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A liquid metal catalyst turns carbon dioxide into solid carbon.

Peter Clarke/RMIT University

New way to turn carbon dioxide into coal could ‘rewind the emissions clock’

If humans hope to limit climate change to just 2°C of warming, we’ve got a lot of work to do, scientists say: reducing emissions, planting trees, and scrubbing carbon dioxide (CO2) from the skies with the latest technologies. Now, a new process can convert gaseous CO2—the product of burning fossil fuels—into solid carbon at room temperature, using only a trickle of electricity. But getting it to work on a planet-wide scale will be a formidable challenge.

In recent years, researchers have discovered a handful of solid metal catalysts—compounds that speed up chemical reactions—that can convert CO2 into solid carbon. But these work only above 600°C, and providing that heat requires a lot of energy—and money. The catalysts also gum up quickly, when the carbon they produce builds up, limiting their ability to keep the reactions going.

To get around this, chemists Dorna Esrafilzadeh and Torben Daeneke at RMIT University in Melbourne, Australia, turned to a new class of catalysts made from metal alloys that are liquid at room temperature. One such catalyst, first reported in Nature Chemistry in 2017, consists of catalytically active palladium mixed with liquid gallium. (The liquid allows the palladium to keep converting low-value hydrocarbons called alkanes into a higher value ones known as alkenes, without gumming up.) Esrafilzadeh, Daneneke, and their colleagues wanted to see whether something similar would work with CO2.

They first made an alloy of gallium, indium, and tin that is liquid at room temperature and conducts electricity. They spiked the silvery mixture with a sprinkling of catalytically active cerium and placed it inside a glass tube, along with a splash of water that helps CO2 convert to carbon.

When they inserted a wire into the liquid metal, some of the cerium atop the liquid surface reacted with oxygen from the surrounding air, forming an ultrathin layer of cerium oxide. But most of the cerium remained protected by the liquid metal. Next, the researchers piped pure CO2 into the glass tube and sent a jolt of electricity into the wire. CO2 diffused into the liquid metal where the cerium metal and electricity converted it into solid carbon, Esrafilzadeh and her colleagues report today in Nature Communications.

The researchers say the exact mechanism of the reaction is not yet clear, but it likely involves five separate steps as the cerium interacts with oxygen, CO2, and water, ultimately releasing solid carbon and pure oxygen as the only byproducts. The big benefit to this new approach is that the cerium catalyst doesn’t gum up. Instead, the carbon forms small black flakes on the liquid metal surface that then slough off and move to the sides and bottom of the tube, allowing the catalytic reaction to continue.

Bert Weckhuysen, a chemist at the Utrecht University in The Netherlands, calls the work “novel” and “quite nice.” He says the carbon that’s produced could find use in a wide variety of materials in things like battery electrodes, tennis rackets, golf clubs, and airplane wings.

The big payoff, however, would be if this technology could be scaled up to suck CO2 out of the air and permanently store it in a solid. “While we can’t literally turn back time, turning carbon dioxide back into coal and burying it back in the ground is a bit like rewinding the emissions clock,” Daeneke says. But first, the team’s tabletop experiment, which Esrafilzadeh calls “a first step,” would have to be duplicated on a massive scale. In 2017 alone, humans released more than 32 billion tons, or gigatons, of CO2 into the air, according to the International Energy Agency. Converting that amount to solid carbon would essentially re-create the mountains of coal that miners dig out of the ground.

“The gigatons magnitude makes it seem daunting,” Douglas MacFarlane, another study co-author and a chemist at Monash University in Melbourne, wrote in an email. “But if the economics become encouraging … [then it] seems very possible.”

*Correction, 26 February, 7:20 p.m.: The original version of this story used "cesium" instead of "cerium." This has been corrected throughout the story.