Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F1: Focus Session: Surface Chemistry and Catalysis III |
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Sponsoring Units: DCP Chair: Feng Tao, University of Notre Dame Room: 103/105 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F1.00001: Parahydrogen Induced Polarization Reactions on Supported Metal Nanoparticle Catalysts Clifford Bowers, Ronghui Zhou, Wei Cheng, Luke Neal, Helena Hagelin-Weaver ALTADENA type parahydrogen induced polarization (PHIP) signals were acquired using various oxide (e.g. Al$_{2}$O$_{3}$, TiO$_{2})$ supported Pt and Ir nanoparticle catalysts in the hydrogenation of small alkenes. The hydrogenation reactions were performed using a home-built mini-reactor installed on top of a 9.4 Tesla superconducting NMR magnet. Precise control of the gas mixture (i.e. alkene, para-H$_{2}$ and carrier gas) was achieved using mass flow controllers. Hyperpolarized adducts were delivered down the magnet bore from the reactor to the NMR probe for NMR detection. For certain substrates, long-lived hyperpolarized states were generated and detected. The PHIP signal enhancement and pairwise H$_{2}$ addition selectivity was measured as a function of the reactant partial pressures and reaction temperature. Activation energies and reaction kinetics were obtained for both pairwise and random addition. The reaction conditions and metal nanoparticle characteristics favoring pairwise selectivity were thus identified. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F1.00002: Nanosecond Dynamics in Pt Nanoparticles F.D. Vila, J.M. Moore, J.J. Rehr Understanding the physical and chemical behavior of supported catalysts is of fundamental and technological importance. However, due to the complex nature of their structure and dynamics at operando temperatures, their nanoscale behavior remains poorly understood. We have shown that DFT/MD calculations provide fundamental insight into the few ps dynamic structure of the nanoparticles, but such methods can be very computationally intensive.\footnote{F. Vila \textit{et al.}, Phys. Rev. B {\bf78}, 121404(R) (2008).}\footnote{F. Vila \textit{et al.}, J. Phys. Chem. C {\bf117}, 12446 (2013).} In order to examine relaxation dynamics in the ns regime here we present finite temperature MD simulations based on a modified Sutton-Chen (SC) model potential, supplemented with Lennard-Jones potentials for the interaction with the support. We find that bulk SC parameters tend to produce nanoparticles with less fluxional dynamics than those in ab initio simulations. To address this issue, we have determined modified SC parameters that capture the DFT dynamics. Nanosecond simulations reveal regimes controlled by internal particle melting and activation of surface mobility. The approach is illustrated for nano-catalysts of Pt/$\gamma$-alumina and compared with ab initio DFT/MD results. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F1.00003: First principles molecular dynamics of metal/water interfaces under bias potential Luana Pedroza, Pedro Brandimarte, Alexandre Rocha, Marivi Fernandez-Serra Understanding the interaction of the water-metal system at an atomic level is extremely important in electrocatalysts for fuel cells, photocatalysis among other systems. The question of the interface energetics involves a detailed study of the nature of the interactions between water-water and water-substrate. A first principles description of all components of the system is the most appropriate methodology in order to advance understanding of electrochemically processes. In this work we describe, using first principles molecular dynamics simulations, the dynamics of a combined surface(Au and Pd)/water system both in the presence and absence of an external bias potential applied to the electrodes, as one would come across in electrochemistry. This is accomplished using a combination of density functional theory (DFT) and non-equilibrium Green's functions methods (NEGF), thus accounting for the fact that one is dealing with an out-of-equilibrium open system, with and without van der Waals interactions. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F1.00004: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F1.00005: Manipulation of Single Molecular Hydrogen in a Size-tunable Nanogap Hui Wang, Haiyan He, Shaowei Li, Wilson Ho, Ruqian Wu The determination of weak bonds in physisorption systems remains as a major challenge for modern density functional approaches. In addition, it becomes important to use the tip of scanning tunneling microscope (STM) to manipulate chemical bonds. Here we study the adsorption geometries, translational and rotational motions, and vibrations of a single H2 molecule trapped in the gap of STM-tip and Au (110) reconstructed surface, using the density functional theory calculations. The tip-substrate separation is used as an adjustable parameters. We find that the stable adsorption geometry, H2 bondlength, H-H stretching frequency, and H2-Au bouncing frequency strongly depends on the tip-substrate distance. Computational results agree well with STM data, both indicate the strong role of STM tip on the behavior H2 motions. The new insights established through this work are useful for the understanding of puzzling observations, and should be applicable for the analysis of other physisorption systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F1.00006: A model of the ideal molecular surface Bryan Henson, Laura Smilowitz We utilize two manifestations of the phenomena of the quasiliquid phase on the surface of molecular crystals to formulate a universal thermodynamic theory describing the thickness of the layer as a function of the liquid phase activity. We use direct measurements of the liquid thickness as a function of temperature and measurements of the acceleration of thermal decomposition as a function of temperature approaching the melting point to illustrate the mechanism. We show that given the existence of a liquid phase below the melting point the ideal liquid activity is necessarily a fixed function of the free energies of sublimation and vaporization. We use this activity to create a reduced formula for the liquid thickness generally applicable to the molecular surface. We provide a prediction of the mechanism and kinetics of quasiliquid formation and show that the phase exists as a metastable kinetic steady state. We show that to first order the principle controlling feature of the system is the configurational entropy of the liquid/solid interface, rather than the specifics of the surface potential energy. This is analogous to other bulk colligative phenomena such as ideal gas and solution theories, and is thus an ideal, universal formulation of inherent, thermodynamically driven, surface disorder. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F1.00007: Origin of unexpected attractive adsorbate-adsorbate interactions between negatively charged ions on Mg (0001) surfaces Su-Ting Cheng, Mira Todorova, J{\"o}rg Neugebauer Electrostatic repulsion usually leads to an increase in work function and a decrease in binding energy when the coverage of electronegative elements adsorbed on a metal surface is increased. Using density-functional theory we investigate the adsorption of \{N,O,F,Cl\} on Mg$(0001)$ and find that only Cl complies with this expectations. All the considered $2^{\rm nd}$ row elements cause a decrease in work-function and an increase in binding energy with increasing coverage. We show that these counterintuitive phenomena can be understood in terms of an efficient embedding of the adsorbate atoms into the unusually large electronic surface spill-out of Mg$(0001)$. The described mechanism is based on purely electrostatic arguments and thus expected to be a generic feature on surfaces consisting of highly electropositive elements. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F1.00008: Multiscale Studies of Surface Chemistry of Catalysis: Au-Ag alloys Invited Speaker: Cynthia Friend Multiscale studies of gold-based materials spanning materials complexity and gas phase pressure demonstrate the predictive value of fundamental studies for selective oxidative transformations of organic oxygenates (alcohols and aldehydes) on gold-based materials. Model studies on single crystal surfaces under ultrahigh vacuum are used to understand surface structure and reaction mechanism on a molecular scale. The model studies use a combination of spectroscopy and imaging with scanning tunneling microscopy. The principles are derived from these used as a basis for predicting and understanding reactivity on complex, nonporous gold catalysts under steady-state conditions pressure. These nonporous materials are Au alloys with $\sim$3{\%} Ag. This work illustrates the predictive value of model studies and the potential for improving reaction selectivity in important catalytic reactions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F1.00009: Fe adsorption on the hematite (0001) and magnetite (111) surface Adam Kiejna, Tomasz Pabisiak A detailed ab initio investigation of the structural, electronic and magnetic properties of Fe-atom adsorption on the hematite (0001) and magnetite (111) surfaces is presented. Spin-polarized density functional theory calculations are applied accounting for strong electron correlation effects by including a Hubbard-type on-site Coulomb repulsion (the DFT+U approach). For each oxide surface, the adsorption on two terminations has been studied: one terminated with Fe and the other with oxygen. The binding sites and coordination geometry of Fe adatoms are identified. Different adatom coverages were considered. The Fe atoms bind strongly to the Fe-oxide surfaces and induce large changes in their near surface geometry, and the electronic and magnetic properties. The binding of Fe is distinctly stronger at the O- than at the Fe-terminated surfaces of both oxides. The resulting adsorption energetics, structure and bonding are discussed based on the calculated local density of states and electron charge transfer. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F1.00010: Temperature programmed desorption of a binary gas mixture Nayeli Zuniga-Hansen, M. Mercedes Calbi Temperature programmed desorption (TPD) is an experimental technique that is widely used to determine the adsorption properties of a surface. Many existing theoretical studies have focused on the desorption of a single gas species, but the desorption of binary mixtures is a subject that has been relatively less explored. We perform computer simulations of the thermal desorption of binary gas mixtures using a kinetic Monte Carlo scheme. We start with a simple structure formed by a single line of adsorption sites and two species of adsorbates which bind to the surface with different energies. By varying the initial surface coverage, the particle-particle interactions and the concentration of the different adsorbates, we study the kinetics of desorption of the mixture and compare our results to available experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F1.00011: Compton Scattering from Bulk and Surface of Water Wenjie Wang, Ivan Kuzmenko, David Vaknin Elastic and Compton scattering at grazing angle X-ray incidence from water show distinct behaviors below and above the critical angle for total reflections suggesting surface restructuring of the water surface.~ Using X-ray synchrotron radiation in reflectivity mode, we collect the Thomson and Compton scattering signals with energy dispersive detector at various angles near the normal to surface as a function of the angle of incidence. Analysis of the ratio between the Thomson and Compton intensity above the critical angle (which mainly probes bulk water) is a constant as expected from incoherent scattering from single water molecule, whereas the signal from the surface shows strong angular dependence on the incident angle. Although we do not fully understand the phenomena, we attribute the observation to more organized water at the interface. [Preview Abstract] |
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