In a recent study from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, scientists used a photocatalyst largely made of copper to transform carbon dioxide (CO₂) to methanol.

The researchers used a photocatalyst made of cuprous oxide (Cu₂O), a semiconductor that when exposed to light can produce electrons that can react with, or reduce, many compounds. After excitation, the electrons leave a positive hole in the catalyst’s lower-energy valence band that, in turn, can oxidize water. “This photocatalyst is particularly exciting because it has one of the most negative conduction bands that we’ve used, which means that the electrons have more potential energy available to do reactions,” researcher Tijana Rajh said.

Previous attempts to use photocatalysts such as titanium dioxide (TiO₂) to reduce CO₂ tended to produce a mishmash of products, ranging from aldehydes to methane. The lack of selectivity of these reactions made it difficult to segregate a usable fuel stream. By switching from TiO₂ to Cu₂O, the Argonne scientists were able to develop a catalyst that not only had a more negative conduction band but that would also be more selective in terms of its products. This capacity for selectivity resulted not only from the chemistry of Cu₂O but also from the geometry of the catalyst itself. “With nanoscience, we start having the ability to meddle with the surfaces to induce certain hot spots or change the surface structure,” Rajh said.

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Looking into the hard x-ray nanoprobe synchrotron chamber while measuring a response of an individual cuprous oxide particle to the exposure of carbon dioxide, water, and light. Courtesy of Tijana Rajh/Argonne National Laboratory.

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The Cu₂O microparticles developed by the researchers have different facets, many of which are inert. However, one facet is very active in driving the reduction of CO₂ to methanol. The reason this facet is so active, the team said, is because the CO₂ molecule bonds to it in such a way that the structure of the molecule bends slightly, diminishing the amount of energy it requires to reduce. Also, water molecules are absorbed very near to where the CO₂ molecules are absorbed.

“In order to make fuel, you not only need to have carbon dioxide to be reduced, you need to have water to be oxidized,” Rajh said. “Also, adsorption conformation in photocatalysis is extremely important — if you have one molecule of carbon dioxide absorbed in one way, it might be completely useless. But if it is in a bent structure, it lowers the energy to be reduced.”

The scientists used scanning fluorescence x-ray microscopy at Argonne’s Advanced Photon Source (APS) and transmission electron microscopy at the Center for Nanoscale Materials (CNM) to reveal the nature of the faceted Cu₂O microparticles. The internal quantum yield for unassisted wireless photocatalytic reduction of CO₂ to methanol using Cu₂O crystals is about 72%, the researchers said.

The research was published in Nature Energy (www.doi.org/10.1038/s41560-019-0490-3).