Oxidation and\nReduction of Polycrystalline Cerium\nOxide Thin Films in Hydrogen

Abstract

This study investigates the oxidation state of ceria\nthin films’\nsurface and subsurface under 100 mTorr hydrogen using ambient pressure\nX-ray photoelectron spectroscopy. We examine the influence of the\ninitial oxidation state and sample temperature (25–450 °C)\non the interaction with hydrogen. Our findings reveal that the oxidation\nstate during hydrogen interaction involves a complex interplay between\noxidizing hydride formation, reducing thermal reduction, and reducing\nformation of hydroxyls followed by water desorption. In all studied\nconditions, the subsurface exhibits a higher degree of oxidation compared\nto the surface, with a more subtle difference for the reduced sample.\nThe reduced samples are significantly hydroxylated and covered with\nmolecular water at 25 °C. We also investigate the impact of water\nvapor impurities in hydrogen. We find that although 1 × 10^–6 Torr water vapor oxidizes ceria, it is probably not\nthe primary driver behind the oxidation of reduced ceria in the presence\nof hydrogen.