Nano-carbon supported cobalt catalysts in Fischer-Tropsch synthesis

Despite of the rapid pace of technological progress and the availability of novel analytic technologies, the principle of operation of solid catalysts in heterogeneous catalysis is still not fully understood. In most cases catalytic systems are still mostly considered as „black boxes“. This point of view is especially pronounced when reactions are comprised of a complex network of consecutive and simultaneous reactions. The hydrogenation of carbon monoxide, namely the Fischer-Tropsch synthesis (FTS), represents such a reaction. In FTS, long chained hydrocarbons are formed via surface polymerization of carbon monoxide and hydrogen, which reveals a promising possibility for the production of transportation fuels from non-crude-oil feedstocks. However, the production of hydrocarbons with a specific chain length is not possible, imposed by the reaction mechanism by itself. Obviously, the selectivity (and also activity) of the applied catalyst predominantly controls the economic efficiency. In this context, nano-carbon utilized as support material has proven its potential. The unique structural properties affect the electronic specification of the anchored metal particles yielding in improvement in selectivity as well as activity. Moreover, the structural features turn this type of support into an auspicious system for examination of factors which are relevant for successful catalyst optimization. Hence, the primary scope of this work is to elucidate how catalyst modification (such as the variation of dispersion of the active metal and introduction of additional promoters) as well as altered feed gas composition influences the catalytic activity and selectivity in FTS. Resting upon this, conclusions are drawn according to the nature of the reaction mechanism and active sites in order to make a further contribution to heterogeneous catalysis involving nano-carbon supported systems. It was found that especially carbon nano-tube (CNT) supported cobalt catalysts reveal a remarkably low methane selectivity. In conjunction with a high chain growth probability, this system offers great possibilities in terms of production of transportation fuels using non-crude-oil feedstocks. Methane production is further reduced by introduction of manganese as promoter which elucidates the potential of Mn in terms of catalyst performance improvements and makes this metal attractive even for non-carbon supports. But, manganese promotion shifts the product selectivity to lower hydrocarbons which lowers the yield of the gasoline and diesel oil fraction. However, this kind of catalytic material features relative insensitivity towards the variation of reactant concentrations potentially facilitating process control in terms of reactor management. Moreover, a method for tailoring Co particle sizes on nano-carbon materials was established, which carries capabilities with it not only for cobalt based systems. Furthermore, the kinetics of these novel nano-carbon based systems were thoroughly determined. Here, a conventional Langmuir-Hinshelwood approach is capable of describing the activity behavior of a Co/CNT catalyst sufficiently, although CNT supported systems reveal an exceptional behavior in activity and selectivity (e. g. significantly reduced methane formation). This is particularly advantageous for (semi-) industrial processes when it is necessary to predict the behavior of the catalyst at several reaction conditions.