Comprehensive Understanding of H Adsorption on MoO3from Systematic Ab Initio Simulations

Among many of its applications (especially as acatalyst support material), MoO3acts as a medium for hydrogenstorageviahydrogen spillover (H atom donation from proton andelectron sources to a support), for which the energetics of H atomson MoO3are of importance. Despite the seeming simplicity ofhydrogen spillover, previously reportedab initioresults for the Hadsorption on MoO3contradict each other as well as experimentalwork. In the present study, we resolve these discrepancies andprovide a comprehensiveab initiounderstanding of H adsorptionfor MoO3in the bulk and on the surface. To this end, wesystematically investigate several exchange-correlation functionalsand several H concentrations, and we carefully track variousrelevant H positions. The asymmetric oxygen site (Oa) is found tobe energetically the most favorable, consistent with experiments. The contradictions in the previousab initiostudies are ascribedmostly to the disregard of the H position in the Oa-Oazigzag chain in the intrabilayer region and to the use of too small supercellsfor which the interaction between close-by H atoms spuriously makes the Oasite less favorable. Using the modern non-empiricalstrongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation, the dilute-limit H adsorptionenergies are obtained as-2.90 eV/(H atom) and-2.98 eV/(H atom) in the bulk and on the surface, respectively. Further, for bulkH0.25MoO3, the activation energy of the rate-controlling H diffusion path between the Oasites is obtained as 0.22 eV/(H atom),reasonably close to experimental values.

Automated summary

This study provides a comprehensive understanding of H adsorption on MoO3 in the bulk and on the surface, resolving discrepancies from previous ab initio studies. The energetically favorable position for H adsorption is found to be the asymmetric oxygen site. The results have important implications for the understanding of MoO3 as a hydrogen storage material.