Monitoring Cobalt Reduction and Carbide Formation of Fischer- Tropsch catalysts using in-situ X-ray diffraction
Different reduction routes to produce a cobalt metal based Fischer-Tropsch catalysts were investigated using in-situ diffraction. The catalyst performance (activity and selectivity) was identified to be directly linked to these early treatments.
Fischer-Tropsch (FT; also known as gas-to-liquids GTL) is a well-established technology converting syngas to liquid hydrocarbons in a fixed-bed reactor. Given FT produces carbon chain lengths from C1 to over C100, cracking and upgrading of the high purity/clean wax product to make fuel fractions such as diesel, gasoline or jet fuel is normally required.
In many cases the fresh loaded catalyst is in form of cobalt oxide spinel (Co3O4) on a support which undergoes multiple steps to reach the active catalyst inside the reactor. Cobalt crystallites of the fresh (calcined) catalyst are in oxidic form, whereas FT occurs on the metallic surfaces. Hence, understanding the behavior of catalyst transformation during reduction is vital in terms of optimizing the selectivity, activity and cycle life of a commercial process. Furthermore, crystallite size, phase and stability under real FT conditions play a crucial role in the final catalyst performance. E.g. Large crystallites leads to a reduction of the surface area which is directly linked to a reduced activity. To compensate this effect, a higher reactor temperature is required which however favors unwanted methanation. More edge sites for a given cobalt loading can be obtained with smaller particles; this favours chain termination and again methane formation. An optimum size of 8 – 10 nm is suggested in FT literature.
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