Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T12: Reaction Kinetics at Surfaces |
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Sponsoring Units: DMP DCMP Chair: John Kitchin, Carnegie Mellon University Room: 308 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T12.00001: Atomic structures and energetics of methanol and its reaction intermediates on the ZnO(0001) surface : A first-principles study Katawut Chuasiripattana, Oliver Warschkow, Bernard Delley, Cathy Stampfl Methanol (CH$_3$OH) is widely used in various chemical synthesis, and in particular it is predicted to be one of the next generation of renewable energy sources as a fuel for fuel cells. Industrially, methanol is mass produced by Al$_2$O$_3$-supported Cu/ZnO catalysts. However, the role of ZnO in the methanol systhesis is still unclear. To provide a better understanding of the mechanisms underlying this process, we present first-principles total- energy calculations of the methanol molecule and its reaction intermediates on the ZnO(0001) surface. A detailed characterization of atomic geometries and associated energetics is presented. The reaction intermediates we consider are CH$_3$O, CH$_2$O, CH$_3$, HCOOH, HCOO, HCO, H$_2$O, CO$_2$, CO, OH and H. These intermediate species are reported from experimental studies to be present during methanol decomposition on the ZnO(0001) surface. We also analyse the vibrational frequencies of each of the adsorbed fragments. The information obtained will be used for investigating the surface chemical reactions of associated with methanol synthesis over the ZnO surface. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T12.00002: Morphology of alloy catalysts in an oxidizing environment: Structure-sensitivity of ethylene epoxidation over Ag-Cu particles Simone Piccinin, Catherine Stampfl, Matthias Scheffler Ag-Cu alloys have been proposed as catalysts for ethylene epoxidation due to their superior selectivity compared to pure silver, the predominant catalyst for this reaction [1]. By means of density-functional theory and atomistic thermodynamics, we study the surface structure and morphology of Ag-Cu particles in thermodynamic equilibrium with an oxygen atmosphere. Contrary to the common assumption of the formation of a 2D surface alloy, we find that at temperatures and pressures of interest for practical applications the particles can display a variety of structures, including thin Cu-surface-oxides in coexistence with clean Ag. At variance with pure Ag in UHV, we find that under reactive conditions the (111) facet is not dominant. We identify different reaction pathways that will compete and/or synergetically interplay in the catalysis. In general, the reaction mechanism is structure-dependent and often the reaction does not proceed through the formation of stable intermediates, in contrast to clean Ag and the 2D alloy. Analyzing the competing reactions, we discuss how the addition of Cu improves the selectivity and stress the overall importance of accounting for the effect of ambient conditions. [1] S. Linic et al. J. Catal. 224, 148 (2004) [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T12.00003: Interaction of benzene and MMA vapors with TiO$_{2}$ surface: Relevance to EUV Lithography Boris Yakshinskiy, Shimon Zalkind, Robert Bartynski We characterize the surface processes that affect the reflectivity and lifetime of TiO$_{2}$ -- capped multilayer mirrors used in EUV lithography. Low energy electron beam is used to mimic excitations initiated by EUV radiation. Temperature programmed desorption, x-ray photoelectron spectroscopy, and low energy ion scattering are used to analyze the surface reactions. Carbon film growth on TiO$_{2}$ (011) crystalline surface is measured during electron bombardment in benzene or MMA vapor (model background gases) over a wide range of pressures and temperatures near 300 K. The surface temperature, gas pressure and electron energy are shown to be important for growth of carbon. The substrate temperature rise lowers the carbon accumulation rate. Increasing the vapor pressure over the surface enhances the carbon deposition, and variation of the electron energy shows a pronounce influence on the reaction rate. Low energy secondary electrons excited by EUV photons contribute substantially to carbon accumulation on clean TiO$_{2}$ cap layers. The steady-state coverage of the molecules on the surface and the cross-sections for electron-stimulated dissociation are key parameters for understanding and modeling the processes on the EUVL mirrors. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T12.00004: Selective Oxidation of Ammonia on RuO$_{2}$(110): a combined DFT and KMC study Sampyo Hong, Altaf Karim, Sergey Stolbov, Talat Rahman Motivated by the experiments of Wang et al [1] on the selective oxidation of ammonia on RuO$_{2}$(110), we have performed first principles electronic structure calculations based on the density functional theory (DFT) and the pseudopotential method to calculate the activation energy for the associated reaction processes, and used the DFT results in kinetic Monte Carlo (KMC) simulations of the reaction rates. We find the overall energy barriers for NH$_{3}$ + O $\to $ N + H$_{2}$O, N + N $\to $ N$_{2}$, and N + O $\to $ NO$_{ }$to be about 1.0, 0.6, and 0.56 eV, respectively. We also find, in agreement with experiment, intermediate products such as NH to be short-lived (i.e. not detectable in experiment). For a simple model of the surface dominated by the two end reactions above, our KMC simulations show indeed almost 100{\%} selectivity toward NO, in excellent agrement with experimental findings. For more realistic simulation, in which we include several intermediates and their reactions (over 20 processes), KMC simulations show about 60{\%} selectivity toward NO within the experimental O$_{2}$ pressure range. We compare our calculated reaction rates to those obtained in experiment and rationalize our results through details of the surface electronic structure. [1] Y. Wang, K. Jacobi, W.-D. Schoene, and G. Ertl, J. Phys. Chem. B 2005, 109, 7883-7893. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T12.00005: Surface Chemistry of Cyanogen on Copper Everett Lee, James Lallo, Erkan Ciftlikli, Sylive Rangan, Alexei Ermakov, B.J. Hinch The surface chemistry of cyanogen/cyanide species has been observed on Cu(100) crystal through Temperature Programmed Desorption, Helium Atom Scattering and soft X-ray Photoelectron Spectroscopy. Cyanogen (C2N2) dissociates on copper to form a mixture of cyanogen and cyanide (CN) species depending at cryogenic temperatures but pure cyanide at around ambient temperatures, leading to c(10x6) superstructure in two domains at saturation. Temperature Program Desorption spectra observed indicated that cyanide recombinatively desorbs as cyanogen at relatively high temperatures with no additional species observed. The desorption results were analysed using a variety of techniques in order to determine the activation energy of desorption (Ed) as well as its coverage dependency. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T12.00006: Identifying surface intermediates with TPD: Methylchlorosilanes on Cu(001) James Lallo, Everett Lee, B.J. Hinch, Dan Strongin Various methylchlorosilane molecules (SiH$_{x}$ Me$_{y}$ Cl$_{z}$ , x+y+z=4) were exposed to a Cu(001) surface. Dissociative adsorption was observed for several hydrogen containing species, at surface temperatures above 160K. The chemistry and thermal stability of the surface bound fragments were then studied as a means for understanding intermediates of the commercially important ``Direct Process,'' namely production of Cl2(CH3)2Si, from Si and CH3Cl, with a Cu catalyst.~ Temperature programmed desorption indicates that methyl groups are readily transferred between absorbed Si-containing species. A large fraction of Cl remains on the surface after observation of typically 2 or 3 distinct TPD features below 450K. By a comparison of~ the product species distributions to the parent species, we are then able to propose key intermediates common to many of the adsorption/desorption mechanisms. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T12.00007: Surface-catalyzed O2 adsorption on quantum thin films Jisun Kim, Alexander Khajetoorians, Wenguang Zhu, Zhenyu Zhang, Chih-Kang Shih Pure crystalline Pb is well known to be inert with respect to oxygen gas. By using scanning tunneling microscopy we demonstrate that the oxygen adsorption on Pb films is greatly increased by Cs adsorbates acting as catalysts. Our previous studies show that Cs atoms can be easily incorporated into the surface layer of thin Pb films grown on Si(111). In addition, Cs adsorbates are able to adjust the surface energy so as to initiate stable Pb nanoislands on Pb flat top mesas of unstable thickness. Because of this unique property of Cs adsorbates on originally inert Pb films, they are a natural choice of template to investigate surface catalysis of quantum thin films. Bare Pb films do not oxidize until much higher coverage (1000L), but in the presence of Cs we see adsorption of oxygen at much lower exposures of only a few Langmuir. The oxidation first occurs preferentially at sites of Cs adsorption. After oxygen clusters nucleate, oxidation of the Pb film increases in proportion to the amount of oxygen exposure. During this process the formation of oxygen clusters forces changes in the film morphology. First-principles density functional theory calculations of the O binding energies on the alloyed surface layers will be done and compared with experiment. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T12.00008: First principle studies of the oxygen reduction reaction on the CrN electrocatalyst Sergey Stolbov These fuel cells offer great advantages for various applications, but severe obstacles remain to their large scale implementation. The Pt-based catalysts, used in both electrodes make them unacceptably expensive. Furthermore, the low rate of the oxygen reduction reaction (ORR) on the Pt cathode significantly reduces efficiency of the device. In this work, ORR on alternative CrN electrocatalyst has been studied from first principles. It is found that, in contrast to Pt, oxygen molecules dissociate spontaneously on the CrN surfaces, which switch on simple 1-electron reduction mechanism. Based on the adsorption energy calculations the free energies of intermediates were obtained as functions of the electrode potential. This diagram explain a relatively high rate of ORR obtained experimentally [1] for this material. The effects of coverage and co-adsorbates have been also studied.\\[3pt] [1] H. Zhong, et al., Appl. Phys. Lett. 91, 163103 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T12.00009: Hydrogen Reduction at Room Temperature of Partially Oxidized Co Cluster Films Capped with Pd A. L. Cabrera, J. I. Avila, R. Trabol, C. Romero, M.J. Van Bael, P. Lievens Co clusters with mean size of 1.8 nm were deposited on sapphire substrates at 25$^{o}$C, 300$^{o}$C and 500$^{o}$C respectively. They were capped by a continuous 15 nm Pd film. Visible light transmission and reflection, in the range 400 to 900 nm, were measured when the samples were exposed to different hydrogen pressures up to 130 Torr. D.C. electrical resistance of the films was also measured as an independent property of the films to confirm hydrogen absorption by the samples. In all the samples the transmission and resistance of the films increased, reaching saturation at around 35-40 Torr hydrogen pressure. The relative change in the resistance of all Co cluster samples is smaller than the change in pure Pd films, indicating that hydrogen absorption is limited to the Pd capping layer only. We observed a significant decrease in the transmittance and the resistance during the first hydrogenation cycle of the sample prepared at 25$^{o}$C suggesting that a reduction of the partially oxidized Co clusters occurred at room temperature. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T12.00010: H$_{2}$ dissociation of H$_{2}$ on Co layers on Cu(111) from abinitio studies Duy Le, Sergey Stolbov, Talat Rahman Through first principles electronic calculations, based on the spin-polarized density functional theory using the generalized gradient approximation and the ultrasoft pseudopotential method in the plane wave representation, we studied the adsorption and the dissociation of H$_{2}$ on two Co layers grown on Cu(111). As H$_{2}$ approaches the surface with the H-H bond parallel to that surface, it dissociates at a distance of about 1.7? from the Co layer, and constituent H atoms proceed to occupy neighboring fcc and hcp sites. The ``adsorption'' energy barrier for H$_{2}$ is 0.14 eV and the ``adsorption'' energy is about 0.80eV. On the Co surface, H diffuses from an fcc site to an hcp site, or vise versa, with diffusion barriers of 0.17eV and 0.12eV respectively. We find no evidence of subsurface H. By analyzing the local electronic density of state, we establish, in agreement with suggestion from experiments [1], that the surface electronic states and magnetic moment of Co atoms depend very much on the H coverage. \\[3pt] [1] M. Sicot \textit{et al}, Phys. Rev. B \textbf{77}, 035417 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T12.00011: Structural and catalytic properties of thin CeO$_{2}$ films on TiO$_{2}$ substrate I.G. Batyrev, S.N. Rashkeev Structural properties of CeO$_{2}$(111) thin films on top of a TiO$_{2}$(110) substrate have been investigated by first-principles calculations. We found the special orientation of the ceria film relative to the rutile substrate that corresponds to a small (few percent) lattice mismatch between the two oxide surfaces. The positions of interfacial Ti atoms undergo some structural modulations during the relaxation process. Also, the relaxation of the interface resulted in the appearance of elongated Ce-O bonds at some sites of the O-terminated surface of CeO$_{2}$/TiO$_{2}$ films. These oxygen sites at the surface show low activation energy for loosing oxygen atoms and, therefore, they have higher catalytic activity in CO oxidation by the Mars-van Krevelen mechanism. We predict from the simulations that higher rate of the catalytic CO oxidation should have CeO$_{2}$(111)/TiO$_{2}$(110) films compared with CeO$_{2}$(111) surface, which may also be attributed to the elongation of Ce-O bonds at some sites of O-terminated film in comparison with a pure ceria surface. We discuss the role of oxygen vacancies in the CO oxidation at reduced films and investigate the interplay between migration of O bulk atoms to the surface O vacancy sites and the structural phase transition from the CeO$_{2 }$(111) (Ce$^{4+)}$ and Ce$_{2}$O$_{3 }$(0001) (Ce$^{3+})$ structures. This work was supported by the U. S. Department of Energy Contract DE-AC07-051D14517. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T12.00012: Adsorption and Dissociation of Molecular Hydrogen on the (0001) Surface of DHCP Americium Pratik Dholabhai, Asok Ray Hydrogen molecule adsorption on the (0001) surface of double hexagonal closed packed americium has been studied in detail within the framework of density functional theory. Weak molecular hydrogen adsorptions were observed. The most stable configuration corresponded to a Hor2 approach molecular adsorption at the one-fold top site where the molecule's approach is perpendicular to a lattice vector. Adsorption energies and adsorption geometries for different adsorption sites will be discussed. The change in work functions, magnetic moments, partial charges inside muffin-tins, difference charge density distributions and density of states for the bare Am slab and the Am slab after adsorption of the hydrogen molecule will be discussed. Reaction barrier for the dissociation of hydrogen molecule will be presented. The implications of adsorption on Am 5$f$ electron localization-delocalization will be summarized. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T12.00013: Atomistic Thermodynamic Approach for Determining Cu Catalyst Morphologies Under Reactive Water-Gas-Shift Conditions Nilay Inoglu, John Kitchin The water gas shift reactions (WGS) are widely used in several industrial processes and are catalyzed by Cu catalysts. In coal derived syngas, there are sulfur (S) impurities which can adsorb on the Cu surfaces and block the active sites of the catalyst and can result in catalyst deactivation. The sulfur adsorption will compete with other possible adsorbates such as CO, O, H and CO$_{2}$ making a quantitative estimate of the impact of sulfur on reactivity difficult. To determine the interactions between these possible adsorbates with Cu, the adsorption properties of these different adsorbates on low Miller index facets of Cu surfaces were studied in the limit where these surfaces are in thermodynamic equilibrium with the reactive environment. The effect of S adsorption on the surface electronic structure was significant and coverage dependent. In addition to a site-blocking deactivation mechanism, we found that sulfur modifies the surface electronic structure in a way that makes the Cu less reactive. Our results show that the adsorbates play a key role in defining the catalyst morphologies. We conclude that the presence of sulfur and other reactive adsorbates can impact the reactivity of the catalysts through several mechanisms including site-blocking, surface electronic structure modification and changes in the distribution and types of reactive sites through catalyst morphology changes. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T12.00014: The nonlocal correlation as the solution of the CO puzzle problem Predrag Lazic, Mojtaba Alaei, Nicolae Atodiresei, Vasile Caciuc, Radovan Brako, Stefan Bluegel For the last 20 years the Density Functional Theory (DFT) has been the standard approach for the calculation of chemisorption, adsorption, chemical reactions and electronic structure in general. Despite the great successes of the theory in predicting adsorption energies and other properties for many systems it turns out that the theory fails to predict correctly the adsorption site preference for the CO molecule on (111) surfaces of Pt, Cu and Rh, for example. The DFT calculations predict that the highly coordinated FCC (hollow) site adsorption is preferred over the top site adsorption, while experiments show unambiguously that CO adsorbs into the top site. Also, the calculated adsorption energies do not match well the experimental values. CO molecule chemisorption on these surfaces is a type of system in which one would expect DFT theory in its present implementation with semi-local (GGA) functionals to work well. We show that the inclusion of the nonlocal correlation into the DFT calculations of CO chemisorption through vdW-DF functional largely solves the discrepancies known as the CO puzzle problem. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T12.00015: ABSTRACT WITHDRAWN |
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