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.