2-1 Photocatalysis and Photocatalysts
2-1-1 TiO2 photocatalysts
In 1972, photocatalysis of TiO2 nanoparticles has been found that can be used for degrading pollutions in water or air. Afterward, many investigations have been carried out with the aim of enhancing photocatalytic efficiencies of the process. In order to improve the efficiency of photoactivity, impurities including transition metal ions including Cr, V, Mn, Fe, or Co were doped into the TiO2 lattice to modify its electronic structures and microstructures.
[3-5, 12] Nowadays, TiO2 has been widely used in industrial application such as solar energy cell[13], photocatalysis[14], gas sensors[15] and CO2 photoreduction.[16]
The energy difference between the energies of the valence band and the conduction band of electron energy in semiconductors is called the band gap. Generally, semiconductor can be excited by energy in terms of photon energy, hν. A photon will excite an electron from the valence band to the conduction band, thereby the excited semiconductor has electrons and holes pairs for further redox reactions.[17-19] This process also shows in Figure 2-1.
When the heterogeneous photocatalyst absorbs UV energy which is higher or equal to the band gap energy of semiconductors, it will generate the electron-hole pair (e--h+ pair).
While several situations of the electron-hole pair may occur. The electron-hole pair can recombine in the bulk or on the surface of the particle in a few nanoseconds. The electron-hole pair can migrate to the photocatalyst surface and then be trapped in surface states. The electron-hole pair trapped in surface states can proceed the redox reaction with the compounds adsorbed on the catalyst. Figure 2-2 shows the basic electron transitions in an activated semiconductor.
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Figure 2-1 Simplified diagram of the photocatalytic processes occurring on an illuminated semiconductors.[20]
Figure 2-2 Schematic photoexcitation in a solid followed by deexcitation events.[17]
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Thermodynamically, the adsorbed pollutants can be photoreduced by conduction band (CB) electrons if they have redox potentials more positive than the flatband potential of the CB. Also, they can be oxidized by holes in the valence band (VB) if they have redox potentials more negative than the flatband potential of the VB. The proposed mechanisms can be expressed by the following set of simplified equations step by step.[20]
Step I: Band gap illumination (hv) onto a photocatalyst causes the electronic transitions.
+
− +
⎯→
⎯ e h
TiO2 hv
Step II: Organic molecule adsorbed on the catalyst surface and lattice oxygen (OL2-).
ads
where R1 represents an organic molecule, R1ads represents an adsorbed organic molecule Step III: Photogenerated holes oxidize the adsorbed OH- and water and electrons react with adsorbed O2.
The free radicals attack the organic molecule under different conditions:
Case I Case II Case III Case IV
In recent years, numerous photocatalyst materials, such as TiO2, ZnO, ZnS, SnO2 and CdS, have been widely discussed and applied in dealing with water pollution or air pollution.
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Figure 2-3 shows the redox potentials of various semiconductors. The titanium dioxide has been attracting more attention due to its non-toxicity, chamical stability, high photocatalytic activity, optical properties, low cost and suitable band gap energy.[21, 22]
TiO2 has two polymorphs: anatase and rutile. Figure 2-4 shows the crystalline structures of the anatase and rutile TiO2. Each Ti4+ ion is surrounded by an octahedron of six O2- ions. In the anatase structure, each octahedron contacts with 8 neighboring octahedrons, while in the rutile structure each octahedron contacts with 10 neighbors. The metastability of the anatase phase can transfer leads phase transformation to the rutile phase when calcination temperature is higher than 550 °C. The band gap of anatase and rutile are 3.2 eV and 3.0, respectively. Generally, anatase shows higher adsorptive ability and lower rate of charge recombination than rutile.[14]
Figure 2-3 The band edge position of various semiconductors.[17]
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Figure 2-4 Structure of rutile and anatase TiO2.[17]
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2-1-2 Modified TiO2 photocatalysts
Photocatalytic activity of a particular semiconductor system for the stated purpose was measured by some factors like the efficiency of the photocatalytic process, the stability of the semiconductor under illumination, the wavelength range response, and the selectivity of the products. Therefore, the limitations of a particular semiconductor as a photocatalyst for a particular use can be overcomed by modifying the semiconductor. There are three benefits of modifications to photocatalytic semiconductor systems: (1) inhibiting e--h+ recombination by increasing the charge separation and increasing the efficiency of the photocatalytic process;
(2) increasing the wavelength response range and (3) changing the selectivity or yield of a particular product.
The typical modifications include doping transition metal ions (Cr, V, Fe)[3-5] or non metal ions (C, N)[23, 24] into TiO2 lattice and combining TiO2 with another semisonductor (CdSe)[7]. The modifications change the microstructures and electronic structures, so that alter the physicochemical properties and photocatalytic acitivity. Among those researches, doping vanadium seems to be an effective route in the theoretical viewpoint. Zhao et al. and Wu et al. [25, 26] found that V-doped TiO2 resulted in a red shift of the absorption band edge.
Anpo et al. [27] modified TiO2 catalysts by bombarding V, Cr, Mn, Ni, or Fe, respectively, with high-energy metal ions. The metal ion-implanted TiO2 showed the V ions had the highest effectiveness in the red shift. Table 2-1 lists some important literatures of TiO2 and V-doped TiO2.
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Table 2-1 The development of titanium dioxide.
Year Authors The results and findings Ref.
1972 Fujishima et al. First developed electrochemical photolysis of water at a semiconductor electrode.
[28]
1992 Davidson et al. Investigated temperature-induced diffusion V ions into the TiO2
by ESR techniques
[29]
1994 Choi et al. Summarized metal-ion dopants in quantum-sized TiO2 [9]
1995 Linsebigler et al. Summarized the mechanisms of photocatalysis of TiO2 [17]
1999 Litter et al. The mechanisms of photocatalysis of metal ions doped in TiO2 [20]
1999 Zhao et al. Sol-gel preparation of Ti1-xVxO2 solid solution film electrodes with conspicuous photoresponse in the visible region [25]
2001 Rodella et al. Chemical and structural characterization of V2O5/TiO2 catalysts [30]
2002 Zhao et al. Photoelectrochemical properties of sol-gel-derived Ti1-xVxO2 [31]
2004 Wu et al. A visible-light response vanadium-doped titania nanocatalyst
by sol–gel method [26]
2005 Anpo et al. The preparation and characterization of highly efficient titanium oxide-based photofunctional materials
[27]
2006 Kemp et al. Characterisation of transition metal-doped TiO2 [4]
2007 Bouras et al. The structural of pure and metal-ion-doped nanocrystalline titania for photocatalysis
[5]
2008 Izumi et al. Photo-oxidation over mesoporous V-TiO2 catalyst under visible light monitored by vanadium K β 5,2-selecting XANES spectroscopy
[32]
2010 Xu et al. Photocatalytic activity of vanadium-doped titania-activated carbon composite film under visible light [33]
2010 Hoffmann et al. Combinatorial doping of TiO2 with platinum (Pt), chromium (Cr), vanadium (V), and nickel (Ni) to achieve enhanced photocatalytic activity with visible light irradiation
[34]
2011 Chang et al. Surface doping is more beneficial than bulk doping to the photocatalytic activity of vanadium-doped TiO2
[35]
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2-2 V-doped TiO2 photocatalyst
2-2-1 Physicochemical properties of V/TiO2
Transition metal doping can change the physicochemical properties of TiO2. In order to understand the structure of vanadium in titanium dioxide, FT-Raman can help us to understand the structure of the vanadium doped TiO2 catalysts. Figure 2-5 and Figure 2-6 show the structure of bulk and surface doped materials. There are two forms of VOX
species attached to the TiO2 surface for surface doped materials: monomeric vanadyl and polymeric vanadates.[30, 36, 37] In addition, the potentially active oxygen sites proposed to be interface V-O-Ti, surface Ti-O-Ti (638 cm-1), bridging V-O-V( 822 cm-1), and vanadyl V=O (1030 cm-1). When the number of vanadium in the polyvanadates increase, the number of terminal V=O (930 cm-1) decreases and forms the number of accommodate V-O-V (822 cm-1) linkages.[30, 36, 37] It shows that the vanadium ions are preferred to perform V2O5 under higher vanadium concentration. The doped vanadium ions diffused to the sample surface and formed V2O5 crystals when the calcination temperature was higher than their Tammann temperature(i.e. 209°C).[29, 38]
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Figure 2-5 Proposed V site transformations for V/TiO2 in reactant/product gas (A) and for mesoporous V-TiO2 by changing the molar ratio of V/Ti (B).[H for models a and c indicates the presence/absence of hydrogen cannot be determined.[37]
Figure 2-6 The structure of vanadium attached to the TiO2 surface.[39]
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2-2-2 Photocatalytic behavior of V/TiO2
When doping the impurity into TiO2, the defect can trap an electron or a hole alone and decrease the recombination time. Generally, The photochemical mechanisms, including charge recombination, charge-trapping, and migration mechanism, in the existence of transition metal ion dopants is showed as Figure 2-7 where Mn+ is a metal ion dopant, R is an electron donor, and O is an electron acceptor:
Figure 2-7 The photochemical mechanism in the present of transition metal ions.[9]
The addition of moderate transition metals into TiO2 can increases the rate of photocatalytic oxidation, because the electron scavenges by the transition metal ions at surface through the following reaction: Mn+ + ecb- → M(n-1)+. The transition metal ions prevent electron-hole recombination and result in an increase rate of formation of OH‧
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radical. In addition, the valence of the transition metal ions can also affect charge trapping.
If the dopants act as holes trapping and electrons trapping, it can decrease the rate of recombination. For example, V4+ can act as both an electron trap and a hole trap in TiO2
lattice. Thus, the photoactivity of V4+ is significantly higher than that of V5+ since V5+ can only trap electrons.[9]
Figure 2-8 Band model of Ti1-xVxO2 film electrodes at bias potential in an electrolyte solution.[31]
Besides, the atomic ratio of V/Ti can affect the electronic structure. Figure 2-8 shows the band model of Ti1-xVxO2 at bias potential at various atomic ratio of V/Ti. The filled V 3d level acted as a donor level in the band gap. For V/Ti between 0 and 0.05 samples, the recombination time was longer than without V sample. That is because the V 3d level in the band gap inhibited the electron-hole recombination. However, when the amount of the V increased (V/Ti 0.1), the donor level≧ may connect with the conduction band. The crystals may shorten the distance between the V 3d level and the top of the valence band. Summary, changing the atomic ratio of V/Ti affect the electronic structure of materials.[31]
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2-3 Photoreduction of CO2
2-3-1 Reduction behavior
Fossil fuels are the most important source of energy in the world because of their stability and high energy density (33GJ/m3 for gasoline). Unfortunately, Due to mass consumption of fossil fuels, it could release a large amount of CO2 which is a kind of greenhouse gas. Many researchers suggest that the average global temperature will increase by about 6 °C in the end of this century. Hansen et al. used paleoclimate data to find out that an average global temperature change of 6 °C can lead to melting the ice in Antarctica and Rising sea levels.[40] Therefore, fossil fuels depletion and global warming have become the urgent environmental problems in the world.
In order to reduce the amount of CO2, scientists use a lot of methods to collect CO2 or convert CO2 into hydrocarbon fuels. Biomass to fuel conversion shows the most promising way to biofuel production. Chisti et al. [41] shows that the microalgae have a oil content of more than 30%. Unfortunately, microalgae has some drawbacks: the energy conversion efficiency of photosynthesis is only approximately 1% and the microalgae required great land and water areas. Thermochemical has been used in converting CO2 into CO. Galvez et al.
and Bamberger et al.[42, 43] use Zn/ZnO cycle and CeO2, respectively, to reduce CO2 to CO.
However, a number of materials challenges associated with large energy requirement in the reaction need to be solved for its development.
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Using solar energy to convert CO2 and water vapor into hydrocarbon fuels by photocatalysts become an attractive prospect. Be able to reduce carbon dioxide by using the photocatalysts, the conduction band of photocatalyst must be higher than the reduction potential of CO2. The excited electronic from the conduction band can be transferred to CO2 and then reduced CO2. Inoue et al.[44] suggested that conversion of CO2 to methane
which e- and h+ mean photogenerated electrons and holes, respectively. Figure 2-9 shows the bandedge positions of the different semiconductor materials and redox potentials of the different chemical species. If the conduction band edge lies at a higher position than the redox potential, that is believed to be responsible for the high rates of product formation.
The reaction also need a hole-scavengers to inhibit the recombination of hole–electron pairs.
Scientific studies on photoreduction of CO2 by semiconductor are listed in Table 2-2.
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Figure 2-9 The bandedge positions of the different semiconductor materials and redox potentials of the different chemical species.
Table 2-2 Summary of some important reports on photoreduction of CO2.
Catalyst Light source medium products Ref.
p-GaP crystal Hg lamp CO2-saturated buffered solution
Methanol, formic acid, formaldehyde
[45]
Cu-TiO2 UV lamp Aqueous suspension Methane, ethylene [8]
TiO2 on zeolite UV lamp H2O vapor Methanol [46]
Cu-ZnO2 UV lamp NaHCO3 solution CO [47]
P25, TiO2 UV lamp H2 and H2O CO, methane,
methanol
[48]
Ag/Cu-TiO2 UV 365 nm H2O vapor Methanol [6]
CdSe/Pt/TiO2 vis 420 nm H2O vapor methane [7]
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2-3-2 Reaction mechanism in liquid phase
Some reseachers focus on the CO2 reduction by photocatalysts in the liquid phase.
Halmann et al. used the SrTiO3 catalyst powder suspended in a liquid phase through which CO2 was bubbled, to produce formic acid, formaldehyde, and methanol by natural sunlight.[49] Sayama et al. reported that use of 1% Cu-loaded ZrO2 catalyst for photocatalytic reduction of CO2 to produced CO in NaHCO3 solutions under UV irradiation.[47] Tseng et al. used Cu-loaded titania to photoreduce CO2 to produce methanol. The methanol yield was Greatly increased by adding NaOH because NaOH in liquid could act as strong hole-scavengers, form OH radicals and enhance the solubility of CO2.[50] Pressure is also a very important parameter on photocatalytic reduction of CO2 in solutions. Takayuki showed that the optimum value of CO2 pressure on photoreduction can produce the highest methanol.[51] In summary, adjusted some important Parameters in photoreaction of solution phase, like the solubility, the optimum value of CO2, and high active hole-scavengers, can increase the photoreduction of CO2 to produce hydrocarbon fuels.
2-3-3 Reaction mechanism in gas phase
In recent years, more and more attention has been focus on the photocatalytic reduction of gaseous CO2. The general selection of hole-scavengers are H2 gas or water vapor. The photoactivity of CO2 reduction are effective by H2. Lo et al.[48] reduced CO2 with H2 and H2O to produce methane, ethane and CO. But hydrogen is artificial production which need Additionally input the energy. Therefore, a lot of researchers focus on the photoreduction of CO2 with H2O. Wang et al.[7] shows that CdSe quantum dot (QD)-sensitized TiO2
heterostructures are capable of catalyzing the photoreduction of CO2 using visible light illumination (λ> 420 nm) in the presence of H2O. However, carbon dioxide and water vapor
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are competitive adsorptions. In Figure 2-10, Anpo et al.[46] showed the reaction mechanism of the photoreduction of CO2 with H2O on the anchored titanium oxide catalyst.
H2O and CO2 molecules interacted with the excited state of photoinduced (Ti3+-O-)* species, the decomposition of H2O and the reduction of CO2 proceed competitively, depending on the ratio of CO2 to H2O. These interactions resulted in the formation of OH radicals, H atoms and carbon species, and these intermediate radical species react with each other to form CH4
and CH3OH. In summary, the choice of hydrogen and water vapor to be the hole-scavenger can effect the mechanism of photoreduction of CO2. The ratio of CO2 to H2O must be considered because it will affect the yield of products and variety of products.
Figure 2-10 The photocatalytic reduction of CO2 with H2O on the titanium oxide.
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