Electrosynthesis of nylon-6 precursor via heteroatom-doping-regulated oxygen vacancies engineering over ZnO

doi:10.1016/S1872-2067(26)65062-0

Abstract

Abstract:

Electrocatalysis is a green alternative to directly synthesize cyclohexanone oxime (CHOX, Nylon-6 precursor) via electrocatalytic reduction of nitrogen oxides to NH₂OH and spontaneous C-N coupling with cyclohexanone, but suffering from a low-yield or poor Faradaic efficiency (FE) because of the scaling relationship. Herein, a strategy of heteroatom-doping-regulated oxygen vacancies (Vo) engineering was proposed to design a robust Cu-ZnO_1‒_x catalyst for efficient electrosynthesis of cyclohexanone oxime (ESCO). The complete characterizations and theoretical studies revealed that the doped-Cu can reduce Vo sites formation energy and regulate their local electronic state. The synergistic effect between doped Cu and Vo led to the break of the scaling relationship, presenting the enhancement of NO₃^- adsorption/dissociation, the weakness of H* adsorption, and the balance of the NH₂OH adsorption for further C-N coupling reaction. Therefore, the hydrogen evolution reaction and NH₂OH reduction to NH₃ side reactions can be suppressed. The optimized Cu-ZnO_1‒_x delivered an outstanding activity with a 1238.8 μmol h^‒1 mg_cat.^‒1 yield and 68.2% FE. Lastly, the reaction mechanism was established following *NO₃ → *NO₃H → *NO₂ → *NO₂H → *NO → *HNO → *NHOH → *NH₂OH and spontaneous C-N coupling with cyclohexanone to form CHOX. The outstanding ESCO performance on Cu-ZnO_1‒_x catalyst demonstrates the effectiveness of this heteroatom-doping-regulated Vo engineering strategy.

Key words: Cyclohexanone oxime, Oxygen vacancy, Cu-doping, C-N coupling reaction, Electrocatalytic

Suwei Lu, Hongping Yan, Hongwei Zhang, Yuying Cheng, Xinxin Jiang, Xuyun Peng, Junwei Huang, Yuanjin Li, Xin Wang, Shijing Liang, Lilong Jiang. Electrosynthesis of nylon-6 precursor via heteroatom-doping-regulated oxygen vacancies engineering over ZnO[J]. Chinese Journal of Catalysis, 2026, 87: 243-253.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65062-0

https://www.cjcatal.com/EN/Y2026/V87/I8/243

Figures/Tables 6

Scheme 1. Synthesis routes for CHOX.

Fig. 1. Morphology and structural characterizations of catalysts. XRD patterns (a) and Raman spectra (b) of ZnO, ZnO1?x and Cu-ZnO1?x. HRTEM (c) and HAADF-STEM and corresponding mapping image (d) of Cu-ZnO1?x. (e) EPR spectra for ZnO, ZnO1?x and Cu-ZnO1?x. (f) Schematic diagram of Cu2+ introducing and replacing Zni to achieve Cu-ZnO1?x. O 1s XPS spectra (g) and Zn 2p XPS spectra (h) for ZnO, ZnO1?x and Cu-ZnO1?x. (i) Cu 2p XPS spectra for Cu-ZnO1?x.

Fig. 2. Electrochemical performance. (a) LSV curves of ZnO, ZnO1?x, and Cu-ZnO1?x in 1 mol L?1 KOH solution containing 0.2 mol L?1 NO3- and 0.1 mol L?1 cyclohexanone. Yield (b) and FE (c) of CHOX over ZnO, ZnO1?x, and Cu-ZnO1?x at different potentials. (d) Distribution of electrocatalytic products over ZnO, ZnO1?x, and Cu-ZnO1?x at -0.8 V vs. RHE. (e) Time-dependent FEs over Cu-ZnO1?x for ESCO. (f) Comparison of the reported catalytic performance.

Fig. 3. Theoretical investigations of various ZnO models. Local structures and Vo formation energy (ΔE) of Pristine ZnO (a), ZnO1?x (b), and Cu-ZnO1?x (c). All structural diagrams are top views. Color scheme: Gray for Zn, orange for Cu, and red for O. (d?f) ZnO, ZnO1?x, Cu-ZnO1?x surface models and atomic charges derived from Bader charge analysis. (g) Free energy diagram of hydrogen production in these models.

Fig. 4. Adsorption of NO3? on various catalyst models. Adsorption free energies (a) and charge density difference (b) of NO3? adsorbed on ZnO, ZnO1?x, Cu-ZnO and Cu-ZnO1?x. (c) Adsorption free energies of NH2OH on ZnO, ZnO1?x, Cu-ZnO and Cu-ZnO1?x. The color code is the same as in Fig. 1, yellow and blue contours denote the accrual and expenditure of electrons, respectively.

Fig. 5. Investigations of mechanism. (a) In-situ DEMS measurements for ESCO with Cu-ZnO1?x at -0.8 V vs. RHE over five continuous cycles. The mass signals increased rapidly upon the reduced potential and gradually decreased after the potential was removed. In-situ Raman spectra (b) and in-situ FTIR spectra (c) for ESCO with Cu-ZnO1?x in 1 mol L?1 KOH solution containing 0.2 mol L?1 NO3? and 0.1 mol L?1 cyclohexanone. (d) Operando EPR spectra of ZnO, ZnO1?x and Cu-ZnO1?x in 1 mol L?1 KOH solution containing 0.2 mol L?1 NO3? and 0.1 mol L?1 cyclohexanone. (each line was collected after 10 min electrolysis at -0.8 V vs. RHE). (e) The ESCO pathway, validated through in-situ characterization results. (f) Reaction-free energy profiles of ESCO over ZnO, ZnO1?x and Cu-ZnO1?x surfaces.

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