(Invited) New Directions to Development of Photocatalytic CO2 Reduction
Authors: Ishitani Osamu 
Published: May 2020

The artificial systems which can convert CO2 to useful and energy-rich compounds by using renewable energy especially solar energy should be developed because the human beings have produced a huge amount of CO2 mainly from fossil resources for making energy. We have been tackling this target by using transition-metal complexes as main players. In this presentation, I mainly talk about three projects, i.e., (1) highly efficient supramolecular photocatalysts with both functions as photosensitizer and catalysts, (2) reduction of low concentration of CO2 using metal-complex catalysts, and (3) photocatalytic CO2 coupled to water oxidation.
1. Highly efficient supramolecular photocatalysts for CO2 reduction1

Since CO2 is the most oxidized state of carbon, one-electron reduction of CO2 is an extraordinarily high endergonic reaction. From the viewpoints of low-energy light application, the multi-electron reduction of CO2 via chemical reactions promises to be a more valuable process. This is a main reason why both a photosensitizer and a catalyst are required to promote efficient photocatalytic CO2 reduction.
We have developed so-called supramolecular photocatalyts where the photosensitizer and the catalyst are combined to each other with a bridging ligand. This bonding in the supramolecular photocatalysts can accelerate electron transfer between the two components, which improves the performance of the photocatalytic system. In particular, this advantage should be heightened on the surface of photofunctional solid materials as described below.
2. Reduction of low concentration of CO2 using metal-complex photocatalysts2

Exhaust gases from such as thermal power plants and iron manufactures contained several % - 20% of CO2. Although several methods such as adsorption and desorption methods using amines and membrane filtration have been already developed for enrichment of CO2, these procedures require high energy consumption. If low concentration of CO2 can be directly reduced by artificial methods, it should give a new direction of researches for artificial photosynthesis.
In this presentation, I report such systems using CO2-capturing properties of metal complexes. It has been reported that fac-[ReI(diimine)(CO)3X]n+ can work as both photocatalyst and electrocatalysts for CO2 reduction. We found that the Re complexes also have CO2-capturing properties with the assistance of triethanolamine (TEOA) as shown in the eq.1. Although this reaction is in equilibrium, the equilibrium constant is very large (K = 1.7 x 103 M-1). This means that the Re complex can efficiently capture CO2 from gases containing low concentration of CO2 such as 1% CO2. This type of CO2adducts work as a catalyst for photochemical reduction of CO2. A Ru(II)-Re(I) supramolecular photocatalyst consisting of a [Ru(diimine)3]+ type photosensitizer unit and the Re complex as a catalyst unit efficiently photocatalyzed reduction of low concentration of CO2 (even 1% CO2).
3. Photocatalytic CO2 coupled to water oxidation3

The molecular photocatalytic systems have another weakness, i.e., weak oxidation power in the excited state. For practical application, water should be used as an electron donor. For overcoming this problem, we have developed a hybrid system consisting of the supramolecular photocatalyst and semiconductor electrodes. The hybrid photoelectrochemical cell consisting of a NiO-RuRe hybrid photocathode and a CoOx/TaON photoanode showed the activity for visible-light-driven CO2 reduction with water as a reductant to generate CO and O2 (Figure).

References

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Figure 1