Spectroscopic investigation of bismuth–molybdenum–oxide catalysts

Abstract

Bismuth–molybdenum–oxide catalysts have been studied by i.r. and u.v. reflectance spectroscopy. Both tetrahedral and octahedral oxomolybdenum(VI) species were at the catalyst surface. The proportion of octahedral species increased with increasing molybdenum concentration and with increasing calcining temperature. The i.r. spectra showed the presence at the surface of an octahedral oxomolybdenum(VI) species with terminal and bridging oxygen. The most active catalysts, including silica-supported catalysts, had characteristic u.v. and i.r. spectra. Molybdenum–terminal oxygen stretching frequencies were in the range found in model compounds with three terminal oxygens per molybdenum. The effect of silica in supported catalysts was to promote formation of the same active species as in the unsupported catalyst but at a higher molybdenum concentration. The relevance of our results to the mechanism of the catalysis of oxidative dehydrogenation of olefines is discussed. We suggest that the bifunctional nature of the bismuth molybdate catalyst is due to the presence of two types of molybdenum–oxygen bond in the particular structure adopted by MoO₃ in association with Bi₂O₃.