CHARACTERIZATION OF CHROMIUM-OXIDE SUPPORTED ON AL2O3, ZRO2, TIO2, AND SIO2 UNDER DEHYDRATED CONDITIONS

In the present investigation, Raman and IR spectroscopy were used to study the surface structures of chromium oxide supported on alumina, titania, zirconia, and silica, as a function of the loading under dehydrated conditions. It was found, that the dehydrated surface structures of chromium oxide differ strongly from those previously reported under ambient conditions, in which the surfaces are hydrated. Two species, each possessing one short terminal Cr=O bond, and one (or more) oligomer (s) are proposed to be present on the dehydrated alumina, titania, and zirconia surfaces. The relative concentrations of these different chromium oxide species is independent of the surface coverage. The chromium oxide species present on the dehydrated silica surface are completely different from those observed on the other three supports. The Raman and IR spectra indicate the presence of an isolated chromium oxide species possessing two short Cr=O bonds together with a small amount of surface species possessing a terminal CrO3 unit, isolated or not. The gradual disappearance of the surface hydroxyl groups of all four supports upon addition of chromium oxide, as monitored by IR spectroscopy, suggests that the chromium oxide species interacts with the surface by removal of the surface hydroxyl groups.