Download TiO2奈米光觸媒濾網應用於空調系統分解甲醛氣體之研究

Decomposition of Formaldehyde by TiO2 Nanocatalyst Filters in a Heating
Ventilation Air Conditioning System
ABSTRACT
This paper investigates the effectiveness of TiO2 photocatalytic oxidation of
formaldehyde under different heating ventilation air conditioning (HVAC)-related air
conditions. Formaldehyde, at a specific concentration, was injected into an enclosed
HVAC test chamber, and circulated under UV irradiation though TiO2 nanofilm-coated
stainless steel air filter. After two hours of HVAC operation under preset temperature,
relative humidity (RH), and air velocity conditions, the formaldehyde photocatalyst
decomposition effectiveness was 65–87%. At constant RH and air velocity,
photocatalysis increased with an increase in air temperature. At 40–60% RH, and
constant temperature and air velocity, the photocatalytic degradation rates were 78–89%.
Under constant temperature and RH conditions, and air velocities of 0.4–1.3 m/s (1.3–4.3
ft/s), the formaldehyde decomposition efficiency was 70–86%. Therefore, in a sealed
system, when one or all of air temperature, RH, and air velocity are increased, the
formaldehyde photodegradation rate will be increased.
Keywords: Formaldehyde, Photocatalyst, Filter, HVAC, Photocatalytic Oxidation
1
INTRODUCTION
Due to deficiencies in natural ventilation or air exchange in the indoor environment
of buildings, mechanical ventilation equipment may be installed to maintain quality
standards, and health and comfort levels of the indoor air. However, when excessive
indoor related materials or chemical-based products are present in the indoor environment,
harmful chemical substances may be produced (Chiang, 2006), including volatile organic
compounds (VOCs) from solvent-based paint and formaldehyde from wooden or other
construction materials. These substances are just two indoor factors that may affect
human health.
A common pollutant of the indoor environment is Volatile organic compounds
(VOCs) and formaldehyde (Zhang, 2007), which may originate from smoking by
building occupants and/or from construction or related materials. In studies of the
relationships between temperature, relative humidity (RH), and formaldehyde
concentration indoors, it has been reported that formaldehyde can be removed by
photocatalysts (Yang, 2007). However, Matthews et al. (Matthews, 1986) showed a
seasonal pattern, with formaldehyde concentrations in summer 6–9 times higher than in
winter. Yu et al. (2006) studied air change rates (ACR), RH, and photocatalytic filters in a
simplified heating ventilation air conditioning (HVAC) system. Their results showed that
first-order decay of toluene and formaldehyde ranged from 0.381 to 1.01 h-1 under
different total ACR, from 0.34 to 0.433 h-1 under different RH, and from 0.381 to 0.433
h-1 for different photocatalytic filters. However, increasing outdoor airflow rate increased
the cooling load of the HVAC system (Amal, 2001). In order to effectively solve the
above problem, it is possible to increase purification equipments of indoor is
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needed, as to reduce the volume of external air induction, and then to save energy
and enhance indoor air quality.
Recently, a lot of research into the removal of pollutants by photocatalysts has been
performed (Ao, 2004). The photocatalyst titanium dioxide (TiO2) has many advantages
related to its high level of photocatalytic activity, high photocatalytic efficiency, low
operation temperature, high specific surface area, good chemical stability, and thermal
stability, TiO2 used for reaction are inexpensive. Studies have shown that it completely
oxidized pollutant into CO2 and H2O (Michael, 1996). An experiment using a TiO2
photocatalyst-attached air cleaner showed formaldehyde removal efficiency of 70–80%,
and toluene removal efficiency of 35–40% (Zhang, 2003). Esswein et al. reported that an
ozone-attached air cleaner had poor formaldehyde removal efficiency (Esswein, 1994).
Moreover, Ao et al (2005). reported that the volume of pollutants absorbed by activated
carbon decreases with a rise in temperature. Furthermore, when RH is high, re-emission
of formaldehyde adhered to activated carbon woven cloth and to the surface of the filter
wall may occur. Therefore, high temperature conditions may not be favorable to the
adsorption of pollutants (Wark, 1998). However, Formic acid is the most commonly
found intermediate from the conversion of formaldehyde by photocatalytic and
photooxidation (Ao, 2004; Veyret, 1989), photocatalysis in aqueous phase (Shin, 1996)
and gaseous phase (Yang, 2000). In general it has been found that using photocatalysts to
remove indoor VOCs is quite effective (Fujishima, 1972; Ao, 2003; Obee, 1995; Jardim,
1994; Peill, 1996).
In this study a small-sized, simulated air-circulation chamber system, was used to
simulate an indoor air quality in an enclosed environment, and measure formaldehyde
3
removal efficiency of a photocatalyst-attached air filter. The TiO2 photocatalyst was
combined with a stainless steel air filter, using a water-soluble polymer binder to firmly
fix the TiO2 film on the surface of the stainless steel filter. The intention of this filter was
to remove VOC nanoparticles from indoor air as they pass though a HVAC system. Such
filtering, if effective, would prevent the particles from entering the nostrils, depositing in
the lung, and resulting in pulmonary fibrosis or other pathologies to the human occupants
of the building. Another advantage of a TiO2-attached air filter is that it can be reused
after being washed with pressurized clean air. When fiber-based filters are tested, air and
dust passing though a loaded filter have been reported to allow the propagation of
pathogenic bacteria, resulting in formaldehyde and acetone production (Hans, 1999). This
study used a stainless steel-based air filter to avoid the propagation of pathogenic bacteria
within the filter (ASHRAE, 2005).
EXPERIMENTAL DETAILS
The commercial TiO2 photocatalyst used was Ti-1125A (QFnano Co., Ltd, Taiwan,
Specific surface area (BET): 75±15 m2/g (366,262.13±73,252.43 ft2/lbm), Al2O3-content:
≦0.003%, SiO2-content: ≦0.006%). Fig. 1 shows the X-ray diffraction (XRD) analysis
of the TiO2 nanofluid. The nanoparticles were anatase TiO2, and TEM analysis indicated
good nanoparticle dispersion with a mean particle size of below 10 nm (32.8 nft). After
using water-soluble polymer binder to affix the TiO2 coating on the stainless steel filter,
the filter was installed in a temperature, RH, air velocity-adjustable HVAC system (Fig.
3). The experiments simulated an enclosed indoor air conditioning environment that was
polluted by formaldehyde. Experiments were conducted with variations in temperature,
4
RH, and air velocity conditions. Measurement of formaldehyde concentrations used a
Formaldemeter 400 (PPM Technology, Caernarfon, Wales, UK) with a detection range of
0–10 ppm (0–37.85 gal/L), a detection limit of 0.01 ppm (0.0378 gal/L), and a precision
of 10% at 2 ppm (7.57 gal/L). The meter was suitable for use over a temperature range of
5–40°C (41–104°F), and a relative humidity range of 40–60%. The study first determined
the background adsorption decline of formaldehyde in the test HVAC system and
determined the portion of the decline caused by natural adsorption of formaldehyde by
the materials within the testing system. Subsequently, ultraviolet (UV) irradiation was
added to determine the effects of UV photocatalysis of the formaldehyde pollutant using
a filter without a TiO2 coating. Next, a direct photolytic experiment was completed using
a TiO2-attached stainless steel filter irradiated with UV light. These separate experiments
were used to determine formaldehyde adsorption by the HVAC system’s materials, as
well as by UV light only, and by UV light on a TiO2 photocatalysis on formaldehyde
concentrations.
Presetting of parameters
The American Society of Heating, Refrigerating and Air conditioning Engineers
(ASHRAE) standards suggest values for indoor environment’s temperature and RH
during winter and summer (ASHRAE, 2005). In summer, the most comfortable
temperature range was 25–27°C (77–80.6°F) dry-bulb (DB), and the most comfortable
RH was 45–55%. In winter, the most comfortable temperature range was 20-22°C
(68–71.6°F) DB, and the most comfortable RH was 45–55%. Therefore, our tests were
performed under the common summer and winter indoor temperatures. Under a fixed RH
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of 50% and a fixed air velocity of < 0.7m/s (2.3ft/s), the study investigated the reaction
efficiencies of the photocatalytic decomposition of formaldehyde at temperatures of
18(64.4), 20(68), 22(71.6), 24(75.2), 26(78.8) and 28°C (82.4°F). The study also
investigated the influence of different RH values (40, 50 and 60%) on the photocatalytic
removal efficiency of formaldehyde.
For determination of photocatalytic decomposition effectiveness, the UV light
should sufficiently irradiate the surface of the photocatalyst, and the organic pollutant
needs sufficient contact time with the photocatalyst surface. Thus, air velocity in the test
system can have an influence on the time during which organic substances are in contact
with the photocatalyst surface. Therefore, this study determined the effects of air velocity
in the test system. Here, we studied the influence of weak (0.4 m/s)(1.3 ft/s), medium (0.7
m/s)(2.3ft/s) and strong (1.3m/s)(4.3ft/s) air speeds, on the effectiveness of photocatalytic
degradation of formaldehyde.
Preparation of stainless steel photocatalytic filters
Due to the photochemical nature of photocatalysts, photocatalysis only occurs on
surfaces that receive light. If a photocatalyst is embedded within a fixed area or in a
fluidized bed, effectiveness of the photocatalyst can be reduced due to shadowing of the
available light. This study selected hole-punched stainless steel filters as the base plate
for the photocatalyst. That selection avoided adsorption on the base plate (Jardim, 1994),
and allowed the added TiO2 film to be distributed evenly and coating speedy. The filter
preparation procedure was as follows:
a. The hole-punched stainless steel filter (0.21m (0.689 ft)× 0.21m (0.689 ft) × 1 mm
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(0.00328 ft) thick; model, SUS304), with pore diameter of 2.0 mm (0.00656ft) and
distance between pores of 1 mm (0.00328ft), was cleaned using ethanol and DI-water.
b. A 1 wt. % solution of water-soluble polymer binder (QFnano Co., Ltd, Taiwan) was
prepared, and evenly applied by spray onto the stainless steel metallic filters, which
were then baked at 50°C (122°F) for 10 min to increase viscosity, strengthen the binder
coating layer, and allow TiO2 nanoparticles to firmly affix to the stainless steel.
c. The TiO2 nanofluid (TiO2 in DI-water) was blended to have a TiO2 concentration of 3
wt. %, and was then vibrated using a supersonic vibrator (DC150H, DELTA) for
10–20 min at beaker to ensure even distribution of TiO2 within the solution.
d. The vibrated TiO2 nanofluid was then poured into a solvent tank, and the prepared
stainless steel base plate was submerged in the tank for 5–10 min to allow the
suspended TiO2 nanoparticles to adhere to the filter surface. The TiO2-stainless steel
filter was then sintered at 100°C (212°F) for 10 min.
e. To ensure similarity of test filters, a microbalance (resolution 0.001g, Precisa) was used
to weigh five TiO2 catalyst coated stainless steel filters base plates both before and
after TiO2 coating. The allowable difference among the five was 0.2 g (441μlbm) ±
5%. In addition, scanning electronic microscopy (SEM) was used to observe the
distribution of TiO2 particles on the base plates. As shown in a representative SEM
image (Fig. 2(c)), TiO2 particles were evenly distributed and formed an even layer of
nanoparticles on the base plate.
f. UV light irradiation was produced by five UVC lamps (5W (17.05Btu/hr), PHILIPS)
with a combined luminous intensity of ~2.35 mW/cm2 (7.445 Btu/hr/ft2).
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Apparatus for simulation of an air conditioning system
A schematic diagram of the experimental apparatus is shown in Fig. 3. The volume
of the test chamber was 0.29 m3 (10.24 ft3). The apparatus comprised a fan, pre-heater,
evaporator, re-heater, humidifier, and a pre-filter. This enclosed HVAC chamber was a
modification of an existing temperature- and humidity-adjustable air conditioning system
(model, P.A. Hilton Ltd., Stockbridge, UK). The modifications (temperature, RH, and air
velocity) allowed accurate control of temperature, RH and air speed; required to meet our
experimental
needs.
Temperature
control
was
performed
using
proportional–integral–derivative controller (TTM 100, TOHO Electronics
a
Inc.,
Kanagawa, Japan) was attached to a solid state relay (SSR)-controlled heater.
In this way, linear temperature increases could be acquired. After mutual
temperature chasing by the heater and cooler, a preset stable temperature value can be
reached in the test condition. The controls allowed a temperature control error of ±0.1°C
(32.2°F) to be achieved. Resistance temperature detectors (PT-100) were used to monitor
temperatures.
A humidifier was installed near the lower side of the cooling coil pipes; thereby
allowing water or steam to be directly sprayed into the passing air to control the RH
inside the test chamber. Humidity was controlled at ±2% RH by controller. Air velocity
within the system was controlled using a voltage-adjustable controller to change the
rotational speed of the fan.
Calculation and analysis
The study investigated the influence of a TiO2-UV-stainless steel air filter reactor on
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the removal efficiency of specific formaldehyde concentrations under different
temperatures, RH values, and air velocities.
During testing, a fixed volume (0.1 ml) (26.4 μgal) of formaldehyde liquid
solution (100% formalin) was dropped into the air-circulation test chamber, which led to
a target initial formaldehyde concentration of about 4.5 ppm (17.03 gal/L) within the test
chamber. Subsequently, the test system was operated at preset experimental parameters
for 2 hours. During system operation, the concentration change of formaldehyde was
measured and recorded every 15 min by the aforementioned formaldehyde analysis
detector. After sampling was completed, the wall surfaces inside the chamber were
cleaned using distilled water. Prior to re-use of the apparatus for further formaldehyde
concentration monitoring, a test chamber formaldehyde level of 0 ppm (0 gal/L) was
obtained. In this study, the contaminant mass-balance equation for chamber concentration
decay test, assuming well-mixing in chamber, is often written as:
V
dC
  Qc ηC  Qv C  k nV C with C  Co
dt
at t = 0
where,
C – contaminant concentration in chamber
C0 – initial contaminant concentration in chamber
Qc – the air flow rate passing through the air cleaner
η – single-pass removal efficiency
V – volume of test chamber
QV – the ventilation rate of test chamber
kn – natural decay rate (due to surface sink effect, etc.)
For test in sealed chamber, QV is zero and Equation E1 becomes:
9
(1)
V
dC
 - Qc ηC - k nV C
dt
or C(t)  C o  e
(
CADR
 k n )t
V
(2a)
 Co  e
(
CADR
 k n )t
V
 C o  eka t
(2b)
The total formaldehyde removal rate at the end of each experiment (background, direct
photolysis, and UV-PCO) can then be expressed as:
de  ( Ini. conc.
 Final conc.
Ini. conc.
k t

e a end
)  100%  C o C o 
 1  e  k a tend
Co
(3)
where, ηde is the formaldehyde photocatalyst decomposition effectiveness, kn is the
natural decay constant, ka is decay constant, the coefficients of ka and kn calculated using
the concentration point at the end of experiment only or using the entire concentration
decay curve by fitting it exponentially.
The result of this study can be transferred to other HVAC applicable parameters
(e.g., clean air delivery rate, CADR) or removal efficiency before being used in a real
system. CADR is relevant to the formaldehyde removal efficiency of the photocatalytic
filter, and is related to the airflow rate passing though the photocatalytic filter. Here,
CADR represents the volume of filtered air passing though the photocatalytic filters per
unit time, and is expressed as:
CADR  (ka - k n )V
(4)
CADR  Rr  ηde  Qc  Rr  ηde  A  v
(5)
CADR
 Rr  ηde  v
A
(6)
where A is the filtration area of the photocatalytic filter, v is the face velocity, ηde is the
total formaldehyde removal rate at the end of each experiment, Rr is the recirculation rate
10
(in this study, Rr=1). Equation 6 represents the CADR per unit area (CADR/A) of the
photocatalytic filter. This parameter can be applied to evaluate the CADR of an interested
real system.
RESULTS AND DISCUSSION
Formaldehyde background adsorption experiment
Each experiment lasted two hours under controlled temperatures. In the absence of
UV illumination, the influence of temperature changes inside the chamber on the
adsorption decline of formaldehyde concentration was assessed. The influence of
temperature on formaldehyde adsorption decline effectiveness was not large. Thus, we
suggest that formaldehyde adsorption decline in the test system was mainly influenced by
formaldehyde adsorption on the surfaces of the air conditioning system. Figure 4 shows
that when the temperature was 28°C (82.4°F), the initial effusion of formaldehyde
concentration was faster, making the initial concentration in the system higher. Moreover,
after the system had operated for 60 min at temperatures of 28°C (82.4°F) and 26°C
(78.8°F), The maximum formaldehyde adsorption decline effectiveness decline, as
influenced by intra-system temperature was 10.68%, and the minimum was 8.27%. The
average formaldehyde concentration decline was 0.5 ppm (1.89 gal/L) inside the
air-circulation test chamber at different temperatures. The formaldehyde concentration
appeared to become stable. This implies that when the temperature is low, formaldehyde
molecules are not easily effused within the test chamber.
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Direct photolysis of formaldehyde
Influence of temperature
After the abovementioned background adsorption tests, and under the same
experimental conditions (i.e., with a non-TiO2 coated filter), UV lamps (2.35mW/cm2
(7.445 Btu/hr/ft2)) were turned on to allow the assessment of direct UV photolysis of
formaldehyde. The experimental results are shown in Fig. 5. After two hours of UV
photolysis of formaldehyde, the initial concentration (4.6 ppm (17.41 gal/L) and 4.5 ppm
(17.03gal/L)) has declined between 0.87 ppm (3.29 gal/L) and 1.44 ppm (5.45gal/L)
from 18 (64.4°F) and 28°C (82.2°F), with an associated photodegradation rate range of
19.03-31.23%. As the temperature rises, the formaldehyde concentration tended to fall.
Also, over the duration of the experiments, degradation of the formaldehyde
concentration continued.
Influence of humidity
As shown in Fig. 6(a), when the temperature was constant at 22°C (71.6°F), and the
RH was varied (40, 50, and 60%), the declines in formaldehyde concentrations were
1.26(4.77), 1.11(4.20), and 1.07(4.05) ppm (gal/L), respectively. After system operation
for two hours, the system’s formaldehyde adsorption decline effectiveness values under
40, 50, and 60% RH were 27.81, 24.23, and 23.41% respectively. Fig. 6(b) shows that, at
a constant temperature of 25°C (77°F), and under 40, 50 and 60% RH, the photocatalytic
decline of formaldehyde concentrations were 1.19 (4.50), 1.14 (4.31), and 1.1 (4.16) ppm
(gal/L), respectively. After two hours of testing, the system’s formaldehyde adsorption
decline effectiveness values under 40, 50, and 60% RH conditions were 28.82, 27.25, and
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26.31%, respectively. The results show that the photocatalytic decomposition
effectiveness under UV illumination was higher when the humidity was low. An
ASHRAE Psychometric Chart (ASHRAE, 2005) shows that, at a temperature of 22°C
(71.6°F) and with RH of 40, 50, and 60%, that humidity ratios would be 6.5 (0.0062),
8.25 (0.008), and 10 (0.0098) gv/kga (lbmv/lbma) , respectively, and when the temperature
was 25°C (77°F), the humidity ratios would be 7.8 (0.008), 9.9 (0.01), and 12 (0.012)
gv/kga (lbmv/lbma), respectively. Those data suggest that when humidity is low, water
vapor occupies a smaller proportion of the total weight of air. Thus, implying that when
UV illumination is presented under low humidity, that a greater heat source (TiO2 reactor
temperature of was raised 0.5°C (32.9°F) under UV illumination than high humidity)
maybe caused formaldehyde molecule to be distribution in the system. But the water
molecules of high humidity instead absorbance formaldehyde molecules, leading to
decreased
thermolysis.
Therefore,
under
low
humidity
conditions
(40%RH)
direct-photolytic speeds become higher than 60%RH in this study.
It is generally thought that direct photolysis by high-energy UV light is related to
the chemical bonding energy of the molecular structure of formaldehyde. This implies
that, for degradation, the light energy absorbed by formaldehyde should be higher than
the chemical bonding energy. The maximum wavelengths to break the C=O bond
(aldehydes) and the C-H bond in the molecular structure of formaldehyde are 162.4 and
289.7 nm respectively (Legan, 1982). However, our study investigated direct photolysis
under a light source irradiation wavelength of 253.7 nm. Therefore, we conclude that
direct photolysis does not cause a marked change in the structure of formaldehyde.
13
Influence of air velocity
The direct UV photolysis experiment was also done under different air movement
conditions, with the other experimental conditions remaining the same. The results are
shown in Fig. 7(a). When the temperature was 22°C (71.6°F), and air velocities were 0.4
(1.3), 0.7 (2.3), and 1.3 (4.3) m/s (ft/s), the formaldehyde concentration adsorption
declines were 1.03 (3.90), 1.11 (4.20), and 1.06 (4.01) ppm (gal/L), respectively. After
two hours of operation, the system’s formaldehyde adsorption decline effectiveness
values, under air velocities of 0.4 (1.3), 0.7 (2.3), and 1.3 m/s (4.3ft/s), were 23.3, 24.23,
and 23.66%, respectively. Fig. 7(b) shows that when the temperature was 25°C (77°F),
and the air velocities were 0.4 (1.3), 0.7 (2.3), and 1.3 (4.3) m/s (ft/s), the adsorption
declines in formaldehyde concentrations were 1.33 (5.03), 1.24 (4.70), and 1.24 (4.70)
ppm(gal/L), respectively. After two hours of system operation, the formaldehyde
adsorption decline effectiveness values, under air velocities of 0.4(1.3), 0.7(2.3), and 1.3
(4.3) m/s (ft/s), were 29.49, 27.25, and 28.18%, respectively. The results suggest that the
effect of air velocity on direct photolysis is not marked.
In summary, for direct UV photolysis of formaldehyde over the temperature range
tested, photolysis was 31.23%; while over the RH range tested, photolysis was 28.82%;
and over the tested air velocities, photolysis was 28.18%.
Formaldehyde photocatalysis experiment using a TiO2 photocatalyst
Influence of temperature
Fig. 8 plots the reaction rates of formaldehyde versus reaction temperatures under
UV irradiation. As mentioned, formaldehyde did not degrade at reaction temperatures of
14
18–28°C(64.4–82.4°F) DB in the background adsorption experiment (i.e., without UV
irradiation; Fig. 4). On the contrary, rapid decomposition of formaldehyde was observed
with UV irradiation, and the formaldehyde reaction rate increased as the reaction
temperature increased (Fig. 8). Comparing the reaction rates of formaldehyde in both
dark and UV irradiation conditions, shows that the decomposition of formaldehyde
results mainly from a photocatalytic reaction. Based on gas-solid catalytic theory (Yang,
2000), such surface reactions include three steps: reactant adsorption, chemical reaction,
and product desorption. Increasing reaction temperature not only increases the chemical
reaction and product desorption rates, but also reduces the reactant adsorption rate.
Increases in the chemical reaction and product desorption rates result in an increase in the
overall reaction rate. Correspondingly, a decrease in the reactant adsorption rate
decreases the reaction rate.
Here, the formaldehyde reaction rate increased with increasing temperature (Fig. 8)
and the decline in formaldehyde concentration following photocatalysis of the TiO2 in the
test chamber was 2.34–2.84 ppm (8.86–10.75 gal/L). After the system operation for two
hours, the system’s formaldehyde photocatalytic decomposition effectiveness was
66.67–85.97%. Furthermore, the experimental results show that as the photocatalytic
temperature rises, photocatalytic speed increased, thereby increasing the influence of the
photocatalyst surface on the degradation of the formaldehyde. Table 1 shows the kn and ka
values of the formaldehyde concentration, with the photocatalytic filter present and under
temperatures of 18(64.4), 20(68), 22(71.6), 24(75.2), 26(78.8) and 28 (82.4)°C (°F) DB,
ranged from 0.043 to 0.057 h-1 and from 0.549 to 0.982 h-1, respectively. When the
temperature increased from 18 to 28°C DB, ka rose by 78.8%. The CADR and CADR/A
15
values ranged from 0.145 (5.12) to 0.272 (9.6) m3/h (ft3/h) and from 3.286 (10.79) to
6.171 (20.24) m3/h/m2 (ft3/h/ft2), respectively. The results indicated that a reduction of the
reactant adsorption rate did not affect the overall reaction, and that the overall reaction
remained photocatalytic for the duration of the test. Meanwhile, the chemical reaction
and product desorption rates were the slowest steps in the overall reaction, even when
temperature increased.
Influence of humidity
Subsequent to the abovementioned background humidity adsorption test, and under
the same experimental conditions (i.e. the temperature was at 22°C (71.6°F) and 25°C
(77°F), the RH was varied (40, 50, and 60%)), experiments on formaldehyde UV
photocatalysis of the TiO2-attached stainless steel filter were performed under different
RH values (Fig. 9). When the temperature was 22°C (71.6°F), and the RHs were 40, 50,
and 60%, the formaldehyde concentration reductions in the test system over the two-hour
experimental period were 2.31 (8.74), 2.48 (9.39), and 2.81 (10.64) ppm (gal/L),
respectively. After system operation for two hours, the formaldehyde adsorption decline
effectiveness values at 40, 50 and 60% RH were 74.03, 77.01, and 84.13%, respectively.
As shown in Fig. 10, when the temperature was 25°C (77°F), the photocatalytic declines
in formaldehyde concentrations were 2.65 (10.03), 2.68 (10.14), and 2.92 (11.05) ppm
(gal/L), respectively. After the system had operated for two hours, the formaldehyde
decline effectiveness values at 40, 50, and 60% RH were 78.17, 84.27, and 89.02%
respectively. The results demonstrate that an increase in RH had a positive effect on the
formaldehyde removal effectiveness of the TiO2 photocatalytic filters. As mentioned
16
previously, this result is because, when the humidity was high, there are more water
molecules in the tested air. Thus, electron-hole pairs, which were stimulated and formed
after UV irradiation of the photocatalyst, have more opportunities to collide with water
molecules and produce hydroxyl radicals. Due to the strong photocatalysis of hydroxyl
radicals, photocatalyst removal efficiencies are improved under high humidity conditions
(Yu, 2006).
As shown in Table 1, the kn and ka of the formaldehyde concentration with
photocatalytic filters present under 40, 50, and 60% RH ranged from 0.041 to 0.060 h-1
and from 0.674 to 1.105 h-1, respectively. When the RH increased from 40% to 60%, at
both 22°C (71.6°F) DB and 25°C (77°F) DB the ka rose 36.5% and 45.2%, respectively.
The CADR, CADR/A, and the efficiency values for formaldehyde also increased with
RH. Those results demonstrate that an RH increase has a positive effect on the
effectiveness of the photocatalytic filter for removing formaldehyde. A possible
explanation for this result is that an RH increase can enhance the formation of hydroxyl
radical during direct gas-phase photolysis. When an electron on the valence band of the
photocatalyst absorbs a photon with a energy higher than the band gap between the
valence band and the conduction band, it will be promoted to the conduction band, and an
electron-hole pair (e- and h-) is created (Obee, 1995). Thus, in the presence of humidity,
the surface of TiO2 photocatalyts is hydroxylated, and hydroxyl groups are formed.
Influence of air velocity
As shown in Fig. 11, under constant temperature (22°C) (71.6°F) and relative
humidity (50%), but with air velocities of 0.4 (1.3), 0.7 (2.3), and 1.3 (4.3) m/s (ft/s), the
17
declines in formaldehyde concentrations were 2.43 (9.20), 2.48 (9.39), and 2.7 (10.22)
ppm (gal/L), respectively. After system operation for two hours, the system’s
formaldehyde adsorption decline effectiveness values, under air velocities of 0.4 (1.3),
0.7 (2.3), and 1.3 (4.3) m/s (ft/s), were 70.43, 77.01, and 85.71% respectively. As shown
in Fig. 12, when the temperature was 25°C (77°F), RH was 50%, and air velocities were
0.4 (1.3), 0.7 (2.3), and 1.3 (4.3) m/s (ft/s), the declines in formaldehyde concentration
were 2.57 (9.73), 2.68 (10.14), and 2.67 (10.11) ppm (gal/L), respectively. After the
system had operated for two hours, the formaldehyde adsorption decline effectiveness
values, at air velocities of 0.4 (1.3), 0.7 (2.3), and 1.3 (4.3) m/s (ft/s), were 76.71, 84.27,
and 85.85%, respectively. The results show that when the velocity of the circulating air is
increased, the formaldehyde concentration decreased and the formaldehyde removal
effectiveness increased. As shown in Table 1, the kn and ka of the formaldehyde
concentration with UV illuminated photocatalytic filters present, and under air velocities
of 0.4 (1.3), 0.7 (2.3), and 1.3 (4.3) m/s (ft/s), ranged from 0.047 to 0.059 h-1 and from
0.609 to 0.978 h-1, respectively. Table 1 also shows that, under these conditions, the
CADR and CADR/A ranged from 0.16(5.7) to 0.267(9.43) m3/h (9.4ft3/h) and from 3.643
(11.95)to 6.040 (19.81) m3/h/m2 (ft3/h/ft2), respectively. When the photocatalysis duration
is fixed and air velocity in the system increases, the circulation frequency of the
contained pollutants increases; thereby resulting in more contact of pollutant with the
photocatalyst (Yang, 2000). Thus, a formaldehyde pollutant circulating under higher air
velocities would have a higher frequency of contact with the TiO2 photocatalyst, resulting
in increased photocatalytic decomposition effectiveness.
These experiments simulated a room without general air exchange rate, e.g., an
18
office using a HVAC system without external air exchange. In such a situation, the
CADR/A of air velocity asymptotically reaches an upper limit at the system’s face
velocity corresponds with the actual surface reaction rate. Thus, the CADR/A in a typical
HVAC systems remains as expected.
CONCLUSIONS
According to the experimental results, the TiO2 photocatalyst was combined with a
stainless steel air filter, using simulated air-circulation chamber system with temperatures,
relative humidities and air velocity. To confirm photocatalytic TiO2 coated air filters are
capable of controlling formaldehyde contaminations by UV illuminated in above case.
When the circulating air temperature is raised, the adsorption decline effectiveness of
relative humidity also increases. In addition, when the circulating air’s air velocity
increased, the adsorption of the system increased, and reaches saturation. An increase in
air velocity shortens the time that a pollutant particle may contact the photocatalytic
surface. The ka, kn and CADR were all affected by air velocity, RH, and presence of the
photocatalytic filter. Overall, formaldehyde removal efficiency of the UV illuminated
TiO2 photocatalytic filter increased with temperature, relative humidity, and air velocity.
Acknowledgements
This research was supported by the Grant No. NSC 96-2221-E-027 -040, offered
by the National Science Council, Taiwan, Republic of China.
19
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22
Table 1. Summary of experimental conditions, results, and calculated parameters.
DB
°C
(°F)
18
(64.4)
20
(68)
22
(71.6)
24
(75.2)
26
(78.8)
28
(82.4)
22
(71.6)
25
(77)
22
(71.6)
25
(77)
RH
%
50
A, Filter
Q
v
m2
3
3
m
/h
(ft
/h)
m/h
(ft/h)
(ft2)
0.221
165.6
2520
(2.379) (5848.1) (8267)
ηde
ka
(h-1)
kn
(h-1)
0.67
0.549
0.051
50
0.221
165.6
2520
0.73
0.657
0.057
50
0.221
165.6
2520
0.77
0.735
0.047
50
0.221
165.6
2520
0.81
0.820
0.053
50
0.221
165.6
2520
0.85
0.878
0.043
50
0.221
165.6
2520
0.86
0.982
0.046
40
0.221
165.6
2520
0.74
0.674
0.041
50
0.221
165.6
2520
0.77
0.735
0.047
60
0.221
165.6
2520
0.84
0.920
0.057
40
0.221
165.6
2520
0.78
0.761
0.046
50
0.221
165.6
2520
0.84
0.925
0.049
60
0.221
165.6
2520
0.89
1.105
0.060
50
0.221
0.70
0.609
0.057
50
0.221
0.77
0.735
0.047
50
0.221
0.86
0.973
0.053
50
0.221
93.6
1440
0.77
0.729
0.059
50
0.221
165.6
2520
0.84
0.925
0.049
50
0.221
306.0
4080
0.86
0.978
0.061
93.6
(3305.5)
165.6
(5848.1)
306.0
(10806)
1440
(4724)
2520
(8267)
4080
(13386)
23
CADR
m3/h
(ft3/h)
0.145
(5.121)
0.174
(6.144)
0.200
(7.063)
0.223
(7.875)
0.242
(8.546)
0.272
(9.606)
0.184
(6.498)
0.200
(7.063)
0.250
(8.829)
0.207
(7.310)
0.254
(8.970)
0.303
(10.70)
0.160
(5.650)
0.200
(7.063)
0.267
(9.429)
CADR/A
m3/h/m2
(ft3/h/ft2)
0.654
(2.145)
0.788
(2.584)
0.904
(2.965)
1.007
(3.302)
1.096
(3.594)
1.229
(4.031)
0.831
(2.725)
0.904
(2.965)
1.133
(3.716)
0.939
(3.079)
1.150
(3.772)
1.371
(4.496)
0.725
(2.378)
0.904
(2.965)
1.207
(3.958)
0.194
(6.851)
0.879
(2.883)
0.254
(8.970)
0.266
(9.394)
1.150
(3.772)
1.203
(3.945)
List of Figures
Fig. 1. (a)XRD and (b)TEM structural analysis chart of the TiO2 used in the experiments.
Fig. 2. Images before and after using a water-soluble polymer binder to apply a TiO2
coating to a stainless steel air filter: (a) original stainless steel filter (optical
microscope, magnification = ×40) (b) TiO2 coated stainless steel filter (optical
microscope, magnification = ×40) (c) TiO2 coating on the stainless steel filter
(SEM image, magnification = × 1000).
Fig. 3. Schematic diagram of the air-circulation test chamber system used to test
degradation of gaseous formaldehyde by a reactor consisting of a TiO2 film on a
stainless steel air filter.
Fig. 4.Background formaldehyde concentration adsorption under different temperatures
and constant relative humidity and air velocity.
Fig. 5. Direct formaldehyde photolysis by UV light under different temperatures with
constant relative humidity and air velocity.
Fig. 6. Direct formaldehyde photodegradation by UV light under different relative
humidities (a) 22°C (71.6°F) dry bulb, winter condition, and (b) 25°C (77°F) dry
bulb, summer conditions.
Fig.7. Direct formaldehyde photodegradation by UV light under different air velocities (a)
22°C (71.6°F) dry bulb, 50% RH; winter conditions, and (b) 25°C (77°F) dry bulb,
50% RH; summer conditions.
Fig. 8. Formaldehyde photocatalysis by UV illumination on TiO2 on a stainless steel filter
under different temperatures with constant relative humidity and air velocity.
Fig. 9. Formaldehyde photocatalysis by UV illumination of TiO2 on a stainless steel filter
under different relative humidities (22°C (71.6°F) dry bulb; winter conditions).
Fig. 10. Photocatalysis of formaldehyde by UV illumination of TiO2 on a stainless steel
filter under different relative humidities (25°C (77°F) dry bulb; summer
conditions).
Fig. 11. Formaldehyde photocatalysis over time by UV illumination of TiO2 on a stainless
steel filter under different air velocities (22°C(71.6°F) drybulb, 50% RH; winter
conditions).
Fig. 12. Formaldehyde photocatalysis over time by UV illumination of TiO2 on a stainless
24
steel filter under different air velocities (25°C(77°F) dry bulb and 50% RH; summer
conditions).
25