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

Stefania Benedetti,Rita Magri,Giulia Righi,Annabella Selloni,Paola Luches,Sergio D’Addato

doi:10.1021/acsami.0c03968

Stefania Benedetti, Rita Magri + Show 4 more

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https://doi.org/10.1021/acsami.0c03968

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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.

Highlights

A wealth of research efforts has been devoted to the search of earth-abundant electrode materials that might replace platinum, the best-known catalyst currently employed in industrial applications
We can conclude that the formation of an oxygen vacancy on the Ag-modified surface causes the reduction of only one Ce atom, whereas two Ce atoms are reduced on the pristine surface. These results indicate that loss of surface oxygen due to thermal treatment under ultra-high vacuum (UHV) conditions causes the creation of more Ce3+ cations in pure ceria than in Ag-modified ceria, consistent with the experimental data (Fig. 1c)
In this work we aimed to obtain insight into the reactivity of Ag-modified ceria films towards H2 by investigating the changes occurring on the film surface after thermal treatments in vacuum and in hydrogen partial pressure

Summary

Introduction

A wealth of research efforts has been devoted to the search of earth-abundant electrode materials that might replace platinum, the best-known catalyst currently employed in industrial applications. Motivated by the need to decrease the amount of Pt used in electrochemical devices, numerous studies have shown that Pt-modified CeO2 is a promising electrode material for proton-exchange membrane FCs (PEMFCs), due to its high activity in mediating the formation of protons from gas-phase H26. In this context, it has been proposed that single atom catalysts can provide a viable route to further decrease the concentration of noble metal on the oxide surface[7]

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