# Shaping and Stabilizing the Active Phase: The Role of Carbon Surface Defects in Carbon-Supported Co Fischer–Tropsch Synthesis Catalysts

## TL;DR

This paper explores how carbon surface defects stabilize cobalt nanoparticles in Fischer–Tropsch synthesis catalysts, improving their performance.

## Contribution

The study identifies carbon gasification-mediated anchoring as a key mechanism for stabilizing Co nanoparticles on carbon supports.

## Key findings

- Defect-rich carbon supports stabilize Co nanoparticles more effectively than defect-poor ones.
- Carbon gasification at the Co/C interface generates reactive bonds that anchor nanoparticles.
- Co phase transformations correlate with CO2 and CH4 evolution during reduction.

## Abstract

Carbon supports offer a promising alternative to conventional
oxide
supports for cobalt-based Fischer–Tropsch synthesis (FTS) catalysts.
However, unlike well-studied oxide systems (e.g., Co/Al2O3, Co/TiO2), the fundamental interactions
between cobalt nanoparticles (Co NP’s) and unfunctionalized
carbon surfaces remain poorly understood, largely due to the structural
and chemical diversity of carbon materials. Establishing a universal
“baseline” interaction for Co/C interfaces has therefore
remained elusive. In this work, we investigated Co anchoring mechanisms
on two carbon black model supports that differ by a factor of 20 in
surface defect (chemisorption) site density but exhibit otherwise
similar properties. On this basis, Co-based catalysts were synthesized
using size-controlled colloidal Co nanoparticles and conventional
incipient wetness impregnation. Employing high-resolution SEM and
HAADF STEM imaging, we could show that Co NP sintering occurs predominantly
via nanoparticle migration and coalescence during catalyst reduction,
with negligible additional growth under FTS conditionsimplying
that Co NP anchoring is established in the reduction step. Combined
in situ XANES/XRD experiments during reduction, coupled with off-gas
analysis by online mass spectrometry, showed that Co phase transformations
coincided with significant CO2 and CH4 evolution.
This was attributed to carbothermal reduction and carbon hydrogasification
at the Co/C interface, which appeared to correlate with the density
of carbon surface defect (chemisorption) sites. We hypothesize that
carbon gasification at the Co/C interface is directly linked to the
immobilization of Co NP, as it generates highly reactive “dangling
bonds” at the Co/C interface, which act as anchoring points.
Overall, the defect-rich carbon support stabilized Co nanoparticles
more effectively than its defect-poor counterpart, resulting in most
cases in higher FTS activity. Our results imply carbon gasification-mediated
anchoring as a “baseline” interaction for Co/C catalysts
and suggest that the chemisorption site densityas measurable
by simple TPD or TPOcan serve as a practical descriptor for
designing more stable carbon-supported FTS catalysts.