Kinetic modeling and transient DRIFTS–MS studies of CO2 methanation over Ru/Al2O3 catalysts

CO2 methanation was investigated on 5% and 0.5% Ru/Al2O3 catalysts (Ru dispersions: ∼18% and ∼40%, respectively) by steady-state kinetic measurements and transient DRIFTS–MS. Methanation rates were higher over 5% Ru/Al2O3 than over 0.5% Ru/Al2O3. The measured activation energies, however, were lower on 0.5% Ru/Al2O3 than on 5% Ru/Al2O3. Transient DRIFTS–MS results demonstrated that direct CO2 dissociation was negligible over Ru. CO2 has to first react with surface hydroxyls on Al2O3 to form bicarbonates, which, in turn, react with adsorbed H on Ru to produce adsorbed formate species. Formates, most likely at the metal/oxide interface, can react rapidly with adsorbed H forming adsorbed CO, only a portion of which is reactive toward adsorbed H, ultimately leading to CH4 formation. The unreactive CO molecules are in geminal form adsorbed on low-coordinated sites. The measured kinetics are fully consistent with a Langmuir–Hinshelwood type mechanism in which the H-assisted dissociation of the reactive CO∗ is the rate-determining step (RDS). The similar empirical rate expressions (rCH4=kPCO20.1PH20.3-0.5) and DRIFTS–MS results on the two catalysts under both transient and steady-state conditions suggest that the mechanism for CO2 methanation does not change with Ru particle size under the studied experimental conditions. Kinetic modeling results further indicate that the intrinsic activation barrier for the RDS is slightly lower on 0.5% Ru/Al2O3 than on 5% Ru/Al2O3. Due to the presence of unreactive adsorbed CO on low-coordinated Ru sites under reaction conditions, the larger fraction of such surface sites on 0.5% Ru/Al2O3 than on 5% Ru/Al2O3 is regarded as the main reason for the lower rates for CO2 methanation on 0.5% Ru/Al2O3.