THE INFLUENCE OF THE PREPARATION TEMPERATURE OF COLLOIDAL RUTHENIUM DIOXIDE ON THE PHOTOSENSITIZED REDUCTION OF H+ IONS

Samples of colloidal RuO2 were prepared by thermal decomposition of RuCl3 at 300, 400 and 500-degrees-C. X-ray diffraction showed that they all consisted of stoichiometric and crystalline RuO2. The specific surface area decreases strongly with increasing preparation temperature. Catalytic properties for the photochemical production of hydrogen were tested in an aqueous system containing a photosensitizer (ruthenium trisbipyridyl), an electron relay (methylviologen) and an electron donor (EDTA). In this solution the RuO2 particles form aggregates which have approximately the same size for all preparation temperatures. The rate of hydrogen gas production was measured as a function of weight concentration of RuO2. For RuO2 samples prepared at 300-degrees-C and at 400-degrees-C identical curves were obtained; for the 500-degrees-C sample the production rate is slightly lower only for low RuO2 concentrations. The activation energies for hydrogen evolution are the same for all RuO2 samples, and do not change upon addition of anthracene carboxylate, which changes the mechanism of the homogeneous reactions. It is concluded that the small variation in hydrogen production rate with preparation temperature of the catalyst is due to changes in mass transfer of methylviologen radicals to the catalytic surface. Differences in catalytic properties of the RuO2 samples are negligible.