Role of H₂O Adsorption in CO Oxidation over Cerium-Oxide Cluster Anions (CeO₂)O^- (n = 1-4)

Water (H₂O) is ubiquitous in the environment and inevitably participates in many surface reactions, including CO oxidation. Acquiring a fundamental understanding of the roles of H₂O molecules in CO oxidation poses a challenging but pivotal task in real-life catalysis. Herein, benefiting from state-of-the-art mass-spectrometric experiments and quantum chemical calculations, we identified that the dissociation of a H₂O molecule on each of the cerium oxide cluster anions (CeO₂)O^- (n = 1-4) at room temperature can create a new atomic oxygen radical (O^•-) that then oxidizes a CO molecule. The size-dependent reactivity of H₂O-mediated CO oxidation on (CeO₂)O^- clusters was rationalized by the orbital compositions (O_2p) and energies of the lowest unoccupied molecular orbitals of active O^•- radicals modified by H₂O dissociation. Our findings not only provide new insights into H₂O-mediated CO oxidation but also demonstrate the importance of H₂O in modulating the reactivity of the O^•- radicals.