Edge functionalization of terminal amino group in carbon nitride by in-situ C–N coupling

Photoreforming of biomass into H2

Significance 

Stringent measures on the use of fossil fuels have led to the rapid development of alternative renewable energy sources. In particular, hydrogen has been identified as promising renewable energy. Among the available hydrogen generation methods, the solar water splitting process is commonly used for large-scale hydrogen generation.  It requires additional sacrificial agents to achieve high reaction rates and enhanced overall water splitting efficiency.  These agents are, however, non-renewable and costly. For sustainable hydrogen generation, alternative strategies such as photo-reforming of biomass have been explored. Unlike water splitting, photo-reforming uses less activation energy and does not require sacrificial agents. Instead, it makes use of photocatalysts such as carbon nitride to accelerate the photo-reforming reactions. Nevertheless, the performance of carbon nitride photocatalysts is hindered by its poor visible light absorption properties.

In an attempt to address this challenge, a team of researchers at Guangdong Institute of Analysis (China National Analytical Center Guangzhou), Guangdong Academy of Sciences: Dr. Qiong Liu, Liling Wei, Qiaoyue Xi, Yongqian Lei, and led by Dr. Fuxian Wang fabricated an edge functionalized carbon nitride via in-situ C-N coupling method. In their work, N-acetylethanolamine (NA) was used as an additive to react with the amino groups, including urea, dicyandiamide, and melamine, to form the C-N bonding. Consequently, the terminal amino groups were replaced, by thermal pyrolysis, to graft the ethyl alcohol groups to heptazine rings. The work is published in the Chemical Engineering Journal.

Compared to the pristine CN, the modified CN exhibited superior photo-reforming performance for several monosaccharides. The partial replacement of the terminal amino group resulted in enhanced visible light absorption and improved charge carrier density. DFT calculations revealed that the enhanced visible light absorption resulted from the formation of a new discrete energy level due to the introduction of the ethyl alcohol group. The electrons donated by the ethyl alcohol group improved the overall charge separation by modulating the electron distribution to form the internal electric field. Furthermore, the authors observed that the presence of ethyl alcohol groups favored the absorption of biomass entity. The ethyl alcohol grafting is a versatile strategy and can be successfully applied to increase the photo-reforming performance of other CN-NAx systems involving the reacting NA with other precursors like dicyandiamide and melamine.

In summary, the research team provided a facile and highly effective strategy for tunning the band structures of amino group-based semiconductors, thus providing a novel method for fabricating high-performance photocatalytic materials for efficient photo-reforming of biomass to hydrogen.  The tuned CN exhibits superior performance and does not require the use of sacrificial agents. Additionally, it is versatile and allows for the visible-light-driven conversion of several monosaccharides into hydrogen. In a statement to Advances in Engineering, Dr. Qiong Liu said the strategies presented in their study would allow sustainable and large-scale generation of hydrogen for use as an alternative renewable energy.

Edge functionalization of terminal amino group in carbon nitride by in-situ C–N coupling for photoreforming of biomass into H2 - Advances in Engineering
Fig. 1. (a) UV–vis DRS spectra and digital picture of resultant samples with different NA addition (ranging from 0 to 4.0 mL of NA, from left to right). Inset is the digital picture of obtained sample. Calculated band structure of UCN (b) and UCN-NA (c) by using density functional theory. (d) Photocatalytic H2 production using UCN and UCN-NAx (20 mg, x from 0.2 to 4.0NA addition) and Pt (0.8 wt%) with D-xylose (100 mg) in 30 mL alkaline aqueous solution under a 5 W white LED irradiation at 25 °C. (d) Photocatalytic H2 production using UCN-NA0.5 under 380, 420, 465, 500 and 550 nm monochromatic light irradiation for 4 h, combined with the DRS spectrum of UCN-NA. (f) Photocatalytic H2 production using UCN and UCN-NA0.5 with 100 mg xylose, glucose, fructose and arabinose under white LED irradiation for 4 h.

About the author

Qiong liu is now working in the Guangdong Institute of Analysis, received the Hundred-Talent Program (Guangdong Academy of Sciences), supported by the GDAS’ Project of Science and Technology Development. He earned his his Ph.D. degree in from School of Chemistry and Chemical Engineering at the South China University of Technology in 2018. And he worked as a joint postdoctoral fellow in the Department of Chemistry at the National University of Singapore from 2018 to 2020.

Current research is focused on design and synthesis of 2D functional conjugated polymeric-based semiconductors and their applications in energy storage, biomass conversion, and organic photoredox by using solar energy.

Reference

Liu, Q., Wei, L., Xi, Q., Lei, Y., & Wang, F. (2020). Edge functionalization of terminal amino group in carbon nitride by in-situ C–N coupling for photoreforming of biomass into H2. Chemical Engineering Journal, 383, 123792.

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