Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Joint Center for Artificial Photosynthesis (JCAP) have shown that recycling carbon dioxide into valuable chemicals and fuels can be economical and efficient using a single copper catalyst. The work appears in the journal Nature Catalysis.
… by reducing mixtures of 13CO and 12CO2, we show that oxide-derived Cu catalysts have three different types of active sites for C–C coupled products, one that produces ethanol and acetate, another that produces ethylene and yet another that produces 1-propanol.—Lum & Ager
Researchers at Berkeley Lab and the Joint Center for Artificial Photosynthesis (JCAP) have demonstrated that recycling carbon dioxide into valuable chemicals such as ethylene and propanol, and fuels such as ethanol, can be economical and efficient through product-specific “active sites” on a single copper catalyst. (Credit: Ager and Lum/Berkeley Lab)
These active sites are where electrocatalysis takes place: electrons from the copper surface interact with carbon dioxide and water in a sequence of steps that transform them into products such as ethanol, ethylene, and propanol, an alcohol commonly used in the pharmaceutical industry.
Ever since the 1980s, when copper’s talent for converting carbon into various useful products was discovered, it was always assumed that its active sites weren’t product-specific—in other words, one could use copper as a catalyst for making ethanol, ethylene, propanol, or some other carbon-based chemical, but one would also have to go through a lot of steps to separate unwanted, residual chemicals formed during the intermediate stages of a chemical reaction before arriving at the chemical end-product.
The goal of ‘green’ or sustainable chemistry is getting the product that you want during chemical synthesis. You don’t want to separate things you don’t want from the desirable products, because that’s expensive and environmentally undesirable. And that expense and waste reduces the economic viability of carbon-based solar fuels.—Joel Ager, a researcher at JCAP who led the study
Previous studies had shown that “oxidized” or rusted copper is an excellent catalyst for making ethanol, ethylene, and propanol. The researchers theorized that if active sites in copper were actually product-specific, they could trace the chemicals’ origins through carbon isotopes, “much like a passport with stamps telling us what countries they visited,” Ager said.
The 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. According to their methodology, 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 state-of-the-art mass spectrometry and NMR (nuclear magnetic resonance) spectroscopy at JCAP to analyze the results, the researchers found that three of the products—ethylene, ethanol, and propanol—had different isotopic signatures showing that they came from different sites on the catalyst.
This discovery motivates future work to isolate and identify these different sites. Putting these product-specific sites into a single catalyst could one day result in a very efficient and selective generation of chemical products.—co-author Yanwei Lum
The Joint Center for Artificial Photosynthesis is a DOE Energy Innovation Hub. The work was supported by the DOE Office of Science.
Yanwei Lum & Joel W. Ager (2018) “Evidence for product-specific active sites on oxide-derived Cu catalysts for electrochemical CO2 reduction” Nature Catalysis doi: 10.1038/s41929-018-0201-7