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, Ov) 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 Ov 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 Ov 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.