Structure-performance relationships of cobalt supported on carbon materials in the Fischer-Tropsch synthesis
Have, I.C. ten
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Fischer-Tropsch synthesis over supported cobalt catalysts is an important industrial process for the conversion of synthesis gas to hydrocarbons. Although enormous efforts have yet been devoted to understand the fundamental aspects of the Fischer-Tropsch synthesis, the influence of the support structure still remains subject of debate. This is largely caused by poorly reported synthesis procedures and reaction conditions, which makes reproducibility and mutual comparison difficult. In this research, mesoporous carbon supports with well-defined ordered pore structures were synthesized. These carbon materials ruled out effects induced by variations in pore size and mixed metal-support phases. Thereby, quantitative analysis of the results and visualization with electron microscopy was facilitated. After examination of the catalyst synthesis procedure, it was found that defects in the carbon structure and oxygen functionalities on the carbon surface could serve as anchoring points and prevent undesired particle growth during catalysis. Confinement of cobalt nanoparticles in support pores was also found to be a powerful tool for preparing stable catalysts. Two phenomena had to be considered when pore confinement was used to stabilize cobalt nanoparticles during catalysis. Firstly, the pores should be large enough to enclose at least 6 nm sized cobalt particles. Smaller particles mainly produced methane, which is a valueless Fischer-Tropsch synthesis product. Secondly, pore blocking should be considered. Especially when pores are only accesible from two sides, pore blocking could shield part of the active metal species from reactants, resulting in low catalytic activity. When cobalt nanoparticles were insufficiently stabilized by the support material, particle growth occurred. If this happened to a large extent, the amount of catalytically active sites decreased, resulting in lower catalytic activity. Carbon-supported cobalt catalysts seemed to display two different particle growth mechanisms. Ostwald ripening was proposed when cobalt nanoparticles had initially a bi- or multimodal size distribution. Migration and coalescence was proposed when cobalt particles were uniformly sized. Finally, a true understanding of structure-performance relationships will provide rational catalyst design and ultimately the development of highly active, selective, and stable catalysts, both for fundamental research and large-scale industrial processes.