Construction of Functionally Compartmental Inorganic Photocatalyst–Enzyme System via Imitating Chloroplast for Efficient Photoreduction of CO₂ to Formic Acid

Abstract

Inorganic photocatalyst-enzyme systems are a prominent platform for the photoreduction of CO₂ to value-added chemicals and fuels. However, poor electron transfer kinetics and enzyme deactivation by reactive oxygen species in the photoexcitation process severely limit catalytic efficiency. In chloroplast, enzymatic CO₂ reduction and photoexcitation are compartmentalized by the thylakoid membrane, which protects enzymes from photodamage, while the tightly integrated photosystem facilitates electron transfer, promoting photocatalysis. By mimicking this strategy, we constructed a novel functionally compartmental inorganic photocatalyst-enzyme system for CO₂ reduction to formate. To accomplish efficient electron transfer, we first synthesized an integrated artificial photosystem by conjugation of the cocatalyst (a Rh complex) onto thiophene-modified C₃N₄ (TPE-C₃N₄), demonstrating an NADH regeneration rate of 9.33 μM·min^-1, 2.33 times higher than that of a homogeneous counterpart. The enhanced NADH regeneration activity was caused by the tightly conjugated structure of the artificial photosystem, enabling rapid electron transfer from TPE-C₃N₄ to the Rh complex. To protect formate dehydrogenase (FDH) from photoinduced deactivation, FDH was encapsulated into MAF-7, a metal-organic framework (MOF) material, to compartmentalize FDH from the toxic photoexcitation process, similar to the function of the thylakoid membrane. Moreover, the triazole linkers of MAF-7 possess both hydrophilicity and pH-buffering capacity providing a stable microenvironment for FDH, which could enhance enzyme stability in photosynthesis. The synergy between the enhanced electron transfer of TPE-C₃N₄ for NADH cofactor regeneration and MOF-protection of the redox enzyme enables the construction of a functionally compartmental inorganic photocatalyst-enzyme association system, promoting CO₂ photoconversion to formic acid with a yield of 16.75 mM after 9 h of illumination, 3.24 times greater than that of the homogeneous reaction counterpart.