SURFACE-TYPE AND BULK-TYPE(II) CATALYSIS IN CATALYTIC OXIDATIONS OVER 12-MOLYBDOPHOSPHORIC ACID AND ITS ALKALI SALTS

Several oxidation reactions have been studied over 12-molybdophosphoric acid and its salts in bulk form having different specific surface areas, and in supported form on silica at different loading levels. It has been confirmed experimentally that there are two extreme cases regarding the contribution of processes in catalyst bulk to the overall rate: surface-type and bulk-type (II) catalysis. The rates of bulk-type reactions, such as oxidative dehydrogenation of cyclohexene and oxidation of H2, changed little with the specific surface area of the unsupported catalysts, while the rates for the supported catalysts increased in proportion to the loading amount up to high loading levels. The results are explained by assuming that diffusion of redox carriers (protons and electrons) is much faster than the rate of overall oxidation. On the other hand, in the case of surface-type reactions such as oxidations of acetaldehyde and CO, the rates were proportional to the specific surface area, and therefore the rates for supported catalysts leveled off at high loadings due to an increase in particle size. The increase was confirmed by narrowing of X-ray diffraction lines. It was presumed that in this case the redox carriers are oxide ions which diffuse relatively slowly. © 1990.