Synthesis of H2O2 by oxygen reduction and water oxidation under near-infrared irradiation

Recently, Professor Pan Chengsi’s team from Jiangnan University and Professor Zhu Yongfa of Tsinghua University published a research result entitled “H2O2 generation from O2 and H2O on a near-infrared absorbing porphyrin supramolecular photocatalyst” in the journal Nature Energy.

This study reports a novel porphyrin-based supramolecular platform for the synthesis of H2O2 using near-infrared light, with quantum efficiencies of 14.9% and 1.1% at 420 nm and 940 nm, respectively. This study also proposes a new path of H2O2 generated by photooxidation of carboxylic acid groups to peroxycarboxylic acid groups and hydrolysis by hot water, which promotes the efficient conversion of solar energy to chemical energy in the system with a conversion efficiency of 1.2%.

The corresponding authors of the paper are Professor Pan Chengsi and Professor Zhu Yongfa; The co-first authors are Dr. Yaning Zhang and Professor Chengsi Pan.

Hydrogen peroxide (H2O2) is an important chemical that is widely used in areas such as bleaching, green chemical synthesis and wastewater treatment, and is an integral part of a sustainable society. In addition, it can be used as an alternative liquid fuel for hydrogen or fossil fuels in fuel cells. The global H2O2 market is expected to grow to 5.7 million tons by 2027 due to increasing demand. However, the current production of H2O2 mainly relies on the unenvironmentally friendly anthraquinone (AO) method, which uses a noble metal palladium-based catalyst, consumes a lot of energy, and produces a large number of toxic by-products, which puts a huge burden on the environment.

The synthesis of H2O2 by photocatalysis is seen as an alternative to the AO method because, ideally, the technique only consumes oxygen, water and sunlight, and is environmentally friendly and economically viable. However, there are still some key problems to be solved in the production of H2O2 by photocatalytic method. For example, in most systems, H2O2 synthesis requires the addition of additional organic sacrifices, while in a few systems where H2O2 can be produced using only oxygen and water, the pathway to photogenerated holes remains unclear. In addition, the insufficient spectral response wavelength (< 600 nm, which is only 31% of solar energy), also limits its application.

In this work, the research team of Professor Pan Chengsi/Zhu Yongfa of Jiangnan University reported a photocatalyst, the self-assembled medium-tetrakis(4-carboxyphenyl) porphyrin supramolecular photocatalyst (SA-TCPP). SA-TCPP has been shown to efficiently synthesize H2O2 through a dual-channel pathway of oxygen reduction and heat-assisted water oxidation. Specifically, under illumination, photogenerated electrons reduce oxygen adsorbed on the pyrrole ring N-H group, while photogenerated holes oxidize the carboxylic acid group (-COOH) to peroxycarboxylic acid intermediate (-CO3H). Taking advantage of the thermal instability of the intermediate, the research design realizes the efficient conversion of -CO3H intermediate to H2O2 by increasing the reaction temperature and promoting hydrolysis. The experimental results show that the average rate of H2O2 generation by SA-TCPP supramolecular photocatalyst at 353 K is as high as 1.72 mM/h, and its quantum efficiency is about 14.9% at 420 nm and 1.1% at 940 nm. In addition, the light absorption wavelength of the catalyst can reach 1100 nm, and under simulated sunlight irradiation, the thermal effect of infrared light can be used to make the reaction liquid temperature reach 328 K, and the conversion efficiency (SCC) from solar energy to chemical energy reaches 1.2%. To further increase the cumulative concentration of H2O2, the research team used a laboratory-made flow reactor to separate H2O2 and concentrated it in an evaporation dish. The experimental results show that the concentration of H2O2 can accumulate to about 1.1 wt%, close to the concentration of household H2O2 disinfectant solution (about 3.0 wt%).

Figure 1: Structural and morphological characterization of SA-TCPP nanosheets.

Figure 2: Band structure of SA-TCPP supramolecular photocatalyst.

Figure 3: Performance of SA-TCPP photocatalytic production of H2O2 and accumulation of CO3H reaction intermediates at room temperature (293 K).

Figure 4: Performance of SA-TCPP supramolecular photocatalyst to produce H2O2 after heating (353 K).

Figure 5: Comparison of CO3H intermediate accumulation at 293 K and 353 K.

In summary, in addition to the widely studied two-electron reduction oxygen to produce H2O2 pathway on SA-TCPP supramolecular photocatalysts, this study also revealed a hole-induced H2O2 production pathway, including the photooxidation of the -COOH group of SA-TCPP to -CO3H intermediate and its two processes of hydrolysis. This study not only provides a new material platform for the synthesis of H2O2 without sacrifices, but also provides theoretical guidance for the design of organic photocatalysts that make efficient use of solar energy. (Source: Science Network)

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