Anchoring Small Au Clusters on the Dehydroxylated and Hydroxylated SiO2 α-Quartz (001) Surface via Ti-Alloying

The adsorption of AuxTiy (x + y = 4, 5) bimetallic clusters on the hydroxyl-free and fully hydroxylated alpha-quartz (001) surfaces was investigated via density functional theory calculations including van der Waals (vdW) forces. The pure gold clusters adsorb only weakly on the silica surfaces via vdW forces. The interaction with the silica surfaces changes dramatically when Au atoms of the clusters are replaced by Ti. For y >= 2, the clusters react with the surface via oxygen spillover (on the hydroxyl-free surface) and hydrogen spillover (on the hydroxylated surface). These bimetallic clusters with more than one Ti atom exhibit very large adsorption energies in the range of 3 to 9 eV. The substitution of only one Au atom by Ti (y = 1) results in an intermediate effect on the adsorption strength. It may, however, be sufficient for the in situ formation of gas-phase hydrogen via reaction of the clusters with surface hydroxyl groups, resulting in strongly bound equivalent to Si-O-AuxTiy complexes.