First-Principles Assessment of H2S and H2O Reaction Mechanisms and the Subsequent Hydrogen Absorption on the CeO2(111) Surface

The main goal of this study is to assess the resistance of ceria against hydrogen penetration into its bulk, in the context of its application as a protective surface coating against hydrogen embrittlement in metals. We evaluate the reaction mechanisms between the H2S and H2O molecules and the CeO2(111) surface and their kinetic descriptors, using first principles based calculations in the density functional theory framework. Our approach is validated by performing an extensive comparison with the available experimental data. We predict that hydrogen penetration into CeO2(111) is a surface-absorption-limited process with a high-energy barrier (1.67 eV) and endothermicity (1.50 eV), followed by a significantly lower bulk dissolution energy and diffusion barrier (0.67 and 0.52 eV, respectively). We find that the presence of surface vacancies and higher coverages affects significantly the energetics of H2S/H2O adsorption, dissociation, and hydrogen subsurface absorption, facilitating most of these processes and degrading the protectiveness of ceria against hydrogen penetration. The reasons behind these effects are discussed. Overall we expect ceria to hinder the hydrogen incorporation significantly due to the effectively large energy barrier against subsurface absorption, provided vacancy formation is suppressed.