Gas-Phase Impurities in Hydrogen: DFT Calculations and Experimental Analysis of Interactions with Gadolinium Hydride Surfaces

Abstract

The interactions between gaseous hydrogen and metallic compounds are of significant basic scientific interest as well as important in various practical applications, including metal hydride formation, energy storage systems, and catalysis. Typically, the initial hydrogenation process involves surface hydrogen dissociation on the native oxide layers followed by atomic hydrogen penetration through those layers into the underlying metallic compounds. Recent research has demonstrated that common impurities (e.g., CO, CO₂, O₂) in the hydrogen stream inhibit the initial hydrogenation process but do not prevent it completely. Even at relatively high impurity concentrations, localized hydride “patches” may form, which in some instances are sufficiently large to disrupt the oxide overlayer. Consequently, during the hydrogenation process hydrogen may interact with two distinct surface types: the native oxide of the metallic compound and the hydride patches formed during the initial hydrogenation stage. This study investigates the impact of gaseous impurities on hydrogen–hydride surface interactions through both experimental and theoretical approaches. Density Functional Theory (DFT) calculations were employed to evaluate the molecular adsorption energies and interaction energies between hydrogen and impurities on the hydride surface. Application of the Langmüir model, incorporating calculated adsorption energies at various pressures and temperatures, indicates complete impurity coverage of the hydride surface, even at very low impurity concentrations. This suggests that, when the hydrogen gas phase contains such impurities, the essential H₂ dissociation step is likely to be inhibited on the regular GdH₂ surface. Furthermore, if dissociation occurs (at surface defects or on the coexisting oxide surface near the hydride), the penetration of the hydridic moiety (H^–δ) through the hydride is generally suppressed due to its capture by impurities, forming new adsorbed species such as ·H₂CO, ·HCO₂, and ·OH. The results of these calculations were experimentally compared. The influence of the above impurities on hydrogenation kinetics was studied using gravimetric analysis combined with X-ray diffraction (XRD) measurements. The results corroborate with the theoretical findings, demonstrating that pure hydrogen yields the highest formation rate, while the presence of low impurity concentrations in the hydrogen stream suppresses and limits the progress of the hydrogenation process on the dihydride surface. The elucidated hydrogenation mechanisms of the impurity effect on the gadolinium hydride development provide valuable insights for the deployment of novel hydrogen storage procedures.