Mechanistic insight into bifunctional thermocatalytic CO2 hydrogenation by Oxide-Supported Palladium

Hydrogenating CO₂ into clean fuels and hydrocarbons such as methane, formic acid, and C₂⁺ products is a viable strategy for mitigating anthropogenic carbon dioxide. Based on density functional theory calculations, we elucidate the mechanism of CO₂ hydrogenation by Pd nanoparticles supported on MgO or CaO. To provide fundamental insight into the rational design of active and selective CO₂ hydrogenation catalysts, we combined Pd, a hydrogen activator, and MgO or CaO, a CO₂ binder. We found that Pd preferentially binds and dissociates H_2, and the Pd-oxide interface activates CO_2, completing a bifunctional CO₂ hydrogenation reaction. The Pd-CaO interface strongly binds CO₂ compared to the Pd-MgO interface. Therefore, the weakened C-O bond enables hydrogenation of the oxygen of CO₂, activating the carboxyl pathway of CO₂ hydrogenation and producing CO and methanol. In contrast, the formate pathway through direct hydrogenation of the carbon of CO₂ operates in the Pd-MgO catalyst due to the relatively weak interaction with CO₂. Producing formic acid and methanol is thermodynamically more accessible in Pd/MgO. Our results show that the catalyst-CO₂ interaction steers the overall reaction pathway of thermocatalytic CO₂ hydrogenation by metal/oxide class heterogeneous catalysts.