Highly Efficient Quasi-Homogeneous System for Photocatalytic H<sub>2</sub>O<sub>2</sub> Production

Citation: Huinan Che, Yanhui Ao. Highly Efficient Quasi-Homogeneous System for Photocatalytic H₂O₂ Production[J]. Chinese Journal of Structural Chemistry, 2022, 41(5): 220509. doi: 10.14102/j.cnki.0254-5861.2022-0107 shu

Highly Efficient Quasi-Homogeneous System for Photocatalytic H₂O₂ Production

作者简介: Huinan Che received her Ph.D. degree at Jiangsu University in June 2020. Then she entered the post-doctoral mobile station of environmental science and engineering of Hohai University to engage in post-doctoral research under the supervision of Prof. Yanhui Ao in August 2020. Her current research interests focus on the photocatalysis water treatment and H₂O₂ production;
Yanhui Ao in August 2020. Her current research interests focus on the photocatalysis water treatment and H₂O₂ production. Yanhui Ao obtained his Ph.D. degree at Southeast University in 2008. In 2009, he entered the post-doctoral mobile station of environmental science and engineering of Hohai University to engage in post-doctoral research. In December 2010, he was specially employed as an associate professor in College of Environment, Hohai University. He is now a full professor in HHU. His current research interests focus mainly on the exploitation of various semiconductor materials and their applications in the fields of energy and environment;

通讯作者: , andyao@hhu.edu.cn

English

Highly Efficient Quasi-Homogeneous System for Photocatalytic H₂O₂ Production

Huinan Che ^1, ,
Yanhui Ao ^{1,
,}

1.
Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China

Author Bio: Huinan Che received her Ph.D. degree at Jiangsu University in June 2020. Then she entered the post-doctoral mobile station of environmental science and engineering of Hohai University to engage in post-doctoral research under the supervision of Prof. Yanhui Ao in August 2020. Her current research interests focus on the photocatalysis water treatment and H₂O₂ production;Yanhui Ao in August 2020. Her current research interests focus on the photocatalysis water treatment and H₂O₂ production. Yanhui Ao obtained his Ph.D. degree at Southeast University in 2008. In 2009, he entered the post-doctoral mobile station of environmental science and engineering of Hohai University to engage in post-doctoral research. In December 2010, he was specially employed as an associate professor in College of Environment, Hohai University. He is now a full professor in HHU. His current research interests focus mainly on the exploitation of various semiconductor materials and their applications in the fields of energy and environment;

Corresponding author: Yanhui Ao, andyao@hhu.edu.cn

Received Date: 03 May 2022
Accepted Date: 06 May 2022
Available Online: 20 May 2022

Abstract: The quasi-homogeneous photocatalytic system can be formed by iodide ions induced fragmented and ultrathin structured TP-PCN. The TP-PCN possesses abundant edge active sites, which can greatly enhance the O₂ adsorption/activation capacity and the 2e^- ORR selectivity. As expected, the quasi-homogeneous system affords a remarkably increased photocatalytic H₂O₂ production activity.

Hydrogen peroxide (H₂O₂) has received much attention as a promising highly efficient and environmentally benign oxidant, which is widely used in organic synthesis, wastewater treatment and energy field. Especially, H₂O₂ is completely soluble in water and easy to transport, making it an ideal energy carrier to replace H₂. To date, the traditional methods of producing H₂O₂ mainly include anthraquinone oxidation and direct synthesis from H₂ and O₂. However, the above methods still need to input high energy, produce environmentally harmful substances and have explosion risk, which hinder their practical applications. Conversely, H₂O₂ production by photocatalytic two-electron O₂ reduction process (2e^- ORR) has attracted much attention due to its simple mechanism (Figure 1a), eco-friendliness and low-cost. It is verified that 2e^- ORR process consists of a sequential two-step single-electron reduction or a one-step two-electron reduction (Figure 1b).^[1] So far, various photocatalysts have been researched for H₂O₂ production, such as TiO₂ and metal organic frameworks. In particular, polymeric carbon nitride (PCN) has been regarded as a promising 2e^- ORR photocatalyst by virtue of its high stability, unique structure and excellent electronic property.^[2] However, the efficiency of PCN for H₂O₂ production is still greatly limited by its poor activity and 2e^- ORR selectivity.

Figure 1

Figure 1. (a) Photoexcitation and charge decay pathway. (b) Mechanism diagram of the photocatalytic production of H₂O₂.^[1]

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Note that the alkali metal doped PCN can effectively improve interlayer charge transfer. In addition, the diffusion distance at which electron migration from the bulk to the surface of PCN is shortened by its ultrathin structure, thus enhancing the photocatalytic activity of H₂O₂ production. Nevertheless, the strong π-π interaction can cause the ultrathin PCN to restack and agglomerate during the stripping process, which severely reduces the availability of active sites. It is an effective strategy for the construction of quasi-homogeneous system with abundant edge active sites to increase the number of surface active sites, improve the adsorption and activation ability of O₂, and promote 2e^- ORR selectivity.^[3] Recently, writing in Angewandte Chemie International Edition, Ao and colleagues reported that a quasi-homogeneous photocatalytic system for the efficient generation of H₂O₂ was constructed by using iodide ions as invisible inhibitors to interrupt PCN (TP-PCN) polymerization.

The quasi-homogeneous system with a fragmented as well as ultrathin structure of TP-PCN exposes rich marginal active sites (cyano and hydroxyl group, Figure 2a). As a result, compared with PCN, the rich edge active sites of TP-PCN highly improve the separation efficiency of charge carriers. Besides, the abundance of marginal active sites also highly promotes the ability of O₂ adsorption and the selectivity of 2e^- ORR. More importantly, the electrons of TP-PCN in β spin-orbital can transfer directly to the π* orbital of O₂, thus promoting the activation of O₂ (Figure 2b). That is to say, the quasi-homogeneous system possesses high dispersion and hydrophilicity, which determines the concentration of surface-active sites. Small size and ultra-thin structure are conducive to expose abundant edge active sites. The O₂ adsorption capacity of different positions is investigated by density functional theory (DFT). The results show that finding the edge active sites (-C≡N and -OH) is more favorable to improving the O₂ adsorption capacity and reducing the reaction energy barrier of H₂O₂ formation.

Figure 2

Figure 2. (a) Schematic diagram of iodide-induced hydrophilic carbon nitride for high performance quasi-homogeneous photocatalytic H₂O₂ production. (b) Mechanism of electron transition from β-HOMO orbitals to O₂ after TP-PCN excitation.^[3]

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In summary, experimental and calculational results confirm that the obtained quasi-homogeneous photocatalytic system significantly improves the photocatalytic H₂O₂ production activity. The excellent photocatalytic H₂O₂ production activity is mainly due to the small size and ultrathin structure, which significantly improves the separation/transfer efficiency of charge carriers. In addition, enhancing the adsorption and activation capacity of O₂ and promoting the selectivity of 2e^- ORR are important for highly efficient H₂O₂ production. Overall, the current work provides new insights into the establishment of a new method for building high-performance PCN with abundant edge active sites for quasi-homogeneous photocatalytic H₂O₂ production.

COMPETING INTERESTS
The authors declare that they have no conflict of interest.
Full paper can be accessed via http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0107
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1. [1]
  Hou, H. L.; Zeng, X. K.; Zhang, X. W. Production of hydrogen peroxide by photocatalytic processes. Angew. Chem. Int. Ed. 2020, 59, 17356-17376. doi: 10.1002/anie.201911609
2. [2]
  Wu, Y.; Chen, J.; Che, H. N.; Gao, X.; Ao, Y. H.; Wang, P. F. Boosting 2e⁻ oxygen reduction reaction in garland carbon nitride with carbon defects for high-efficient photocatalysis-self-Fenton degradation of 2, 4-dichlorophenol. Appl. Catal. B: Environ. 2022, 307, 121185. doi: 10.1016/j.apcatb.2022.121185
3. [3]
  Che, H. N.; Gao, X.; Chen, J.; Hou, J.; Ao, Y. H.; Wang, P. F. Iodide-induced fragmentation of polymerized hydrophilic carbon nitride for high-performance quasi-homogeneous photocatalytic H₂O₂ production. Angew. Chem. Int. Ed. 2021, 60, 25546-25550.
Figure 1 (a) Photoexcitation and charge decay pathway. (b) Mechanism diagram of the photocatalytic production of H₂O₂.^[1]

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Figure 2 (a) Schematic diagram of iodide-induced hydrophilic carbon nitride for high performance quasi-homogeneous photocatalytic H₂O₂ production. (b) Mechanism of electron transition from β-HOMO orbitals to O₂ after TP-PCN excitation.^[3]

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