O2Activation over Ag-Decorated CeO2(111) and TiO2(110) Surfaces: A Theoretical Comparative Investigation

Periodic spin-polarized hybrid density functional theory calculations have been performed to investigate the reactivity of pristine, O-defective, and Ag-decorated CeO2(111) and TiO2(110) surfaces with a small Ag10 cluster toward O2. The adsorption of O2 and its subsequent dissociation have been studied in order to provide a better understanding of the role of the oxide, the metallic nanoparticle, and their interaction in the reactivity of composite metal/metal oxide materials toward O2, as potential catalysts to this reaction. Structural, energetic, electronic, and vibrational properties of all species involved in the different reaction paths considered have been fully characterized. On the stoichiometric surfaces, Ag10 is oxidized and reduces surface Ce4+/Ti4+ ions, while on the O-defective surfaces, the adhesion of silver is promoted only on CeO2 but unfavored on TiO2. On the other hand, on the silver-free supports, O2 strongly adsorbs at vacancies and preferentially reduces to peroxide. When no O vacancies are considered on the Ag10-decorated supports, the net positive charge on Ag10 actually prevents the adsorption and reduction of O2. Instead, when O vacancies are included, two reaction pathways are observed; oxygen molecules can weakly absorb on the silver cluster as a superoxide moiety or strongly adsorb at the vacancy as peroxide. The dissociation of the O-O bond of the peroxide is favored both from the thermodynamic and kinetic points of view in silver-decorated surfaces, in contrast with the silver-free cases. In addition, Ag10/CeO2 shows higher activity toward the O2 adsorption and dissociation than Ag10/TiO2, which can be related both to the higher ionicity and superior electron storage/release ability of ceria with respect to titania, thus leading to the weakening of the O-O bond and providing lower activation barriers for oxygen reduction. These results deepen the current understanding of the reactivity of metal/metal oxide composites toward O2, especially elucidating how the surface stoichiometry affects the charge state of the metal clusters, and hence the reactivity of these interfaces toward O2, with potential important consequences when such composites are considered for catalytic applications.