Adsorbate-Induced Changes in the Surface Composition of Bimetallic Clusters: Pt-Au on TiO2(110)

The growth, surface composition, and chemical activity of bimetallic Pt-Au clusters on TiO2(110) have been investigated. Scanning tunneling microscopy (STM) experiments demonstrate that the deposition of Au on Pt clusters results in the formation of bimetallic Pt-Au clusters due to the seeding of the mobile Au atoms at existing Pt nuclei. The composition of the top surface layer of the clusters was studied by low energy ion scattering (LEIS) for bulk compositions ranging from 25%-87.5% Pt with total metal coverages of 0.25 and 0.50 ML. For both coverages, the cluster surfaces consisted of nearly pure Au at Pt compositions of 50% and lower; however, a mix of Au and Pt atoms were found at the cluster surfaces at higher fractions of deposited Pt. These results are consistent with bulk thermodynamics, which predicts a Pt core-Au shell structure based on the lower surface free energy of Au compared to Pt and the large bulk miscibility gap for the two metals. The adsorption of CO on the Pt-Au clusters at room temperature promotes the diffusion of Pt to the surface of the clusters, and this phenomena is most pronounced for the clusters that are initially pure Au at the surface. Density functional theory calculations demonstrate that it is thermodynamically favorable for Pt to diffuse to the cluster surface in order to bind to CO. In contrast, the extent of CO2 production via sequential adsorption of O-2 and CO on the Pt-Au clusters reflects the surface Pt content before adsorption. For CO oxidation, the first step in the reaction is the dissociation of O-2 at Pt sites. Since this process requires more than one contiguous Pt site, it is not surprising that O-2 dissociation cannot occur on the Pt-Au clusters that are similar to 100% Au at the surface before CO exposure, given the low probability for ensembles of Pt sites to form at the surface.