Au-based catalysts present excellent low temperature activity and selectivity for partial oxidation reactions, but the fundamental issue of atomic oxygen availability, the key parameter to obtain such reactivity, remains present. To enable the O2 dissociation reaction, Au nanoparticles must be smaller than 5 nm, which induces structural issues for upscaled applications. Alloying Au creates synergistic catalytic effects, and this option is investigated here: using residual amounts of Ag enables the O2 dissociation, regardless of the size of the Au-Ag catalyst. Questions remain about the precise interplays between the surface structure, the composition and the observed reactivity and selectivity. The aim of this thesis is to investigate the phenomena occurring during oxidation catalysis on Au-Ag surfaces, at a molecular scale, using surface science techniques.Nanoporous (np) Au structures with residual amounts of Ag (1-5 at.%) are increasingly used as catalyst for oxidation reactions. They are made of an interconnected array of three-dimensional ligaments (20-70 nm in diameter) presenting highly crystalline structures and exposing different crystallographic facets with different properties. These structural features allow for a bottom-up surface science investigation using Au-Ag samples with increasing structural complexity. The used experimental approach provides representative data on single crystals (using photoemission electron microscopy (PEEM) and quadrupole mass spectrometry (QMS)), on model nanoparticles (using field ion microscopy (FIM) and field electron microscopy (FEM)) and on np-Au samples (using temporary analysis of products (TAP) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)) and allows to corroborate results from the different techniques. During experiments with the respective techniques, these samples are exposed to O2, NO2, H2, CO, CH4, CH3OH as well as their analogous reactive gas mixtures to study the corresponding oxidation reactions.PEEM experiments allowed to determine the crucial importance of low coordinated surface atoms to achieve reactivity towards O2 dissociation. This assessment became even clearer when no reactivity towards oxidation of any of the probed reactions was determined on fully Ag covered Au(111) surfaces. Model nanoparticles used in FIM and FEM expose a multitude of crystallographic facets, allowing to simultaneously study facets with varying catalytic properties and understand the influence of connective properties such as surface diffusion of adsorbates or the effects of surface reconstructions on neighbouring facets. On pure Au, the catalytic performances in oxidation reactions are highlighted but an external source of O(ads) was shown to be necessary. Pure Ag samples present a high susceptibility of oxidation which allows for reactivity towards all of the probed reactants, but ultimately leads to the deactivation of the surface by permanent oxidation. Au-8.8 at.%Ag samples present intermediate properties with an intrinsic ability to provide O(ads) from O2, and reactive regimes over prolongated periods of time are possible. Differences in activity are discussed with respect to various parameters such as the underlying crystallographic structures, the chemical composition and repartition of the adsorbates, the temperature, the reconstructions and compositional changes of the surface. Finally, experiments in TAP on np-Au-1.5 at.%.Ag confirmed the selectivity changes in the case of oxidising pre-treatments during methanol oxidation. These changes are corroborated in DRIFTS under ambient pressure conditions during CO oxidation.This work contributes to the clarification of elementary steps during the oxidation processes on Au-Ag surfaces. The possibility to extrapolate results from single crystal surfaces under reduced pressure conditions up to nanoporous structures under ambient pressure is shown. This allows the partial bridging of the materials and pressure gaps between studies undertaken by the surface science and the applied catalysis approaches. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Reprinted figure 3 with permission from American Physical Society. Readers may view, browse, and/or downloadmaterial for temporary copying purposes only, provided these uses are for noncommercial personal purposes.Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted,performed, displayed, published, or sold in whole or part, without prior written permission from the AmericanPhysical Society. / In dieser Arbeit wurden Gold- und PlatinNanopartikel in sphärischen Polyelektolyt-Bürsten (SPB) synthetisiert. Diese wurden zu mechanistischen Untersuchungen der p-Nitrophenol-Reduktion mittels Natriumborhydrid herangezogen. Dabei konnte der Mechanismus der Reaktion auf der Oberfläche der Nanopartikel aufgeklärt werden. Die Reaktion folgt einem Langmuir Hinshelwood (LH) Mechanismus. Hierbei adsorbieren beide Edukte auf die Oberfläche, bevor sie im zu p-Aminophenol umgesetzt werden. Nach der Reaktion desorbiert das Reaktionsprodukt. Mittels des LH Modells konnten für verschiedene Temperaturen die intrinsische Geschwindigkeitskonstante, sowie die Adsorptionskonstanten der Edukte bestimmt werden. Mit diesen Daten konnten dann die Enthalpie und Entropie der Adsorption der Edukte und die Aktivierungsenergie berechnet werden. Neben dem Reaktionsmechanismus wurde die Induktionszeit der p-Nitrophenol Reduktion untersucht. Hierbei konnte gezeigt werden, dass diese Totzeit der Reaktion wahrscheinlich auf eine Restrukturierung der Nanopartikeloberfläche zurückzuführen ist. Sie ist unabhängig von den eingesetzten Konzentrationen des Borhydrids, hingegen abhängig von der Konzentration an p-Nitrophenol auf der Oberfläche der Nanopartikel, was auf Restrukturierung der Nanopartikel durch p-Nitrophenol hindeutet. Zudem wurden Hinweise auf eine spontane Rekonstruktion der Nanopartikel gefunden, die unabhängig von der Konzentration des p-Nitrophenols ist. Des Weiteren wurde die katalytische Oxidation von Morin mit Manganoxid Nanopartikeln untersucht. Diese sind in der Polyelektrolytschale der SPB immobilisiert. Analysen der Reaktionskinetik der Morin Oxidation ergaben, dass auch in diesem Fall der LH Mechanismus vorliegt. Hierbei konnten die Adsorptionskonstanten und Geschwindigkeitskonstanten für verschiedene Temperaturen ermittelt werden und somit die Aktivierungsenergie der Oxidation sowie die Adsorptionsenthalpie und Entropie der Edukte. / In this work, gold and platinum nanoparticles were synthesized into spherical polyelectrolyte brushes (SPB) in order to apply them as catalysts for kinetic studies of the reduction of p-nitrophenol by sodium borohydride. It was found that the reaction follows the Langmuir-Hinshelwood (LH) mechanism where both educts must adsorb onto the surface of the catalyst in order to react. Thereby, the rate determining step is the surface reaction of both educts. After the reaction, the product desorbs from the surface and a free active site is formed. With this model the intrinsic reaction rate and the adsorption constants for both educts could be determined. The measurements at different temperatures allowed the calculation of the activation energy and the adsorption enthalpy and entropy of the educts. Besides the reaction mechanism, the induction time of the reaction was analyzed. Here, it was shown that the reason of this delay time is a restructuring of the nanoparticle surface. The induction time is solely dependent on the concentration of p-nitrophenol on the surface of the nanoparticles and independent of the applied concentrations of borohydride. Moreover, hints for a spontaneous reconstruction of the nanoparticles without p-nitrophenol were found. In the second part, the catalytic oxidation of morin by manganese oxide has been studied. These nanoparticles were embedded inside the polyelectrolyte layer of the SPB. These nanoparticles were used for systematic studies of the oxidation of morin with hydrogen peroxide. It was shown that in this case the reaction followed a LH kinetics as well. Here, the intrinsic rate constants and the adsorption constants could be obtained for different temperatures. The activation energy and the adsorption enthalpy and entropy could be determined accordingly. The adsorption enthalpy is exothermic in both cases.

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