A team of researchers from the Tokyo Institute of Technology, Paris Diderot University, and the French National Centre for Scientific Research (CNRS) have discovered a way to reduce carbon dioxide (CO2) into carbon monoxide (CO) using only commonly occuring elements and sunlight.

Their method mimics photosynthesis — the natural process by which plants derive sustenance using only water, sunlight, and carbon dioxide — in a lab setting.

Their technology helps advance the wide search for an effective method to reduce the persistent buildup of carbon dioxide — the most common greenhouse gas and main source of climate change — while also creating a material that could be used in industrial processes.

The artificial synthesis method

The researchers developed a photocatalyst composed of an organic semiconductor material and an iron complex.

After testing several photosensitizers and catalysts, the researchers settled on using carbon nitride as the semiconductor.

The carbon nitride photosensitizer was developed by a Tokyo Tech research team led by Osamu Ishitani, a professor of chemistry, and Kazuhiko Maeda, an associate professor of chemistry, while the iron complex was developed by a team led by Marc Robert, professor of chemistry at Paris Diderot University and CNRS.

With carbon nitride — composed of carbon and nitrogen — and an iron complex as the only materials in the reaction, the process only requires common organic materials.

Furthermore, using visible light as the only energy source, the reaction occurs at high efficiency under ordinary temperature and pressure.

The reaction occurs as follows.

When the materials are exposed to visible light, the carbon nitride, acting as a redox photosensitizer, absorbs visible light.

Electrons from the reducing agent then move to the iron complex — the catalyst.

The iron complex then uses the electrons to reduce carbon dioxide into carbon monoxide.

Once reduced, the carbon monoxide can be used to synthesize hydrocarbon, which can be repurposed as a fuel, through the Fischer-Tropsch process.

The results

The researchers were able to successfully reduce carbon dioxide with a selectivity of about 99 percent, meaning that the ratio of the desired product to the total product formed by the reaction was about equal.

The reaction also performed well on other metrics of efficiency, such as turnover number (155) and external quantum efficiency (4.2%).

These metrics demonstrate that the reaction was just as successful as methods that require the use of precious metals or rare metal complexes.

It is also 10 times more successful than other methods using base metals or organic materials as photocatalysts.

The reaction had the highest durability and highest efficiency for carbon dioxide reduction using exclusively elements abundant on earth, said Ishitani.

But there is still a long way to go before artificial photosynthesis can be applied on a large scale.

“I believe that our result will have a strong impact on research in the field of artificial photosynthesis (solar fuels),” said Ishitani. “However, for developing practically useful systems, we have to add many functions into our system, such as an oxidation system which can use water as a reductant.”