  During 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 both experimental work and other ab initio
results. 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 various
exchange-correlation functionals and various H concentrations, and we carefully
track the various relevant H positions. For a dilute H concentration, the
asymmetric oxygen site (Oa) is found to be energetically the most favorable.
With increasing H content, the difference of the H adsorption energies between
the terminal (Ot) and the Oa sites becomes smaller. Previous contradictions are
ascribed mostly to the disregard of the H position along the Oa-Oa zig-zag
chains in the intrabilayer region. Using the modern non-empirical
strongly-constrained and appropriately-normed (SCAN) meta-generalized gradient
approximation (GGA), the dilute-limit H adsorption energies are obtained as
-2.89 eV/(H atom) and -2.97 eV/(H atom) in the bulk and on the surface,
respectively, and the activation energy of H diffusion between the Oa sites as
0.11-0.15 eV/(H atom), consistent with previous experiments.
