Evidence for Rh electron-deficient atoms (Rhδ+) as the catalytic species for CO oxidation when supported on Ce0.68Zr0.32O2: A combined N2-FTIR, benzene hydrogenation, and kinetic study

The characterization of two low-loaded Rh(wt%)/Ce0.68Zr0.32O2 samples (Rh(0.29)/CZ and Rh(0.32)/CZ) was performed via N2-FTIR and benzene hydrogenation before being investigated for CO oxidation activity and kinetics. N2-FTIR allows characterization of both Rhδ+ (image) and Rh0 (image) centers and revealed that the concentration of Rhδ+ does not vary to a significant extent on both samples, whereas the concentration of Rh0 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 Rh0 centers, but more likely by the Rhδ+ centers. Such a conclusion is supported by the estimated reaction orders with respect to CO and O2, which are obviously different from those expected for Rh0 catalytic sites. The dramatic increase in the concentration of the Rh0 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 N2, do not catalyze CO oxidation either.