Co Metal Based Catalysts Probed by Ferromagnetic 59Co NMR
An interest of Co metal based catalysts characterization has risen simultaneously to Fischer-Tropsch synthesis (FTS) due to new opportunities of using biomass for syngas production. New perspectives require novel catalysts to overcome existing limitations of industrially utilized FTS catalysts. Co FTS catalysts are known to produce not only diesel or lubrication oil, but also more valuable products such as alcohols, ethers and olefins. Despite many techniques have been implemented  to probe the structure of Co FTS catalysts, the application of ferromagnetic 59Co NMR (f-NMR) is still challenging . The main advantage of f-NMR consists in the type of information providing by the method. First, it gives structural insight into studied active component of FTS catalysts reflecting different Co metal stacking as hcp, fcc or stacking faults (sfs). Second, f-NMR is sensitive to different magnetically ordered parts of sample as magnetic domains, domain walls and single-domain particles. And the last but not least, it is highly sensitive to various substitutions in the
Co coronation sphere, which causes a substantial shift of resonance lines due to increase/decrease of magnetic moments on Co nuclei. A set of different types of Co FTS catalysts has been examined both ex situ and in situ. Co supported on γ-Al2O3 has been studied ex situ after activation (fresh catalyst) and after the redox treatment represented by reduction-oxidation-reduction procedure to follow up the catalyst stability to partial uncontrolled oxidation during FTS synthesis. The obtained results on a set of Co/γ-Al2O3 catalysts activated in a different way reveal the drastic particle size changes because of the redox treatment. On the other hand, in situ examination of Co/SiC catalyst performed by sealed glass ampoule heating inside the NMR probe in the range of 300-850 K reveals wide temperature range of sample stability with increasing temperature. The fact is important in terms of legitimation of room and low temperature spectra acquisition of catalytic species, which are actually exposed to reagent mixture at heightened temperature.