Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q10: Chemisorption and Surface Reactions |
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Sponsoring Units: DCMP Chair: Yevgeniy Puzyrev, Vanderbilt University Room: D221 |
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q10.00001: Catalytic Reactions of DNT and TNT Molecules on Porphyrin Complexes Keith Warnick, Bin Wang, Sokrates Pantelides Reactions of molecules with substrates can be used to identify them, as in sensor applications. Here we examine reactions of DNT and TNT molecules on porphyrin and metal-porphyrin via first-principles DFT calculations. We find that the oxidation of DNT by O$_{2}$ using Fe-porphyrin as a catalyst is exothermic. The affinity of O$_{2}$ to Fe-porphyrin weakens the O$_{2}$ intramolecular bond, which lowers the oxidation reaction barrier is lowered by $\sim$1 eV. Substrate effects on this process are accounted for. One way to use this selective oxidation reaction for DNT/TNT sensor applications is to exploit the metal-semiconductor transition in thin-film VO$_{2}$ to detect the energy deposited by the exothermic reaction between the adsorbed molecules. This work was supported in part by DTRA grant HDTRA1-10-0047. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q10.00002: Reactivity of TiO$_{2}$ Rutile and Anatase Surfaces toward Nitroaromatics Shao-Chun Li, Ulrike Diebold The Au-TiO$_{2}$ system is a promising catalyst for the synthesis of nitro-aromatic compounds. The adsorption of azobenzene (C$_{6}$H$_{5}$N-NH$_{5}$C$_{6})$ and aniline (C$_{6}$H$_{5}$NH$_{2})$ on two single-crystalline TiO$_{2}$ surfaces, anatase (101) and rutile (110), has been investigated with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoemission spectroscopy (XPS), and synchrotron Ultraviolet photoemission (UPS). While azobenzene adsorbs as an intact molecule at low coverages, ordered overlayers of phenyl imide (C$_{6}$H$_{5}$N) form at saturation coverage, indicating that TiO$_{2}$ surfaces cleave the N-N bond even without the presence of Au. The same superstructures, p(1 $\times $ 2) on anatase and c(2 $\times $ 2) on rutile and the same electronic structures, form upon adsorption of aniline, suggesting the formation of the same, or a very similar, reaction intermediate. These results suggest that the main role of the supported Au in catalytic aniline $\leftrightarrow $ azobenzene conversion is the activation of O$_{2}$/H$_{2}$ for de/hydrogenation reactions. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q10.00003: The role of subsurface oxygen in the selectivity enhancement of ethylene epoxidation on Ag-Cu Catalysts Ngoc Linh Nguyen, Stefano de Gironcoli, Simone Piccinin The role of subsurface oxygen on the Ag-Cu alloy catalysts for the ethylene epoxidation reaction has been studied by means of first principles Density Functional Theory (DFT) calculations. We find that in presence of oxygen and ethylene reactants, the subsurface oxygen adsorption is energetically favorable on fcc sites under the thin oxide-like CuO layer formed at the catalyst surface. On this substrate the reaction proceeds via the formation of a common oxametallacycle precursor. The calculated activation energies show favorable energetics for the pathway leading to the formation of the desired product, ethylene oxide, with respect to the one leading to the formation of the undesired product, acetaldehyde, while the opposite order is obtained on pure Ag catalyst. These findings provide an understanding, at the atomistic level, of the selectivity enhancement of Ag-Cu alloy with respect to pure Ag catalysts. Furthermore, we find that under temperature and partial pressure conditions close to the experimental ones, the ethylenedioxy intermediate is present on the phase diagram of Ag-Cu (111) surface. Our calculations indicate, however, that the formation of this structure could poison the catalyst surface. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q10.00004: Surface defects activate new reaction paths: formation of formate during methanol oxidation on defective Ru(0001) I. Palacio, O. Rodr\'Iguez de la Fuente An optimum understanding of the existing molecular mechanisms taking place while reactions occur on surfaces, should preferably be based on a correct identification of the intermediate species and the reaction paths, so to avoid trial-and-error approaches. Otherwise, a good control of the chemical activity is not easily attainable. We have adsorbed methanol on Ru(0001), with surfaces having a variable density of defects. In this way, with Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Infrared Reflection-Absorption Spectroscopy (IRAS) we have identified reaction paths in the methanol/Ru(0001) system. While the sole methanol adsorption leads to its complete dehydrogenation towards CO, we show that oxygen coadsorption stabilizes intermediate products, namely methoxy (CH$_{3}$O), formaldehyde (CH$_{2}$O) and formyl (CHO). We show as well that a new reaction path appears just on the defective surface: the formation of formate (HCOO). The presence of the defects (mainly steps) catalyzes the oxidation of formaldehyde to formate. This particular case shows how surface defects profoundly affect the catalytic activity, opening new reaction channels which are not available when the density of defects is low. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q10.00005: Dynamics of Low-Energy Electron Induced Reactions in Condensed Michael Boyer, ChanMyaeMyae Soe, Kristal Chamberlain, Yomay Shyur, Christopher Arumainayagam We present insights into the dynamics of low-energy electron-induced reactions in thin films of methanol (CH$_{3}$OH). Low-energy electrons in matter can initiate chemical reactions though electron impact ionization of a molecule, electron impact excitation of a molecule, or through dissociation of a transient negative ion formed by electron attachment to a molecule. Our studies focus on the dynamics by which low-energy electron interaction with condensed methanol initiates chemical reactions which lead to the formation of methoxymethanol (CH$_{3}$OCH$_{2}$OH) and ethylene glycol (HOCH$_{2}$CH$_{2}$OH). The results of our post-irradiation temperature programmed desorption experiments indicate that both products can form from irradiating methanol with electrons at subionization energies. In addition, we find evidence that dissociative electron attachment plays a role in the formation of methoxymethanol but not in ethylene glycol. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q10.00006: X-ray Induced Reorganization/Polymerization of Resorcinol on the TiO$_{2}$ Rutile (110) Surface Vinod Kumar Paliwal, Shao-Chun Li, Ulrike Diebold The room-temperature adsorption of resorcinol (1, 3 benzenediol, C$_{6}$H$_{4}$(OH)$_{2})$ on the (110) surface of rutile TiO$_{2}$ was investigated with STM and x-ray photoemission (XPS). The saturation coverage of resorcinol is smaller as compared to catechol (1,2 benzenediol) with a C1s/Ti2p$_{3/2}$ ratio of $\sim $ 7.3{\%} and 12{\%}, respectively. This indicates that resorcinol occupies on average more than two Ti sites on the surface. STM suggests that resorcinol molecules are mobile at lower coverage, whereas a weakly-ordered overlayer with a periodicity of 3 unit-cells along [001] is observed at higher coverages. Interestingly, exposure of resorcinol-saturated TiO$_{2}$ surface to an XPS Mg-K$\alpha $ beam (1253.6 eV) induces a reorganization of adsorbed resorcinol molecules. STM shows well-resolved double chains that run across [001]-oriented rows of TiO$_{2}$(110) surface. These results suggest that irradiation induces a polymerization reaction of adsorbed resorcinol molecules, where neighboring aromatic rings are arranged in a zig-zag configuration. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q10.00007: Propane-1,3-diol Adsorption and Dissociation on Rutile TiO$_{2}$(110): A Scanning Tunneling Microscopy Study Zhenrong Zhang, Xiao Lin, Bruce Kay, Zdenek Dohn\'alek Titanium Dioxide (TiO$_{2})$ has attracted great attention in the past decades due to its importance in heterogeneous catalysis. Here the adsorption and dissociation of Propane-1,3-diol molecules on partially reduced rutile TiO$_{2}$(110) surfaces are studied via variable temperature scanning tunneling microscopy (VT-STM). STM images of TiO$_{2}$(110) surfaces obtained before and after \textit{in-situ} doses of melecules at room temperature show that the molecules preferentially bind in bridge-bonded oxygen vancanies (BBO$_{V}$'s) \textit{via} one O-H bond scission. The dynamics of Propane-1,3-diol molecules motion has been investigated at room and elevated temperatures. Propane-1,3-diol molecules swing on TiO$_{2}$ surface with one end (-CH$_{2}$-O$^{-})$ anchored on vacancies. Strong interaction of the other end (-O-H) with Ti$_{5c}$ reduces the swing rate when compared with octanol. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q10.00008: First-Principles Studies of Electric Field Effects in Heterogeneous Catalysis: NH$_{3 }$on Ru(0001) Aaron Sisto, Alexey Zayak, Jeffrey Neaton The catalytic dissociation of NH$_{3}$ has been the focus of recent studies due to the prospect of efficient hydrogen storage and generation. The effects of a static electric field on the surface electronic structure and energy barriers of reactions are examined using density functional theory calculations with gradient corrections. It is found that the interaction strength between the adsorbate and surface can be tuned based on the magnitude and polarity of the field, as evidenced by a field-induced shift of the d-electron band. Correspondingly, energy barriers along minimum energy pathways for desorption and dissociation reactions are significantly affected by the change in substrate-adsorbate interaction. It is concluded that the application of an electric field enhances the catalytic performance of Ru through increased activity and selectivity of NH$_{3}$ dissociation. We acknowledge support from DOE, DOE CSGF Fellowship. Computational resources provided by NERSC. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q10.00009: Adsorption and dissociation of molecular oxygen on $\alpha $-Pu (020) surface: A density functional study Jianguang Wang, Asok Ray Molecular and dissociative oxygen adsorption on the (020) surface of $\alpha $-Pu have been studied using the full-potential linearized augmented-plane-wave plus local orbitals (FP-LAPW+lo) basis method. Four adsorption sites and three approaches of O$_{2}$ molecule have been considered. Adsorption energies have been optimized according to the distance of the adsorbates from the Pu surface as well as the oxygen dimer bond length. Dissociative adsorption is found for two horizontal approaches (O$_{2}$ is parallel to the surface and parallel/perpendicular to a lattice vector) and to be more energetically favorable at the scalar and ``fully'' relativistic (NSOC vs. SOC) levels of theory. Hor2 approach on the top site was the preferred adsorption site among all cases studied here. Molecular adsorption occurs at the Vert (O$_{2}$ is vertical to the surface) approach. The work functions, net spin magnetic moments, and charge transfer are also calculated. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q10.00010: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q10.00011: Structure and Adsorption on Hydrated Alumina Surfaces Vincenzo Lordi, Patrick Huang, Eric Schwegler Understanding the mechanisms of adsorption of chemical agents on environmental materials under different atmospheric conditions is important for applications in environmental remediation, industrial catalysis, and protection against chemical warfare. In this work, we study molecular adsorption of the chemical agent simulant dimethyl-methylphoshonate (DMMP) on various alumina surfaces, using density functional theory-based molecular dynamics simulations. Both alpha and gamma alumina surfaces of different orientations (and thus surface terminations/reconstructions) are studied, under both wet and dry conditions. Adsorption from the gas phase onto dry and hydroxylated surfaces is compared to adsorption from an aqueous layer in the limit of a fully bulk-like liquid water layer. Interfacial structure and dynamics are directly compared to previous synchrotron X-ray scattering and sum-frequency vibrational spectroscopy experiments, from which specific contributions of different surface functional groups are identified and resolved. Differences in site reactivity on the various surfaces are also compared. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q10.00012: Ag/Al$_2$O$_3$/FeAl(110): Electronic structure and NO$_2$ adsorption Matthew Patterson, Orhan Kizilkaya, Richard Kurtz, Phillip Sprunger Ag/Al$_2$O$_3$ systems are widely studied as catalysts in the selective catalytic reduction of NO$_x$ with hydrocarbons. The exact nature of the active sites and the role of the Ag clusters in such reactions is still not fully understood. In this study, we characterize thermally evaporated Ag clusters on the ultrathin alumina film produced by oxidizing FeAl(110). ARPES demonstrates the evolution of Ag cluster electronic structure and morphology with increasing Ag coverage. Changes in electronic binding energy distinguish charged from metallic clusters. Vibrational EELS of NO$_2$ adsorbed on the Ag/Al$_2$O$_3$/FeAl(110) system elucidates the nature of the NO$_x$ binding site and the changes in the surface chemistry both as a function of Ag cluster size and NO$_x$ adsorption temperature. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q10.00013: Neutron and Thermodynamic Studies of Hydrogen on Pd Decorated Metal Oxides Paige Landry, A. Ramirez-Cuesta, E. Cruz Silvia, B. Sumpter, J.Z. Larese We report our investigations of thermodynamic, inelastic and quasielastic neutron scattering (INS and QENS) studies of H2 adsorbed on bare and Pd decorated metal oxide (MO) surfaces, specifically ZnO, SBA-15 silica, and alumina. Guided by our volumetric adsorption measurements, we used INS and QENS to probe the dynamics of the adsorbed hydrogen molecules. These measurements provide insight into how the microscopic behavior of hydrogen is changed when it is confined at interfaces or interacts with a Pd catalyst. Using INS, the motion of the adsorbed hydrogen are examined as a function of surface adsorbate composition. For rotational motion we use the ortho-to-para transition as a guide and find that the rotational barrier for H2 adsorbed on some of these MO surfaces shift to lower energy (relative to bulk H2). For comparison, the hydrogen adsorption and microscopic behavior when the MO are decorated with 1{\%} Pd metal will be discussed. Evidence for the presence of adsorbed H2, Zn hydroxide and the potential role of spillover will be discussed. This work was partially supported by the U.S. DOE, BES under contract No. DE-AC05-00OR22725 with ORNL managed and operated by UT-Battelle, LLC, the NSF under grant DMR-0412231 and a grant from the University of Tennessee, JINS. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q10.00014: Atomistic Mechanism of Surface Oxide Formation on Pt(111) Zhengzheng Chen, Chao Wu, William Schneider A detailed understanding of the interaction of oxygen with Pt surfaces is essential for understanding its catalytic activity and deactivation in oxidizing environments. Here we analyze the transition between metallic and oxidized Pt surfaces. Using first-principles calculations, we characterize the chain-like oxide reconstruction on the Pt(111) surface associated with O coverage $>$50\%. We describe the sensitivity of the reconstruction energy to the occupancy of adjacent fcc and hcp sites and present a phenomenological model that relates the reconstruction to the balance between elastic strain energy and screening of O-O repulsions. Core level shift calculations indicate the reconstruction generates two O states with different binding energies and reactivity. Finally, we analyze the thermodynamic stability and equilibrium states of the reconstruction a cluster expansion model. The results are important in developing models of oxidation catalysis on Pt (111) surface. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q10.00015: Tuning Semiconductor Band Edge Energies via Surface Ligand Passivation Shenyuan Yang, David Prendergast, Jeffrey Neaton Semiconductor band gaps and band edge energies are key parameters that can dictate the efficiency of photocatalysis in solar energy conversion applications. CdSe is a representative semiconducting system with an ideal band gap for solar photon absorption, but with band edge energies that are not positioned for efficient water splitting. Using first-principles calculations within density functional theory, we present a study of the electronic structure of passivated CdSe surfaces and nanostructures, exploring the ability to tune band edge energies in this system via chemisorbed ligands. We predict substantial shifts in band edge energies that are electrostatic in origin, and due to the induced dipole at the CdSe-ligand interface and the intrinsic dipole of the ligand. We further show that, by changing the size and orientation of the ligand's intrinsic dipole moment via novel functionalization strategies, we can control the magnitude and direction of the shifts of CdSe energy levels. The effect of ligands on energy levels of two-dimensional CdSe surfaces and nanocrystal surfaces are thoroughly discussed. [Preview Abstract] |
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