Thermodynamic stability of transition metals on the Mg-terminatedMgB2(0001) surface and their effects on hydrogen dissociation and diffusion

Abstract

The hydrogenation of MgB 2 is a critical step in the reversibility of several well-known hydrogen storage reactions. Of the many processes that must occur during rehydrogenation, at least two of them take place near the surface: the dissociation of H 2 molecules and the subsequent diffusion of atomic hydrogen. Using first-principles calculations, we determine the energetic barriers for these processes on the ideal Mg-terminated MgB 2 (0001) surface, as well as on surfaces containing transition metal dopants (Sc -Zn, Y -Cd, Pt, and Au). The calculated dissociation barrier for H 2 on the clean surface is 0.89 eV, and the surface diffusion barrier is 0.17 eV. However, we find examples of dopants that significantly decrease the activation barrier for the dissociation of H 2 . Our calculations suggest that Ni, Cu, and Pd are good catalytic candidates for the surface processes involved in MgB 2 rehydrogenation.

Hydrogen diffusion in bulk MgB2

“…is observed. From the standpoint of reference [45] and this work, the H 2 dissociation rate is low on the pure layered MgB 2 surface, and it is also hard to form the hydrogen defects in the bulk of MgB 2 . Both of the two steps potentially prevent further rehydrogenation.…”

“…In principle, diffusion can occur through any of the de/re-hydrogenated phases in the Mg(BH 4 , but other kinetic processes should also be considered, such as hydrogen dissociation and diffusion on the surface. A prior study of MgB 2 rehydrogenation [45] reported that the activation barrier for hydrogen molecule dissociation and atomic diffusion of hydrogen on the MgB 2 (0001) surface are 0.89 eV and 0.17 eV, respectively. The diffusion barrier on the surface is similar to the migration barrier in MgB 2 bulk, but the dissociation barrier is much larger than both the diffusion barriers.…”

“…Therefore, the low hydrogen diffusion barriers are in favor of MgB 2 rehydrogenation, however, the high dissociation barrier and large hydrogen defect formation energies may hinder the adsorption process. In previous work [45], transition metal as dopants on MgB 2 surface have been studied, and several viable catalysts (Ni, Cu, and Pd) have been proposed to reduce the dissociation barrier without simultaneously increasing the surface diffusion barrier. Since the dissociation barrier can be modified by catalysts, the large hydrogen defect formation energy must be reduced, if the reaction rate is to be improved.…”

Electronic structure and surface properties ofMgB2(0001) upon oxygen adsorption

“…50 These experimental observations offer useful insight and further motivate to investigate a probable metastable oxide phase of B-terminated surface, despite the Mg-terminated surface is found to have lower surface energy in its pristine phase. 14 In our study, we use DFT to investigate the oxygen adsorption on both B-and Mg-terminated surfaces. We performed a full evaluation of the potential energy surface (PES) for oxygen adsorption across the entire unit cell of MgB 2 (0001), as opposed to evaluating the adsorption energy at only a few most-stable sites.…”

“…A stable low-energy surface orientation is more thermodynamically probable and experimentally relevant, 13 therefore, we focus on the MgB 2 (0001) surface as a first step. Interestingly, calculations show the Mg-terminated MgB 2 (0001) is more thermodynamically stable in terms of surface energy, 14 but experimental studies reported in several cases that boron-rich surface phase is observed in MgB 2 , as well. 5,15,16 The present work explores a possible reconstruction pathway of the B-terminated surface triggered by surface-adsorbate interaction, causing formation of complex surface oxide phase.…”