CO-Induced Diffusion of Ni Atoms to the Surface of Ni-Au Clusters on TiO2(110)

The growth, surface composition, and chemical activity of Ni-Au clusters on TiO2(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS), and temperature-programmed desorption (TPD), as well as density functional theory (DFT) calculations and ab initio molecular dynamics simulations. STM images of similar coverages of pure Au and pure Ni on TiO2(110) illustrate that Au clusters are larger with lower cluster densities, indicating that Au is more mobile on the surface than Ni. Consequently, bimetallic Ni Au clusters can be grown by nucleating Au at existing Ni clusters. A sequence of STM images acquired from the same region of the surface after various depositions of Au on Ni seed clusters demonstrates that new clusters of pure Au are not formed on the surface. Furthermore, the size of the existing clusters increases with each Au deposition due to the incorporation of incoming Au atoms. For bimetallic clusters of varying compositions with a total coverage of 0.25 ML, the addition of Ni has a minor effect in suppressing cluster sintering. LEIS studies indicate that the surface of the clusters are Au-rich (85-95% Au) for bulk Au fractions \u3e = 50%. For annealed bimetallic clusters, the presence of Au at the cluster surface does not significantly inhibit the encapsulation of Ni by titania, while surface Au is not encapsulated. TPD investigations of CO. desorption show that CO desorbs from pure Ni clusters in a molecular peak at similar to 400 K and a recombinant peak at similar to 790 K. Although CO does not adsorb onto titania or pure Au clusters at room temperature, significant CO desorption occurs from bimetallic clusters even for surfaces with only a small fraction of Ni at the surface; this result suggests that CO induces the diffusion of Ni to the surface of the clusters. DFT calculations for unsupported Ni1Au121 clusters confirm that in the presence of a CO molecule, the lowest energy structure involves CO bonding to a Ni atom at the surface. In contrast, in the absence of CO, the most stable cluster surface is pure Au with all of the Ni atoms in the interior of the cluster. Ab initio molecular dynamics simulations show that Ni will migrate to the cluster surface at 300 Kin the presence of CO, but Ni migration to the surface does not occur even at higher temperatures in the absence of CO