Dual-Path Mechanism for Catalytic Oxidation of Hydrogen on Platinum Surfaces

The catalytic formation of water from adsorbed hydrogen and oxygen atoms on Pt(111) was studied with scanning tunneling microscopy and high resolution electron energy loss spectroscopy. The known complexity of this reaction is explained by the strongly temperature dependent lifetime of the product H2O molecules on the surface. Below the desorption temperature water reacts with unreacted O adatoms to OHad, leading to an autocatalytic process; at higher temperatures sequential addition of H adatoms to Oad with normal kinetics takes place

Vibrational and structural properties of OH adsorbed on Pt(111)

OH species adsorbed on Pt(111) were studied in a combined investigation using scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). OH was formed by two different reactions, by reaction of H2O with O, and as an intermediate in the reaction of O with hydrogen to H2O. In both cases, two ordered OH phases were observed, a (√3×√3)R30° and a (3×3) structure, for which models are proposed. Both structures have OH coverages of 2/3, and their formation is driven by hydrogen bond formation between the adparticles; the OH adsorption site is most likely on top. OH molecules at defects in the adlayer, in particular at island edges, are spectroscopically distinguishable and contribute significantly to the vibrational spectra in disordered OH layers. This is important for the water formation reaction, where the OH islands are small. The discrepancies between previous HREELS studies on OH can be explained by the different degree of order under the various formation conditions

Detection of the Natural Alpha Decay of Tungsten

The natural alpha decay of 180W has been unambiguously detected for the first time. The alpha peak is found in a (gamma,beta and neutron)-free background spectrum. This has been achieved by the simultaneous measurement of phonon and light signals with the CRESST cryogenic detectors. A half-life of T1/2 = (1.8 +- 0.2) x 10^18 y and an energy release of Q = (2516.4 +- 1.1 (stat.) +- 1.2 (sys.)) keV have been measured. New limits are also set on the half-lives of the other naturally occurring tungsten isotopes.Comment: Submitted to Physical Review C Revised versio

CO oxidation on Pt(111) – Scanning tunneling microscopy experiments and Monte Carlo simulations

The catalytic reaction between adsorbed oxygen atoms and CO molecules on Pt(111) was investigated by scanning tunneling microscopy and modeled by Monte Carlo simulations. Experiments were performed by dosing preadsorbed Oad layers with CO between 237 and 274 K. Two stages were observed during dosing with CO, an initial reordering and compression of (2Å~2)Oad islands, and a subsequent shrinking of the islands by the reaction of Oad to give CO2. The reaction occurs exclusively at boundaries between (2Å~2)Oad and c(4Å~2)COad domains. The reaction order with respect to the oxygen coverage is 0.5; the reactivity of the boundary increases during the reaction. The Monte Carlo simulations included surface diffusion of Oad atoms, attractive interactions between Oad atoms, the Oad–COad reaction probability (with parameters from quantitative scanning tunneling microscopy measurements), adsorption/desorption of CO, and a high mobility of COad. The experimentally observed domain shapes, the reaction order of 0.5, and the increasing boundary reactivity could only be reproduced by additionally including an Oad coordination-dependent activation energy E of 25 meV per Oad neighbor that accounts for the attractive Oad–Oad interactions. The initial ordering stage could be modeled by incorporating an additional repulsive interaction between Oad and COad. The fact that no reaction occurs in the interior of the (2Å~2)Oad domains, although they are covered by a layer of interstitial COad molecules, is attributed to the crucial role of reactive Oad–COad configurations that only exist at the domain boundaries

Direct Observation of Surface Reactions of Acetylene on Pd(111) with Scanning Tunneling Microscopy

The cyclotrimerization of acetylene to benzene on a Pd(111) surface has been studied by scanning tunneling microscopy (STM). The formation of benzene becomes visible upon increasing the acetylene coverage at 140 K. Initially, the acetylene molecules adsorb in a (2 × 2) ordered layer, which is compressed into an ordered (√3 × √3)R30° structure when more acetylene is added. The cyclotrimerization reaction is observed just before saturation of the (√3 × √3)R30° structure and stops when the saturation coverage is reached. Further exposure to acetylene does not result in a reaction, indicating that the cyclotrimerization reaction involves a transient adsorption state, different from that in the (√3 × √3)R30° layer. The (√3 × √3)R30° layer itself is stable up to 230 K, even when a background pressure of acetylene is present. At 230 K, the (√3 × √3)R30° layer decays, a process which is related to the isomerization and further decomposition of acetylene. The results are consistent with the available spectroscopic data and indicate that the limited conversion of acetylene to benzene is determined by the relative rates of (√3 × √3)R30° domain formation and cyclotrimerization

Atomic and Macroscopic Reaction Rates of a Surface-Catalyzed Reaction

The catalytic oxidation of carbon monoxide (CO) on a platinum (111) surface was studied by scanning tunneling microscopy. The adsorbed oxygen atoms and CO molecules were imaged with atomic resolution, and their reactions to carbon dioxide (CO2) were monitored as functions of time. The results allowed the formulation of a rate law that takes the distribution of the reactants in separate domains into account. From temperature-dependent measurements, the kinetic parameters were obtained. Their values agree well with data from macroscopic measurements. In this way, a kinetic description of a chemical reaction was achieved that is based solely on the statistics of the underlying atomic processes

Photographie d'une consultation mémoire à l'hôpital neurologique de Lyon et description des pathologies diagnostiquées

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