Pulling carbon dioxide (CO2) from the atmosphere and economically converting it into renewable fuels and useful materials has become a sort of Holy Grail for researchers. Catalysts are available that can accomplish such transformations, but they are typically made from rare and expensive metals. The search is on for inexpensive catalysts that will selectively produce desirable materials.
That’s why a discovery by researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Joint Center for Artificial Photosynthesis (JCAP) is so significant. They showed that recycling carbon dioxide into valuable chemicals and fuels can be economical and efficient—all through a single, inexpensive copper catalyst.
Researchers have demonstrated that recycling carbon dioxide into valuable chemicals, such as ethylene and propanol and fuels like ethanol, can be accomplished through product-specific “active sites” on a single copper catalyst. (Image source: Joel Ager and Yanwei Lum/Berkeley Lab)
During the 1980s, it was discovered that copper could act as a catalyst to convert CO2 into other compounds. A smooth piece of copper metal has, on a microscopic scale, a surface roughness that acts as a series of activation sites. The activation sites are where electro-catalysis takes place. Electrons from the copper surface interact with CO2 and water in a sequence of steps that transforms them into products like ethanol fuel; ethylene, the precursor to plastic bags; and propanol, an alcohol commonly used in the pharmaceutical industry.
The problem is that all of these reactions occur more or less simultaneously, resulting in a mixture of materials and compounds that must be separated from one another and from unwanted residual chemical compounds that are formed during the stages of the chemical reactions. You could use copper as a catalyst for making ethanol, ethylene, propanol, or some other carbon-based chemical. But you would have to go through a lot of steps to separate unwanted, residual chemicals from the desired chemical end product. What was not known was whether the various desirable compounds were catalyzed at product-specific sites.
Could They Be Product Specific?
If the activation sites were product specific, could they be switched on or off in some manner to produce only the desired chemicals? “What if, like photosynthesis in nature, we could use electrons from solar cells to drive specific active sites of a copper catalyst to make a pure product stream of a carbon-based fuel or chemical?” said Joel Ager in a Berkeley Lab news release. Ager, a researcher at JCAP who led the study, is a staff scientist in Berkeley Lab’s Materials Sciences Division and an adjunct professor in the Department of Materials Science and Engineering at UC Berkeley.
The problem was how to determine if copper created specific compounds at specific activation sites. According to the news release, “The researchers theorized that if active sites in copper were actually product-specific, they could trace the chemicals’ origins through carbon isotopes.” This was “much like a passport with stamps telling us what countries they visited,” according to Ager.
“When we thought of the experiment, we thought that this is such an in-obvious idea, that it would be crazy to try it,” Ager said. “But we couldn’t let it go, because we also thought it would work, as our previous research with isotopes had enabled us to discover new reaction pathways.”
Creating Something Useful
As described in the news release, the research team ran a series of experiments using two isotopes of carbon—carbon-12 and carbon-13—as “passport stamps.” Carbon dioxide was labeled with carbon-12, and carbon monoxide—a key intermediate in the formation of carbon-carbon bonds—was labeled with carbon-13. The researchers reasoned that the ratio of carbon-13 versus carbon-12—the “isotopic signature” found in a product—would determine from which active sites the chemical product originated.
After dozens of experiments and using state-of-the-art mass spectrometry and NMR (nuclear magnetic resonance) spectroscopy at JCAP to analyze the results, the team found that three of the products that they wanted—ethylene, ethanol, and propanol—had different isotopic signatures showing that they came from different sites on the catalyst.
Ager describes the work as “straightforward chemistry with an environmental and economic twist” and hopes that it “can be a new beginning for green chemical manufacturing, where a solar cell could feed electrons to specific active sites within a copper catalyst to optimize the production of ethanol fuels.” He went on to say, “Perhaps one day, this technology could make it possible to have something like an oil refinery, but powered by the sun, taking carbon dioxide out of the atmosphere and creating a stream of useful products.”
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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