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Within this work a detailed study on the deposition of precious metals on carbonaceous supports based on carbide-derived carbons for the application in heterogeneous catalysis is presented. The advantage of carbide-derived carbons in contrast to common carbon-support materials like activated carbon or carbon black is their high purity, reducible material properties from batch to batch and tailorable pore size distribution. Scope of the study is to clarify, whether theses novel materials can be used as carbon model catalyst support. Therefore, porous carbons were derived from silicon carbide and titanium carbide, the pore size distribution varied from a narrow microporous up to a broad mesoporous one, the deposition of platinum and palladium as catalytically active species by the ion adsorption studied and different catalytic experiments carried out. For the deposition of catalytically active species oxygen is introduced to the carbon surface via post-treatment of the carbon with HNO3, H2SO4 or KOH. The degree of functionalization hereby depends strongly on the graphitic character of the carbon as well as the oxidizing character of the agent. Functionalization parameters hence have to be adapted towards the respective material used in order to preserve its previously adjusted pore structure and were derived for the materials studied in this work. It could be shown that the fundamentals of ion-adsorption can be adapted to these oxygen functionalizes carbide-derived carbon, to deposit the platinum precursor within the pore structure. As a consequence it was possible to tailor the metal cluster sizes by the impregnation and reduction conditions. The catalysts were applied in the hydrogenation of thymol, a competitive reaction of different aromates, the aqueous phase reforming and the direct-methanol fuel cell. In some of the studies shape-selective effects and structure-property relationships could be deduced. In the thymol hydrogenation the catalytic activity showed a strong dependence on metal cluster size and the pore size of the support. A molecular sieving character which can be induced by the support get’s obvious. Yet, due to the complex reaction network no structure- selectivity relationships could be derived. In contrast, the competitive reaction of the differently sterical demanding substrates tri-tert-butylbenzene and trimethylbenzene obviously shows shape-selective effects of these support materials. Also in the energy-relevant applications of the aqueous phase reforming and the direct-methanol fuel cell catalysts supported on carbide-derived carbon feature highly promising activity and selectivity, the later one due to the exceptional low acidity of the support. This work shows that carbide-derived carbons can be used effectively to deduce structure-property relationships of porous carbon materials, if the pore structure is not altered to much during the active metal deposition and that the resulting catalysts feature a promising potential in various applications. |
