Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y26: Surfaces, Interfaces, Colloids and Catalysis I |
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Sponsoring Units: DCP Chair: Jinglie Ping, University of Pennsylvania Room: 204A |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y26.00001: CO Adsorption on Pd(111) at 0.5ML: a First Principles Study Zahra Hooshmand Gharehbagh, Duy Le, Talat S. Rahman While the adlayer structures formed by CO molecules on Pd(111) are well-understood both experimentally and theoretically, for low and high coverages, it is still a matter of discussion for medium coverage (0.5ML). At this medium coverage, it is well-known that the c(4 $\times$ 2) phase is formed but the adsorption sites of CO molecules have been reported differently by various studies: at the bridge sites, at the hollow sites, or at both bridge and hollow sites. Using density functional theory calculations we studied the overlayer structure of CO at 0.5ML on Pd(111) with all possible highly symmetric adsorption sites leading to c(4 $\times$ 2) structures. We will show that, on the same surface, CO molecules adsorb either only on bridge or hollow sites and that there is no overlayer structure in which CO binds at both bridge and hollow sites. By means of \textit{ab initio} thermodynamics simulation, we will also report the conditions (temperature, pressure) in which each overlayer structure exists. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y26.00002: Theoretical investigation of oxygen adsorption on Pu-Ga alloy (111) surface Sarah C. Hernandez, Thomas J. Venhaus, Muhammad N. Huda All electron density functional theory was implemented to study the adsorption of atomic oxygen on a 3.125 at. {\%} Ga stabilized $\delta $-Pu (111) surface. A 4-layer periodic slab, with 8 atoms per layer, was used to model the surface, and the location of the Ga within the surface was considered. High symmetry on-surface and interstitials adsorption sites were explored, which also included the adatom placed in different local environments (i.e. oxygen coordinated with/without a Ga atom). Full relaxation of the atomic positions of the Pu-Ga slab and O atom were employed. The inclusion of spin-orbit-coupling was preformed for the lowest energetic structure. The goal of these calculations was to the probe the effects that Ga may have within the surface when O is adsorb. We found that oxygen binds strongly at an on-surface site with chemisorption energy of -5.06 eV and prefers to be three-fold coordinated in a Pu-rich environment. However, when Ga is participating in chemical bonding with the O adatom chemisorptions energies were unfavorable. Interstitials sites were also unfavorable, which implies that diffusion of O into the subsurface is an activated process. Furthermore, a geometric analysis of the slab after O adsorption showed local oxygen-induced distortions, which will be discussed in detail. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y26.00003: Stabilization of CH$_2$ on Ru(0001) by hydrogen co-adsorption Sergey V. Levchenko, Xunhua Zhao, Matthias Scheffler Based on indirect experimental evidence, CH$_2$ was proposed as a buidling block for hydrocarbon chain growth on the Ru(0001) surface during the Fischer-Tropsch process. However, previous calculations agreed that CH$_2$ is not stable on Ru(0001) at the reaction conditions, and should quickly convert into CH. Employing density-functional theory, we show that this disagreement can be reconciled if coadsorbed hydrogen is present on the surface. The atomic structure of various CH$_x$+H$_y$ phases is obtained with genetic algorithm. CH$_x$ dissociation barriers are calculated using the string method. We further demonstrate, by calculating the surface phase diagram for one-carbon species on Ru(0001) as a function of H$_2$ chemical potential, that the stabilization of CH$_2$ by co-adsorbed hydrogen requires non-equilibrium conditions. The calculated barrier for the CH$_2$ dissociation in the presence of hydrogen is significantly increased, and is close to the one recently measured by vibrational sum-frequency generation spectroscopy [1]. Our results also explain why CH$_2$ was not observed when C or CH are hydrogenated on Ru(0001), although it is observed after methane decomposition.\\[4pt] [1] H. Kirsch {\em et al.}, J. Catal. \textbf{320}, 89 (2014). [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y26.00004: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y26.00005: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y26.00006: Structure and Electron Localization of Reduced Ceria Surfaces Renat Sabirianov, Khaldoun Tarawneh, Nabil Al-Aqtash, Nan Shao, Wai-Ning Mai, Chin Li Cheung Ceria (CeO$_{2})$ is an extensively used industrial catalyst. However, the mechanism for its catalytic activity, especially in aqueous media, is not yet well-understood. While high density of oxygen vacancy defects (OVDs) are often cited as the major factor in enhancing the activity of nanostructured ceria, the synergistic influences between the surface defects, subsurface defects and internal defects has not been explored systematically. Using density-functional theory (DFT) with DFT$+$U approach, we study the local structure of surface and subsurface oxygen vacancies on the (100), (111) and (110) surfaces of ceria. The removal of a neutral surface oxygen atom leaves back excess electrons that are shown to localize on cerium ions neighboring the defect. The preferential defect formation and the different chemical reactivity of the (100), (111) and (110) surfaces are discussed in terms of defect formation energies. DFT$+$U calculations predict the preferential subsurface formation of OVDs for (111) surface by energy difference of 0.10 eV, in agreement with previous GGA$+$U results, while in case of (110) surface calculations predict the surface vacancy to be more stable by energy difference of 1.03 eV. The calculated atomistic and electronic structures of ceria the reduced surfaces are shown to agree with spectroscopic and STM measurements. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y26.00007: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y26.00008: Towards Controlling the Structural Evolution and Kinetic Properties of Monolayer (Hydr)oxide-Metal Interfaces Zhenhua Zeng, Joseph Kubal, Jeff Greeley Ultrathin (hydr)oxide films grown on transition metal surfaces have widespread applications in processes related to electrocatalysis, photocatalysis, coating and corrosion inhibition, and heterogeneous catalysis, among others. These hybrid systems with novel properties are often prepared and characterized under very specific conditions, such as ultrahigh vacuum. When subsequently used in various applications, however, significant structural evolution, which depends strongly on the reaction conditions in-situ, is expected and has been widely observed. Nevertheless, the atomic-level details of these structural changes are generally unknown, particularly in the case of electrocatalytic environments. In order to identify structure-property relationships and, ultimately, predict new materials with improved performance, the development of such understanding is essential. In the present study, on the basis of detailed density functional theory calculations, and using Ni (hydr)oxide films on Pt(111) and Au(111) electrodes as model systems, we describe a detailed structural analysis of film growth and electrocatalytic analysis of hydrogen evolution at three-phase boundaries under alkaline electrochemical conditions. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y26.00009: Study of Xenon Adsorption on Zeolitic Imidazolate Framework -- 8 (ZIF-8) Dinuka Gallaba, Brice Russell, Aldo Migone We have investigated Xe adsorption on ZIF-8 for temperatures in the range between 138 and 158 K. ZIF-8 is known to undergo a structural (``gate-opening'') transition as a function of increasing pressure (or loading) for a number of adsorbates (N$_{\mathrm{2}}$, Ar, CO, O$_{\mathrm{2}})$. For isotherms measured at sufficiently low temperatures, the gate-opening transition manifests itself as an additional (higher pressure) substep in the adsorption isotherm data. Xe isotherms measured above 145 K do not show the additional isotherm feature, while those measured below do. The extra adsorption step is a consequence of the ``gate opening'' transition that occurs due to the re-orientation of the organic linkers in the ZIF-8. This re-orientation increases the size of the apertures in the ZIF-8 structure, and consequently allows more Xe atoms to adsorb in the material, thus producing the additional adsorption step. The adsorption isotherm data were used to determine the effective surface area of ZIF -8 through application of the ``point B'' method. The isosteric heat of adsorption of Xe on ZIF -8 was determined from the isotherm data. We will also report on the kinetics of adsorption of Xe on ZIF-8. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y26.00010: O$_{2}$ Adsorption on ZIF-8: Temperature Dependence of the Gate-Opening Transition Aldo Migone, Brice Russell, Jhonny Villaroel, Karim Sapag We present the results of an adsorption isotherm study of O$_{2}$ on the metal-organic framework ZIF-8. This material undergoes a structural transition (``gate-opening'') as a function of increasing pressure and sorbent loading which manifests itself in the isotherm data as a quasi-vertical substep. We used this feature to explore the temperature dependence of the structural transition. We have found that the transition occurs below the saturated vapor pressure only for temperatures below 93.93 K. The adsorption isotherm data measured at various temperatures was also used to determine the isosteric heat of adsorption of O$_{2}$ on this sorbent for different sorbent loading values. We have studied the adsorption kinetics for this system, i.e., how the equilibration times for adsorption change as a function of sorbent loading. The sorbent loading dependence of the equilibration time is non-monotonic. This unusual characteristic appears to be related to the structural transition present in the sorbent. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y26.00011: Ferroelectric based catalysis: Switchable surface chemistry Arvin Kakekhani, Sohrab Ismail-Beigi We describe a new class of catalysts that uses an epitaxial monolayer of a transition metal oxide on a ferroelectric substrate. The ferroelectric polarization switches the surface chemistry between strongly adsorptive and strongly desorptive regimes, circumventing difficulties encountered on non-switchable catalytic surfaces where the Sabatier principle dictates a moderate surface-molecule interaction strength. This method is general and can, in principle, be applied to many reactions, and for each case the choice of the transition oxide monolayer can be optimized. Here, as a specific example, we show how simultaneous NO$_{x}$ direct decomposition (into N$_{2}$ and O$_{2}$) and CO oxidation can be achieved efficiently on CrO$_{2}$ terminated PbTiO$_{3}$, while circumventing oxygen (and sulfur) poisoning issues. One should note that NO$_{x}$ direct decomposition has been an open challenge in automotive emission control industry. Our method can expand the range of catalytically active elements to those which are not conventionally considered for catalysis and which are more economical, e.g., Cr (for NO$_{x}$ direct decomposition and CO oxidation) instead of canonical precious metal catalysts. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y26.00012: Acceleration of non-PGM Electrocatalyst Design For Fuel Cells Through Site Specific XPS Predictions from First-principles Simulations Boris Kiefer, Sadia Kabir, Kateryna Artyushkova, Plamen Atanassov One of the most pressing problems in the 21$^{\mathrm{st}}$ century is the provision of environmentally consistent energy technologies especially for space limited non-stationary applications. Fuel Cells are promising candidates for addressing and mastering this challenge. Alternative materials to platinum catalysts which continue to attract significant attention are non-PGM FeN$_{\mathrm{x}}$/C based materials. The understanding of the geometry and chemistry of catalytically active defect moieties is a prerequisite for the rational improvement and design of non-PGM electrocatalysts. XPS, a widely used surface analytical technique, generally shows broad N1s peaks with abundance weighted contributions from every defect motif. Due to the lack of appropriate reference materials it is currently impossible to de-convolute the N1s peak into defect specific contributions. In order to remove this limitation we have performed density-functional-theory (DFT) based calculations for a variety of in-plane Fe-N$_{\mathrm{x}}$ (x$=$2-4) defects. Using DFT we predict, for the first time, defect chemistry and geometry induced N1s binding energy shifts in these materials. In combination with our XPS experiments and catalyst performance we find that higher FeN$_{3}$ defect abundance correlates with improved catalyst performance. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y26.00013: Electrolytes near structured dielectric interfaces Huanxin Wu, Yufei Jing, Francisco Solis, Monica Olvera de la Cruz, Erik Luijten The ion distribution in an electrolyte near a dielectric interface has important consequences for numerous applications. To date, most studies have focused on planar interfaces, where, e.g., simulations can take advantage of the image-charge method. However, for surfaces that display structure on the nanoscale, dielectric effects may be significantly different. Here, we investigate such interfaces via a combination of computer simulations and Poisson--Boltzmann theory. We demonstrate how, even for systems with piecewise uniform dielectric constant, surface structure affects the induced polarization charge as well as the ion distribution near the interface, in particular for asymmetric salts. We explore the role of ion concentration, dielectric mismatch and characteristic length scale of the surface structure. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y26.00014: Charge dependent condensation of macro-ions at air-water interfaces Mrinal Bera, Mark Antonio Ordering of ions at and near air-water interfaces is a century old problem for researchers and has implications on a host of physical, chemical and biological processes. The dynamic nature of water surface and the surface fluctuations created by thermally excited capillary waves have always limited measurement of near surface ionic-distributions. We demonstrate that this limitation can be overcome by using macro-ions of sizes larger than the capillary wave roughness $\sim $3{\AA}. Our attempts to measure distributions of inorganic macro-ions in the form of Keggin heteropolyanions (HPAs) of sizes $\sim $10{\AA} have unraveled novel charge-dependent condensation of macro-ions beneath air-water interfaces. Our results demonstrate that HPAs with -3 charges condense readily beneath air-water interfaces. This is in contrast to the absence of surface preference for HPAs with -4 charges. The similarity of HPA-HPA separations near air-water interfaces and in bulk crystal structures suggests the presence of the planar Zundel ions (H$_{5}$O$_{2}^{+})$, which interact with HPAs and the water surface to facilitate the charge dependent condensation beneath the air-water interfaces. [Preview Abstract] |
Friday, March 6, 2015 10:48AM - 11:00AM |
Y26.00015: Water-A New Player of the Solid Surface Yi Gao, Yadong Li, Beien Zhu It is well-known that water configuration and behaviors are highly affected by the solid surface. On the other hand, water is generally considered to have negligible effects on the solid surface. But it might not be the case. Here, we theoretically present two examples to show water could significantly affect the surface structure and properties. The water molecule adsorption could induce the migration of the subsurface vacancies and the change of the surface elements population. These observations might give us a new perspective to understand the properties of liquid/solid interfaces. [Preview Abstract] |
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