Solar energy can be used as “green” energy by photocatalysis for the nitrogen fixation under the atmospheric conditions compared with the traditional energy-intensive industrial production of ammonia. However, the complex kinetics and high reaction barriers greatly hinder the development of the photocatalytic N2 reduction reaction. Herein, a BiOBr/MXene-Ti3C2 composite catalyst is prepared by the simple electrostatic adsorption and self-assembly method. The as-prepared 10 wt % BiOBr/Ti3C2 exhibits the best performance for N2 fixation to NH3 by photocatalysis. The evolution rate of NH3 is up to 234.6 μmol·g–1·h–1, which is approximately 48.8 times and 52.4 times higher than those of pure BiOBr and Ti3C2, respectively. It is found that the designed double vacancies of oxygen and titanium for BiOBr/Ti3C2 composites, with the availability of localized electrons, have the ability to adsorb and activate N2, which can be efficiently reduced to NH3 by the interfacial electrons transferred from the excited BiOBr/Ti3C2 composite. In addition, the results of in situ Fourier transform infrared show the generation of NxHy species by the continuous protonation processes. Moreover, titanium vacancy (VTi) induces a strong absorption energy for nitrogen atoms on the surface of BiOBr/Ti3C2 according to the density functional theory calculations. In particular, the P-electron feedback caused by VTi could effectively promote the weakening of the N≡N triple bond and elongate the N2 bond length by ∼31.6%. This work might provide new insights into the synergistic effect of double defects and inspiration for the rational design of catalysts by defect engineering in the field of catalytic synthesis of ammonia.
Zhenqing Zhao,Changhyeok Choi,Song Hong,Huidong Shen,Chao Yan,Justus Masa,Yousung Jung,Jieshan Qiu,Zhenyu Sun
Solar N 2 fixation under visible light offers a promising method toward sustainable NH 3 production at benign conditions. However, it still remains a formidable challenge to activate and cleave N N bonds and promote the separation and transport of electrons and holes during photocatalysis . To address these issues, the discovery and design of high-performance and robust photocatalysts is imperative. Here, we report the defect engineering of two-dimensional oxidized Sb nanosheets to activate intrinsically inactive Sb for efficient visible light-driven N 2 reduction to NH 3 . Impressively, the Sb nanosheets rich in Sb and oxygen vacancies afford a remarkable NH 3 formation rate of up to 388.5 μg NH3 h −1 g cat. −1 without cocatalyst in visible light, 8 times higher than that for bulk Sb and also significantly outperforming many previously reported photocatalysts. The defective Sb nanosheets exhibit excellent stability after five successive reaction cycles. Further density functional theory calculations reveal a considerably strong interaction between N 2 and defects on the surface and edge of Sb nanosheets , which facilitates the formation of *NNH (N 2 + (H + + e - ) → *NNH, where * denotes an adsorption site), thus promoting photocatalytic N 2 reduction. This finding opens a novel avenue to enhancing N 2 photofixation over inherently inactive surfaces by synergistically engineering defect sites.
Yang Hu,George Z. Chen,Lin Zhuang,Zhiyong Wang,Xianbo Jin
Artificial N 2 fixation is one of the most important industrial processes for human beings. However, the current Haber-Bosch process based on fossil fuel suffers from unsustainability and a low N 2 to NH 3 conversion. Here, we report a magnesium chloride cycle for the synthesis of NH 3 from N 2 and H 2 O at atmospheric pressure. In this cycle, N 2 reacts with Mg to form Mg 3 N 2 . Mg and Cl 2 are produced by the electrolysis of molten MgCl 2 . NH 4 Cl is used to hydrogenolyze Mg 3 N 2 to produce NH 3 , with anhydrous MgCl 2 being regenerated. The cycle is closed via the reverse Deacon reaction to convert Cl 2 to HCl, which is captured as NH 4 Cl with the recyclable NH 3 . This indirect electrosynthesis produces NH 3 at an industry-acceptable rate with zero CO 2 emission, a coulombic efficiency >92%, and an energy consumption of ∼14.1 kWh/kg-NH 3 .
Palladium (Pd) has long been known to be inactive for dinitrogen activation and hence has received little attention for ammonia synthesis. Herein, we report that the reactivity of Pd is changed when reacting with lithium hydride (LiH) forming a series of lithium palladium hydrides such as LiPdH0.7 and Li2PdH2 (denoted as [Li–Pd–H]). LiPdH0.7 promotes N2 fixation by LiH forming Li2NH and LiPd, and the subsequent hydrogenation of Li2NH and LiPd leads to ammonia and regeneration of LiH and [Li–Pd–H]. Based on these experimental results, an ammonia loop mediated by a [Li–Pd–H]–LiH and LiPd–Li2NH composite pair is proposed and demonstrated. An ammonia production rate of 693 μmolNH3 g–1 h–1 was achieved at 300 °C and 1 bar, which is ca. 7 times that of the neat LiH/Li2NH pair at 350 °C and 1 bar and 2 times that of the thermocatalytic process catalyzed by Ru/MgO at 300 °C and 10 bar.
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