Chemists design novel method for generating sustainable fuel

Chemists have been working to synthesize high-value materials from waste molecules for years. Now, an international collaboration of scientists is exploring ways to use electricity to streamline the process.

In their study, recently published in Nature Catalysis, researchers demonstrated that carbon dioxide, a greenhouse gas, can be converted into a type of liquid fuel called methanol in a highly efficient manner.

This process happened by taking cobalt phthalocyanine (CoPc) molecules and spreading them evenly on carbon nanotubes, graphene-like tubes that have unique electrical properties. On their surface was an electrolyte solution, which, by running an electrical current through it, allowed CoPc molecules to take electrons and use them to turn carbon dioxide into methanol.

Using a special method based on in-situ spectroscopy to visualize the chemical reaction, researchers for the first time saw those molecules convert themselves into either methanol or carbon monoxide, which is not the desired product. They found that which path the reaction takes is decided by the environment where the carbon dioxide molecule reacts.

Tuning this environment by controlling how the CoPc catalyst was distributed on the carbon nanotube surface allowed carbon dioxide to be as much as eight times more likely to produce methanol, a discovery that could increase the efficiency of other catalytic processes and have a widespread impact on other fields, said Robert Baker, co-author of the study and a professor in chemistry and biochemistry at The Ohio State University.

"When you take carbon dioxide and convert it to another product, there are many different molecules you can make," he said. "Methanol is definitely one of the most desirable because it has such a high energy density and can be used directly as an alternative fuel."

While transforming waste molecules into useful products isn't a new phenomenon, until now, researchers have often been unable to watch how the reaction actually takes place, a crucial insight into being able to optimize and improve the process.

"We might empirically optimize how something works, but we don't really have an understanding of what makes it work, or what makes one catalyst work better than another catalyst," said Baker, who specializes in surface chemistry, the study of how chemical reactions change when they occur on the face of different objects. "These are very difficult things to answer."

But with the help of special techniques and computer modeling, the team has come significantly closer to grasping the complex process. In this study, researchers used a new type of vibrational spectroscopy, which allowed them to see how molecules behave on the surface, said Quansong Zhu, the lead author of the study and former Ohio State Presidential Scholar whose challenging measurements were vital to the discovery.

"We could tell by their vibrational signatures that it was the same molecule sitting in two different reaction environments," said Zhu. "We were able to correlate that one of those reaction environments was responsible for producing methanol, which is valuable liquid fuel."

According to the study, deeper analysis also found these molecules were directly interacting with supercharged particles called cations that enhanced the process of methanol formation.

More research is needed to learn more about what else these cations enable, but such a finding is key to achieving a more efficient way to create methanol, said Baker.

"We're seeing systems that are very important and learning things about them that have been wondered about for a long time," said Baker. "Understanding the unique chemistry that happens at a molecular level is really important to enabling these applications."

Besides being a low-cost fuel for vehicles like planes, cars and shipping boats, methanol produced from renewable electricity could also be utilized for heating and power generation, and to advance future chemical discoveries.

"There's a lot of exciting things that can come next based on what we've learned here, and some of that we're already starting to do together," said Baker. "The work is ongoing."

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Clean, sustainable fuels made 'from thin air' and plastic waste

Researchers have demonstrated how carbon dioxide can be captured from industrial processes -- or even directly from the air -- and transformed into clean, sustainable fuels using just the energy from the Sun.

The researchers, from the University of Cambridge, developed a solar-powered reactor that converts captured CO2 and plastic waste into sustainable fuels and other valuable chemical products. In tests, CO2 was converted into syngas, a key building block for sustainable liquid fuels, and plastic bottles were converted into glycolic acid, which is widely used in the cosmetics industry.

Unlike earlier tests of their solar fuels technology however, the team took CO2 from real-world sources -- such as industrial exhaust or the air itself. The researchers were able to capture and concentrate the CO2 and convert it into sustainable fuel.

Although improvements are needed before this technology can be used at an industrial scale, the results, reported in the journal Joule, represent another important step toward the production of clean fuels to power the economy, without the need for environmentally destructive oil and gas extraction.

For several years, Professor Erwin Reisner's research group, based in the Yusuf Hamied Department of Chemistry, has been developing sustainable, net-zero carbon fuels inspired by photosynthesis -- the process by which plants convert sunlight into food -- using artificial leaves. These artificial leaves convert CO2 and water into fuels using just the power of the sun.

To date, their solar-driven experiments have used pure, concentrated CO2 from a cylinder, but for the technology to be of practical use, it needs to be able to actively capture CO2 from industrial processes, or directly from the air. However, since CO2 is just one of many types of molecules in the air we breathe, making this technology selective enough to convert highly diluted CO2 is a huge technical challenge.

"We're not just interested in decarbonisation, but de-fossilisation -- we need to completely eliminate fossil fuels in order to create a truly circular economy," said Reisner. "In the medium term, this technology could help reduce carbon emissions by capturing them from industry and turning them into something useful, but ultimately, we need to cut fossil fuels out of the equation entirely and capture CO2 from the air."

The researchers took their inspiration from carbon capture and storage (CCS), where CO2 is captured and then pumped and stored underground.

"CCS is a technology that's popular with the fossil fuel industry as a way to reduce carbon emissions while continuing oil and gas exploration," said Reisner. "But if instead of carbon capture and storage, we had carbon capture and utilisation, we could make something useful from CO2 instead of burying it underground, with unknown long-term consequences, and eliminate the use of fossil fuels."

The researchers adapted their solar-driven technology so that it works with flue gas or directly from the air, converting CO2 and plastics into fuel and chemicals using only the power of the sun.

By bubbling air through the system containing an alkaline solution, the CO2 selectively gets trapped, and the other gases present in air, such as nitrogen and oxygen, harmlessly bubble out. This bubbling process allows the researchers to concentrate the CO2 from air in solution, making it easier to work with.

The integrated system contains a photocathode and an anode. The system has two compartments: on one side is captured CO2 solution that gets converted into syngas, a simple fuel. On the other plastics are converted into useful chemicals using only sunlight.

"The plastic component is an important trick to this system," said co-first author Dr Motiar Rahaman. "Capturing and using CO2 from the air makes the chemistry more difficult. But, if we add plastic waste to the system, the plastic donates electrons to the CO2. The plastic breaks down to glycolic acid, which is widely used in the cosmetics industry, and the CO2 is converted into syngas, which is a simple fuel."

"This solar-powered system takes two harmful waste products -- plastic and carbon emissions -- and converts them into something truly useful," said co-first author Dr Sayan Kar.

"Instead of storing CO2 underground, like in CCS, we can capture it from the air and make clean fuel from it," said Rahaman. "This way, we can cut out the fossil fuel industry from the process of fuel production, which can hopefully help us avoid climate destruction."

"The fact that we can effectively take CO2 from air and make something useful from it is special," said Kar. "It's satisfying to see that we can actually do it using only sunlight."

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