Hybrid Photocatalytic System with Enzymes and Pt Nanoparticles for Visible Light-Controlled Hydrogen Production from Formate Under Mild Conditions
Authors: Yoshikawa Shintaro , Amao Yutaka 
Published: December 2024

Hydrogen is a promising clean energy source replacing largely consumed fossil fuels which emit greenhouse gases such as carbon dioxide causing global warming because water is the only substance after hydrogen burning and a wide variety of energies can be utilized for stable supply of hydrogen. However, hydrogen has serious drawbacks, low energy density and explosive nature, which promotes the use of hydrogen carriers to realize hydrogen storage and transportation with efficiency. Though some hydrogen carriers such as ammonia, liquid hydrogen, and methylcyclohexane have been reported, formic acid is increasingly attractive among them since it can be synthesized from carbon dioxide and hydrogen produced by water-splitting and is a nontoxic organic acid.
A number of studies on hydrogen production from formic acid with metal complex catalysts have been developed[1], where a strong acidic solution is needed to decompose formic acid into hydrogen and carbon dioxide efficiently. Moreover, the reaction control is mainly conducted by pH, temperature and pressure. Thus, a simple reaction control alternative to these factors under mild condition is desirable in terms of safety and sustainability.
We report herein hydrogen production based on formate decomposition using hybrid catalytic system composed of enzyme (FDH: formate dehydrogenase), photosensitizer (ZnTPPS: Zn (II) meso-tetra(4-sulfonatophenyl)porphyrin), hydrogen evolution catalyst (Pt-PVP: platinum nanoparticles dispersed by polyvinylpyrrolidone), NAD+ and methyl viologen (MV) as electron mediators as shown in Figure 1. By using this system, hydrogen production in neutral pH region was controlled with visible light irradiation.
A mixed solution of 200 mM phosphate buffer (4.9 mL, pH 7.0) containing HCOONa (50 µmol), NAD+ (25 µmol), ZnTPPS (50 nmol), MV (250 nmol) and Pt-PVP (250 nmol) was deaerated by freeze-pump-thaw cycle repeated six times, and then gas phase was replaced by argon. 0.10 mL FDH (0.95U) was added to the solution. Finally, the solution was irradiated with 250 W halogen lamp at 30.5 oC. Gas phase was analyzed by gas chromatography to qualify and quantify produced hydrogen and carbon dioxide. Formate in the solution was also analyzed with ion chromatography.
In this condition, hydrogen production and formate consumption were on the increase constantly and nearly equal, while apparent carbon dioxide production was half of them. The possible cause is that a part of the produced carbon dioxide which induces global warming was captured in the reaction solution as pH of the solution had been adjusted to 7.0. After 25-hour-irradiation, formate was converted into hydrogen completely. This result illustrates hydrogen production was derived from formate decomposition. In addition, the optimum pH for hydrogen production was investigated in this system ranging from pH 6.0 to pH 8.0. The highest catalytic activity was observed at pH 7.0 because formate oxidation catalyzed by FDH proceeded the most smoothly in the region of pH 7.0-8.0 and more adequate protons to be reduced were in this system at pH 7.0 than pH 8.0. Finally, Figure 2 shows the visible light response of gas production and formate consumption. Hydrogen was produced only in the presence of visible light irradiation whereas carbon dioxide production and formate consumption increased consistently. This means formate oxidation proceeded regardless of the visible light irradiation or non-irradiation producing carbon dioxide while photocatalytic hydrogen production from NADH regenerated by formate dehydrogenation was driven by visible light irradiation.
A hybrid catalytic system for visible light-controlled hydrogen production from formate in neutral pH range was constructed successfully, leading to the unharmful and green utilization of formic acid as a useful hydrogen carrier.
In this hybrid catalytic system, MV is used as an electron mediator for charge separation, which has been known to inhibit light absorption of photosensitizer due to the absorption overlap between MV• and ZnTPPS[2]. As a result, this system becomes slightly complicated including four redox reactions. Hence, hybrid photocatalytic system without MV will be constructed in the future for efficient and simple hydrogen evolution.
Reference
[1] W. H. Wang, Y. Himeda, J. T. Muckerman, G. F. Manbeck, E. Fujita, Chem. Rev. 2015, 115, 12936.
[2] M. E. El-Khouly, E. El-Mohsnawy, S.Fukuzumi, J. Photochem. Photobiol. C. 2017, 31, 36.

Figure 1