Effect of γ-alumina crystallinity on the state of rhodium during high-temperature treatments in oxygen and hydrogen

Rhodium catalysts, supported on six γ-Al₂O₃ supports with different crystallinities, were exposed to sequential treatments in hydrogen at 500°C, in oxygen at 760°C, in hydrogen at 500°C and at 760°C, respectively. Samples were characterized by X-ray diffraction and hydrogen chemisorption at various stages in the sequential treatment. Based on the characterization results, it is concluded that the formation of crystalline Rh₂O₃ is a function of γ-Al₂O₃ crystallinity; formation of crystalline Rh₂O₃ increased with increasing crystallinity of γ-Al₂O₃ during treatment in oxygen at 760°C. The crystalline Rh₂O₃ formed during treatment in oxygen at 760°C was reduced to Rh metal by hydrogen at 500°C, but most of the Rh did not adsorb hydrogen at room temperature. Subsequent treatment in hydrogen at 760°C increased the hydrogen adsorption capacity by as much as a factor of three. X-ray line broadening measurements showed that oxygen treatment of reduced Rh/γ-Al₂O₃ at 760°C followed by hydrogen reduction at 500°C resulted in significant increases in Rh crystal size; further treatment in hydrogen at 760°C resulted in additional sintering of Rh.