Temperature-programmed reduction of Al2O3-, SiO2-, and carbon-supported Re2O7 catalysts

Temperature-Programmed Reduction (TPR) has been applied to characterize the reducibility of Al2O3-, SiO2-, and carbon-supported Re2O7 catalysts, over a wide range of transition metal content. Dried catalysts are found to contain a so-called monolayer-type Re7+ surface phase as well as crystalline NH4ReO4. Calcination at 575 or 825 K resulted in decomposition of NH4ReO4, formation of the Re7+ surface phase and Re2O7 clusters, and Re loss via sublimation of Re2O7. Differences in reducibility of the various catalyst samples are ascribed to variations in the strength and the heterogeneity of the Re7+-support interaction. The strength of the interaction was found to depend on the support material applied and decreased in the order: Al2O3 > SiO2 > carbon. The heterogeneity was essentially the same for all three supports. The largely varying literature data on the reducibility of Re2O7/Al2O3 catalysts is supposedly related with the presence of additives, such as chlorides, which may increase the Re7+-support interaction