Published online 14 January 2010 | Nature | doi:10.1038/news.2010.12

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Pollutants plucked from air with copper

Fortuitous catalyst discovery offers a new way to suck carbon dioxide from the atmosphere.

smokestackCatalytic copper chemistry might one day help scrub the air of carbon dioxideBrand X

In an accidental discovery, chemists have stumbled on a catalyst that strips only carbon dioxide from the air — ignoring oxygen — and converts it into a useful compound.

The copper-based compound is nowhere near being a practical air-scrubber for removing CO2 — not least because the catalyst takes hours to be recycled to its original state. But its innovative chemistry offers a faint hope that a catalyst could one day selectively and efficiently remove the greenhouse gas from the air, turning it into organic chemicals.

Many catalysts with a structure based around a metal centre — such as a copper atom — are able to grab CO2 from a pure stream of the gas. But when faced with air, they prefer to couple with the more abundant and more reactive oxygen. So the selectivity of the new compound is "completely unexpected", says Elisabeth Bouwman at Leiden University in the Netherlands, who led the team that discovered the catalyst. They publish their results in this week's Science1.

A catalyst that strips CO2 instead of oxygen from the air is "definitely unusual, probably unprecedented", agrees Cliff Kubiak, an electrochemist from the University of California, San Diego, who was not involved in the work.

Chance discovery

Copper compoundThe structure of the copper compound, before it reduces carbon dioxideScience

Bouwman's team was investigating compounds that mimicked the activity of biological enzymes. Bouwman took the chemical shell off the nickel centre of one such mimic, and tried wrapping it around copper for comparison. This structure produced a yellow solution, which turned green-blue after sitting in the open air for a few days.

Analysis of the green-blue product showed that it contains a segment called oxalate — made of two CO2 molecules — which form a bridge linking two copper atoms together. This fragment could occur only if CO2, not oxygen, had oxidized the copper compound.

Bouwman says that she doesn't know why the copper complex prefers CO2 to oxygen, but it could be because the oxalate bridge within the molecular structure of the green-blue product is extremely stable.

Back to square one

But the compound doesn't just grab CO2: with a little input of electrical energy, it also acts as a catalyst. The copper complex can be recycled and returned to its low oxidation state, and the oxalate can be pulled off the molecule.

In an electrochemical cell, Bouwman could rip away the oxalate from the copper's grasp by adding lithium ions. The bare copper complex left behind is returned to its original state thanks to an electrode — this adds electrons that the copper is missing after it loses its oxalate fragment.

The cell requires a small amount of electrical energy — 0.03 volts — to drive this process. But this is much lower, for example, than the 2 volts needed to add an electron to CO2 at an electrode (an alternative way of making dissolved CO2 reactive so that it forms useful chemicals).

Once stripped off the catalyst, the oxalate salt can also form the basis of several chemicals that have practical applications. These include oxalic acid — commonly used in many laboratories and in household products such as rust-proofing treatments — and, after chemical conversion, ethylene glycol, which is used as an antifreeze in cars and as a building block for chemical synthesis.

Neat, but impractical

Still, the system is far from being a practical method of cleaning CO2 from the air to combat global warming. "The efficiency of the compound is not good enough," says Bouwman. So far her team has cycled the system just six times in seven hours — and that rate is only achieved in pure CO2 in the laboratory, not in air. An efficient catalyst needs to be capable of tens of thousands of cycles an hour, Bouwman says.

"It is certainly elegant chemistry," says Fraser Armstrong, a chemist from the University of Oxford, UK. But he agrees that the conversion rates are too low to be of use for removing CO2 from the atmosphere.

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By contrast, large-scale systems to remove CO2 from air — if they ever become practical or affordable — are much more likely to rely on physical membranes that suck in the gas selectively, or on sodium hydroxide scrubbers that chemically trap the gas but that require large amounts of energy to regenerate. Both of these systems — some of which are approaching commercial reality, albeit expensive — simply concentrate CO2 rather than converting it into a useful chemical as Bouwman's electrocatalytic concept does.

Meanwhile, Bouwman has returned to her enzyme studies. But she continues to investigate her chance discovery, wondering if changes in the chemical side groups of the copper molecule might improve the catalyst's efficiency. 

  • References

    1. Angamuthu, R., Byers, P., Lutz, M., Spek, A. L. & Bouwman, E. Science 327, 313-315 (2010).
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