contrast to the corona, the electric field in the glow discharge is
high along the whole inter-electrode gap, and purple light is now emitted in the whole gap. As
a certain voltage is reached, the glow discharge turns into sparking. When sparking starts, the
experiment is stopped and the voltage is turned off to avoid damaging of the electrodes.
Ozone production
Another important plasma parameter is the ozone production. The amount of ozone produced
in NTP is a measure for the plasma reactivity since its occurrence can be correlated with reactive
species concentrations in the NTP. In addition, ozone monitoring is crucial due to its negative
effect on human health. The ozone outlet concentration is measured with an ozone monitor, and
can be used to calculate the ozone production (g/h) by the following formula:
𝑂𝑧𝑜𝑛𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 =
𝑂𝑧𝑜𝑛𝑒 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
𝑃
· 𝑀𝑂𝑧𝑜𝑛𝑒 ·
·𝑄
6
10
𝑅 · 𝑇𝑔𝑎𝑠 𝑔𝑎𝑠
( 39 )
46
with ozone concentration in [ppm], Mozone the molecular weight in [g·mol-1], P the pressure in
[Pa], Tgas the gas temperature in [K], R the gas constant (R = 8.314 J·K-1·mol-1) and Qgas the
gas flow rate in [m3·s-1].
In what follows, current-voltage curves and ozone production will be discussed based on plasma
experiments under different conditions. The experiments are performed with air and the
following parameters will be investigated: gas flow rate, humidity and TCE concentration.
Influence of gas flow rate
The experiments show that the gas flow rate has an effect on the voltage-current curves, as can
be seen in Figure 27. For a constant applied voltage, higher currents are measured with
increasing gas flow rate. For a voltage of 11 kV, a current of 0.42 and 0.67 mA is measured for
a gas flow rate of 0.5 and 2 L·min-1, respectively. At higher voltages the difference in current
becomes more pronounced. In addition, the maximum achievable current is higher for a gas
flow rate of 2 L·min-1. This phenomenon is also observed by other researchers [39, 105] who
investigated the influence of gas flow rate on corona-to-glow and glow-to-spark threshold
currents in a multi-pin-to-plate DC corona discharge.
The increase of the threshold current for sparking with increasing gas flow rate can be explained
as follows. Sparks are formed by the occurrence of local ionization instabilities present in the
plasma at high voltages. A gas flow is able to blow away these instabilities and increases the
current at which sparks are formed. Higher gas flow rates, resulting in higher turbulence, are
able to better disperse these instabilities compared to lower gas flow rates. Next to ionization
instabilities, the threshold current for spark formation is also determined by the occurrence of
discharge thermal instabilities. These instabilities mainly come from the discharge thermal load.
The temperature at the outlet of the plasma reactor is slightly higher than at the inlet due to
heating of the gas by the plasma. A higher temperature in the discharge volume will increase
the reduced field E/N, in which E is electric field and N is the density of the gas. The increase
of the reduced field will in turn increase the ionization rate and more heat will be produced.
The higher ionization rate at the outlet of the reactor results in a higher electron density, and
thus in a higher current. These conditions of increased current lead to the formation of anode
spots which in turn cause spark formation. During the experiments, the formation of a spark
discharge near the exit of the plasma reactor was observed in most of the cases. Again, by
applying a fast gas flow the heat produced by the discharge can be blown away and the thermal
instability can be effectively avoided.
Another parameter that affects the voltage-current curves is the electrode geometry. Akishev et
al. [39] investigated the effect of electrode geometry on the threshold currents for the coronato-glow and glow-to-spark transitions. The highest threshold currents were obtained by
applying hollow crown-shaped cathode pins and an anode plate with a spherical crater, with an
47
inter-electrode distance of 9 mm. Therefore, this optimal electrode geometry is also applied in
this thesis.
Reduced current-voltage curves are plotted at the right hand side of Figure 27 to visualize the
corona-to-glow transition. For a gas flow rate of 2 L·min-1 the glow regime occurs around 10.5
kV. In case of the lower gas flow rate no clear demarcation between the corona and glow regime
is visible.
2.0 L/min
0.5 L/min
0.07
12
0.06
Reduced current (mA/kV)
Plasma voltage (kV)
0.5 L/min
13
11
10
9
8
7
2.0 L/min
0.05
0.04
0.03
0.02
0.01
6
0.00
0.0
0.1
0.2
0.3
0.4
0.5
Current (mA)
0.6
0.7
7
8
9
10
11
Plasma voltage (kV)
12
Figure 27. Voltage-current (left) and reduced current-voltage (right) characteristics for two different gas flow rates.
(dry air, CTCE = 0 ppm, T = 294 K, atmospheric pressure)
In addition to the current, the ozone production is monitored as function of the plasma voltage
for both gas flow rates. As can be seen in Figure 28, a higher gas flow rate results in a higher
ozone production for a given energy density. The energy density is defined as the applied
electric power (Pel) divided by the gas flow rate (Q), and expresses the amount of energy
supplied to the gas per unit of gas volume. An increasing ozone production at higher gas flow
rates can be explained due to the occurrence of a dilution effect when applying higher gas flow
rates. Indeed, the ratio of the ozone production between a gas flow rate of 0.5 and 2 L·min-1 is
around four at a given energy density. The ozone production also increases with increasing
energy density. This is due to the increased production of reactive electrons in the plasma when
applying a higher energy density. When looking at the ozone outlet concentration as function
of the energy density (Figure 28, right), it becomes clear that the ozone concentration is
independent of the gas flow rate. Vandenbroucke [106] reported a linear correlation between
the ozone production and the energy density. The curves in Figure 28 deviate from linear
behavior, which is possibly caused by measurement errors of the ozone monitor, due to a poor
calibration.
48
0.5 L/min
2.0 L/min
0.5 L/min
0.20
2.0 L/min
800
700
O3 concentration (ppm)
O3 production (g/h)
0.16
0.12
0.08
0.04
600
500
400
300
200
100
0.00
0
0
50
100
150
200
Energy density (J/L)
250
0
50
100
150
Energy density (J/L)
200
250
Figure 28. Ozone production (left) and the ozone outlet concentration (right) as function of the energy density for two
different gas flow rates. (dry air, CTCE = 0 ppm, T = 294 K, atmospheric pressure)
Influence of humidity
The influence of humidity on the plasma characteristics is investigated by adding a certain
amount of water vapor to dry air. The relative humidity (RH) is monitored with a humidity
meter and seven different values of humidity were investigated. Figure 29 illustrates the effect
of humidity on the reduced current-voltage curves. With increasing humidity a higher plasma
voltage is needed to achieve the same reduced current. For a reduced current of 0.02 mA·kV-1
the plasma voltage increases from 8.7 to 12.1 kV when the humidity increases from 5 to 50 %
RH. The increase in plasma voltage is more pronounced for relative humidity between 2.8 and
20 % compared to higher humidity. It is also observed that an increase in humidity led to an
unstable plasma, which resulted in a decrease of the threshold current for spark formation.
The increase of the plasma voltage can be explained by the electron capturing behavior of water
molecules [107]. It is well recognized that water can trap energetic electrons via electron impact
dissociative attachment:
( 40 )
𝐻2 𝑂 + e− → 𝑂𝐻 + H −
An increase of the relative humidity leads to a higher number of H2O attachment reactions, and
thus a decrease in current. To maintain the same current this electron loss has to be compensated
by electron producing processes, such as ionizing collisions. To increase the rate of these
collisions, the electric field has to be increased.
49
2.8% RH
5% RH
10% RH
20% RH
50% RH
80% RH
15% RH
0.07
Reduced current (mA/kV)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
7
8
9
10
11
Plasma voltage (kV)
12
13
Figure 29. Reduced current-voltage characteristics for different levels of air humidity. (humid air, CTCE = 0 ppm, Q =
2 L·min-1, T = 294.4 K, atmospheric pressure)
Humidity also affects the ozone outlet concentration, as can be seen in Figure 30.
Measurements showed a nonlinearly decreasing ozone production at increased humidity levels,
with a larger rate of reduction at lower humidity. The ozone outlet concentration for an energy
density of 100 J·L-1 is 414 ppm for dry air, while this decreased to 281 ppm at 50 % RH and
228 ppm at 80 % RH.
2.8% RH
5% RH
10% RH
20% RH
50% RH
80% RH
15% RH
800
O3 concentration (ppm)
700
600
500
400
300
200
100
0
0
50
100
150
Energy density (J/L)
200
250
Figure 30. Ozone outlet concentration as function of the energy density for different air humidity levels. (humid air,
CTCE = 0 ppm, Q = 2 L·min-1, T = 294.4 K, atmospheric pressure)
The decrease in ozone production with increasing humidity was also observed by Morent and
Leys [69], who intensively studied ozone generation in air by a multi-pin-to-plate DC corona
50
discharge. Chen and Wang [108] modified existing models for ozone production by DC coronas
in dry air and incorporated the effect of water. They unveiled the critical pathway for ozone
production and indicated the following reactions as most important ozone production reactions:
𝑂 + 𝑂2 + 𝑂2 → 𝑂3 + 𝑂2
( 41 )
𝑂 + 𝑂2 + 𝑁2 → 𝑂3 + 𝑁2
( 42 )
𝑂2 ∗ + 𝑂2 → 𝑂3 + O
( 43 )
At higher humidity levels, the contribution of reactions (41) and (42) to the ozone formation is
strongly decreased due to the reduction of oxygen radicals in humid air by
𝐻2 𝑂 + 𝑂 → 𝑂𝐻 + 𝑂𝐻
( 44 )
The rate of reaction (43) is proportional to the concentration of 𝑂2 ∗ and relates to the electron
distribution. Since the electron distribution is independent of relative humidity, the
concentration of 𝑂2 ∗ , and thus the rate of reaction (43) is the same for both dry and humid air.
Besides the ozone producing reactions, the change in rate of the ozone destruction reactions
also contributes to the decreasing ozone outlet concentration with increasing humidity. The
most important ozone destruction reactions are listed below.
𝑁𝑂 + 𝑂3 → 𝑁𝑂2 + 𝑂2
( 45 )
𝑂𝐻 + 𝑂3 → 𝐻𝑂2 + 𝑂2
( 46 )
In dry air, 𝑂3 is primarily destructed by NO through reaction (45). At higher relative humidity
the contribution of reaction (46) strongly increases and becomes the primary ozone destruction
reaction. However, the relative production of OH radicals reaches a peak value between 40 and
60 % RH. This phenomenon was observed by Ge et al. [109] and can be explained as follows.
Next to reaction (46), the amount of OH radicals varies with the collision frequency between
energetic electrons and water molecules. At low humidity, the collision frequency between
energetic electrons and water molecules will stay low but increases with increasing humidity.
On the other hand, with increasing humidity, more electrons will be absorbed by water due to
its electronegativity. This results in a reduction of the mean density of energetic electrons,
which in turn leads to a reduction of the collision frequency between energetic electrons and
water molecules. This explains the smaller rate of reduction in ozone outlet concentration at
higher humidity levels (Figure 30).
51
Influence of TCE
Since TCE will be used during the abatement experiments, it is interesting to investigate the
influence of the presence of TCE on the plasma characteristics. First, the influence of the TCE
concentration on the reduced current-voltage characteristic will be discussed. Addition of TCE
leads to a decrease of the reduced current, as illustrated in Figure 31. A TCE concentration of
250 ppm and a plasma voltage of 11 kV resulted in a reduced current of 0.03 mA·kV-1,
compared to 0.05 mA·kV-1 for dry air. A further increase of the TCE concentration has only
minor effects on the reduced current. The current at which the corona-to-glow transition occurs
is hard to determine based on Figure 31, but is around 0.68 mA for all four curves. The
measurements were performed until sparking occurred, which means that the glow-to-spark
transition is marked by the last data point on the curve.
0 ppm TCE
250 ppm TCE
500 ppm TCE
0.08
Reduced current (mA/kV)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
7
8
9
10
11
12
Plasma voltage (kV)
13
14
15
16
Figure 31. Reduced current-voltage characteristics for different TCE concentrations. (dry air, Q = 2 L·min-1, T = 294
K, atmospheric pressure)
As in case of the presence of water, a reduction of the threshold current for spark formation is
expected with increasing TCE concentration, due to the destabilizing influence of this
electronegative compound. However, addition of TCE led to a slight increase of the threshold
current from 0.82 mA (dry air) to 0.9 mA (500 ppm TCE). A possible explanation for this is
that the addition of TCE improves the uniformity of the current distribution in the direction of
the gas flow. This effect was observed by Vertriest et al. [37] who investigated the distribution
of the current, drawn by the individual cathodes in a multi-pin-to-plate plasma reactor,
consisting of 5 rows of 20 cathode pins. Addition of 160 ppm TCE to dry air led to a less steep
current profile (Figure 32), which can be explained by the increase of the reduced field (E/N)
dependent electron attachment rates to TCE and its by-products.
52
Figure 32. Effect of TCE on the distribution of the current drawn by individual cathode pins in a multi-pin-to-plate
plasma reactor. [37]
The presence of TCE also affects the ozone produced in the plasma reactor, as can be seen in
Figure 33. For an energy density of 200 J·L-1, addition of 250 ppm TCE to dry air led to a
reduction of the ozone concentration from 670 to 409 ppm. Further increase of the TCE
concentration to 500 ppm further decreased the ozone concentration to 305 ppm. A plausible
explanation for this effect lays in the competition between TCE and O2 to react with oxygen
radicals. A higher TCE concentration results in a higher consumption of oxygen radicals
towards numerous end by-products, which inhibits the reaction between O2 and O towards O3.
The most common TCE abatement reactions in the presence of oxygen radicals, suggested by
Vandenbroucke et al. [63], are listed below.
𝐶2 𝐻𝐶𝑙3 + 𝑂 → 𝐶𝐻𝑂𝐶𝑙 + 𝐶𝐶𝑙2
( 47 )
𝐶2 𝐻𝐶𝑙3 + 𝑂 → 𝐶𝑂𝐶𝑙 + 𝐶𝐻𝐶𝑙2
( 48 )
𝐶2 𝐻𝐶𝑙3 + 𝑂 → 𝐶2 𝐶𝑙3 + 𝑂𝐻
( 49 )
53
0 ppm TCE
250 ppm TCE
500 ppm TCE
900
800
O3 concentration (ppm)
700
600
500
400
300
200
100
0
0
100
200
Energy density (J/L)
300
400
Figure 33. Ozone outlet concentration as function of the energy density, for different TCE concentrations. (dry air, Q =
2 L·min-1, T = 294 K, atmospheric pressure)
54
8 Plasma-assisted TCE abatement
In this part the removal of TCE in the multi-pin-to-plate plasma reactor will be investigated.
Focus is on the removal efficiency and the selectivity of the abatement process towards
oxidation products. In an ideal process under humid conditions, TCE is completely oxidized by
reaction with oxygen and hydroxyl radicals via [110]
𝐶2 𝐻𝐶𝑙3 + 4𝑂𝐻 → 𝐶𝑂2 + 3𝐻𝐶𝑙 + 𝐻2
( 50 )
𝐶2 𝐻𝐶𝑙3 + 4𝑂 → 2𝐶𝑂2 + 𝐻𝐶𝑙 + 𝐶𝑙2
( 51 )
The end products CO2 and H2 can be exhausted to the atmosphere. HCl can be easily removed
from the outlet gas by a water scrubber. This results in an acidic solution which can be
neutralized with a base. Scrubbing with water alone is impractical for the removal of Cl2 due to
its limited solubility in water. Cl2 can be scrubbed with an aqueous NaOH solution resulting in
products NaCl and NaOCl (hypochlorite). The latter can be used as bleaching agent. The
stoichiometry of reactions (50) and (51) is, however, difficult to achieve in NTP. As explained
before, incomplete oxidation leads to the formation of harmful chlorinated by-products.
The goal of the plasma-assisted experiments is to investigate the influence of several parameters
on the performance of the TCE abatement process, in order to maximize the conversion of TCE
towards oxidation products CO2, H2O, HCl and Cl2. Two parameters will be investigated during
the experiments: gas flow rate and humidity. By-products formed during TCE abatement will
be identified based on the FT-IR and MS spectra of the outlet gas.
Identification of TCE abatement products
Before investigating the influence of gas flow rate and humidity on the performance of the TCE
removal process, it is interesting to identify the range of by-products formed during this process.
To do this, a preliminary experiment is performed on basis of which the products present in the
outlet gas are determined by FT-IR in combination with MS.
Table 6. Characteristic IR bands of TCE [111]
Wavenumber range (cm-1)
Vibration
-
3115 – 3065
A
B
C
D
E
F
1615 – 1530
1305 – 1215
970 – 885
865 – 815
785
655 – 605
C–H olefinic C–H stretch
vibration
C=C stretch vibration
C–H deformation
C–H out of plane deformation
No unambiguous explanation
C–Cl vibration
C–Cl vibration
55
TCE inlet
TCE outlet
0.09
D
0.08
0.07
C
0.05
0.04
E
Absorbance (-)
0.06
0.03
0.02
A
F
B
0.01
0
3000
2800
2600
2400
2200
2000 1800 1600 1400
Wavenumber (cm-1)
1200
1000
800
600
Figure 34. FT-IR spectra between 600 and 3050 cm-1 of TCE inlet (black) and plasma treated TCE gas (red) at an energy
density of 50 J·L-1 (dry air, CTCE = 500 ppm, Q = 2.0 L∙min-1, T = 293 K, atmospheric pressure)
Figure 34 shows the FT-IR spectra of both the TCE inlet (black line) and outlet gas (red line)
when the plasma reactor is operated at 50 J·L-1. The characteristic IR bands of TCE are indicated
with letters, and listed in Table 6. Out of these characteristic IR bands, the peak at 945 cm-1 is
chosen to calculate the TCE removal efficiency since this band is not disturbed by interference
of bands originating from by-products. The reduction in surface area of this peak and the
occurrence of new peaks in the outlet spectrum indicates the decomposition of TCE into a range
of by-products. By comparing the outlet spectrum with reference compound spectra of NIST,
the most important by-products will be identified. Figure 36, Figure 37 and Figure 38 show
zoomed outlet spectra to study the plasma treated TCE gas in more detail. Numbers are used to
allocate different IR bands to the corresponding product.
In addition to FT-IR, mass spectra of the inlet and outlet gas are recorded in order to identify
additional by-products. The presence of some products cannot be proved with FT-IR due to
interference with bands of other products. Also, diatomic molecules such as Cl2 are IRtransparent and cannot be identified with FT-IR. The mass spectrum of the TCE inlet gas is
shown in Figure 35, and only contains the characteristic TCE fragment ions. The typical
fragment ions formed during collisions between electrons and TCE are C2HCl+ (m/z: 60, 62),
C2HCl2+ (m/z: 95, 97, 99) and C2HCl3+ (m/z: 130, 132, 134, 136). The mass spectrum of the
outlet gas, treated at an energy density of 200 J·L-1, shows a decrease in the abundance of the
TCE fragment ions, indicating partial TCE degradation. The products corresponding with these
peaks are identified by comparison to the NIST mass spectral library. A description of the
different products identified with FT-IR and MS is given below.
56
Figure 35. Mass spectrum of the TCE inlet gas (a) and the plasma treated outlet gas (b) at an energy density of 200 J·L1 (dry air, C
-1
TCE = 500 ppm, Q = 0.5 L∙min , T = 293 K, atmospheric pressure)
Dichloroacetylchloride (DCAC)
The formation of DCAC can be confirmed by comparing the outlet FT-IR spectrum with the
reference spectrum of DCAC (Appendix, Figure 77). The characteristic IR bands of this
compound appear at 1820, 1789, 1225, 1076, 989, 800 and 740 cm-1. In the mass spectrum of
the outlet gas, the characteristic fragment ions of m/z 83, 85 (CHCl2+), m/z 63, 65 (CClO+), m/z
48, 50 (CHCl+) can be ascribed to DCAC. DCAC belongs to the group of acid chlorides and is
also a VOC. Studies have shown negative effects of DCAC on human health [112].
57
The presence of DCAC in the outlet of plasma treated TCE gas is also confirmed by other
researchers [113-115]. Kirkpatrick et al. [116] detected DCAC as the primary by-product of
TCE decomposition and suggested the following reaction pathways.
𝐶2 𝐻𝐶𝑙3 + 𝐶𝑙𝑂 → 𝐶𝐻𝐶𝑙2 𝐶𝑂𝐶𝑙 + 𝐶𝑙
( 52 )
𝐶2 𝐻𝐶𝑙3 + 𝑂𝐻 → 𝐶𝐻𝐶𝑙2 𝐶𝑂𝐶𝑙 + 𝐻
( 53 )
DCAC can further decompose by attack of Cl radicals, leading to the formation of CO, HCl,
CCl4, CHCl3 and COCl2 as final products. However, the formation of CCl4 and CHCl3 could
not be unambiguously proven based on the outlet spectra.
Trichloroacetaldehyde (TCAA)
Many researchers [9, 71, 110, 113] who studied the abatement of TCE in NTP reported the
presence of TCAA in the outlet gas. Due to the absence of a reference IR spectrum in the NIST
databank, TCAA could not be identified with FT-IR. However, analysis of the mass spectrum
can reveal the formation of this chlorinated VOC as by-product. Indeed, the characteristic
fragment ions of m/z 82, 84 (CCl2+) can be ascribed to TCAA. Fragment ions of m/z 48, 50
(CHCl+) are also formed by collision between electrons and TCAA, but can also originate from
DCAC. TCAA is toxic when inhaled or absorbed through skin and causes respiratory tract
irritation [117]. Therefore, the formation of this compound has to be minimized.
Phosgene (COCl2)
Phosgene is a highly toxic acid chloride that can cause suffocation by inhalation [118]. The
presence of this compound can be indicated by the characteristic bands at 1827, 1685 and 850
cm-1 (Appendix, Figure 78). At 850 cm-1, the band of phosgene is interfered with a TCE
characteristic band. The presence of phosgene can be indicated since this band of TCE does not
decrease to the same extent as the non-interfered TCE band at 945 cm-1. Phosgene is also
detected by the characteristic fragment ions 63 and 65 m/z (CO35Cl+ and CO37Cl+) present in
the mass spectrum of the outlet gas.
The formation of phosgene is also confirmed by Vandenbroucke et al. [63], who modelled the
abatement of TCE in a negative multi-pin-to-plate corona discharge. For an energy density of
300 J·L-1, the model predicted that phosgene accounts for about 30 % of the formed by-products
in dry air. Phosgene is mainly produced by reaction between TCE and ClO radicals, as shown
below.
𝐶2 𝐻𝐶𝑙3 + 𝐶𝑙𝑂 → 𝐶𝑂𝐶𝑙2 + 𝐶𝐻𝐶𝑙2
( 54 )
Phosgene can further decompose by Cl abstraction of chlorine, oxygen or other radicals forming
CO, Cl2 and Cl radicals [25]. A possible way to remove phosgene from the outlet gas is by
58
passing it through a water scrubber leading to the formation of HCl and CO2. HCl can be
neutralized by addition of a base [110].
Molecular chlorine (Cl2)
The production of Cl2 in NTP has to be stimulated since the presence of this compound indicates
total oxidation of TCE (reactions (50) and (51)). However, a large amount of the Cl radicals
formed in NTP does not recombine towards Cl2, but further react with other compounds or
radicals. An important intermediate is ClO, formed by collision between Cl and O radicals. This
radical further reacts with TCE to form toxic phosgene and methyldichloride, which in turn
rapidly reacts with oxygen via
𝐶2 𝐻𝐶𝑙3 + 𝐶𝑙𝑂 → 𝐶𝑂𝐶𝑙2 + 𝐶𝐻𝐶𝑙2
( 55 )
𝐶𝐻𝐶𝑙2 + 𝑂 → 𝐶𝐻𝑂𝐶𝑙 + 𝐶𝑙
( 56 )
Cl2 is IR-transparent and cannot be identified in a FT-IR spectrum, but the mass spectrum of
the outlet gas revealed the formation of Cl2 by the peaks at m/z 70 (35Cl35Cl+), 72 (35Cl37Cl+)
and 74 (37Cl37Cl+).
Hydrogen chloride (HCl)
The presence of HCl is clearly visible in Figure 38 by looking at the characteristic bands of
HCl (Appendix, Figure 79) between 3050 and 2700 cm-1. Chlorine radicals, mainly produced
by dissociation of COCl, are very important for controlling the TCE destruction chemistry, and
largely contribute to the production of Cl-containing by-products. A part of the formed chlorine
radicals reacts via different reaction pathways towards HCl.
𝐶𝐻𝑂𝐶𝑙 + 𝐶𝑙 → 𝐶𝑂𝐶𝑙 + 𝐻𝐶𝑙
( 57 )
𝐶𝐻𝐶𝑙2 𝐶𝑂𝐶𝑙 + 𝐶𝑙 → 𝐶𝐶𝑙2 𝐶𝑂𝐶𝑙 + 𝐻𝐶𝑙
( 58 )
𝐶2 𝐻𝐶𝑙3 + 𝐶𝑙 → 𝐶2 𝐶𝑙3 + 𝐻𝐶𝑙
( 59 )
𝐻𝑂𝐶𝑙 + 𝐶𝑙 → 𝐶𝑙𝑂 + 𝐻𝐶𝑙
( 60 )
Carbon monoxide (CO)
CO can be easily recognized in a FT-IR spectrum by the occurrence of two typical peaks at
2120 and 2180 cm-1, as shown in the reference spectrum of CO (Appendix, Figure 80). Indeed,
these peaks are also present in the zoomed FT-IR spectrum (Figure 38) of the plasma treated
outlet gas. Inhalation of CO can cause harmful health effects by reducing oxygen delivery to
the bodies organs and tissues. Despite these negative effects, CO is less harmful than several
chlorinated by-products (e.g. DCAC, TCAA and phosgene), explaining the aim for a high COx
selectivity. The CO concentration in the outlet depends on the amount of COCl formed during
plasma treatment, as this is the main source for CO production. In both dry and humid air, most
of the COCl is converted into CO by oxidation with O2 and ClO. Another reaction pathway is
the decomposition of COCl into Cl and CO.
59
𝐶𝑂𝐶𝑙 + 𝑂2 → 𝐶𝑙𝑂2 + 𝐶𝑂
( 61 )
𝐶𝑂𝐶𝑙 + 𝐶𝑙𝑂 → 𝐶𝑙2 + 𝐶𝑂 + 𝑂
( 62 )
𝐶𝑂𝐶𝑙 → 𝐶𝑙 + 𝐶𝑂
( 63 )
Carbon dioxide (CO2)
CO2 is the most favorable by-product formed during TCE abatement, and can be identified by
the presence of a peak around 2350 cm-1 (Appendix, Figure 81). This peak is present in the
zoomed spectrum of the outlet gas (Figure 38), but is split into two smaller bands. CO2 can be
produced by oxidation of active species (e.g. CO, COCl, OH) formed in NTP.
𝐶𝑂𝐶𝑙 + 𝑂2 → 𝐶𝑙𝑂 + 𝐶𝑂2
( 64 )
𝐶𝑂 + 𝑂𝐻 → H + CO2
( 65 )
𝐶𝑂 + 𝑂 → CO2
( 66 )
𝐶𝑂 + 𝐶𝑙𝑂 → 𝐶𝑙 + 𝐶𝑂2
( 67 )
Ozone (O3)
The presence of ozone in the outlet gas can be confirmed based on the two characteristic bands
around 1022 cm-1 (Appendix, Figure 82) in the FT-IR spectrum. The most dominant production
and destruction pathways for ozone are already discussed in 3.6. The ozone concentration in
the treated gas has to be minimized due to their negative effects on human health. A lot of
research is done on the use of a plasma in combination with a catalyst, which is able to
efficiently decompose ozone, leading to the production of active oxygen species. These species
further oxidize the by-products, which results in an increased COx selectivity. Plasma-catalytic
TCE abatement will be studied later.
60
TCE & phosgene
0.07
0.06
DCAC
TCE
DCAC
0.05
0.04
DCAC
DCAC
O3
TCE
Absorbance (-)
TCE
DCAC
0.03
TCE
0.02
0.01
0
1300
1200
1100
1000
900
Wavenumber (cm-1)
800
700
600
Figure 36. Detailed FT-IR spectrum between 1350 and 600 cm-1 of the plasma treated TCE gas at an energy density of
50 J·L-1 (dry air, CTCE = 500 ppm, Q = 2.0 L∙min-1, T = 293 K, atmospheric pressure)
DCAC
0.05
phosgene
0.045
0.04
0.03
TCE
0.025
phosgene
0.02
Absorbance (-)
0.035
0.015
0.01
0.005
2050
1950
1850
1750
1650
Wavenumber (cm-1)
1550
1450
0
1350
Figure 37. Detailed FT-IR spectrum between 2050 and 1350 cm-1 of the plasma treated TCE gas at an energy density of
50 J·L-1 (dry air, CTCE = 500 ppm, Q = 2.0 L∙min-1, T = 293 K, atmospheric pressure)
61
0.004
CO2
CO
0.0035
0.0025
HCl
0.002
0.0015
Absorbance (-)
0.003
0.001
0.0005
3050
2950
2850
2750
2650
2550
2450
Wavenumber (cm-1)
2350
2250
2150
0
2050
Figure 38. Detailed FT-IR spectrum between 3050 and 2050 cm-1 of the plasma treated TCE gas at an energy density of
50 J·L-1 (dry air, CTCE = 500 ppm, Q = 2.0 L∙min-1, T = 293 K, atmospheric pressure)
Now that the different degradation products of TCE are identified, the plasma-assisted
experiments can be performed. For each set of experiments, the removal efficiency and the
formation of products CO, CO2, ozone, HCl, phosgene and DCAC are investigated.
Influence of gas flow rate
The plasma-assisted experiments were performed at different values of energy density of the
plasma. In addition, two values for the gas flow rate were investigated: 0.5 and 2 L∙min-1, which
correspond to a residence time of 17.2 and 4.2 seconds, respectively. All TCE abatement
experiments were performed with a TCE concentration of 500 ppm. The influence of the energy
density on the removal efficiency is presented in Figure 39, for both flow rates. The error bars
indicate the 95 % confidence interval, based on the standard deviation of two measurements.
An increase in removal efficiency is observed upon higher energy density, which is due to the
higher density of the radicals responsible for destruction of TCE, such as O, ClO and CHCl2.
These radicals are produced by electron impact reactions with the background gas in plasma,
and the rates of these reactions increase at higher energy density, because of the higher electron
density. Until an energy density of 175 J∙L-1, the gas flow rate has no visible effect on the
removal efficiency. A further increase of the energy density results in a slower rise of the
removal efficiency for a gas flow rate of 0.5 L∙min-1, compared to 2 L∙min-1. Indeed, an increase
of the energy density from 150 to 350 J∙L-1 results in an increase of the removal efficiency from
81 % (0.5 L∙min-1) and 82 % (2.0 L∙min-1) to 92 % (0.5 L∙min-1) and 97 % (2.0 L∙min-1),
respectively. So, if more than 80 % of TCE has to be removed in polluted air, it is better to
62
operate at higher gas flow rates to minimize the energy to be supplied per unit of gas volume.
Moreover, more polluted air can be treated per unit of time when operating at a higher gas flow
rate. Actually, the increase of the removal efficiency at higher gas flow rates was not expected.
A higher residence time should increase the collision probability for electron impact reactions
and for reactions between VOCs and plasma generated radicals. However, the measurement
results in Figure 39 are not very accurate, due to the overlap of the 95 % confidence interval
for several measurement values, and could explain the unexpected trend in the curve.
0.5 L/min
2.0 L/min
100
Removal efficiency (%)
90
80
70
60
50
40
0
200
400
600
Energy density (J/L)
800
1000
Figure 39. TCE removal efficiency as function of the energy density, for a gas flow rate of 0.5 L∙min-1 and 2.0 L∙min-1
(dry air, CTCE = 500 ppm, T = 293 K, atmospheric pressure)
The increase in energy density also leads to an increase of the selectivity towards oxidation
products CO and CO2 (Figure 40). Nevertheless, the COx selectivity remains low and did not
exceeded 31 %. This means that the main part of TCE is decomposed towards chlorinated byproducts phosgene, TCAA and DCAC. As previously discussed, some of these by-products are
more toxic than TCE itself and have to be avoided. The influence of the gas flow rate on the
COx selectivity is not obvious. At low energy density, the COx selectivity is around 4 % higher
when operating at 0.5 L∙min-1. At an energy density of 300 J∙L-1, the difference in COx
selectivity between both gas flow rates becomes smaller and the 95 % confidence intervals start
to overlap. The increase in COx formation can be explained by the higher residence time when
decreasing the gas flow rate. The highest COx selectivity is obtained for a gas flow rate of 0.5
L∙min-1 due to the higher energy density at which sparking occurs.
A lower gas flow rate also resulted in a higher CO2 selectivity due to the higher residence time.
However, the difference in CO2 selectivity is very small. The 95 % confidence interval is not
visible in the curves of the CO2 selectivity, due to the negligible standard deviation. An energy
density of 1050 J∙L-1 and a gas flow rate of 0.5 L∙min-1 resulted in a maximal CO2 selectivity
of 6 %. In order to increase the COx selectivity of the TCE abatement process, a NTP in
63
combination with a catalyst in a PPC configuration will be investigated. This will be discussed
later.
COx selectivity (0.5 L/min)
COx selectivity (2.0 L/min)
CO2 selectivity (0.5 L/min)
CO2 selectivity (2.0 L/min)
35
30
Selectivity (%)
25
20
15
10
5
0
0
200
400
600
Energy density (J/L)
800
1000
Figure 40. COx and CO2 selectivity of the TCE abatement process as function of the energy density, for a gas flow rate
of 0.5 L∙min-1 and 2.0 L∙min-1 (dry air, CTCE = 0 ppm, T = 294 K, atmospheric pressure)
Another interesting product to monitor during plasma-assisted TCE abatement is HCl. The
presence of this compound indicates total oxidation via reaction x. In addition, the formation of
HCl is wanted, since it can be easily removed from the outlet gas. A quantitative determination
of HCl by FT-IR or MS is not possible, due to the absence of a standard. However, the influence
of the gas flow rate on the formation of HCl can be visually observed by comparing FT-IR
spectra for the same TCE conversion. The amount of HCl present in the outlet gas is observed
by zooming in on the characteristic bands of HCl (3050 – 2700 cm-1), as illustrated in Figure
41. These characteristic bands indicate a small decrease of the produced HCl with an increase
in gas flow rate from 0.5 to 2.0 L·min-1. This can be explained by the higher residence time,
resulting in an increased formation of stable products.
64
2 L/min
HCl
0.004
0.003
0.002
Absorbance (-)
0.5 L/min
0.001
3000
2900
2800
Wavenumber (cm-1)
0
2700
Figure 41. FT-IR spectra between 3050 and 2700 cm-1 of the plasma treated outlet gas for two gas flow rates, at a TCE
conversion of 64 %. (dry air, CTCE = 500 ppm, T = 294 K, atmospheric pressure)
Another harmful compound produced in the plasma reactor is ozone, formed by reaction
between plasma generated oxygen radicals and molecular oxygen. The ozone outlet
concentration is measured as a function of the energy density for both gas flow rates, as
illustrated in Figure 42. A higher energy density leads to an increase in ozone concentration
due to the increased production of active electrons in the plasma. There is no influence of the
gas flow rate on the ozone outlet concentration. This determination was also obtained during
the experiments with pure air, as discussed in 7.1. Ozone is well known to be a respiratory
hazard having adverse effects on human health. Therefore, a lot of researchers investigated the
use of plasma in combination with a catalyst that is able to decompose ozone into reactive
oxygen radicals, which in turn enhances the oxidation of TCE towards CO and CO2.
65
0.5 L/min
2.0 L/min
700
Ozone concentration (ppm)
600
500
400
300
200
100
0
0
200
400
600
Energy density (J/L)
800
1000
Figure 42. Ozone outlet concentration as a function of the energy density, for a gas flow of 0.5 L∙min -1 and 2.0 L∙min-1
(dry air, CTCE = 500 ppm, T = 294 K, atmospheric pressure)
The formation of chlorinated by-products (DCAC, phosgene, HCl, TCAA, Cl2) during TCE
abatement has been proven in 8.1 based on FT-IR and MS. The production of DCAC and
phosgene will be studied in more detail due to its toxicity. The influence of the gas flow rate on
the formation of DCAC and phosgene can be visually observed by comparing FT-IR spectra
for the same TCE conversion. The formation of DCAC is investigated by zooming in on the
characteristic bands around 740 cm-1, as shown in the right hand side of Figure 43. A higher
residence time resulted in a decrease of the DCAC formation, due to the improved oxidation as
mentioned above. For the study of phosgene is zoomed in on the characteristic band at 850 cm1
(Figure 43, left). Here, an increase of the residence time resulted in a small increase of the
phosgene production. This was not expected and could not be explained.
0.5 L/min
0.07
2 L/min
DCAC
0.06
0.05
0.03
0.02
0.05
Absorbance (-)
0.04
0.04
0.03
0.02
phosgene
0.01
0.01
0
0
765
755
745
735
725
Wavenumber (cm-1)
715
Absorbance (-)
2 L/min
0.5 L/min
0.06
875
865
855
845
835
Wavenumber (cm-1)
825
Figure 43. FT-IR spectra between 770 – 715 and 875 – 825 cm-1 of the plasma treated outlet gas for two gas flow rates,
at a TCE conversion of 64 %. (dry air, CTCE = 500 ppm, T = 294 K, atmospheric pressure)
66
Influence of humidity
The effect of humidity on TCE decomposition is also investigated, and is of great interest
because water plays an important role in the plasma chemistry. Water limits the electron density
due to its electronegative character, thereby inhibiting the formation of active plasma species
[25]. The humidity of the gas inlet is adjusted by passing the gas through a H2O containing
bubbling bottle, controlled by a mass flow controller. Seven different values of humidity were
investigated.
During the experiments, it was noted that brown/yellow spots were deposited on the spherical
surface segments on the anode plate of the reactor. This deposition was formed as a result of
the humidity since it was only visible during the experiments at 50 and 80 % RH. The spots led
to an increase of the resistance in the electric circuit, thereby reducing the current through the
plasma at a constant voltage. The latter negatively influenced the TCE abatement. Therefore,
the anode plate was cleaned after each experiment. The brown/yellow spots on the anode plate
of the reactor are illustrated in Figure 44.
Figure 44. Anode plate of the plasma reactor profiled with spherical surface segments. Left: spots deposited after
performing a TCE abatement experiment at a relative humidity of 80 %. Right: anode plate after cleaning.
Figure 45 shows the humidity effect on the TCE removal efficiency. For an energy density
higher than 80 J·L-1, a relative humidity of 15 % resulted in the highest removal efficiency. A
further increase of the humidity resulted in a decrease in removal efficiency. For an energy
density of 80 J·L-1, the removal efficiency drops by ± 17 % as the humidity increases from 15
% to 80 %. A relative humidity between 0 - 10 % has no significant influence on the TCE
abatement.
67
The humidity effect on TCE abatement can be explained by looking at the decomposition
reactions of water in plasma [25], shown below.
𝐻2 𝑂 + 𝑒 − → 𝐻 + 𝑂𝐻 + 𝑒 −
( 68 )
+
3
𝐻2 𝑂 + 𝑁2 (𝐴 ∑ ) → 𝑁2 + 𝐻 + 𝑂𝐻
( 69 )
𝑢
𝐻2 𝑂 + 𝑂 → 2 𝑂𝐻
( 70 )
Water decomposes into OH radicals that possess a stronger oxidation power than other oxidants
such as oxygen and peroxyl radicals, resulting in an increase of the TCE removal. On the other
hand, water has a negative effect on TCE abatement since OH radicals strongly react with ClO
and oxygen radicals via
𝑂𝐻 + 𝑂 → 𝑂2 + 𝐻
( 71 )
𝑂H + ClO → 𝐻𝐶𝑙 + 𝑂2
( 72 )
Since ClO and oxygen radicals significantly decompose TCE, a reduced concentration of these
radicals by OH leads to a suppression of the removal efficiency [110, 119]. In addition, water
negatively influences TCE removal due to its electronegative characteristics. Higher humidity
lowers the collision frequency between electrons and molecules (water, TCE, O2, …), thereby
limiting the formation of active plasma species such as OH and O radicals. The latter was
confirmed by Ge et al. [109], who investigated the effect of relative humidity on the relative
production of OH radicals.
Depending on the humidity level of polluted air, the presence of water leads to an enhancement
or inhibition of the removal efficiency. When looking at Figure 45, the enhancement and
inhibition effect seem to balance each other out at a humidity level between 15 – 20 %.
It should be noted that humidity negatively influenced the stability of the plasma. For low
energy density, an increase of the humidity level resulted in a reduction of the amount of
cathode pins that emitted purple light. This can be explained by the electron capturing behavior
of water, impeding the formation of stable plasma between the electrodes. For an energy density
until 80 J·L-1, this explains why the removal efficiency at a humidity of 10 and 15 % RH is
lower than in case of 2.8 and 5 % RH. A decrease of the number of active plasmas leads to a
reduction of the active reaction volume and results in a decrease of the removal efficiency. At
higher energy density, the whole interelectrode gap is filled with plasma due to the higher
electron density.
68
2.8% RH
5% RH
10% RH
20% RH
50% RH
80% RH
15% RH
100
90
Removal efficiency (%)
80
70
60
50
40
30
20
10
0
0
50
100
150
200
Energy density (J/L)
250
300
350
Figure 45. TCE removal efficiency as function of the energy density for for different humidity levels. (humid air, CTCE
= 500 ppm, Q = 2 L·min-1, T = 294.5 K, atmospheric pressure)
The COx selectivity of the removal process as function of energy density is shown in Figure
46, for different humidity levels. An increase of the energy density leads to a higher COx
selectivity. Moreover, the humidity also has a positive effect on the COx selectivity and can be
explained by the strong oxidizing power of OH radicals. An increase of the humidity from 5 to
80 % RH led to a shift in COx selectivity from 8.5 to 18.5 %, at an energy density of 75 J·L-1.
However, for all experiments, the COx selectivity remained low. Even at a humidity of 80 %
RH, the COx selectivity did not exceed 20 %, meaning that TCE is mainly decomposed into
chlorinated by-products. Noteworthy is the significant difference in COx selectivity when
comparing a humidity of 2.8 and 5 %. A possible explanation for these results is that the
experiments with humid air (RH > 2.8 %) were carried out after the plasma reactor was cleaned.
The presence of dust or deposits on the electrodes may cause a reduction in performance of the
reactor, leading to a lower COx selectivity. However, during cleaning there were no visible
deposits present on the electrodes. Energy density nor humidity had an effect on the CO/CO2
ratio, which ranged between 3.5 and 5.
69
2.8% RH
20% RH
5% RH
50% RH
10% RH
80% RH
15% RH
20
18
COx selectivity (%)
16
14
12
10
8
6
4
2
0
0
50
100
150
200
Energy density (J/L)
250
300
350
Figure 46. COx selectivity of the TCE abatement process as function of the energy density for different humidity levels.
(humid air, CTCE = 500 ppm, Q = 2 L·min-1, T = 294.5 K, atmospheric pressure)
Humidity also has a positive effect on the formation of HCl, as shown in Figure 47. The FTIR spectrum of the outlet gas show a significant increase of the characteristic bands of HCl with
increasing humidity. Only for a relative humidity of 50 %, the presence of HCl was not
observed. The absence of HCl is hard to believe, since hydroxyl radicals react with ClO radicals
towards HCl and O2 via (72). A reason for this phenomenom can be that the characteristic bands
of HCl are interfered with a band of water in the FT-IR spectrum. The reference spectrum of
water (Appendix, Figure 83) shows a significant IR absorption band between 3600 and 3000
cm-1. During the experiments at high humidity (> 50 % RH), it was difficult to keep the relative
humidity stable. Due to this instability, water was present in the background spectrum and this
pulled down/up the region where HCl absorbs infrared. This interference could not be removed
by the water compensation method in OPUS and possibly explains why the characteristic bands
of HCl are not visible in the outlet spectrum at 50 % RH.
70
5% RH
10% RH
20% RH
50% RH
0.008
HCl
0.004
Absorbance (-)
0.006
0.002
3000
0.000
2700
2900
2800
Wavenumber (cm-1)
Figure 47. FT-IR spectra between 3050 and 2700 cm-1 of the plasma treated outlet gas for different humidity levels, at
a TCE conversion of 60 %. (humid air, CTCE = 500 ppm, Q = 2 L·min-1, T = 294.5 K, atmospheric pressure)
The influence of humidity on the ozone outlet concentration is illustrated in Figure 48. Just as
in the experiments with pure air, an increase of the humidity leads to a lower ozone outlet
concentration.
2.8% RH
20% RH
5% RH
50% RH
10% RH
80% RH
15% RH
450
ozone concentration (ppm)
400
350
300
250
200
150
100
50
0
0
50
100
150
200
Energy density (J/L)
250
300
350
Figure 48. Ozone outlet concentration as function of the energy density for different humidity levels. (humid air, CTCE
= 500 ppm, Q = 2 L·min-1, T = 294.5 K, atmospheric pressure)
Humidity also influences the formation of DCAC and phosgene. Under humid conditions,
DCAC formation is suppressed when comparing the FT-IR spectra of the outlet gas for different
71
humidity levels (Figure 49). This effect was also reported by other researchers [71, 110] and
can be explained as follows. DCAC can be produced by oxidation of TCE with OH or ClO
radicals via the following reactions [63, 110].
C2 HCl3 + ClO → CHCl2 COCl + HCl
k = 1.66 · 10−13 cm3 molecules −1 s−1
( 73 )
C2 HCl3 + OH → CHCl2 COCl + H
k = 2.4 · 10−14 cm3 molecules −1 s−1
( 74 )
Based on reaction (74), an increase of the DCAC production is expected under humid
conditions. However, OH radicals strongly react with ClO radicals towards HCl and O2
(reaction (72)), leading to a suppression of the DCAC production. The rate of reaction (73) is
one order higher than reaction (74) and will thus determine the total production of DCAC.
ClO radicals also play an important role in the formation of phosgene via
𝐶2 HCl3 + ClO → COCl2 + CHCl2
( 75 )
A decreased formation of phosgene with increasing humidity would be expected due to the
suppression of ClO radicals by OH radicals. However, the results of the experiments show an
enhanced production of phosgene when looking at the right hand side of Figure 49. The
enhancement is rather low and can be explained by reaction of OH radicals with the carboncarbon double bond of TCE leading to CHCl(OH)-CCl2 radicals [71]. This unstable
intermediate further react with O2 towards peroxyl radicals and transform into chloroethoxyl
radicals after radical coupling. The latter radicals can be further decomposed towards phosgene.
An increased formation of phosgene was also observed by Nguyen Dinh et al. [71], who
investigated the production of TCE degradation products in dry and humid air in a multi-pinto-plate reactor.
20% RH
DCAC
80% RH
20% RH
0.09
0.06
50% RH
0.08
80% RH
0.07
0.05
0.04
0.03
0.06
Absorbance (-)
50% RH
0.07
0.05
0.04
0.03
0.02
phosgene
0.02
0.01
0.01
0
0
765
755
745
735
725
Wavenumber (cm-1)
715
Absorbance (-)
5% RH
5% RH
875
865
855
845
835
Wavenumber (cm-1)
825
Figure 49. FT-IR spectra between 770 – 715 and 875 – 825 cm-1 of the plasma treated outlet gas for different humidity
levels, at a TCE conversion of 60 %. (humid air, CTCE = 500 ppm, Q = 2 L·min-1, T = 294.5 K, atmospheric pressure)
72
Influence of TCE
The effect of the TCE inlet concentration on the abatement process is studied since the VOC
concentration in industrial waste gases strongly varies. The experiments were performed with
dry air (2.8 % RH), and the TCE concentration was varied between 250 and 750 ppm.
Figure 50 shows the TCE removal efficiency as function of the energy density for three
different TCE inlet concentrations. The removal efficiency decreases with increasing TCE
concentration for an energy density lower than 250 J·L-1, and can be explained as follows. A
higher amount of TCE molecules causes a noticeable difference in gas composition in the
plasma reactor. Each TCE molecule shares fewer electrons and reactive plasma species, thereby
reducing the probability of reaction between these species and a certain TCE molecule.
Different researchers [120-125] who investigated the abatement of VOCs in NTP confirmed
this effect.
At an energy density of 250 J·L-1 or more, the effect of the TCE concentration on the TCE
removal efficiency is not visible anymore. The concentration of electrons and active species is
so high that it has no influence on the removal efficiency.
250 ppm TCE
500 ppm TCE
750 ppm TCE
100
Removal efficiency (%)
90
80
70
60
50
40
30
0
200
400
600
Energy density (J/L)
800
1000
Figure 50. TCE removal efficiency as function of the energy density for different TCE inlet concentrations. (dry air, Q
= 0.5 L·min-1, T = 294 K, atmospheric pressure)
Figure 51 shows the selectivity of the TCE abatement process towards CO and CO2. The COx
selectivity increases linearly with the energy density and is strongly influenced by the TCE inlet
concentration. At 250 ppm TCE, a COx selectivity of almost 50 % is reached, while for the
higher concentrations tested the COx selectivity remained below 31 %. The production of CO
and CO2 is mainly determined by the amount of atomic oxygen present in the plasma reactor.
73
However, atomic oxygen is also an important TCE decomposing compound (reactions (47-49)).
Since a higher TCE inlet concentration leads to a higher amount of decomposed TCE
molecules, the atomic oxygen concentration in the reactor decreases with an increasing TCE
concentration. Magureanu et al. [125] investigated the influence of the initial concentration on
the TCE removal process in a dielectric barrier discharge, and also observed this effect.
250 ppm TCE
500 ppm TCE
750 ppm TCE
50
COx selectivity (%)
40
30
20
10
0
0
200
400
600
Energy density (J/L)
800
1000
Figure 51. COx selectivity of the TCE abatement process as function of the energy density for different TCE inlet
concentrations. (dry air, Q = 0.5 L·min-1, T = 294 K, atmospheric pressure)
The influence of the initial TCE concentration on the formation of HCl is shown in Figure 52.
A higher initial TCE concentration resulted in an increase of the HCl production, when the same
amount of TCE is converted (240 ppm).
250 ppm TCE
500 ppm TCE
750 ppm TCE
HCl
0.005
0.003
0.002
Absorbance (-)
0.004
0.001
3050
3000
2950
2900
2850
2800
Wavenumber (cm-1)
2750
0
2700
Figure 52. FT-IR spectra between 3050 and 2700 cm-1 of the plasma treated outlet gas for TCE inlet concentrations, at
a conversion of 240 ppm TCE. (dry air, Q = 0.5 L·min-1, T = 294 K, atmospheric pressure)
74
The TCE inlet concentration also affects the ozone formation in the plasma reactor, as shown
in Figure 53. A reduction of the TCE concentration from 750 to 500 ppm resulted in an increase
of the O3 outlet concentration from 252 to 330 ppm, when operating at 250 J·L-1. A further
reduction of the TCE concentration to 250 ppm led to a further increase towards 448 ppm. This
effect can be explained by the competition between TCE and O2 to react with oxygen radicals,
as discussed in 7.3.
250 ppm TCE
500 ppm TCE
750 ppm TCE
1000
ozone concentration (ppm)
900
800
700
600
500
400
300
200
100
0
0
200
400
600
Energy density (J/L)
800
1000
Figure 53. Ozone outlet concentration as function of the energy density for different TCE inlet concentrations. (dry air,
Q = 0.5 L·min-1, T = 294 K, atmospheric pressure)
The amount of DCAC and phosgene present in the outlet gas, for different TCE inlet
concentrations, is shown in Figure 54. The FT-IR spectra were monitored at a constant amount
of converted TCE (240 ppm) to compare these spectra. The TCE inlet concentration negatively
influences the formation of DCAC and phosgene. This was expected since an increase of the
TCE concentration led to an increase of the selectivity towards CO and CO2, as discussed
above. From this it can be concluded that NTP is most suited for the abatement of low
concentrated TCE gases.
75
0.05
250 ppm TCE
500 ppm TCE
750 ppm TCE
DCAC
Phosgene
0.14
0.04
0.02
0.1
Absorbance (-)
0.03
0.12
0.08
0.06
Absorbance (-)
250 ppm TCE
500 ppm TCE
750 ppm TCE
0.04
0.01
0.02
0
765
755
745
735
725
Wavenumber (cm-1)
715
0
875
865
855
845
835
Wavenumber (cm-1)
825
Figure 54. FT-IR spectra between 770 – 715 and 875 – 825 cm-1 of the plasma treated outlet gas for different TCE inlet
concentrations, at a conversion of 240 ppm TCE. (dry air, Q = 0.5 L·min-1, T = 294 K, atmospheric pressure)
76
9 Plasma-catalytic TCE abatement
The plasma-assisted experiments showed that TCE can be removed by NTP without the
addition of considerable energy. However, the application of NTP for TCE abatement is
impeded by low COx selectivity due to the formation of polychlorinated compounds (DCAC,
TCAA, phosgene), which can be more toxic than TCE itself. Moreover, oxygen radicals
generated in the plasma quickly react with molecular oxygen to produce ozone. By placing a
catalyst downstream of the discharge zone, O3 can be decomposed in reactive oxygen radicals,
able to greatly improve the oxidation of TCE and its hazardous by-products.
Different catalysts will be studied and compared based on its performance to decompose TCE
into oxidation products at low energy input. The catalyst temperature is an important parameter
that determines the rate constant of the removal reactions via the Arrhenius equation
𝑘 = 𝐴 · 𝑒 −𝐸𝐴/(𝑅𝑇)
( 76 )
with
k
Reaction constant [depends on reaction order]
( 77 )
A
Pre-exponential factor [depends on reaction order]
( 78 )
Ea
Activation energy [J·mol-1]
( 79 )
R
Universal gas constant [J·(mol·K)-1]
( 80 )
T
Temperature [K]
( 81 )
Based on this equation, the optimal catalyst temperature will be a tradeoff between increasing
TCE removal/oxidation with increasing temperature, and decreasing energy input with
decreasing temperature. Next to the catalyst temperature, the influence of humidity on the
performance of the removal process will be investigated.
Cerium oxide (CeO2)
CeO2 is one of the most commonly used components in the three-way catalysts for purification
of exhaust gases in cars [126]. CeO2 acts as an ‘oxygen store’ during the lean operation
conditions, while it can provide oxygen for the oxidation of H2, CO and organic compounds. In
addition, CeO2 is able to decompose ozone, due to the promoting role of Ce during oxygen
dissociation [127, 128]. For all plasma-catalytic experiments, 0.5 g of CeO2 powder was
introduced in the cylindrical glass reactor located in a temperature controlled oven.
77
9.1.1 Dry air experiments
The first series of experiments are performed with TCE polluted dry air. The plasma is operated
at an energy density of 40 and 80 J·L-1 and the catalyst temperate ranges between 100 – 300
°C. The choice for a low energy density in combination with a low catalyst temperature is to
improve the energy efficiency of the plasma-catalytic system. The TCE abatement
measurements were performed after thermal balance was reached.
Figure 55 shows the TCE removal efficiency of the PPC experiments. The results of the
plasma-alone experiment (performed at room temperature) are also shown to compare them
with the PPC experiments. The error bars indicate the 95% confidence interval, based on the
standard deviation of three measurements. The PPC system clearly enhanced the TCE
abatement compared to the plasma-alone system, obtaining a value of 64 % (plasma-catalysis)
and 51 % (plasma-alone) at 40 J·L-1. TCE abatement experiments were also performed in a
catalyst-alone system. Regardless of the catalyst temperature, the TCE removal efficiency never
exceeded 1 %. This means that the combination of NTP with CeO2 induces a synergetic effect
on the removal efficiency. To evaluate the synergy in our process, a synergy factor f for TCE
abatement is introduced as
𝑓𝑇𝐶𝐸 =
(𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦)𝑝𝑙𝑎𝑠𝑚𝑎−𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑖𝑠
(𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦)𝑝𝑙𝑎𝑠𝑚𝑎−𝑎𝑙𝑜𝑛𝑒 + (𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦)𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡−𝑎𝑙𝑜𝑛𝑒
( 82 )
Thus, the synergy factor gives the relation of the TCE removal efficiency for plasma-catalysis
with respect to the sum of its individual values for plasma and catalyst alone conditions. If this
value exceeds 1, a synergetic effect is observed. Table 7 shows that the synergy factor for TCE
abatement is in the range of 1.17 – 1.29. The synergy factor is slightly lower when operating at
an energy density of 80 J·L-1. This can be explained by the higher TCE abatement with
increasing energy density. At 80 J·L-1, NTP is able to decompose 65 % of TCE. This means
that only 35 % of the TCE is available for further decomposition by the catalyst. While in case
of an energy density of 40 J·L-1, still 50 % of the initial TCE is available for further degradation.
The effect of the catalyst temperature on the synergy factor is much smaller. The small decrease
of the synergy factor with increasing temperature can be possibly explained by little
deactivation of the catalyst. All PPC experiments were started at a catalyst temperature of 100
°C and the temperature was further increased in steps of 50 °C. So, during the performance of
the experiment at 300 °C it could be that a part of the active sites were poisoned by TCE
degradation products, formed during previous experiments, and thereby inhibiting the
decomposition of TCE.
78
Table 7. Synergy factors for plasma-catalytic TCE abatement
Temperature [°C]
100
150
200
250
300
90
80
fTCE
40 J·L-1
1.27
1.29
1.26
1.25
1.23
80 J·L-1
1.20
1.22
1.20
1.18
1.17
40 J/L
80 J/L
Removal efficiency (%)
70
60
50
40
30
20
10
0
NTP
PPC 100°C
PPC 150°C
PPC 200°C
PPC 250°C
PPC 300°C
Figure 55. TCE abatement in a plasma-catalytic (CeO2) and a plasma-alone system for different temperatures, at an
energy density of 40 and 80 J·L-1. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, atmospheric pressure)
Figure 56 shows the selectivity of the TCE abatement process towards CO, CO2 and
chlorinated by-products for the plasma-alone and PPC system. Only the results of the
experiments at an energy density of 40 J·L-1 are displayed. An increase of the energy density to
80 J·L-1 had no significant effect on the COx selectivity. The PPC system at a low temperature
increased the COx selectivity compared to the plasma-alone system. Indeed, a catalyst
temperature of 100 °C resulted in a COx selectivity of 14.3 %, compared to 9.3 % in NTP. When
increasing the catalyst temperature to 300 °C, the COx selectivity further increases to 60 % due
to the activated ozone dissociation reaction on the catalyst surface.
79
Cl by-products
90
CO
80
CO2
Selectivity (%)
70
60
50
40
30
20
10
0
NTP
PPC 100°C
PPC 150°C
PPC 200°C
PPC 250°C
PPC 300°C
Figure 56. The selectivity of the TCE removal process towards CO, CO2 and chlorinated by-products in a plasmacatalytic (CeO2) and a plasma-alone system for different temperatures, at an energy density of 40 J·L-1. (dry air, CTCE
= 500 ppm, Q = 0.5 L·min-1, atmospheric pressure)
The increased COx selectivity in the PPC system can be explained by the decomposition of
ozone on the catalyst surface. Ozone can be cleaved at the CeO2 surface, leading to the
formation of active oxygen species via [25, 129]
𝑂3 + ∗ → O2 + O∗
( 83 )
𝑂3 + O∗ → O2 + O2 ∗
( 84 )
The oxygen species will desorb and react with chlorinated by-products towards oxidation
products CO, CO2, HCl and Cl2, explaining the increased COx selectivity. The ozone
decomposing ability of CeO2 is clearly visible in Figure 57, that shows the ozone concentration
in the outlet gas of the plasma-alone and the PPC system. The use of a catalyst (at 100 ° C)
downstream of the plasma (operated at 40 J·L-1) resulted in a decrease of the ozone outlet
concentration from 119 to 74 ppm, compared to the plasma-alone system. An increase of the
catalyst temperature further decreased the ozone concentration, due to the activated ozone
dissociation on the catalyst. The experiments at an energy density of 80 J·L-1 resulted in a higher
ozone production, due to the presence of more electrons in the plasma when operating at a
higher energy density.
80
175
40 J/L
80 J/L
O3 concentration (ppm)
150
125
100
75
50
25
0
NTP
PPC 100°C
PPC 150°C
PPC 200°C
PPC 250°C
PPC 300°C
Figure 57. Ozone outlet concentration of the plasma treated TCE gas in a plasma-catalytic (CeO2) and a plasma-alone
system for different temperatures, at an energy density of 40 J·L-1. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1,
atmospheric pressure)
As previously discussed, the use of NTP in combination with a catalyst clearly reduced the
selectivity towards chlorinated by-products. It is interesting to identify the chlorinated byproducts and to investigate whether new products are formed in the PPC system. Nguyen Dinh
et al. [130] studied the removal of TCE in a PPC system using Ce-Mn based oxides as catalyst,
and detected the presence of CCl4 (794 cm-1) and HCCl3 (773 cm-1) in the outlet FT-IR spectrum
of all PPC experiments. However, when zooming in on this specific region in the outlet
spectrum of the PPC process, none of these absorption bands were visible. The comparison of
the FT-IR outlet spectrum (Figure 58) of the plasma-alone and PPC system confirms the
presence of the following chlorinated by-products: phosgene, DCAC, TCAA and HCl. The
formation of Cl2 is identified based on the MS spectrum of the outlet gas. The production of
phosgene, DCAC and HCl will be discussed more in detail.
81
TCE inlet
NTP
PPC-100°C
0.2
0.18
0.16
0.12
0.1
0.08
Absorbance (-)
0.14
0.06
0.04
0.02
0
2900
2700
2500
2300
2100
1900 1700 1500
Wavenumber (cm-1)
1300
1100
900
700
Figure 58. FT-IR spectra between 700 and 3050 cm-1 of TCE inlet (black), plasma-alone (red) and plasma-catalytic
(blue) treated TCE gas at an energy density of 40 J·L-1 (dry air, CTCE = 500 ppm, Q = 0.5 L∙min-1, Tplasma = 293 K,
atmospheric pressure)
Figure 59 shows the characteristic IR bands of HCl in the outlet FT-IR spectrum of the plasmaalone and PPC system at different catalyst temperatures. The use of CeO2 (at 100 °C) in
combination with NTP led to an increased formation of HCl, compared to the plasma-alone
system. This can be explained by the production of oxygen radicals on the catalyst surface,
enhancing the rate of the following oxidation reaction
C2 HCl3 + 4O → 2CO2 + HCl + Cl2
( 85 )
Operating at a higher catalyst temperature further increased the HCl production, due to
activation of the catalyst.
82
NTP
PPC-200°C
PPC-100°C
PPC-250°C
PPC-150°C
PPC-300°C
HCl
0.04
0.02
Absorbance (-)
0.03
0.01
3050
3000
2950
2900
2850
Wavenumber (cm-1)
2800
2750
0
2700
Figure 59. FT-IR spectra between 3050 and 2700 cm-1 of the plasma and plasma-catalytic treated outlet gas for different
temperatures, at a TCE conversion of 64 %. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, Tplasma = 293 K, atmospheric
pressure)
As expected, the addition of a catalyst resulted in a decrease of the DCAC production (Figure
60, left). An increasing catalyst temperature further enhanced the decomposition of DCAC.
However above a catalyst temperature of 200 °C, this effect was strongly reduced.
Next to DCAC, researchers [71, 129, 130] also observed a decreased production of phosgene
during TCE abatement in a plasma-catalytic system when using metal oxides as catalyst.
However, Figure 60 (right) indicates that the addition of a catalyst (at 100 °C) to the plasma
does not influence the phosgene production. However, an increase of the catalyst temperature
slightly enhanced the formation of phosgene. This was unexpected since phosgene should be
decomposed by oxygen radicals via
𝐶𝑂𝐶𝑙2 + O → 𝐶𝑂𝐶𝑙 + 𝐶𝑙𝑂
( 86 )
A possible explanation can be that reaction (86) is inhibited due to competition with other
oxidation reactions, such as the oxidation of DCAC, TCAA and TCE. In addition, the increased
production of oxygen species on the catalyst surface with increasing catalyst temperature will
enhance the production of ClO radicals, which can further react towards phosgene.
𝐶𝑙2 + O → Cl + ClO
( 87 )
𝐶2 HCl3 + ClO → COCl2 + CHCl2
( 88 )
However, this explanation could not be justified due to the absence of information about the
rate constants of the above reactions.
83
NTP
PPC-100°C
PPC-150°C
PPC-200°C
PPC-250°C
PPC-300°C
0.04
0.03
0.02
0.1
0.08
0.06
0.04
0.01
0.02
phosgene
0
765
755
745
735
725
Wavenumber (cm-1)
715
Absorbance (-)
0.05
DCAC
Absorbance (-)
NTP
PPC-100°C
PPC-150°C
PPC-200°C
PPC-250°C
PPC-300°C
0
875
865
855
845
835
Wavenumber (cm-1)
825
Figure 60. FT-IR spectra between 770 – 715 and 875 – 825 cm-1 of the plasma-alone and plasma-catalytic treated TCE
gas for different catalyst temperatures, at a TCE conversion of 64 %. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, Tplasma
= 293 K, atmospheric pressure)
9.1.2 Influence of humidity
The effect of humidity on the performance of the PPC removal process is studied by performing
experiments with TCE polluted air at different humidity levels. The plasma-alone experiments
(8.3) already showed the positive effect of humidity on the COx selectivity due to the strong
oxidizing power of hydroxyl radicals. Four different relative humidity levels (10, 20, 50 and 80
% RH) were studied and the experiments were performed at an energy density of 280 and 400
J·L-1. The reason why such high energy densities were used is because of the negative effect of
humidity on the stability of the plasma. At low energy density, an increase of the humidity
resulted in a reduction of the amount of cathode pins that emitted purple light. This can be
explained by the electron capturing behavior of water, impeding the formation of stable plasma
between the electrodes. So to be able to compare the results of the experiments at different
humidity levels it is important that during each experiment the same amount of electrodes light
up. Tests showed that all 10 electrodes light up at an energy density higher than 250 J·L-1,
regardless of the humidity.
The effect of humidity on the TCE removal efficiency is shown in Figure 61. The experiments
were performed at 280 and 400 J·L-1, and the catalyst temperature was set on 200 °C. At first
sight, an increase of the humidity leads to a small decrease of the removal efficiency. Indeed,
an increase of the humidity from 2.8 to 80 % RH led to a decrease of the removal efficiency
from 95.5 to 86.5 %. The adverse effect of humidity on the TCE removal efficiency is also
observed by other researchers [107, 132, 133], and can be partly ascribed to a decrease in
catalytic ozone decomposition under humid conditions (Figure 63), since the amount of
converted ozone is directly related to the concentration of newly formed active oxygen species
over the catalyst surface. Another possible explanation is that the competitive adsorption of
water also inhibits the adsorption of TCE, thereby reducing the catalytic removal of TCE.
However, the latter explanation seems unlikely since the catalyst-alone experiments showed a
84
small enhancement of the TCE removal with increasing humidity. However, the removal
efficiency of the catalyst-alone experiments was negligible and never exceeded 5 %.
PPC 280 J/L
PPC 400 J/L
Removal efficiency (%)
100
90
80
70
60
0
10
20
30
40
50
Relative humidity (%)
60
70
80
Figure 61. TCE removal efficiency in a plasma-catalytic (CeO2) system as function of the relative humidity, at an
energy density of 280 and 400 J·-1. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 294.5 K, atmospheric pressure)
The influence of the humidity and catalyst temperature on the COx selectivity of the PPC system
is plotted in Figure 62. The experiments were performed at an energy density of 280 J·L-1. The
effect of water is rather complex and strongly depends on the catalyst temperature. At a catalyst
temperature of 100 °C, humidity positively influences the COx selectivity. The COx selectivity
is rather low and reaches a maximal value of 38 %, at 80 % RH. The enhanced TCE oxidation
with increasing humidity can be explained by the strong oxidizing power of water. When
looking at the COx selectivity at a catalyst temperature of 250 and 300 °C, water has a negative
effect on the oxidation of TCE for a humidity higher than 50 % RH. This can be explained as
follows. An increase of the temperature activates the catalyst resulting in an increase of the
ozone decomposition rate on the catalyst surface. This effect is clearly visible in Figure 63.
Ozone is decomposed in active oxygen species, enhancing the production of oxidation products
CO and CO2. However an increase of the humidity inhibits the ozone formation via reaction
(46) resulting in a decrease of active oxygen species on the catalyst surface. At low humidity
(< 50 % RH), water has a positive effect on the COx selectivity due to the combination of strong
oxidizing hydroxyl radicals and a high amount of ozone decomposed on the catalyst surface.
So at a high catalyst temperature, the optimal humidity is a tradeoff between increasing
hydroxyl radicals with increasing humidity, and increasing oxygen species produced by the
catalyst with decreasing humidity. The importance of ozone is clearly visible when comparing
the COx selectivity for dry air and humid air at 80 % RH. An increase of the catalyst temperature
from 100 to 300 °C leads to an increase of the COx selectivity from 24 to 82 % (dry air) and 38
to 72 % (humid air at 80 %). The smaller increase at 80 % RH is caused by the inhibition of the
ozone formation. The highest COx selectivity is 89 %, reached at a catalyst temperature of 300
°C and a humidity of 20 % RH.
85
PPC 100°C
PPC 250°C
100
PPC 150°C
PPC 300°C
PPC 200°C
90
COx selectivity (%)
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
Relative humidity (%)
60
70
80
Figure 62. The selectivity of the TCE removal process towards COx in a plasma-catalytic (CeO2) system as a function
of the relative humidity, at an energy density of 280 J·L-1. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 294.5 K,
atmospheric pressure)
PPC 280 J/L (100°C)
PPC 400 J/L (100°C)
PPC 280 J/L (150°C)
PPC 400 J/L (150°C)
PPC 280 J/L (200°C)
PPC 400 J/L (200°C)
200
ozone concentration (ppm)
160
120
80
40
0
0
10
20
30
40
50
Relative humidity (%)
60
70
80
Figure 63. Ozone outlet concentration of the plasma-catalytic (CeO2) treated TCE gas as function of the relative
humidity, for different temperatures and energy densities. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 294.5 K,
atmospheric pressure)
The influence of humidity on the production of HCl is visualized in Figure 64. The left hand
side shows the outlet FT-IR spectra between 2700 – 3050 cm-1 at a catalyst temperature of 100
°C for three different humidity levels. The spectra were measured at a removal efficiency
between 86 – 89 %. No peaks were observed, indicating the absence of HCl in the treated outlet
86
gas. An increase of the catalyst temperature to 300 °C resulted in an enhancement of the HCl
formation (Figure 64, right), except for a humidity of 80 % RH. This indicates that CeO2
enhances the formation of HCl (if the catalyst is activated), while water suppresses its
formation. The latter was unexpected since the plasma-alone experiments showed the opposite
effect. Humidity enhances the formation of HCl due to the temperature independent reaction
between OH and ClO radicals [110].
𝑂𝐻 + 𝐶𝑙𝑂 → 𝐻𝐶𝑙 + O2
k = 2.5 · 10-11 cm3 molecules −1 s−1
( 89 )
However, the HCl production can be suppressed by the occurrence of reaction [133]
𝐻𝐶𝑙 + 𝑂𝐻 → 𝐻2 O + Cl
k = 8.4 · 10-13 cm3 molecules −1 s−1
( 90 )
At room temperature, the rate constant of the HCl consuming reaction is 2 orders of magnitude
lower as the HCl production reaction, but can be increased with increasing temperature.
Moreover, it is possible that CeO2 activates this HCl consuming reaction explaining the
decrease of HCl formation with increasing humidity. However, this does not explain the
absence of HCl at a catalyst temperature of 100 °C.
10% RH (300 °C)
50% RH (300 °C)
80% RH (300 °C)
0.014
0.012
0.012
0.01
0.01
0.008
0.006
0.008
0.006
0.004
0.004
0.002
0.002
0
3050 3000 2950 2900 2850 2800 2750 2700
Wavenumber (cm-1)
Absorbance (-)
0.014
Absorbance (-)
10% RH (100°C)
50% RH (100 °C)
80% RH (100 °C)
0
3050 3000 2950 2900 2850 2800 2750 2700
Wavenumber (cm-1)
Figure 64. FT-IR spectra between 3050 and 2700 cm-1 of the plasma-catalytic treated outlet gas for different humidity
levels, at a catalyst temperature of 100 °C (left) and 300 °C (right). The spectra are measured at a TCE conversion
between 86 – 89 %. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, Tplasma = 294 K, atmospheric pressure)
An increase of the humidity also suppresses the formation of DCAC, as illustrated in Figure
65. The left hand side shows the characteristic peak of DCAC in the FT-IR outlet spectrum for
three humidity levels at a catalyst temperature of 100 °C. For a humidity of 80 % RH, the DCAC
outlet concentration is three times higher than for a humidity of 10 %. This effect was also
observed during the plasma-alone experiments (8.3), and can be explained by the decreasing
formation of ClO with increasing humidity via reaction (89). ClO is an important radical for
the decomposition of TCE into DCAC.
87
An increase of the catalyst temperature from 100 to 300°C (Figure 65, right) strongly activates
the catalyst and resulted in a complete removal of DCAC that was formed in the NTP.
10% RH (100 °C)
10% RH (300 °C)
0.04
50% RH (100 °C)
0.04
50% RH (300 °C)
80% RH (100°C)
80% RH (300 °C)
Absorbance (-)
0.02
0.02
0.01
0.01
0
0
765
755
745
735
725
Wavenumber (cm-1)
Absorbance (-)
0.03
0.03
765
715
755
745
735
725
Wavenumber (cm-1)
715
Figure 65. FT-IR spectra between 770 and 715 cm-1 of the plasma-catalytic treated outlet gas for different humidity
levels, at a catalyst temperature of 100 °C (left) and 300 °C (right). The spectra are measured at a TCE conversion
between 86 – 89 %. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, Tplasma = 294 K, atmospheric pressure)
In contrast to DCAC, humidity enhances the production of phosgene in the PPC process.
However, the increased phosgene formation with increasing humidity is quiet low, as can be
seen in Figure 66. These results are consistent with the results of the plasma-alone experiments
in 8.3. The outlet FT-IR spectra at a catalyst temperature of 300 °C show a strong reduction of
the phosgene production, compared to 100 °C. This can be explained by the enhanced ozone
decomposition on the catalyst surface leading to an increased concentration of active oxygen
species.
10% RH (300 °C)
50% RH (300 °C)
80% RH (300 °C)
50% RH (100 °C)
0.07
0.07
0.06
0.06
0.05
0.05
0.04
0.03
0.02
Absorbance (-)
80% RH (100 °C)
0.04
0.03
0.02
0.01
0.01
0
875
865
855
845
835
Wavenumber (cm-1)
825
Absorbance (-)
10% RH (100 °C)
0
875
865
855
845
835
Wavenumber (cm-1)
825
Figure 66. FT-IR spectra between 875 and 825 cm-1 of the plasma-catalytic treated outlet gas for different humidity
levels, at a catalyst temperature of 100 °C (left) and 300 °C (right). The spectra are measured at a TCE conversion
between 86 – 89 %. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, Tplasma = 294 K, atmospheric pressure)
88
Cryptomelane (K-OMS-2)
A second mineral that is studied as potential ozone decomposing catalyst in the plasma-catalytic
abatement of TCE is cryptomelane. This potassium manganese oxide mineral, with formula
KMn8O16, is composed of 2 x 2 edge-shared MnO6 octahedral chains, which are corner
connected to form one-dimensional tunnels of ca. 4.7 x 4.7 Ȧ2 (Figure 67). The synthetic
counterpart of cryptomelane is known as octahedral molecular sieve (K-OMS-2). Due to the
presence of manganese in different oxidation states within the framework of OMS materials and
their ability to transfer oxygen, these materials are widely investigated as catalysts for partial
and total oxidation reactions [134]. In addition, OMS-type catalysts have been proved to be a
cost-effective and environmentally benign alternative to conventional catalysts for the
oxidation of VOCs such as acetone [135] and ethyl acetate [136].
Figure 67. 2D dimensional projection of K-OMS-2 structure [137]
For all (plasma-)catalytic experiments, 0.5 g of K-OMS-2 powder and 3 g of carborundum were
introduced in the cylindrical glass reactor located in a temperature controlled oven. Before each
experiment, K-OMS-2 was calcinated at 350 °C for 4 hours.
9.2.1 Influence of humidity
TCE abatement is investigated at 150 °C with catalyst alone, NTP at ambient conditions and
with a PPC system. Figure 68 shows the TCE removal efficiency in NTP and PPC system as
function of the energy density. The PPC experiments were performed with dry (2.8 % RH) and
humid (15 % RH) air to investigate the effect of humidity on the TCE abatement. Regarding
the NTP experiment, the TCE abatement increases with energy density due to the formation of
more energetic electrons, reaching a removal efficiency of 90 % at 250 J·L-1. The evolution of
the TCE abatement as function of energy density in PPC experiments with dry air displays a
similar behavior to the one observed for NTP alone, but are translated to higher values for a
89
given energy density. The TCE removal efficiency in the catalyst-alone system was very low
and did not exceeded 3 %. The sum of the removal efficiency in NTP and catalyst-alone system
is lower than the removal efficiency in the PPC system, which indicates a synergetic effect by
combining both NTP and K-OMS-2 catalyst. The synergy factors of the PPC system for
different energy densities are given in Table 8. An increase of the energy density resulted in
lower values of the synergy factor since the removal efficiency is getting closer to its maximal
value. Comparison of the synergy factors of dry and humid air shows a negative effect of
humidity on the performance of the PPC system. An increase of the energy density from 40 to
200 J·L-1 resulted in a decrease of the synergy factor from 1.37 to 0.89. This means that the
synergetic effect disappears with increasing energy density. This effect is also observable in
Figure 68. In contrast, humidity has a positive effect on TCE abatement in the plasma-alone
system. A possible explanation for the negative effect of humidity in the PPC system is the
competitive adsorption of water, inhibiting the adsorption of TCE on the catalyst surface.
Table 8. Synergy factors for plasma-catalytic (K-OMS-2) TCE abatement
Energy density [J·L-1]
fTCE
dry air
1.60
1.37
1.18
-
40
80
100
150
200
PPC - dry
NTP - dry
15 % RH
1.37
1.16
1.07
0.95
0.89
PPC - 15% RH
NTP - 15% RH
100
Removal efficiency (%)
90
80
70
60
50
40
30
0
50
100
150
Energy density (J/L)
200
250
Figure 68. TCE removal efficiency in a plasma-alone and plasma-catalytic (K-OMS-2) system as function of the energy
density, for two different humidity levels. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 295.5 K, atmospheric
pressure)
90
Figure 69 shows the carbon mass-balance in terms of COx selectivity in function of energy
density. The PPC system clearly improved the complete oxidation of TCE towards CO2. During
the PPC experiments with dry air, a maximal CO2 selectivity of 47 % was obtained, while in
NTP only 3 % could be reached. The increased CO2 selectivity in the PPC system indicates the
ozone decomposing power of the catalyst. The decomposition of ozone on the catalyst can also
be seen when looking at the ozone concentration of the outlet gas, shown in Figure 70. The
ozone concentration of the plasma-catalytic treated outlet gas is very low, and never exceeded
3 ppm. In contrast, the ozone concentration of the plasma treated outlet gas is much higher and
ranges between 50 and 350 ppm, depending on the energy density.
COx - dry (PPC)
COx - 15% RH (PPC)
COx - dry (NTP)
COx - 15% RH (NTP)
CO2 - dry (PPC)
CO2 - 15% RH (PPC)
CO2 - dry (NTP)
CO2 - 15% RH (NTP)
70
COx selectivity (%)
60
50
40
30
20
10
0
0
50
100
150
Energy density (J/L)
200
250
Figure 69. The selectivity of the TCE removal process towards COx in a plasma-alone and plasma-catalytic (K-OMS-2)
system as function of the energy density, for two different humidity levels. (humid air, CTCE = 500 ppm, Q = 0.5 L·min1, T = 295.5 K, atmospheric pressure)
For both the NTP and PPC experiments, humidity enhanced the COx selectivity. At 40 J·L-1, an
increase of the humidity from 2.8 to 15 % resulted in an increase of the COx selectivity from
57 to 71 %. This is due to the oxidizing power of hydroxyl radicals formed in NTP.
91
PPC - dry
PPC - 15% RH
NTP - dry
NTP - 15% RH
350
ozone concentration (ppm)
300
250
200
150
100
50
0
0
50
100
150
200
Energy density (J/L)
250
300
Figure 70. Ozone outlet concentration of the (plasma-)catalytic (K-OMS-2) treated TCE gas as function of the energy
density, for two different humidity levels. (humid air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 294.5 K, atmospheric
pressure)
9.2.2 Catalyst deactivation
When looking at the evolution of the COx selectivity as function of energy density in Figure
69, something remarkable was noticed. For the NTP experiments, it can be seen that the CO x
selectivity increased with the increase of the energy density, indicating that higher energy
density promotes the conversion of TCE to total oxidation. This trend was also observed during
the plasma-catalytic experiments with CeO2. However, in case of K-OMS-2, the opposite effect
was observed. An increase of the energy density from 25 to 250 J·L-1 resulted in a decrease of
the COx selectivity from 71 to 61 %. The most plausible explanation for this phenomenon is
deactivation of the catalyst, since the energy density was increased in time. K-OMS-2 possesses
strong adsorption properties due to the variable valence state for manganese in its structure
[138]. However, adsorption is selective and depends on the adsorption enthalpy of the adsorbing
species. In the ideal case, ozone is selective adsorbed and decomposed on the catalyst,
increasing the total oxidation of TCE and chlorinated by-products formed in NTP. However,
the decrease of the COx selectivity in time suggests poisoning of the catalytic sites with
chlorinated by-products. In order to prove the deactivation of the catalyst the TCE abatement
experiment was performed in the reverse direction. The PPC experiment was started at an
energy density of 250 J·L-1, and was further decreased in steps of 25 J·L-1. The results of the
reverse experiment are shown in Figure 71. As can be seen, the COx selectivity again decreases
in function of time, indicating the deactivation of the catalyst. The reverse experiment took
around 5 hours, and resulted in a decrease of the COx selectivity from 62 to 37 %.
92
COx - dry (normal)
COx - dry (reverse)
start
70
start
COx selectivity (%)
60
50
40
30
20
10
0
0
50
100
150
Energy density (J/L)
200
250
Figure 71. The COx selectivity of the plasma-catalytic (K-OMS-2) experiment as function of the energy density,
performed in the normal and reverse direction. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 295.5 K, atmospheric
pressure)
The deactivation of the catalyst is also visible in Figure 72. In this figure, the blue line shows
the evolution of the TCE removal efficiency of the catalyst-alone system as a function of time.
After 1 hour, a removal efficiency of 4.5 % was reached. After 5 hours of passing TCE through
the catalyst, the TCE removal was measured again and resulted in a value of 3.3 % (red line).
So during 5 hours of passing TCE through the catalyst, the removal efficiency dropped 1.2 %.
During a second series of measurements, the catalyst-alone experiment was performed after the
PPC experiments. The latter experiments took 5 hours and the removal efficiency in the
catalyst-alone experiment now reached a value of only 1.4 % (green line). This value is halved
compared to the removal efficiency of the catalyst-alone system (perfomed after 5 hours), and
shows that the by-products formed in NTP are the most responsible for the deactivation of KOMS-2.
93
50
45
Removal efficiency (%)
40
35
Catalyst alone - dry
(immediate)
30
Catalyst alone - dry
(after 5 h)
25
20
Catalyst alone - dry
(after PPC exp.)
15
10
5
0
0
500
1000
1500
2000
Time (s)
2500
3000
3500
Figure 72. The removal efficiency of the catalyst-alone system as function of time. A first experiment was performed
immediately (blue line) and after 5 h of passing TCE through the catalyst (red line). A second experiment was executed
after the PPC experiments (green line). (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 295.5 K, atmospheric pressure)
Fe3O4/K-OMS-2
This cryptomelane composite is synthesized to improve the TCE abatement to total oxidation,
since Fe3O4 is known for its oxidizing properties [139]. The catalyst is produced by grafting
Fe3O4 nanoparticles onto OMS-2 via a precipitation method, as explained in 6.4.1. The TCE
abatement experiments were performed under the same conditions as the experiments with pure
K-OMS-2 in order to compare the results.
The TCE removal efficiency as function of the energy density is shown in Figure 73. Placing
Fe3O4/K-OMS-2 downstream of the plasma source slightly enhanced the TCE abatement
compared to the plasma-alone system. Operating at 40 J·L-1 resulted in an enhancement of 11
%, but this effect decreased with increasing energy density to only 3 % at 250 J·L-1. The
catalyst-alone system resulted in a removal efficiency of 3.5 %, which indicates that the
synergetic effect of the PPC system is negligible. The PPC system with pure K-OMS-2 on the
other hand, shows a clear synergetic effect. The latter was already quantified by the synergy
factors in 9.2.1. (Table 8).
94
PPC (Fe3O4/K-OMS-2)
PPC (K-OMS-2)
NTP
100
Removal efficiency (%)
90
80
70
60
50
40
30
0
50
100
150
Energy density (J/L)
200
250
Figure 73. TCE removal efficiency in a plasma-alone and plasma-catalytic system with K-OMS-2 and Fe3O4/K-OMS-2
as function of the energy density. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 295.5 K, atmospheric pressure)
Figure 74 shows the carbon mass-balance in terms of COx selectivity as function of the energy
density. In contrast to the TCE removal efficiency, Fe3O4/K-OMS-2 gives better results in terms
of COx selectivity compared to K-OMS-2. Operating at 40 J·L-1 resulted in a COx selectivity of
74 and 57 % for Fe3O4/K-OMS-2 and K-OMS-2, respectively. This indicates that the addition
of Fe3O4 to K-OMS-2 results in a compromise between increasing COx selectivity and
decreasing TCE removal.
When looking at the evolution of the COx selectivity as function of the energy density, the same
decreasing trend as in case of K-OMS-2 is visible. This can again be explained by the
deactivation of the catalyst by chlorinated intermediates formed in NTP. An increase of the
energy density from 40 to 150 J·L-1 resulted in a decrease of the COx selectivity from 74 to 62
%.
95
Figure 74. The selectivity of the TCE removal process towards COx in a plasma-alone and plasma-catalytic system with
K-OMS-2 and Fe3O4/K-OMS-2 as function of the energy density. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, T = 295.5
K, atmospheric pressure)
The high COx selectivity in the PPC system with Fe3O4/K-OMS-2 clearly affects the
concentration of hazardous chlorinated by-products in the treated outlet gas, as shown in Figure
75. The HCl concentration in the Fe3O4/K-OMS-2 system is slightly increased compared to KOMS-2, while the formation of DCAC and phosgene is more inhibited for Fe3O4/K-OMS-2.
During the analysis of the FT-IR outlet spectra, two new bands at 794 and 772 cm-1 have been
detected in the PPC experiments with K-OMS-2 and Fe3O4/K-OMS-2. These bands can be
ascribed to the C-Cl vibrations of CCl4 and CHCl3 (chloroform), when comparing with
reference compound spectra of NIST (Appendix, Figure 84 and 85). The bands of these two
compounds were not visible in the outlet spectra of the plasma-alone and PPC system with
CeO2. CCl4 and CHCl3 may be likely formed through carbon-carbon cleavage of DCAC and
TCAA. This explains the larger decrease of the DCAC outlet concentration compared to
phosgene, when comparing the PPC and plasma-alone experiments.
96
Fe3O4/K-OMS-2
K-OMS-2
NTP
Fe3O4/K-OMS-2
K-OMS-2
phosgene
NTP
0.04
0.1
0.08
Absorbance (-)
0.04
0.01
1100
1090
1080
1070
1060
Wavenumber (cm-1)
Fe3O4/K-OMS-2
K-OMS-2
NTP
0.02
0
0
1050
875
HCl
CCl4
825
0.14
0.12
CHCl3
0.1
Absorbance (-)
0.008
0.004
855
845
835
Wavenumber (cm-1)
Fe3O4/K-OMS-2
K-OMS-2
CeO2
NTP
0.01
0.006
865
0.08
0.06
0.04
0.002
0
3050 3000 2950 2900 2850 2800 2750 2700
Wavenumber (cm-1)
Absorbance (-)
0.02
DCAC
0.06
Absorbance (-)
0.03
0.02
0
810
800
790
780
770
Wavenumber (cm-1)
760
750
Figure 75. Zoomed FT-IR outlet spectra of the chlorinated by-products in the plasma-alone and PPC system with KOMS-2 and Fe3O4/K-OMS-2, at a catalyst temperature of 150 °C. The spectra were measured at a TCE conversion
between 85 – 87 %. (dry air, CTCE = 500 ppm, Q = 0.5 L·min-1, Tplasma = 295.5 K, atmospheric pressure)
Plasma-catalytic TCE abatement scheme
To finish the chapter on the plasma-catalytic TCE abatement, a simplified TCE degradation
scheme is presented in Figure 76. When passing TCE polluted air through plasma, electronmolecule collisions convert N2 and O2 molecules to a mixture of ionized, excited, metastable
and radical species that are able to decompose TCE to polychlorinated intermediates (phosgene,
DCAC, TCAA) and total oxidation products (CO, CO2, HCl, Cl2). Molecular oxygen is also
involved in a three body reaction leading to the formation of O3. Downstream of the plasma
source, O3 comes into contact with the catalyst surface and dissociates towards molecular and
radical oxygen species. These surface species promote the further oxidation of residual TCE
and the chlorinated by-products towards CO, CO2, HCl and Cl2 via pathway 1. In case of
97
cryptomelane-type catalysts (K-OMS-2 and Fe3O4/K-OMS-2) an additional reaction path
(pathway 2) is present, leading to the formation of CCl4 and CHCl3 through carbon-carbon
cleavage of DCAC and TCAA.
Figure 76. Simplified TCE degradation scheme in a plasma-catalytic process
98
10 Conclusion
In this thesis, the abatement of TCE in a plasma-catalytic system was studied. Three different
catalysts were studied and compared based on the removal efficiency, COx selectivity and the
formation of harmful by-products. Furthermore, the influence of the following process
parameters on the TCE abatement process was studied: gas flow rate, humidity and TCE inlet
concentration. Focus was on operating the PPC system at low energy density in combination
with a low catalyst temperature to improve the energy efficiency of the TCE abatement process.
First, the abatement of TCE in a plasma-alone process was studied. The plasma was generated
in a multi-pin-to-plate reactor connected with a DC power source. FT-IR and MS spectrometry
revealed the presence of hazardous products phosgene, DCAC, TCAA and ozone in the plasma
treated outlet gas. The ozone outlet concentration was monitored because it indicates the
oxidative power of NTP.
The degradation experiments were performed for two different gas flow rates: 0.5 and 2 L·min1
. An increase of the gas flow rate enhanced the removal efficiency. However, this effect
disappeared when operating at an energy density lower than 175 J·L-1. For the removal of 95
% of the initial TCE in the inlet gas, an energy density of 300 J·L-1 (2 L·min-1) and 650 J·L-1
(0.5 L·min-1) was needed. In contrast, a lower gas flow rate resulted in an increase of the
selectivity towards CO and CO2. However, the COx selectivity remained low. Even at 1050 J·L1
(0.5 L·min-1) the COx selectivity did not exceeded 31 %. An increase of the gas flow rate
slightly inhibited the formation of HCl and phosgene, but resulted in a small increase of the
DCAC production. The ozone formation was not affected by the gas flow rate.
The experiments with humid TCE polluted air showed a maximal removal efficiency at a
relative humidity of 15 %. This optimal humidity level is a compromise between two opposite
effects of water on TCE removal. On the one hand, addition of water enhances TCE removal
due to the production of strong oxidizing OH radicals. On the other hand, water has a negative
effect on TCE abatement since OH radicals strongly react with other TCE oxidizing species
such as ClO and oxygen radicals. In addition, water lowers the formation of active species due
to its electronegative characteristics. An increase of the humidity also increased the COx
selectivity. A maximal COx selectivity of 19 % was reached for a humidity of 80 % RH, when
operating at 100 J·L-1. Under humid conditions, the formation of ozone was suppressed since
OH radicals strongly react with oxygen radicals. Furthermore, humidity enhanced the formation
of HCl, and inhibited the production of DCAC due to the strong oxidizing power of OH radicals.
Humidity also affected the stability of the NTP. Addition of water led to an unstable plasma,
which resulted in a decrease of the threshold current for spark formation.
During the experiments, it was noted that brown/yellow spots were deposited on the spherical
surface segments of the anode plate of the reactor. This deposition was formed as a result of the
humidity since it was only visible during the experiments at 50 and 80 % RH.
99
The effect of the TCE inlet concentration is studied since the VOC concentration in industrial
waste gases strongly varies. At an energy density of 40 J·L-1, an increase from 250 to 750 ppm
TCE resulted in a decrease of the removal efficiency from 58 to 40 %. With increasing
concentration, each TCE molecule shares fewer electrons and reactive plasma species, thereby
reducing the probability of reaction between these species and a certain TCE molecule. The
lowest TCE inlet concentration resulted in the highest COx selectivity and lowest amount of byproducts phosgene and DCAC. The formation of CO and CO2 strongly depends on the amount
of oxygen radicals in NTP. Since a higher TCE inlet concentration leads to a higher amount of
decomposed TCE molecules, the amount of oxygen radicals decreases with an increasing TCE
concentration. This also explains the lower ozone concentration in the outlet gas with an initial
TCE concentration of 750 ppm.
A second series of TCE abatement experiments were performed in a post plasma-catalytic
process. The performance of three different catalysts (CeO2, K-OMS-2 and Fe3O4/K-OMS-2)
was investigated and compared with the plasma-alone system. The combination of NTP and a
catalyst clearly enhanced the TCE abatement, and induced a synergetic effect. Ozone plays an
important role in this. The catalysts studied are able to decompose ozone in active oxygen
species on the catalyst surface. These active species desorb from the surface and oxidize
chlorinated by-products and residual TCE, thereby enhancing the removal efficiency and COx
selectivity.
For the PPC system with CeO2, the highest TCE removal efficiency was achieved for a catalyst
temperature of 150 °C. Operating at 40 and 80 J·L-1 resulted in a removal efficiency of 66 and
81 %, respectively. This corresponds to an enhancement of 15 % (for both energy density)
compared to the plasma-alone system. An increase of the catalyst temperature from 150 to 300
°C slightly reduced the TCE removal from 81 to 77 %, at 80 J·L-1. This is possibly caused by
catalyst deactivation due to irreversible adsorption of chlorinated by-products. In contrast, a
catalyst temperature increase clearly enhanced the COx selectivity due to activation of the ozone
decomposition reactions on the catalyst surface. An increase from 150 to 300 °C led to a shift
in COx selectivity from 23 to 60 %, when operating at 40 J·L-1. The plasma-alone system (room
temperature) only reached a COx selectivity of 9 %. The enhanced oxidation rate with
increasing temperature resulted in an increased and decreased formation of HCl and DCAC,
respectively. Remarkable was the increased phosgene production at higher catalyst
temperature. A possible explanation is the increased formation of ClO radicals with temperature
which further react with TCE towards phosgene. The experiments with humid air were
performed at an energy density of 280 and 400 J·L-1. Due to electronegativity of water, this
high energy density was needed to light up all cathode pins. An increase of the humidity from
2.8 (dry) to 80 % resulted in a decrease of the removal efficiency from 95 to 87 %, for an energy
density of 280 J·L-1. This can be partly ascribed to a decrease in catalytic ozone decomposition
under humid conditions since the amount of converted ozone is directly related to the
concentration of newly formed active oxygen species over the catalyst surface. Another
possible explanation is that the competitive adsorption of water inhibits the adsorption of TCE,
thereby reducing the catalytic removal of TCE. The effect of water on the selectivity towards
CO and CO2 is rather complex and strongly depends on the catalyst temperature. At low catalyst
100
temperature (< 200 °C) humidity positively influences the COx selectivity due to the strong
oxidizing power of OH radicals. The effect of humidity on the formation of oxygen species on
the catalyst surface is negligible since the catalyst is only little activated at low temperature. An
increase of the catalyst temperature resulted in a maximal COx selectivity at a humidity of 20
% RH. This humidity level led to the highest COx selectivity due to the combination of strong
oxidizing hydroxyl radicals and a high amount of ozone decomposed on the catalyst surface. A
higher humidity inhibits the ozone formation resulting in a decrease of active oxygen species
formed on the catalyst surface. Just as in the plasma-alone experiments, an increase of humidity
resulted in an increase and decrease of phosgene and DCAC, respectively. An increase of the
catalyst temperature towards 300 °C completely removed DCAC.
The experiments with K-OMS-2 in dry air resulted in the highest removal efficiency. At a
catalyst temperature of 150 °C, operating at 40 and 80 J·L-1 resulted in a removal efficiency of
86 and 94 %, respectively. Just as in case of CeO2, humidity negatively influenced the removal
efficiency. The COx selectivity of the PPC system with K-OMS-2 reaches higher values
compared to CeO2. At 150 °C and 40 J·L-1, a value of 57 % was reached. An increase of the
humidity from 2.8 to 15 % resulted in a further increase to 71 %. For both dry and humid air,
an increase of the energy density in time from 40 to 250 J·L-1 resulted in a decrease of the COx
selectivity with 13 %. This was remarkable since the opposite effect was observed during the
plasma-alone and PPC experiments with CeO2. The decreasing COx selectivity can be explained
by deactivation of the catalyst.
In order to further improve the total oxidation of TCE, Fe3O4/K-OMS-2 is synthesized. This
cryptomelane composite led to an increase of the COx selectivity with 15 % compared to KOMS-2. However, an increase of the energy density in time again resulted in a reduction of the
COx selectivity due to catalyst deactivation. In contrast to COx selectivity, Fe3O4/K-OMS-2
resulted in a lower TCE removal efficiency compared to K-OMS-2. The synergetic effect in the
PPC system with Fe3O4/K-OMS-2 was negligible. Analysis of the FT-IR outlet spectra of the
PPC experiments with K-OMS-2 and Fe3O4/K-OMS-2 revealed two new bands at 794 and 772
cm-1, corresponding to CCl4 and CHCl3, respectively. These compounds were not detected in
the plasma-alone and PPC system with CeO2, and may be likely formed through carbon-carbon
cleavage of DCAC and TCAA.
This work shows the possibility to decompose TCE to a large extent towards total oxidation
products (CO, CO2, HCl, Cl2) by combination of NTP at low energy density (40 – 80 J·L-1) and
a catalyst at relative low temperature (150 – 200 °C). A removal efficiency and COx selectivity
of 86 and 57 % was reached with K-OMS-2, when operating at 40 J·L-1 and a catalyst
temperature of 150 °C. Addition of Fe3O4 nanoparticles to K-OMS-2 further increased the CO2
selectivity with 15 %, but led to a decrease of the removal efficiency with 24 %. The
disadvantage of cryptomelane type catalysts is the fast deactivation rate. CeO2 is less sensitive
to deactivation but resulted in a lower removal efficiency (66 %) and COx (23 %) selectivity,
under the same conditions.
101
Further investigation is required to get more insight into the chemical and physical interactions
between the catalyst and products formed in NTP, which allows to search more directed for the
optimum catalyst composition that enables total oxidation of TCE. In order to prevent the
reduction of catalyst performance due to deactivation, catalyst regeneration procedures can be
developed.
102
Appendix
Figure 77. FT-IR spectrum of dichloroacetylchloride (DCAC) [140]
Figure 78. FT-IR spectrum of phosgene [141]
103
Figure 79. FT-IR spectrum of HCl [141]
Figure 80. FT-IR spectrum of CO [141]
104
Figure 81. FT-IR spectrum of CO2 [141]
Figure 82. FT-IR spectrum of ozone [141]
105
Figure 83. FT-IR spectrum of water [141]
Figure 84. FT-IR spectrum of chloroform [141]
106
Figure 85. FT-IR spectrum of CCl4 [141]
107
References
[1]
“Council Directive 1999/13/EC of 11 March 1999 on the limitation of emissions of
volatile organic compounds due to the use of organic solvents in certain activities and
installations”, Official Journal L 085, 29/03/1999. pp. 0001 – 0022.
[2]
European Environment Agency, "Air quality in Europe", Denmark, 2013.
[3]
T. Stavrakou, J.-F. Muller, I. De Smedt, M. Van Roozendael, G. R. van der Werf, L.
Giglio, A. Guenther, "Evaluating the performance of pyrogenic and biogenic emission
inventories against one decade of space-based formaldehyde columns,” Atmospheric
Chemistry and Physics, 2009. 9: pp. 1037-1038.
[4]
A. M. Evuti, "A synopsis on biogenic and anthropogenic volatile organic compounds
emissions: hazards and control," International Journal of Engineering Sciences,
2013. 2(5): pp. 145-153.
[5]
L. Vancraeynest and H. Van Hooste, "verspreiding van niet-methaan vluchtige
organische stoffen," MIRA, Brussels, 2013.
[6]
U.S. Energy Information Administration, "Emissions of greenhouse gases in the
U.S.," 31 March 2011. [Online]. Available:
http://www.eia.gov/environment/emissions/ghg_report/ghg_methane.cfm.
[Accessed 12 October 2014].
[7]
S. Han and T. Oda, "Decomposition mechanism of trichloroethylene based on byproduct distribution in the hybrid barrier discharge plasma process," Plasma Sources
Science and Technology, 2007. 16(2): pp. 413-421.
[8]
C. Subrahmanyam, A. Renken and L. Kiwi-Minsker, "Novel catalytic non-thermal
plasma reactor for the abatement of VOCs," Chemical Engineering Journal, 2007.
134(1-3): pp. 78-83.
[9]
M. Magureanu, N.B. Mandache, V.I. Parvulescu, C. Subrahmanyam, A. Renken, L.
Kiwi-Minsker, "Improved performance of non-thermal plasma reactor during
decomposition of trichloroethylene: optimization of the reactor geometry and
introduction of catalytic electrode," Applied Catalysis B: Environmental, 2007. 74:
pp. 270-277.
[10]
R. Morent, J. Dewulf, N. Steenhaut, C. Leys, H. Van Langenhove, "Hybrid plasmacatalyst system for the removal of trichloroethylene in air," Journal of Advanced
Oxidation Technologies, 2006. 9(1): pp. 53-58.
108
[11]
A. M. Vandenbroucke, R. Morent, N. De Geyter, M. T. Nguyen Dinh, J.-M. Giraudon,
J.-F. Lamonier, C. Leys, "Plasma-catalytic decomposition of TCE," International
Journal of Plasma Environmental Science & Technology, 2010. 4(2): pp. 135-138.
[12]
C. Most, "Locating and estimating air emissions from sources of perchloroethylene
and trichloroethylene," U.S. Environmental protection agency, United States, 1989.
[13]
Australian Government Department of the Environment, "Trichloroethylene," 2011.
[Online]. Available: http://www.npi.gov.au/resource/trichloroethylene. [Accessed 12
October 2014].
[14]
National Institute for Occupational Safety and Health, "International chemical safety
card
trichloroethylene,"
cdc,
October
2004.
[Online].
Available:
http://www.cdc.gov/niosh/ipcsneng/neng0081.html. [Accessed 12 October 2014].
[15]
E. Smet and S. Deboosere, "Behandeling van vluchtige
chloorverbindingen in afgassen," Kluwer, 2013. 19(1): pp. 1-3.
[16]
NCE-Crawford-EMCOTEK, "Crawford's regenerative thermal oxidizer systems,"
2013. [Online]. Available: http://www.animalcremation.com/crawfordindustrial/voc/thermal_regenerative/regenerative_thermal_o
xidizers.html. [Accessed 12 October 2014].
[17]
R. Lylykangas, "Thermal or catalytic VOC oxidation?," 22 September 2010. [Online].
Available: http://formiasmartvoc.com/thermal-or-catalytic-voc-oxidation. [Accessed
12 October 2014].
[18]
D. Volckaert, P. M. Heynderickx, S. Wuytens, H. Van Langenhove, “Removal of
dimethylsulfide, n-hexane and toluene from waste air in a flat membrane bioreactor
under continuous conditions,” Gent University, 2013.
[19]
H. H. Bengtson, "Carbon adsorbers," in Air Pollution Control: Carbon Adsorption
for VOCs, U.S. Environmental Protection Agency, 2002. pp. 3-11.
[20]
I. Langmuir and L. Tonks, "Oscillations in ionized gases," Physical Review, 1929. 33:
pp. 195.
[21]
H. Rauscher, M. Perucca and G. Buyle, "Introduction to plasma technology for
surface functionalization," in Plasma Technology for Hyperfunctional Surfaces.
Food, Biomedical and Textile Applications, Germany, 2010. WILEY-VCH, pp. 3-6.
[22]
H. O. Rucker, Introduction to plasma physics, Austria: Karl-Franzens-University
Graz, 2007.
organische
109
[23]
A. Fridman, "Introduction to theoretical and applied plasma chemistry," in Plasma
Chemistry, Unites States, 2008. Cambridge University Press, pp. 1-2.
[24]
J. Van Durme, "Development of heterogeneous plasma catalysis for the abatement of
health damaging organic micropollutants in indoor environments," Ghent University,
Bioscience engineering, 2008. pp. 18.
[25]
A. M. Vandenbroucke, R. Morent, N. De Geyter, C. Leys, "Non-thermal plasmas for
non-catalytic and catalytic VOC abatement," Journal of Hazardous Materials, 2011.
195: pp. 30-54.
[26]
T. Callebaut, I. Kochetov, Y. Akishev, A. Napartovich, C. Leys, "Numerical
simulation and experimental study of the corona and glow regime of a negative pinto-plate discharge in flowing ambient air," Plasma Sources Science and Technology,
2003. 13: pp. 245-250.
[27]
J. Chang, "Corona Discharge Processes," IEEE Transactions On Plasma Science,
1991. 19(6): pp. 1152.
[28]
A. Jaworek and A. Krupa, "Corona discharge from a multipoint electrode in flowing
air," Journal of Electrostatics, 1996. 38: pp. 187-188.
[29]
J. S. Townsend, Philosophical Magazine, 1914. 28.
[30]
K. Satoh, T. Matsuzawaa and H. Itoh, "Decomposition of benzene in a corona
discharge at atmospheric pressure," Thin Solid Films, 2008. 516(13): pp. 4423-4429.
[31]
X. Zhang, W. Feng, Z. Yu, S. Li, J. Zhu, K. Yan, "Comparison of styrene removal in
air by positive and negative DC corona discharges,” International Journal of
Environmental Science and Technology, 2013. 10: pp. 1377-1382.
[32]
Y. Akishev, M. Grushin, I. Kochetov, A. Napartovich, M. Pan`kin, N. Trushkin,
"Transition of a multipin negative corona in atmospheric air to a glow discharge,"
Plasma Physics Reports, 2000. 26(2): pp. 157-163.
[33]
Z. Machala, E. Marode, C. Laux, C. Kruger, "DC Glow Discharges in atmospheric
pressure air," Journal of Advanced Oxidation Technologies, 2004. 7(2): pp. 133-137.
[34]
A. Mohamed, R. Block and K. Schoenbach, "Direct current glow discharges in
atmospheric air," IEEE Transactions On Plasma Science, 2002. 30(1): pp. 182-183.
[35]
D. M. El-zeer and A. A. Garamoon, "Atmospheric pressure glow discharge plasma in
air at frequency 50 Hz," Plasma Sources Science and Technology, 2009. 18(4).
110
[36]
D. Trunec, A. Brablec and F. Šťastný, "Experimental study of atmospheric pressure
glow discharge," Contributions to Plasma Physics, 1998. 38(3): pp. 435-445.
[37]
R. Vertriest, R. Morent, J. Dewulf, C. Leys, H. Van Langenhove, "Multi-pin-to-plate
atmospheric glow discharge for the removal of volatile organic compounds in waste
air," Plasma Sources Science and Technology, 2003. 12: pp. 412-416.
[38]
Y. Akishev, M. Grushin, I. Kochetov, V. Karal´nik, A. Napartovich, "Negative
corona, glow and spark discharges in ambient air and transitions between them,"
Plasma Sources Science and Technology, 2005. 14: pp. S18-S25.
[39]
Y. Akishev, O. Goossens, T. Callebaut, C. Leys, A. Napartovich, N. Trushkin, "The
influence of electrode geometry and gas flow on corona-to-glow and glow-to-spark
threshold currents in air," Journal of Physics: Applied Physics, 2001. 34: pp. 28752882.
[40]
G. Zheng, J. Hou and J. Jiang, "Degradation of flowing low-density gaseous toluene
by non-equilibrium plasma," Journal of Fudan University (Natural Science), 2001.
40(4): pp. 364-367.
[41]
J. Jiang, J. Hou and G. Zheng, "The decomposition of benzene and xylene under
normal atmospheric pressure by dielectric barrier discharge," Journal of
Environmental Sciences, 2001. 21(6): pp. 531-534.
[42]
M. Chang and C. Lee, "Destruction of formaldehyde with dielectric barrier discharge
plasmas," Environmental Science & Technology, 1995. 29(1): pp. 181-186.
[43]
W. Liang, W. Jiang, L. Zhang, J. Wua, J. Zhang, D. Yang, "Experimental studies of
removing typical VOCs by dielectric barrier discharge reactor of different sizes,"
Process Safety and Environmental Protection, 2014.
[44]
C. Subrahmanyam, M. Magureanu, A. Renken, L. Kiwi-Minsker, "Catalytic
abatement of volatile organic compounds assisted by non-thermal plasma," Applied
Catalysis B: Environmental, 2006. 65: pp. 150-156.
[45]
U. Kogelschatz, B. Eliasson and W. Egli., "Dielectric-barrier discharges. Principle
and applications," Journal de Physique, 1997. 7(4): pp. 47-66.
[46]
T. Oda, R. Yamashita, I. Haga, T. Takahashi, S. Masuda, "Decomposition of gaseous
organic contaminants by surface discharge induced plasma chemical processing,"
IEEE Transactions on Industry Applications, 1996. 32(1): pp. 118-124.
[47]
T. Oda, T. Takahashi, H. Nakano, S. Masuda, "Decomposition of fluorocarbon
gaseous contaminants by surface discharge induced plasma chemical processing,"
IEEE Transactions on Industry Applications, 1993. 29(4): pp. 787-792.
111
[48]
S. Masuda, K. Akutsu, M. Kuroda, Y. Awatsu, Y. Shibuya, "A ceramic based
ozonizer using high frequency discharge," IEEE Transactions on Industry
Applications, 1988. 24(2): pp. 223-231.
[49]
A. A. Assadia, A. Bouzazaa, M. Lemaslea, D. Wolbert, "Removal of trimethylamine
and isovaleric acid from gas streams in a continuous flow surface discharge plasma
reactor (In Press, Corrected Proof)," Chemical Engineering Research and Design,
2014.
[50]
Y. Oishi, Y. Ehara and T. Yamamoto, "VOC removal using adsorption and surface
discharge," in 11th International Conference on Electrostatic Precipitation,
Hangzhou, 2008.
[51]
F. Holzer, F.-D. Kopinke and U. Roland, "Influence of ferroelectric materials and
catalysts on the performance of non-thermal plasma (NTP) for the removal of air
pollutants," Plasma Chemistry and Plasma Processing, 2005. 25(6): pp. 595-611.
[52]
U. Roland, F. Holzer and F.-D. Kopinke, "Improved oxidation of air pollutants in a
non-thermal plasma," Catalysis Today, 2002. 73: pp. 315-323.
[53]
W. Liang, J. Li and Y. Jin, "Abatement of toluene from gas streams via ferroelectric
packed bed dielectric barrier discharge plasma," Journal of Hazardous Materials,
2009. 170: pp. 633-638.
[54]
K. Takaki, S. Takahashi, S. Mukaigawa, T. Fujiwara, K. Sugawara, T. Sugawara,
"Influence of pellet shape of ferro-electric packed-bed plasma reactor on ozone
generation and NO removal," International Journal of Plasma Environmental Science
and Technology, 2009. 3(1): pp. 28-34.
[55]
T. Yamamoto, K. Ramanathan, P. Lawless, D. Ensor, J. Newsome, N. Plaks, G.
Ramsey, "Control of vollatile organic compounds by an ac energized ferroelectric
pellet reactor and a pulsed corona reactor," IEEE Transactions on Industry
Applications , 1992. 28(3): pp. 528-533.
[56]
A. Ogata, N. Shintani, K. Mizuno, S. Kushiyama, "Decomposition of benzene using
a nonthermal plasma reactor packed with ferroelectric pellets," IEEE Transactions on
Industry Applications, 1999. 35: pp. 753-759.
[57]
A. M. Vandenbroucke, R. Morent, N. De Geyter, C. Leys, "Decomposition of
trichloroethylene with plasma-catalysis: A review," journal of advanced oxidation
technologies, 2011. 14(1): pp. 1-9.
112
[58]
T. Zhu, "VOCs decomposition via modified ferroelectric packed bed dielectric barrier
discharge plasma," IEEE Transactions on Plasma Science, 2011. 39(8): pp. 16951700.
[59]
V. Parvulescu, M. Magureanu and P. Lukes, “Plasma Chemistry and Catalysis in
Gases and Liquids,” John Wiley & Sons, 2013.
[60]
P. Lukes, M. Clupek, V. Babicky, V. Janda, P. Sunka, "Generation of ozone by pulsed
corona discharge over water surface in hybrid gas–liquid electrical discharge reactor,"
Journal of Physics D: Applied Physics, 2005. 38(3): pp. 409-416.
[61]
J-S. Chang, P. Lawless and T. Yamamoto, "Corona discharge processes," IEEE
Transactions on Plasma Sciences, 1991. 19(6): pp. 1152-1166.
[62]
I. Park and W. Koh, "Numerical Simulation of a Pulsed Corona Discharge Plasma,"
Journal of the Korean Physical Society, 2003. 42: pp. S920-S924.
[63]
A. M. Vandenbroucke, R. Aerts, W. Van Gaens, N. De Geyter, C. Leys, R. Morent,
A. Bogaerts, "Modeling and Experimental Study of Trichloroethylene Abatement
with a Negative Direct Current Corona Discharge," Plasma Chemistry and Plasma
Processing, 2015. 35: pp. 217-230.
[64]
W. Siemens, "Ueber die elektrostatische Induction und die Verzögerung des Stroms
in Flaschendrähten," poggendorff annalen der physik und chemie, 1857. 102: pp. 66122.
[65]
J. Chen and J. Davidson, "Ozone Production in the Positive DC Corona Discharge:
Model and Comparison to Experiments," Plasma Chemistry and Plasma Processing,
2002. 22(4): pp. 495-496.
[66]
D. Braun, U. kuchler and G. Pietsch, "Microdischarges in Air-Fed Ozonizers,"
Journal of Physics D: Applied Physics, 1991. 24: pp. 564-572.
[67]
B. Eliasson, U. Kogelschatz and P. Baessler, "Dissociation of O2 in N2/O2 mixtures,"
Journal of Physics B: Atomic and Molecular Physics, 1984. 17(22): pp. L797-L801.
[68]
M. Benyamina, K. Khodja, F. Ghaleb, A. Belasri, "Influence of the Gas Temperature
in Ozone Production of Mixture N2-O2," Journal of Chemistry and Chemical
Engineering, 2012. 6: pp. 391-392.
[69]
R. Morent and C. Leys, "Ozone Generation in Air by a DC-Excited Multi-Pin-toPlane Plasma Source," Ozone: Science and Engineering, 2005. 27: pp. 239-245.
113
[70]
H. Einagaa and A. Ogata, "Benzene oxidation with ozone over supported manganese
oxide catalysts: Effect of catalyst support and reaction conditions," Journal of
Hazardous Materials, 2009. 164: pp. 1236-1241.
[71]
M. Nguyen Dinh, J. Giraudon, J. Lamonier, A. Vandenbroucke, N. De Geyter, C.
Leys, R. Morent, "Plasma-catalysis of low TCE concentration in air using LaMnO3+δ
as catalyst," Applied Catalysis B: Environmental, 2014. 147: pp. 904-911.
[72]
H.H. Kim, A. Ogata, S. Futamura, "Complete Oxidation of Volatile Organic
Compounds (VOCs) Using Plasma-Driven Catalysis and Oxygen Plasma,"
International Journal of Plasma Environmental Science & Technology, 2007. 1(1):
pp. 46-51.
[73]
H. An, T. Huu, T. Van, J. Cormier, A. Khacef, "Application of atmospheric non
thermal plasma-catalysis hybrid system for air pollution control: Toluene removal,"
Catalysis Today, 2011. 176(1): pp. 474-477.
[74]
H. Sekiguchi, "Catalysis assisted plasma decomposition of benzene using dielectric
barrier discharge," The Canadian Journal of Chemical Engineering, 2009. 79(4): pp.
512-519.
[75]
R. Morent, J. Dewulf, N. Steenhaut, C. Leys, H. Van Langenhove, "Hybrid plasmacatalyst system for the removal of trichloroethylene in air," Journal of Advanced
Oxidation Technologies, 2006. 9(1): pp. 1-6.
[76]
U. Roland, F. Holzer and F.-D. Kopinke, "Combination of non-thermal plasma and
heterogeneous catalysis for oxidation of volatile organic compounds: Part 2. Ozone
decomposition and deactivation of γ-Al2O3," Applied Catalysis B: Environmental,
2005. 58(3-4): pp. 217-226.
[77]
M. Sugasawa and S. Futamura, "Synergistic Effect of Nonthermal Plasma and
Catalysts on the Decomposition of VOCs," Industry Applications Conference, 2007.
pp. 1485-1488.
[78]
H. H. Kim, A. Ogata and S. Futamura, "Oxygen partial pressure-dependent behavior
of various catalysts for the total oxidation of VOCs using cycled system of adsorption
and oxygen plasma," Applied Catalysis B: Environmental, 2008. 79: pp. 356-367.
[79]
K. Hensel, S. Katsura and A. Mizuno, "DC microdischarges inside porous ceramics,"
IEEE Transactions on Plasma Science, 2005. 33(2): pp. 574-575.
[80]
S. Chavadej, W. Kiatubolpaiboon, P. Rangsunvigit, T. Sreethawong, "A combined
multistage corona discharge and catalytic system for gaseous benzene removal,"
Journal of Molecular Catalysis A: Chemical, 2007. 263(1-2) pp. 128-136.
114
[81]
J. Chae, V. Demidiouk, M. Yeulash, I. Choi, T. Jung, "Experimental study for indoor
air control by plasma-catalyst hybrid system," IEEE Transactions on Plasma Science,
2004. 32: pp. 493-497.
[82]
Y. Guo, D. Yea, K. Chenb, J. Hea, W. Chena, "Toluene decomposition using a wireplate dielectric barrier discharge reactor with manganese oxide catalyst in situ,"
Journal of Molecular Catalysis A: Chemical, 2006. 245(1-2): pp. 93-100.
[83]
A. Pribytkov, G. Baeva, N. Telegina, A. Tarasov, A. Stakheev, A. Tel’nov, V.
Golubeva, "Effect of electron irradiation on the catalytic properties of supported Pd
catalysts," Kinetics and Catalysis, 2006. 47(5): pp. 765-769.
[84]
H. H. Kim, A. Ogata and S. Futamura, "comparative assessment of different
nonthermal plasma reactors on energy efficiency and aerosol formation from the
decomposition of gas-phase benzene," IEEE transactions on industry applications,
2005. 41(1): pp. 206-214.
[85]
Y. Guo, D. Ye, Y. Tian, K. Chen, "Humidity Effect on Toluene Decomposition in a
Wire-plate Dielectric Barrier Discharge Reactor," Plasma Chemistry and Plasma
Processing, 2006. 26(3): pp. 237-249.
[86]
M. Peluso, L. Gambaro, E. Pronsato, D. Gazzoli, H. Thomas, J. Sambeth, "Synthesis
and catalytic activity of manganese dioxide (type OMS-2) for the abatement of
oxygenated VOCs," Catalysis Today, 2008. 133-135: pp. 487-492.
[87]
P. Ruetschi, "Cation‐Vacancy Model for MnO2," Journal of the Electrochemical
Society, 1984. 131(12): pp. 2737-2744.
[88]
H. Einaga and S. Futamura, "Catalytic oxidation of benzene with ozone over Mn ionexchanged zeolites," Catalysis Communications, 2007. 8(3): pp. 557-560.
[89]
S. Futamura and A. Gurusamy, "Synergy of nonthermal plasma and catalysts in the
decomposition of fluorinated hydrocarbons," Journal of Electrostatics, 2005. 63: pp.
949-954.
[90]
J. Jarrige and P. Vervisch, "Plasma-enhanced catalysis of propane and isopropyl
alcohol at ambient temperature on a MnO2-based catalyst," Applied Catalysis B:
Environmental, 2009. 90: pp. 74-82.
[91]
Y. Li, Z. Fan, J. Shi, Z. Liu, W. Shangguan, "Post plasma-catalysis for VOCs
degradation over different phase structure MnO2 catalysts," Chemical Engineering
Journal, 2014. 241: pp. 251-258.
115
[92]
Y.-H. Song, S.-J. Kim, K.-I. Choi, T. Yamamoto, "Effects of adsorption and
temperature on a nonthermal plasma process for removing VOCs," Journal of
Electrostatics, 2002. 55(2): pp. 189-201.
[93]
A. Ogata, K. Yamanouchi, K. Mizuno, S. Kushiyama, T. Yamamoto, "Oxidation of
Dilute Benzene in an Alumina Hybrid Plasma Reactor at Atmospheric Pressure,"
Plasma Chemistry and Plasma Processing, 1999. 19(3).
[94]
D.-W. Park, S.-H. Yoon, G.-J. Kim, H. Sekiguchi, "The Effect of Catalyst on the
Decomposition of Dilute Benzene Using Dielectric Barrier Discharge," Journal of
Industrial and Engineering Chemistry, 2002. 8(4).
[95]
A. Ogata, H.H. Kim, S.M. Oh, H. Einaga and S. Futamura, "Relationship Between
Ozone Formation and Toluene Decomposition Efficiency in a Zeolite-hybrid Plasma
Reactor," in International Society for Electrostatic Precipication, Australia, 2006.
[96]
H.H. Kim, J.H. Kim and A. Ogata, "Adsorption and Oxygen Plasma-Driven Catalysis
for Total Oxidation of VOCs," International Journal of Plasma Environmental
Science and Technology, 2008. 2(2): pp. 106-112.
[97]
L. Intriago, E. Dı´az, S. Ordo´n˜ez, A. Vega, "Combustion of trichloroethylene and
dichloromethane over protonic zeolites: Influence of adsorption properties on the
catalytic performance," Microporous and Mesoporous Materials, 2006. 91: pp. 161169.
[98]
U.S. Secretary of Commerce on behalf of the United States of America, "NIST:
National Institute of Standards and Technology," DOW CHEMICAL COMPANY,
2011.
[Online].
Available:
http://webbook.nist.gov/cgi/cbook.cgi?ID=C50000&Type=IR-SPEC&Index=1.
[Accessed 23 November 2014].
[99]
M. Loudon, "Infrared Spectroscopy: Theory," in Handbook for Organic Chemistry
Lab, Colorado, 2002. pp. 155-164.
[100] T. Nicolet, "FT-IR vs. Dispersive Infrared," 2012. [Online]. Available:
http://www.thermo.com.cn/resources/200802/productpdf_21615.pdf. [Accessed 24
November 2014].
[101] "Infra-red absorption spectroscopy," Sheffield Hallam University, 2008. [Online].
Available: http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec3.htm.
[Accessed 24 November 2014].
[102] U.S. Secretary of Commerce on behalf of the United States of America, "NIST,
National Institute of Standards and Technology," 2014. [Online]. Available:
116
http://webbook.nist.gov/cgi/cbook.cgi?ID=C79016&Mask=200.
November 2014].
[Accessed
24
[103] P.
Biosoft,
"Mass
Spectrometry,"
2011.
[Online].
Available:
http://www.premierbiosoft.com/tech_notes/mass-spectrometry.html. [Accessed 24
November 2014].
[104] Scripps Center for Metabolomics and Mass Spectrometry, "What is Mass
Spectrometry?,"
2010.
[Online].
Available:
http://masspec.scripps.edu/mshistory/whatisms_details.php#massanalyzers.
[Accessed 24 November 2014].
[105] C. Ren, T. Ma, D. Wang, J. Zhang, Y. Wang, “A study of cross-gas-flow to stabilize
an atmospheric pressure glow plasma in a multi-pin-to-multi-cupped-plane negative
corona discharge,” Journa of Electrostatics, 2005. 64: pp. 23-28.
[106] A. Vandenbroucke, “Studie van plasmakatalytische afbraakprocessen van vluchtige
organische stoffen,” Masterthesis at University Ghent, Department Applied Physics,
2009. p. 125.
[107] X. Zhu, X. Gao, C. Zheng, Z. Wang, M. Ni, X. Tu, “Plasma-catalytic removal of a
low concentration of acetone in humid conditions,” The Royal Society of Chemistry,
2014. 4: pp. 37796-37805.
[108] P. Wang and J. Chen, “Effect of relative humidity on electron distribution and ozone
production by DC coronas in air,” Transactions on Plasma Science, 2005. 33(2): pp.
808-812.
[109] H. Ge, L. Zhang, L. Yan, D. Mi, Y. Zhu, “Parameter optimization of excited OH
radical in multi-needle to plate negative DC corona discharge,” Journal of
Electrostatics, 2011. 69: pp. 529-532.
[110] D. Evans, L. A. Rosocha, G. K. Anderson, J. J Coogan, “Plasma remediation of
trichloroethylene in silent discharge plasmas,” Journal of Applied Physics , 1993.
74(9): pp. 5378-5386.
[111] G. Socrates, “Infrared and Raman characteristic group frequencies: Tables and
charts,” West Sussex, England: Wiley, 2004.
[112] TCI America, “Material safety data sheet DCAC,” [Online]. Available:
https://www.spectrumchemical.com/MSDS/TCI-D0313.pdf. [Accessed 15 March
2015].
[113] A. Vandenbroucke, M. T. N. Dinh, J-M. Giraudon, R. Morent, N. De Geyter, J-F.
Lamonier, C. Leys, “Qualitative by-product identification of plasma-assisted TCE
117
abatement by mass spectrometry and fourier-transform infrared spectroscopy,”
Plasma Chem Plasma Process, 2011. 31: pp. 707-718.
[114] L. Prager, H. Langguth, S. Rummel, R. Mehnert, “Electron beam degradation of
chlorinated hydrocarbons in air,” Radiation Physics and Chemistry, 1995. 46(4-6):
pp. 1137-1142.
[115] S. A. Vitale, K. Hadidi, D. R. Cohn, P. Falkos, “The effect of a carbon-carbon double
bond on electron beam-generated plasma decomposition of trichloroethylene and
1,1,1-trichloroethane,” Plasma Chemistry and Plasma Processing, 1997. 17(1): pp.
59-78.
[116] M. J. Kirkpatrick, W. C. Finney and B. R. Locke, “Chlorinated organic compound
removal by gas phase pulsed streamer corona electrical discharge with reticulated
vitreous carbon electrodes,” Plasmas and Polymers, 2003. 8(3): pp. 165-177.
[117] Sigma-Aldrich,
“chemblink,”
2009.
[Online].
Available:
http://www.chemblink.com/MSDS/MSDSFiles/75-87-6_Sigma-Aldrich.pdf.
[Accessed 28 March 2015].
[118] J. S. Gift, R. McGaughy, D. V. Singh, B. Sonawane, “Health assessment of phosgene:
approaches for derivation of reference concentration,” Regul Toxicol Pharmacol,
2008. 51(1): pp. 98-107.
[119] Y. Nakagawa, Y. Tomimura, R. Ono, T. Oda, “Dilute trichloroethylene
decomposition by non-thermal plasma-catalyst and humidity effect, and OH radical
behavior,” in Joint Electrostatics Conference 1, 2012.
[120] A. S. Chiper, N. Blin-Simiand, M. Heninger, H. Mestdagh, P. Boissel, F. Jorand, J.
Lemaire, J. Leprovost, S. Pasquiers, G. Popa, C. Postel, “Detailed characterization of
2-heptanone conversion by dielectric barrier discharge in N2 and N2/O2 mixtures,”
Journal of Physical Chemistry, 2010. A114: pp. 397-407.
[121] N. Blin-Simiand, F. Jorand, L. Magne, S. Pasquiers, C. Postel, J.R. Vacher, “Plasma
reactivity and plasma-surface interactions during treatment of toluene by a dielectric
barrier discharge,” Plasma Chemistry and Plasma Processing, 2008. 28: pp. 429-466.
[122] S. Delagrange, L. Pinard and J. M. Tatibouet, “Combination of a non-thermal plasma
and a catalyst for toluene removal from air: manganese based oxide catalysts,”
Applied Catalysis B: Environmental, 2006. 68: pp. 92-98.
[123] Sugasawa and S. Futamura, “Additive effect on energy efficiency and byproduct
distribution in VOC decomposition with nonthermal plasma,” IEEE Transactions on
Industry Applications, 2008. 44: pp. 40-45.
118
[124] K. Abedi, F. Ghorbani-shahna, B. Jaleh, A. Bahrami, R. Yarahmadi, “Enhanced
performance of non-thermal plasma coupled with TiO2/GAC for decomposition of
chlorinated organic compounds: influence of a hydrogen-rich substance,” Journal of
Environmental Health Science & Engineering, 2014. 12(119): pp. 1-13.
[125] M. Magureanu, N. B. Mandache and V. I. Parvulescu, “Chlorinated organic
compounds decomposition in a dielectric barrier discharge,” Plasma Chem Plasma
Process, 2007. 27: pp. 679-690.
[126] B. Harrison, A. F. Diwell and C. Hallett, “Promoting platinum metals by ceria,”
Platinum Metals Review, 1988. 32(2): pp. 73-83.
[127] A. Naydenov, R. Stoyanova and D. Mehandjiev, “Ozone decomposition and CO
oxidation on CeO2,” Journal of Molecular Catalysis A: Chemical, 1995. 98: pp. 9-14.
[128] B. Dhandapani and S.T. Oyama, “Gas phase ozone decomposition catalysts,” Applied
Catalysis B: Environmental, 1997. 11: pp. 129-166.
[129] A. M. Vandenbroucke, M. Mora, C. Jiménez-Sanchidrián, F. J. Romero-Salguero, N.
De Geyter, C. Leys, R. Morent, “TCE abatement with a plasma-catalytic combined
system using MnO2 as catalyst,” Applied Catalysis B: Environmental, 2014. 156-157:
pp. 94-100.
[130] M. T. Nguyen Dinh, J.-M. Giraudon, A. M. Vandenbroucke, R. Morent, N. De Geyter,
J. -F. Lamonier, “Post plasma-catalysis for total oxidation of trichloroethylene over
Ce-Mn based oxides synthesized by a modified "redox-precipitation route",” Applied
Catalysis B: Environmental, 2015. 172-173: pp. 65-72.
[131] X. Fan, T. Zhu, Y. Wan and X. Yan, “Effects of humidity on the plasma-catalytic
removal of low-concentration BTX in air,” Journal of Hazardous Materials, 2009.
180: pp. 616-621.
[132] T. Oda, Y. Nakagawa, R. Ono, H. Fujisawa, “Non-thermal plasma decomposition of
dilute trichloroethylene in air - catalyst effect,” in International Society for
Electrostatic Precipitation, 2008. China.
[133] R. Buszek, J. Barker and J. Francisco, “Water effect on the OH + HCl reaction,
”Journal of Physical Chemistry, 2012. 116(19): pp. 4712-4719.
[134] F. Schurz, J. M. Bauchert, T. Merker, T. Schleid, H. Hasse, R. Gläser, “Octahedral
molecular sieves of the type K-OMS-2 with different particle sizes and morphologies:
Impact on the catalytic properties in the aerobic partial oxidation of benzyl alcohol,”
Applied Catalysis A: General, 2009. 355(1-2): pp. 42-49.
119
[135] D. Frías, S. Nousir, L. Barrio, M. Montes, T. López, M.A. Centeno, J.A. Odriozola,
“Synthesis and characterization of cryptomelane- and birnessite-type oxides:
Precursor effect,” Materials Characterization, 2007. 58: pp. 776-781.
[136] A. R. Gandhe, J. S. Rebello, J. L. Figueiredo, J.B. Fernandes, “Manganese oxide
OMS-2 as an effective catalyst for total oxidation of ethyl acetate,” Applied Catalysis
B: Environmental, 2007. 72: pp. 129-135.
[137] J. E. Post, R. B. Von Dreele and P. R. Buseck, “Symmetry and cation displacements
in hollandites: structure refinements of hollandite, cryptomelane and priderite,” Acta
Crystallographica Section B, 1982. 38: pp. 1056-1065.
[138] A. Lu, X. Gao, C. Wang, Y. Gao, D. Zheng, T. Chen, D. Zhao, S. Qin, “Natural
cryptomelane and its potential application in the adsorption of heavy metal cadmium,”
Journal of Mineralogical and Petrological Sciences, 2007. 102: pp. 217-225.
[139] S. Zhang, X. Zhao, H. Niu, Y. Shi, Y. Cai, G. Jiang, “Superparamagnetic Fe3O4
nanoparticles as catalysts for the catalytic oxidation of phenolic and aniline
compounds,” Journal of Hazardous Materials, 2009. 167: pp. 112-119.
[140] AcrosOrganics,
„Chemexper,”
[Online].
Available:
https://www.chemexper.com/searchResult.shtml?format=ccd2013%2Cccd&target=s
tructure&options=brandqtyoffercrm&i=e7fad7&country=BE&searchValue=Dichlor
oacetyl+chloride&searchTemplate=rn.value%3D%22%3F%22+elsor+iupac.value%
3D%22%3F%22+elsor+mol.value%3D%22%3F%. [Accessed 15 March 2015].
[141] “NIST: National Institute of Standards and Technology,” [Online]. Available:
http://webbook.nist.gov/Chemistry/. [Accessed 15 March 2015].
120