Chemisorption - Induced 2 D 3 D 2 D Structural Transitions in Gold Heptamer : ( CO ) n Au 7 ( n = 1 4 )

S the discovery of high catalytic activities for CO oxidation by gold nanoparticles, 3 a growing list of important reactions has been found to be catalyzed by gold nanoparticles, such as epoxidation, hydrogenation/reduction, C Cbond formation, and water gas shift. Extensive research has been devoted to the CO oxidation reaction. Many factors have been reported to influence the rate of CO oxidation, including cluster size, shape, and oxidation/charge state, 15 as well as support and methods for preparation of gold nanoparticles. 18 Numerous models have been proposed, but the exact mechanisms of CO oxidation are still under debate. 35 Many studies have been focused on CO or O2 binding to structurally well-defined gold clusters. For example, small-sized gold clusters in anionic, neutral, and cationic states show good CO binding ability. Corner or apex atoms are found to be the preferred sites for theCO binding, for example, on Au6 , Au16 , and Au20. 36 40 In general, gold clusters are capable of binding more than one COmolecules. The binding efficiency can be altered, depending on the flexibility of the geometry. Previous studies have shown that the gold clusters are usually robust against adsorption of a single CO molecule without major structural transformation, although adsorption of multiple COs on gold clusters has been observed to induce significant structural changes. For example, Au6 can maintain its triangular structure even when it binds up to three COmolecules. To our knowledge, there have been few instances where a coinage metal cluster exhibits a significant structural change while binding to a specific ligand. For Au6 , for example, we showed in previous studies that chemisorption of at least four CO molecules was required to induce a major structural change. For Ag5 , Manard et al. have shown that at least three C2H4 molecules are required to induce a major structural change. In this Letter, we show experimental and theoretical evidence that the anionic gold heptamer Au7 is an unusually flexible cluster; upon binding with 1 4 COmolecules, isomers of the cluster can exhibit 2D to 3D and then back to 2D transitions. The PES experiments were carried out using a magneticbottle-type apparatus equipped with a laser vaporization supersonic cluster source. Briefly, the (CO)nAu7 (n = 1 4) cluster anions were generated using laser vaporization of a pure gold target in the presence of a He carrier gas seeded with 2% CO. (CO)nAum clusters were analyzed using a time-of-flight mass spectrometer. The (CO)nAu7 (n = 1 4) clusters of interest were each mass selected and decelerated before being photodetached by a 193 nm (6.424 eV) laser beam. Photoelectrons were collected at nearly 100% efficiency by the magnetic bottle