f-d-σ* Orbital Relay Enables Homolytic H ₂ Dissociation and CeO 2– x Oxidation

Ceria (CeO 2 ) is a widely employed catalytic material in hydrogenation catalysis, and experimental studies have reported that reduced ceria (CeO 2– x ) can be oxidized by H 2; 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 H 2 and the oxidation of the surface. Depending on the spatial arrangement of oxygen vacancies, H 2 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 Ce 3+ ions to Ce 4+ . 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 H 2 dissociation, where the empty Ce-5d orbital serves as an Electronic Relay Orbital (ERO) that bridges the partially occupied Ce-4f states and the H 2 -σ* orbital. These findings identify the active sites for H 2 -induced oxidation of CeO 2– x and highlight the central role of the f-d-σ* relay mechanism in H 2 activation on f-electron oxides.