Porphyrin-Based Covalent Organic Frameworks Anchoring Au Single Atoms for Photocatalytic Nitrogen Fixation

Abstract

The development of efficient photocatalysts for N2 fixation to produce NH3 under ambient conditions remains a great challenge. Since covalent organic frameworks (COFs) possess predesignable chemical structures, good crystallinity, and high porosity, it is highly significant to explore their potential for photocatalytic nitrogen conversion. Herein, we report a series of isostructural porphyrin-based COFs loaded with Au single atoms (COFX–Au, X = 1–5) for photocatalytic N2 fixation. The porphyrin building blocks act as the docking sites to immobilize Au single atoms as well as light-harvesting antennae. The microenvironment of the Au catalytic center is precisely tuned by controlling the functional groups at the proximal and distal positions of porphyrin units. As a result, COF1–Au decorated with strong electron-withdrawing groups exhibits a high activity toward NH3 production with rates of 333.0 ± 22.4 μmol g–1 h–1 and 37.0 ± 2.5 mmol gAu –1 h–1, which are 2.8- and 171-fold higher than that of COF4–Au decorated with electron-donating functional groups and a porphyrin–Au molecular catalyst, respectively. The NH3 production rates could be further increased to 427.9 ± 18.7 μmol g–1 h–1 and 61.1 ± 2.7 mmol gAu –1 h–1 under the catalysis of COF5–Au featuring two different kinds of strong electron-withdrawing groups. The structure–activity relationship analysis reveals that the introduction of electron-withdrawing groups facilitates the separation and transportation of photogenerated electrons within the entire framework. This work manifests that the structures and optoelectronic properties of COF-based photocatalysts can be finely tuned through a rational predesign at the molecular level, thus leading to superior NH3 evolution.

Postsynthetic Annulation of Three-Dimensional Covalent Organic Frameworks for Boosting CO2 Photoreduction

“…To confirm the formation of imine linkages in NJU-318 and NJU-319Fe, Fourier transform infrared (FTIR) spectra and solid-state nuclear magnetic resonance (ssNMR) spectra were collected (Figures S4–S7). The diminished CO stretching vibration at 1701 cm –1 of the aldehydes and the appearance of characteristic CN stretching vibrations at 1625 cm –1 for NJU-318 and NJU-319Fe corroborated the formation of imine bonds. − Furthermore, the CN peaks (158 ppm for NJU-318 and 159 ppm for NJU-319Fe) in the ssNMR spectra further verified the existence of imine groups in both NJU-318 and NJU-319Fe.…”

Photochemical Ammonia Synthesis over Reduced Polyoxovanadate-Intercalated Defective ZnAl Layered Double Hydroxide Nanosheets

“…Ammonia is not only a highly important ingredient of the global industrial economy and agricultural development but also a valuable candidate for hydrogen storage owing to its carbon-free and liquefiable feature. − The Haber–Bosch process serves as the classical industrial NH 3 synthesis technology usually performing the dissociative nitrogen reduction pathway under high energy input, which produces approximately 1% of the total CO 2 emission on the world. , In recent years, nitrogen photofixation achieves solar-to-chemical energy conversion under ambient conditions, indicating a feasible pathway for ammonia synthesis. − Suitable photocatalysts, the core of such nitrogen photofixation processes, directly harvest solar energy to generate an abundance of electrons, facilitating the NN triple-bond activation and dissociation. − In fact, the ammonia synthesis efficiency is still far from satisfactory mainly due to limited light absorption and poor charge carrier separation. , Therefore, the development of brand-new photocatalysts featuring a wide absorption range as well as highly efficient electron transport efficiency is crucial to realize efficient nitrogen fixation under ambient conditions.…”

Natural Ellagic Acid-Derived Nanoporous Organic Polymers for CO2 and Organic Dye Adsorption

“…Porous organic polymers (POPs) are an emerging class of lightweight robust nanoporous materials constructed from organic monomers via strong covalent bonds. − The tunable pore structure, designable skeletons, and tailored functionality have offered POPs great potential in diverse applications including adsorption/separation, − catalysis, − sensing, − and energy storage. − Over the past few decades, great efforts have been devoted to exploring new functional organic monomers to construct functional POPs for task-specific applications. − However, the functional organic monomers derived from fossil feedstocks are still few and usually expensive because of the complex synthesis routes. Therefore, the exploration of new organic monomers with high functionality and low cost is still highly desirable in the field of POPs.…”