Hydroxyl speciation and water phases on sub-nanometer Ru10 clusters supported on anatase TiO2(101)

Ab initio thermodynamics of water on flat Ru(0001) predicts full dissociation to atomic O and H, with no stable hydroxyl, contradicting the experimental consensus that OH is an essential intermediate on Ru/TiO 2 hydrodeoxygenation catalysts. Grand-canonical sampling of a sub-nanometer Ru 10 /TiO 2 (1 0 1) cluster across 0 ≤ n H2O ≤ 9 on pristine and oxygen-vacancy supports shows hydroxyl returning as the dominant adsorbate-O species (45–50%), bound at single-Ru top sites on under-coordinated periphery atoms with no flat-metal analogue. An OH-rich 6-water phase is the predicted operational state across the 400–530 K aqueous-catalysis window. Hydrogen spillover to the support is an order of magnitude more probable on the reduced support at saturation coverage, but lies below the sampling resolution at the operational six-water plateau on both supports; Bader analysis shows the vacancy also changes its character, with the transferred hydrogen arriving hydridic on an under-coordinated Ti rather than protic on a lattice oxygen. The active site is the cluster periphery and the relevant adsorbate is OH, not the fully dissociated O + H of the flat-Ru paradigm.