Formation of oxygen active species in Ag-modified CeO2 catalyst for soot oxidation: A DFT study

In this paper, we analyze from a theoretical point of view, based on the existing literature and on specific first principles DFT calculations, some aspects of the reactivity of Ag-modified CeO2 surfaces used for soot combustion in particulate diesel filters. We have considered various species adsorbed and coadsorbed on the CeO2(1 1 1) stoichiometric and reduced surface: O2 molecules, small carbon clusters, isolated Ag atoms and small Ag clusters. Carbonaceous species react with the ceria surface with formation of oxygenated carbon fragments and subsequent formation of reduced ceria but the mechanism which occurs to replenish the oxygen content of the oxide catalyst is still unclear. The formation of peroxo and superoxo species is discussed. The role of silver in modifying the properties of CeO2(1 1 1) has also been considered. Ag atoms and Ag5 clusters act as donor species, become oxidized and donate the valence electron to the oxide with formation of reduced Ce3+ ions. A charge transfer in the opposite direction occurs when Ag atoms are bound to O vacancies. In the second case, we notice that in the presence of gas-phase oxygen the formation of superoxo species can be mediated by small Ag particles. However, the stronger affinity for oxygen of the vacancy compared to Ag always leads as a final result to filling the cavity by ambient oxygen.