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Surface Reactivity of Ag-Modified Ceria to Hydrogen: A Combined Experimental and Theoretical Investigation

Cite this: ACS Appl. Mater. Interfaces 2020, 12, 24, 27682–27690
Publication Date (Web):June 7, 2020
https://doi.org/10.1021/acsami.0c03968
Copyright © 2020 American Chemical Society
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Abstract

We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce3+ ions, O vacancies, and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria toward H2 oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce3+ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electrochemical devices.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c03968.

  • Fit of Ce 3d spectra from CeO2 film on Pt reduced at 770 K in H2 (Figure S1); Ce 3d and O 1s on pristine CeO2 annealed at 770 K in H2 measured at normal and grazing emission (Figure S2); Ce 3d spectra for pure CeO2 in UHV and H2 and Ag-modified CeO2 in the same environments (Figure S3); Ce3+ concentration obtained by the fitting procedure described in Figure S1 and reported in Figure 1 for the pure CeO2 and Ag-modified films after annealing in UHV and H2 (Table S1); STM images of Ag-modified CeO2 film (Figure S4) (PDF)

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