INFLUENCE OF THE REDUCTION EVACUATION CONDITIONS ON THE RATE OF HYDROGEN SPILLOVER ON RH/CEO2 CATALYSTS

The hydrogen chemisorption was studied on a Rh/CeO2 catalyst after a reduction/evacuation treatment at 773 K. In addition to volumetric and TPD-H-2 measurements, magnetic susceptibility and semiconductivity experiments were performed to determine the Ce3+ content and the electron concentration, respectively, in order to discriminate the contribution of the support from the total hydrogen adsorption. By comparison with previous results obtained after reduction/evacuation at 623 K, the reduction at 773 K induces a strong decrease of the apparent H/Rh value determined at 295 K. The variations of the Ce3+ content calculated from magnetism indicate a very limited hydrogen chemisorption on the support at 295 K. It increases significantly upon heating under H-2 at 473 K. The conductivity data are in good qualitative agreement with magnetic measurements. These results show that the main effect of increasing the reduction temperature from 623 to 773 K is to slow down the hydrogen spillover rate on the support. In agreement with previous suggestions, they indicate that hydrogen chemisorption on the rhodium is not suppressed after reduction at 773 K as it could be expected for a metal under the classical SMSI state.