Does CO₂ Oxidize Ni Catalysts? A Quick X-ray Absorption Spectroscopy Answer

MgAl₂O₄-supported Ni materials are highly active and cost-effective CO₂ conversion catalysts, yet their oxidation by CO₂ remains dubious. Herein, NiO/MgAl₂O₄, prepared via colloidal synthesis (10 wt % Ni) to limit size distribution, or wet impregnation (5, 10, 20, and 40 wt % Ni), and bare, i.e., unsupported, NiO are examined in H₂ reduction and CO₂ oxidation, using thermal conductivity detector-based measurements and in situ quick X-ray absorption spectroscopy, analyzed via multivariate curve resolution-alternating least-squares. Ni reoxidation does not occur for bare Ni but is observed solely on supported materials. Only samples with the smallest particle sizes get fully reoxidized. The Ni-MgAl₂O₄ interface, exhibiting metal-support interactions, activates CO₂ and channels oxygen into the reduced lattice. Oxygen diffuses inward, away from the interface, oxidizing Ni entirely or partially, depending on the particle size in the applied oxidation time frame. This work provides evidence for Ni oxidation by CO₂ and explores the conditions of its occurrence and the importance of metal-support effects.