RESUMO
The identification of a Rh-oxidised species of a Rh(0.29)/Ce0.68Zr0.32O2 catalyst that exhibits peculiar CO-O2 kinetics [I. Manuel, J. Chaubet, C. Thomas, H. Colas, N. Matthess and G. Djéga-Mariadassou, J. Catal. 2004, 224, 269] is addressed. For this purpose, various catalysts are studied by XANES, CO- and N2-FTIR, and benzene hydrogenation. The results obtained, particularly from N2-FTIR, which is, to our knowledge, reported for the first time on this kind of catalyst, suggest that Rh is mainly stabilised as electron-deficient clusters (Rhndelta+) on a Rh(0.29)/Ce0.68Zr0.32O2 catalyst after reduction at 500 degrees C under H2. The existence of these species, which may be caused by either electron perturbation induced on the metal by the reduced support or electron withdrawal from the metal clusters by an inductive effect of the neighbouring Cl anions, is also revealed through CO-FTIR experiments. In the presence of CO, however, evidence of RhI(CO)2 species is also provided.
RESUMO
The low-temperature adsorption of N(2) on Rh/SiO(2) samples of various particle-size distributions was followed by FTIR. The addition of O(2) pulses on Rh(0) surfaces saturated with chemisorbed N(2) allowed us to reassign stretching frequencies attributed originally to N(2)-Rh(0) to N(2)-Rh(delta+). The formation of the latter oxidized Rh species is assumed to be induced by an electron withdrawal from adsorbed oxygen species on Rh surface centers neighboring those onto which N(2) species are chemisorbed. The present work, thus, enables us to delimit ranges of frequencies for which the adsorption of N(2) can be considered to occur on either Rh(0) or Rh(delta+) centers for nu(N2) lower or higher than 2243 cm(-1), respectively. The N(2)-FTIR experiments performed on the studied catalysts also suggest a lattice plane selectivity for N(2) adsorption on metallic Rh planes of different natures which, to our knowledge, has not been reported yet for Rh.