The synthesis of methane from CO2 and H2 was investigated on supported Rh catalysts. The adsorbed species present on the catalysts were characterized by infrared spectroscopy. The hydrogenation of CO2 on Rh Al2O3 occurred at a measurable rate above 443 K, yielding exclusively methane. The rate of methane formation on Rh Al2O3 is described by the expression NCH4 = 2.69 × 106 exp( -16.200 RT)PH20.61 · PCO20.26. From a comparison of the specific activities of Rh Al2O3 in the H2 + CO2 and H2 + CO reactions it appears that the hydrogenation of CO2 occurs much faster than that of CO. The support exerted a marked influence on the specific activity of Rh. The most effective support was TiO2 and the least effective one SiO2. Infrared spectroscopic measurements revealed that linearly bonded CO (perturbed by hydrogen adsorbed on the same Rh atom) and adsorbed formate species were present on the catalyst surface during the reaction. Evidence is presented to show that the surface formate is located not on the Rh, but rather on the support. The formation of surface C was also detected. Its amount slightly increased during the conditioning period and also with temperature elevation. It is proposed that the important steps in CH4 formation are the dissociation of CO2 promoted by adsorbed hydrogen, the subsequent dissociation of CO into reactive surface carbon and hydrogenation of the latter. The possible reasons for the different methanation rates of CO2 and CO are discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry