Evidence for Rh electron-deficient atoms (Rh δ+ ) as the catalytic species for CO oxidation when supported on Ce 0.68Zr 0.32O 2: A combined N 2-FTIR, benzene hydrogenation, and kinetic study

Abstract

The characterization of two low-loaded Rh(wt%)/Ce 0.68Zr 0.32O 2 samples (Rh(0.29)/CZ and Rh(0.32)/CZ) was performed via N 2-FTIR and benzene hydrogenation before being investigated for CO oxidation activity and kinetics. N 2-FTIR allows characterization of both Rh δ+ ( ν N 2 = 2295 cm −1 ) and Rh 0 ( ν N 2 = 2192 cm −1 ) centers and revealed that the concentration of Rh δ+ does not vary to a significant extent on both samples, whereas the concentration of Rh 0 sites increases dramatically on Rh(0.32)/CZ, as also supported by benzene hydrogenation activities. In contrast, CO oxidation activity and kinetics are nearly identical on both catalysts. This clearly indicates that CO oxidation is catalyzed not by the Rh 0 centers, but more likely by the Rh δ+ centers. Such a conclusion is supported by the estimated reaction orders with respect to CO and O 2, which are obviously different from those expected for Rh 0 catalytic sites. The dramatic increase in the concentration of the Rh 0 centers on Rh(0.32)/CZ suggests the presence of electron-enriched Rh clusters on Rh(0.29)/CZ, owing to the basicity of CZ. These electron-enriched Rh clusters, which neither catalyze benzene hydrogenation nor chemisorb N 2, do not catalyze CO oxidation either.