Comprehensive Understanding of H Adsorption on MoO3 from Systematic Ab Initio Simulations

Among many of its applications (especially as a catalyst support material), MoO3 acts as a medium for hydrogen storage via hydrogen spillover (H atom donation from proton and electron sources to a support), for which the energetics of H atoms on MoO3 are of importance. Despite the seeming simplicity of hydrogen spillover, previously reported ab initio results for the H adsorption on MoO3 contradict each other as well as experimental work. In the present study, we resolve these discrepancies and provide a comprehensive ab initio understanding of H adsorption for MoO3 in the bulk and on the surface. To this end, we systematically investigate several exchange–correlation functionals and several H concentrations, and we carefully track various relevant H positions. The asymmetric oxygen site (Oa) is found to be energetically the most favorable, consistent with experiments. The contradictions in the previous ab initio studies are ascribed mostly to the disregard of the H position in the Oa–Oa zigzag chain in the intrabilayer region and to the use of too small supercells for which the interaction between close-by H atoms spuriously makes the Oa site less favorable. Using the modern non-empirical strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation, the dilute-limit H adsorption energies are obtained as −2.90 eV/(H atom) and −2.98 eV/(H atom) in the bulk and on the surface, respectively. Further, for bulk H0.25MoO3, the activation energy of the rate-controlling H diffusion path between the Oa sites is obtained as 0.22 eV/(H atom), reasonably close to experimental values.