Plants are great at pulling carbon dioxide out of the air. But they are slow, and researchers would love to speed up the removal of carbon dioxide (CO2)—a key greenhouse gas—from the atmosphere. Today, researchers in California report that they’ve taken the first step to doing just that, by developing a porous material that converts CO2 into carbon monoxide (CO) and oxygen. Not only could the new material clean our skies, but it might also serve as the starting point for making fuel from renewable power sources.
Chemists have been trying to do something useful with CO2 for decades. But CO2 is a very stable, unreactive molecule. To split it into CO and oxygen, researchers have to add energy, typically electricity. That’s not done now, because it’s far cheaper to make fuel by refining oil. But some catalysts—substances that speed up chemical reactions—could make the process cheaper by reducing the amount of energy that needs to be pumped in.
One promising catalyst is a ring-shaped organic molecule with a cobalt atom at its core, called a porphyrin. When added to a solution with two electrodes, an electrolyte, and some dissolved CO2, the porphyrin nuzzles up close to the negatively charged electrode and ferries electrons to the CO2, splitting it into CO and oxygen that bubble away. But the setup works only when the porphyrins are dissolved in an environmentally questionable organic solvent. And there are other problems: Porphyrins tend to clump together over time, destroying their electron-ferrying abilities.
To get around these problems, researchers at the University of California, Berkeley, led by chemists Omar Yaghi and Chris Chang, found a way to link porphyrins together into a porous solid material called a covalent organic framework (COF). Yaghi and his colleagues have developed a variety of COFs as filters for separating different gases from one another. But in hopes of taking a first step to making renewable fuels, they wanted to see if their cobalt COF could split CO2 as well. Porphyrins seemed like a natural choice, because they’re not only good at ferrying electrons to CO2, but they can also conduct electricity. In theory, using a porphyrin COF as a catalyst would allow the material to conduct electrons from an electrode to porphyrin components throughout a thick film of the material. And the COF’s porous nature would allow CO2 to percolate through and gain access to the catalytically active cobalt atoms at the porphyrin’s core.
After synthesizing their new COF, Yaghi, Chang, and their colleagues placed a layer atop an electrode. Because their catalyst was already in contact with the electrode, they didn’t need the organic solvent that’s required for the molecular porphyrin catalyst, and they could use a simple water-based electrolyte instead. When they applied an electric current, they found that the porphyrin COF did a better job than the molecular version of splitting CO2 into CO and oxygen.
The key to the molecular splitting process is when CO2 molecules bind to the cobalt atoms at a porphyrin’s core. But Yaghi’s team found that in their cobalt porphyrin COF not all of the CO2 molecules found their cobalt targets. So they rebuilt their COF frameworks so they had slightly larger pores to ease the passage of CO2. They also added a bit of copper. CO2molecules tend to avoid copper in favor of cobalt, and adding copper caused the CO2 molecules to crowd around the remaining cobalt atoms, like groupies swarming a rock star. That increased the likelihood that CO2 molecules would make actual contact with a cobalt atom and be split.
The upshot—which the Berkeley team reports online today in Science—was that the dual-metal COF split CO2 molecules 60 times as well as the free-floating cobalt porphyrin molecules. The COFs also proved highly efficient, using 90% of the electrons to split CO2 molecules into CO. Finally, the catalysts were extremely active, fracturing some 240,000 CO2 molecules per hour, 25 times as fast as the cobalt-only COFs. That makes the material among the best CO2-splitting catalysts out there.
“This is very nice work,” says Paul Kenis, a chemist at the University of Illinois, Urbana-Champaign. He notes that lots of groups are trying to improve their CO2-to-CO conversions by using porous electrode materials. But this work makes the electrode itself out of a catalytically active material. “So you get two for one,” Kenis says.
Ultimately, the CO created could be combined with hydrogen (made by splitting water into hydrogen and oxygen) to produce hydrocarbon fuels from renewable energy sources like wind and solar, Kenis and Yaghi say. That’s not economical today, because it’s still far cheaper to refine oil. But if countries ever want to make fuels from renewable energy only, and avoid dumping more fossil fuel CO2 into the air, catalysts such as this one could be an essential component.