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

Silica supported Rh catalysts (Rh/SiO2) with metal loading ranging from 1 to 10 wt.% were prepared by impregnation method and applied to CO2 hydrogenation reaction. Characterization of surface species by TEM, H-2 chemisorption, 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 transformed 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 carbonyl species occurred on 1 wt.% Rh/SiO2 catalyst. The concentration of Rh particles 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 species, which were derived from dissociative CO2 adsorption, reacted with surface hydroxyl groups to form relatively fine Rh carbonyl clusters, and adsorbed CO was not subjected to further hydrogenation, resulting in desorption as molecular CO. On the other hand, 10 wt.% Rh/SiO2 catalyst achieved about 5.8 limes more surface coverage of Rh species than 1 wt.% Rh/SiO2, based 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 reserved.