Carbon-Anchored Molybdenum Oxide Nanoclusters as Efficient Catalysts for the Electrosynthesis of Ammonia and Urea

The electrochemical NO₃⁻ reduction and its coupling with CO₂ can provide novel and clean routes to synthesize NH₃ and urea, respectively. However, their practical application is still impeded by the lack of efficient catalysts with desirable Faradaic efficiency (FE) and yield rate. Herein, we report the synthesis of molybdenum oxide nanoclusters anchored on carbon black (MoO_x/C) as electrocatalyst. It affords an outstanding FE of 98.14 % and NH₃ yield rate of 91.63 mg h⁻¹ mg_cat.⁻¹ in NO₃⁻ reduction. Besides, the highest FE of 27.7 % with a maximum urea yield rate of 1431.5 μg h⁻¹ mg_cat.⁻¹ toward urea is also achieved. The formation of electron-rich MoO_x nanoclusters with highly unsaturated metal sites in the MoO_x/C heterostructure is beneficial for enhanced catalytic performance. Studies on the mechanism reveal that the stabilization of *NO and *CO₂NOOH intermediates are critical for the NH₃ and urea synthesis, respectively.     

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。     

Carbon-Based Electron Buffer Layer on ZnOx−Fe5C2−Fe3O4 Boosts Ethanol Synthesis from CO2 Hydrogenation

The conversion of CO₂ into ethanol with renewable H₂ has attracted tremendous attention due to its integrated functions of carbon elimination and chemical synthesis, but remains challenging. The electronic properties of a catalyst are essential to determine the adsorption strength and configuration of the key intermediates, therefore altering the reaction network for targeted synthesis. Herein, we describe a catalytic system in which a carbon buffer layer is employed to tailor the electronic properties of the ternary ZnO_x−Fe₅C₂−Fe₃O₄, in which the electron-transfer pathway (ZnO_x→Fe species or carbon layer) ensures the appropriate adsorption strength of −CO* on the catalytic interface, facilitating C−C coupling between −CH_x* and −CO* for ethanol synthesis. Benefiting from this unique electron-transfer buffering effect, an extremely high ethanol yield of 366.6 g_EtOH kg_cat⁻¹ h⁻¹ (with CO of 10 vol % co-feeding) is achieved from CO₂ hydrogenation. This work provides a powerful electronic modulation strategy for catalyst design in terms of highly oriented synthesis.     

Tandem Electrocatalytic Nitrate Reduction to Ammonia on MBenes

We demonstrate the great feasibility of MBenes as a new class of tandem catalysts for electrocatalytic nitrate reduction to ammonia (NO₃RR). As a proof of concept, FeB₂ is first employed as a model MBene catalyst for the NO₃RR, showing a maximum NH₃-Faradaic efficiency of 96.8 % with a corresponding NH₃ yield of 25.5 mg h⁻¹ cm⁻² at −0.6 V vs. RHE. Mechanistic studies reveal that the exceptional NO₃RR activity of FeB₂ arises from the tandem catalysis mechanism, that is, B sites activate NO₃⁻ to form intermediates, while Fe sites dissociate H₂O and increase *H supply on B sites to promote the intermediate hydrogenation and enhance the NO₃⁻-to-NH₃ conversion.     

Fe-Doped CoS2 Nanoarrays: Efficient Electrocatalytic Nitrate Reduction to Ammonia under Ambient Conditions

The electrocatalytic nitrate reduction reaction (NO₃⁻RR) enables the reduction of nitrate to ammonium ions under ambient conditions. It was considered as an alternative reaction for the production of ammonia (NH₃) in recent years. In this paper, we report that the Fe doping CoS₂ nanoarrays can effectively catalyze the formation of NH₃ from nitrate (NO₃⁻) under ambient conditions. This is mainly due to the increase of the NO₃⁻ reaction active site by Fe doping and the porous nanostructure of the catalyst, which greatly improves the catalytic activity. Specifically, at −0.9 V vs. RHE, the NH₃ yield rate (R_NH3) of Fe−CoS₂/CC is 17.8×10⁻² mmol h⁻¹ cm⁻² with Faraday Efficiency (FE) of 88.93 %. Besides, such catalyst shows good durability and catalytic stability, which provides the possibility for the future application of electrocatalytic NH₃ production.     

Dual Metal Active Sites in an Ir1/FeOx Single-Atom Catalyst: A Redox Mechanism for the Water-Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water-gas shift (WGS) reaction on Ir₁/FeO_x single-atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first-neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second-neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺−O⋅⋅⋅Ir²⁺−O_vac to Fe²⁺−O_vac⋅⋅⋅Ir³⁺−O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

Non-Interacting Ni and Fe Dual-Atom Pair Sites in N-Doped Carbon Catalysts for Efficient Concentrating Solar-Driven Photothermal CO2 Reduction

Solar-to-chemical energy conversion under weak solar irradiation is generally difficult to meet the heat demand of CO₂ reduction. Herein, a new concentrated solar-driven photothermal system coupling a dual-metal single-atom catalyst (DSAC) with adjacent Ni−N₄ and Fe−N₄ pair sites is designed for boosting gas-solid CO₂ reduction with H₂O under simulated solar irradiation, even under ambient sunlight. As expected, the (Ni, Fe)−N−C DSAC exhibits a superior photothermal catalytic performance for CO₂ reduction to CO (86.16 μmol g⁻¹ h⁻¹), CH₄ (135.35 μmol g⁻¹ h⁻¹) and CH₃OH (59.81 μmol g⁻¹ h⁻¹), which are equivalent to 1.70-fold, 1.27-fold and 1.23-fold higher than those of the Fe−N−C catalyst, respectively. Based on theoretical simulations, the Fermi level and d-band center of Fe atom is efficiently regulated in non-interacting Ni and Fe dual-atom pair sites with electronic interaction through electron orbital hybridization on (Ni, Fe)−N−C DSAC. Crucially, the distance between adjacent Ni and Fe atoms of the Ni−N−N−Fe configuration means that the additional Ni atom as a new active site contributes to the main *COOH and *HCO₃ dissociation to optimize the corresponding energy barriers in the reaction process, leading to specific dual reaction pathways (COOH and HCO₃ pathways) for solar-driven photothermal CO₂ reduction to initial CO production.     

Urea Production on Metal-Free Dual Silicon Doped C9N4 Nanosheet Under Ambient Conditions by Electrocatalysis: A First Principles Study

The development of cheap, eco-friendly electrocatalysts for urea synthesis which avoids the traditional nitrogen reduction to form ammonia, is very important to meet our growing demand for urea. Herein, based on density functional theory, we propose a novel electrocatalyst (dual Si doped C₉N₄ nanosheet) composed of totally environmentally benign non-metal earth abundant elements, which is able to adsorb N₂ and CO₂ together. Reduction of CO₂ to CO happens, which is then inserted into activated N−N bond, and it produces *N(CO)N intermediate, which is the crucial step for urea formation. Eventually following several proton coupled electron transfer processes, urea is formed under ambient conditions. The limiting potential value for urea formation is found to be lower than that of NH₃ formation and HER (hydrogen evolution reaction). Moreover, the faradaic efficiency of our proposed catalyst system is 100 % for urea formation, which suggests greater selectivity of urea formation over other competitive reactions.     

Single-atom Iron Catalyst with Biomimetic Active Center to Accelerate Proton Spillover for Medical-level Electrosynthesis of H2O2 Disinfectant

Electrosynthesis of H₂O₂ has great potential for directly converting O₂ into disinfectant, yet it is still a big challenge to develop effective electrocatalysts for medical-level H₂O₂ production. Herein, we report the design and fabrication of electrocatalysts with biomimetic active centers, consisting of single atomic iron asymmetrically coordinated with both nitrogen and sulfur, dispersed on hierarchically porous carbon (Fe_SA-NS/C). The newly-developed Fe_SA-NS/C catalyst exhibited a high catalytic activity and selectivity for oxygen reduction to produce H₂O₂ at a high current of 100 mA cm⁻² with a record high H₂O₂ selectivity of 90 %. An accumulated H₂O₂ concentration of 5.8 wt.% is obtained for the electrocatalysis process, which is sufficient for medical disinfection. Combined theoretical calculations and experimental characterizations verified the rationally-designed catalytic active center with the atomic Fe site stabilized by three-coordinated nitrogen atoms and one-sulfur atom (Fe-N₃S-C). It was further found that the replacement of one N atom with S atom in the classical Fe-N₄-C active center could induce an asymmetric charge distribution over N atoms surrounding the Fe reactive center to accelerate proton spillover for a rapid formation of the OOH* intermediate, thus speeding up the whole reaction kinetics of oxygen reduction for H₂O₂ electrosynthesis.     

Nitrite Electroreduction to Ammonia Promoted by Molecular Carbon Dioxide with Near-unity Faradaic Efficiency

Electrochemical reduction of nitrite (NO₂⁻) offers an energy-efficient route for ammonia (NH₃) synthesis and reduction of the level of nitrite, which is one of the major pollutants in water. However, the near 100 % Faradaic efficiency (FE) has yet to be achieved due to the complicated reduction route with several intermediates. Here, we report that carbon dioxide (CO₂) can enhance the nitrite electroreduction to ammonia on copper nanowire (Cu NW) catalysts. In a broad potential range (−0.7∼−1.3 V vs. RHE), the FE of nitrite to ammonia is close to 100 % with a 3.5-fold increase in activity compared to that obtained without CO_2. In situ Raman spectroscopy and density functional theory (DFT) calculations indicate that CO₂ acts as a catalyst to facilitate the *NO to *N step, which is the rate determining step for ammonia synthesis. The promotion effect of CO₂ can be expanded to electroreduction of other nitro-compounds, such as nitrate to ammonia and nitrobenzene to aniline.     

Highly Efficient Electrochemical Nitrate and Nitrogen Reduction to Ammonia under Ambient Conditions on Electrodeposited Cu-Nanosphere Electrode

The electrochemical reduction reaction of nitrogenous species such as NO₃⁻ (NO₃RR) and N₂ (NRR) is a promising strategy for producing ammonia under ambient conditions. However, low activity and poor selectivity of both NO₃RR and NRR remain the biggest problem of all current electrocatalysts. In this work, we fabricated Cu-nanosphere film with a high surface area and dominant with a Cu(200) facet by simple electrodeposition method. The Cu-nanosphere film exhibits high electrocatalytic activity for NO₃RR and NRR to ammonia under ambient conditions. In the nitrate environment, the Cu-nanosphere electrode reduced NO₃⁻ to yield NH₃ at a rate of 5.2 mg/h cm², with a Faradaic efficiency of 85 % at −1.3 V. In the N₂-saturated environment, the Cu-nanosphere electrode reduced N₂ to yield NH₃ with the highest yield rate of 16.2 μg/h cm² at −0.5 V, and the highest NH₃ Faradaic efficiency of 41.6 % at −0.4 V. Furthermore, the Cu-nanosphere exhibits excellent stability with the NH₃ yield rate, and the Faradaic efficiency remains stable after 10 consecutive cycles. Such high levels of NH₃ yield, selectivity, and stability at low applied potential are among the best values currently reported in the literature.     

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO₃⁻RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe−N/P−C) as a NO₃⁻RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO₃⁻RR process. The Fe−N/P−C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h⁻¹ mg_cat⁻¹, greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO₃⁻RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic-level precision modulation by heteroatom doping for the NO₃⁻RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.     

Boosting Electrocatalytic Nitrate-to-Ammonia via Tuning of N-Intermediate Adsorption on a Zn−Cu Catalyst

The renewable-energy-powered electroreduction of nitrate (NO₃⁻) to ammonia (NH₃) has garnered significant interest as an eco-friendly and promising substitute for the Haber–Bosch process. However, the sluggish kinetics hinders its application at a large scale. Herein, we first calculated the N-containing species (*NO₃ and *NO₂) binding energy and the free energy of the hydrogen evolution reaction over Cu with different metal dopants, and it was shown that Zn was a promising candidate. Based on the theoretical study, we designed and synthesized Zn-doped Cu nanosheets, and the as-prepared catalysts demonstrated excellent performance in NO₃⁻-to-NH₃. The maximum Faradaic efficiency (FE) of NH₃ could reach 98.4 % with an outstanding yield rate of 5.8 mol g⁻¹ h⁻¹, which is among the best results up to date. The catalyst also had excellent cycling stability. Meanwhile, it also presented a FE exceeding 90 % across a wide potential range and NO₃⁻ concentration range. Detailed experimental and theoretical studies revealed that the Zn doping could modulate intermediates adsorption strength, enhance NO₂⁻ conversion, change the *NO adsorption configuration to a bridge adsorption, and decrease the energy barrier, leading to the excellent catalytic performance for NO₃⁻-to-NH₃.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN⁻), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN⁻ or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN⁻, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

Simultaneous Generation of H2O2 and Formate by Co-Electrolysis of Water and CO2 over Bifunctional Zn/SnO2 Nanodots

Hydrogen peroxide (H₂O₂) and formate are important chemicals used in various chemical manufacturing industries. One promising approach for the simultaneous production of these chemicals is coupling anodic two-electron water oxidation with cathodic CO₂ reduction in an electrolyzer using nonprecious bifunctional electrocatalysts. Herein, we report an innovative hybrid electrosynthesis strategy using Zn-doped SnO₂ (Zn/SnO₂) nanodots as bifunctional redox electrocatalysts to achieve Faradaic efficiencies of 80.6 % and 92.2 % for H₂O₂ and formate coproduction, respectively, along with excellent stability for at least 60 h at a current density of ≈150 mA cm⁻². Through a combination of physicochemical characterizations, including operando attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), isotope labeling mass spectrometry (MS)/¹H NMR and quasi-in situ electron paramagnetic resonance (EPR), with density functional theory (DFT) calculations, we discovered that the Zn dopant facilitates the coupling of *OH intermediates to promote H₂O₂ production and optimizes the adsorption of *OCHO intermediates to accelerate formate formation. Our findings offer new insights into designing more efficient bifunctional electrocatalyst-based pair-electrosynthesis system for the coproduction of H₂O₂ and formate feedstocks.     

Atomically Dispersed Iron Regulating Electronic Structure of Iron Atom Clusters for Electrocatalytic H2O2 Production and Biomass Upgrading

The integration of highly active single atoms (SAs) and atom clusters (ACs) into an electrocatalyst is critically important for high-efficiency two-electron oxygen reduction reaction (2e⁻ ORR) to hydrogen peroxide (H₂O₂). Here we report a tandem impregnation-pyrolysis-etching strategy to fabricate the oxygen-coordinated Fe SAs and ACs anchored on bacterial cellulose-derived carbon (BCC) (FeSAs/ACs-BCC). As the electrocatalyst, FeSAs/ACs-BCC exhibits superior electrocatalytic activity and selectivity toward 2e⁻ ORR, affording an onset potential of 0.78 V (vs. RHE) and a high H₂O₂ selectivity of 96.5 % in 0.1 M KOH. In a flow cell reactor, the FeSAs/ACs-BCC also achieves high-efficiency H₂O₂ production with a yield rate of 12.51±0.18 mol g_cat⁻¹ h⁻¹ and a faradaic efficiency of 89.4 %±1.3 % at 150 mA cm⁻². Additionally, the feasibility of coupling the produced H₂O₂ and electro-Fenton process for the valorization of ethylene glycol was explored in detail. The theoretical calculations uncover that the oxygen-coordinated Fe SAs effectively regulate the electronic structure of Fe ACs which are the 2e⁻ ORR active sites, resulting in the optimal binding strength of *OOH intermediate for high-efficiency H₂O₂ production.     

Enhancing Electrochemical Nitrate Reduction to Ammonia over Cu Nanosheets via Facet Tandem Catalysis

Electrochemical conversion of nitrate (NO₃⁻) into ammonia (NH₃) represents a potential way for achieving carbon-free NH₃ production while balancing the nitrogen cycle. Herein we report a high-performance Cu nanosheets catalyst which delivers a NH₃ partial current density of 665 mA cm⁻² and NH₃ yield rate of 1.41 mmol h⁻¹ cm⁻² in a flow cell at −0.59 V vs. reversible hydrogen electrode. The catalyst showed a high stability for 700 h with NH₃ Faradaic efficiency of ≈88 % at 365 mA cm⁻². In situ spectroscopy results verify that Cu nanosheets are in situ derived from the as-prepared CuO nanosheets under electrochemical NO₃⁻ reduction reaction conditions. Electrochemical measurements and density functional theory calculations indicate that the high performance is attributed to the tandem interaction of Cu(100) and Cu(111) facets. The NO₂⁻ generated on the Cu(100) facets is subsequently hydrogenated on the Cu(111) facets, thus the tandem catalysis promotes the crucial hydrogenation of *NO to *NOH for NH₃ production.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single-Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen-doped carbon supported single-atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single-atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm⁻²) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single-atom configurations for the H₂ and CO evolution. The results present a useful case on how non-precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell

Molecular catalysts have been shown to have high selectivity for CO₂ electrochemical reduction to CO, but with current densities significantly below those obtained with solid-state materials. By depositing a simple Fe porphyrin mixed with carbon black onto a carbon paper support, it was possible to obtain a catalytic material that could be used in a flow cell for fast and selective conversion of CO₂ to CO. At neutral pH (7.3) a current density as high as 83.7 mA cm⁻² was obtained with a CO selectivity close to 98 %. In basic solution (pH 14), a current density of 27 mA cm⁻² was maintained for 24 h with 99.7 % selectivity for CO at only 50 mV overpotential, leading to a record energy efficiency of 71 %. In addition, a current density for CO production as high as 152 mA cm⁻² (>98 % selectivity) was obtained at a low overpotential of 470 mV, outperforming state-of-the-art noble metal based catalysts.     

Cu-Doped MoSi2N4 Monolayer as a Potential NH3 Sensor

Cu doped MoSi₂N₄ monolayer (Cu-MoSi₂N₄) was firstly proposed to analyze adsorption performances of common gas molecules including O₂, N₂, CO, NO, NO₂, CO₂, SO₂, H₂O, NH₃ and CH₄ via density functional theory (DFT) combining with non-equilibrium Green's function (NEGF). The electronic transport calculations indicate that Cu-MoSi₂N₄ monolayer has high sensitivity for CO, NO, NO₂ and NH₃ molecules. However, only NH₃ molecule adsorbs on the Cu-MoSi₂N₄ monolayer with moderate strength (−0.55 eV) and desorbs at room temperature (2.36×10⁻³ s). Thus, Cu-MoSi₂N₄ monolayer is demonstrated as a potential NH₃ sensor.