Abstract: The photocatalytic degradation of five various dyes has been investigated in TiO 2 /UV aqueous suspensions. It was attempted to determine the feasibility of such a degradation by varying the chemical structures, either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)). In addition to a prompt removal of the colors, TiO 2 /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO 2 . Sulfur heteroatoms were converted into innocuous SO 4 2− ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the −3 oxidation state, such as in amino-groups, remain at this reduction degree and produced NH 4 + cations, subsequently and very slowly converted into NO 3 − ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central NN azo-group was converted in gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any physicochemical method. The aromatic rings were submitted to successive attacks by photogenerated OH radicals leading to hydroxylated metabolites before the ring opening and the final evolution of CO 2 induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. These results suggest that TiO 2 /UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters not only for decolorization, but also for detoxification, in particular in textile industries in semi-arid countries.

Abstract: TiO2 and SrTiO3 codoped with antimony and chromium showed intense absorption bands in the visible light region and possessed 2.2 and 2.4 eV of energy gaps, respectively. TiO2 codoped with antimony and chromium evolved O2 from an aqueous silver nitrate solution under visible light irradiation, while SrTiO3 codoped with antimony and chromium evolved H2 from an aqueous methanol solution. The activity of TiO2 photocatalyst codoped with antimony and chromium was remarkably higher than that of TiO2 doped with only chromium. It was due to that the charge balance was kept by codoping of Sb5+ and Cr3+ ions, resulting in the suppression of formation of Cr6+ ions and oxygen defects in the lattice which should work as effectively nonradiative recombination centers between photogenerated electrons and holes.

Abstract: Advanced oxidation processes (AOPs) are characterized by a common chemical feature: the capability of exploiting the high reactivity of OH radicals in driving oxidation processes which are suitable for achieving the complete abatement and through mineralization of even less reactive pollutants. This paper reviews the use of sunlight to produce OH radicals. The experimental systems necessary for performing pilot-plant scale solar photocatalytic experiments are described. It outlines the basic components of these pilot plants and the fundamental parameters related to solar photocatalysis reactions. This paper summarizes also most of the research carried out at Plataforma Solar de Almeria (PSA) related with solar photocatalytic degradation of water contaminants. A description is given of how solar photocatalysis could become a significant segment of the wastewater treatment technologies related with the degradation of very persistent toxic compounds. It outlines also the decomposition of organic and inorganic contaminants and different examples are also shown for better comprehension of the ability of solar energy for carrying out oxidation and reduction processes. These examples include chlorophenols, chlorinated solvents, pesticides and cyanide. Besides, the possibility of using the photo-Fenton reaction illuminated with solar light opens the boundary where solar photocatalysis could be applied.

Abstract: We have investigated the effect of H2O2 on the behavior of O2•- and OH• produced in photocatalysis of aqueous TiO2 suspensions by means of luminol chemiluminescence probing and terephthalic acid fluorescence probing, respectively. The reduction of O2 by photoinduced conduction band electrons (e-) was increased by the addition of H2O2, since the consumption of photoinduced valence band holes (h+) in the oxidation of H2O2 caused the repression of e-−h+ recombination. After the end of the light irradiation, the amount of O2•- decreased based on the fractal-like kinetics at the heterogeneous surface of the TiO2 particle. The decay process might be caused by trapped h+, which cannot react with water and then remains on the TiO2 particle after the irradiation. The energy level of the trapped h+ was estimated to be above the redox potential of SCN-, since it could react with the adsorbed H2O2 and I- ions but not with other ions such as SCN-, Br-, and Cl-. The formation rate of OH• was increased by the addition o...

Abstract: Thin films of various metal oxides were prepared on glass substrates by a wet process to determine their photocatalytic ability to decompose adsorbed dye and to evaluate their photoinduced hydrophilicity under UV illumination. The metal oxides used in this study are classified into four categories based on their behavior over the two photochemical reaction: (1) active in both photocatalytic oxidation and photoinduced hydrophilicity (TiO2, SnO2, ZnO); (2) only active in photocatalytic oxidation (SrTiO3); (3) only active in photoinduced hydrophilicity (WO3, V2O5); (4) and inactive over both processes (CeO2, CuO, MoO3, Fe2O3, Cr2O3, In2O3). X-ray photoelectron spectroscopy revealed that oxygen defect sites were produced by Ar+ bombardment on the surface of metal oxides, showing photoinduced hydrophilicity. These results indicate that photoinduced hydrophilicity is not induced by the photocatalytic oxidation of organic compounds adsorbed on the surface, but is based on structural changes of the metal oxide s...

Abstract: Carbon dioxide was photocatalytically reduced to produce methanol in an aqueous solution using 254 nm UV irradiation. Titania and Cu-loaded titania were synthesized by an improved sol–gel method using a homogeneous hydrolysis technique. The grain size of TiO 2 and Cu/TiO 2 were uniform and average diameters were approximately 20 nm. Photocatalytic reduction was conducted in a quartz reactor with a UV lamp irradiating at the center. XPS analysis reveals that Cu 2p 3/2 is 933.4 eV indicating primary Cu 2 O species on the TiO 2 supports. EDX and XPS revealed that most copper clusters were on the TiO 2 surface. The optimum amount of copper loading was 2.0 wt.% for the highest dispersion among catalysts. The methanol yield of 2.0 wt.% Cu/TiO 2 was 118 μmol/g following 6 h of UV illumination. The yield was much higher than those of sol–gel TiO 2 and Degussa P25, whose yields were 4.7 and 38.2 μmol/g, respectively. The methanol yield reached a steady-state 250 μmol/g after 20 h of irradiation. Experimental results indicated that the methanol yield was significantly increased by adding NaOH. The caustic solution dissolved more CO 2 than did pure water. In addition, the OH − in aqueous solution also served as a strong hole scavenger. The redistribution of the electric charge and the Schottky barrier of Cu and TiO 2 facilitates electron trapping via supported Cu. The photocatalytic efficiency of Cu/TiO 2 was markedly increased because of the lowering the re-combination probability for hole–electron pairs. The highest quantum and energy efficiencies achieved were 10 and 2.5%, respectively.

Abstract: The textile industry consumes considerable amounts of water during the dyeing and finishing operations. Dyes are extensively used and hence wastewaters discharged in rivers or public sewage treatment plants are highly contaminated. In this work, a detailed investigation of the adsorption and photocatalytic degradation of the safira HEXL dye, an anionic azo dye of reactive class, is presented. H 2 O 2 and UV light have a negligible effect when they are used on their own. The adsorption of dye on the semiconductor shows a strong dependence on the pH and follows a Langmuir adsorption model. The photodegradation kinetics is discussed in terms of the Langmuir–Hinshelwood model. The effect of pH, amount of photocatalyst, UV light intensity and hydrogen peroxide concentration is discussed. The relative photonic efficiency of the system is reported using phenol as a standard organic compound.

Abstract: Transparent anatase mesoporous TiO2 (MTiO2) and TiO2 nanometer thin films were prepared on soda-lime glass and fused quartz via the reverse micellar method and sol–gel method, respectively. The as-prepared MTiO2 and TiO2 films were then treated by dipping them in a H2SO4 solution. The MTiO2 and TiO2 films before and after surface acid treatment were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), BET surface area and UV–VIS spectrophotometry. The photocatalytic activity of the samples was evaluated by photocatalytic oxidation of acetone in air. It was found that MTiO2 thin films showed higher photocatalytic activity than that of the TiO2 thin films. This was attributed to the fact that MTiO2 thin films were composed of smaller monodisperse spherical particles about 15 nm and had higher specific surface areas. Furthermore, the monodispersity of TiO2 particles was beneficial to transfer and separation of photo-generated electrons and holes in the inner of and on the surface of TiO2 particle and reduced the recombination of photo-generated electrons and holes. The films deposited on quartz showed the highest photocatalytic activity because films deposited on quartz exhibited a better crystallization and had no sodium contaminant. The photocatalytic activity of MTiO2 and TiO2 thin films deposited on different substrates after treated with H2SO4 solution was significantly enhanced. Acid treatment was particularly effective for MTiO2/glass and TiO2/glass, which showed activity enhancement of four and over two times, respectively. This increase in activity has been correlated with the reduction of sodium ions and the increase in the adsorbed hydroxyl content on the surface of TiO2 films.

Abstract: Introduction.- Photoelectrochemical Processes of Seomiconductors.- Design, Preparation and Characterization of Highly Active Metal Oxide Photocatalysts.- Photoelectrochemistry at Semiconductor/Liquid Interfaces.- Photoelectrochemical Reactions at the Semiconductor Microparticle.- New Approaches in Solution-Phase Processing of Semiconductor Thin Films.- Self-Cleaning Properties of TiO2-coated Substrates.- Cleaning the Atomospheric Environment.- Water Purification - Degradation of Aqueous Pollutant and Applications to Water Treatment.- Second-Generation TiO2 Photocatalysts Able to Initiate Reactions Under Visible Light Irradiations.- Photocatalytic Organic Syntheses Using Semiconductor Particles.- Further Development to Stereoselective Organic Synthesis of Nitrogen-Containing Compounds.- ... 20 Photoelectric TiO2 Solar Cells.

Abstract: Titanium dioxide (TiO2) nanoparticles of both anatase and rutile phases were synthesized by hydrothermal treatment of microemulsions, and their photocatalytic activity for wet oxidation of phenol w ...

Abstract: A transition metal nitride, Ta3N5, is a novel photocatalyst responding to visible light. Ta3N5 evolves H2 and O2 by visible light (λ<600 nm) irradiation in the presence of sacrificial electron donor and acceptor, respectively without any noticeable photo-anodic or cathodic corrosion, representing a candidate for overall water splitting by visible light.

Abstract: The simultaneous photocatalytic degradation of organic compounds and reduction of silver ions in titanium dioxide suspensions at pH 3.0-3.5 has been studied. The organic compounds of interest were sucrose and salicylic acid. The presence of silver ions in TiO 2 suspensions was found to enhance the photooxidation of high loadings (2001) μgC) of sucrose. However, for low sucrose loadings (100 μg C), pure TiO 2 particles performed as well as modified TiO 2 particles. An optimum silver ion loading of 2.0 at.% Ag + was observed for the mineralisation of 2000 μg C sucrose. At this silver ion loading, the mineralisation of 2000 μg C sucrose was enhanced by a factor of approximately 4.0 (based on 90% overall oxidation rates). In contrast, the addition of silver ions to TiO 2 suspensions did not have any significant effect on the photocatalytic mineralisation of salicylic acid to carbon dioxide, for both low and high loadings of salicylic acid in the suspension. It was also observed that pure TiO 2 particles performed as well as silver-modified TiO 2 particles for the degradation of 2000 μg C salicylic acid. The higher activity of silver ion-modified titanium dioxide suspensions for sucrose mineralisation is predominantly due to the presence of small silver particles on the titania surface, rather than due to the trapping of electrons during the reduction of silver ions. Approximately 50% of the initial mass of silver ions added to TiO 2 suspensions were reduced to metallic silver deposits in the presence of sucrose and salicylic acid mineralisation at the 2.0 at.% Ag + loading. Nanosize silver deposits on TiO 2 particles act as sites of electron accumulation where the reduction of adsorbed species such as oxygen occur. The enhanced reduction of oxygen through better electron-hole separation in Ag/TiO 2 particles compared to pure TiO 2 particles increases the rate of sucrose mineralisation. Therefore, it is proposed that the rate-limiting step in the sucrose photooxidation reaction is the transfer of electrons to dissolved oxygen molecules, whereas in the case of salicylic acid degradation and mineralisation, the rate-limiting step is the attack of salicylic acid molecules and its degradation intermediate products by holes and hydroxyl radicals. Hence silver deposits on TiO 2 particles are not beneficial for the photocatalytic degradation and mineralisation of salicylic acid but are advantageous for the mineralisation of sucrose.

Abstract: The nano-sized coupled oxides ZnO/SnO 2 in a molar ratio of 2:1 (Z 2 S) and 1:1 (ZS) were prepared using the co-precipitation method and characterized with X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy and specific surface area (Brunauer–Emmett–Teller (BET)). Their photocatalytic activities were also evaluated using methyl orange (MO) as a model organic compound. The isothermal adsorption behavior of MO on Z 2 S and the factors affecting the photocatalytic activity, such as the heat-treating temperature for the photocatalyst, the pH value of the reaction suspension and the addition of NaCl, KNO 3 and K 2 SO 4 into the suspension, have been studied. It is found that the change in phase of Z 2 S occurs with the calcination temperature. The band gap energy value observed by UV–VIS diffuse reflectance spectroscopy is 3.15 eV for Z 2 S compared with those of 3.12 eV for ZS, 3.17 eV for ZnO and 2.53 eV for SnO 2 . The isothermal adsorption behavior is a two-stage process. The photocatalytic degradation rate of MO on Z 2 S is faster than that on ZS and ZnO by 40.2 and 66.1%, respectively. A schematic diagram of photocatalytic activity is also presented to explain the results.

Abstract: Gas–solid heterogeneous photocatalytic decomposition of benzene, toluene, cyclohexane and cyclohexene over TiO 2 was studied at room temperature, and their reactivities were compared. Catalyst deactivation was ascribed to the formation of the carbon deposits on TiO 2 surface, and the formation and decomposition behavior of the carbon deposits affected the decomposition rate. Deactivated TiO 2 catalysts were photochemically regenerated in the presence of water vapor, and the carbon deposits were decomposed to CO x .

Abstract: A “sense and shoot” approach has been developed for the photocatalytic degradation of organic contaminants from aqueous solutions. The presence of aromatic compounds such as chlorinated phenols is readily detected from the quenching of visible emission of ZnO semiconductor film. The emission quenching is quantitative and can be analyzed on the basis of adsorption equilibrium between ZnO and organic molecules. High apparent association constant values (Ka = (1−5) × 104 M-1) make possible the detection of these organic molecules at very low concentration levels. For example, 1 ppm of 4-chlorocatechol in water results in a 15% decrease of ZnO emission. Interestingly, the same ZnO film is capable of degrading aromatic compounds present in water under UV irradiation. The concept of adopting a strategy of simultaneous sensing and degradation in photocatalysis is illustrated in this paper.

Abstract: The photocatalytic oxidation of an azo-dye acid black 1 (AB1) in water has been investigated over modified titanium dioxide (Tytanpol A11, “Police” Chemical Factory, Poland). The effect of operational parameters, i.e. pH of the solution, photocatalyst content, initial dye concentration on the photocatalytic process has been examined. It can be stated that the complete removal of color, after selection of optimal operational parameters, using modified Tytanpol A11 can be achieved in relatively short time, about 1 h. The catalytic material has been characterized by several analytical methods, like XRD, FTIR, UV–VIS/DRS.

Abstract: Titanium dioxide (TiO 2 ) was used as a photocatalyst for the detoxification of water containing methyl orange (MO), which was used as a model compound. Solar radiation was used as an irradiation source. It was found that there was no degradation for the MO in the dark and in the presence of TiO 2 . Also no degradation was observed for MO when the solution placed under solar radiation but without TiO 2 . Several experiments were used to optimize the experimental parameters. In the first set of experiments variable amounts of TiO 2 were used with a fixed concentration of MO. It was found that 0.4% of TiO 2 gave the highest degradation rate constant, 0.619 h -1 . In the second set of experiments TiO 2 concentration was fixed at 0.4% and the MO concentration was varied, the highest rate constants was obtained when the concentration of MO was 4 x 10 -5 M and it was found to be 0.639h - The degradation became negligible in the presence of high concentrations of MO. The highest degradation rate was obtained at pH = 3 with a rate constant K = 2.6683h -1 , followed by that at pH = 9 where the calculated K = 0.7585 h -1 .