When it’s exposed to the elements, the surface of copper turns green because it reacts with oxygen. But now scientists have discovered a copper-based material with a surprising property: it reacts with carbon dioxide in air rather than oxygen. Though the reaction is not a practical way to remove large quantities of carbon dioxide from the atmosphere, it does provide an alternative new route, using a cheap, nonpetroleum feedstock, to make useful chemicals.
Researchers have been looking for such a material for a long time, taking a cue from plants, which use atmospheric carbon dioxide to produce a wide range of useful materials. But previous approaches have fallen short in a variety of ways. For example, they’ve required large amounts of energy and concentrated streams of carbon dioxide rather than the trace amounts found in air. One of the big challenges is that materials tend to preferentially react with oxygen, which is much more reactive than carbon dioxide and far more abundant. Oxygen makes up over 20 percent of the atmosphere, whereas there are only a few hundred parts per million of carbon dioxide.
With the new material, “the energy you need to put in is very low,” says Daniel DuBois, a senior scientist at the Pacific Northwest National Laboratory in Richland, WA, who was not involved with the research. “And the fact that it will bind and reduce CO2 directly from the atmosphere is pretty startling. I wouldn’t have thought that you could do that.”
When the copper material is exposed to air, it binds two molecules of carbon dioxide to form oxalate. The researchers then expose the material to a lithium salt, which removes the oxalate from the material, forming lithium oxalate. By applying a low voltage to the copper material, the researchers reduce it to its original state, and it can again bind carbon dioxide. Lithium oxalate can easily be converted to oxalic acid, a ingredient in household cleaners such as rust removers, says Elisabeth Bouwman, a senior lecturer in inorganic chemistry at the University of Leiden in the Netherlands, who led the group that discovered the material. Oxalic acid can also be used to make ethylene glycol, an antifreeze and a precursor to some common plastics.
But Bouwman notes that the material is “a long way” from commercial applications. For one thing, it is very slow–it takes an hour to do each cycle. It’s also unlikely that the process, or indeed any other process for turning carbon dioxide into commodity chemicals, will significantly reduce atmospheric levels of carbon dioxide. Even if the process can be made much faster–and if expensive lithium salts can be replaced with sodium to reduce costs–there’s not enough demand for such chemicals to make a dent in carbon dioxide levels. For example, the chemicals produced in the largest volumes in the United States are only made at the scale of tens of millions of tons. Annual carbon dioxide emissions in the United States are around six billion tons.
The researchers are currently experimenting with the new material, changing it in small ways to learn more about how it works and to improve its performance.