Preparation of high surface area reduced molybdenum oxide catalysts

Abstract

High surface area molybdenum oxides have been prepared by the thermal decomposition and reduction of molybdenum (VI) oxalate. It has been found that the initial trioxide formed is highly oxygen deficient and that the composition depends on the method of preparation, varying from MoO_2.98, for an oxide prepared by decomposing the oxalate in nitrogen, to MoO_2.82 for an oxide prepared in vacuum. The rate of reduction was found to be dependent on: the method used to decompose the oxalate; the temperature of the decomposition; the partial pressure of hydrogen; and the partial pressure of water vapour. Vacuum-prepared trioxides are reduced in a single stage to MoO_2–x. Nitrogen-prepared trioxides reduce first to Mo₄O₁₁ or Mo₂O₅ depending respectively on whether “wet” or “dry” conditions are used. In most cases the reduction-time curves are essentially sigmoidal in character. Possible reduction mechanisms and rate-determining steps are discussed.