On the support dependency of the CO2 methanation – decoupling size and support effects

Abstract

The influence of the support basicity, according to the Lewis and Brønsted definition, was investigated for the Ru catalyzed CO₂ methanation in the temperature range from 200 °C to 400 °C. Due to the structure-sensitivity of the reaction, a novel building block approach was used to ensure a constant Ru particle size, while varying the support material. In this way, differences in the catalytic behaviour could be directly related to support effects. Eight oxides – the rare earth metal oxides Gd₂O₃, Sm₂O₃ and Y₂O₃ (REOs) as well as TiO₂, ZrO₂, Al₂O₃, MgO and SiO₂ as a non-basic oxide – were chosen to cover different types and combinations of basic surface sites on the support, such as Brønsted basic hydroxyl groups, Lewis basic oxygen atoms and oxygen vacancies. Above 310 °C, the REO supported catalysts showed the highest methane formation rates. The consumption of carbonate species formed upon CO₂ adsorption on all three types of basic sites indicated their catalytic involvement in the high temperature regime. Below 310 °C, TiO₂ and – to a lesser extent – ZrO₂ excelled the other supports. For ZrO₂ the enhanced performance could be related to the sole presence of Lewis basic oxygen vacancies, acting as additional CO₂ adsorption and activation sites on the support. On contrary, in case of TiO₂ they seemed not to be directly but only indirectly involved by facilitating the conversion on the Ru particles on the basis of a favourable electronic metal–support interaction. The inferior catalytic results obtained with the other supports were in accord with the absence of basic sites or a spectator role of the carbonates formed – except for Al₂O₃ which stood out probably due to Brønsted basic OH-groups formed under reaction conditions. Overall, the study reveals that basic supports can noticeably contribute to the catalytic turnover by opening new support-related pathways in addition to the Ru-related pathway evidenced in all cases and/or by promoting the latter. Their impact is dependent on the type, density and strength of basic sites available and varies with temperature.