Nanostructures in Catalysis: Au/TiO2 for the Oxidation of CO at Low Temperature and Confinement Effect in Siloxane Nanocages

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Two subjects are the focus of this dissertation: Au/TiO2 catalyst used for oxidation of CO at low temperature and siloxane nanocages, a novel material (~2nm) recently developed in our group While bulk gold is chemically inert, gold supported as nanoparticles is remarkably active for the oxidation of carbon monoxide at low temperature. The reaction has been intensively studied in the last years; however, the origin of this unusual activity is not yet fully understood. In particular, the nature of active site and the reaction mechanism have not been clearly identified. In this work, a combination of in-situ XANES, EXAFS and FTIR with microreactor studies are used to develop a structure-activity relationship by monitoring the structural changes during the activation of an inactive catalyst. In addition, the reaction pathway is investigated by identifying the surface species directly involved in the reaction and quantifying their transformation rates under transient and steady-state conditions. It is demonstrated that metallic nanosized gold is a necessary component of the active site. Its role is to activate CO, which is subsequently oxidized at the catalytic centers. Information on the nature of the catalytic centers is obtained through the identification of adsorbed hydroxycarbonyl species, via a combination of in-situ FTIR, 18O2 isotopic labeling and mass spectrometry. All together, this study suggests that nanosized gold provides the necessary unsaturated sites where CO is activated Key properties of the novel siloxane nanocages, including its protonation response to variations in the external pH, are investigated using Au3+ as a probe. The binding of Au3+ to the amine groups inside the nanocage is studied by a combination of XANES, EXAFS, UV-VIS and cyclic voltammetry. A remarkable reduction of five pH units on the Brønsted basicity of propylamine groups within the nanocage, consequence of their nano-confinement, occurs. When the amine groups inside the nanocage are in excess with respect to gold, they act as a chelating ligand. In addition, all the Au3+ inside the nanocage are chemically bound to the amine groups, for all amine/gold ratios

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  • 08/28/2018
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