Catalysis at the metal-support interface: exemplified by the photocatalytic reforming of methanol on Pd/TiO2

Some types of catalytic reaction may take place at the boundary between a metal and a support. In this paper we describe the mathematical relationships between observed reaction rates and the loading of the supported phase. The reaction rate shows a sharp maximum with increasing loading, and at high loadings the rate is reduced to zero, due to the lack of active sites (the support is completely covered). We report a study of a particular reaction where the kinetic data indicate rather clearly that the active site is at the boundary between such phases. The particular reaction is the photocatalytic degradation of methanol on a Pd/TiO2 catalyst under anaerobic conditions. The reaction produces CO2 and hydrogen and only proceeds at steady state when light is introduced to the system. We propose that this is due to blockage of the Pd by adsorbed CO, and the role of light is to produce a reactive form of oxygen on the support, which cleans off CO from the Pd at the boundary, thus allowing a catalytic cycle to be completed. The larger the extent of this boundary, the higher the reaction rate. A general mathematical model of such reactions is given and applied to these data.