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Photocatalytic ammonia synthesis (PAS) is an emerging zero carbon emission technology, which is critical for mitigating energy crises and achieving carbon neutrality. Herein, we summarize the recent advances and challenges in PAS from an engineering perspective based on its whole chain process, i.e., materials engineering, structure engineering and reaction engineering. For materials engineering, we discuss the commonly used photocatalytic materials including metal oxides, bismuth oxyhalides and graphitic carbon nitride and emerging materials, such as organic frameworks, along with the analysis of their characteristics and regulation methods to enhance the PAS performance. For structure engineering, the design of photocatalysts is described in terms of morphology, vacancy and band, corresponding to the crystal, atom and electron scales, respectively. Moreover, the structure–performance relationship of photocatalysts has been deeply explored in this section. For reaction engineering, we identify three key processes from the chemical reaction and mass transfer, i.e., nitrogen activation, molecule transfer and electron transfer, to intensify and optimize the PAS reaction. Hopefully, this review will provide a novel paradigm for the design and preparation of high-efficiency ammonia synthesis photocatalysts and inspire the practical application of PAS. [ABSTRACT FROM AUTHOR]

• Nitrogenase-inspired MIL-53(FeII/FeIII) is first synthesized for N 2 photo fixation. • Mixed-valence FeII/FeIII clusters mimic the active center in nitrogenase. • FeII are in-situ formed by ethylene glycol reductant in solvothermal process. • FeII/FeIII ratio is vital to coordinate catalytic activity and framework stability. • MIL-53(FeII/FeIII) stably yields 306 μmol h−1 g−1 NH 3 under visible light. Biological nitrogenases exhibit superior nitrogen fixation efficiency owing to their unique multi-iron metallocluster (Fe2+ 3 Fe3+ 4 M3+, M = Mo, V, Fe) coordinated by organic polypeptide. Herein, we design a kind of metal organic framework (MOF) photocatalyst, MIL-53(FeII/FeIII) (MIL = Material from Institute Lavoisier), in which the FeII and FeIII constitute the mixed-valence metalloclusters to mimic the Fe2+ active sites and high-valence metal ions in nitrogenases, respectively. Both the FeII and FeIII are coordinated by organic ligands (terephthalic acid), which afford the electron transfer chains as well as the support of monodispersed FeII active sites. The FeIII in MIL-53(FeII/FeIII) is partly in-situ reduced into FeII by ethylene glycol (EG) via one-step solvothermal method, and the FeII/FeIII ratio is regulated from 0.18:1 to 1.21:1 by varying the EG content. Our results show that the FeII/FeIII ratio can significantly affect the photocatalytic activity and structure stability, and MIL-53(FeII/FeIII)-1 with optimal FeII/FeIII ratio (1.06:1) achieves the highest ammonia evolution rate up to 306 μmol h−1 g−1, nearly 10-fold higher than that of other framework-based materials, while remaining stable after 24 h irradiation. Such nitrogenase-like design in MIL-53(FeII/FeIII) endows the efficient electron transfer, exposed active sites, and in particular rational synergy between the catalytic function and non-catalytic function. This work may open a new avenue to the rational design of nitrogen fixation photocatalysts based on framework materials. [ABSTRACT FROM AUTHOR]

• NM@NHM hetero-framework photocatalysts are designed and constructed. • The optimal photocatalyst exhibits a H 2 evolution rate of 2.25 mmol g−1 h−1 under visible light. • Both dual-ligand push-pull effect and heterojunction structure promote the electron transfer. • This study provides a new strategy to intensify the electron transfer in photocatalysis. The efficient transfer of photogenerated carriers is essential to the photocatalytic H 2 evolution. Herein, we designed and synthesized a kind of novel NH 2 -MIL-125@NH 2 /H-MIL-125 hetero-frameworks with nanopore structure to drive efficient charge transfer for photocatalytic H 2 evolution. Single-ligand NH 2 -MIL-125 (NM) and dual-ligand NH 2 /H-MIL-125 (NHM), which have similar composition, structure and well-matched energy band, were selected as the core and shell to prepare the NM@NHM hetero-framework as a proof-of-principle system. The hetero-framework strategy is proposed to intensify the charge transfer and thus enhance the photocatalytic activity. The H 2 evolution rate of optimal NM@NHM hetero-framework can reach 2.25 mmol g−1 h−1 under visible light, much higher than pristine NM (0.726 mmol g−1 h−1) and NHM (1.27 mmol g−1 h−1). Such high performance can be attributed to the hetero-framework structure consisting of NM and NHM with similar composition and structure, which reduces the interfacial resistance in the charge transfer, thus facilitating the photocatalytic H 2 evolution. Our work may provide a new strategy for the rational design of high-performance photocatalysts. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A multi-ligand strategy was proposed to design high-efficient MOF photocatalysts. • The three-ligand MOFs, EY/NH 2 /H-MIL-125, were synthesized to verify our strategy. • The optimal three-ligand MOF can yield H 2 of 2.15 mmol g−1 h−1 under visible light. • Electron transfer is enhanced by a push–pull effect induced by the NH 2 -H 2 BDC and H 2 BDC ligands. • The third ligand EY facilitates the electron photoexcitation under visible light. The design and construction of high-performance photocatalysts with both strong visible light harvesting and effective electron transfer are highly desired in the photocatalysis field. Herein, a multi-ligand strategy based on the MOF materials is proposed. A three-ligand Ti-MOF EY/NH 2 /H-MIL-125 (ENHM) was prepared via a facile hydrothermal method, in which 2-aminoterephthalic acid (NH 2 -H 2 BDC), terephthalic acid (H 2 BDC) and Eosin Y (EY) were employed as three ligands. The push–pull effect induced by different substituent groups (–NH 2 and –H) on the skeleton of NH 2 -H 2 BDC and H 2 BDC promotes the efficient transfer of photogenerated electrons from organic ligands to metal nodes; while the third ligand EY increases the number of photoexcited electrons by enhancing the visible light absorption ability. Among the three-ligand MOF photocatalytic materials, ENHM-2 performs the optimal H 2 production activity of 2.15 mmol g−1 h−1, much higher than dual-ligand NH 2 /H-MIL-125 and EY/NH 2 -MIL-125 and single-ligand NH 2 -MIL-125. A probable mechanism was also proposed for the three-ligand MOFs. This multi-ligand strategy may open up a new direction toward the preparation of MOF-based photocatalytic materials with excellent photocatalytic performances. [ABSTRACT FROM AUTHOR]

• Porous cPTA/CNT/CNU nanorod is prepared by using PTA as both template and adhesive. • A g-C 3 N 4 isotype heterojunction forms on the cPTA nanorod surface in cPTA/CNT/CNU. • Optimal cPTA/CNT/CNU nanorod can degrade 95% RhB within 1 h under visible light. • A multistep electron transfer path from CNU, CNT to cPTA forms in cPTA/CNT/CNU. • Porous cPTA nanorod increases light harvesting, active sites and molecule channels. Graphite carbon nitride (g-C 3 N 4) has a huge application potential in the photocatalysis field due to its unique conjugated structure. Inspired by the mussel adhesion protein, the cPTA/CNT/CNU (cPTA, carbonized poly(tannic acid); CNT, g-C 3 N 4 prepared by thiourea); CNU, g-C 3 N 4 prepared by urea) nanorod was synthesized by using PTA as the adhesive and template, and thiourea and urea as g-C 3 N 4 precursors. The CNT/CNU isotype heterojunction is assembled on the surface of porous cPTA nanorod, which effectively solves the problem of high photogenerated electron-hole recombination of pure g-C 3 N 4. The optimal cPTA/CNT/CNU photocatalyst can degrade 95% of rhodamine B (RhB) within 1 h under visible light, which is much higher than CNT, CNU and CNT/CNU. With the conduction band potential difference of cPTA, CNT and CNU, a multistep electron transfer path forms in the cPTA/CNT/CNU, which can accelerate the electron transfer through the interface, thereby increasing the photocatalytic activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nitrogenase-inspired modular synthesis is designed to construct bimetallic UiO-66. • Zr-Hf clusters are constructed to coordinate the electron transfer and utilization. • U(0.5Hf)-2SH with TPA-2SH as the ligand can absorb and utilize the visible light. • A tandem ligand-to-metal-to-metal charge transfer mechanism (LMMCT) is proposed. • The resultant bimetallic UiO-66 yields 116.1 μmol h−1 g−1 NH 3 under visible light. Inspired by nitrogenase, we designed a kind of metal-organic framework (MOF) photocatalysts, U(Zr-Hf)-X, composed of bimetallic Zr-Hf nodes and different functional ligands. In the Zr-Hf bimetallic cluster, the Zr species serves as the active site of nitrogen fixation, while the Hf species acts as electron buffer tank to optimize electron transfer and utilization. In the organic ligands, the light absorption and utilization of MOFs is improved via introducing target functional groups to efficiently generate electrons. Notably, the organic ligand containing two thiol groups (-SH) can extend the absorption edge to the visible region. The optimal U(0.5Hf)-2SH (metal nodes: 0.5Zr: 0.5Hf; linkers: TPA-2SH) photocatalyst exhibits nitrogen fixation rate up to 116.1 μmol g−1 h−1 under visible-light, attributed to the ligand-to-metal-to-metal electron transfer (LMMET) pathway and Zr-based π-backbonding mechanism. Our nitrogenase-inspired strategy establishes a novel paradigm for rational design of visible-light-driven nitrogen fixation catalysts. [ABSTRACT FROM AUTHOR]