INFLUENCE OF THE CARRIER ON THE INTERACTION OF H-2 AND CO WITH SUPPORTED RH

The influence of the carrier (SiO2, Al2O3, and TiO2) on the interaction of supported Rh with H2 and CO and on kinetic parameters in CO hydrogenation is investigated employing transient and steady-state techniques. TPD of H2 revealed two modes of adsorption on Rh/Al2O3 and Rh/TiO2, one of which is activated. This was not observed over Rh/SiO2. TPD spectra obtained following CO adsorption reveal significant differences, depending on the carrier. Three major states of desorbing CO were detected, one of which desorbs as CO2 accompanied by H2 evolution. In Rh/SiO2 and Rh/Al2O3 the weakly adsorbed CO exhibits peak maximum temperatures in the range 90-100°C, while for the strongly adsorbed CO these are in the range 263-270°C in Rh/SiO2 and 210-245°C in Rh/Al2O3. In both cases, the CO2 peak maximum appears in the range 340-355°C. The activity of the catalysts toward CO decomposition was found to decrease in the order: Rh/TiO2 ≫ Rh/Al2O3 > Rh/SiO2. This observation has been attributed to the creation of new adsorption sites at the Rh-TiO2 interface and carrier participation in the process of CO decomposition. The reactivity of adsorbed CO toward hydrogenation, as investigated by TPR experiments, was found to follow the same order as that for CO decomposition. The Rh/TiO2 catalyst was found to be 15-20 times more active in steady-state CO hydrogenation, as compared to Rh/SiO2 and Rh/Al2O3, and to exhibit lower activation energies for methanation and water-gas shift reactions. © 1993 Academic Press, Inc.