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The growth, surface composition, and chemical activity of bimetallic Pt-Au clusters on TiO 2 (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 CO 2 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 ∼100% Au at the surface before CO exposure, given the low probability for ensembles of Pt sites to form at the surface.

The thermal decomposition of hydroxyl-terminated generation-4 polyamidoamine dendrimer (G4OH) films deposited on Au surfaces has been compared with decomposition of the same dendrimer encapsulating an approximately 40-atom Pt particle (Pt-G4OH). Infrared absorption reflection spectroscopy studies showed that, when the films were heated in air to various temperatures up to 275 degrees C, the disappearance of the amide vibrational modes occurred at lower temperature for the Pt-G4OH film. Dendrimer decomposition was also investigated by thermogravimetric analysis (TGA) in both air and argon atmospheres. For the G4OH dendrimer, complete decomposition was achieved in air at 500 degrees C, while decomposition of the Pt-G4OH dendrimer was completed at 400 degrees C, leaving only platinum metal behind. In a nonoxidizing argon atmosphere, a greater fraction of the G4OH decomposed below 300 degrees C, but all of the dendrimer fragments were not removed until heating above 550 degrees C. In contrast, Pt-G4OH decomposition in argon was similar to that in air, except that decomposition occurred at temperatures approximately 15 degrees C higher. Thermal decomposition of the dendrimer films on Au surfaces was also studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum conditions. Heating the G4OH films to 250 degrees C during the TPD experiment induced the desorption of large dendrimer fragments at 55, 72, 84, 97, 127, 146, and 261 amu. For the Pt-G4OH films, mass fragments above 98 amu were not observed at any temperature, but much greater intensities for H(2) desorption were detected compared to that of the G4OH film. XPS studies of the G4OH films demonstrated that significant bond breaking in the dendrimer did not occur until temperatures above 250 degrees C and heating to 450 degrees C caused dissociation of C=O, C-O, and C-N bonds. For the Pt-G4OH dendrimer films, carbon-oxygen and carbon-nitrogen bond scission was observed at room temperature, and further decomposition to atomic species occurred after heating to 450 degrees C. All of these results are consistent with the fact that the Pt particles inside the G4OH dendrimer catalyze thermal decomposition, allowing dendrimer decomposition to occur at lower temperatures. However, the Pt particles also catalyze bond scission within the dendrimer fragments so that decomposition of the dendrimer to gaseous hydrogen is the dominant reaction pathway compared to desorption of the larger dendrimer fragments observed in the absence of Pt particles.

It has been well established that bimetallic systems can exhibit activity different than that of pure metals, and there are many examples in the catalysis literature illustrating the ability of a second metal to promote the desired catalytic activity and selectivity. 1À6 Consequently, there is much interest in basic understanding of the chemical activity on bimetallic surfaces in order to develop catalysts with properties that can be tuned by changing compositions. In some cases, reactions are promoted via a bifunctional mechanism, in which the reaction requires the different activities provided by each metal. 7À12 Furthermore, electronic effects associated with the formation of new me-talÀmetal bonds may alter surface chemical properties, such as CO adsorption strength, 6,13À16 hydrogenation activity, 3,17À19 dehydrogenation activity, 20,21 and reforming selectivity. 3,22,23 Bimetallic surfaces may also provide mixed-metal sites with activity different from that of the pure metal sites, such as on the SnÀPt alloy surfaces. 24À26 In addition, interactions between the metal clusters and the oxide support may also be used to control surface chemistry on the clusters, with lattice oxygen participating in reactions on the oxide-supported clusters. For example, atomic carbon on Ni clusters recombine with lattice oxygen from the titania support to produce gaseous CO, 27,28 and gaseous products containing lattice oxygen are observed in reactions on metal clusters supported on ceria. 29À34 Also for noble metals on ceria supports, ceria plays an important role in oxygen storage in the three-way catalysts for the conversion of CO, NO x , and hydrocarbons into CO 2 , water, and N 2 . 35À37 In other cases, it has been reported that chemical activity occurs at metal clusterÀoxide interfacial sites. 29,38À42 In order to probe the nature of metalÀmetal and metalÀsupport interactions, we have chosen to study a model system consisting of vapor-deposited NiÀAu bimetallic clusters supported on rutile TiO 2 (110). In this model system, the relationships between morphology, composition, and chemical activity can be explored on a fundamental level. The AuÀtitania system is one of chemical interest due to the unusual catalytic properties of

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