Methanation of CO2 over alkali-promoted Ru/TiO2 catalysts: I. Effect of alkali additives on catalytic activity and selectivity

The effect of alkali promotion of TiO2 on the CO2 hydrogenation activity and selectivity has been investigated over 0.5%Ru/TiO2 and 5%Ru/TiO2 catalysts of different promoter type (Li, Na, K, Cs) and variable promoter loading (0.0–0.40 wt.%). The effect of alkali promotion was found to be more pronounced over catalysts of low Ru loading (or small Ru crystallites). In the case of 0.5%Ru/TiO2 catalysts, both catalytic activity and methane selectivity are strongly enhanced with the addition of small amounts (0.2 wt.%) of alkalis following the order of TiO2 (unpromoted) < Li ∼ K < Cs < Na. The intrinsic reaction rate exhibits a volcano-type dependence on the sodium content. Optimal results were obtained for 0.5%Ru/TiO2 catalyst containing 0.2 wt.% Na, the specific activity of which is about 3 times higher, compared to that of the unpromoted sample. In the case of 5%Ru/TiO2 catalysts, alkali addition does not practically affect catalytic activity and selectivity. Turnover frequency of 0.5%Ru/0.2%Na-TiO2 catalyst is higher than that measured over 5%Ru/TiO2, indicating that small amounts of alkalis can replace part of precious metals, reducing the cost and increasing the efficiency of the CO2 methanation process.