Session W11: Focus Session: Aerosols, Clusters, Droplets: Physics and Chemistry of Nanoobjects VI: Nanocatalysis, Supported Clusters

Chemical Functionalities in the non-scalable size-regime

The reactivity and optical properties of nanoscale systems are mainly dominated by quantum-size effects that govern the electronic spectra of clusters, by the structural dynamical fluxionality of clusters, as well as by impurity-doping effects. In this talk these fundamental and unique cluster properties will be illustrated by specific examples obtained from molecular beam experiments in the gas phase and experiments on size-selected clusters on surfaces. Where possible, concepts for their understanding are given. Specifically, in the first part of the talk new results on the optical properties of small gold clusters on amorphous silica will be presented, where Cavity Ringdown Spectroscopy is used to measure optical transitions of clusters at surfaces with extremely high sensitivity. In the second part of the talk a new approach for obtaining thermodynamic properties of chemical reactions by using micromechanical devices is introduced and an overview of results on the catalysis of gold clusters is presented. By combining these experimental data with ab-initio calculations, a picture of the peculiar catalytic behavior of gold is now emerging.   

Structure and Reactivity of M$_{x}$S$_{y}^{+}$ (M= Mo,W) Clusters with CO in the Gas Phase: an Experimental and DFT Study

We have recently constructed a cluster deposition apparatus which employs a magnetron sputtering source for generating gas-phase cation clusters of pure metals and metallic compounds. Of particular interest are clusters of the transition metal sulfides, M$_{x}$S$_{y}^{+}$ (M = Mo, W), which are known in their bulk form to be active catalysts for a wide range of heterogeneous reactions. The work reported here examines the gas-phase reactivity of small transition metal sulfide clusters as a first step towards investigations of model catalysts prepared by size-selected deposition. Specifically, we have used density functional theory (DFT) along with mass spectroscopy and gas-phase collision studies to examine the structure and stability of small sulfide clusters, M$_{x}$S$_{y}^{+}$ (x/y = 2/6, 3/7, 4/6, 5/7, 6/8, 7/10, 8/12). The number of metal sites was probed through the formation of adducts with CO, which was introduced into a hexapole collision cell. Calculated binding energy curves for the addition of CO onto available metal sites were compared with experiment to give insight as to which geometry for the bare clusters fit best. Results will be presented for the calculated structures and stabilities of the prominent clusters as well as their adducts.   

Oxidation of CO on various Fe$_{2}$O$_{3 }$ surfaces: A Theoretical Study

Recent experiments indicate that Fe$_{2}$O$_{3}$ nanoparticles can oxidize CO to CO$_{2}$ in the absence or presence of O$_{2}$. Depending on the size and experimental conditions, a Fe$_{2}$O$_{3}$ nanoparticle can have different faces that correspond to bulk surfaces of various orientations; which in turn can affect the catalytic activity of the nanoparticle.~ Hence,~an understanding of the reaction pathways, transition barriers, and the feasibility of CO oxidation on bulk surfaces of different orientations are critical to optimize the selection of nanoparticles. Theoretical investigations of the oxidation of CO on various Fe$_{2}$O$_{3}$ surfaces using gradient corrected density functional approach has been carried out. BPW91 functional form and double numeric basis set (DNP), as implemented in Dmol3 code are employed here. Different Fe$_{2}$O$_{3}$ (corundum) faces/surfaces are modeled by a cluster where the edge atoms are saturated by H atoms to simulate the effect of the infinite surface. Results corresponding to the reaction of CO with (100) and oxygen terminated (0001) surfaces at various surface sites; oxidation of CO, both in the presence as well as absence of oxygen, will be presented and discussed.   

Efficient Low-Temperature Oxidation of Carbon-Cluster Anions by SO$_{2}$

Carbon-cluster anions, C$_{N}^{-}$, are very reactive toward SO$_{2}$ (sticking probability of 0.012 $\pm $ 0.005 for C$_{27}^{-}$ at 25 $^{o}$C), in contrast to their inertness toward other common atmospheric gases and pollutants. In flow-reactor experiments at ambient temperature and near atmospheric pressure, primary adsorption of SO$_{2}$ by the carbon cluster anions, N = 4 -- 60, yields C$_{N}$SO$_{2}^{-}$ or C$_{N-1}$S$^{-}$. The inferred elimination of neutral CO$_{2}$ is also detected as meta-stable decay in collision-induced dissociation. At higher temperatures, the reaction of SO$_{2}$ with nascent carbon clusters yields C$_{N-1}$SO$^{-}$ as well as undetected CO. Such carbon clusters are formed in sooting flames and may act as nuclei for the formation of primary soot particles, and serve as models for the local structural features of active soot particle sites for black-carbon soot. The facile generation of reactive carbon-sulfide and --sulfinate units may therefore have implications for understanding the health and environmental effects attributed to the coincidence of soot and SO$_{2}$.   

Oxidation of magnesia-supported Pd-clusters leads to the ultimate limit of epitaxy with a catalytic function

Oxide-supported transition metal clusters and nanoparticles have recently attracted significant attention due to their important role as components of model-catalysts, sensors, solar-cells and magnetic recording devices. For small clusters, functionality and structure are closely interrelated. However, knowledge of the structure of the bare cluster is insufficient since the interaction with the chemical environment might cause drastic structural changes. Here we show by ab initio simulations based on the density functional theory that the reaction with molecular oxygen transforms small, non-crystalline, magnesia-supported Pd-clusters to crystalline Pd$_{x}$O$_{y}$ nano-oxide clusters that are in epitaxy with the underlying support [1]. Restructuring of the Pd backbone is controlled by the electrostatic interaction with magnesia leading to a strong reduction of the O$_{2 }$dissociation barrier. The supported Pd$_{x}$O$_{y}$ clusters are likely to serve as Mars-van-Krevelen oxygen reservoirs in catalytic oxidation reactions as observed previously for PdO overlayers and demonstrated here for the oxidation of CO molecules. [1] B.Huber, P.Koskinen, H.H\"{a}kkinen, M.Moseler, Nature Materials, advanced online publication 4. Dec. 2005   

Size effects on catalytic activity of supported metal clusters

Size-selected cluster deposition is used to prepare and study model catalysts with size-selected gold and iridium clusters supported on single crystal oxide supports. Chemistry is found to be strongly size-dependent and a combination of ion scattering and xray photoemission is used to probe the origins of the effects.   

The origin of catalytic activity of supported noble-metal nanoparticles

Supported Au nanoparticles $<$5 nm show a sharp rise in the low-temperature catalytic oxidation of CO while the reverse occurs for Pt nanoparticles. Subsequent experimental and theoretical investigations focused on Au nanoparticles and reached conflicting conclusions, attributing the enhanced Au activity to particular nanoscale features such as perimeter sites and low-coordination atoms, or to a particular bilayer structures, independent of particle size. Here we report atomically-resolved Z-contrast images of on TiO$_{2}$- supported Au nanoparticles and theoretical results on an ensemble of rutile- and anatase- supported Au and Pt nanoparticles. We show that high catalytic activity requires that (i) reaction barriers are small, (ii) reaction barriers are smaller than desorption energies of reacting molecules. As nanoparticle size is reduced, attached molecules induce reconstruction of Au clusters, resulting in looser Au-Au bonding and higher desorption energies and smaller reaction barriers for the attached molecules. Pt clusters get tighter by attached molecules, resulting in larger reaction barriers. The bilayer gold structures (M. S. Chen and D. W. Goodman, Science \textbf{306}, 252 (2004)) are extremely active because local reconstruction significantly reduces coordination of Au atoms. This work was supported in part by DOE Grant DE-FC02-01CH11085 and by DOE Division of Chemical Sciences under contract No. DE-AC05-00OR22725 with ORNL.   

Supported Gold and Platinum Clusters: Stability under Vacuum and Hydrogen at Elevated Temperatures; Optical Properties

The Achilles heal of supported clusters remains their low stability at elevated temperatures or when exposed to reactive gases. In this paper, the stability of Au$_{n}$ and Pt$_{n}$ clusters (n=6-10) supported on SiO$_{2}$, Al$_{2}$O$_{3}$ {\&} TiO$_{2}$ films is addressed. The clusters were heated in vacuum and in H$_{2}$ atmosphere, their stability monitored by synchrotron grazing incidence small angle X-ray scattering. Pt clusters supported on Al$_{2}$O$_{3}$ did not undergo sintering in vacuum and when exposed to hydrogen during a lengthy heat treatment reaching 400C; Au clusters on SiO$_{2}$ remained stable up to 350C. These temperatures are considerably higher than those characteristic for the onset of the catalytic activity of these clusters. Results on heat-induced structural isomerization of clusters will be shown. Single-particle UV-VIS spectra of Au-particles obtained by dark-field microscopy will be presented as well.   

Clusters at Surfaces Studied with Low-Temperature STM and UPS

We study the electronic structure of cluster/surface systems and the nature of charge transfer processes between the cluster and the surface. STM/STS and UPS on size selected large clusters in contact with a surface will be combined with photoemission on the same clusters in the gas phase. For metal islands (Au, Pb) on different surfaces (HOPG, Au(111) and Pb(111)) we observed significant energetic shifts in UPS if the islands were decoupled from the surface by a thick rare gas layer and different materials for the substrate and the islands were used [1]. In addition we measure mass spectra of size selected Ag clusters with a cluster machine consisting of a magnetron sputter gas aggregation source, a differential pumping stage with a cryo pump and a high transmission infinite range mass selector. In current experiments we extend these studies to the deposition of mass selected clusters on rare gas layers and different substrate systems. For these samples low-temperature STM and UPS will be compared with photoemission on the same clusters in a free cluster beam. [1] T. Irawan, D. Boecker, F. Ghaleh, C. Yin, B. v. Issendorff and H. H\"{o}vel, Appl. Phys. A (published online Sept. 2005)   

Size-Selected Au$_{n}$ and Ag$_{n}$ Nanoclusters on Rutlie TiO$_{2}$(110)-1x1 Surfaces Probed by UHV-STM.

Catalysis of the oxidation of CO and small olefins by Au$_{n}$ and Ag$_{n}$ nanoclusters on oxide supports is known to be strongly dependent on the size of the cluster and its interaction with the oxide surface. In our group we have probed this size dependence by depositing size-selected clusters of Ag$_{n}^{+}$ and Au$_{n}^{+}$(n = 1-7) from the gas phase onto single crystal rutile TiO$_{2}$ (110) (1x1) surfaces at room temperature under soft-landing ($<$ 2 eV/atom) conditions. We analyze the clusters on the surface using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and compare the resulting structures with theory. In the case of Au$_{n}^{+ }$, Ag$^{+ }$and Ag$_{2}^{+}$ clusters deposited under soft-landing conditions we observe large, sintered clusters indicating high mobility for these species on the surface. For Au$_{n}^{+}$ (n $\ge $ 2) and Ag$_{n}^{+}$ (n $\ge $ 3) clusters deposited under soft-landing conditions, however, we observe a high density of intact clusters bound to the surface and no sintered clusters indicating that these species have very limited mobility on the surface. For the intact clusters we can also observe the binding site and geometry of the cluster in the STM image and compare these with structures calculated using density functional theory (DFT) as well as with structures observed in the gas phase.   

Angle-resolved photoemission of Au clusters on graphite: quantized surface states on cluster facets

We present an experimental study for the electronic properties of metal clusters on surfaces. For the specific case of the confined Shockley surface state on the top (111) facets of gold clusters on graphite [1] we were able to detect the quantized electronic structure with two independent experimental techniques, scanning tunneling spectroscopy (STS) and ultraviolet photoelectron spectroscopy (UPS). Here we present new UPS data and their analysis which shows a quantitative agreement if we compare the density of states, extracted from the STS spectra by averaging over the cluster size distribution, with the UPS spectra using a deconvolution to compensate the dynamic final state effect [2] which leads to a systematic asymmetric broadening of all valance band UPS features [3]. \newline [1] I. Barke, H. H\"{o}vel, Phys. Rev. Lett. 90, 166801 (2003). \newline [2] H. H\"{o}vel, B. Grimm, M. Pollmann, B. Reihl, Phys. Rev. Lett. 81, 4608 (1998). \newline [3] H. H\"{o}vel, I. Barke, Prog. Surf. Sci., submitted for publication.