# Mn-Promoted Co/TiO2 Catalysts: Quantitative Analysis of Cobalt Polymorphs and Stacking Faults and Its Effect on Fischer-Tropsch Synthesis Performance

## TL;DR

This paper investigates how manganese affects cobalt catalysts in Fischer-Tropsch synthesis, showing how it changes cobalt structures to improve fuel production efficiency.

## Contribution

The study quantitatively links Mn promotion to cobalt polymorphs and stacking faults, revealing how these structural changes enhance catalytic performance in Fischer-Tropsch synthesis.

## Key findings

- Mn increases HCP cobalt domains and Co2C formation, correlating with higher alcohol and olefin selectivity.
- Mn reduces FCC stacking faults while increasing HCP stacking faults, promoting structural transformations under reaction conditions.
- 3% Mn loading optimizes HCP content and catalytic performance, highlighting Mn's role in stabilizing and modulating cobalt structures.

## Abstract

The transition to
net-zero emissions hinges on circular
economy
strategies that valorize waste and enhance resource efficiency. Among
X-to-liquid (XTL) technologies, the Fischer-Tropsch (FT) process stands
out for converting biomass, waste, and CO2 into hydrocarbons
and chemicals, especially when powered by renewable hydrogen. Cobalt-based
catalysts are preferred in FT synthesis due to their efficiency and
CO2 tolerance, yet their catalytic performance is closely
tied to their polymorphic structuresface-centered cubic (FCC),
hexagonal close-packed (HCP), and stacking-faulted intergrowths thereof.
HCP cobalt has been shown to exhibit high activity and selectivity
for higher hydrocarbons and oxygenates, particularly when transformed
into cobalt carbide (Co2C), which forms more readily at
low H2/CO ratios. This study presents a quantitative analysis
of cobalt polymorphs and stacking faults in Mn-promoted Co/TiO2 FT catalysts from in situ powder X-ray diffraction (XRD)
data and X-ray Diffraction Computed Tomography (XRD-CT) data from
spent catalysts in order to obtain a more complete correlation of
structural features with catalytic performance. By modeling stacking
fault probabilities using supercell simulations, the proportion of
faulted FCC and HCP domains was determined across varying Mn loadings
(0–5%). Increased Mn loading was found to decrease stacking
faults in the FCC phase while increasing them in HCP, promoting the
formation of HCP domains and ultimately Co2C under reaction
conditions. Notably, the 3% Mn-loaded sample showed a marked rise
in HCP content and Co2C formation, correlating with the
highest observed alcohol and olefin selectivity. These findings highlight
a critical structure–function relationship: Mn facilitates
a transformation from FCC to HCP and then to Co2C, this
final transition driven by similar stacking sequences and metal–support
interactions. The findings show that Mn promotion not only stabilizes
smaller Co particles and enhances its dispersion, but also modulates
the distribution of Co polymorphs and stacking faults, leading to
altered catalytic behavior. This highlights the importance of stacking
fault characterization for optimizing FT catalyst design and performance,
and suggests pathways to more efficient and selective carbon-neutral
fuel production through engineered polymorphic and interfacial structures.