Methanation of CO2 on supported Ru catalysts

Abstract

The transformation of C in the form of CO₂ into hydrocarbons was investigated on supported Ru catalysts. Special attention was paid to the surface processes occurring during the removal of the O atoms of CO₂ by H₂ and on the identification of surface species formed during the reaction. Infrared spectroscopic measurements revealed that chemisorbed CO and formate ion are formed during the coadsorption of H₂+ CO₂ at 373 K and also during the methanation of CO₂ at higher temperatures. The CO formed produced a weak absorption band at lower frequencies (1990–2000 cm^–1) than did the CO alone (2030–2040 cm^–1). This shift was attributed to the effect of hydrogen adsorbed on the same Ru atoms and to that of surface C formed during the reaction. Evidence is presented to show that formate ion forms on the Ru but migrates rapidly onto the supports. It is considered as an inactive species in the methanation of CO₂.

The hydrogenation of CO₂ on Ru/Al₂O₃ occurred at a measurable rate above 443 K yielding almost exclusively CH₄. The formation of surface carbon was detected during the reaction at a level ca. 1.5 orders of magnitude less than in the H₂+ CO reaction. The rate of CH₄ formation is described by the expression N_CH4= 2.7 × 10⁶ exp (–16.1/RT)×P_H2× 0.47 P_CO2. It is proposed that the synthesis of CH₄ from H₂+ CO₂ occurs via the formation of surface C and its subsequent hydrogenation.