The structure of surface rhenium oxide on alumina from laser raman spectroscopy and x-ray absorption near-edge spectroscopy

Abstract

The interaction of rhenium oxide with an alumina support is examined over a wide range of conditions (rhenium oxide loading, calcination temperature and environment) using laser Raman spectroscopy (LRS) and X-ray absorption near-edge spectroscopy (XANES). These structural probes reveal that supported rhenium oxide on alumina is present as an atomically dispersed surface [ReO 4] ads species coordinated to the alumina support. The [ReO 4] ads species possesses C 3v symmetry which is consistent with the presence of three equivalent terminal Re-O bonds and one inequivalent Re-O bond as part of the Re-O-Al linkage to the alumina support. The [ReO 4] ads species are solvated by water molecules at ambient conditions, and the solvating water molecules readily desorb on heating in dry environments. The surface coverage of the [ReO 4] ads species declines with increasing calcination temperature, because the surface rhenium oxide species can apparently recombine in dry environments and elevated temperatures to yield gaseous dimeric Re 2O 7. Additional studies are required to determine if a [ReO 4] ads surface species can be isolated on the alumina support in dry environments and at high rhenium oxide surface coverages. The supported rhenium oxide on alumina system is dynamic, and the state of rhenium oxide depends on temperature, moisture/water content and rhenium oxide loading.