3% Ru/Al2O3catalyst is active in converting CO2 into methane at atmospheric pressure. At 673 K and above the thermodynamic equilibrium is nearly attained. At 623 K CH4 yield is above 85%. CO selectivity increases by decreasing reactants partial pressure apparently more than expected by thermodynamics. The reaction order for CO2 partial pressure is confirmed to be zero, while that related to hydrogen pressure is near 0.38 and activation energy ranges 60-75 kJ/mol. Arrhenius plot demonstrates that only at reduced reactant partial pressure (3% CO2) or high contact times, a contribution due to some diffusional limitation is present. IR study shows that the H2-reduced catalyst has high-oxidation state Ru oxide species able to oxidize CO to CO2at 173-243 K, while after oxidation/reduction cycle the alumina surface acido-basic sites are freed and the catalyst surface contains both extended Ru metal particles and dispersed low valence Ru species. IR studies show that the formation of methane, both from CO and CO2, occurs when both surface carbonyl species and surface formate species are observed. Starting from CO2, methane is formed already in the low temperature range, i.e., 523-573 K, even when CO is not observed in the gas phase.6
