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

The interaction of sintered low-loaded Re/γ-Al2O3 catalysts with oxygen has been studied over a wide temperature range, 20–800 °C. The structure of the catalyst was characterized using different techniques: H2 chemisorption, O2 uptake, BET, TEM, and also by Raman and XPS spectroscopy. In the catalyst reduced in H2 at 800 °C, 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 °C only 2% of total Re remains in the reduced state which implies high affinity of highly dispersed Re to oxygen. At 200–300 °C, oxidation of Re hastens giving a mixture of Re4+, Re6+, and Re7+ ions, while at 500 °C, complete oxidation to Re2O7 occurs. At elevated temperatures, Re2O7 sublimates and instantaneously adsorbs on γ-alumina surface as monomeric ReO4 species. At 500–800 °C, 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 °C. Redispersion of rhenium was observed after a mild H2 treatment of the oxidized rhenium catalyst.