Sunlight can Convert carbon dioxide to methanol, study

The concentration of carbon dioxide in the atmosphere is steadily increasing, and many scientists believe that it is causes impacts in the environment. However, carbon dioxide can now be harvested from the atmosphere and be converted into a usable fuel, methanol — which could be the holy grail for sustainable energy production, the new study found.

In a recent study by the Argonne National Laboratory of the U.S. Department of Energy, researchers have discovered that sunlight together with a copper catalyst can be used to transform carbon dioxide to methanol. A liquid fuel, methanol offers the potential for industry to find an additional source to meet America’s energy needs.

Carbon dioxide is such a stable molecule. It is the by-product from burning of basically everything, and a known culprit of the worsening problem of climate change. “Instead of being dreadful of the steadily increasing carbon dioxide in the atmosphere, this could be an opportunity as well to discover new ways to generate sustainable energy,”  said Tijana Rajh, author of the study and an Argonne Distinguished Fellow.

The study expounded that in the process of photocatalysis, cuprous oxide (Cu­2O), a semiconductor that when exposed to light can produce electrons and become available to react with, or reduce, many compounds. After being excited, 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,” said Rajh.

Previous attempts to use photocatalysts, such as titanium dioxide, to reduce carbon dioxide tended to produce a whole mish-mash of various products, ranging from aldehydes to methane. The lack of selectivity of these reactions made it difficult to segregate a usable fuel stream, Rajh explained.

The idea for transforming carbon dioxide into useful energy comes from nature itself, where this happens regularly. ​“Look at the process of photosynthesis, which uses carbon dioxide to make food, so why couldn’t we use it to make fuel. This hypothesis turns out to be a complex problem, because in order to make methanol, it needs not just one electron but six,” study explained.

By switching from titanium dioxide to cuprous oxide, scientists developed a catalyst that not only had a more negative conduction band but that would also be dramatically more selective in terms of its products. This selectivity results not only from the chemistry of cuprous oxide but from the geometry of the catalyst itself.

Applying nanoscience, the study was able to meddle with the surfaces to induce certain hotspots or change the surface structure of the target object,” Rajh said.

With this ​“meddling,” technique Rajh together with co-researcher Yimin Wu, an assistant professor at the University of Waterloo, managed to create a catalyst with a bit of a split personality. They developed a cuprous oxide micro-particles that have different facets, much like a diamond has different facets. Many of the facets of the microparticle are inert, but one is very active in driving the reduction of carbon dioxide to methanol.

According to the study, the reason that this facet is so active lies in two unique aspects.  First, the carbon dioxide molecule bonds to it in such a way that the structure of the molecule actually bends slightly, diminishing the amount of energy it takes to reduce. Second, water molecules are also absorbed very near to where the carbon dioxide molecules are absorbed.

In the process, “In order to make fuel, carbon dioxide must be reduced, while water must have to be oxidized,” Rajh said. Likewise, “the adsorption conformation in photocatalysis is extremely important — if 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.”

Argonne researchers also used scanning fluorescence X-ray microscopy at Argonne’s Advanced Photon Source (APS) and transmission electron microscopy to reveal the nature of the faceted cuprous oxide micro-particles.

The study entitled “Facet-dependent active sites of a single Cu2O particle photocatalyst for CO2 reduction to methanol,” published in the November 4 online edition of Nature Energy. Other contributors to the study include Argonne’s Ian McNulty, Cong Liu, Kah Chun Lau, Paul Paulikas, Cheng-Jun Sun, Zhonghou Chai, Jeff Guest, Yang Ren, Vojislav Stamenkovic, Larry Curtiss, Yuzi Liu and Qi Liu of the City University of Hong Kong.