Understanding Photocatalytic Overall Water Splitting of β-Ketoamine COFs through the N–C Site Synergistic Mechanism

Overcoming the sluggish reaction kinetics of the oxygen evolution reaction (OER) is a determining factor for the practical application of photocatalysts for overall water splitting. Two-dimensional covalent organic frameworks (2D-COFs) offer an ideal platform for catalyst design in the field of overall water splitting for their exceptional chemical tunability and high efficiency of light capture. In this work, four β-ketoamine 2D-COFs, consisting of 1,3,5-triformylphloroglucinol (Tp) groups and different linkers with pyridine segments, were constructed and optimized. By means of first-principles calculations, the band structures, free energy changes of photocatalytic hydrogen evolution reaction (HER) and OER, and charge density distributions were calculated and investigated systemically to discuss the visible-light response, overall water splitting activities on active sites, and the characteristic of charge transfer and separation. The protonated pyridine N derived from the double-H2O closed-ring H-bond adsorption model could efficiently induce N-C sites' synergistic effect between pyridine N and its ortho-position C to minimize the OER energy barrier and to enhance the charge transfer and separation. A N-C site synergistic mechanism has been proposed to provide a comprehensive explanation for the experimental results and a new strategy to design novel 2D-COF photocatalysts for overall water splitting.