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In this thesis, the selective oxidation of glycerol in the aqueous phase over heterogeneous catalysts is investigated. The work is divided into two main parts, which are based on conventional thermocatalysis and surface plasmon-assisted photocatalysis, respectively. In the first part, the study focuses on the use of non-precious Copper-Cobalt-based catalysts for the conventional thermocatalytic oxidation reaction. The second part is devoted to the exploitation of the plasmonic properties of TiO2 supported gold nanoparticles in order to assist the thermocatalytic reaction by plasmonic photocatalysis, i.e., photon energy is used together with thermal energy to accelerate the reaction.
In the first part of the thesis (Chapter 4), the use of copper-cobalt-based materials for the glycerol oxidation in the aqueous phase is described. Two different synthesis procedures were employed for the catalyst preparation. For the first procedure, metallic copper particles supported on ordered mesoporous cobalt monoxide were prepared through a novel approach by using a nanocasting method with subsequent post-treatments. A 3D ordered mesoporous silica, KIT-6, was employed as removable template for the preparation of ordered mesoporous copper cobalt oxide spinels. A mild reduction heat treatment with ethanol vapor was used to reduce the copper-cobalt oxide spinel to form metallic copper nanoparticles supported on ordered mesoporous cobalt monoxide. As a counterpart, non-ordered cobalt monoxide supported copper particles were prepared by the second procedure presented herein, consisting of a facile co-precipitation method with subsequent post-treatments. Both catalyst types were investigated for the glycerol oxidation reaction. Catalysts prepared by the facile co-precipitation method exhibited a superior catalytic performance with decent recyclability compared to the corresponding mesostructured samples. Careful characterization of the materials before and after the reaction provided important insights into the potentially active crystalline phases of the materials and explained the distinguished differences observed in the catalytic performance between the mesostructured and non-structured catalysts. It was shown that the in situ generation of cobalt oxyhydroxide in close contact with the formed copper oxide has a decisive impact on the catalytic activity. The synergy between these two crystal phases seemed to enhance the catalytic activity – whereas each phase alone showed strongly diminished glycerol conversions. Furthermore, different solvents (apart from water) were introduced for the oxidation reaction and it was shown that certain alcohols as co-solvents have a beneficial impact on the catalytic activity of the materials.
The second part of this thesis (Chapter 5) is dedicated to the relatively new field of plasmonic photocatalysis and its efficient integration into the well-studied thermocatalytic process of selective glycerol oxidation over supported gold catalysts. The photocatalytic part of this reaction process supports the thermocatalytic oxidation reaction, augmenting the overall catalytic performance. Hence, the term surface plasmon-assisted glycerol oxidation is coined in this work. Titania supported gold catalysts with two different morphologies were investigated for the reaction process. It was shown that a core-shell morphology with a gold core and a titania shell exhibited no photocatalytic enhancement for the glycerol oxidation. On the other hand, small gold nanoparticles deposited on commercial titania showed a several fold increase in conversion for reactions illuminated by visible light compared to the analogous reactions conducted in the dark. Generally, high selectivities toward dihydroxyacetone, a highly desired product used in the cosmetic industry, were obtained. Moreover, hydrogen peroxide was identified as a key intermediate and essential for obtaining improved catalytic activities with visible light irradiation.
