Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P21: Focus Session: Fundamental Issues in Catalysis I |
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Sponsoring Units: DCP Chair: Francisco Zaera, University of California, Riverside Room: Morial Convention Center 213 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P21.00001: The role of hot electrons in catalysis science Invited Speaker: One long-standing observation in the field of heterogeneous catalysis is that the activity and selectivity in certain reactions is dramatically affected by the oxide onto which the metal nanoparticles are deposited, even though the oxide itself is not active in catalysis. Recently, studies which detected hot electron formation at metal surfaces helped to explain these curious findings. Pulse probe experiments have detected hot electron formation within femtoseconds when photons are incident on a metal surface. Experiments indicate that the mean free path of these hot electrons is on the order of 5 nm, which is in the range of the size of catalyst nanoparticles. Further studies indicate that exothermic catalytic reactions can also produce hot electrons readily, for example CO oxidation or the reaction of hydrogen and oxygen to form water. We have constructed a ``catalytic nanodiode'' in our laboratory whereby we carry out catalytic reactions at high and continuous turnover and, using a Schottky barrier, collect hot electrons. Simultaneous measurement of turnover frequency and hot electron current during CO oxidation has shown that the hot electron current and the turnover rate for the reaction are correlated. This implies that the catalytic activity at the oxide-metal interface in certain catalytic reactions is associated with the hot electron flow. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P21.00002: CO oxidation over noble metals: The continuum from ultrahigh vaccuum to atmospheric pressures Invited Speaker: Catalytic oxidation of CO has been investigated for many decades by numerous researchers and is considered to be one of the best understood catalytic reactions. Because of its importance in pollution control, fuel cells, etc., this reaction has received considerable attention for fundamental and practical reasons. Removal of CO from automobile exhaust is accomplished by catalytic converters using supported Pt-group metals of Pt, Pd and Rh catalysts. The catalytically removal of traces of CO from H$_{2}$ by Pt-group metals to the few ppm level is required for efficient operation in fuel cells. Efforts in our laboratory have addressed the adsorption of CO and the kinetics of CO-oxidation on single crystals and supported metal catalysts over a wide temperature (400--650 K) and pressure (1$\times $10$^{-7}\sim $500 Torr) range. Two active phases, CO-dominated and O-dominated, have been identified for which the mechanisms for CO catalytic oxidation are entirely different. The highly active phase formed in oxygen-rich reaction condition exhibits CO$_{2}$ formation rates several orders higher than the rates found for stoichiometric reaction conditions. This highly active surface was determined to consist of approximate one monolayer of surface oxygen using Auger spectroscopy and X-ray photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P21.00003: Gold atoms, chains and islands on oxide films: looking at orbitals and counting electrons. Invited Speaker: Low-temperature STM measurements combined with DFT calculations are employed to analyze the adsorption of gold on alumina/NiAl(110). The binding of Au monomers involves breaking of an oxide Al-O bond below the adatom and stabilizing the hence under-coordinated O ion by forming a new bond to an Al atom in the NiAl. The adsorption implies negative charging of the adatom. The linear arrangement of favorable binding sites induces the self-organization of Au atoms into chains. For every ad-chain, the number of electrons, in particular of transfer-electrons from the support, is determined by analyzing the node structure of its HOMO. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P21.00004: Theoretical Insights into C1 Surface Chemistry Invited Speaker: Reforming and partial oxidation of methane as well as other C1 fuels are important processes in the production of hydrogen and synthesis gas and will likely play important roles future energy strategies. Herein we use theory and simulation to examine the reactivity of methane, methanol and dimethyl ether with CO$_2$, H$_2$O, or O$_2$ over supported transition metals. We systematically probe the elementary C-H bond activation as well as the oxidation pathways involved in both reforming as the oxidation of methane and other C1 intermediates over well defined transition metal surfaces, metal alloys and metal nanoparticles. The calculations demonstrate well-established trends in C-H bond activation as the result of changes in the metal, the activating molecule (methane, methanol, and DME) as well as the reaction conditions. The reaction conditions ultimately dictate the surface coverage of carbon and oxygen which have important consequences on the surface reactivity. The theoretical and simulation results are compared with well defined experiments carried out at Berkeley over supported particles. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P21.00005: Atomic ordering periodicity and catalytic properties of nanoparticles Valeri Petkov Often nanosized particles of crystals are catalytically very active while the corresponding crystals are not. A typical example is gold. The enhanced catalytic performance of nanosized particles, however, does not come merely from their greatly enhanced surface-to-volume ratio. We would like to draw attention to the often overlooked fact that nanosized particles of crystals do not necessarily possess the periodic 3D structure of their bulk counterparts, and this too may impact their catalytic properties substantially. In particular, nanoparticles that do not have a periodic 3D structure may not come in a well-defined, faceted shape, i.e. may not be terminated by well-defined (usually high energy) atomic planes, as crystalline objects of the same size would be. Hence, nanoparticles may be catalytically more (or less) active than ``nanosized'' crystals. Results from recent structure studies (synchrotron XRD and computer simulations) on 1 -- 5 nm Ru, Au and Pt particles will be presented as evidence. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P21.00006: First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment Simone Piccinin, Catherine Stampfl, Matthias Scheffler By means of density-functional theory, together with concepts from atomistic thermodynamics, we present a theoretical procedure for describing the structure and stoichiometry of a binary alloy in contact with a surrounding gas phase environment. We apply the approach to the Ag-Cu alloy in an oxygen atmosphere, for which recent results report a superior selectivity for ethylene epoxidation compared to pure silver, the predominant catalyst for this reaction. We first show that the presence of oxygen leads to copper segregation to the surface. Then, considering the surface free energy as a function of the surface Cu composition, we construct the ``convex hull''. By including the dependence of the surface free energy of the oxygen chemical potential, we determine the phase diagram of the alloy as a function of temperature, pressure, and Cu surface content. We predict that for conditions typical of the epoxidation reaction, a number of structures can be present on the surface of the alloy including the clean silver surface, thin copper-oxide-like structures, and thick copper oxides. These findings are consistent with, and help explain the recent experimental results. We envisage this approach will be useful and generally applicable for the study of other alloys in contact with a gas or liquid phase. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P21.00007: The Cu/ZnO(0001) Surface under Oxidative and Reducing Conditions: A First-principles Study Katawut Chuasiripattana, Oliver Warschkow, Bernard Delley, Catherine Stampfl The Cu/ZnO(0001) surface is widely used as a catalyst for the production of H$_{2}$-gas from methanol and is thus of considerable relevance to the emergent hydrogen economy. A key to the further development of this catalyst system is a detailed atomic-scale understanding of the relation between surface structure and function versus environmental conditions such as copper content and state of surface oxidation. Towards this goal, we use density functional theory within the framework of ab initio atomistic thermodynamics to conduct a detailed survey of conceivable surface structures under variety of Cu exposures. This produces a surface phase diagram that reveals several distinct regimes of surface reconstruction under oxygen-rich and poor conditions. We correlate our findings with experimental studies, including recent scanning tunneling microscopy results by Dulub et al [1]. \newline References: \newline [1] O. Dulub, M. Batzill, and U. Diebold, Topics in Catalysis 36 (2005) 65. [Preview Abstract] |
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