Photocatalytic Reduction of CO₂

To some extent, carbon dioxide has been cited as the main culprit of the greenhouse effect. However, it is undeniable that carbon dioxide is also a potential source of carbon energy. The use of photocatalytic CO₂ reduction to valuable products (e.g., hydrocarbons) is beneficial in alleviating the energy crisis. The wavelength range of light commonly used in photochemistry is between 200 and 700 nm. Since carbon dioxide cannot absorb visible light and ultraviolet light from 200 nm to 900 nm, a catalyst with photocatalytic activity is required to complete the artificial photosynthesis of carbon dioxide. A good photocatalyst should have a long-lasting redox ability, and there should be photoelectrons and photo voids on the surface of the catalyst itself. Because the coordination complexes possess these properties, scientists have started to work on synthesizing various novel metal coordination complex catalysts to reduce carbon dioxide to valuable products (e.g., hydrocarbons) using photocatalysis, thus alleviating the energy crisis to some extent.

Reaction mechanism

Photocatalytic reaction is essentially a redox process, which draws on the photosynthesis of green plants in nature. Green plants convert CO₂ in the air into carbohydrates through a Calvin cycle under light. The photocatalytic reduction process of CO₂ in chemistry is to convert CO₂ into CO, methane, and methanol. The essence of photocatalytic reaction is a chemical process in which the electron-hole pairs generated on the surface of the coordination complexes interact with oxidizing and reducing substances respectively under light excitation. The specific reaction mechanism is that the enriched CO₂ is on the surface of the coordination complexes, and the coordination complexes is excited to generate photogenerated electrons under light conditions.

Figure 1. Diagram of photosynthesis of green plants and photocatalytic reduction of CO₂

Application

Studies have shown that many precious metal complexes are used as photocatalysts for CO₂ reduction. For example, Osamu Ishitani's group ^[1] experimentally synthesized a series of biradical and multiradical supramolecular coordination complexes of Ru-Re. The photocatalytic activities of these supramolecular complexes were strongly influenced by the choice of both the peripheral and the bridging ligands. Interesting results were obtained using the above synthesized complexes as photocatalyst. Features include high selectivity of production of CO over H₂, relatively high quantum yields, and a surprisingly large turnover number for CO formation.

Figure 2. Schematic diagram of the photocatalytic reduction of CO₂ by Ru-Re binuclear coordination complexes catalyst

In addition to precious metal complexes, non-precious metal complexes have also received a lot of attention. Common non-precious metals include Fe, Mn, Cu, Co, Ni and so on. After continuous investigation, researchers were surprised to find that these non-precious metals can also form complexes for photocatalytic reduction of CO₂, and the effect is also good.

What can we do?

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Reference

• Bobak G.; et al. Architecture of supramolecular metal complexes for photocatalytic CO₂ reduction: ruthenium-rhenium bi- and tetranuclear complexes. Inorg.Chem. 2005,44(7): 2326-2336.