Influence of the oxide support reducibility on the CO₂ methanation over Ru-based catalysts

Catalysts consisting of 5 wt.% ruthenium dispersed on four different metal oxide supports were synthesized with single-step flame spray pyrolysis and assessed for the hydrogenation of CO₂. The chosen supports covered a variety of characteristics expected to alter the catalytic performance, ranging from amphoteric and irreducible Al₂O₃, to amphoteric and reducible ZnO and MnOx, to basic and reducible CeO₂. For the pristine metal oxide supports, the catalytic activity correlated with the oxide basicity and ZnO showed the highest CO₂ conversion. However, CO formation through the reverse water-gas shift reaction dominated over the CO₂ methanation due to a lack of H₂ dissociation sites. The addition of Ru created such H₂ dissociation sites and a significant increase in CO₂ conversion and CH₄ selectivity was observed. Combining the results of H₂ temperature-programmed reduction, quantitative H₂ and CO chemisorption, in situ DRIFTS, and the CO₂ hydrogenation reaction kinetics revealed that the Ru coverage with CO decreased with the support reducibility. For the irreducible Al₂O₃ support, the Ru particles were quasi-saturated with CO at low temperatures and the reaction was limited by the competitive Langmuir-type co-adsorption of H₂. The ZnO support lead to the lowest Ru-CO coverage associated with a weak CO adsorption and the reaction was dominated by the reverse water-gas shift reaction. CeO₂ showed a Ru-CO coverage in between Al₂O₃ and ZnO, which ensured the presence of H₂ dissociation sites while the Ru-CO adsorption strength remained sufficiently high.