Effect of metal loading on CO2 hydrogenation reactivity over Rh/SiO2 catalysts

Silica supported Rh catalysts (Rh/SiO2) with metal loading ranging from 1 t o 10 wt.% were prepared by impregnation method and applied to CO2 hydrogena tion reaction. Characterization of surface species by TEM, H-2 chemisorptio n, XPS, EXAFS and FT-IR was carried out in order to elucidate the effect of metal loading on the CO2 hydrogenation reactivity. The main product transf ormed from CO to CH4 with the loading amount of Rh, retaining similar total activity. Judging from the results of EXAFS and FT-IR, formation of carbon yl species occurred on 1 wt.% Rh/SiO2 catalyst. The concentration of Rh par ticles on SiO2 surface had a significant influence on the reactivity. For 1 wt.% Rh/SiO2 catalyst, the concentration of surface Rh particles was low, so Rh species were surrounded by hydroxyl groups of SiO2. CO-saturated Rh s pecies, which were derived from dissociative CO2 adsorption, reacted with s urface hydroxyl groups to form relatively fine Rh carbonyl clusters, and ad sorbed CO was not subjected to further hydrogenation, resulting in desorpti on as molecular CO. On the other hand, 10 wt.% Rh/SiO2 catalyst achieved ab out 5.8 limes more surface coverage of Rh species than 1 wt.% Rh/SiO2, base d on the results of H-2 chemisorption. Therefore, too few surface hydroxyl groups of SiO2 existed around Rh particles not to form Rh carbonyl clusters . The Rh surface was occupied by more hydride species and fewer CO species, resulting in CH4 formation. (C) 2000 Elsevier Science B.V. All rights rese rved.