A photo-enzyme coupled system for carbon dioxide conversion to solar fuel: The rate-matching and compatibility between photocatalysis and enzyme catalysis

Using reduced nicotinamide adenine dinucleotide (NADH) as cofactor, formate dehydrogenase (FDH) can reversely catalyze CO₂ reduction to formic acid. However, the industrialized application of FDH is fairly limited due to its high cost and stoichiometric cofactor consumption. Enzyme immobilization and cofactor regeneration are vital solutions to these problems. Ascribed to the benefits of utilizing a cheap, environmentally friendly, and renewable energy source, photocatalytic NADH regeneration is naturally sustainable and promising. Nonetheless, the rate-matching and compatibility between photocatalysis and enzyme catalysis have not been well revealed. In this study, polyethyleneimine (PEI) modified hollow fiber membrane (HFM) as the enzyme support was in-situ integrated with a visible-light-driven NADH regeneration system using thiophene-modified macroporous graphitic carbon nitride (ATCN-CN) as the catalyst. First, NADH regeneration conditions were regulated to accommodate enzyme catalysis. Then, the photo-enzyme coupled system (PECS) for the sustainable synthesis of formic acid with triethanolamine (TEOA) or H₂O as electron donor was constructed, and the influencing factors were investigated. At last, the PECS was compared with other methods as well as a system that used FDH immobilized on silica nanoparticles. The results show that the reaction pH, electron donor, and catalyst concentration have a significant impact on NADH regeneration. For the PECS with TEOA as electron donor, the optimal pH value and photocatalyst concentration for formic acid synthesis are 7.0 and 1.2 mg mL⁻¹, respectively. With the initial addition of 1 mmol L⁻¹ NAD⁺, a formic acid yield of 35.3% was obtained after 5 h, standing out among the reported values. H₂O as a green and low-cost electron donor meets the desire for low-cost photocatalysis. Although the production of formic acid after 5 h was lower in the PECS with H₂O as electron donor than in the TEOA system, it was still 14.6 times that of the immobilized enzyme system without cofactor regeneration, and the yield of formic acid could reach 14.8% and 98.3% with 1 or 0.1 mmol L⁻¹ NAD⁺, validating that current PECS is feasible to a sustainable and green synthesis of solar fuel from CO₂.