CHEMICAL SCIENCE

Supramolecular biomimetic constructs artificial photosynthetic spherical pigment bodies for aqueous CO2 reduction


On May 18, 2023, Professor Tian Jia of the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, together with the team of Professor Ye Ruquan of City University of Hong Kong, Professor David Phillips of the University of Hong Kong and Professor Lili Du of Jiangsu University, published an article entitled “Artificial spherical chromatophore nanomicelles for selective CO2″ in the journal Nature Catalysis reduction in water”.

This achievement reports a new strategy for photocatalytic conversion of photoselective CO2 by simulating the key motifs and assembly structure of natural photosynthetic purple pigment bodies by supramolecular self-assembly, and proposes possible reaction mechanisms through isotope tracing experiments, steady-state/transient spectroscopy experiments and DFT calculations, which proves the influence of assembly confinement effect on photocatalytic reactions. This work provides novel solutions for the accurate simulation of biological structure and function of supramolecular assemblies and the photocatalytic energy conversion of supramolecules.

The corresponding authors of the paper are Professor Tian Jia, Ye Ruquan, David Phillips and Professor Du Lili; The co-first authors are Yu Junlai and Huang Libei. The Shanghai Institute of Organic Sciences, Chinese Academy of Sciences, is the first completion unit of the paper.

Photosynthesis provides the material and energy basis for life. Simulating the natural development of artificial photosynthetic systems and converting solar energy into chemical energy and storing it through the “zero-carbon cycle” pathway is an effective means to alleviate the energy crisis and reduce carbon emissions. However, because the energy generated by the natural photosynthetic system needs to supply many life processes, the number of catalytic centers is limited and the distance from the photosensitive system is far away, resulting in the total quantum efficiency of its photoenergy-chemical energy conversion being less than 0.1~1% (the annual average of plants is ~0.1%, and the harvest season is ~1%). How to use synthetic chemistry and supramolecular assembly methods “bottom-up” to simulate the key molecular blocks and exquisite assembly structures in natural photosynthetic systems, and then construct artificial photosynthetic assembly systems and ultimately realize the efficient conversion and storage of light energy to chemical energy is a very challenging research hotspot today (see Group Review: Mater. Futures 2022, 1, 042104. DOI: 10.1088/2752-5724/aca346)。

Inspired by the spherical chromosome structure of Rhodobacter sphaeroides, the researchers created an artificial photosynthetic spherical chromosome nanomicelle system based on the idea of supramolecular assembly simulation, and applied it to the selective catalytic conversion of aqueous photo-promoted CO2 (Figure 1). This work creatively synthesized triblock porphyrin amphiphilic molecules introducing oligo-arylamide fragments (bodyarmor-like Kevlar molecule fragments), which greatly enhanced the structural stability through hydrophobicity, hydrogen bonding and π-π stacking, spontaneously assembled in the aqueous phase to form spherical nanomicelle assemblies with a diameter of about 14 nm and a uniform distribution (Figure 2). The assembly not only has excellent light capture and good anti-photobleaching properties, but also forms a 4.2 nm diameter porphyrin ring array substructure with a positive electric properties on the micelle surface, resulting in the “nanofence” and “spherical antenna” effects, which greatly promote the efficient injection of photogenerated electrons to the catalytic site (Figure 3).

Figure 1: Schematic diagram of the assembly path and catalytic function of artificial spherical chromosome nanomicelles. (a) Triblock porphyrin amphiphilic monomer structure, supramolecular spherical nanomicelle assembly process and schematic diagram of its photocatalytic CO2 reduction with TSPP-Co; (b) Natural photosynthetic purple bacteria (R. sphaeroides) spherical chromosome structure.

Figure 2: Structural characterization of artificial spherical chromosome nanomicelles. (a) TEM photograph of spherical micelles; (b-c) STEM and HR-STEM photographs, the latter of which observes surface annular array structures; (d) Liquid-phase AFM characterization; (e-f) Cryo-TEM and single-particle analytical characterization.

Figure 3: Assembly-bounded catalytic structure of artificial spherical chromosomal nanomicelles. (a) Schematic diagram of spherical nanomicellar binding to catalyst; (b) Assembled 12-porphyrin ring array conical structure; (c) Schematic diagram of nano-“molecular fence” structures formed by assembly confinement effects to promote photocatalytic CO2 reduction.

The researchers selected the water-soluble anionic tetraphenylporphyrin Co complex (TSPP-Co) as the catalyst to shorten the spatial distance between the two and improve the electron transport efficiency through electrostatic force. It was found that the catalytic system could catalyze the conversion of CO2 to CH4 (TON > 6600, electron-selectivity > 89%) for 30 consecutive days under visible light irradiation (Xe lamp, AM 1.5 G, λ > 420 nm). In the first 2 hours, the apparent quantum efficiency of photo-chemical energy conversion measured at 450 nm is higher than 15%; Long-term experiments and 300 cycle experiments show that the catalytic stability of the system is excellent. In addition, the system has the ability to reduce low atmospheric concentrations of CO2 (approximately 410 ppm) to CH4 (total conversion > 96%, CO2 to CH4 electron selectivity >92%) (Figure 4).

Figure 4: Photocatalytic CO2 reduction performance of artificial spherical chromosomal nanomicelles. (a) 30-day long-acting experiment at 1 atm CO2; (b) 300 catalytic cycle experiments; (c) Catalytic effect of CO2 at atmospheric concentrations (410 ppm).

Finally, through isotope labeling experiments, steady-state and ultrafast absorption spectroscopy experiments and DFT calculations, the authors proved that the process of CO2 generation of CH4 is divided into two steps: first, CO2 undergoes 2e-reduction process to generate CO intermediates; The CO is then reduced to CH4 by 6e-reduction, which yields CH4 > 13,000 μmol h−1 g−1; At the same time, the authors elaborate on the processes of TSPP-Co binding to substrates, catalysis, and valence changes of central metals (Figure 5).

Figure 5: Schematic diagram of the mechanism of photocatalytic reduction of CO2 by artificial spherical pigment body nanomicelles.

The research was strongly funded and supported by the National Key Research and Development Program of the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Shanghai Municipal Science and Technology Commission, the Shanghai Institute of Organic Biology, the Key Laboratory of Organic Functional Molecular Synthesis and Assembly Chemistry, and Professor Li Zhanting’s team. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41929-023-00962-z



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