MoP-ing up the CO2-petition

Converting carbon dioxide to useful products presents a difficult task, but metal phosphides are up to the challenge.
MoP-ing up the CO2-petition

The ongoing rise in global temperatures requires action. One strategy to reduce carbon dioxide emissions is for humanity to curb its fossil-fuel-fueled endeavors, but this is easier said than done. Societies need cheap, abundant energy sources and burning long-dead things satisfies this need, for now. Renewable energy technologies, including solar and wind power, have made vast leaps in the last few years, so there is a foreseeable end to massive-scale carbon dioxide release. But industrial practices won’t shift overnight, so the transition from carbon-intensive to carbon-neutral activities will be slow. Until then, carbon dioxide capture may be the best way to stay below the Paris Accord’s “2 ˚C” threshold (if that is even possible).

One way to convert carbon dioxide to more interesting, useful products is to reduce the molecule using electrochemistry. Carbon monoxide, formate, methanol ,and methane are among the simplest C1 products that are or have the potential to be industrial feedstocks. However, carbon dioxide is fairly inert, relatively unreactive, and requires multiple electrons, drastically complicating kinetics. These hurdles render carbon dioxide reduction research particularly tricky, on top of the desire to employ abundant, inexpensive materials that can perform the process on an industrial scale.

Professor Buxing Han and colleagues at the Institute of Chemistry, Chinese Academy of Sciences in Beijing have recently published a report on a metal phosphide capable of reducing carbon dioxide electrochemically. The authors prepare an indium-doped carbon scaffold from an indium-containing metal-organic framework. Molybdenum addition during the carbonization process produces MoO2 nanocrystals that are subsequently phosphidized by heating with ammonium phosphate. These procedures result in a porous, carbonaceous In-doped material with embedded MoP nanoparticles.

In an ionic liquid electrolyte, this material predominantly reduces carbon dioxide to formate, an industrially relevant product. The formate selectivity reaches 96.5% at 43.8 mA cm-2, one of the highest current densities reported at such high Faradaic efficiencies. Control experiments suggest that the indium, MoP, and ionic liquid electrolyte are all crucial for the high activity and selectivity. Furthermore, binding-studies with sulfate ions suggest that MoP nanoparticles have a high binding and stabilization propensity for the 1-electron reduced carbon dioxide intermediate, providing a clue as to this composite material’s strong carbon dioxide reduction performance.

This contribution provides an interesting stepping stone for electrocatalytic carbon dioxide conversion, but with several caveats. Whereas carbon, molybdenum, and phosphorous are fairly abundant, indium is somewhat scarce. Furthermore, this composite’s high selectivity for formate, a liquid, could prove advantageous simply for ease of transport, although separation from the surrounding water may prove troublesome. And, while formic acid is produced on industrial scales, it is not clear whether carbon dioxide electroreduction could replace or even simply bolster the traditional methods. I look forward to seeing how this field progresses.

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