Au, Pt, and Au−Pt clusters were grown on TiO2(110) at room temperature and studied by scanning tunneling microscopy. For the same metal coverages, the deposition of pure Pt produces smaller clusters and higher cluster densities compared to pure Au because of the greater mobility of Au on the surface. Heating the surface causes greater sintering of the Au clusters compared to Pt; this behavior is explained by the stronger metal−metal bonds for Pt and the fact that atom detachment is the rate-limiting step in cluster sintering. For the deposition of 0.024 ML of Pt followed by 0.072 ML of Au, bimetallic clusters are formed from the nucleation of Au at existing Pt clusters, whereas the reverse order of deposition results in pure Pt clusters and pure Au clusters coexisting on the surface. The presence of Pt in the bimetallic Pt−Au clusters inhibits sintering, and the average size of the clusters after annealing decreases with increasing Pt composition. Low energy ion scattering experiments demonstrate that the deposition of Au on Pt does not produce core−shell structures with Au on top. Bulk thermodynamics predicts that the cluster surfaces should be pure Au, given that the Au surface free energy is lower than that of Pt, and Au and Pt are immiscible at the compositions studied here. However, surface compositions of the Au−Pt clusters are 10−30% richer in Pt compared to the overall compositions for total coverages of 0.10 ML and 25−75% Pt. These results demonstrate that Au and Pt atoms can intermix at room temperature and the surface properties of Au−Pt nanoclusters are different from those of the bulk. Grazing angle X-ray photoelectron spectroscopy experiments show that annealed Au−Pt clusters are covered by reduced titania. Annealing the Au−Pt clusters to temperatures above 600 K induces encapsulation of the clusters, but the presence of Au at the cluster surface decreases the extent of encapsulation compared to that of pure Pt clusters.

Combining the results from experimental (STM and IRAS) and theoretical (DFT) studies of water adsorption on gold, we show that the Au(111) surface is hydrophobic. The weak interaction of water with Au induces the formation of a unique double bilayer, which itself is hydrophobic due to the internal locking of all hydrogen bonds within the bilayer and between the two bilayers of the water clusters.

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The reactions of CO, NO, and NO with CO have been studied on Pt, Rh, and bimetallic Pt-Rh clusters deposited on TiO 2 (110). The following four cluster surfaces were investigated: 4 ML of Rh, 4 ML of Pt, 2 ML of Rh + 2 ML of Pt (Rh + Pt), and 2 ML of Pt + 2 ML of Rh (Pt + Rh). Scanning tunneling microscopy studies demonstrated that the surfaces exhibited similar cluster sizes and densities, and low-energy ion scattering experiments showed that the surfaces of the bimetallic clusters were Pt-rich (20-30% Rh) regardless of the order of metal deposition; therefore, both Pt and Rh atoms are capable of diffusing to the cluster surface at room temperature. Notably, heating the surface caused substantial encapsulation of the metal clusters by titania at 700 K and complete encapsulation at 800 K. In temperature programmed desorption experiments, the activities of the Pt and Rh clusters for CO and NO dissociation were found to be higher than those of the (111) surfaces of the corresponding single crystals. For both reactions, the activities of the Rh + Pt and Pt + Rh clusters were identical to each other and intermediate between that of pure Rh and pure Pt. For the reaction of NO with CO, the bimetallic clusters exhibited the greatest production of CO 2 and the highest fraction of NO dissociation. On pure Rh clusters, CO 2 production is inhibited by the preferential adsorption of NO over CO, whereas on the pure Pt clusters, CO adsorption is favored over NO. Only the Pt-Rh surfaces can provide sites for both NO dissociation and CO adsorption that are necessary for facilitating CO 2 formation.