Tech Briefs Magazine - May 2024 - 30

" It's very unrealistic for large-scale CO₂
mitigation, " Chen said. " In contrast, we
found a process that can occur at about
400 °C, which is a much more practical,
industrially achievable temperature. "
The trick was to break the reaction into
stages and to use two different types of catalysts
- materials that make it easier for
molecules to come together and react.
" If you decouple the reaction into several
sub-reaction steps you can consider
using different kinds of energy input and
catalysts to make each part of the reaction
work, " said Brookhaven Lab and Columbia
Research Scientist Zhenhua Xie, lead
author on the paper.
The scientists started by realizing that
Scientists have devised a strategy for converting carbon dioxide (CO2
) from the atmosphere into
valuable carbon nanofibers. The process uses tandem electrocatalytic (blue ring) and thermocatalytic
(orange ring) reactions to convert the CO2
(teal and silver molecules) plus water (purple and
teal) into " fixed " carbon nanofibers (silver), producing hydrogen gas (H2, purple) as a beneficial
byproduct. The carbon nanofibers could be used to strengthen building materials such as cement
and lock away carbon for decades. (Image: Zhenhua Xie/Brookhaven National Laboratory and Columbia
University; Erwei Huang/Brookhaven National Laboratory)
carbon monoxide (CO) is a much better
starting material than CO₂ for making
carbon nanofibers (CNF). Then they
backtracked to find the most efficient way
to generate CO from CO₂.
Earlier work from their group steered
them to use a commercially available electrocatalyst
made of palladium supported
on carbon.
Electrocatalysts drive chemical reactions
using an electric current. In the
presence of flowing electrons and protons,
the catalyst splits both CO₂ and water
(H₂O) into CO and H₂.
HAADF
C
Ce
5nm
Fe
Co
Fe Co Ce C
High-resolution transmission electron microscopy (TEM) shows the tip of the resulting carbon nanofiber
(left) on the iron-cobalt/cerium oxide (FeCo/CeO2
high-angle
annular dark
) thermocatalyst. Scientists mapped the strucfield
(HAADF)
imaging,
ture and chemical composition of newly formed carbon nanofibers (right) using scanning transmission
electron microscopy (STEM),
and
energy-dispersive x-ray spectroscopy (EDS) (scale bar represents 8 nanometers). The images show
that the nanofibers are made of carbon (C), and reveal that the catalytic metals, iron (Fe) and cobalt
(Co), are pushed away from the catalytic surface and accumulate at the tip of the nanofiber. (Image:
Center for Functional Nanomaterials/Brookhaven National Laboratory)
The idea of capturing CO₂ or converting
it to other materials to combat climate
change is not new. But simply storing CO₂
gas can lead to leaks. And many CO₂ conversions
produce carbon-based chemicals
or fuels that are used right away, which
releases CO₂ right back into the atmosphere.
" The novelty of this work is that
we are trying to convert CO₂ into something
that is value-added but in a solid,
useful form, " Chen said.
30
Such solid carbon materials - including
carbon nanotubes and nanofibers
with dimensions measuring billionths of a
meter - have many appealing properties,
including strength and thermal and electrical
conductivity. But it's no simple matter
to extract carbon from carbon dioxide
and get it to assemble into these fine-scale
structures. One direct, heat-driven process
requires temperatures in excess of
1,000 °C.
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For the second step, the scientists
turned to a heat-activated thermocatalyst
made of an iron-cobalt alloy. It operates at
temperatures around 400 °C, significantly
milder than a direct CO₂-to-CNF conversion
would require. They also discovered
that adding a bit of extra metallic cobalt
greatly enhances the formation of the carbon
nanofibers.
" By coupling electrocatalysis and
thermocatalysis, we are using this tandem
process to achieve things that
cannot be achieved by either process
alone, " Chen said.
For the second stage, " We wanted to
know what's the structure of the iron-cobalt
system under reaction conditions
and how to optimize the iron-cobalt catalyst, "
Xie said. The x-ray experiments
confirmed that both an alloy of iron and
cobalt plus some extra metallic cobalt are
present and needed to convert CO to carbon
nanofibers.
" The two work together sequentially, "
said Liu, whose DFT calculations helped
explain the process.
" According to our study, the cobalt-iron
sites in the alloy help to break the C-O
bonds of carbon monoxide. That makes
atomic carbon available to serve as the
source for building carbon nanofibers.
Tech Briefs, May 2024

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Tech Briefs Magazine - May 2024

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