A Japanese research group saw its way clear to the practical application of ruthenium-rhenium (Ru-Re) supramolecular complex as a photocatalyst that uses sunlight to reduce CO2 to CO, which can be used as a chemical engineering material.
The group is led by Osamu Ishitani, professor at the Graduate School of Science and Engineering of the Tokyo Institute of Technology. It combined the Re complex, which can efficiently reduce CO2, with the Ru complex, which can absorb light (photon) in the visible region of the sunlight spectrum, in a sophisticated manner.
With a photocatalyst that can convert CO2 to CO by using sunlight, "it would be possible to perform artificial photosynthesis to create useful substances from CO2 as if plants photosynthesize carbohydrates such as starch and sucrose from CO2," Ishitani said. The new technology, which enables to effectively use CO2 as a raw material, is drawing attention as a countermeasure against global warming.
The Ru complex absorbs visible light with a wavelength of 500nm or longer in sunlight with a high quantum efficiency (ratio of product to one photon) of 0.21. The Re complex acts as a high performance photocatalyst that can reduce CO2 to CO with a quantum efficiency of 0.59, which Ishitani claims to be "the highest quantum efficiency (of a Re complex) in the world."
The Re complex has a low absorption capacity for sunlight in the visible region, although it has an excellent photocatalytic performance to reduce CO2. Therefore, the research group designed a supramolecular complex that combines the Re complex with the Ru complex, which has a high absorption capacity for the light in the visible region.
The group focused on a Re complex because it, as a photocatalyst, seldom reduces water (H2O) to H2 and O2 even in an environment where water exists and electively reduces CO2 to CO. As a result, it successfully developed a molecular design of a rhenium-diimine tricarbonium complex that reduces CO2 with a high quantum efficiency of 0.59 in an environment where a reductant such as amine coexists.
The remaining problem was that the complex only absorbed light in the ultraviolet region with a wavelength of 450nm or shorter because it had a low absorption capacity for light in the visible region with a wavelength of 400-800nm, which is the main component of sunlight. In addition, the complex was unstable.
Therefore, the group focused on a Ru complex, which is known as an excellent sensitizer, and succeeded in developing a molecular design in which a rhenium-trisdiimine complex effectively absorbs the sunlight in the visible region with a quantum efficiency of 0.21. Although the details have yet to be disclosed, the group recently "paved the way to improve the quantum efficiency of the Ru complex to 0.34," Ishitani said.
There are many candidates for the compositions and structures to loosely combine the Re complex, which has a high photocatalytic performance, and the Ru complex, which is an excellent sensitizer. The photocatalytic performance of the Re complex deteriorates when the two complexes are simply bonded. Thus, the research group closely examined the bridging ligand between the Re and Ru complexes and discovered that the non-conjugated system is superior to the conjugated system.
The group intends to develop a practical supramolecular complex by improving the molecular design to optimize the length of the ligand in the non-conjugated system and the Re complex structure. The next major challenge is to "stabilize the supramolecular complex," Ishitani said.