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Abstract: The photocatalytic degradation of butanedioic acid (BDA), hydroxybutanedioic acid (OH-BDA) and 2,3-dihydroxybutanedioic acid (diOH-BDA) (succinic acid, malic acid and tartaric acid, respectively) in a TiO2 aqueous suspension at pH 3 was investigated to determine the effects of hydroxyl substitution. Temporal variations in the concentrations of the main intermediate products are reported. The use of a set of reactions involving decarboxylation and formation of alkylperoxy and alkoxy radicals has allowed us to account for the occurrence of these products, and to suggest the preferential decarboxylation of the COOH/COO− group adjacent to the OH group in OH-BDA. The adsorbed amounts in the dark increased with the number of OH groups, while the irradiation time necessary for total disappearance of the initial diacid, decreased. However, the evolution rate of CO2, which was initially the same for the three acids, was then lower for the hydroxylated diacids for a significant period of irradiation time. This difference is tentatively attributed predominantly to the effect of hydroxyl substitution on the adsorption mode. One of the carboxyl groups would remain away from the surface (and hence not be available for direct electron transfer) due to the restricted mobility of the adsorbed diacid caused by hydrogen-bonding of TiO2 with the alcohol group adjacent to the other carboxyl group. This hypothesis is also effective to qualitatively interpret how the zeta potential varied in the course of the degradation of each diacid. The present study further illustrates the essential role of distinct adsorption modes in photocatalytic events. [Copyright &y& Elsevier]

Abstract: The photocatalytic activity of TiO2 is the result of an interplay between a considerable number of parameters, e.g., phase composition, electronic structure, particle size, exposed surface area, degree of aggregation, mobility of charge carriers, presence of impurities, amount and kind of defects, adsorption of molecules from gas or aqueous phase, lateral interactions between adsorbed species, nature of solvent, etc. Furthermore, these parameters can be broadly subdivided into those that are intrinsic to the photocatalytic material, and those that are extrinsic being influenced by the surrounding environment and conditions. The specific function and influence of a given feature for the photocatalytic performance of a TiO2 sample is difficult to characterize since many of the before-mentioned parameters are strongly coupled. For example, while the degree of aggregation could be inherent to a given material, it is also simultaneously influenced by pH. The degree of aggregation can then influence adsorption of molecules, light scattering and photon adsorption, charge carrier dynamics etc. The plurality of variables driving the nature of the photocatalytic activity, presents a challenge when trying to understand the kinetics and mechanisms underlying photocatalytic processes. It is of primary importance to develop a method to understand and control these properties (or at least some of them). In this paper, we also discuss the relevance of quantum-integrated systems in which the local environment where the molecule is adsorbed is different from the “lonely” photocatalyst or the molecule in solution, and could be treated as a whole. [Copyright &y& Elsevier]

Abstract: This study compares the photocatalytic (PC) and photoelectrocatalytic (PEC) oxidation of glucose by Pt–TiO2 films under similar conditions. The overall oxidation efficiency of the PEC process was better than the PC process, for both TiO2 and Pt–TiO2 films. The Pt deposits enhanced the PEC oxidation efficiency of glucose at low cathodic (−0.1V) and low anodic potential bias (+0.1V). As the applied anodic bias was increased to >+0.5V, the undoped-TiO2 films outperformed their Pt–TiO2 counterparts. This suggests that in the studied systems, at an anodic bias of >+0.5V, more holes were available for the oxidation of glucose on the surface of TiO2 compared to Pt–TiO2 films. In the PEC systems, above a certain applied bias, Pt deposits may have a detrimental effect on the performance of a PEC system because the Pt deposits could merely block the surface of TiO2 (particularly at the highest Pt loadings). Lower measured photocurrents from Pt–TiO2 films compared to undoped-TiO2 and a decrease in the photocurrent as the Pt loading increased corroborate this postulation. [Copyright &y& Elsevier]

The stability of Ag-TiO2 photocatalysts was examined for the photocatalytic degradation of dichloroacetic acid (DCA) as a function of the recycling times. The photocatalytic activity was investigated by measuring the rate of H+ ions released during the photodegradation of DCA and confirmed by measuring the total organic carbon removal. The photodegradation reactions were studied at pH 3 and pH 10 for a series of Ag-TiO2 photocatalysts as different with Ag loadings . All the Ag-TiO2 and bare TiO2 photocatalysts showed a decrease in photocatalytic activity on recycling for the DCA photodegradation reaction. The decrease in activity can be attributed to poisoning of active sites by Cl- anions formed during the photocatalytic DCA degradation. The photocatalytic activity was, however, easily recovered by a simple washing technique. The reversibility of the poisoning is taken as evidence to support the idea that the recycling of Ag-P25 TiO2 photocatalysts does not have a permanent negative effect on their photocatalytic performance for the degradation of DCA. The choice of the preparation procedure for the Ag-TiO2 photocatalysts is shown to be of significant importance for the observed changes in the photocatalytic activity of the Ag-TiO2 particles. [ABSTRACT FROM AUTHOR]