Oxidation and redispersion of a low-loaded Re/γ-Al2O3 catalyst

The interaction of sintered low-loaded Re/y-Al2O3 catalysts with oxygen has been studied over a wide temperature range, 20-800 degreesC. The structure of the catalyst was characterized using different techniques: H-2 chemisorption, O-2 uptake, BET, TEM, and also by Raman and XPS spectroscopy. In the catalyst reduced in H-2 at 800 degreesC, Re is present as a nonuniform phase consisting of metallic particles with sizes of 1-4 nm and a certain amount of very small clusters (below 1 nm) undetectable by TEM. Oxidation at room temperature causes dissociative chemisorption of oxygen on rhenium with possible formation of a superficial oxide on the Re particles and complete oxidation of the clusters to Re4+-Re7+ species. After oxidation at 150 degreesC only 2% of total Re remains in the reduced state which implies high affinity of highly dispersed Re to oxygen. At 200-300 degreesC, oxidation of Re hastens giving a mixture of Re4+, Re6+, and Re7+ ions, while at 500 degreesC, complete oxidation to Re2O7 occurs. At elevated temperatures, Re2O7 sublimates and instantaneously adsorbs on gamma-alumina surface as monomeric ReO4 species. At 500-800 degreesC, these species are strongly bound to alumina forming a kind of a surface compound with an Al-O-ReO3 structure. This accounts for the low loss of Re observed even at 800 degreesC. Redispersion of rhenium was observed after a mild H-2 treatment of the oxidized rhenium catalyst. (C) 2003 Elsevier Inc. All rights reserved.