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.