Reactive Surface Explored by NAP-XPS: Why Ionic Conductors Are Promoters for Water Gas Shift Reaction

Abstract

Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments have been carried out in N₂ and N₂–H₂O atmospheres on a Pt-based catalyst physically mixed with an Eu-doped ZrO₂ ionic conductor as a function of temperature under realistic conditions of the water gas shift (WGS) reaction. This work aims to demonstrate the significant effect of having active H₂O on the ionic conductor surface at reaction temperatures to provide it to Pt metal sites. The ionic conductor, Eu-doped zirconia matrix, presents defects (oxygen vacancies, O_v) that allows upon H₂O dissociation the formation of a hydrogen-bonded molecular water layer favoring diffusion through a Grotthuss mechanism below 300 °C. In the presence of H₂O, the O_v are occupied by hydroxyl species as observed in the Eu 4d spectra, which differentiate two types of Eu oxidation states. The Eu³⁺-to-Eu²⁺ atomic ratio increases with the occupancy of the O_v by hydroxyls. Moreover, while the Pt-based catalyst alone is unable to create Pt–OH bonds, the physical mixture of the Pt-based catalyst and the ionic conductor allows the formation of Pt–OH bonds from room temperature up to 300 °C. These data demonstrate that the increase in molecular water concentration on the ionic conductor surface up to 300 °C acts as a reservoir to provide water to the Pt surface, enhancing the catalyst performance in the WGS reaction, supporting the importance of the surface H₂O concentration in the reaction kinetics.