The nature of supported-molybdena catalysts. Evidence from a Raman and X-ray diffraction investigation of pyridine adsorption

Abstract

Raman spectroscopy and X-ray diffraction have been used to characterise molybdena catalysts supported on γ-alumina and silica. Raman spectroscopy of pyridine molecules adsorbed on these samples was used to characterise the surface acid sites. Loading levels of molybdena on both support materials were varied from 0.5 to 16 mol %. On silica, a separate MoO3 phase was formed at a loading level of 8%, whereas on alumina, no separate molybdena phase was formed up to a loading level of 16%. It is suggested that molybdenum ions were present in the latter sample as MoO6 6- groups, via surface substitution of Al by Mo atoms. The molybdena phase on silica acted as a generator of Lewis acidity. Evidence was found that even at 16% loading, some silica surface was still free, and that consequently the molybdena phase tended to grow at localised sections of the surface. Coordination of pyridine to molybdenum ions involved a change in coordination. The 16 mol% MoO3/γ-alumina sample had no acid sites strong enough to bind pyridine.