“…In comparison with the vacancy formation energies for Co3Mo3N in the literature 15 , the corresponding Co3Mo3N(111) surface N vacancy formation energy was calculated to be 1.68 eV, only 0.21 eV less endothermic than for Fe3Mo3N. Whilst this is in agreement with the experimental findings that lattice N in Fe3Mo3N is generally less active compared to Co3Mo3N 38,39,59 , it is notable that the modest difference in the calculated vacancy formation energies for the two compounds suggests that formation of surface N vacancies on Fe3Mo3N is at the very least highly plausible, which is consistent with the experimental observation that bulk lattice N in Fe3Mo3N is inactive, if loss of lattice N is confined only to the surface layers, and therefore accounts for only a negligible loss of total lattice nitrogen from the experimental sample. Hence, this finding suggests that the experimentally observed resistance of Fe3Mo3N to loss of lattice N under high temperature hydrogenation conditions can be attributed to the diffusion of bulk and subsurface lattice nitrogen to the surface being kinetically, rather than thermodynamically, unfeasible.…”