Exploring the kinetic mechanism of dehydrogenation based on yttrium hydride surfaces: First principles calculations

Abstract

A fundamental understanding of the dehydrogenation mechanisms of yttrium hydride (YH₂) is of great importance for its applications in neutron moderator. In this study, we have employed density-functional theory (DFT) to systematically investigate the surface structures and the relative stabilities of low Miller-index YH₂ facets, with particular emphasis on their dependence on environmental parameters (hydrogen partial pressure $P_{H_2}$and temperature T). Based on the computational results, a surface phase diagram of YH₂ was obtained for a wide $P_{H_2}$ range (300 -1100 K). The YH₂ surfaces tend to be dominated by the stoichiometric (111) facet with the 111-stoi-4H termination and the surface energy of 0.78 J/m². Two additional surfaces, 100-non-2Y and 110-non-2Y1H, will be exposed at the extremely low H partial pressure condition. Then, several possible dehydrogenation pathways were explored to elucidate the dehydrogenation mechanism based on these three surfaces. The results show that the dehydrogenation process in YH₂ primarily involves the formation of interstitial hydrogen atoms within the bulk, followed by the migration of these interstitial atoms from the bulk to the surface, ultimately leading to the release of hydrogen atoms into the surrounding environment.