Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L2: Focus Session: Surface Chemistry and Catalysis V |
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Sponsoring Units: DCP Chair: Jill Millstone, University of Pittsburgh Room: 102 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L2.00001: Structural Characterization of Mg/Al hydrotalcite-like Compounds and their Thermal Stability Shuhua Zhang, Siyuan Yang, Cheng Wang, Weijun Liu, Xiaodan Gu, Wenjun Gan, Xiaoyu Xue Hydrotalcite-like compounds, repersented by the formula [M$^{2+}_{\mathrm{1-x}}$M$_{\mathrm{x}}^{3+}$(OH)$_{2}$]X$_{\mathrm{x/n}}^{\mathrm{n-}}$ $\cdot$ nH$_{2}$O (M$^{2+}=$ Ni$^{2+}$, Mg$^{2+}$, Cu$^{2+}$,etc; M$^{3+}=$Al$^{3+}$, Fe$^{3+}$, etc; X$^{\mathrm{n-}}=$CO$_{3}^{2-}$, NO$_{3}^{-}$, etc) possess the brucite-like layers [Mg(OH)$_{2}$] with positive charge and anionic compounds in the interlayer to form neutral materials. Catalytic effects to decompose NO$_{\mathrm{x}}$ from automobile exhaust were highly related with the difference of M$^{2+}$ and thermal stability because the catylists locate are about 200 $\sim$ 500${^\circ}$. In this paper, Mg-Al-Cu and Mg-Al-Ni hydrotalcite-like compounds were characterized by XRD and FT-IR spectra and the thermal stability were analyzed by TGA and DTA. Even though they both have the typical diffraction peaks of hydrotalcites, but their interlayer spaces are different. Some weak chemical bonds were observed to be formed in Mg-Al-Ni hydrotalcites by FT-IR. Mg-Al-Ni hydrotalcite-like compound degraded at lower temperature, by contrast, Mg-Al-Cu hydrotalcite has the better structural stablilty and thermal stability. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L2.00002: The Role of Cluster Size and Composition, the Nature of the Support / Interface on the Performance of gas Phase Heterogeneous Catalysts and Electrocatalysts Stefan Vajda In this paper, we will discuss the catalytic performance of with atomic precision size- and composition selected supported clusters consisting of a handful to several dozen atoms in 1) gas phase reactions of selective C$=$C bond activation, C-H bond breaking and CO oxidation; and 2) under electrochemical reactions, such as water splitting. Catalysts' performance will be evaluated as function of cluster size and composition, while the chemistry of the support material / interface will be used to fine-tune catalytic activity and selectivity. In situ X-ray techniques are used to monitor the size, shape and oxidation state of the catalyst under reaction conditions. As time will allow, results will be presented on the use of additives/modifiers which allow controlling the nature of the catalytic site under reaction conditions, and accordingly, its activity and selectivity. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L2.00003: Self-assembly of one-dimensional metal-organic nanoarchitectures on a surface Martina Capsoni, Agustin Schiffrin, Adam Shaw, Sarah Burke Supramolecular chemistry holds promise for designing low-dimensional nanostructures with predefined functionalities. In particular, the interface between metal-organic complexes and surfaces is relevant in applications such as photovoltaics, photocatalysis, molecular electronics, etc. The structural, chemical and electronic properties of these systems can be dramatically altered by the interaction with the underlying surface. It is therefore of great relevance to achieve morphological control of functional nano-assemblies on a substrate at the single molecule and atom level. Here, we investigate the \textit{in situ} coordination of bisterpyridine molecules with transition metal adatoms on Ag(111), by means of low-temperature scanning tunneling microscopy. The bare ligand adsorbs following specific orientations with respect to the substrate atomic lattice. Ordered supramolecular domains emerge via parallel adjacent non-covalent binding of the molecules. Coordination between deposited iron adatoms and terpyridine ligands is activated at room temperature, likely mediated by an intramolecular conformational change of the pyridine groups. The resulting self-assembled one-dimensional nanostructures are described. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L2.00004: Unraveling the Role of Metal-Support Interactions in Heterogeneous Catalysis Invited Speaker: J.R. Schmidt We examine the role of the metal-support interaction in modulating the activity and selectivity of oxide-supported metal nanoparticles, focusing specifically on the Fischer-Tropsch (FT) synthesis of ethanol (EtOH). Although it is well-known that oxide supports can play a non-innocent role in heterogeneous catalysis, a comprehensive and predictive picture of the role of such supports remains elusive. Using realistic computational models of supported nanoparticles, we decouple the electronic and geometric aspects of the metal-support interaction, and we show that the former can be largely understood in terms of charge transfer between support and nanoparticle. The resulting metal-support interactions induce significant changes in adsorbate binding energies, and thus significantly influence reaction thermodynamics and kinetics. For the specific case of FT, we show how our model can be used to understand the observed increase in EtOH selectivity when switching from silica to titania supports. More generally, we illustrate how these ideas can be used to crudely predict the influence of a support even in the absence of detailed calculations and provide a general framework for understanding the influence of various oxide supports on elementary association / dissociation reactions. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L2.00005: Controlling Surface-Mediated Interactions at the Organic Semiconductor / Metal Interface Oliver Monti, Nahid Ilyas, Rocio Cortes-Rodriguez, Percy Zahl, Peter Sutter We show by a combination of two-photon photoemission spectroscopy (2PPE) and low-temperature scanning tunneling microscopy (LT-STM) that surface-mediated interactions at the organic semiconductor / metal interface can be controlled by the molecular orientation on the surface. For the dipolar molecule boron subphthalocyanine chloride (dipole moment of 4.5 D) on Cu (111), nearest-neighbour distance distributions show radically different properties for the two different molecular orientations on the surface, ``Cl-up'' and ``Cl-down''. We are able to model the respective interaction potentials by a combination of Friedel oscillation-induced surface mediated interactions and van der Waals (Cl-up) vs. screened Coulomb interactions (Cl-down). These interaction modes, substantially different for the two molecular orientations, lead to completely different growth modes at higher coverages. They result from selective charge-transfer to Cl-down molecules, as corroborated by 2PPE. Our results suggest a pathway towards control of the interfacial electronic structure and molecular assembly at organic semiconductor / metal interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L2.00006: Trapping of Excited Electrons at the NaCl/Ag(100) Interface David Suich, Benjamin Caplins, Alex Shearer, Charles Harris Understanding metal/insulator systems, such as alkali halides on noble metals, and their properties are important for the fields of catalysis, devices, and the emerging field of nanoelectronics. To date, however, there remain few time-resolved studies of these systems. Time- and angle- resolved two-photon photoemission is used to study the dynamics of the electronic states at the NaCl/Ag(100) interface. We observe the n=1, 2, and 3 image potential states. Electrons in the n=1 state undergo a trapping with a high probability, as shown by a dynamic change in their energy. Momentum resolved measurements show this state is initially delocalized, but becomes localized within a few hundred femtoseconds. The source of trapping is believed to occur at step edges of the NaCl islands. Our studies correlate the coverage and temperature dependence with the dynamics and magnitude of electron trapping in this state. Qualitatively similar behavior has been observed for several other alkali halides on both the Ag(100) and Ag(111) surfaces, proving the generality of the phenomenon. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L2.00007: NO$_{x}$ Direct Decomposition: Potentially Enhanced Thermodynamics and Kinetics on Chemically Modified Ferroelectric Surfaces Arvin Kakekhani, Sohrab Ismail-Beigi NO$_{x}$ are regulated pollutants produced during automotive combustion. As part of an effort to design catalysts for NO$_{x}$ decomposition that operate in oxygen rich environment and permit greater fuel efficiency, we study chemistry of NO$_{x}$ on (001) ferroelectric surfaces. Changing the polarization at such surfaces modifies electronic properties and leads to switchable surface chemistry. Using first principles theory, our previous work has shown that addition of catalytic RuO$_{2}$ monolayer on ferroelectric PbTiO$_{3}$ surface makes direct decomposition of NO thermodynamically favorable for one polarization. Furthermore, the usual problem of blockage of catalytic sites by strong oxygen binding is overcome by flipping polarization that helps desorb the oxygen. We describe a thermodynamic cycle for direct NO decomposition followed by desorption of N$_{2}$ and O$_{2}$. We provide energy barriers and transition states for key steps of the cycle as well as describing their dependence on polarization direction. We end by pointing out how a switchable order parameter of substrate,in this case ferroelectric polarization, allows us to break away from some standard compromises for catalyst design(e.g. the Sabatier principle). This enlarges the set of potentially catalytic metals. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L2.00008: First principles analysis of metal and oxide-metal interfacial catalysis for hydrogen production Invited Speaker: Jeffrey Greeley Current and growing interest in the development of new catalytic materials for complex chemistries has challenged the methods traditionally employed by practitioners of computational catalysis. Explicit Density Functional Theory (DFT) analysis of all possible reaction pathways in biomass reaction networks, for example, is computationally prohibitive, and to make progress at a reasonable rate, strategies to accelerate the predictions made by DFT-based methods must be developed. In this talk, we will review some recent work in our group focusing on first principles analyses of the production of hydrogen from the decomposition of biomass-derived oxygenated hydrocarbons on heterogeneous catalytic surfaces. We will discuss, in particular, the development of accelerated DFT-based strategies to map the complex reaction networks associated with biomass decomposition at metal and oxide-metal interfaces, and we will show how these strategies can efficiently produce semi-quantitative predictions of activity and selectivity trends in hydrogen production on these surfaces. We will also briefly describe the development of reactivity trends for another chemical process that is relevant to biomass chemistry, the water-gas shift reaction, at metal-oxide interfaces, and will describe how bifunctional properties of these interfaces may promote this important chemistry. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L2.00009: Spectroscopic Analysis of Ion Concentration Profile at Electrode/Electrolyte Interface by Interferometry David Moore, Ravi Saraf Owing to the difference in Fermi levels at an electrode/electrolyte interface, ions form an electrical double layer (EDL) with ion concentrations well over 10-fold compared to bulk. The concentration profile of the EDL intrinsically affects the electrochemical reaction rates at the electrode, which is of great significance in many applications, such as batteries and biosensors. Conventionally, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), the electrical properties of the EDL are represented as ``equivalent circuits'' consisting of the resistance to charge transfer (Rct), the double layer capacitance (Cdl) and a ``Warburg (constant phase) diffusion element'' that represents the long range diffusion of ions to the electrode. The translation to the well-understood physical structure can be lost as complicated effects are often lumped together. For example, the effect of subtle modification of the electrode surface by say, redox compounds, enzymes, or polymers is not directly measured, and must be inferred by capacitance changes. An interferometer method will be described to directly measure changes in concentration at the interface during redox process. This method in concert with CV or EIS performed concomitantly will lead to more information to model the diffuse layer for improved understanding of the kinetics of the reaction at different distances from the electrode. Applications to DNA and polymer adsorption binding will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L2.00010: Density functional theory simulation of hydrogen-bonding vibrational densities of states at the quartz (101)-water interface and its relation to dissolution in aqueous solutions of ions Mark DelloStritto, James Kubicki, Jorge Sofo Physical processes at aqueous interfaces are strongly dependent on structure at the interface, and for water/metal-oxide interfaces, the structure of the local H-bond network dominates the behavior at the interface. In particular, for silica it has been hypothesized that the increase in dissolution rate due to dissolved salts is due to a reorganization of the H-bond network by ions near the surface. This would explain the order of magnitude increase in dissolution rate despite the fact that the activation energy of the dissolution reaction does not change with the addition of salts. We investigate two hypotheses of the dissolution of SiO$_{2}$ in ionic solutions using ab-initio molecular dynamics simulations. These hypotheses are 1) that the presence of ions induces orientations in interfacial H$_{2}$O molecules which are preferential for proton transfer to bridging oxygen (BO) atoms, and 2) the presence of ions induces stronger H-bonding between terminal hydroxyl (TH) groups and BO atoms, allowing proton transfer. It is found that although elements of these hypotheses are true, the model structures produced by density functional theory simulations do not support the former as valid mechanisms of dissolution. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L2.00011: Directly Grafting Alkanethiol on Bare Si (111) by UV-assisted Photochemical Reaction Lo-Yueh Chang, Hung-Wei Shiu, Shangjr Gwo, Chia-Hao Chen Self-assembled monolayers (SAMs) are organic molecules that self-assembled and closely packed on substrate surface. The surface physic and chemical properties are dependent on the controllable tail of SAMs. Therefore, SAMs is attracting a lot of attention in bio-sensing, nano-manipulating, and microfluidic field. The alkanethiol on noble metal surface, such as gold and silver, is a well-known SAM system to understand the fundamental properties. However, alkanethiols grown on semiconductor surfaces was less systematically studied, especially on bare silicon surface, despite their prospective applications. To have in-depth understanding of such system, we tried to grow alkanethiol SAMs on hydrogen-terminated Si surface by UV-assisted photochemical reaction. The resulting monolayer was studied by means of water contact angle measurement, synchrotron radiation based X-ray photoemission spectroscopy, and polarization dependent near-edge X-ray absorption fine structure. The combined characterization probes revealed a hydrophobic ambient surface, and the n-alkanethiols were directly attached on Si through Si-S bond that formed a highly order monolayer to prevent the air oxidation and contamination. [Preview Abstract] |
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