Role of H 2 O Adsorption in CO Oxidation over Cerium-Oxide Cluster Anions (CeO 2 ) n O - ( n = 1-4).

Water (H 2 O) is ubiquitous in the environment and inevitably participates in many surface reactions, including CO oxidation. Acquiring a fundamental understanding of the roles of H 2 O 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 H 2 O molecule on each of the cerium oxide cluster anions (CeO 2 ) n 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 H 2 O-mediated CO oxidation on (CeO 2 ) n O - clusters was rationalized by the orbital compositions (O 2p ) and energies of the lowest unoccupied molecular orbitals of active O •- radicals modified by H 2 O dissociation. Our findings not only provide new insights into H 2 O-mediated CO oxidation but also demonstrate the importance of H 2 O in modulating the reactivity of the O •- radicals.