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      Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

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          Abstract

          The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465 K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513 K. A change in the front propagation mechanism is proposed to explain the observed effects.

          Highlights

          ► Kinetics of the CO oxidation on (100)-type grains of a polycrystalline Pt foil was studied in situ. ► Photoemission electron microscopy was used to study the local reaction kinetics on a mm-scale. ► Local kinetic phase diagrams for individual Pt(100) grains were obtained. ► Anisotropy of propagating reaction fronts on the apparently isotropic Pt(100) surface is explained.

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          Rapid planetesimal formation in turbulent circumstellar discs

          Anders Johansen, Jeffrey S. Oishi, Mordecai-Mark Mac Low … (2007)
          The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.
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            Particle mesh Ewald: An N⋅log(N) method for Ewald sums in large systems

            Tom Darden, Darrin York, Nancy Pedersen (1993)
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              The Dicke Quantum Phase Transition with a Superfluid Gas in an Optical Cavity

              Ferdinand Brennecke, Tilman Esslinger, Christine Guerlin … (2009)
              A phase transition describes the sudden change of state in a physical system, such as the transition between a fluid and a solid. Quantum gases provide the opportunity to establish a direct link between experiment and generic models which capture the underlying physics. A fundamental concept to describe the collective matter-light interaction is the Dicke model which has been predicted to show an intriguing quantum phase transition. Here we realize the Dicke quantum phase transition in an open system formed by a Bose-Einstein condensate coupled to an optical cavity, and observe the emergence of a self-organized supersolid phase. The phase transition is driven by infinitely long-ranged interactions between the condensed atoms. These are induced by two-photon processes involving the cavity mode and a pump field. We show that the phase transition is described by the Dicke Hamiltonian, including counter-rotating coupling terms, and that the supersolid phase is associated with a spontaneously broken spatial symmetry. The boundary of the phase transition is mapped out in quantitative agreement with the Dicke model. The work opens the field of quantum gases with long-ranged interactions, and provides access to novel quantum phases.
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                Author and article information

                Journal
                Journal ID (nlm-ta): Surf Sci
                Journal ID (iso-abbrev): Surf Sci
                Title: Surface Science
                Publisher: North-Holland Pub. Co
                ISSN (Print): 0039-6028
                Publication date PMC-release: December 2011
                Publication date (Print): December 2011
                Volume: 605
                Issue: 23-24
                Pages: 1999-2005
                Affiliations
                [a ]Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria
                [b ]Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
                [c ]Photonics Institute, Vienna University of Technology, Gußhausstr. 27-29, 1040 Vienna, Austria
                Author notes
                [* ]Corresponding author. Tel.: + 43 1 58801 165106; fax: + 43 1 58801 16599. yuri.suchorski@ 123456imc.tuwien.ac.at
                Article
                Publisher ID: SUSC19420
                DOI: 10.1016/j.susc.2011.07.018
                PMC ID: 3191274
                PubMed ID: 22140277
                SO-VID: a5edc24d-d90b-42dd-8a2d-6e9db7835d27
                Copyright statement: © 2011 Elsevier B.V.
                License:

                This document may be redistributed and reused, subject to certain conditions.

                History
                Date received : 6 May 2011
                Date accepted : 25 July 2011
                Categories
                Subject: Article

                ScienceOpen disciplines: Thin films & surfaces
                Keywords: atomic force microscopy,photoemission electron microscopy,surface reactions,co oxidation,platinum
                Data availability:
                ScienceOpen disciplines: Thin films & surfaces
                Keywords: atomic force microscopy, photoemission electron microscopy, surface reactions, co oxidation, platinum

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