Effects of substituents on photocatalytic ethanol reforming to hydrogen production over ketoenamine COFs

Photocatalytic reforming of ethanol provides an effective way to produce hydrogen energy using natural and nontoxic ethanol as raw material. However, photocatalytic performance remains limited by the rapid recombination of photogenerated charge carriers and insufficient light-harvesting capability. Herein, we synthesized a series of ketoenamine-based COFs with different electron-withdrawing and electron-donating groups, namely NUS-14-X (X = -H, -CH₃, and -NO₂). The effects of introducing functional groups on the lattice structure, morphology, and electronic structure of 2D COFs with the same host framework were systematically investigated. Results show that introducing of electron-withdrawing groups (i.e., -NO₂) significantly enhances the hydrogen production efficiency. The photocatalytic efficiency of the optimal sample is 5.218 mmol·h⁻¹·g⁻¹, which is 2.7 times that of the pure NUS-14-H. A series of photoelectrochemical characterizations confirmed that NUS-14-NO₂ exhibits a broader light absorption range and significantly improved photogenerated electron-hole separation efficiency. Furthermore, DFT calculations revealed that electron-withdrawing groups can promote stronger π-conjugation, enhancing light absorption and charge carrier mobility within the in-plane π-electron system, thereby contributing to the overall photocatalytic activity. These results emphasize the critical role of electron-withdrawing and electron-donating substituents in ketoenamine-based COFs for enhancing photocatalytic hydrogen production from ethanol reforming.