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 recombinationcenters----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 productionrate 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 dopingIt 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 , impurityenergy 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 ismore 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 metalion doping methods , doping content and depth

• Anion dopingDoping 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 differentmodification methods can contribute to a higher efficiency

• Metal ion-implantationMetal 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 reductionThe 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 2-propanol. 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 useof semiconductor powders for CO2 reduction, including TiO2, ZnO, CdS, SiC, and WO3, suspended in CO2 saturated water illuminated by a Xe lamp. Smallamounts 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 donorsAdding electron donors (sacrificial reagents or hole scavengers) to react irreversibly with the photo-generated 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 SO3

2- added can dissolve S into S2O3

2- 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 saltsIt 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 C2O6

2-. 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 temperatureGenerally, for photocatalysts, photon irradiation is the primary source of energy for electron-hole pair formation at ambient temperature, because the band-gap energy is too high for thermal excitation to overcome. However, at high temperatures, thereaction 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-treatmentImportant 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

Thanks!