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

TLDR

X‑ray absorption spectroscopy showed cobalt remained metallic after reduction, a prerequisite for catalysis that is hard to achieve on oxidic supports, and it did not support alternative explanations involving CoO or Co carbide formation on small particles. The study investigates how cobalt particle size (2.6–27 nm) affects Fischer–Tropsch synthesis performance using well‑defined carbon‑nanofiber‑supported catalysts. The authors prepared well‑defined cobalt catalysts on inert carbon nanofibers and examined their size‑dependent activity in Fischer–Tropsch synthesis. Catalysts with cobalt particles larger than 6–8 nm exhibited size‑independent turnover frequencies, whereas smaller particles showed reduced TOF and C5⁺ selectivity, with TOF dropping from 23 × 10⁻³ to 1.4 × 10⁻³ s⁻¹ and selectivity from 85 % to 51 % at 35 bar as size decreased from 16 to 2.6 nm, indicating a minimal effective size of 6–8 nm and suggesting nonclassical structure sensitivity coupled with CO‑induced surface reconstruction that may guide future catalyst design.

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 × 10-3 to 1.4 × 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.