Laser raman characterization of oxidized Re/gamma-Al2O3 catalysts: Effect of calcination temperature

The interaction of oxygen with sintered 1.04 and 10.4 wt% Re/gamma -Al2O3 c atalysts was studied over a wide temperature range, 300-800 degreesC. The m olecular structure of the oxidated catalysts under dehydration and hydratio n conditions was examined using laser Raman spectroscopy. Irrespective of t he temperature and time of calcination, Raman spectra revealed that rhenium in the oxidated state is present as ReO4 monomers coordinated to the alumi na support. Under dehydration conditions, ReO4 monomers form two slightly d ifferent species. The first occurs for both loadings, while the second occu rs for high loading only. Additionally, formation of the second type of sur face rhenium oxide species depends on the temperature and time of calcinati on. The structural details of these dehydrated rhenium oxide species are no t, however, clear. During oxidation of the high-loading catalyst at 300-500 degreesC, ReO4 monomers form a surface compound probably exibiting Al-(O-R eO3)(3) structure, which is resistant to moisture but not resistant to the high-temperature treatment. The surface coverage of the rhenium declines wi th increasing oxidation temperature to 800 degreesC. The remaining rhenium oxide is present as adsorbed ReO4 monomers forming the first type of surfac e rhenium oxide species. (C) 2001 Academic Press.