INSITU RAMAN-SPECTROSCOPY OF ALUMINA-SUPPORTED METAL-OXIDE CATALYSTS

The molecular structures of the surface overlayers of rhenium(VII) oxide, molybdenum(VI) oxide, tungsten(VI) oxide, chromium(VI) oxide, vanadium(V) oxide, niobium(V) oxide, and titanium(IV) oxide on gamma-alumina were determined by in situ Raman spectroscopy under dehydrated conditions. It was found that the dehydrated surface metal oxide structures of all the systems under study, except for supported titanium oxide, are different from those under ambient condition where moisture is present on the surface. Supported molybdenum oxide, tungsten oxide, vanadium oxide, and niobium oxide show the presence of a highly distorted mono-oxo species (one M=O bond) at all loadings. At higher surface coverage these supported systems also reveal the presence of M-O-M linkages which are indicative of polymeric structures. Above approximately 20% metal oxide loading, monolayer coverage is exceeded and crystalline MoO3, WO3, V2O5, and Nb2O5 particles are found on the alumina surface together with the dehydrated surface species. Crystalline Re2O7 is not formed at high loadings and two slightly different surface rhenium oxide species are observed as a function of surface coverage. Both possess three terminal Re=O bonds and one bridging Re-O-Al bond. Polymeric chromium oxide surface species are observed at all loadings, 0.5-5% CrO3/Al2O3, as well as for titanium oxide which also forms a surface metal oxide overlayer of polymeric species up to a loading of 17%. The polymeric titanium oxide surface species are, however, not sensitive to moisture and only possess Ti-O-Ti bonds and not Ti=O bonds. Crystalline TiO2 (anatase) is found to be present at the alumina surface in the 17% TiO2/Al2O3 sample.