A radical new catalyst design has been shown to efficiently convert carbon dioxide into carbon monoxide, potentially allowing it to be translated into a valuable energy source.
Carbon monoxide (CO) has a wide range of uses in industry. It can be combined with water to produce super-potent hydrogen gas, or combined with hydrogen itself to produce hydrocarbons and alcohols.
However, converting it from carbon dioxide (CO2) has been either impossible or impractical, depending on the technology used.
That may all be about to change, thanks to a team of researchers led by Eli Stavitski from the US Department of Energy’s Brookhaven National Laboratory in New York.
In a paper published in the journal Energy & Environmental Science, Stavitski and colleagues reveal a proof-of-concept CO2-CO converter that uses atoms embedded in a graphene sheet and achieves an efficiency of up to 97%.
Current model CO2 converters, known as electrocatalysts, don’t produce carbon monoxide because their inherent chemistry forces them to perform a different operation, known as the hydrogen evolution reaction (HER) or “water-splitting”.
It is technically possible to build an electrocatalyst that avoids HER, but only by using gold, platinum or one of a couple of noble gases – an approach that is far too costly to carry out on any scale.
To make their new type of catalyst, Stavitski’s team opted for what at first seems like a surprising choice: nickel. This is a metal known for inducing a particularly robust form of HER, and also quickly becomes “poisoned” by carbon monoxide molecules adhering to its surface.
Instead of deploying it in bulk, however, the scientists opted to use it in the form of individual atoms – producing a very different kind of reaction.
“The surface of a metal has one energy potential – it is uniform,” explains co-author Klaus Attenkofer, “whereas on a single atom, every place on the surface has a different kind of energy.”
Because of this, the catalytic result is very different.
“Single atoms prefer to produce CO, rather than performing the competing HER, because the surface of a bulk metal is very different from individual atoms,” says Stavitski.
To further suppress any remaining tendencies towards HER, and to provide a practical substrate, the researchers embedded the nickel atoms in a sheet of graphene.
Using scanning transmission electron microscopy (STEM) to monitor the behaviour of the nickel-graphene combination, the scientists discovered the atom-thin carbon sheet was ensuring the efficiency of the catalytic conversion by preventing the nickel moving around.
“Single atoms are usually unstable and tend to aggregate on the support,” says co-author Dong Su. “However, we found the individual nickel atoms were distributed uniformly, which accounted for the excellent performance of the conversion reaction.”
With the excellent conversion rate achieved by the proof-of-concept, the Brookhaven team is now looking to find ways to scale up, with a view to large-scale production.
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