Ammonia Synthesis Under Ambient Conditions: Selective Electroreduction of Dinitrogen to Ammonia on Black Phosphorus Nanosheets

Constructing efficient catalysts for the N₂ reduction reaction (NRR) is a major challenge for artificial nitrogen fixation under ambient conditions. Herein, inspired by the principle of “like dissolves like”, it is demonstrated that a member of the nitrogen family, well‐exfoliated few‐layer black phosphorus nanosheets (FL‐BP NSs), can be used as an efficient nonmetallic catalyst for electrochemical nitrogen reduction. The catalyst can achieve a high ammonia yield of 31.37 μg h^?1 mg^?1_cat. under ambient conditions. Density functional theory calculations reveal that the active orbital and electrons of zigzag and diff‐zigzag type edges of FL‐BP NSs enable selective electrocatalysis of N₂ to NH₃ via an alternating hydrogenation pathway. This work proves the feasibility of using a nonmetallic simple substance as a nitrogen‐fixing catalyst and thus opening a new avenue towards the development of more efficient metal‐free catalysts.     

A theoretical study of electrocatalytic ammonia synthesis on single metal atom/MXene

Electrocatalytic ammonia synthesis under mild conditions is an attractive and challenging process in the earth's nitrogen cycle, which requires efficient and stable catalysts to reduce the overpotential. The N₂ activation and reduction overpotential of different Ti₃C₂O₂-supported transition metal (TM) (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Cd, and Au) single-atom catalysts have been analyzed in terms of the Gibbs free energies calculated using the density functional theory (DFT). The end-on N₂ adsorption was more energetically favorable, and the negative free energies represented good N₂ activation performance, especially in the presence Fe/Ti₃C₂O₂ (?0.75 eV). The overpotentials of Fe/Ti₃C₂O₂, Co/Ti₃C₂O₂, Ru/Ti₃C₂O₂, and Rh/Ti₃C₂O₂ were 0.92, 0.89, 1.16, and 0.84 eV, respectively. The potential required for ammonia synthesis was different for different TMs and ranged from 0.68 to 2.33 eV. Two possible potential-limiting steps may be involved in the process: (i) hydrogenation of N₂ to *NNH and (ii) hydrogenation of *NH₂ to ammonia. These catalysts can change the reaction pathway and avoid the traditional N–N bond-breaking barrier. It also simplifies the understanding of the relationship between the Gibbs free energy and overpotential, which is a significant factor in the rational designing and large-scale screening of catalysts for the electrocatalytic ammonia synthesis.     

Defect Engineering Metal‐Free Polymeric Carbon Nitride Electrocatalyst for Effective Nitrogen Fixation under Ambient Conditions

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions provides an intriguing picture for the conversion of N₂ into NH₃. However, electrocatalytic NRR mainly relies on metal‐based catalysts, and it remains a grand challenge in enabling effective N₂ activation on metal‐free catalysts. Here we report a defect engineering strategy to realize effective NRR performance (NH₃ yield: 8.09 μg h^?1 mg^?1_cat., Faradaic efficiency: 11.59 %) on metal‐free polymeric carbon nitride (PCN) catalyst. Illustrated by density functional theory calculations, dinitrogen molecule can be chemisorbed on as‐engineered nitrogen vacancies of PCN through constructing a dinuclear end‐on bound structure for spatial electron transfer. Furthermore, the N?N bond length of adsorbed N₂ increases dramatically, which corresponds to “strong activation” system to reduce N₂ into NH₃. This work also highlights the significance of defect engineering for improving electrocatalysts with weak N₂ adsorption and activation ability.     

Transition metal decorated bismuthene for ammonia synthesis: A density functional theory study

The electrochemical nitrogen reduction reaction (NRR) for the ammonia production under ambient conditions is regarded as a sustainable alternative to the industrial Haber–Bosch process. However, the electrocatalytic systems that efficiently catalyze nitrogen reduction remain elusive. In the work, the nitrogen reduction activity of the transition metal decorated bismuthene TM@Bis is fully investigated by means of density functional theory calculations. Our results demonstrate that W@Bis delivers the best efficiency, wherein the potential-determining step is located at the last protonation step of *NH₂ + H⁺ + e^– → *NH₃ via the distal mechanism with the limiting potential U_L of 0.26 V. Furthermore, the dopants of Re and Os are also promising candidates for experimental synthesis due to its good selectivity, in despite of the slightly higher U_L of NRR with the value of 0.55 V. However, the candidates of Ti, V, Nb and Mo delivered the relative lower U_L of 0.35, 0.37, 0.41 and 0.43 V might be suffered from the side hydrogen evolution reaction. More interestingly, a volcano curve is established between U_L and valence electrons of metal elements wherein W with 4 electrons in d band located at the summit. Such phenomenon originates from the underlying acceptance-back donation mechanism. Therefore, our work provides a fundament understanding for the material design for nitrogen reduction electrocatalysis.     

Tuning the reactivity of mononuclear nonheme manganese(iv)-oxo complexes by triflic acid

Triflic acid (HOTf)-bound nonheme Mn(iv)-oxo complexes, [(L)Mn^IV(O)]²⁺–(HOTf)₂ (L = N4Py and Bn-TPEN; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine and Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine), were synthesized by adding HOTf to the solutions of the [(L)Mn^IV(O)]²⁺ complexes and were characterized by various spectroscopies. The one-electron reduction potentials of the Mn^IV(O) complexes exhibited a significant positive shift upon binding of HOTf. The driving force dependences of electron transfer (ET) from electron donors to the Mn^IV(O) and Mn^IV(O)–(HOTf)₂ complexes were examined and evaluated in light of the Marcus theory of ET to determine the reorganization energies of ET. The smaller reorganization energies and much more positive reduction potentials of the [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes resulted in greatly enhanced oxidation capacity towards one-electron reductants and para-X-substituted-thioanisoles. The reactivities of the Mn(iv)-oxo complexes were markedly enhanced by binding of HOTf, such as a 6.4 × 10⁵-fold increase in the oxygen atom transfer (OAT) reaction (i.e., sulfoxidation). Such a remarkable acceleration in the OAT reaction results from the enhancement of ET from para-X-substituted-thioanisoles to the Mn^IV(O) complexes as revealed by the unified ET driving force dependence of the rate constants of OAT and ET reactions of [(L)Mn^IV(O)]²⁺–(HOTf)₂. In contrast, deceleration was observed in the rate of H-atom transfer (HAT) reaction of [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes with 1,4-cyclohexadiene as compared with those of the [(L)Mn^IV(O)]²⁺ complexes. Thus, the binding of two HOTf molecules to the Mn^IV(O) moiety resulted in remarkable acceleration of the ET rate when the ET is thermodynamically feasible. When the ET reaction is highly endergonic, the rate of the HAT reaction is decelerated due to the steric effect of the counter anion of HOTf.     

The influence of phase and morphology of molybdenum nitrides on ammonia synthesis activity and reduction characteristics

The reactivities of a series of ternary and binary molybdenum nitrides have been compared. Data have been obtained for the catalytic synthesis of ammonia at 400 °C and ambient pressure using a 3:1 H₂:N₂ mixture. Amongst the ternary nitrides, the mass normalised activity is in the order Co₃Mo₃N>Fe₃Mo₃N?Ni₂Mo₃N. For the binary molybdenum nitrides, the ammonia synthesis activity is significantly lower than that of Co₃Mo₃N and Fe₃Mo₃N and varies in the order γ-Mo₂N∼β-Mo₂N_0.78?δ-MoN. Nanorod forms of β-Mo₂N_0.78 and γ-Mo₂N exhibit generally similar activities to conventional polycrystalline samples, demonstrating that the influence of catalyst morphology is limited for these two materials. In order to characterise the reactivity of the lattice nitrogen species of the nitrides, temperature programmed reactions with a 3:1 H₂:Ar mixture at temperatures up to 700 °C have been performed. For all materials studied, the predominant form of nitrogen lost was N₂, with smaller amounts of NH₃ being formed. Post-reaction powder diffraction analyses demonstrated lattice shifts in the case of Co₃Mo₃N and Ni₂Mo₃N upon temperature programmed reaction with H₂/Ar. Incomplete decomposition yielding mixtures of Mo metal and the original phase were observed for Fe₃Mo₃N and γ-Mo₂N, whilst β-Mo₂N_0.78 transforms completely to Mo metal and δ-MoN is converted to γ-Mo₂N.     

Catalytic Performance of Spherical MCM-41 Modified with Copper and Iron as Catalysts of NH3-SCR Process

Spherical MCM-41 with various copper and iron loadings was prepared by surfactant directed co-condensation method. The obtained samples were characterized with respect to their structure (X-ray diffraction, XRD), texture (N₂ sorption), morphology (scanning electron microscopy, SEM), chemical composition (inductively coupled plasma optical emission spectrometry, ICP-OES), surface acidity (temperature programmed desorption of ammonia, NH₃-TPD), form, and aggregation of iron and copper species (diffuse reflectance UV-Vis spectroscopy, UV-Vis DRS) as well as their reducibility (temperature programmed reduction with hydrogen, H₂-TPR). The spherical MCM-41 samples modified with transition metals were tested as catalysts of selective catalytic reduction of NO with ammonia (NH₃-SCR). Copper containing catalysts presented high catalytic activity at low-temperature NH₃-SCR with a very high selectivity to nitrogen, which is desired reaction products. Similar results were obtained for iron containing catalysts, however in this case the loadings and forms of iron incorporated into silica samples very strongly influenced catalytic performance of the studied samples. The efficiency of the NH₃-SCR process at higher temperatures was significantly limited by the side reaction of direct ammonia oxidation. The reactivity of ammonia molecules chemisorbed on the catalysts surface in NO reduction (NH₃-SCR) and their selective oxidation (NH₃-SCO) was verified by temperature-programmed surface reactions.     

Reaction of the intermetallide ZrV<Subscript>2</Subscript> with ammonia

The reaction of the intermetallic compound ZrV₂ with ammonia within a temperature range of 150–500°C in the presence of NH₄Cl as an activator of the process was studied. Depending on the reaction temperature, intermetallide hydrides and compositions of metal hydrides and nitrides or metal nitrides were obtained in the form of finely dispersed powders with particle sizes of less than 1.0 μm.     

Insights into the selective catalytic reduction of NO by NH<Subscript>3</Subscript> over Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>(110): a DFT study coupled with microkinetic analysis

Nitric oxide (NO_x), as one of the main pollutants, can contribute to a series of environmental problems, and to date the selective catalytic reduction (SCR) of NO_x with NH₃ in the presence of excess of O₂ over the catalysts has served as one of the most effective methods, in which Mn-based catalysts have been widely studied owing to their excellent low-temperature activity toward NH₃-SCR. However, the related structure-activity relation was not satisfactorily explored at the atomic level. By virtue of DFT+U calculations together with microkinetic analysis, we systemically investigate the selective catalytic reduction process of NO with NH₃ over Mn₃O₄(110), and identify the crucial thermodynamic and kinetic factors that limit the catalytic activity and selectivity. It is found that NH₃ prefers to adsorb on the Lewis acid site and then dehydrogenates into NH₂* assisted by either the two- or three-fold lattice oxygen; NH₂* would then react with the gaseous NO to form an important intermediate NH₂NO that prefers to convert into N₂O rather than N₂ after the sequential dehydrogenation, while the residual H atoms interact with O₂ and left the surface in the form of H₂O. The rate-determining step is proposed to be the coupling reaction between NH₂* and gaseous NO. Regarding the complex surface structure of Mn₃O₄(110), the main active sites are quantitatively revealed to be O_3c and Mn_4c.     

Enhanced Electrocatalytic N2 Reduction via Partial Anion Substitution in Titanium Oxide–Carbon Composites

The electrochemical conversion of N₂ at ambient conditions using renewably generated electricity is an attractive approach for sustainable ammonia (NH₃) production. Considering the chemical inertness of N₂, rational design of efficient and stable catalysts is required. Therefore, in this work, it is demonstrated that a C‐doped TiO₂/C (C‐Ti_xO_y/C) material derived from the metal–organic framework (MOF) MIL‐125(Ti) can achieve a high Faradaic efficiency (FE) of 17.8 %, which even surpasses most of the established noble metal‐based catalysts. On the basis of the experimental results and theoretical calculations, the remarkable properties of the catalysts can be attributed to the doping of carbon atoms into oxygen vacancies (OVs) and the formation of Ti?C bonds in C‐Ti_xO_y. This binding motive is found to be energetically more favorable for N₂ activation compared to the non‐substituted OVs in TiO₂. This work elucidates that electrochemical N₂ reduction reaction (NRR) performance can be largely improved by creating catalytically active centers through rational substitution of anions into metal oxides.     

Phosphorene-Supported Transition-Metal Dimer for Effective N2 Electroreduction

The electrochemical reduction of N₂ to NH₃ at ambient conditions is a promising alternative to the energy-intensive, high-temperature, high-pressure Haber-Bosch process. But it is extremely challenging to find an electrocatalyst that can effectively activate N₂ and reduce it to NH₃. From first principles density functional theory, we found that the Ti dimer supported on single-layer phosphorene can be used as a promising electrocatalyst for N₂ capture and conversion to NH₃. The overpotential (relative to the standard hydrogen electrode) was found to be as low as 0.20, much lower than those predicted on the Ti surface (1 to 1.5 V) or their nitrides (0.5 to 1 V). In addition, we found that hydride is involved in the N₂ reduction on the Ti dimer catalyst via formation of Ti₂-H species, and the hydride would favorably transfer onto the adsorbed N₂* to form *NNH intermediate and further reduced to NH₃. Moreover, we also examined other first-row transition metal dimers, and found that Sc and Fe dimer to be potential catalysts which could catalyze N₂ reduction at a low overpotential of about 0.21 and 0.45 V, respectively. Our predictions hence suggest Ti, Sc and Fe dimer clusters supported on phosphorene as promising electrocatalysts for N₂ reduction to NH₃.     

Roads less traveled: Nitrogen reduction reaction catalyst design strategies for improved selectivity

Direct electrochemical nitrogen reduction for ammonia production is necessary to reduce the use of fossil fuels from conventional Haber–Bosch methods. Applications of nitrogen reduction electrocatalysts remain inhibited by slow reaction kinetics and low faradaic efficiencies because of competitive H₂ production pathways. Current strategies to address this challenge in selectivity have focused on catalyst design, reactor configuration, and electrolyte conditions. This brief review discusses the thermodynamic and kinetic challenges in the field as well as current underused approaches for selective catalyst development including bimetallic catalysts, transition metal nitrides, and carbon supports.     

Promotional effect of Ce on the SCR of NO with NH<Subscript>3</Subscript> at low temperature over MnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> supported by nitric acid-modified activated carbon

Different amounts of Mn and Ce oxides were loaded onto nitric acid-modified activated carbon (ACN) by wet impregnation. The series of catalysts were employed for the selective catalytic reduction of NO _x by NH₃ at temperatures between 100 and 250 °C. Cerium-modified catalysts exhibited higher de-NO _x performance than those modified with Mn/ACN, even with the same total loadings. The precursor solution with a molar ratio for Ce/(Mn + Ce) of 0.4 exhibited the highest catalytic activity. Enhanced resistance to SO₂ and H₂O and better stability were observed for 10%Mn–Ce(0.4)/ACN relative to 10%Mn/ACN. The catalysts were further characterized by N₂ physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H₂-TPR), and temperature-programmed desorption of ammonia (NH₃-TPD). The N₂ physisorption and XRD results suggested that co-doping Ce with Mn increased the surface area and promoted the dispersion of Mn–Ce binary metal oxides. H₂-TPR the NH₃-TPD results demonstrated that the interaction between manganese oxide and cerium oxide species enhanced the redox and surface acidity of 10%Mn–Ce(0.4)/ACN.     

<Emphasis Type="Italic">In situ</Emphasis> IR Spectroscopic and XPS Study of Surface Complexes and Their Transformations during Ammonia Oxidation to Nitrous Oxide over an Mn-Bi-O/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalyst

Surface complexes resulting from the interaction between ammonia and a manganese-bismuth oxide catalyst were studied by IR spectroscopy and XPS. At the first stage, ammonia reacts with the catalyst to form the surface complexes [NH] and [NH₂] via abstraction of hydrogen atoms even at room temperature. Bringing the catalyst into contact with flowing air at room temperature or with helium under heating results in further hydrogen abstraction and simultaneous formation of [N] from [NH₂] and [NH]. The nitrogen atoms are localized on both reduced (Mn²⁺) and oxidized (Mn^δ+, 2 < δ < 3) sites. Atomic nitrogen is highly mobile and reacts readily with the weakly bound oxygen of the oxidized (Mn^δ+-N) active site. The nitrogen atoms localized on oxidized sites play the key role in N₂O formation. Nitrous oxide is readily formed through the interaction between two Mn^δ+-N species. N₂ molecules result from the recombination of nitrogen atoms localized on reduced (Mn²⁺-N) sites.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 590–600.Original Russian Text Copyright © 2005 by Slavinskaya, Chesalov, Boronin, Polukhina, Noskov.     

MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen‐Doped Carbon Matrix for High‐Performance Bifunctional Oxygen Electrodes

Reversible interconversion of water into H₂ and O₂, and the recombination of H₂ and O₂ to H₂O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn₃O₄ and Co₃O₄ nanoparticles embedded in nitrogen‐doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N₄ macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M–N_x coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1 M ) than those for RuO₂, IrO₂, Pt, NiO, Mn₃O₄, and Co₃O₄, thus placing them among the best non‐precious‐metal catalysts for reversible oxygen electrodes reported to date.     

基于尿素电合成反应的电催化剂研究进展

尿素是一种重要的化工原料并作为氮源广泛应用于化肥生产。工业合成尿素由氮气加氢合成氨气以及氨气和二氧化碳转化为尿素两步实现,存在高能耗和高污染等问题。通过电催化碳氮偶联,将二氧化碳和氮源（氮气、硝酸根、亚硝酸根、一氧化氮等）转化为尿素,可直接跳过合成氨反应并在温和的反应条件下同时实现人工固氮和固碳。因此,尿素电合成技术不仅避免了高能耗和高污染,还能够实现惰性气体分子的高效利用,对于加快实现“碳达峰碳中和”战略有着重要的意义。本文聚焦尿素电合成这一前沿研究热点,结合领域内最新研究进展,首先介绍了不同电催化剂的设计策略及其催化机制,随后总结了电催化碳氮偶联合成尿素的反应机理,并对尿素电合成的后续研究方向进行了展望。     

Two dimensional inorganic electride-promoted electron transfer efficiency in transfer hydrogenation of alkynes and alkenes

A simple and highly efficient transfer hydrogenation of alkynes and alkenes by using a two-dimensional electride, dicalcium nitride ([Ca₂N]⁺·e^–), as an electron transfer agent is disclosed. Excellent yields in the transformation are attributed to the remarkable electron transfer efficiency in the electride-mediated reactions. It is clarified that an effective discharge of electrons from the [Ca₂N]⁺·e^– electride in alcoholic solvents is achieved by the decomposition of the electride via alcoholysis and the generation of ammonia and Ca(OⁱPr)₂. We found that the choice of solvent was crucial for enhancing the electron transfer efficiency, and a maximum efficiency of 80% was achieved by using a DMF mixed isopropanol co-solvent system. This is the highest value reported to date among single electron transfer agents in the reduction of C–C multiple bonds. The observed reactivity and efficiency establish that electrides with a high density of anionic electrons can readily participate in the reduction of organic functional groups.     

Single‐Atom Catalysts for the Electrocatalytic Reduction of Nitrogen to Ammonia under Ambient Conditions

Powered by renewable electricity, the electrochemical reduction of nitrogen to ammonia is proposed as a promising alternative to the energy‐ and capital‐intensive Haber–Bosch process, and has thus attracted much attention from the scientific community. However, this process suffers from low NH₃ yields and Faradaic efficiency. The development of more effective electrocatalysts is of vital importance for the practical applications of this reaction. Of the reported catalysts, single‐atom catalysts (SACs) show the significant advantages of efficient atom utilization and unsaturated coordination configurations, which offer great scope for optimizing their catalytic performance. Herein, progress in state‐of‐the‐art SACs applied in the electrocatalytic N₂ reduction reaction (NRR) is discussed, and the main advantages and challenges for developing more efficient electrocatalysts are also highlighted.     

Single-Atom Catalysis Using Chromium Embedded in Divacant Graphene for Conversion of Dinitrogen to Ammonia

Reduction of dinitrogen to ammonia under ambient conditions is a long-standing challenge. The few metal-based catalysts proposed have conspicuous disadvantages such as high cost, high energy consumption, and being hazardous to the environment. Single-atom catalysis has emerged as a new frontier in heterogeneous catalysis and metal atoms atomically dispersed on supports receive more and more attention owing to rapid advances in synthetic methodologies and computational modeling. Herein, we propose metal atoms embedded in divacant graphene as a catalyst for N₂ fixation based on density functional calculations. We systematically investigate the potential of using transition metal like Cr, Mn, Fe, Mo and Ru as catalysts and our study reveals that Cr embedded in graphene exhibit good catalytic activity for N₂ fixation. The synergy between the metal atoms and graphene surface provides a stable support to the metal center that has a high spin density to promote adsorption of N₂ and activation of its N≡N triple bond. Our study deciphers the mechanism of conversion of N₂ to ammonia following two possible reaction pathways, distal and enzymatic routes, via sequential protonation and reduction of activated N₂. The study provides a rational framework for conversion of dinitrogen to ammonia using single atom catalyst.     

A New Route to Metal Azides

Beside several other applications, metal azides can be used for the synthesis of nitridophosphates and binary nitrides. Herein we present a novel synthetic access to azides: Several metals, such as main‐group, transition metals, and rare‐earth metals, react with silver azide in liquid ammonia as a solvent giving the corresponding metal azides. In this work Mn(N₃)₂, Sn(N₃)₂, and Eu(N₃)₂, as well as their ammonia complexes were synthesized for the first time through low‐temperature methods. Also a simpler access to Zn(N₃)₂ was possible. At room temperature and the respective vapor pressure of NH₃, it became possible to grow single crystals of the dinuclear holmium azide [Ho₂(μ‐NH₂)₃(NH₃)₁₀](N₃)₃?1.25 NH₃. We are confident that this new route could lead to novel metal azides as well as nitrides of the main‐group, the transition, and the rare‐earth metals upon careful decomposition.