MOLYBDENA ON SILICA CATALYSTS - ROLE OF PREPARATION METHODS ON THE STRUCTURE SELECTIVITY PROPERTIES FOR THE OXIDATION OF METHANOL

A series of MoO3/SiO2 samples prepared by several research groups, emp loying different silicas and preparation methods, have been studied by in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and met hanol oxidation as a probe reaction. The in situ Raman spectroscopy st udies show that under dehydrated conditions the molecular structure of the silica-supported molybdenum oxide surface species is independent of the preparation methods used in the present study. The surface moly bdenum oxide species is assigned to an isolated, highly distorted octa hedral mono-oxo Mo structure. The appearance of new structures in some samples is due to the presence of calcium impurities in the silica su pports which result in the formation of calcium molybdate. Neither the preparation method nor the specific silica used affected the methanol oxidation activity of the MoO3/SiO2 catalysts. The surface molybdenum oxide coverage on silica is the only relevant factor that determines the catalytic properties during methanol oxidation. In siu Raman spect roscopy during methanol oxidation shows aggregation of surface molybde num oxide species to crystalline beta-MoO3. The extent of aggregation increases with the surface molybdenum coverage, even at very low cover ages, and accounts for the decrease in the methanol oxidation catalyti c activity with increasing surface molybdenum coverage. (C) 1994 Acade mic Press, Inc.