Catalyseurs à base de métaux nobles supportés sur carbone pour l’oxydation sélective en phase liquide : préparation par adsorption contrôlée

(eng) This work deals with the optimization of the synthesis of carbon-supported catalysts based on Pd, Pt and/or Au, prepared by impregnation in aqueous solution. These catalysts are active and selective in the oxidations of glyoxal into glyoxalic acid and of glucose into gluconic acid. The first part of this work is organized in three sections. First, we determined the physicochemical characteristics of the two active carbons used as supports, and particularly their isoelectric point and their point of zero charge. Secondly, we identified the pH windows where the metal adsorption on the support in the aqueous phase was maximized. This has been realized for metal precursors differing in their charge and the oxidation state of the metal and with the two carbonaceous supports. Thirdly, we synthesized mono- and bimetallic catalysts by impregnation in aqueous solution by controlling the synthetic pH throughout the synthesis process, in order to keep it within the optimal adsorption range. The main factors varied during the preparation were the impregnation and activation pH values, the nature of the solubilizing agent, the incorporation order of the metals and the oxidation state of the first metal introduced when adsorbing the second one in the case of the bimetallic systems. Each sample prepared was characterized by physical and chemical techniques, which showed that the preparation method used in this work can produce materials with a homogeneous distribution of small metal particles of Pt and Pd (< 10 nm) on the carbon surface. The second part of this study has been dedicated, on the one hand, to the measurement of the catalytic performances of the materials in the liquid phase selective oxidation of glyoxal and glucose, and on the other hand, to the determination of the rate constants for each consecutive and parallel step in glyoxal oxidation, which displays a complex reaction scheme. In glyoxal oxidation, convincing results have been obtained for Pd/C and Au/Pd/C systems. In the second reaction, a conversion of 97% and a selectivity of 100% have been observed after 20 h of reaction in the case of Au/Pd/C catalytic system. The XPS, CO chemisorption, SEM/EDAXS, TEM and XRD characterizations of the most active catalysts have indicated that the catalytic performances are related to the presence of small Pd particles (> 10 nm) on the external surface of carbon. In the case of Pt/C materials, particles were very small but localised inside the carbon pores and the activity is disappointing. All Au/C catalysts were inactive. However, gold has shown a promoter effect on platinum and palladium in both oxidation reactions.