Fischer—Tropsch synthesis catalyst particle size effects

A.A. Khassin, T.M. Yurieva, V.N. Parmon, Fischer Tropsch Synthesis over Cohalt Containing Catalysts in Slurry Reactor. Effect of the Metallic Co Particle Size on the Selectivity, React. Kinet. Catal. Lett. 64 (1998) 55 62. 

A controversial issue related to cobalt catalysts in Fisher-Tropsch synthesis is the structure-sensitive character of this reaction. Iglesia and co-workers [126,127] reported a large increase in activity when the cobalt particle size was decreased from 200 nm to 9 nm, whereas the specific activity [turnover frequency (TOF)] was not influenced by the cobalt particle size. However, other authors have reported that the TOF suddenly decreased for catalysts with cobalt particle sizes smaller than 10 nm [122,128]. Bezemer et al. [125] were the first to investigate the influence of cobalt particle size in the range 2.6 to 27 nm on performance in Fischer-Tropsch synthesis on well-defined catalysts supported on carbon nanofibers. It was found that the TOF for CO hydrogenation was independent of cobalt particle size for catalysts with particles larger then 6 nm (at atmospheric pressure) or 8 nm (at 35 bar). But both the TOF and the C5+ selectivity decreased for catalysts with smaller particles. It was proposed that the cobalt particle size effects could be attributed to a strucmre-sensitivity characteristic of the reaction, together with a CO-indnced reconstmction of the cobalt surface. 

Co/lTQ-2 catalysts prepared by reverse micellar synthesis using a surface silylated lTQ-2 delaminated zeolite [103] is characterized by the uniform Co° particle size distribution (5-11 nm). The authors revealed the effect of the Co particle size on the catalyst performance. At the Fischer-Tropsch synthesis conditions (493 K, 2.0 MPa), TOF increases from 1.2 x 10 to 8.6x 10 s when /(Co°) increased from 5.6 to 10.4nm, and then it remains constant up to a particle size of 141 nm. The Co surface reconstruction occurs during the reaction, no matter what the metal particle size is. Interfacial Co + sites are likely the reason behind the decreased TOF observed for d Co°) < lOnm (Fig. 9). 

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