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Summary of Literatures
Reporter: Xiang Tianyu
Supervisor: Xin Feng
1.Introduction of photocatalysis
Photocatalysis makes use of semiconductors to
promote reactions in the presence of light radiation.
The generation of electron-hole pairs and its reverse
process are:
where hv is the photon energy, e- represents a
conduction band electron, and h+ represents a hole in
the valence band.
Figure 2 illustrates the photoexcitation process :
The optimal characteristics required for photocatalysts:
(1) The redox potential of the hole must be sufficiently
positive.
(2) The redox potential of the electron must be
sufficiently negative.
(3) Not be prone to photocorrosion or produce toxic
byproducts.
(4) Commercially and economically available.
2. Selection and modification of photocatalyst
Fig.3 Elements constructing heterogeneous photocatalysts.
Fig. 4 Relationship between band structure of semiconductor and
redox potentials of water splitting.
Fig 5. Conduction band and valence band potentials of semiconductor
photocatalysts relative to energy levels of the redoxcouples in water.
Semiconductors absorb light radiation with a
threshold wavelength that provides sufficient
photon energy to overcome the band gap between
the valence and conduction bands.The minimum
wavelength required to promote the excited state
depends on the band-gap energy is:
Large-band-gap semiconductors are the most
suitable photocatalysts for CO2 reduction, because they
provide sufficient negative and positive redox
potentials in conductance bands and valence bands,
respectively.
Presently, the energy conversion efficiency is still
low, mainly due to the following reasons:
(1) Recombination of photo-generated electron/hole
pairs.
(2) Inability to utilize visible light.
• Noble metal loading
Noble metals, including Pt, Au, Pd, Rh, Ni, Cu and Ag,
have been reported to be very effective for
enhancement of photocatalyst . As the Fermi levels of
these noble metals are lower than that of catalyst ,
photo-excited electrons can be transferred from CB to
metal particles deposited on the surface of catalyst ,
while photo-generated VB holes remain on the catalyst.
These activities greatly reduce the possibility of
electron-hole recombination, resulting in efficient
separation and stronger photocatalytic reactions.
Loading of Pt worked better than loading of Au
Au loading----deposition worked better than
deposition–precipitation and impregnation----better
contact with active sites
Pt-loaded TiO2:less sensitive to the preparation
methods
Too much metal particle---reduce photon
absorption/become electron-hole recombination
centers----resulting in lower efficiency
Since Pt is very expensive , more research is needed to
identify low-cost metals
Cu loading was found to be almost comparable to Pt
loading
At the optimal loading of Cu, hydrogen production
rate was enhanced as much as 10 times higher
Other low-cost metals, such as Ni and Ag, were also
found to be effective for photocatalytic activity
enhancement.
• Metal ion doping
It was found that doping of metal ions could expand
the photo-response of catalyst into visible spectrum. As
metal ions are incorporated into the lattice , impurity
energy levels in the band gap are formed. Metal ions
should be doped near the surface of catalyst particles
for a better charge transferring. In case of deep doping,
metal ions likely behave as recombination centers,
since electron/hole transferring to the interface is
more difficult.
Fe, Mo, Ru, Os, Re, V, and Rh ions can increase
photocatalytic activity
Dopants Co and Al ions can cause detrimental effects
Doping of either Cu or Fe ions could be recommended
for enhancement of photocatalytic activity
Cu, Mn and Fe ions can trap both electrons and holes,
Cr, Co and Ni ions can only trap one type of charge
carrier
Rare earth metal ions (La, Ce, Er , Pr, Gd, Nd and Sm)
can enhance photocatalytic activities and cause red
shift of photo-response , Gd ions were found to be
most effective
Doping Fe, Co, Ni, Cu and Zn on WO3 to form FeO, CoO,
NiO and Cu2O with more negative CB levels, ZnO could
not capture electrons from CB of WO3
Doping of 1% and 10% Ni ions is optimal
Wu reported that doping of Cu and Fe ions in TiO2 was
more effective than doping of Ni ions, the discrepancy
could be explained by different forms of doping
Be ion doped TiO2----near the surface/beneficial---deep doping/poor performance
Photocatalytic effect is very sensitive to the metal
ion doping methods , doping content and depth
• Anion doping
Doping of anions(N, F, C, S etc.)in catalyst could shift its
photo-response into visible spectrum
Mixing of p states of N with 2p of O could shifts VB
edge upwards to narrow down the band gap of TiO2
The ionic radius of S was too large to be incorporated
into the lattice
Dopants C and P were found to be less effective as the
introduced states were so deep that photo-generated
charge carriers were difficult to be transferred to the
surface of the catalyst
For efficient photo catalytic reaction , coupling with
other technologies, such as noble metal loading or
electron donor addition, is necessary
• Dye sensitization
Dye sensitization is widely used to utilize visible light
for energy conversion. Some dyes having redox
property and visible light sensitivity can be used in
solar cells as well as photocatalytic systems .Under
illumination by visible light, the excited dyes can inject
electrons to CB of semiconductors to initiate the
catalytic reactions as illustrated in Fig. 6.
Semiconductors and sacrificial agents are needed.
Fig. 6. Mechanism of dye-sensitized photocatalytic hydrogen production
under visible light irradiation.
• Composite semiconductors
Semiconductor composition (coupling) is another
method to utilize visible light for hydrogen production.
When a large band gap semiconductor is coupled with
a small band gap semiconductor with a more negative
CB level, CB electrons can be injected from the small
band gap semiconductor to the large ban d gap semi
conductor. Thus , a wide electron-hole separation is
achieve d as shown in Fig. 7.
Fig. 7. Electron injection in composite semiconductors.
Successful coupling of the two semiconductors should
met the following conditions:
(i)Semiconductors should be photocorrosion free
(ii)The small band gap semiconductor should be able to
be excited by visible light
(iii)The CB of the small band gap semiconductor should
be more negative than that of the large band gap
semiconductor
(iv)The CB of the large band gap semiconductor should
be more negative than CO2 redox potential
(v)Electron injection should be fast as well as efficient
CdS(2.4eV)/SnO2(3.5eV)----visible irradiation----EDTA as
hole scavenger
CdS/TiO2 ----UV irradiation----2-chlorophenol
degradation----better charge separation
CdS/ZnS---- solar irradiation----Na2S/Na2SO3 solution
TiO2/WO3(2.7eV)/SiC(3.0eV)----UV irradiation---electrons transferred from SiC to TiO2 to WO3 ---efficient charge separation
The hydrogen production rate of the coupled
Pt/ZnO/SnO2/dye was found to be much higher
(0.92ml)than Pt/ZnO/dye (0.04ml)
Nitrogen-doped ZnO was coupled with WO3 ,V2O5 and
Fe2O3 for acetaldehyde decomposition under visible
light, N-doped ZnO-WO3 and ZnO-V2O5 work better
It is expected that suitable coupling of different
modification methods can contribute to a higher
efficiency
• Metal ion-implantation
Metal ion-implantation was recently reported to be an
effective method to modify semiconductor electronic
structures to improve visible light response.
The catalyst was prepared by ionized cluster beam (ICB)
method.
The qualitative effectiveness of red shift was observed to be
in the following order: V＞Cr＞Mn＞Fe＞Ni.
Red shift could be realized only when implantation was
followed by calcinations in an O2 atmosphere at around
723–823K.
3.Products of CO2 reduction
The photoreduction of CO2 by water is readily available
and inexpensive. Two important species involved in CO2
photoreduction are H·(hydrogen atom) and ·CO2
(carbon dioxide anion radical) produced by electron
transfer from the conduction band as follows:
These radicals will also form other stable substances：
The solubility of CO2 in water is particularly low, and
the CO2 photoreduction process is competing with H2
and H2O2 formation, which consumes H+ and e- as
follows:
Low-dielectric-constant solvents/low-polarity solvents---·CO2- not well dissolved in solvents----strongly
adsorbed on the surface----CO is major product
High-dielectric-constant solvent----·CO2- greatly
stabilized by the solvent----weak interactions with the
photocatalyst surface----the carbon atom of the radical
tends to react with a proton to produce formic acid.
It was noted that the amount of H+ in the reductant
controls the direction and selectivity of the CO2
photoreduction products
Products depends on various aspects such as the
proportion of H2O and CO2,the type of photocatalyst and
the reaction temperature.
Cu/TiO2 suspension----water----CH4
Hg and Pt/TiO2 ----CHOH
Pd, Rh, Pt, Au, Cu and Ru/TiO2 ----CH4 and CH3COOH
Pd/TiO2 ----high selectivity for CH4
Pt/Ti-containing zeolite----increased CH4 and CH3OH
production
Single crystal TiO2 (100)----CH4 , CH3OH
Single crystal TiO2 (110)----CH3OH with a low yield
Pd/ TiO2----the highest amount of CH4
Cu,Ru/TiO2 ----a substantial amount of CH3COOH
Ru/TiO2 ----H2 (proton sourse)----CH4 (3 times to the
dark condition)
At high pressures CO2 ,CH3COOH, CH3OH, and HCOOH
in the liquid phase and CH4 as a major product, along
with minor yields of C2H6 and C2H4 in the gas phase,
have been observed on the photolysis of TiO2
suspended in aqueous solutions. Under this condition
the yield of methane increases with the increase in CO2
pressure. The entire yield of the gaseous products has
been increased on the addition of electrolytes like
NaOH into the system. CO2 reduction does not produce
CH4 in the absence of any electron donors.
The ambient such as N2 ,air ,O2 and CO2 are important
for chemical reduction of CO2 in the presence of 2propanol. A higher yield of CH4 has been reported both
in the aerated and CO2-saturated systems in contrast to
O2 and N2-purged systems. The yield of methane also
depends on the concentrations of hole scavenger.
In 1979, Inoue and co-workers examined the use
of semiconductor powders for CO2 reduction, including
TiO2, ZnO, CdS, SiC, and WO3, suspended in CO2
saturated water illuminated by a Xe lamp. Small
amounts of formic acid, formaldehyde, methyl alcohol,
and methane were produced.
SiC----Xe lamp----the highest amount of HCHO,CH3OH
WO3 ----Xe lamp----absence of CHOH
SrTiO3 ----visible light----HCOOH,CHOH,CH3OH
Mixture of p-SiC and Cu particles----CH4,C2H4,C2H6
Cu/TiO2----Xe lamp----room temperature----CH4,C2H4
TiO2 highly dispersed on glass----UV light---CH4,CH3OH,CO
TiO2----Cu as cocatalyst----CH3OH
TiO2 loaded zeolite----water vapor----328K----gas phase
HCOH
Pt/TiO2 ----increased CH4 yield
1% Cu-loaded ZrO2 ----NaHCO3 solutions----UV light---CO
Ti-containing porous SiO2 films----Hg lamp----H2O
vapor----CH4,CH3OH
P-type CaFe2O4 ----Hg lamp----NaOH solution---CH3OH,HCHO
A thin film anatase TiO2 layer on one side of a Nafion
substrate and ZnO with Cu electrocatalysts on the
other----Hg lamp/sunlight----CH4,C2H4
Pt-loaded K2Ti6O13 ----sunlight----CH4 , CH3COOH,
CH3CHO , H2
Ru-doped TiO2/SiO2 ----CH3OH---- optimum
concentration is 0.5% Ru and 10wt % TiO2 in SiO2
It was found that ethanol was the major resultant
product when the composite catalyst was prepared by
sol-gel, while formic acid was the major resultant
product when the composite catalyst was prepared via
a hydrothermal technique.
4.Solution system
• Addition of electron donors
Adding electron donors (sacrificial reagents or hole
scavengers) to react irreversibly with the photogenerated VB holes can enhance the photocatalytic
electron/hole separation resulting in higher quantum
efficiency.
The rankings in terms of the degree of hydrogen
production enhancement capability were found to be:
EDTA＞methanol＞ethanol＞lactic acid.
Other inorganic ions, such as S-2/SO3-2,Ce+4/Ce+3 and
IO3-/I- were used as sacrificial reagents
When CdS is used as photocatalyst , S2-can react with 2
holes to form S. The aqueous SO32- added can dissolve
S into S2O32- in order to prevent any detrimental
deposit ion of S onto CdS. There fore, photocorrosion
of CdS is prevented.
IO3-/I- :without consumption of the sacrificial reagent
• Addition of carbonate salts
It is reported that addition of carbonate salts could
consume photo-generated holes by reacting with
carbonate species to form carbonate radicals, which is
beneficial for photo-excited electron/hole separation.
The Infrared (IR) study revealed that the surface of
catalyst was covered by many types of carbonate
species, such as HCO3- ,CO·3- , HCO3· and C2O62-. These
carbonate species were formed through the following
reactions:
However, when Pt-TiO2 was used as photocatalyst ,
addition of Na2CO3 was more effective than addition of
K2CO3 in terms of hydrogen production . The reason to
the above phenomenon is still unknown .
5.Effect of temperature
Generally, for photocatalysts, photon irradiation is the
primary source of energy for electron-hole pair
formation at ambient temperature, because the bandgap energy is too high for thermal excitation to
overcome. However, at high temperatures, the
reaction rate can be increased due to the thermal step
involved in entire reaction process such as collision
frequency , diffusion rate , absorption and desorption.
6. Post-treatment
Important points in the semiconductor photocatalyst
materials are the width of the band gap and levels of
the conduction and valence bands. The band gap of
anatase is 3.2eV while that of rutile is 3.0eV indicating
that the crystal structure determines the band gap
even if the composition is the same. Crystal structure,
crystallinity and particle size strongly affect the reaction.
Crystallinity is increased by calcination, the higher the
crystalline quality is, the smaller the amount of defects
is. But the surface area is decreased with an increase in
particle size through sintering: that is a negative factor. A
high degree of crystallinity is often required rather than
a high surface area for water splitting because
recombination between photogenerated electrons and
holes is especially a serious problem.
Photocatalysts prepared by soft processes sometimes
show higher activities than those prepared by solid state
reaction because they have small particle size and good
crystallinity.
7.Individual Ideas
(1)Using SiC or WO3 as photocatalyst
(2)Preparing catalyst in hydrothermal or sol-gel method
(3)Modifications : N-doping and noble metal loading
(4)Using Na2CO3 , NaHCO3 ,NaOH(-0.059V/pH) as
solution
(5)Illumination : Visible light
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