Structural Variation and Charge-Transfer Dynamics of Protonated β-Ketoenamine-Linked Covalent Organic Framework for Boosted Photocatalytic H ₂ Evolution

The surface characteristics of covalent organic framework (COF) photocatalysts can be intentionally altered by the protonated treatment, a straightforward postmodification process. This study provides some insights into the specific variation in crystallinity, microstructures, chemical bonds, charge-transfer dynamics mechanism, electronic distribution, and activation energy resulting from the introduction of protons in a β-ketoenamine-linked COF (TpPa-1). It demonstrates that the introduced protons are attached to the –NH of β-ketoenamine groups to form the –NH···H+ groups. Moreover, it is found that the protonated TpPa-1 treated with 0.5 M HOAc solution exhibits a photocatalytic hydrogen evolution rate of 0.238 mmol h–1 in 100 mL ascorbic acid solution at the stationary point with the photocatalyst concentration of 400 mg L–1 without any cocatalysts under visible-light irradiation, equivalent to the mass-normalized value of 5.95 mmol h–1 g–1, which is about 40 and 20 times higher than that of pristine TpPa-1 and TpPa-1 treated with HCl solution, respectively. The isotopic labeling measurement demonstrated that both H2O and ascorbic acid molecules served as the sources of evolved H2. Furthermore, the significantly improved photocatalytic hydrogen evolution performance could be associated with the introduced protons and the acid–base adducts presented in the sample of TpPa-1-HOAc, as demonstrated by X-ray absorption near-edge spectra (XANES), which contributed to the promoted charge separation efficiency as well as the longer lifetime of photoexcited electrons. Additionally, density functional theory (DFT) calculations reveal that the protonated TpPa-1-HOAc shows special electron density redistribution and favorable Gibbs free energy (ΔGH*), which contributed to the significantly boosted photocatalytic H2 evolution performance.