Oxidation and redispersion of a Re/gamma-Al2O3 catalyst

The interaction of oxygen with rhenium crystallites supported on gamma-alum ina over a wide temperature range, 20-800 degrees C has been studied. Hydro gen chemisorption, oxygen uptake, and HRTEM, SAED, and XRD methods were app lied to monitor morphology changes created upon heat treatment in H-2 and O -2. The structure of the surface rhenium oxide species was determined by HR TEM, Raman spectroscopy, and leaching with the cold water. The results obta ined allowed us to propose a detailed mechanism of the oxidation and redisp ersion of Re crystallites on alumina. At 20-150 degrees C dissociative chem isorption of oxygen occurs with the possible formation of a superficial Re oxide. At 200-300 degrees C the process of oxidation of Re accelerates, wit h instantaneous sublimation of Re2O7 and its simultaneous adsorption as the ReO4 species on gamma-alumina. Metallic Re is still observed in this tempe rature range. At 400-600 degrees C, the ReO4 species on the support form tw o kinds of a surface compound exhibiting Al-O-ReO3 and Al-(O-ReO3)(3) struc tures. At 800 degrees C the surface aluminium perrhenate decomposes and a p art of the Re oxide sublimes as Re2O7. The redispersion of rhenium was obse rved after H-2 treatment of the oxidized rhenium catalyst. (C) 1999 Academi c Press.