CO{sub 2} methanation under transient and steady-state conditions over Rh/CeO and CeO{sub 2}-promoted Rh/SiO{sub 2}: The role of surface and bulk ceria
- Universita di Udine (Italy)
- Universitat de Barcelona (Spain)
A series of Rh/SiO{sub 2} samples promoted with CeO{sub 2} have been prepared and used as catalyst in CO{sub 2} methanation. Their behavior has been compared to that of unpromoted Rh/SiO{sub 2} and Rh/CeO{sub 2} catalysts. The activity has been monitored under transient and steady-sate 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(NO{sub 3}){sub 3}. 6H{sub 2}O followed by calcination at 923 K, aggregates of CeO{sub 2} particles with size ranging from 35 to 150 nm (depending on initial cerium loading) form on the surface. These larger agglomerates are constituted by individual, smaller (5-15 nm), crystalline CeO{sub 2} particles. Redispersion of CeO{sub 2} in the presence of H{sub 2} at 773 K is observed in all samples. The presence of Rh, deposited by impregnation from RhCl{sub 3} {center_dot} 3H{sub 2}O solutions, accelerates the process. The reducibility of ceria is strongly enhanced by deposition on silica: complete reduction to Ce{sub 2}O{sub 3} is observed for CeO{sub 2}-supported examples at temperatures lower than 1100 K, while a maximum of 50% reduction (corresponding to CeO{sub 1.75}) is observed for unsupported CeO{sub 2} 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 CeO{sub 2} 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 CO{sub 2}, under reaction conditions, restores the normal catalytic behavior. 45 refs., 8 figs., 5 tabs.
- OSTI ID:
- 75799
- Journal Information:
- Journal of Catalysis, Vol. 151, Issue 1; Other Information: PBD: Jan 1995
- Country of Publication:
- United States
- Language:
- English
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