Photocatalytic oxidation of deposited sulfur and gaseous sulfur dioxide by TiO2 films

Thick (4 mu m) films of anatase titania are used to photocatalyze the removal of deposited films of amorphous sulfur, similar to 2.8 mu m, thick and under moderate illumination conditions (I = 5.6 mW cm(-2)) on the open bench the process is complete within similar to 8 or 18 h using UVC or UVA light, respectively. Using UVA light, 96% of the product of the photocatalytic removal of the film of sulfur is sulfur dioxide, SO2. The photonic efficiency of this process is similar to 0.16%, which is much higher (> 15 times) than that of the removal of soot by the same films, under similar experimental conditions. In contrast to the open bench work, in a closed system the photocatalytic activity of a titania film toward the removal of sulfur decreased with repeated use, due to the accumulation of sulfuric acid on its surface generated by the subsequent photocatalytic oxidation of the initial product, SO2. The H2SO4-inactivated films are regenerated by soaking in water. The problems of using titania films to remove SO2 from a gaseous environment are discussed briefly.

Characterization of novel Ag on TiO2 films for surface-enhanced Raman scattering

Novel Ag on TiO₂ films are generated by semiconductor photocatalysis and characterized by ultraviolet-visible (UV/Vis) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), as well as assessed for surface-enhanced Raman scattering (SERS) activity. The nature and thickness of the photodeposited Ag, and thus the degree of SERS activity, is controlled by the time of exposure of the TiO₂ film to UV light. All such films exhibit the optical characteristics (λ_max ≅ 390 nm) of small (<20 nm) Ag particles, although this feature becomes less prominent as the film becomes thicker. The films comprise quite large (>40 nm) Ag islands that grow and merge with increasing levels of Ag photodeposition. Tested with a benzotriazole dye probe, the films are SERS active, exhibiting activity similar to that of 6-nm-thick vapordeposited films. The Ag/TiO₂ films exhibit a lower residual standard deviation (∼25%) compared with Ag vapor-deposited films (∼45%), which is, however, still unacceptable for quantitative work. The sample-to-sample variance could be reduced significantly (<7%) by spinning the film during the SERS measurement. The Ag/TiO₂ films are mechanically robust and resistant to removal and damage by scratching, unlike the Ag vapor-deposited films. The Ag/TiO₂ films also exhibit no obvious loss of SERS activity when stored in the dark under otherwise ambient conditions. The possible extension of this simple, effective method of producing Ag films for SERS, to metals other than Ag and to semiconductors other than TiO₂, is briefly discussed. 

The photo-oxidation of water by sodium persulfate, and other electron acceptors, sensitised by TiO2

A number of different electron acceptors are tested for efficacy in the oxidation of water to oxygen, photocatalysed by titanium dioxide. The highly UV-absorbing metal ion electron acceptors, Ce4+ and Fe3+, appear ineffective at high concentration (10(-2) M), due to UV-screening, but more effective at lower concentrations (10(-3) M). The metal-depositing electron acceptor, Ag+, is initially effective, but loses activity upon prolonged irradiation due to metal deposition which promotes electron-hole recombination as well as UV-screening the titania particles. Most striking of the electron acceptors tested is persulfate, particularly in alkaline solution (0.1 M NaOH). The kinetics of the photo-oxidation of water by persulfate, photocatalysed by titania are studied as a function of pH, [S2O82-] and incident light intensity (I). The initial rate of water oxidation increases with pH, is directly proportional to the concentration of persulfate present and depends upon I-0.6. The TiO2/alkaline persulfate photosystem is robust and shows very little evidence of photochemical wear upon repeated irradiation. The results of this work are discussed with regard to previous work in this area and current mechanistic thinking. The formal quantum efficiency of the TiO2/alkaline persulfate photosystem was estimated as ca. 2%. (C) 2004 Elsevier B.V. All rights reserved.

Photo-oxidation of water sensitized by TiO2 and WO3 in presence of different electron acceptors

The photo-oxidation of water is studied in presence of UV-light (lambda

Novel TiO2 CVD films for semiconductor photocatalysis

A novel CVD film of titanium(IV) oxide has been prepared on glass, via the reaction of titanium(IV) chloride and ethyl acetate, using a CVD technique. The film is clear, very robust mechanically and comprised of a thin (24 nm) layer of nanocrystalline anatase titania that absorbs light of lambda

Photobleaching of methylene blue sensitised by TiO2: an ambiguous system?

The photobleaching of methylene blue MB, sensitised by TiO2, in an aqueous solution is studied in the absence and presence of oxygen, in the absence of oxygen and in the presence of a sacrificial electron acceptor (SED), MB is photoreduced to its colourless leuco form, LMB, by the TiO2 photocatalyst. This same photoreduction process is observed even if an SED is not present, indicating that MB itself can act as an SED. The oxidation of LMB by oxygen to regenerate MB is significantly slower if the aqueous solution is acidified (0.01 mol dm(-3) HClO4) and, at low partial pressures, the rate of reaction depends directly upon the concentration of dissolved oxygen. The TiO2-sensitised photobleaching of MB is irreversible in an oxygen-saturated aqueous solution, as expected, since the bleaching was due to an oxidative process. However, in an acidified solution (0.01 mol dm(-3) HClO4), the photobleaching process, in an oxygen-saturated solution, generates LMB initially. The latter situation arises because, under acidic conditions, LMB reacts only very slowly with oxygen to form MB. The significance of these findings with respect to the popular use of photobleaching of MB as a demonstration of semiconductor photomineralisation is discussed. (C) 1999 Elsevier Science S.A. All rights reserved.

Photomineralisation of 4-chlorophenol sensitised by TiO2 thin films

The results of a study of the oxidative mineralisation of 4-CP by oxygen, sensitised by thin films of Degussa P25 TiO2, are reported. The films are used under conditions in which the kinetics of photomineralisation are independent of mass transfer effects and stable towards repeated irradiation. Using a TiO2 film, the process goes through the same mechanism as a TiO2 dispersion, generating the same intermediates, namely: 4-chlorocatechol and hydroquinone. The kinetics of photomineralisation show clear differences between a TiO2 film and a dispersion. With TiO2 films the initial rate of photomineralisation is strongly dependent upon photocatalyst loading, (units; g dm(-3)) reaching a distinct maximum, which appears to be associated with the formation of a monolayer of aggregated particles - the diameter of the aggregated particles is estimated as 0.44 mu m. A simple 2D model is presented to help illustrate the features of such a system. With TiO2 dispersions the rate usually reaches a plateau at ca. 0.5 g dm(-3) of TiO2. For TiO2 films the initial rate depends directly upon the incident light intensity, implying that the photocatalytically active particles are under low illumination conditions, partially shielded by the other particles making up each aggregated particle. In contrast, with TiO2 dispersions R-i depends upon I-0.64, implying that the different light intensities used spanned both the high (R(i)proportional to I-1/2) and low (R(i)proportional to I) intensity kinetic regions. The kinetics of photomineralisation of 4-CP, sensitised by TiO2 films obey the same Langmuir-Hinshelwood expressions as found in most semiconductor photocatalyst work conducted with TiO2 dispersions. However, in a study of the variation R-i as a function of [4-CP] and [O-2] the values for the maximum rates were larger, and those for the apparent Langmuir adsorption coefficients were smaller, than those found for TiO2 dispersions. (C) 1998 Elsevier Science S.A. All rights reserved.

ACID ENHANCEMENT EFFECT IN THE CLEAN OXIDATION OF TOLUENES PHOTOCATALYZED BY TIO2

The yield of substituted benzaldehydes and benzoic acids formed by the aerial oxidation of a range of substituted toluenes photocatalysed by titanium dioxide in acetonitrile is dramatically improved by the addition of small amounts of sulfuric acid.

ACTIVATION-ENERGIES IN SEMICONDUCTOR PHOTOCATALYSIS FOR WATER-PURIFICATION - THE 4-CHLOROPHENOL TIO2 O-2 PHOTOSYSTEM

The variation in the activation energy for the initial stage of photomineralization of 4-chlorophenol (4-CP), sensitized by Degussa P25 TiO2 was investigated as a function of P-O2 and [4-CP]. A model was developed based on the incorporation of Arrhenius-type functions in a general rate equation for the initial stage of photomineralization. Values of the essential constants in the model were derived from a few simple experiments. Positive, negative and zero apparent activation energies were predicted using the model, and verified experimentally, under moderate reaction conditions. The general applicability of the model is briefly discussed.

EFFECT OF PH ON THE STABILITY OF TIO2 COATINGS ON GLASS PHOTOCATALYSIS REACTORS FOR WATER-PURIFICATION

TiO2 coated glass shows excellent stability in the range pH 2-9, however, there is rapid and complete stripping of the TiO2 coating between pH 11 and 12.

PHOTOCATALYTIC DEGRADATION OF 4-CHLOROPHENOL MEDIATED BY TIO2 - A COMPARATIVE-STUDY OF THE ACTIVITY OF LABORATORY MADE AND COMMERCIAL TIO2 SAMPLES

The photocatalytic efficiencies of laboratory made and commercial TiO2 samples were compared using a standard test reaction: the photomineralization of 4-chlorophenol (4-CP) to CO2, H2O and HCl mediated by Degussa P25 TiO2 in a batch reactor. The results show that the rate of photodegradation of 4-CP, sensitized by a sample of TiO2, shows no clear simple dependence on physical characteristics such as the degree of crystallinity, the surface area and the percentage of H2O.

PHOTOMINERALISATION OF 4-CHLOROPHENOL SENSITIZED BY TITANIUM-DIOXIDE - A STUDY OF THE EFFECT OF ANNEALING THE PHOTOCATALYST AT DIFFERENT TEMPERATURES

The results of a study of the variation in photocatalytic activity of TiO2, as measured by its ability to photomineralise 4-chlorophenol, as a function of temperature used to anneal the TiO2, are reported. Heat treatment of the TiO2 leads to a marked decrease in its photocatalytic activity at annealing temperatures above 600-degrees-C. This decrease is associated with a concomitant drop in the specific surface area of the TiO2, owing to particle sintering, rather than the anatase to rutile transformation, which occurs largely at temperatures above 700-degrees-C. There is a reasonable correlation between photocatalytic activity and the surface area of the aggregate particles in the dispersions of the different heat-treated TiO2 samples.