Cobalt Particle Size Effects in the Fischer−Tropsch Reaction Studied with Carbon Nanofiber Supported Catalysts

Abstract

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size 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. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. 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 the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.

Exploring the phase space of time of flight mass selected PtxY nanoparticles

“…16 Furthermore, Au nanoparticles showed a remarkably high activity for CO oxidation below 5 nm particle diameter, reaching a maximum in activity at 2-3 nm size. 17 Ruthenium has interesting catalytic properties for CO oxidation if prepared in the form of nanoparticles of diameter between 2 nm and 6 nm.…”

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…Recently, the increase in demand for transportation fuels and the large reserves of natural gas have made FTS an attractive industrial process. In the FTS process, cobaltbased catalysts, owing to low water-gas shift activity and remarkable stability, is considered to be the favorable catalyst for the synthesis of long-chain hydrocarbons from synthesis gas with a H 2 /CO ratio about 2 [5][6][7].…”

Fischer‒Tropsch Conversion of Biomass‐Derived Synthesis Gas to Liquid Fuels

“…However, it has been demonstrated how site time yield decreases dramatically for very small cobalt particles, i.e. below a cobalt particle size of 6-8 nm 27 , which defines an optimum on how efficiently cobalt can be used. Readers who are interested in more information on preparation and characteristics of cobalt catalysts are referred to the review by Khodakov et al 11 .…”

“…In contrast to the effect on Co catalysts, alkali is known to promote desirable effects in iron catalysts and potassium is commonly added as a promotor 27,33 . A recent study have also shown that in operando addition of 400 ppbw KCl and NaCl, and also 40 ppmw NaHCO3 and KHCO3 with the synthesis gas, did not have any measureable impact on catalytic activity on a precipitated iron catalyst after 400 hours of operation 77 .…”