Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V32: Surface Reactions and Dynamics |
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Sponsoring Units: DMP Chair: Alexey Zayak, Lawrence Berkeley National Laboratory Room: E142 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V32.00001: Scanning Tunneling Microscopy Study of CO diffusion on stepped Pt(111) surface Kedong Wang, Xudong Xiao A time-dependent tunneling current mode based on scanning tunneling microscopy/spectroscopy (STM/STS) was used to study the tracer diffusion of CO molecules along steps and on terraces of Pt(111). The results show that the hopping rate of CO molecules along steps is about 10 times faster than that on terraces in the measured temperature range. The diffusion activation energies are 5.1 kcal/mol and 3.8 kcal/mol on terraces and along steps, respectively. The lower activation energy and faster hopping rate for CO molecules diffusing along steps provide direct evidence that steps provide fast diffusion channels for CO molecules on stepped Pt(111) surfaces. [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V32.00002: Electric Field-Induced Dissociation of NH$_3$ on Ru(0001) from First Principles Aaron Sisto, Alexey T. Zayak, Jeffrey B. Neaton Recent experimental evidence suggests that an applied electric field from an STM tip can induce rapid dissociation of NH3 on a Ru(0001) surface. Here, density functional theory calculations, within the generalized gradient approximation, are used to understand the experimental observations and develop a clear picture of this complex reaction. The desorption and dissociation thresholds are found to occur at E=-0.06 V/\AA and E=0.12 V/\AA, respectively, in good agreement with experiment. Calculation of the energy barrier along a potential reaction pathway indicates a decrease of 0.50 eV in the barrier at the dissociation threshold. A field-induced softening of the ammonia umbrella mode is found to explain the initial dissociation. It is concluded that an applied electric field greatly enhances the dissociation reaction rate through a reduction of the reaction energy barrier. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V32.00003: Surface Reactions following Ultrafast Substrate Excitation: A Path towards Atomic Scale Resolution of High-Temperature Reactions at Metal Surfaces? Christophe Huchon, Zhihai Cheng, Ludwig Bartels The diffusion of CO molecules on a copper surface triggered by femtosecond optical excitations was monitored by means of a low temperature scanning tunnelling microscope (LT-STM). The method uses an experimental setup including an amplified Ti:Sa oscillator providing ~85 fs laser pulses at 800nm, which are subsequently frequency-doubled. The optical system is rigidly coupled to an LT-STM. This work follows initial demonstration of ~the desorption and the anisotropic diffusion dynamics of CO molecules on a Cu (110) surface at a fixed laser fluence. Our adsorbate/substrate model is intended as a starting point in unveiling the energy transfer mechanism underlying picosecond and femtosecond surface processes that occur under equilibrium conditions. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V32.00004: ``QM/Me'' - a novel embedding approach for adsorbate dynamics on metal surfaces J\"org Meyer, Karsten Reuter The dissociative adsorption of oxygen molecules on metal surfaces is a commonly known, highly exothermic reaction and in its slow or fast form of great importance for corrosion or oxidation catalysis, respectively. However, knowledge about atomistic details of the heat dissipation, a central conceptual concern, is very limited at best. Even on the level of Born-Oppenheimer potential energy surfaces, accurate dynamical \emph{ab-initio} descriptions of such reactions are quite challenging from a computational point of view: Modeling the excitations of substrate phonons within periodic boundary conditions requires huge supercells, whereas traditional ``QM/MM'' embedding schemes would need unfeasibly large metal clusters. In the novel ``QM/Me'' approach presented here, the adsorbate- interaction is obtained from periodic first-principles calculations in convenient supercells and combined with the description of a 'bath-like' substrate based on classical potentials, which are parametrized to seamlessly fit the first-principles data. We apply our approach to the dissociative adsorption of O$_2$ and H$_2$ on Pd(100) using density-functional theory and a modified embedded atom potential. In both cases, a dominant fraction of the released chemisorption energy is dissipated into the bulk already on a femtosecond time scale. Implications for the adsorbate dynamics will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V32.00005: First principles study of the activation of carbon dioxide on catalytic surfaces Sujata Paul, Marco Buongiorno-Nardelli Using calculations from first principles we have elucidated the atomic scale mechanisms of activation and reduction of carbon dioxide on specifically designed catalytic surfaces. Among the many reactions that require the development of novel catalytic materials, the understanding of adsorption, activation and reduction of carbon dioxide (CO$_2$) has become central to the effort to manage greenhouse gas emission and control, if not revert, global warming. In this work we will show how one can use first-principles calculations based on Density Functional Theory to design catalytic surfaces specifically tailored for the activation and reduction of carbon dioxide via the rational manipulation of the surface properties that can be achieved by combining transition metal thin films on oxide substrates. The mechanisms of the molecular reactions on the class of catalytic surfaces so designed will be analyzed in an effort to optimize materials parameters in the search of optimal catalytic materials. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V32.00006: The dynamics of quantum tunneling: effects on the rate and transition path of OH on Cu(110) Erlend Davidson, Ali Alavi, Angelos Michaelides The quantum tunneling of hydrogen is important to many scientific disciplines. Through well-defined experiments at surfaces with for example scanning-tunneling microscopy (STM) it is possible to understand the fundamental mechanisms involved at the atomic level. Recent STM experiments have identified OH on Cu(110) as an interesting model system in which the low temperature dynamics is entirely dominated by proton tunneling. Here we report the results from our analysis of the quantum nuclear tunneling dynamics of this system using density functional theory based techniques. We present computed classical, semi-classical and quantum mechanical transition rates for the flipping of OH between two degenerate energy minima. The classical transition rate is essentially zero at the temperatures used in experiment, and the tunneling rate along the minimum energy path is also much too low compared to experimental observations. However when tunneling is taken into account along a direct path connecting the initial and final states with only a small amount of the oxygen movement the transition rate obtained is in much better agreement with experiment, suggesting quantum tunneling effects cause a deviation of the reaction coordinate from the classical transition path. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V32.00007: The Raising Of A Molecular Flagpole: Acetophenone on the Silicon (001) Surface Oliver Warschkow, Steven R. Schofield, Daniel R. Belcher, Marian W. Radny, Phillip V. Smith The utility of scanning tunneling microscopy (STM) to image chemical processes on semiconductor surfaces is now widely appreciated. In addition to imaging, the STM may also be used as an invasive device to induce chemical reactions in a molecule under observation. This offers enticing prospects for directed single-molecule synthetic chemistry, provided the chemical processes observed are understood. This presentation reports a detailed density functional study to resolve the mechanism behind a series of STM-induced reactions in an acetophenone (PhCOCH$_3$) molecule on the Si(001) surface. Acetophenone follows the general reaction pattern resolved by us earlier for acetone [1] and acetaldehyde [2]; however, the presence of an aromatic ring introduces an important twist. [1] O. Warschkow, I. Gao, S.R. Schofield, D.R. Belcher, M.W. Radny, S.A. Saraireh, and P.V. Smith, Phys. Chem. Chem. Phys. 11 (2009) 2747. [2] D.R. Belcher, S.R. Schofield, O. Warschkow, M.W. Radny, P.V. Smith, J. Chem. Phys. 131 (2009) 104707. [Preview Abstract] |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V32.00008: Reaction mechanism of the oxidation of HCl over RuO$_2$(110) Ari Seitsonen, Herbert Over Density functional theory (DFT) calculations reveal that the oxidation of HCl with oxygen producing Cl$_2$ and water proceeds on the chlorine-stabilized RuO$_2$(110) surface via a one-dimensional Langmuir-Hinshelwood mechanism [S Zweidinger {\it et al}, Journal of Phys.\ Chem.\ C {\bf 112}, 9966 (2008)]. The recombination of two adjacent chlorine atoms on the surface of the catalyst constitutes the rate-determining step in this novel Deacon-like process, having been recently introduced in the industrial chemistry by Sumimoto Chemical [K Iwanaga {\it et al}, Kagaku I, 1 (2004)]. The DFT results explain the high-resolution core level shift and temperature-programmed reaction experiments. [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V32.00009: Ab Initio Nonadiabatic Molecular Dynamics of Wet-Electrons on the TiO$_2$ Surface Sean Fischer, Walter Duncan, Oleg Prezhdo The electron transfer (ET) dynamics of wet-electrons on a TiO$_2$ surface was investigated using state-of-the-art ab initio nonadiabatic molecular dynamics, which includes electronic evolution, phonon motions, and electron-phonon coupling. Delocalized over both water and TiO$_2$, wet-electrons are supported by a new type of state that is created at the interface due to the strong water-TiO$_2$ interaction and that cannot exist separately in either material. Our simulations indicate that the ET is sub-10 fs and driven mainly by low frequency vibrational modes. The high density of TiO$_2$ conduction band states leads to frequent crossings of the strongly coupled donor and acceptor states, which is conducive to fast ET. The average ET dynamics for the system feature essentially equal contributions from both adiabatic and nonadiabatic transfer mechanisms. Similar states are present in a number of other systems with strong interfacial coupling, including certain dye-sensitized semiconductors. The wet-electron state may also have relevance to the electrochemical photolysis of water. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V32.00010: Theoretical study on the diffusion of hydroxyl radical on BaO terminated BaTiO$_3$ (001) surface John Mark Martirez, Wissam Al-Saidi, Andrew Rappe In this study, the role of surface adsorbed hydroxyl radical (OH) in stabilizing ferroelectricity in BaTiO$_3$ thin films was investigated using first-principles density functional theory (DFT). The stable adsorption sites for OH adsorption and the low-energy pathways for OH diffusion were explored. Thin film BaTiO$_3$ was simulated using a five-layer supercell slab model and the plane-wave pseudopotential DFT method. The diffusion pathways were identified using the nudged elastic band (NEB) method. Diffusion barriers and pathways were calculated for different OH coverages and polarization directions, elucidating the relationship between OH diffusion and depolarization of nanoscale ferroelectric materials. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V32.00011: Studies of oxidation and thermal reduction of the Cu(100) surface using low energy positrons N.G. Fazleev, W.B. Maddox, A.H. Weiss Changes in the surface of an oxidized Cu(100) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the Cu M2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 \r{ }C. The intensity then decreases monotonically as the annealing temperature is increased to $\sim $600 \r{ }C. In contrast, the O KLL PAES intensity is the lowest at 300 \r{ }C and it starts to increase again as the temperature is increased further. PAES results are analyzed by performing calculations of positron surface states and annihilation characteristics taking into account the charge redistribution at the surface, surface reconstructions, and changes of electronic properties of the surfaces with adsorbed oxygen. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M2,3VV and O KLL Auger peaks and probabilities of annihilation of surface trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V32.00012: Water Oxidation on GaN Surface Xiao Shen, Jue Wang, Y. A. Small, P. B. Allen, M. V. Fernandez-Serra, M. S. Hybertsen, J. T. Muckerman Efficient solar water-splitting requires a good catalyst to oxidize water into O$_2$ with the photo-holes. Efficient water oxidation catalysts are hard to find. The solid solution of wurtzite GaN/ZnO is a water-splitting photocatalyst which works in visible light, while pure GaN is a water-splitting photocatalyst which works in UV. Unlike other semiconductors, the surfaces of both materials exhibit high efficiencies for water oxidation. However, microscopic details of the reactions are not known. We present a first-principles study of water oxidation on the (10$\bar{1}$0) surface of wurtzite GaN. The semiconductor/aqueous interface is modeled by a passivated Ga$_{15}$N$_{15}$ cluster, with two active sites (Ga atoms which bind OH$^-$ ions, next to N atoms which bind H$^+$),\footnote{X. Shen {\it et al.}, J. Phys. Chem. C {\bf 113}, 3365 (2009)} together with a few explicit water molecules and a polarizable continuum. We propose the key intermediates of a four-step mechanism for water oxidation on the GaN surface. We calculate the standard reduction potentials. The first of the four proton-coupled electron transfer reactions appears likely to be the rate-limiting step. We argue that our mechanism should likely apply to other semiconductor surfaces. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V32.00013: High Coverage Oxidation and diffusion on the Cu(001) Surface Willie Maddox, Nail Fazleev The structures formed on oxidized transition metal surfaces vary from simple adlayers of chemisorbed oxygen to more complex structures which result from the diffusion of oxygen into subsurface regions. In this paper, an \textit{ab-initio} investigation of the stability and electronic structure of the Cu(001) missing row reconstructed surface at various surface and subsurface oxygen coverages ranging from 0.5 to 1.5 monolayers is presented using density functional theory (DFT). Results of calculations regarding geometrical properties such as interatomic distances and changes in interlayer spacing as well as electronic properties including changes in adsorbate binding energy, electron work function, surface dipole moment, and deformation electron density as a function of oxygen coverage are also discussed. The studied structures are all found to be energetically more favorable as compared to structures formed by pure on-surface oxygen adsorption. We observe an increase in the work function and surface dipole moment when oxygen atoms occupy subsurface sites which can be attributed to significant charge redistribution within the first two layers, buckling effects within each layer and interlayer expansion. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V32.00014: Catalytic activity of gold nanoclusters supported by cerium oxide: interplay between cluster reactivity, size, and interface charge transfer revealed by DFT calculations Stefano Fabris, Matteo Farnesi Camellone The parameters controlling the catalytic activity of oxide-supported Au atoms and clusters are studied by means of density functional theory calculations. CeO$_2$(111) surfaces containing positively charged Au ions, either as supported Au$^+$ or as substitutional Au$^{3+}$ ions, are shown to activate molecular CO and to catalyze its oxidation to CO$_2$ via participation of lattice O. For the Au$^+$ adatoms, the limiting rate is determined by the adsorbate spillover. The reaction proceeds with CO oxidation via O vacancy formation. These vacancies readily attract the Au$^+$ adatoms, turn them into negatively charged Au$^{\delta-}$ adspecies that prevent further CO adsorption, thus deactivating the catalyst. The reactivity of gold nanoparticles nucleated at O vacancies can be recovered for cluster sizes as small as Au$_2$. Substitutional Au3+ ions dispersed into the ceria lattice can instead sustain a full catalytic cycle maintaining their charge state and activity along the reaction process. The interplay between the reversible Ce$^{4+}$/Ce$^{3+}$ and Au$^{3+}$/Au$^+$ redox couples underpins the high catalytic activity of dispersed Au atoms into the ceria substrate. Ab-initio surface thermodynamics is used to investigate the stability of different solid solutions and to predict more reactive catalysts. [Preview Abstract] |
Thursday, March 18, 2010 10:48AM - 11:00AM |
V32.00015: Study of atomic/molecular hydrogen diffusion on/desorption from amorphous surfaces at low temperature Gianfranco Vidali, Ling Li We present results of experiments and calculations of the interaction of atomic and molecular hydrogen with amorphous solid surfaces at low (5-20 K) temperature. We obtain information on the mechanisms and energetics of atom/molecule diffusion and desorption, and on the efficiency of molecular hydrogen formation. [Preview Abstract] |
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