Engineering photocatalytic ammonia synthesis.

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.

Dual-Ligand Ti-MOFs with Push-Pull Effect for Photocatalytic H2 Production.

Enhancing the photogenerated electrons transfer efficiency is crucial for photocatalytic reactions. Herein, a dual-ligand-induced push-pull effect was manipulated to intensify the transfer of photogenerated electrons between organic ligands and metal clusters using NH2-MIL-125(Ti), a kind of Ti-based metal-organic framework (MOF), as the model system. The dual-ligand MOF, NH2/Cl-MIL-125, was designed and synthesized based on the Hammett constant (σm), in which -NH2 (σm = -0.16) and -Cl (σm = 0.37) were selected as the electron-pushing group and the electron-pulling group, respectively. Meanwhile, -CH3 (σm = -0.07, electron-pushing) and -H (σm = 0, neither electron-pushing nor electron-pulling) were selected as the reference groups to prepare NH2/CH3-MIL-125 and NH2/H-MIL-125, respectively, to validate the electron push-pull effect. NH2/Cl-MIL-125 (5.32 mmol g-1 h-1) exhibits a higher photocatalytic H2 evolution activity than single-ligand NH2-MIL-125 (1.93 mmol g-1 h-1), NH2/CH3-MIL-125 (4.45 mmol g-1 h-1), and NH2/H-MIL-125 (4.73 mmol g-1 h-1) under full-spectrum irradiation. The result can be attributed to the electron push-pull effect between -NH2 and -Cl, which boosts the electron transfer along the ligand-metal-ligand direction. Our dual-ligand-induced push-pull strategy for enhancing the electron transfer may offer some novel insights into the rational design and synthesis of photocatalysts for many other reactions.