CO₂ hydrogenation on Ni_xMg_1−xAl₂O₄: A comparative study of MgAl₂O₄ and NiAl₂O₄

Due to the increasing attention focused on global warming, many studies on reducing CO₂ emissions and developing sustainable energy strategies have recently been performed. One of the approaches is CO₂ methanation, transforming CO₂ into methane. Such transformation (CO₂ + 4H₂ → CH₄ + 2H₂O) provides advantages of carbon liquification, storage, etc. In this study, we investi-gated CO2 methanation on nickel–magnesium–alumina catalysts both experimentally and compu-tationally. We synthesized the catalysts using a precipitation method, and performed X-ray diffrac-tion, temperature-programmed reduction, and N₂ adsorption–desorption tests to characterize their physical and chemical properties. NiAl2O4 and MgAl₂O₄ phases were clearly observed in the cata-lysts. In addition, we conducted CO₂ hydrogenation experiments by varying with temperatures to understand the reaction. Our results showed that CO₂ conversion increases with Ni concentration and that MgAl₂O₄ exhibits high selectivity for CO. Density functional theory calculations explained the origin of this selectivity. Simulations predicted that adsorbed CO on MgAl₂O₄(100) weakly binds to the surface and prefers to desorb from the surface than undergoing further hydrogenation. Elec-tronic structure analysis showed that the absence of a d orbital in MgAl₂O₄(100) is responsible for the weak binding of CO to MgAl₂O₄. We believe that this finding regarding the origin of the CO selectivity of MgAl₂O₄ provides fundamental insight for the design methanation catalysts.