Steering product selectivity via metallic site-dependent pathways in porphyrin-based covalent organic frameworks for electrocatalytic nitrite reduction

doi:10.1016/S1872-2067(26)64989-3

Abstract

Abstract:

Electrocatalytic nitrite reduction represents a sustainable and efficient alternative to conventional routes for the production of hydroxylamine and ammonia. A comprehensive understanding of the correlation between the nature of catalytic active sites and their electrocatalytic performance is essential, particularly in steering product selectivity. Herein, we report the synthesis of a series of highly crystalline metalated porphyrin-based covalent organic frameworks (M-TAPP-TTF COFs, M = Zn, Fe), enabling systematic investigation of the structure-selectivity relationships and mechanistic pathways dictated by distinct metallic centers during NO₂RR. Experimental evaluations reveal that fully metalated Zn-porphyrins in COFs could efficiently inhibit the deep reduction of NO₂^-to NH₃ even at higher potentials, with NH₂OH as the major product throughout the entire voltage window. The maximum Faradaic efficiency of NH₂OH (FE_NH2OH) is 71.4% and its yield rate could reach up to 542.3 μmol h^-1 mg_COF^-1 at -2.0 V vs. Ag/AgCl. Meanwhile, when the metallic centers in the porphyrins were switched to Fe ions, it exhibits superior selectivity toward NH₃ formation, achieving a maximum FE_NH3 of 72.6% and a yield rate of 867.2 μmol h^-1 mg_COF^-1 at -2.0 V vs. Ag/AgCl. Complementary density functional theory calculations and in-situ Raman spectroscopy reveal that the Zn active sites in Zn-TAPP-TTF COF promote the preferential hydrogenation of *NO to *NHO, followed by the thermodynamically favorable desorption of NH₂OH, thereby enhancing selectivity toward NH₂OH. In contrast, Fe active sites in Fe-TAPP-TTF COF favor the *NO to *NOH pathway, in which *NOH undergoes further reduction via N‒O bond cleavage with following favorable desorption of NH₃. These insights into metallic site-dependent reaction pathways offer a mechanistic basis for the rational design of single-atom catalysts with tunable selectivity in electrocatalytic NO₂RR.

Key words: Covalent organic frameworks, Nitrite reduction reaction, Metallic sites, Electrocatalysts, Hydroxylamine and ammonia

Donghua Li, Hongyin Hu, Jinye Zhou, Hanyun Miao, Yu Wu, Jinyan Wang, Baochun Guo, Mingliang Du, Shuanglong Lu. Steering product selectivity via metallic site-dependent pathways in porphyrin-based covalent organic frameworks for electrocatalytic nitrite reduction[J]. Chinese Journal of Catalysis, 2026, 83: 258-270.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)64989-3

https://www.cjcatal.com/EN/Y2026/V83/I4/258

Figures/Tables 6

Scheme 1. Schematic illustration of site-dependent metalated porphyrin-based COFs toward selective electrocatalytic NO2RR.

Fig. 1. Synthesis and structural characterization of Zn-TAPP-TTF COF. (a) Schematic illustration of the synthesis of Zn-TAPP-TTF COF. (b) PXRD patterns of Zn-TAPP-TTF COF, Zn/H2-TAPP-TTF COF and H2-TAPP-TTF COF. (c) TEM images of Zn-TAPP-TTF COF. (d) TEM-EDS mapping of C, N, Zn, and S elements in selected area of Zn-TAPP-TTF COF.

Fig. 2. (a) FT-IR spectra of TAPP, TTF, Zn-TAPP-TTF COF, Zn/H2-TAPP-TTF COF, and H2-TAPP-TTF COF. (b) High-resolution N 1s XPS spectra of Zn-TAPP-TTF COF, Zn/H2-TAPP-TTF COF, and H2-TAPP-TTF COF. (c) High-resolution Zn 2p XPS spectra of Zn-TAPP and Zn-TAPP-TTF COF. (d) N2 sorption isotherm of Zn-TAPP-TTF COF (inset: Pore-size distribution profile).

Fig. 3. Electrocatalytic performance of Zn-based TAPP-TTF COFs. (a) LSV curves of Zn-based TAPP-TTF COFs in mixed electrolyte of 0.1 mol L?1 KHCO3 with and without 0.1 mol L?1 KNO2. (b) NH2OH FE of Zn/H2-TAPP-TTF COF at each given potential. (c) NH2OH FE of Zn-TAPP-TTF COF at each given potential. (d) NH2OH partial current density of Zn-based TAPP-TTF COFs. (e) NH2OH yield rate of Zn-TAPP-TTF COF at each given potential. (f) Double layer capacitance of Zn-TAPP-TTF COF and Zn/H2-TAPP-TTF COF. (g) The cycling tests of Zn-TAPP-TTF COF for reduction tests at -1.5 V. (h) Radar chart comparing the electrochemical performance parameters of Zn-TAPP-TTF, Zn/H2-TAPP-TTF and H2-TAPP-TTF COFs.

Fig. 4. Structural characterization and electrocatalytic performance of Fe-TAPP-TTF COF. (a) Schematic illustration of Fe-TAPP-TTF COF. (b) XRD pattern of Fe-TAPP-TTF COF. (c) High-resolution Fe 2p XPS spectra of Fe-TAPP-TTF COF. (d) TEM images of Fe-TAPP-TTF COF. (e,f) TEM-EDS mapping of C, N, Fe, and S elements in selected area of Fe-TAPP-TTF COF. (g) LSV curves of Fe-TAPP-TTF, Zn-TAPP-TTF, H2-TAPP-TTF COFs in 0.1 mol L?1 KHCO3-0.1 mol L?1 KNO2 mixed electrolyte. (h) NH3 and NH2OH FE of Fe-TAPP-TTF COF at each given potential. (i) NH3 yield rates of Fe-TAPP-TTF, Zn-TAPP-TTF, H2-TAPP-TTF COFs.

Fig. 5. DFT calculation results of NO2RR on Zn-TAPP-TTF COF and Fe-TAPP-TTF COF. Energy barriers of *NO protonated on Zn-TAPP-TTF COF (a) and Fe-TAPP-TTF COF (b). (c) The free energy of desorption of NH2OH and NH3 on Zn-TAPP-TTF COF and Fe-TAPP-TTF COF. In-situ Raman spectra of Zn-TAPP-TTF COF (d) and Fe-TAPP-TTF COF (e) for NO2RR. (f) Free energies on Zn-TAPP-TTF COF and Fe-TAPP-TTF COF with different pathways and intermediate states.

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