Modulating the direction of catalytic glyoximate sites of covalent organic frameworks towards electrocatalytic nitrate reduction

Abstract

Two-dimensional (2D) covalent organic frameworks (COFs) with metal centers are ideal templates to construct electrocatalysts due to their high degree of structural controllability. However, the metal centers are stacked in columns with limited space, which impedes the mass delivered to catalytic sites across the pore channels. Herein, we demonstrate a topologic synthesis strategy for constructing catalytic sites in three-dimensional (3D) space. The designed 3D COF adopts an ffc topology, with a large space of 1.15 and 1.53 nm between the metal sites along the parallel and vertical directions, respectively. In situ spectroscopy revealed that ∼100% Ni–N₄ sites in 3D frameworks were reconstructed to Ni–N₄–NO, while the reconstruction proportion of Ni–N₄ sites was ∼40% for 2D COF (with a distance of 0.38 nm between metal sites). The catalytic 3D COFs enable the electrochemical synthesis of NH₃ via the reduction of nitrate (NO₃RR) at a rate of 9.51 mg mg_cat⁻¹ h⁻¹, corresponding to 140% of that for the 2D COF at −0.7 V vs. RHE. Theoretical calculations further revealed that the reconstructed Ni–N₄–NO site had a stronger binding ability of the reactants and intermediates than that of the initial Ni–N₄ site and thus contributed to higher activity. This work provides general design strategies for heterogeneous catalysts in electrocatalytic systems.