Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q27: Focus Session: Chemical Physics of Clusters: Bridging from Angstrom-scale Clusters to Micron-scale Aerosol Particles IV |
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Sponsoring Units: DCP Chair: Barbara Wyslouzil, Ohio State University Room: 204B |
Wednesday, March 4, 2015 2:30PM - 3:06PM |
Q27.00001: Stability of Phosphine-Ligated Gold Cluster Ions toward Dissociation: Effect of Ligand and Cluster Size Invited Speaker: Julia Laskin Precise control of the composition of phosphine-ligated gold clusters is of interest to their applications in catalysis, sensing, and drug delivery. Reduction synthesis in solution typically generates a distribution of ligated clusters containing different number of gold atoms and capping ligands. Ligand binding energy is an important factor determining the kinetics of cluster nucleation and growth in solution and hence the resulting cluster distribution. Phosphines are popular capping ligands with tunable electronic and steric properties that affect their binding to the gold core. We examined the effect of the number of gold atoms in the cluster and the properties of the phosphine ligand on the ligand binding energy to the gold core using surface-induced dissociation (SID) of mass selected cluster cations produced through electrospray ionization. SID of vibrationally excited ions is ideally suited for studying gas-phase fragmentation of complex ions such as ligated gold clusters. The energetics, dynamics, and mechanisms of cluster ion fragmentation in the absence of solvent are determined through RRKM modeling of time and kinetic energy dependent SID spectra. This approach provides quantitative information on the ligand binding energies in phosphine-ligated gold clusters important for understanding their formation in solution. Furthermore, ligand binding energies derived from SID data provide the first benchmark values for comparison with electronic structure calculations. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q27.00002: Novel Electrocatalysts Prepared by Soft Landing of Mass-Selected Cluster Ions Grant Johnson, Trevor Moser, Nigel Browning, Mark Engelhard, Julia Laskin Metal clusters, which possess size and composition dependent properties, are promising materials for use as catalysts to promote electrochemical reactions in fuel cells. A physical synthesis technique, magnetron sputtering combined with gas-aggregation, has been employed to produce anionic metal clusters in the gas-phase across a range of sizes, shapes, and compositions for mass-selection and deposition onto glassy carbon electrodes. Sputtering of multiple targets in the same region of gas aggregation is demonstrated to produce uncapped binary and ternary alloy clusters with defined composition and morphology that are not accessible through synthesis in solution. Introduction of reactive gases including alcohols, alkanes, and amines into the sputtering region is shown to result in the formation of complex cluster morphologies containing carbon, nitrogen, and oxygen. A suite of cutting-edge characterization techniques is utilized to demonstrate how the size, shape, elemental composition, and surface density of clusters may be tuned through variations in source parameters such as the sputtering power, gas flow rates, and aggregation distance. The catalytic activity of the soft landed clusters towards the oxygen reduction reaction, a critical process occurring in hydrogen fuel cells, is measured using cyclic voltammetry. Alloy clusters containing reduced quantities of precious metals are shown to exhibit promising catalytic activity. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q27.00003: Quantum Monte Carlo study of charged transition-metal organometallic cluster systems Kamil Tokar, Rene Derian, Ivan Stich Using accurate fixed-node quantum Monte Carlo (QMC) methods we study 1D clusters formed by transition metal atoms separated by benzene molecules (TMBz), both positively and negatively charged. TMBz are among the most important $\pi $-bonded organometallics, which, however, often require charged states for experimental studies. We have performed a systematic study of ground-sate spin multiplets, ionization potentials, electron affinities, and dissociation energies of vanadium-benzene cationic and anionic half- and full-sandwiches. By comparison of QMC and DFT results, we find a very strong impact of electronic correlation on properties of these systems, such as dissociation energies, where $\approx $1 eV energy corrections are found. In particular, the anions are unstable at the DFT level and are stabilized only at the QMC level after sophisticated optimization of the trial wavefunction. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q27.00004: Isolation and structural characterization of a silver-platinum nanocluster, Ag$_{4}$Pt$_{2}$(DMSA)$_{4}$ Anthony Pedicini, Arthur Reber, Scott Biltek, Ayusman Sen, Shiv Khanna Cluster assembled materials offer an attractive prospect of making nanoscale materials with tunable characteristics. Here, we report the synthesis, isolation, and characterization of the ligand-protected bimetallic cluster, Ag$_{4}$Pt$_{2}$(DMSA)$_{4}$ (DMSA$=$meso-2,3,-dimercaptosuccinic acid) and its analogue, Ag$_{4}$Pd$_{2}$(DMSA)$_{4}$. The procedure is similar to the one employed previously for the synthesis of Ag$_{4}$Ni$_{4}$(DMSA)$_{4}$. Theoretical studies show that the Pt and Ni atoms are square planar in configuration. Furthermore, the dependence on the optical spectrum due to congener replacement of the transition metal is highlighted. Since the crystal field splitting of 5d orbitals is typically larger than that for 3d orbitals, we show the Pt-based cluster has an optical spectrum that is significantly blue shifted as compared to the Ni-based cluster. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 4:18PM |
Q27.00005: Cluster size matters: Size-driven performance of subnanometer clusters in catalysis, electrocatalysis and Li-air batteries Invited Speaker: Stefan Vajda This paper discusses the strongly size-dependent performance of subnanometer cluster based catalysts in 1) heterogeneous catalysis, 2) electrocatalysis and 3) Li-air batteries. The experimental studies are based on I. fabrication of ultrasmall clusters with atomic precision control of particle size and their deposition on oxide and carbon based supports; II. test of performance, III. \textit{in situ }and \textit{ex situ} X-ray characterization of cluster size, shape and oxidation state; and IV.electron microscopies. \textit{Heterogeneous catalysis}. The pronounced effect of cluster size and support on the performance of the catalyst (catalyst activity and the yield of C$_{n}$ products) will be illustrated on the example of nickel and cobalt clusters in Fischer-Tropsch reaction. \textit{Electrocatalysis.} The study of the oxygen evolution reaction (OER) on size-selected palladium clusters supported on ultrananocrystalline diamond show pronounced size effects. While Pd$_{4}$ clusters show no reaction, Pd$_{6}$ and Pd$_{17}$ clusters are among the most active catalysts known (in in terms of turnover rate per Pd atom). The system (soft-landed Pd$_{4}$, Pd$_{6}$, or Pd$_{17}$ clusters on an UNCD Si coated electrode) shows stable electrochemical potentials over several cycles, and the characterization of the electrodes show no evidence for evolution or dissolution of either the support Theoretical calculations suggest that this striking difference may be a demonstration that bridging Pd-Pd sites, which are only present in three-dimensional clusters, are active for the oxygen evolution reaction in Pd$_{6}$O$_{6}$. \textit{Li-air batteries.} The studies show that sub-nm silver clusters have dramatic size-dependent effect on the lowering of the overpotential, charge capacity, morphology of the discharge products, as well as on the morphology of the nm size building blocks of the discharge products. The results suggest that by precise control of the active surface sites on the cathode, the performance of Li-air cells can be significantly improved [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q27.00006: Theoretical Study of Chemisorption on Small Palladium Clusters Ajit Hira, Frank Naranjo, Felipe Munoz, Danelle Jaramillo We continue our interest in the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, H$_{2,}$ Li and O adsorbates with Pd$_{n}$ clusters (n $=$ 2 thru 20). Transition-metal clusters are specially suited for the study of quantum size effects and for formation of metallic states, and are ideal candidates for catalytic processes. Hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E), hollow (H), on-top (T), threefold sites and fourfold sites. Effects of crystal symmetries corresponding to the bulk structures are investigated. The capacity of Pd clusters to adsorb H atoms will be compared to Ni clusters. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q27.00007: Role of electronic structure and surface structuring effects in the synergistic catalytic activity of Ni-Pd nanoparticles Linn Leppert, Rhett Kempe, Stephan Kuemmel Nickel-palladium nanoalloys show a drastically enhanced catalytic activity in a variety of hydrogenation reactions as compared to pure nickel or palladium nanoparticles. We explore the mixing behavior, electronic structure and magnetic properties of nickel-palladium clusters using density functional theory to gain insight into this synergistic effect. We show that the binding energy of hydrogen to the metal nanoparticle's surface, which can be tuned via the nickel-palladium composition, is the decisive factor determining how efficiently the reaction can take place. The optimal magnitude of the binding energy for intermediate nickel-palladium ratios can be traced back to a purely electronic effect: a balanced hybridization of the hydrogen \textit{s} with the metal particle \textit{d} and \textit{s} states. This explanation not only holds for small clusters, but also for nickel-palladium surfaces. Finally, we demonstrate that catalytic activity on nickel-palladium surfaces might not only benefit from alloying, but also from the formation of nanostructures on surfaces. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q27.00008: Ultrasmall Carbide Nanospheres - Formation and Electronic Properties Petra Reinke, Ehsan Monazami, John McClimon Metallic nanoparticles are highly coveted but are subject to rapid Ostwald ripening even at moderate temperatures limiting study of their properties. Ultrasmall transition metal carbide ``nanospheres'' are synthesized by a solid-state reaction between fullerene as carbon scaffold, and a W surface. This produces nanospheres with a narrow size distribution below 2.5 nm diameter. The nanosphere shape is defined by the scaffold and densely packed arrays can be achieved. The metal-fullerene reaction is temperature driven and progresses through an intermediate semiconducting phase until the fully metallic nanospheres are created at about 350 C. The reaction sequence is observed with STM, and STS maps yield the local density of states. The reaction presumably progresses by stepwise introduction of W-atoms in the carbon scaffold. The results of high resolution STM/STS in combination with DFT calculations are used to unravel the reaction mechanism. We will discuss the transfer of this specific reaction mechanism to other transition metal carbides. The nanospheres are an excellent testbed for the physics and chemistry of highly curved surfaces. [Preview Abstract] |
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