Abstract
The Fischer-Tropsch (FT) process converts synthesis gas (H2/CO) over a heterogeneous catalyst into hydrocarbons. Generally, cobalt catalysts supported on oxidic carriers are used for the FT process, however it appears to be difficult to obtain and maintain fully reduced cobalt particles. To overcome these problems we started to use carbon
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nanofibers (CNF), a novel support material on which cobalt-support compounds are not expected to form.
In chapter 2 we describe our research on the preparation of catalysts on CNF and on silica using the HDP method. Besides the conventional deposition starting from acidic solution we also performed experiments starting from basic solution. Presented results demonstrate that the newly developed method yields better-dispersed Co catalysts on both supports compared to the conventional low-pH method. Due to the larger cobalt surface area we found 2-4 times higher FT activities for samples prepared from basic environment, with similar or better catalyst selectivities.
In chapter 3 the influence of cobalt particle size on the Fischer-Tropsch reaction was investigated using catalysts with average cobalt particle sizes varying from 2.7 to 27 nm. It was found that the turnover frequency (TOF) was not affected for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in literature, viz. formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence. Interestingly we found with EXAFS a decrease of the cobalt coordination number, which points to reconstruction of the surface. Consequently, it is argued that the cobalt particle size effects can be attributed to CO-induced non-classical structure sensitivity. The profound influence of cobalt dispersion on FT performance found in our work is very relevant for the design of economic Fischer-Tropsch catalysts.
Chapter 4 deals with the preparation and testing of MnO-promoted cobalt catalysts supported on CNF. We demonstrated with XPS and STEM-EELS that in the dried samples the promoter was only present associated with the cobalt particles. After reduction at 350 ºC the manganese phase remained in the oxidized state, localized close to the cobalt particles, as the inert support material lacked sites with significant interaction with metal oxides. This close association between cobalt and MnO resulted in significant improvement in catalytic performances even with very low concentrations of promoter present.
It has been mentioned before that CNF consists of interwoven fibers. In chapter 5 it is shown that these individual fibers have an inner tube that is opened during the activation treatment in nitric acid. The preparation of cobalt and palladium catalysts with impregnation and ion exchange, respectively, resulted in the deposition of metal particles inside the fibers. Using TEM tilt series and quantitative XPS we showed that the weight fraction of metal deposited in the inner tube is related to the fraction of surface area or pore volume related with this tube.
Presented results demonstrate that the use of CNF helped to solve fundamental issues related with Fischer-Tropsch synthesis. Moreover, we discovered ways to enhance both activity and selectivity of these catalysts, which is very interesting for the development of new Fischer-Tropsch catalysts
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