Surface- and bulk-type catalysis of heteropolymolybdates. importance of the concept in the structure-activity relationships for catalyst design

Oxidations of acetaldehyde, H₂ and CO, and oxidative dehydrogenation of cyclohexene over several 12-heteropolymolybdates and molybdovanadates (supported and unsupported) have been studied, in relation to the structure-activity relationships. First, the concept of the surface- and bulk-type catalysis are described with experimental evidence. Then, the experimental results concerning the effects of redox properties and acidity of the catalysts on the catalytic activity for the two types of reactions are given. The catalytic activity for oxidation of acetaldehyde (a surface-type catalysis) is closely related to the surface redox property measured by the rate of reduction of catalyst by CO, while the catalytic activity for bulk-type catalysis (e.g., oxidative dehydrogenation of cyclohexene) is correlated with the bulk redox property measured by the reduction by H₂. It was found that the rates of acetaldehyde oxidation over the acid form dispersed on Cs salt showed a very similar pattern to those for acidic Cs salts, as regards the H/Cs ratio.