ROLE OF PLATINUM DEPOSITS ON TITANIUM(IV) OXIDE PARTICLES - STRUCTURAL AND KINETIC ANALYSES OF PHOTOCATALYTIC REACTION IN AQUEOUS ALCOHOL AND AMINO-ACID SOLUTIONS

Photocatalytic reaction at 298 K try platinum-loaded titanium(IV) oxid e (TiO2-Pt) particles suspended in deaerated aqueous solutions of 2-pr opanol or (S)-lysine (Lys) was investigated. The TiO2 catalysts with v arious amounts of Pt loadings were prepared by impregnation from aqueo us chloroplatinic acid solution onto a commercial TiO2 (Degussa P-25) followed by hydrogen reduction at 753 K. The physical properties of de posited Pt, e.g., particle size, surface area, and electronic state, w ere studied respectively by transmission electron microscopy, volumetr ic gas adsorption measurement, and X-ray photoelectron spectroscopy as well as infrared spectroscopy of adsorbed carbon monoxide. The increa se in Pt amount mainly resulted in an increase of the number of Pt dep osits, not of their size. The catalysts were suspended in the aqueous solutions and photoirradiated at a wavelength >300 nm under an argon ( Ar) atmosphere. The overall rate of photocatalytic reactions for both 2-propanol and Lys, corresponding to the rate of consumption of these substrates, was negligible without Pt loading, increased drastically w ith the loading up to ca. 0.3%, and was almost constant or a little de creased by the further loadings. However, the rate of formation of pip ecolinic acid (PCA) from Lys was improved gradually with a increase of Pt loading up to ca. 2 wt %. These dependences were discussed as a fu nction of Pt surface area, which is employed as a measure that include s the properties of both number and size of Pt deposits. For the photo catalytic dehydrogenation of 2-propanol, the rate dependence could be interpreted semiquantitatively with the model that only the TiO2 parti cles loaded with at least one Pt deposit can photocatalyze, but the re action rate is independent of the number of Pt deposits. Therefore, th e overall rate is proportional to the number of Pt-loaded TiO2 particl es. On the other hand, for the interpretation of the rate of PCA and H z productions, the number of Pt deposits on each TiO2 particle had to be taken into account. The efficient production of PCA at higher Pt lo adings was attributed to the reduction of a Schiff base intermediate p roduced via oxidation of Lys with positive holes and subsequent intram olecular condensation at the Pt deposit that is close to the site for the oxidation. Otherwise, photoexcited electrons are consumed for HZ p roduction and the intermediate remains unreduced or undergoes further oxidation. It was suggested that the intermediate produced at the TiO2 surface sites within a distance of several nanometers from the Pt dep osit undergoes efficient reduction to PCA. Thus, the importance of the distribution of Pt deposits for the preparation of highly active and selective TiO2-Pt photocatalyst has been clearly demonstrated.