Hydroxyl-defect Complexes on Hydrated MgO Smokes

The dissociative adsorption of water vapor on MgO smokes is studied by infrared spectroscopy and first-principles calculations. The MgO smokes are synthesized in controlled conditions so to display a particularly high concentration of defect-free step edges (4-fold coordinated “4C” sites). Exposure to water vapor results in a set of twinned infrared bands at ν A = 3480 and ν B = 3710 cm −1 —the former having never been observed before—with an intensity ratio I (ν A )/ I (ν B ) ≈ 2. Simulated configurations include water adsorption, for various coverages, on terraces of distinct orientations, at step edges and corners. It is shown that OH stretching frequencies can be robustly correlated with OH bond lengths. The twinned (ν A,αν B ) bands are assigned the hydrogen-bonded (O s 4C −H, Mg s 4C −OH) moiety adsorbed at fully decorated monatomic <100> step edges on (100) terraces. The coordination number of adsorbed OH species is shown not to be sufficient to characterize the physicochemical properties of wet MgO smokes and it is suggested that the hydrated surface can be better described in terms of (OH, MgO) complexes.