Rational construction of donor-acceptor covalent heptazine polymers for efficient photocatalytic N2 reduction to NH3

Abstract

Three novel donor-acceptor covalent heptazine polymers were rationally constructed by regulating the donor and the electronic structure of the acceptor. The results show that the photocatalytic N2 reduction to NH3 of Cy-Tq is up to 562 μmol g -1 h -1 through an alternating association mechanism. Ammonia (NH3) plays an important role in agricultural production and medicine as an indispensable energy source. 1 Photocatalytic NH3 production serves as a green and sustainable method, which employs solar energy and photocatalysts to drive N2 conversion by using H2O as the proton source under ambient conditions. 2, 3 However, the stable N≡N triple bond, 6-electron reaction pathway, and the high energy barrier of this process results in low efficiency. 4,5 Hence, designing and developing novel photocatalysts for efficient N2 reduction to NH3 in aqueous phase system is of great significance, while challenging.Conjugated microporous polymers (CMPs) are expected to serve as a new generation of metal-free organic photocatalysts for NH3 production due to their tunable functionality and welldefined active sites. 6 Nevertheless, their overall photocatalytic efficiency is still unsatisfactory because of the high exciton binding energies and slow charge separation/transfer efficiency. 7 To address this issue, constructing special donoracceptor (D-A) structures is proven a facile and efficient strategy, which primarily involves fine-tuning the strength of D-A effect through molecular design of electron-rich conjugated donors and electron-deficient acceptors, thereby providing a powerful driving force for rapid separation/transfer of photo-generated charges. 8 Notably, covalent triazine/heptazine polymers (CTPs/CHPs) are typical CMPs with strong D-A effect, excelling at converting photon energy to chemical transformations. [9][10][11] Due to their outstanding light absorption ability and acceptor