EFFECT OF SILVER DEPOSITS ON THE PHOTOCATALYTIC ACTIVITY OF TITANIUM-DIOXIDE SAMPLES FOR THE DEHYDROGENATION OR OXIDATION OF 2-PROPANOL

Until now, the effect of the deposition of metal particles onto a powdered semiconductor oxide on its photocatalytic activity has been determined for group VIII transition metals only. In this study, Ag-TiO2 samples were prepared by photodeposition, characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) and tested, under band-gap illumination, using the oxidation of 2-propanol (neat liquid phase or mixed with water) in the absence or presence of molecular oxygen. The silver deposits were found to be metallic both before and after use, but the size of the particles and their distribution on the support were markedly influenced by the type of TiO2 sample. A small amount of silver caused the catalytic oxygen-free oxidation of 2-propanol, although the rate was much lower than that with platinum which possesses dehydrogenating properties. This effect is though to be due to the electron-attracting role of the metal particles. For oxidation of 2-propanol by molecular oxygen, an optimum silver content was observed (corresponding to a rate improvement of approximately 50%). This was interpreted to be due to the beneficial effects of silver on the trapping of electrons and/or the reduction of oxygen. At higher than optimum silver contents there was a detrimental increase in the probability of recombination of charges at the numerous silver particles. The optimum was shifted to lower values in the presence of water, which is tentatively explained by competitive hole trapping by 2-propanol, water and silver.