f-d-σ* Orbital Relay Enables Homolytic H₂ Dissociation and CeO_2-x Oxidation

Ceria (CeO₂) is a widely employed catalytic material in hydrogenation catalysis, and experimental studies have reported that reduced ceria (CeO_2-x) can be oxidized by H₂; however, the microscopic origin of this process remains unclear. Density functional theory calculations corrected by on-site Coulomb interaction in the current work show that surface oxygen vacancies at CeO_2-x are essential for both the dissociation of H₂ and the oxidation of the surface. Depending on the spatial arrangement of oxygen vacancies, H₂ can either heterolytically dissociate to yield surface hydroxyls and hydrides while leaving Ce-4f electrons unperturbed or undergo homolytic dissociation to generate two hydrides coupled with the oxidation of two Ce³⁺ ions to Ce⁴⁺. The latter pathway becomes accessible only for cross-layer vacancy pairs, which stabilize the transition state in a Ce-H-H dihydrogen configuration. Electronic structure analysis further demonstrates an f-electron regulated channel for homolytic H₂ dissociation, where the empty Ce-5d orbital serves as an Electronic Relay Orbital (ERO) that bridges the partially occupied Ce-4f states and the H₂-σ* orbital. These findings identify the active sites for H₂-induced oxidation of CeO_2-x and highlight the central role of the f-d-σ* relay mechanism in H₂ activation on f-electron oxides.