Methanation of CO2 on supported Ru catalysts

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 NCH4= 2.7 × 106 exp (–16.1/RT)×PH2× 0.47 PCO2. It is proposed that the synthesis of CH4 from H2+ CO2 occurs via the formation of surface C and its subsequent hydrogenation.