From photosynthesis to photocatalysis: Dual catalytic oxidation/reduction in one system

Lin X. Chen

PNAS April 21, 2020 117 (16) 8672-8673

Natural photosynthetic systems and photocatalysis share several fundamental processes in common including light energy conversion and utilization, such as exciton (excited state) generation/splitting and charge migration. The high efficiency of light conversion to chemical redox equivalents in natural photosynthesis is achieved by an electron transfer cascade resulting in a long-distance hole and electron separation across the membrane. Consequently, the charge geminate recombination is minimized, and the loss is reduced. The machinery of natural photosynthesis enables high-quantum yields of photon to electron/charge conversion efficiency facilitated by a fine-tuned local potential energy gradient by the protein surrounding each chlorophyll or its derivative to ensure unidirectional electron transfer and efficient final charge separation across the membrane. The holes and electrons separated far away from each other can thus go their separate ways to drive water oxidation and quinone reduction, respectively. Using the functions of metal oxide cluster or multiple reduction equivalents in molecules like quinones, natural photosynthesis succeeds in coupling multiple single-photon events with generation of multiple redox equivalents for catalytic reactions, such as water splitting.

Inspired by natural photosynthesis, many artificial photosynthetic systems have been designed to achieve similar functions in photoinduced sequential, unidirectional, and long-distance electron transfer for simultaneous oxidation and reduction reactions at the both ends of the electron transfer chain. However, it has been a long struggle to effectively couple multiple single-photon events with multiple redox equivalent generation required for catalytic reaction such as water splitting. Frequently, only one of the two single half-reactions may take place, which not only uses only electrons or holes but also causes charge imbalance. Some systems may suffer the loss from charge recombination due to insufficient hole/electron separation after the exciton splitting and from the lack of directionality of charge separation. Thus, it has been a dream for photocatalysis or solar fuel community to design and synthetize platforms capable of accumulating both well-separated holes and electrons from photoinduced charge separation for sufficiently long time to carry out effective and simultaneous reduction and oxidation reactions in a single system.

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In summary, Chu et al. in their study (22) simultaneously tackle the following challenges in photocatalysis in a platform of Co₁/AQ/C₃N₄: 1) moving hole and electron away from where they are generated via exciton splitting upon light absorption to minimize the loss due to geminate recombination via a semiconductor 2D C₃N₄ nanosheet; 2) segregating the photocatalytic oxidation cobalt site in the middle and reduction AQ site at the edge so that the two reactions can simultaneously take place without the needs for sacrificial donor/acceptor and external wiring; and 3) selecting proper AQ reduction site so that the reaction of forming H₂O₂ is competitive with other reactions in both energetic and kinetic standpoints.

Because of the above key advances, this research opens an approach for achieving both efficient oxidation and reduction reactions on 2D photocatalysts for many possible reactions that can be driven by renewable solar energy. We look forward to seeing more successful artificial photosynthetic systems and photocatalytic systems for fuel generation to store solar energy effectively.

See https://www.pnas.org/content/117/16/8672

(22). C. Chu et al. Spatially separating redox centers on 2D carbon nitride with cobalt single atom for photocatalytic H₂O₂ production. Proc. Natl. Acad. Sci. U.S.A. 117, 6376–6382 (2020).

Figure: (Left) Directional photoinduced electron transfer among bacterial chlorophylls and pheophytins in a sphaerodis bacterial reaction center protein results in charge separation across the membrane (not shown) with positive charges on one side and negative charges on the other side with reduced quinone derivative. (Right) Similarly, the Co₁C₃N₄AQ system by Chu et al. (22) also achieved directional electron and hole transfer resulting in two catalytic centers with Co to perform oxidative reaction and the AQ to perform reduction reaction in the same system. Right: Adapted from ref. 22, licensed under CC BY-NC-ND 4.0.