Chemical Imaging of Terrace-Based Active Sites on Gold Kevin S. Schneider, ² Kenneth T. Nicholson, ² Bradford G. Orr,* ,‡,§ and Mark M. Banaszak Holl* ,²,§ Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109-1055, and Physics Department and the Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109-1120 Received December 6, 2003 Scanning tunneling microscopy data of a mixed monolayer comprised of a 40:60 ratio of H8Si8O12 and C6H13-H7Si8O12 clusters on gold are presented. The images display a composite monolayer surface with well-defined domain regions of the individual components. Holes present at face-centered cubic (fcc) sites of the starting Au/H 7Si8O12 adsorbate layer indicate the location of active sites for impinging C6H13- H7Si8O12 clusters. Adsorption of a C6H13-H7Si8O12 cluster likely yields a mobile hydrogen atom available to recombine with and desorb an adjacent H8Si8O12 cluster. Hydrogen atom diffusion along substrate [121] directions is the proposed pattern formation mechanism of the mixed monolayer. Imaging of the spherosiloxane cluster domains identifies a novel terrace-based active site located in the fcc regions of the Au(111) 23 ×x3 surface reconstruction. Introduction Gold has long been described as a noble metal and largely considered unreactive relative to most elements. 1 Contributing to its chemically inert reputation are the high dissociation energy and low chemical adsorption energy of molecular hydrogen on gold. 2 Indeed, in reviews published in the 1980s, gold is exceptional among the metals for its lack of utility with respect to heterogeneous catalysis. 3-5 However, the popular adage coined by George Bernard Shaw stating “the golden rule is that there are no golden rules” 6 holds especially true considering that a number of significant results delineating catalytic gold chemistry have been reported in the last two decades. In particular, gold has demonstrated catalytic reactivity with respect to combustion of CO and a variety of hydrocarbons in addition to several hydrogenation reactions. 7-15 These reports ascribe gold particle size, gold/support interactions, and gold particle edge effects as critical to the observed catalytic chemistry. Ambient-temperature Si-H bond activation by gold was first observed in 1999. 16 In this instance, H 8 Si 8 O 12 and H 10 Si 10 O 15 spherosiloxane clusters were observed to form chemisorbed layers on the Au(111) surface. 17,18 X-ray photoelectron spectroscopy (XPS) and reflection absorption infrared spectroscopy (RAIRS) studies indicated a precur- sor kinetic model for spherosiloxane cluster chemisorption. Spectroscopic and chemical evidence suggest that ap- proximately 10% of the clusters desorb upon evacuation of excess cluster pressure from the ultrahigh vacuum (UHV) reaction chamber. Cluster desorption forms holes in the remaining chemisorbed layer, which act as reactive sites for subsequent H 8 Si 8 O 12 /H 8 Si 8 O 12 ,H 8 Si 8 O 12 /D 8 Si 8 O 12 , H 8 Si 8 O 12 /C 6 H 13 -H 7 Si 8 O 12 , 17,18 and H 8 Si 8 O 12 /C 8 H 17 SiH 3 19 molecular exchange and displacement reactions. Spec- troscopic data indicate that silsesquioxane 18 and alkyl- silane 19 molecules actively displace H 8 Si 8 O 12 up to ∼60% upon exposure to the Au/H 7 Si 8 O 12 adsorbate layer. In the case of the monosubstituted C 6 H 13 -H 7 Si 8 O 12 clusters, XPS data display a 15% increase in O(1s) core-level peak area indicating that, in addition to undergoing displacement reactions with H 8 Si 8 O 12 ,C 6 H 13 -H 7 Si 8 O 12 clusters also “fill in” the available adsorption sites (i.e., the adsorbate layer holes) initially present on the Au/H 7 Si 8 O 12 surface. 18,20 Molecular structures for H 8 Si 8 O 12 and C 6 H 13 -H 7 Si 8 O 12 , the two clusters utilized in this paper, are provided in Figure 1. Schematic structures of H 8 Si 8 O 12 and C 6 H 13 - H 7 Si 8 O 12 chemisorbed to Au(111) are provided as a part of Scheme 1. Submonolayer exposures to ambient atmosphere poison the Au(111) surface to H 8 Si 8 O 12 chemisorption reactions even though spectroscopic characterization indicates that the substrate is “clean”. This suggests that active sites present on the Au(111) surface may also play a key role in the chemical reactivity with spherosiloxane clusters. 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Langmuir 2002, 18, 8116-8122. 2250 Langmuir 2004, 20, 2250-2256 10.1021/la0363027 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/11/2004