CO2 METHANATION UNDER TRANSIENT AND STEADY-STATE CONDITIONS OVER RH/CEO2 AND CEO2-PROMOTED RH/SIO2 - THE ROLE OF SURFACE AND BULK CERIA

A series of Rh/SiO2 samples promoted with CeO2 have been prepared and used as catalyst in CO2 methanation. Their behaviour has been compared to that of unpromoted Rh/SiO2 and Rh/CeO2 catalysts. The activity has been monitored under transient and steady-state conditions and the catalysts have been characterized using quantitative temperature programmed reduction, chemisorption, X-ray diffraction, and transmission electron microscopy. By impregnation of amorphous silica with Ce(NO3)(3). 6H(2)O followed by calcination at 923 K, aggregates of CeO2 particles with size ranging from 35 to 150 nm (depending on initial cerium loading) form on the surface. These large agglomerates are constituted by individual, smaller (5-15 nm), crystalline CeO2 particles. Redispersion of CeO2 in the presence of Ha at 773 K is observed in all samples. The presence of Rh, deposited by impregnation from RhCl3.3H(2)O solutions, accelerates the process. The reducibility of ceria is strongly enhanced by deposition on silica: complete reduction to Ce2O3 is observed for CeO2-supported samples at temperatures lower than 1100 K, while a maximum of 50% reduction (corresponding to CeO1.75) is observed far unsupported CeO2 in the range of temperature 295-1400 K. The activity of the virgin catalyst, as tested under unsteady-state conditions, is positively influenced by the reduction temperature and CeO2 crystallite size. We suggest that the observed enhancement of catalytic activity is linked to the presence of bulk. vacancies created on ceria after reduction at high temperatures. Annihilation of these oxygen vacancies by oxygen from CO2, under reaction conditions, restores the normal catalytic behaviour. (C) 1995 Academic Press, Inc.