Lattice oxygen reverse spillover from polar MgO(111) to the surface of supported metal catalysts: a DFT study

Spontaneous lattice oxygen reverse spillover from MgO(111) substrate to the surface of supported transition metal catalysts can influence the mechanisms and activities of this emerging class of catalysts for organic oxidation, hydrogen evolution, and ammonia synthesis or decomposition. In this study, we employed density functional theory calculations to analyze various models of metal/MgO catalysts to understand how metal size, MgO facet, metal species, and exposure to a hydrogen atmosphere could affect the occurrence of such lattice oxygen reverse spillover. Energetic analysis of Rux (x = 10, 4, 1) on MgO(111), (110), and (100) facets shows that lattice oxygen reverse spillover becomes more favorable with decreasing Ru size, but only on the MgO(111) facet. Electronic structure analyses reveal that the charge redistribution from Ru to the surface O species of MgO(111) stabilizes the polar substrate. Consistently, single atom models of all 30 d-block metals on MgO(111) reveal that metal species donating more electrons to the MgO(111) surface during lattice oxygen reverse spillover also facilitate more facile lattice oxygen reverse spillover, from both thermodynamic and kinetic perspectives. Particularly, lattice oxygen reverse spillover is maximized for Cr, Tc, and W among 3d, 4d, and 5d metals, respectively, and becomes less favored for metals with inhibited electron-donating capabilities due to either very few valence electrons or substantial electronegativity. Moreover, we demonstrate that chemisorption of atomic hydrogen intermediates on the MgO(111) substrate, known to occur on metal/MgO(111) catalysts applied under hydrogen-rich conditions, inhibits lattice oxygen reverse spillover. Hence, whereas lattice oxygen reverse spillover on metal/MgO(111) catalysts plays a significant role when applied under oxidative reaction conditions, it is inhibited under reducing hydrogen-rich environments. This study highlights lattice oxygen reverse spillover on MgO(111)-supported metal catalysts and provides a detailed mechanistic understanding that will be important to consider for the rational design and optimization of these MgO(111)-based catalysts.