Hydrogenation of CO2 over Mn-Substituted SrTiO3 Based on the Reverse Mars–van Krevelen Mechanism

Oxygen storage materials (OSMs) such as CeO2 have attracted great attention as catalysts for the reverse water gas shift (RWGS) reaction, replacing metal-supported catalysts from the perspectives of thermal stability and CO selectivity. Because perovskite-type oxides are also known to be powerful OSM candidates, here we employed perovskite-type Mn-substituted SrTiO3 as a catalyst for RWGS and investigated the effects of oxygen vacancies on the activity toward CO generation. Among the solid solutions with different Mn ratios, SrTiO3 with 20% Mn substitution exhibited the highest activity. It is suggested that oxygen vacancies generated by the substitution of Mn ions for Ti ions in SrTiO3 exhibit selective activity toward CO2 splitting, whereas the redox of bulk Mn oxides does not have the capacity for CO generation in the control experiments. Quantitative H2-temperature programmed reduction and CO2-temperature programmed oxidation experiments revealed that half of the oxygen vacancies in Mn-substituted SrTiO3, which were generated by reduction with H2, were used as active sites for CO2 splitting. We conclude that this generated active oxygen vacancy, which is attributed to the valence change of the Mn ions, contributes to efficient CO2 splitting to form CO through the reverse Mars–van Krevelen mechanism.