5490
J. Phys. Chem. C 2008, 112, 5490-5500
Bimetallic Pt-Au Clusters on TiO2(110): Growth, Surface Composition, and Metal-Support Interactions J. B. Park, S. F. Conner, and D. A. Chen* Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: July 30, 2007; In Final Form: December 20, 2007
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.
Introduction Nanosized pure Au clusters on titania have demonstrated outstanding activity and selectivity for a variety of reactions, including low temperature CO oxidation and propylene epoxidation.1-5 The surprising activity of Au nanoclusters less than 40 Å in diameter compared to the catalytically inactive bulk Au surfaces has been a topic of much discussion in the literature.3,6,7 It has been reported that Au clusters that are only 2 monolayers in height are the most active for CO oxidation8 and that interactions between the Au clusters and the titania support are important for catalytic activity.3,5,9 There is some evidence that the electronic properties of Au clusters change with size because of quantum size effects,8 but the unusual activity of the small Au clusters has also been attributed to a greater number of active, high coordination sites on the clusters10,11 and electronic interactions between the support and the clusters.12,13 One problem with nanosized Au catalysts is that Au clusters are sintered easily at high temperatures, resulting in not only a loss of surface area but also a loss of activity because of particle size effects. For example, propylene epoxidation on the Au/ TiO2 catalysts can be carried out at or below room temperature, but these catalysts have relatively short lifetimes before deactivation occurs3,14 because of the formation of carbonates and carboxylates from the decomposition of adsorbed propylene oxide.15 Reactivation requires heating in oxygen at temperatures * Corresponding author. Phone: 803-777-1050. Fax: 803-777-9521. E-mail:
[email protected].
of 200-300 °C,16 and further loss of the original activity occurs from sintering. In the work reported here, the sintering of Au-containing clusters on titania is suppressed by the presence of Pt in AuPt bimetallic clusters. On the basis of the bulk immiscibility of Pt and Au at Pt concentrations between 15 and 98%17 and the lower surface free energy of Au compared to Pt,18-20 the bimetallic clusters were expected to exhibit core-shell structures with Au at the surface; if this were the case, then the Au-Pt clusters would be sinter-resistant but would also retain the catalytic activity of pure Au nanoclusters. Furthermore, the ability to tune cluster-support interactions by changing the composition of the bimetallic clusters is explored in this work because it is known that cluster-support interactions play a major role in determining the activity of supported metal clusters. Pt is a metal that interacts strongly with reducible oxide supports like titania and becomes encapsulated with a reduced layer of the metal oxide upon heating.21-23 In contrast, Au does not exhibit strong interactions with titania and does not undergo encapsulation.24 The Au-Pt bimetallic system is also of chemical interest because it has unusual catalytic properties compared to pure Au and pure Pt. For example, Au-Pt clusters have been found to be good electrocatalysts for methanol oxidation reactions in fuel cells, and it has been proposed that the presence of Au suppresses the adsorption of poisoning species such as CO and modifies the strength of the surface adsorption.25 Hexane conversion reactions on Au-Pt catalysts have greater selectivity for light hydrocarbons over skeletal reforming products,26 and
10.1021/jp076027n CCC: $40.75 © 2008 American Chemical Society Published on Web 03/14/2008
Bimetallic Pt-Au Clusters on TiO2(110) higher activity for cyclohexane dehydrogenation to benzene has been observed on Au-Pt alloy surfaces compared to pure Pt or pure Au.27,28 Although earlier studies of Au-Pt surface alloys suggested that the effect of Au was mainly to break up the Pt ensembles needed for hydrocarbon reaction, more recent investigations of CO on Au-Pt clusters indicate that Au may electronically modify the Pt sites.29 The growth, sintering, and surface composition of bimetallic Au-Pt clusters are studied on TiO2(110) using scanning tunneling microscopy (STM), low energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Rutile TiO2(110) is used as a support because this surface has a well-defined structure and is stable toward reconstruction at temperatures below 1100 K; furthermore, TiO2 can be made conductive for STM, LEIS, and XPS experiments by heating in vacuum to create an n-type semiconductor.30 Bimetallic Au-Pt clusters are formed by the deposition of Au on existing Pt clusters, and the presence of Pt in the Au-Pt clusters inhibits the rate of sintering compared to pure Au clusters. The Au-Pt clusters have surface compositions that are very similar to the overall compositions, demonstrating that Au and Pt atoms can mix within the clusters at room temperature. Upon heating, Au-Pt clusters with a higher surface Pt content undergo more extensive encapsulation from strong metal support interactions (SMSI), and therefore the degree to which reduced titania encapsulates the bimetallic clusters can be controlled by varying the relative amounts of Pt present at the cluster surfaces. Experimental Section All experiments were conducted in an ultrahigh vacuum chamber with a base pressure of
