Surface characteristics and activity of chromia/alumina catalysts prepared by atomic layer epitaxy

A series of CrOx/gamma-Al2O3 catalysts was prepared by atomic layer epitaxy (ALE) from the vapor phase, with use of sequential saturating reactions of Cr(acac)(3) vapor and air. The loading was varied from 1.3 to 8.8 wt% of Cr. Lower loadings were achieved by partially blocking the alumina surface with acetylacetone or dipivaloylmethane before introducing Cr(acac)(3). The catalytically active material was found to be evenly distributed through the catalyst particles, Low energy ion scattering measurements showed that, up to 7.4 wt% of Cr, the Cr species was dispersed in a monolayer. XPS and UV-vis spectrophotometry revealed that, even at the low loadings, both Cr3+ and Cr6+ species were present. This was attributed to a stabilization of Cr3+ On the alumina support during the ALE process. Loading beyond monolayer coverage was not reflected in the dehydrogenation of i-butane to i-butene: the activity continued to increase. (C) 1996 Academic Press, Inc.