Photoinduced reactivity of titanium dioxide

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Photosensitized singlet oxygen and its applications

A surface science perspective on TiO2 photocatalysis

Transparent conductors as solar energy materials: A panoramic review

In this paper, visible light assisted degradation of various pollutants using different methods has been briefly reviewed. These methods have been broadly divided into two main categories. In the first category, the use of TiO2 semiconductor facilitating the photooxidative degradation of organic pollutants has been presented. This semiconductor has been treated in several ways. A major aim of these treatments is to maximize the range of wavelength in the visible light region for wastewater treatment. In the second category, various ways of degrading organic pollutants without the use of TiO2 semiconductor have been briefly outlined. The role of Fe(II)/Fe(III), etc., species in the sensitization process of various substrates helping it in the process to photocatalytic degradation reactions of organic pollutants has been highlighted. Also, the usage of semiconductors other than TiO2 has been critically analyzed.

Visible light induced photocatalytic degradation of organic pollutants

The commercially available TiO2-catalyst (Degussa P25) was modified with nanosized platinum and silver particles by the photoreduction method to obtain different noble metal loading (0.5 and 1 wt.%). The characterization of the synthesized catalysts was carried out by the BET method, XPS, TEM and the adsorption of the model pollutant. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO2, modified with nanosized platinum or silver particles. The photocatalytic activity of TiO2, modified with noble metal, is approximately double that of the semiconducting support. The adsorption properties of the catalysts, as well as the noble metal content on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schottky barriers on the metal–semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.

Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

The commercially available TiO2-photocatalyst (Degussa P25) was modified with nanosized gold particles by the photoreduction method at four different values of the pH factor of the medium. The characterization of the synthesized catalysts was carried out by the BET method, X-ray photoelectron spectroscopy (XPS), TEM and the adsorption of the model pollutant. A remarkable influence of the pH on the particle size of Au was registered upon investigating the catalysts by XPS and TEM methods. The size of the gold nanoparticles on the TiO2 surface decreases with increase in the pH of the medium. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO2, modified with nanosized gold particles. The photocatalytic activity of TiO2, modified with gold, was found to increase with the decrease of the size of the gold nanoparticles on the surface of the photocatalytic material. The maximal value of the photocatalytic activity (twice higher than that of the semiconducting support) is registered in the case of gold photoreduction at pH 7. The adsorption properties of the catalysts, as well as the size of the noble metal nanoparticles on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schotky barriers on the metal-semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.

Influence of the size of gold nanoparticles deposited on TiO2 upon the photocatalytic destruction of oxalic acid

The photooxidation of oxalic acid, catalyzed by nanosized TiO2 or WO3 and composite photocatalysts: Au/TiO2, Au/WO3, WO3/TiO2, Au/WO3/TiO2 was studied under irradiation with UV, visible and combined UV–visible light. The catalysts were characterized by the XRD, XPS, SEM and TEM methods. The photocatalytic mineralization of oxalic acid, catalyzed by WO3/TiO2 or Au/WO3/TiO2, proceeded at a significantly higher rate under UV-A irradiation than that under visible light. This is due to the lower specific surface area of the WO3 and its small amount in the composite catalyst. Doping of the semiconductor materials with gold nanoparticles more than doubles the rates of mineralization of oxalic acid, compared to the un-doped samples, and more significantly in the case of Au/WO3/TiO2. The higher rate constants of oxalic acid decomposition under UV, visible or UV–visible light irradiation with the WO3/TiO2 and Au/WO3/TiO2 catalysts, compared with those measured with the individual oxide photocatalysts, are due to the more efficient separation of the electron–hole charges generated upon irradiation. Especially efficient is the charge separation in the case of the Au/WO3/TiO2 photocatalyst under irradiation with UV or combined UV–visible light, when the rate constants of oxalic acid destruction are approximately 1.7 times higher than that of the process catalyzed by Au/TiO2 and 3 times higher than that catalyzed by pure TiO2.

Enhancement of photocatalytic oxidation of oxalic acid by gold modified WO3/TiO2 photocatalysts under UV and visible light irradiation

Water soluble zinc (II) 2,9,16,23-phthalocyanine tetrasulfonic acid [ZnPc(SO 3 H) 4 ], zinc (II) 2,9,16,23-phthalocyanine tetracarboxylic acid [ZnPc(COOH) 4 ] and sterically hindered bulky zinc(II) 2,9,16,23-tetrakis(8-quinolineoxy-5-sulfonicacid)-phthalocyanine [ZnPc(QnSO 3 H) 4 ] complexes have been studied as photocatalysts for oxidation of 2-mercaptoethanol and sodium thiosulfate. The ZnPc(QnSO 3 H) 4 complex exhibited the highest photocatalytic activity due to its low aggregation degree in aqueous medium. In the presence of tetrabutylammonium chloride, quinoline and ethanol additives the monomerization degree of the ZnPc(SO 3 H) 4 and ZnPc(COOH) 4 complexes is increased, their photocatalytic activity being close to that of ZnPc(QnSO 3 H) 4 . Because of high aggregation degree of ZnPc(SO 3 H) 4 and ZnPc(COOH) 4 in viscous medium, the quantum yields of singlet dioxygen generation and the photocatalytic oxidation rates of the sulfur-containing compounds are decreased. The photocatalytic activity of ZnPc(QnSO 3 H) 4 is slightly changed in viscous medium and in the presence of additives. The low aggregation degree of this complex allowed to establish enhanced and lowered rates of photocatalytic oxidation of 2-mercaptoethanol and sodium thiosulfate, respectively, on increasing medium viscosity. The effect of medium viscosity on ZnPc(QnSO 3 H) 4 photocatalyzed oxidation of the two substrates is discussed.

Effect of metal phthalocyanine complex aggregation on the catalytic and photocatalytic oxidation of sulfur containing compounds

The photooxidation of phenol, catalyzed by superficially modified with phthalocyanine complex (Pc) oxide supports (Al2O3 or TiO2), has been studied upon irradiation with visible light. The rate of substrate photooxidation, catalyzed by Pc/TiO2, is much higher than that over the sample obtained by anchoring the Pc complex on Al2O3. The degree of mineralization of phenol during photocatalysis in the presence of Pc/TiO2 is also higher. The high photocatalytic activity of the sample is explained by the realization of an electron transfer from the conduction band of the excited phthalocyanine semiconductor particles to the conduction band of TiO2 support. The increase of the quantum yield of the redox process comes as a result of the additional formation of superoxide radicals on the TiO2 conduction band.

V. Iliev

Papers

Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

Influence of the size of gold nanoparticles deposited on TiO2 upon the photocatalytic destruction of oxalic acid

Enhancement of photocatalytic oxidation of oxalic acid by gold modified WO3/TiO2 photocatalysts under UV and visible light irradiation

Effect of metal phthalocyanine complex aggregation on the catalytic and photocatalytic oxidation of sulfur containing compounds

Phthalocyanine-modified titania—catalyst for photooxidation of phenols by irradiation with visible light