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

• Published in: Applied catalysis. B, Environmental Vol. 14; no. 3-4; pp. 249 - 259
• Main Authors: Fiedorow, Ryszard M.J., Wanke, Sieghard E.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 29.12.1997
• Subjects: Alumina crystallinity; Rhodium (supported); Rhodium oxide; Sintering; Rhodium oxide; Alumina crystallinity; Sintering; Rhodium (supported)
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/S0926-3373(97)00027-1

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.