Carbon sequestration, the processing and storing of carbon dioxide from the atmosphere, is one of the leading methods for the reduction of greenhouse gases. A group of scientists has now found a copper complex that is capable of reacting with carbon dioxide at a slightly elevated electric potential. The process turns the carbon dioxide into a usable byproduct which can be recycled and reused for this purpose multiple times.

An easy way to get carbon dioxide out of the atmosphere is to find a chemical that combines easily with it, similar to the way that some metals oxidize. For example, compounds involving copper will usually combine with oxygen in the atmosphere voluntarily without catalysts, covering the surface of the copper material with a green patina, like the Statue of Liberty.

Unlike oxygen, carbon dioxide cannot combine so easily with other materials. It is possible to remove one electron from the molecule to facilitate its integration into other molecules, but that removal requires an electric potential of -1.97 volts, which is unreasonably high for the purpose of processing a single molecule.

One group of scientists found a certain dinuclear copper (I) complex that turns green when exposed to air under a slight electric potential (-0.03 volts). At first, they assumed it was from the exposure to oxygen, but upon closer inspection they learned that this particular form of copper was reacting with carbon dioxide.

A few varieties of the same basic complex were able to spontaneously capture carbon dioxide, removing it from the atmosphere and reductively coupling it to the copper to form a tetranulcear copper complex with two oxalate groups, or two sets of two bonded carbon dioxide molecules (the oxalate formula is C 2 O 4 2-). On its own, this is a useful mechanism, but stopping here would result in huge amounts of reacted waste copper.

To cut down on waste, the researchers found that they could wash the oxalate from the copper complex using a lithium salt, then separate the the lithium salt from the oxalate by adding acid. After this process was complete, the copper complex was returned to its original state and was ready to react with more carbon dioxide, and the recovered lithium salts were likewise reusable

Lithium is fairly expensive, but Elisabeth Bouwman, one of the authors of the paper, speculated in a Science podcast that they might be able to use regular sodium chloride. The resulting product, oxalate, is an ingredient found in many household chemicals, such as those used in the removal of rust. It can also be reduced to oxalic acid and made into antifreeze.

A copper complex and salts that are recyclable, the ability to remove carbon dioxide from the atmosphere, and a practical byproduct--it all sounds like the magical solution to climate change. Unfortunately, this chemical dream team works painfully slowly. Each molecule of the copper complex takes about seven hours to process out 12 molecules of oxalate, or 24 carbon dioxide molecules.

These aren't trivial numbers, but by the time the copper has processed that many molecules, the whole mess has to be processed twice (separating the oxalate and then the salts) before it can start over again and process more carbon dioxide. The rate-limiting factor in the process is returning the copper complex to its original state. To satisfactorily improve the process, researchers "need to optimize the electron transfer and the rate of the whole reaction," said Elisabeth Bouwman.

Science, 2010. DOI: 10.1126/science.1177981