An international collaborative research group including Tokyo Institute of Technology, Université PARIS DIDEROT and CNRS says that CO₂ can be selectively reduced to CO when a photocatalyst composed of an organic semiconductor material and an iron complex is exposed to visible light.

CO2 reduction using a photocatalyst combining carbon nitride and an iron complex.In recent years, technologies to reduce CO₂ into a resource using metal complexes and semiconductors as photocatalysts have been under development worldwide. If this technology, called artificial photosynthesis, can be applied, scientists would be able to convert CO₂ into a potentially valuable carbon resource using sunlight as the energy source.

Complexes and inorganic semiconductors containing precious and rare metals such as ruthenium, rhenium and tantalum have been used in highly active photocatalysts reported so far. The latest research effort fused carbon nitride, an organic semiconductor, with a complex made of iron and organic materials.

By combining the organic semiconductor carbon nitride, made of carbon and nitrogen, with an iron complex and using it as a photocatalyst, they found that they could reduce carbon dioxide (CO₂) to carbon monoxide (CO) at high efficiency. This photocatalytic reaction progresses when exposed to visible light, which is the major component in the wavelength band of sunlight.

The carbon nitride absorbs visible light and drives the migration of electrons from the reducing agent to the iron complex, the catalyst. The iron complex uses those electrons to reduce CO₂ to CO. The turnover number, the external quantum efficiency and the selectivity of CO₂ reduction — performance indicators for the formation of CO — reached 155, 4.2 percent and 99 percent, respectively. The researchers say these values are almost the same as when precious metal or rare metal complexes are used, and about 10 times more than photocatalysts reported so far using base metals or organic molecules.

This research also demonstrates that CO₂ can be reduced into a resource efficiently using sunlight as the energy source, even by using materials which exist abundantly on Earth, such as carbon, nitrogen and iron. Tasks remaining are to further improve their function as a photocatalyst and to succeed in fusing them with oxidation photocatalysts which can use water as a reducing agent.