An international collaborative research group including Tokyo Institute of Technology, Universite PARIS DIDEROT and CNRS has discovered that CO₂ is selectively reduced to CO[1] when a photocatalyst[2] composed of an organic semiconductor material and an iron complex is exposed to visible light. They have made clear that it is possible to convert CO₂, the major factor of global warming, into a valuable carbon resource using visible light as the energy source, even with a photocatalyst composed of only commonly occurring elements.

This is CO2 reduction using a photocatalyst combining carbon nitride and an iron compl. Credit: Osamu Ishitani

In recent years, technologies to reduce CO₂into a resource using metal complexes and semiconductors as photocatalysts are being developed worldwide. If this technology called artificial photosynthesis can be applied, scientists would be able to convert CO₂, which is considered the major factor of global warming and is being treated as a villain, into a 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. However, considering the tremendous amount of CO₂, there was a need to create new photocatalysts made only with elements widely available on Earth.

Professor Osamu Ishitani, Associate Professor Kazuhiko Maeda, research staff Ryo Kuriki and others of Tokyo Tech, with the support of JST (Japan Science and Technology Agency)’s Strategic Basic Research Programs (CREST Establishment of Molecular Technology towards the Creation of New Functions) for international collaborative research projects, performed collaborative research with the research group of Professor Marc Robert of Universite PARIS DIDEROT and CNRS. As a result, by fusing carbon nitride, an organic semiconductor, with a complex made of iron and organic materials and using it as a photocatalyst, they succeeded in turning CO₂ into a resource at high efficiency under the condition of exposure to visible light at ordinary temperature and pressure.

By combining the organic semiconductor carbon nitride[3], 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 that electrons to reduce CO₂ to CO. The turnover number[4], the external quantum efficiency[5], and the selectivity[6] of CO₂ reduction–performance indicators for the formation of CO–reached 155, 4.2%, and 99%, respectively. These values are almost the same as when precious metal or rare metal complexes are used, and about ten times more than photocatalysts reported so far using base metals or organic molecules.

This research was the first to demonstrate 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, which exists abundantly on Earth and is inexpensive, as a reducing agent.