Electrocatalytically Active Fe‐(O‐C2)4 Single‐Atom Sites for Efficient Reduction of Nitrogen to Ammonia

Single‐atom catalysts have demonstrated their superiority over other types of catalysts for various reactions. However, the reported nitrogen reduction reaction single‐atom electrocatalysts for the nitrogen reduction reaction exclusively utilize metal–nitrogen or metal–carbon coordination configurations as catalytic active sites. Here, we report a Fe single‐atom electrocatalyst supported on low‐cost, nitrogen‐free lignocellulose‐derived carbon. The extended X‐ray absorption fine structure spectra confirm that Fe atoms are anchored to the support via the Fe‐(O‐C₂)₄ coordination configuration. Density functional theory calculations identify Fe‐(O‐C₂)₄ as the active site for the nitrogen reduction reaction. An electrode consisting of the electrocatalyst loaded on carbon cloth can afford a NH₃ yield rate and faradaic efficiency of 32.1 μg h^?1 mg_cat.^?1 (5350 μg h^?1 mg_Fe^?1) and 29.3 %, respectively. An exceptional NH₃ yield rate of 307.7 μg h^?1 mg_cat.^?1 (51 283 μg h^?1 mg_Fe^?1) with a near record faradaic efficiency of 51.0 % can be achieved with the electrocatalyst immobilized on a glassy carbon electrode.     

Advanced Non-metallic Catalysts for Electrochemical Nitrogen Reduction under Ambient Conditions

The electrochemical reduction of nitrogen into ammonia under ambient conditions is a potential strategy for sustainable ammonia production. At present, one of the main research directions in the field of electrochemical nitrogen fixation is to improve the current efficiency and ammonia yield by developing efficient nitrogen reduction catalysts. To optimise the selectivity and catalytic activity of nitrogen reduction catalysts more efficiently, herein, we systematically summarise the progress of research on nitrogen reduction catalysts in recent years and present some general catalyst design strategies. Considering that it is difficult for metal-based catalysts to balance the competitive reactions of nitrogen activation and hydrogen evolution, we discuss in detail the advantages and application prospects of non-metallic catalysts in electrochemical nitrogen fixation. Moreover, both the design strategy of surface or interface defects, and how this atomic-level control of functionalisation helps to promote selectivity and catalytic activity are also discussed by theoretical and experimental electrochemistry. On this basis, we also discussed the future development direction, opportunities and challenges of nitrogen reduction electrocatalysts.     

基于非贵金属催化剂常温常压电化学合成氨

氨是一种重要的化工原料和能量载体,“哈伯反应”是工业上合成氨最主要的方法,但是该方法存在着能耗高,大量排放温室气体CO₂以及转化率低等问题。近年来,常温常压下基于多相催化剂的电化学还原N₂反应(NRR)来制备氨因其原料(N₂ + H₂O)易得,不依赖传统化石能源以及条件温和等原因而表现出巨大的应用潜能,并受到了科学家的广泛关注。然而目前NRR仍存在着如催化剂以贵金属材料为主,催化效率低和催化机理未明确等问题亟待解决。本综述主要总结了电催化NRR的最新研究成果,首先介绍了电催化NRR热力学和催化机理,接着重点列举了基于非贵金属催化剂的研究进展,包括过渡金属氧化物、氮化物、硫化物、非金属催化剂及单原子催化剂等,然后讨论了几种NRR电催化剂的改性方法,以及常见的产物氨的定性定量方法,最后,就目前该研究方向中仍待解决的问题进行了总结,并对下一步的研究进行了展望。     

Single‐Atom Electrocatalysts

Recent years have witnessed a dramatic increase in the production of sustainable and renewable energy. However, the electrochemical performances of the various systems are limited, and there is an intensive search for highly efficient electrocatalysts by more rational control over the size, shape, composition, and structure. Of particular interest are the studies on single‐atom catalysts (SACs), which have sparked new interests in electrocatalysis because of their high catalytic activity, stability, selectivity, and 100 % atom utilization. In this Review, we introduce innovative syntheses and characterization techniques for SACs, with a focus on their electrochemical applications in the oxygen reduction/evolution reaction, hydrogen evolution reaction, and hydrocarbon conversion reactions for fuel cells (electrooxidation of methanol, ethanol, and formic acid). The electrocatalytic performance is further considered at an atomic level and the underlying mechanisms are discussed. The ultimate goal is the tailoring of single atoms for electrochemical applications.     

Recent Advances in Noble‐Metal‐Free Catalysts for Electrocatalytic Synthesis of Ammonia under Ambient Conditions

Ammonia is an essential chemical for producing fertilizers and energy carriers. However, the industrial Haber–Bosch process causes huge CO₂ emissions and energy waste. As a promising alternative for Haber‐Bosch process, electrochemical synthesis of ammonia has drawn much attention. Catalysts, as a vital part of electrochemical nitrogen reduction reaction (NRR), have developed rapidly in recent years. Compared to noble‐metal catalysts, noble‐metal‐free catalysts possess a low‐cost advantage. In this review, noble‐metal‐free catalysts, including metal‐based materials, metal organic frameworks (MOFs), MXenes, and metal‐free materials, are summarized. In addition, effective design strategies are discussed, along with the main problems and some potential directions of noble‐metal‐free NRR catalysts.     

Electrocatalytically Active Fe-(O-C2)4 Single-Atom Sites for Efficient Reduction of Nitrogen to Ammonia

Single-atom catalysts have demonstrated their superiority over other types of catalysts for various reactions. However, the reported nitrogen reduction reaction single-atom electrocatalysts for the nitrogen reduction reaction exclusively utilize metal–nitrogen or metal–carbon coordination configurations as catalytic active sites. Here, we report a Fe single-atom electrocatalyst supported on low-cost, nitrogen-free lignocellulose-derived carbon. The extended X-ray absorption fine structure spectra confirm that Fe atoms are anchored to the support via the Fe-(O-C₂)₄ coordination configuration. Density functional theory calculations identify Fe-(O-C₂)₄ as the active site for the nitrogen reduction reaction. An electrode consisting of the electrocatalyst loaded on carbon cloth can afford a NH₃ yield rate and faradaic efficiency of 32.1 μg h⁻¹ mg_cat.⁻¹ (5350 μg h⁻¹ mg_Fe⁻¹) and 29.3 %, respectively. An exceptional NH₃ yield rate of 307.7 μg h⁻¹ mg_cat.⁻¹ (51 283 μg h⁻¹ mg_Fe⁻¹) with a near record faradaic efficiency of 51.0 % can be achieved with the electrocatalyst immobilized on a glassy carbon electrode.     

环境条件下电催化氮还原的现状、挑战与展望

氨是一种重要的化肥生产原料和清洁能源载体,在工业上主要通过哈伯法合成,但该工艺反应条件苛刻,需要高温高压并消耗大量的化石能源.因此,开发能耗低、反应温和的合成氨方法,对于缓解能源和环境的双重压力具有重要的现实意义.近年来,在温和条件下通过电催化氮还原反应(NRR)合成氨有望替代哈伯法,但该技术的重点在于设计合理的电催化...     

Maximizing the Number of Interfacial Sites in Single‐Atom Catalysts for the Highly Selective,Solvent‐Free Oxidation of Primary Alcohols

The solvent‐free selective oxidation of alcohols to aldehydes with molecular oxygen is highly attractive yet challenging. Interfacial sites between a metal and an oxide support are crucial in determining the activity and selectivity of such heterogeneous catalysts. Herein, we demonstrate that the use of supported single‐atom catalysts (SACs) leads to high activity and selectivity in this reaction. The significantly increased number of interfacial sites, resulting from the presence of individually dispersed metal atoms on the support, renders SACs one or two orders of magnitude more active than the corresponding nanoparticle (NP) catalysts. Lattice oxygen atoms activated at interfacial sites were found to be more selective than O₂ activated on metal NPs in oxidizing the alcohol substrate. This work demonstrates for the first time that the number of interfacial sites is maximized in SACs, providing a new avenue for improving catalytic performance by developing appropriate SACs for alcohol oxidation and other reactions occurring at metal–support interfacial sites.     

Cobalt Single‐Atom Catalysts with High Stability for Selective Dehydrogenation of Formic Acid

Metal–organic framework (MOF)‐derived Co‐N‐C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co‐N‐C catalyst achieves superior activity, better acid resistance, and improved long‐term stability compared with nanoparticles synthesized by a similar route. High‐angle annular dark‐field–scanning transmission electron microscopy, X‐ray photoelectron spectroscopy, electron paramagnetic resonance, and X‐ray absorption fine structure characterizations reveal the formation of Co^IIN_x centers as active sites. The optimal low‐cost catalyst is a promising candidate for liquid H₂ generation.     

Single‐Atom Iron Boosts Electrochemiluminescence

The traditional luminol–H₂O₂ electrochemiluminescence (ECL) sensing platform suffers from self‐decomposition of H₂O₂ at room temperature, hampering its application for quantitative analysis. In this work, for the first time we employ iron single‐atom catalysts (Fe‐N‐C SACs) as an advanced co‐reactant accelerator to directly reduce the dissolved oxygen (O₂) to reactive oxygen species (ROS). Owing to the unique electronic structure and catalytic activity of Fe‐N‐C SACs, large amounts of ROS are efficiently produced, which then react with the luminol anion radical and significantly amplify the luminol ECL emission. Under the optimum conditions, a Fe‐N‐C SACs–luminol ECL sensor for antioxidant capacity measurement was developed with a good linear range from 0.8 μm to 1.0 mm of Trolox.     

Single‐Atom Iron Catalysts on Overhang‐Eave Carbon Cages for High‐Performance Oxygen Reduction Reaction

Single‐atom catalysts have drawn great attention, especially in electrocatalysis. However, most of previous works focus on the enhanced catalytic properties via improving metal loading. Engineering morphologies of catalysts to facilitate mass transport through catalyst layers, thus increasing the utilization of each active site, is regarded as an appealing way for enhanced performance. Herein, we design an overhang‐eave structure decorated with isolated single‐atom iron sites via a silica‐mediated MOF‐templated approach for oxygen reduction reaction (ORR) catalysis. This catalyst demonstrates superior ORR performance in both alkaline and acidic electrolytes, comparable to the state‐of‐the‐art Pt/C catalyst and superior to most precious‐metal‐free catalysts reported to date. This activity originates from its edge‐rich structure, having more three‐phase boundaries with enhanced mass transport of reactants to accessible single‐atom iron sites (increasing the utilization of active sites), which verifies the practicability of such a synthetic approach.     

Nanoporous Palladium Hydride for Electrocatalytic N2 Reduction under Ambient Conditions

The electrocatalytic nitrogen reduction reaction (NRR) is an alternative eco‐friendly strategy for sustainable N₂ fixation with renewable energy. However, NRR suffers from sluggish kinetics owing to difficult N₂ adsorption and N≡N cleavage. Now, nanoporous palladium hydride is reported as electrocatalyst for electrochemical N₂ reduction under ambient conditions, achieving a high ammonia yield rate of 20.4 μg h^?1 mg^?1 with a Faradaic efficiency of 43.6 % at low overpotential of 150 mV. Isotopic hydrogen labeling studies suggest the involvement of lattice hydrogen atoms in the hydride as active hydrogen source. In situ Raman analysis and density functional theory (DFT) calculations further reveal the reduction of energy barrier for the rate‐limiting *N₂H formation step. The unique protonation mode of palladium hydride would provide a new insight on designing efficient and robust electrocatalysts for nitrogen fixation.     

A Durable Nickel Single‐Atom Catalyst for Hydrogenation Reactions and Cellulose Valorization under Harsh Conditions

Hydrothermally stable, acid‐resistant nickel catalysts are highly desired in hydrogenation reactions, but such a catalyst remains absent owing to the inherent vulnerability of nickel under acidic conditions. An ultra‐durable Ni‐N‐C single‐atom catalyst (SAC) has now been developed that possesses a remarkable Ni content (7.5 wt %) required for practical usage. This SAC shows not only high activities for hydrogenation of various unsaturated substrates but also unprecedented durability for the one‐pot conversion of cellulose under very harsh conditions (245 °C, 60 bar H₂, presence of tungstic acid in hot water). Using integrated spectroscopy characterization and computational modeling, the active site structure is identified as (Ni‐N4)???N, where significantly distorted octahedral coordination and pyridinic N constitute a frustrated Lewis pair for the heterolytic dissociation of dihydrogen, and the robust covalent chemical bonding between Ni and N atoms accounts for its ultrastability.     

基于Li-N2电池体系的“连续式”氮气还原合成氨

电化学合成氨近年来受到较多关注, 直接的电化学固氮法(NRR)存在产氨来源不明的问题, 而间接的锂式合成氨(LiNR)被认为是一种可行的固氮方案. LiNR的研究多为电沉积锂, 本工作以Li-N₂电池体系为基础, 利用电池的放电反应固定N₂, 质子源H₂O同时参与反应, 理论上提高了Li-N₂电池的放电电压. 结合充电反应锂盐分解, 构成了清晰的锂循环方案. 研究发现, 当N₂和H₂O共同通入电池, 可以实现连续式的NH₃生产, 且放电电位与理论值接近. 充放电循环显示, 每个循环均可以产生NH₃, 产氨量随循环次数而增加. 该方案可循环利用锂, 对于开发新型的固氮方式有较大的研究与利用价值.     

An Enzyme‐Mimicking Single‐Atom Catalyst as an Efficient Multiple Reactive Oxygen and Nitrogen Species Scavenger for Sepsis Management

Sepsis, characterized by immoderate production of multiple reactive oxygen and nitrogen species (RONS), causes high morbidity and mortality. Despite progress made with nanozymes, efficient antioxidant therapy to eliminate these RONS remains challenging, owing largely to the specificity and low activity of exploited nanozymes. Herein, an enzyme‐mimicking single‐atom catalyst, Co/PMCS, features atomically dispersed coordinatively unsaturated active Co‐porphyrin centers, which can rapidly obliterate multiple RONS to alleviate sepsis. Co/PMCS can eliminate O₂^.? and H₂O₂ by mimicking superoxide dismutase, catalase, and glutathione peroxidase, while removing ^.OH via the oxidative‐reduction cycle, with markedly higher activity than nanozymes. It can also scavenge ^.NO through formation of a nitrosyl–metal complex. Eventually, it can reduce proinflammatory cytokine levels, protect organs from damage, and confer a distinct survival advantage to the infected sepsis mice.     

Regulating the Coordination Environment of MOF‐Templated Single‐Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single‐atom catalysts (SACs) remains a great challenge. Herein, a general host–guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal–organic framework. As an example, the introduction of Mg²⁺ in MgNi‐MOF‐74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single‐atom Ni catalysts (named Ni_SA‐N_x‐C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni_SA‐N₂‐C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h^?1), far superior to those of Ni_SA‐N₃‐C and Ni_SA‐N₄‐C, in electrocatalytic CO₂ reduction. Theoretical calculations reveal that the low N coordination number of single‐atom Ni sites in Ni_SA‐N₂‐C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.     

Direct Observation of Dynamic Bond Evolution in Single‐Atom Pt/C3N4 Catalysts

Single‐atom catalysts are promising platforms for heterogeneous catalysis, especially for clean energy conversion, storage, and utilization. Although great efforts have been made to examine the bonding and oxidation state of single‐atom catalysts before and/or after catalytic reactions, when information about dynamic evolution is not sufficient, the underlying mechanisms are often overlooked. Herein, we report the direct observation of the charge transfer and bond evolution of a single‐atom Pt/C₃N₄ catalyst in photocatalytic water splitting by synchronous illumination X‐ray photoelectron spectroscopy. Specifically, under light excitation, we observed Pt?N bond cleavage to form a Pt⁰ species and the corresponding C=N bond reconstruction; these features could not be detected on the metallic platinum‐decorated C₃N₄ catalyst. As expected, H₂ production activity (14.7 mmol h^?1 g^?1) was enhanced significantly with the single‐atom Pt/C₃N₄ catalyst as compared to metallic Pt‐C₃N₄ (0.74 mmol h^?1 g^?1).     

Coordination Tunes Selectivity: Two‐Electron Oxygen Reduction on High‐Loading Molybdenum Single‐Atom Catalysts

Single‐atom catalysts (SACs) have great potential in electrocatalysis. Their performance can be rationally optimized by tailoring the metal atoms, adjacent coordinative dopants, and metal loading. However, doing so is still a great challenge because of the limited synthesis approach and insufficient understanding of the structure–property relationships. Herein, we report a new kind of Mo SAC with a unique O,S coordination and a high metal loading over 10 wt %. The isolation and local environment was identified by high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure. The SACs catalyze the oxygen reduction reaction (ORR) via a 2 e^? pathway with a high H₂O₂ selectivity of over 95 % in 0.10 m KOH. The critical role of the Mo single atoms and the coordination structure was revealed by both electrochemical tests and theoretical calculations.