Laser Raman Characterization of Oxidized Re/γAl2O3 Catalysts: Effect of Calcination Temperature

The interaction of oxygen with sintered 1.04 and 10.4 wt% Re/γ-Al2O3 catalysts was studied over a wide temperature range, 300–800°C. The molecular structure of the oxidated catalysts under dehydration and hydration conditions was examined using laser Raman spectroscopy. Irrespective of the temperature and time of calcination, Raman spectra revealed that rhenium in the oxidated state is present as ReO4 monomers coordinated to the alumina support. Under dehydration conditions, ReO4 monomers form two slightly different species. The first occurs for both loadings, while the second occurs for high loading only. Additionally, formation of the second type of surface rhenium oxide species depends on the temperature and time of calcination. The structural details of these dehydrated rhenium oxide species are not, however, clear. During oxidation of the high-loading catalyst at 300–500°C, ReO4 monomers form a surface compound probably exibiting Al–(O–ReO3)3 structure, which is resistant to moisture but not resistant to the high-temperature treatment. The surface coverage of the rhenium declines with increasing oxidation temperature to 800°C. The remaining rhenium oxide is present as adsorbed ReO4 monomers forming the first type of surface rhenium oxide species.