Local structure change of Rh on alumina after treatments in high-temperature oxidizing and reducing environments

The effects of high-temperature oxidization and reduction on the local structures of Rh in Rh/Al2O3 catalysts were investigated. After oxidization at 1200degreesC, XPS measurements indicated that the Rh was in a highly oxidized state. Extended x-ray absorption fine structure (EXAFS) analysis for the highly oxidized Rh showed a shorter distance between the Rh atom and the next cation than that in Rh2O3. The shortened distance agrees with the Rh substitution model, in which Rh substituted for the inner Al site of the alpha-Al2O3 lattice. Stepwise reductions with XPS measurements after oxidization at 1200degreesC revealed that the Rh that diffused into gamma-Al2O3 was more resistant to reduction than that diffused into alpha-Al2O3. The Rh aged at 600degreesC was oxidized but had no specific local structure. Copyright (C) 2005 John Wiley Sons, Ltd.