Metal-Based Electrocatalysts for Selective Electrochemical Nitrogen Reduction to Ammonia

Ammonia (NH3) plays a significant role in the manufacture of fertilizers, nitrogen-containing chemical production, and hydrogen storage. The electrochemical nitrogen reduction reaction (e-NRR) is an attractive prospect for achieving clean and sustainable NH3 production, under mild conditions driven by renewable energy. The sluggish cleavage of N≡N bonds and poor selectivity of e-NRR are the primary challenges for e-NRR, over the competitive hydrogen evolution reaction (HER). The rational design of e-NRR electrocatalysts is of vital significance and should be based on a thorough understanding of the structure-activity relationship and mechanism. Among the various explored e-NRR catalysts, metal-based electrocatalysts have drawn increasing attention due to their remarkable performances. This review highlighted the recent progress and developments in metal-based electrocatalysts for e-NRR. Different kinds of metal-based electrocatalysts used in NH3 synthesis (including noble-metal-based catalysts, non-noble-metal-based catalysts, and metal compound catalysts) were introduced. The theoretical screening and the experimental practice of rational metal-based electrocatalyst design with different strategies were systematically summarized. Additionally, the structure-function relationship to improve the NH3 yield was evaluated. Finally, current challenges and perspectives of this burgeoning area were provided. The objective of this review is to provide a comprehensive understanding of metal-based e-NRR electrocatalysts with a focus on enhancing their efficiency in the future.

Bio-Inspired Iron-Loaded Polydopamine Functionalized Montmorillonite as an Environmentally Friendly Flame Retardant for Epoxy Resin

As an important thermosetting material, flame-retardant epoxy resin has various applications in the aerospace, chemical, and electronics industry, and other fields. However, the flame retardancy of epoxy resins is often improved at the expense of mechanical performance. The contradiction between flame retardancy and mechanical properties seriously impedes the practical applications of epoxy resin (EP). Herein, iron-loaded polydopamine functionalized montmorillonite (D-Mt-Fe³⁺), which was prepared by dopamine, iron chloride and montmorillonite in an aqueous solution, was introduced to prepare iron-loaded polydopamine functionalized montmorillonite/epoxy resin composites (D-Mt-Fe³⁺/EP). As expected, D-Mt-Fe³⁺/EP-10 with 10 phr of D-Mt-Fe³⁺ passed the UL-94 V-0 rating, achieved a limiting oxygen index (LOI) value of 31.0% and reduced the smoke production rate (SPR) and total smoke production (TSP), indicating that the introduction of D-Mt-Fe³⁺ could endow EP with satisfactory flame retardancy through the radical scavenging function of dopamine in the gas phase and the catalytic charring effect of iron ions, respectively. Encouragingly, the mechanical property was also enhanced with the flexural strength increased by 25.5%. This work provided an attractive strategy for improving both the mechanical properties and fire resistance of EP, which greatly broadened their applications in the chemical industry and electronics field, etc.

P-Block Metal-Based Electrocatalysts for Nitrogen Reduction to Ammonia: A Minireview

Electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃ ) using renewable electricity provides a promising approach towards carbon neutral. What's more, it has been regarded as the most promising alternative to the traditional Haber-Bosch route in current context of developing sustainable technologies. The development of a class of highly efficient electrocatalysts with high selectivity and stability is the key to electrochemical NRR. Among them, P-block metal-based electrocatalysts have significant application potential in NRR for which possessing a strong interaction with the N 2p orbitals. Thus, it offers a good selectivity for NRR to NH₃ . The density of state (DOS) near the Fermi level is concentrated for the P-block metal-based catalysts, indicating the ability of P-block metal as active sites for N₂ adsorption and activation by donating p electrons. In this work, we systematically review the recent progress of P-block metal-based electrocatalysts for electrochemical NRR. The effect of P-block metal-based electrocatalysts on the NRR activity, selectivity and stability are discussed. Specifically, the catalyst design, the nature of the active sites of electrocatalysts and some strategies for boosting NRR performance, the reaction mechanism, and the impact of operating conditions are unveiled. Finally, some challenges and outlooks using P-block metal-based electrocatalysts are proposed.

Green Biosynthesis of Tin Oxide Nanomaterials Mediated by Agro-Waste Cotton Boll Peel Extracts for the Remediation of Environmental Pollutant Dyes

The sustainable synthesis of metal oxide materials provides an ecofriendly and more exciting approach in the domain of a clean environment. Besides, plant extracts to synthesize nanoparticles have been considered one of the more superior ecofriendly methods. This paper describes the biosynthetic preparation route of three different sizes of tetragonal structure SnO₂ nanoparticles (SNPs) from the agro-waste cotton boll peel aqueous extract at 200, 500, and 800 °C for 3 h and represents a low-cost and alternative preparation method. The samples were characterized by X-ray diffraction, Fourier transform infrared spectrophotometry, ultraviolet-visible absorption spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy. Surface area and porosity size distribution were identified by nitrogen adsorption-desorption isotherms and Brunauer-Emmett-Teller analysis. The photocatalytic properties of the SNP samples were studied against methylene blue (MB) and methyl orange (MO), and the degradation was evaluated with three different size nanomaterials of 3.97, 8.48, and 13.43 nm. Photocatalytic activities were carried out under a multilamp (125 W Hg lamps) photoreactor. The smallest size sample exhibited the highest MB degradation efficiency within 30 min than the most significant size sample, which lasted 80 min. Similarly, in the case of MO, the smallest sample showed a more superior degradation efficiency with a shorter period (40 min) than the large-size samples (100 min). Therefore, our studies suggested that the developed SNP nanomaterials could be potential, promising photocatalysts against the degradation of industrial effluents.

Carbon-doped tin disulfide nanoflowers: a heteroatomic doping strategy for improving the electrocatalytic performance of nitrogen reduction to ammonia

In this work, hydrophobic carbon-doped tin disulfide (C-SnS2) was fabricated for the first time and adopted as an advanced catalyst for the eNRR.

Efficient Electrocatalytic Nitrogen Reduction to Ammonia on Ultrafine Sn Nanoparticles

Electrocatalytic nitrogen reduction reaction (NRR) at ambient conditions is a promising route for ammonia (NH₃) synthesis but still suffers from low activity and selectivity. Here, ultrafine Sn nanoparticles (NPs) grown on carbon blacks (Sn_SC/C) have been synthesized through a wet-chemical method using sodium citrate dehydrate as a stabilizing agent. Benefiting from the small sizes of Sn NPs, the Sn_SC/C catalyst exhibits excellent electrocatalytic performance for NRR with a high Faradaic efficiency of 22.76% and an NH₃ yield rate of 17.28 μg h^-1 mg^-1 in the 0.1 M Na₂SO₄ electrolyte, outperforming many reported electrocatalysts for NRR under similar conditions. Density functional theory calculation results reveal that the potential-determining step on Sn NPs is the generation of NHNH* through simultaneous hydrogenation of N₂^* by a H* and a H⁺/e^- pair via Langmuir-Hinshelwood plus Eley-Rideal mechanisms.

Emerging two-dimensional nanomaterials for electrochemical nitrogen reduction

Ammonia (NH₃) is essential to serve as the biological building blocks for maintaining organism function, and as the indispensable nitrogenous fertilizers for increasing the yield of nutritious crops. The current Haber-Bosch process for industrial NH₃ production is highly energy- and capital-intensive. In light of this, the electroreduction of nitrogen (N₂) into valuable NH₃, as an alternative, offers a sustainable pathway for the Haber-Bosch transition, because it utilizes renewable electricity and operates under ambient conditions. Identifying highly efficient electrocatalysts remains the priority in the electrochemical nitrogen reduction reaction (NRR), marking superior selectivity, activity, and stability. Two-dimensional (2D) nanomaterials with sufficient exposed active sites, high specific surface area, good conductivity, rich surface defects, and easily tunable electronic properties hold great promise for the adsorption and activation of nitrogen towards sustainable NRR. Therefore, this Review focuses on the fundamental principles and the key metrics being pursued in NRR. Based on the fundamental understanding, the recent efforts devoted to engineering protocols for constructing 2D electrocatalysts towards NRR are presented. Then, the state-of-the-art 2D electrocatalysts for N₂ reduction to NH₃ are summarized, aiming at providing a comprehensive overview of the structure-performance relationships of 2D electrocatalysts towards NRR. Finally, we propose the challenges and future outlook in this prospective area.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Defect-Induced Ce-Doped Bi2WO6 for Efficient Electrocatalytic N2 Reduction

Electrochemical nitrogen reduction reaction (NRR) is a promising method for synthesizing ammonia (NH₃). However, due to the extremely strong N≡N bond and the competing hydrogen evolution reaction (HER), the electrochemical NRR process remains a great challenge in achieving a high NH₃ yielding rate and a high Faradaic efficiency (FE). Recently, either Bi-based or W-based catalysts have been used in N₂ fixation due to lower HER activity. To further promote N₂ activation, we develop a simple protocol to introduce and adjust the crystal defects in the host lattice of Bi₂WO₆ nanoflowers via adjusting the amount of Ce dopant (denoted as xCe-Bi₂WO₆, where x represents the designed mole percentage of Ce). At -0.20 V versus the reversible hydrogen electrode (RHE), 10%Ce-Bi₂WO₆ manifests a high NH₃ yielding rate (22.5 μg h^-1 mg^-1_cat.), a high FE (15.9%), and excellent electrochemical and structure durability. Its performance is better than most previously reported Bi-based and W-based electrocatalysts for NRR in aqueous solutions. According to density functional theory (DFT) calculations, the introduction of crystal defects into Bi₂WO₆ can strengthen the adsorption and activation of N₂, thus leading to a significant increase in NRR activity.

Dual Interface-Engineered Tin Heterostructure for Enhanced Ambient Ammonia Electrosynthesis

Electrocatalytic nitrogen reduction reaction (NRR) represents a promising alternative route for sustainable ammonia synthesis, which currently dominantly relies on the energy-intensive Haber-Bosch process, while it is significantly hampered by the sluggish reaction kinetics due to the short of glorious electrocatalysts. In this work, we report an efficient porous tin heterostructure with intimate dual interfaces for electrosynthesis of ammonia, which exhibits outstanding NRR efficiency with an NH₃ yield rate and Faradaic efficiency as high as 30.3 μg h^-1mg^-1_cat and 41.3%, respectively, and excellent stability as well at a low potential of -0.05 V (vs RHE) in 0.1 M Na₂SO₄ solution under ambient conditions. This matrix value is superior to the analogue Sn-based heterostructures with a single interface and outperforms the currently state-of-the-art Sn-based catalysts. Comprehensive characterizations and theoretical calculations uncovered the formation of the unique intimate dual interfaces in the tin heterostructure promoting the enhancement of the NRR process, which not only effectively exposes more active sites for stronger N₂ chemisorption and activation but also accelerates the interfacial electron transfer and reduces the free energy barrier for the rate-determining *N₂H formation step, highlighting the importance of the dual interface effect for the design of electrocatalysts in catalysis.

Iron-Doped MoO3 Nanosheets for Boosting Nitrogen Fixation to Ammonia at Ambient Conditions

Nitrogen can be electrochemically reduced to produce ammonia, which supplies an energy-saving and environmental-benign route at room temperature, but high-efficiency catalysts are sought to reduce the reaction barrier. Here, iron-doped α-MoO₃ nanosheets are thus designed and proposed as potential catalysts for fixing N₂ to NH₃. The α-MoO₃ band structure is intentionally modulated by the iron doping, which narrows the band gap of α-MoO₃ and turns the semiconductor into a metal-like catalyst. Oxygen vacancies, generated by substituting Mo⁶⁺ for Fe³⁺ anions, are beneficial for nitrogen adsorption at the active sites. In 0.1 M Na₂SO₄, the Fe-doped MoO₃ catalyst reached a high faradaic efficiency of 13.3% and an excellent NH₃ yield rate of 28.52 μg h^-1 mg_cat^-1 at -0.7 V versus reversible hydrogen electrode, superior to most of the other metal-based catalysts. Theoretical calculations confirmed that the N₂ reduction reaction at the Fe-MoO₃ surface followed the distal reaction path.

A peanut shell-derived economical and eco-friendly biochar catalyst for electrochemical ammonia synthesis under ambient conditions: combined experimental and theoretical study

The electrochemical conversion of N₂ to NH₃ under ambient conditions is a highly promising alternative to the energy-intensive Haber–Bosch process. As a catalyst for electrocatalytic N₂ synthesis of NH₃, biochar is a sustainable green material.

Exploration and Investigation of Periodic Elements for Electrocatalytic Nitrogen Reduction

High demand for green ecosystems has urged the human community to reconsider and revamp the traditional way of synthesis of several compounds. Ammonia (NH₃ ) is one such compound whose applications have been extended from fertilizers to explosives and is still being synthesized using the high energy inhaling Haber-Bosch process. Carbon free electrocatalytic nitrogen reduction reaction (NRR) is considered as a potential replacement for the Haber-Bosch method. However, it has few limitations such as low N₂ adsorption, selectivity (competitive HER reactions), low yield rate etc. Since it is at the early stage, tremendous efforts have been devoted in understanding the reaction mechanism and screening of the electrocatalysts and electrolytes. In this review, the electrocatalysts are classified based on the periodic table with heat maps of Faraday efficiency and yield rate of NH₃ in NRR and their electrocatalytic properties toward NRR are discussed. Also, the activity of each element is discussed and short tables and concise graphs are provided to enable the researchers to understand recent progress on each element. At the end, a perspective is provided on countering the current challenges in NRR. This review may act as handbook for basic NRR understandings, recent progress in NRR, and the design and development of advanced electrocatalysts and systems.

Electrochemically shape-controlled synthesis of great stellated dodecahedral Au nanocrystals with high-index facets for nitrogen reduction to ammonia

Au great stellated dodecahedra (GSD), one of the Kepler-Poinsot solids, are synthesized by an electrochemical double-step potential method in a choline chloride-urea based deep eutectic solvent. The as-synthesized Au GSD are bound by high-index {331} facets and exhibit excellent electrocatalytic performance for the nitrogen reduction reaction with a high NH3 yield rate (49.96 μg h-1 cm-2) and faradaic efficiency (28.59%) under ambient conditions.

Current Progress of Electrocatalysts for Ammonia Synthesis Through Electrochemical Nitrogen Reduction Under Ambient Conditions

Ammonia, one of the most important chemicals and carbon-free energy carriers, is mainly produced by the traditional Haber-Bosch process operated at high pressure and temperature, which results in massive energy consumption and CO2 emissions. Alternatively, the electrocatalytic nitrogen reduction reaction to synthesize NH3 under ambient conditions using renewable energy has recently attracted significant attention. However, the competing hydrogen evolution reaction (HER) significantly reduces the faradaic efficiency and NH3 production rate. The design of high-performance electrocatalysts with the suppression of the HER for N2 reduction to NH3 under ambient conditions is a crucial consideration for the development of electrocatalytic NH3 synthesis with high FE and NH3 production rate. Five kinds of recently developed electrocatalysts classified by their chemical compositions are summarized, with particular emphasis on the relationship between their optimal electrocatalytic conditions and NH3 production performance. Conclusions and perspectives are provided for the future design of high-performance electrocatalysts for electrocatalytic NH3 production. The Review can give practical guidance for the design of effective electrocatalysts with high FE and NH3 production rates.

Ambient Ammonia Electrosynthesis: Current Status, Challenges, and Perspectives

Ammonia (NH₃ ) electrosynthesis from atmospheric nitrogen (N₂ ) and water is emerging as a promising alternative to the energy-intensive Haber-Bosch process; however, such a process is difficult to perform due to the inherent inertness of N₂ molecules together with low solubility in aqueous solutions. Although many active electrocatalysts have been used to electrocatalyze the N₂ reduction reaction (NRR), unsatisfactory NH₃ yields and lower Faraday efficiency are still far from practical industrial production, and thus, considerable research efforts are being devoted to address these problems. Nevertheless, most reports still mainly focus on the preparation of electrocatalysts and largely ignore a summary of optimization-modification strategies for the NRR. In this review, a general introduction to the NRR mechanism is presented to provide a reasonable guide for the design of highly active catalysts. Then, four categories of NRR electrocatalysts, according to chemical compositions, are surveyed, as well as several strategies for promoting the catalytic activity and efficiency. Later, strategies for developing efficient N₂ fixation systems are discussed. Finally, current challenges and future perspectives in the context of the NRR are highlighted. This review sheds some light on the development of highly efficient catalytic systems for NH₃ synthesis and stimulates research interests in the unexplored, but promising, research field of the NRR.

Recent advancement in the electrocatalytic synthesis of ammonia

Ammonia can not only be used as an active nitrogen component of nitrogen fertilizers, fibers, explosives, etc., but also provides a high energy density and carbon free energy carrier. Currently, ammonia is industrially synthesized by the Haber Bosch process at high temperature and high pressure, which results in high energy loss and a serious greenhouse effect. The electrocatalytic nitrogen reduction reaction (NRR) is a sustainable and environmentally friendly strategy for the synthesis of ammonia. Although lots of electrocatalysts have been developed for this reaction, further breakthroughs are needed in catalytic activity, selectivity and Faraday efficiency to meet the large-scale commercial demand. In this review, the recent advance in NRR electrocatalysis is thoroughly commented on. Different kinds of electrocatalysts used in ammonia synthesis (including single atom catalysts, metal oxide catalysts, nanocomposite catalysts, and metal free catalysts) are introduced. The reaction mechanism of the NRR is discussed in detail. The structure-function relationship and efficient strategies to improve the ammonia yield are clearly discussed. The effect of the electronic structure and morphology of catalysts on the selectivity of the NRR is highlighted. The research directions and perspectives on the further development of more efficient electrocatalysts for the NRR are provided.

Urchin-like Al-Doped Co3O4 Nanospheres Rich in Surface Oxygen Vacancies Enable Efficient Ammonia Electrosynthesis

Developing cost-efficient electrocatalysts for ambient N₂-to-NH₃ conversion and revealing the reaction mechanism are appealing yet challenging tasks. Some transition metal oxides have been recently used to catalyze the nitrogen reduction reaction (NRR), but their further applications are greatly impeded because of their questionable conductivity, poor dispersion, limited active sites, and so forth. Herein, three-dimensional Ni foam-supported urchin-like Al-doped Co₃O₄ nanospheres rich in surface oxygen vacancies (Al-Co₃O₄/NF) were prepared via a hydrothermal process and subsequent annealing treatment. It is shown that introducing Al atoms into Co₃O₄ effectively tunes the electronic properties of the catalyst, and the increased surface oxygen vacancies induced by Al doping facilitate the activation of nitrogen. What is more, this urchin-like nanostructure, demonstrating an ability to limit the coalescence of gas bubbles, enables the rapid removal of small gas bubbles and better exposure of active sites to N₂, thus yielding an impressive ammonia electrosynthesis activity (NH₃ yield rate: 6.48 × 10^-11 mol s^-1 cm^-2; Faradaic efficiency: 6.25%) in 0.1 M KOH. Electrochemical-based in situ Fourier transform infrared spectroscopy was employed to study the mechanism of NRR, indicating an associative alternating pathway.

Core@shell structured Au@SnO2 nanoparticles with improved N2 adsorption/activation and electrical conductivity for efficient N2 fixation

The design of electrocatalysts with enhanced adsorption and activation of nitrogen (N₂) is critical for boosting the electrochemical N₂ reduction (ENR). Herein, we developed an efficient strategy to facilitate N₂ adsorption and activation for N₂ electroreduction into ammonia (NH₃) by vacancy engineering of core@shell structured Au@SnO₂ nanoparticles (NPs). We found that the ultrathin amorphous SnO₂ shell with enriched oxygen vacancies was conducive to adsorb N₂ as well as promoted the N₂ activation, meanwhile the metallic Au core ensured the good electrical conductivity for accelerating electrons transport during the electrochemical N₂ reduction reaction, synergistically boosting the N₂ electroreduction catalysis. As confirmed by the ¹⁵N-labeling and controlled experiments, the core@shell Au@amorphous SnO₂ NPs with abundant oxygen vacancies show the best performance for N₂ electroreduction with the NH₃ yield rate of 21.9 μg h^-1 mg^-1_cat and faradaic efficiency of 15.2% at -0.2 V_RHE, which surpass the Au@crystalline SnO₂ NPs, individual Au NPs and all reported Au-based catalysts for ENR.

Controlled oxygen vacancy engineering on In2O3−x/CeO2−y nanotubes for highly selective and efficient electrocatalytic nitrogen reduction

Introducing and adjusting the oxygen vacancies (V_O) of transition metal oxides has been proposed as a significant and effective way to tackle the sluggish nitrogen reduction reaction (NRR) in the electrocatalysis process.

Sb2S3 nanoparticles anchored on SnO2 nanofibers: a high-performance hybrid electrocatalyst toward ammonia synthesis under ambient conditions

Herein, we report a novel transition-metal-based electrocatalyst, Sb2S3 nanoparticles, which exhibits electrocatalytic activity toward ammonia synthesis under ambient conditions. These Sb2S3 nanoparticles are further anchored on SnO2 nanofibers, which act as an active substrate to prevent them from aggregation while enhancing the electrocatalytic activity. The obtained Sb2S3@SnO2 nanofibers deliver excellent ammonia yield (22.0 μg h-1 mgcat-1) and faradaic efficiency (15.1%) at -0.4 V vs. RHE in 0.1 M Na2SO4.

WO3 nanosheets rich in oxygen vacancies for enhanced electrocatalytic N2 reduction to NH3

The Haber-Bosch process for industrial-scale NH3 production suffers from harsh conditions and serious CO2 release. Electrochemical N2 reduction is an alternative approach to synthesize NH3 under ambient conditions, but it requires highly-efficient electrocatalysts for the N2 reduction reaction (NRR). In this Communication, we demonstrate that WO3 nanosheets rich in oxygen vacancies (R-WO3 NSs) exhibit greatly enhanced NRR performances. In 0.1 M HCl, such R-WO3 NSs achieve a large NH3 yield of 17.28 μg h-1 mgcat.-1 and a high faradaic efficiency of 7.0% at -0.3 V vs. a reversible hydrogen electrode, much superior to the WO3 nanosheets deficient in oxygen vacancies (6.47 μg h-1 mgcat.-1 and 1.02%). Remarkably, R-WO3 NSs also show high electrochemical stability.

Single molybdenum atom anchored on 2D Ti2NO2 MXene as a promising electrocatalyst for N2 fixation

The electrocatalytic synthesis of ammonia (NH3) at ambient temperature is an attractive and challenging subject in the chemical industry. The synthesis of NH3 under ambient conditions requires efficient and stable electrocatalysts with ultralow overpotential to ensure low energy consumption and high NH3 yield. Herein, electrocatalysts consisting of a single transition metal (TM) atom (TM = Mo, Mn, Fe, Co, Ni, or Cu) anchored on 2D M2NO2 MXene (M = Ti, V, and Cr), designated as TM/M2NO2, are designed for N2 reduction reaction (NRR) by density functional theory calculations. The results show that the bonding strength between Mo and Ti2NO2 is strong. The overpotential (ηNRR) of Mo/Ti2NO2 surface-catalyzed NRR is estimated to be as low as 0.16 V via an enzymatic mechanism, which is lower than those reported to date. For Mo/V2NO2 and Mo/Cr2NO2 catalysts, the NRR occurs through the consecutive mechanism and enzymatic mechanism, with corresponding ηNRR values of 0.38 V and 0.22 V, respectively. In addition, the reaction Gibbs free energy of NH3 desorption from the Mo/Ti2NO2 surface is only 0.12 eV. Electronic structure analysis indicates that Mo/Ti2NO2 shows metallic characteristics, which ensures the efficient transfer of electrons between Mo and Ti2NO2. Ab initio molecular dynamics simulations indicate that the Mo atom can be stably immobilized on the Ti2NO2 substrate to prevent its aggregation into Mo clusters. Further analysis illustrates that hydrogen adsorption is not favored on the Mo/Ti2NO2 surface. Mixing the N2 source with extra gases, such as NO2, NO, SO2, SO, and O2, should be avoided for NRR on Mo/Ti2NO2 surface. These predictions offer a new opportunity for the electrocatalytic synthesis of NH3 by N2 reduction in the future.

Facile Preparation of Carbon Shells-Coated O-Doped Molybdenum Carbide Nanoparticles as High Selective Electrocatalysts for Nitrogen Reduction Reaction under Ambient Conditions

Electrochemical nitrogen reduction reaction (NRR) has been considered as a promising alternative to the traditional Haber-Bosch process for the preparation of ammonia (NH₃) under ambient conditions. The development of cost-effective electrocatalysts with suppressive activity for hydrogen evolution reaction is critical for improving the efficiency of NRR. Herein, oxygen-containing molybdenum carbides (O-MoC) embedded in nitrogen-doped carbon layers (N-doped carbon) can be easily fabricated by pyrolyzing the chelate of dopamine and molybdate. A rate of NH₃ formation of 22.5 μg·h^-1·mg_cat^-1 is obtained at -0.35 V versus reversible hydrogen electrode with a high faradaic efficiency of 25.1% in 0.1 mM HCl + 0.5 M Li₂SO₄. Notably, the synthesized O-MoC@NC-800 also exhibits high selectivity (no formation of hydrazine) and electrochemical stability. The moderate electron structure induced by the interaction between O-MoC and N-doped carbon shells can effectively weaken the activity of hydrogen evolution reaction and increase the faradaic efficiency of NRR. Additionally, by applying the in situ Fourier transform infrared spectroscopy, an associative reaction pathway is proposed on O-MoC@NC-800. This work provides new insights into the rational design of carbon-encapsulated metal nanoparticles as efficient catalysts for NRR at ambient conditions.

Electronically Coupled SnO2 Quantum Dots and Graphene for Efficient Nitrogen Reduction Reaction

Electrocatalytic N₂ reduction reaction (NRR) provides an effective and renewable approach for artificial NH₃ production, but still remains a grand challenge because of the low NH₃ yield and Faradaic efficiency (FE). Herein, we reported that the SnO₂ quantum dots (QDs) supported on reduced graphene oxide (RGO) could efficiently and stably catalyze NRR at ambient conditions. The NRR performance of resulting SnO₂/RGO was studied by both experimental techniques and density functional theory calculations. It was found that the ultrasmall SnO₂ QDs (2 nm) grown on RGO could provide abundant sites for efficient N₂ adsorption. Significantly, the strongly electronically coupled SnO₂ QDs and RGO brought about the enhanced conductivity and the decreased work function, which led to a considerably lowered energy barrier of *N₂ → *N₂H that was the rate-determining step of the NRR process. Meanwhile, the SnO₂/RGO exhibited inferior hydrogen evolution reaction activity. As a result, the SnO₂/RGO delivered a high NH₃ yield of 25.6 μg h^-1 mg^-1 (5.1 μg cm^-2h^-1) and an FE of 7.1% in 0.1 M Na₂SO₄ at -0.5 V (vs RHE), together with the outstanding selectivity and stability, endowing it as a promising electrocatalyst for N₂ fixation.

Incorporating N Atoms into SnO₂ Nanostructure as an Approach to Enhance Gas Sensing Property for Acetone

The development of high-performance acetone gas sensor is of great significance for environmental protection and personal safety. SnO₂ has been intensively applied in chemical sensing areas, because of its low cost, high mobility of electrons, and good chemical stability. Herein, we incorporated nitrogen atoms into the SnO₂ nanostructure by simple solvothermal and subsequent calcination to improve gas sensing property for acetone. The crystallization, morphology, element composition, and microstructure of as-prepared products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Electron paramagnetic resonance (EPR), Raman spectroscopy, UV–visible diffuse reflectance spectroscopy (UV–vis DRS), and the Brunauer–Emmett–Teller (BET) method. It has been found that N-incorporating resulted in decreased crystallite size, reduced band-gap width, increased surface oxygen vacancies, enlarged surface area, and narrowed pore size distribution. When evaluated as gas sensor, nitrogen-incorporated SnO₂ nanostructure exhibited excellent sensitivity for acetone gas at the optimal operating temperature of 300 °C with high sensor response (R_air/R_gas − 1 = 357) and low limit of detection (7 ppb). The nitrogen-incorporated SnO₂ gas sensor shows a good selectivity to acetone in the interfering gases of benzene, toluene, ethylbenzene, hydrogen, and methane. Furthermore, the possible gas-sensing mechanism of N-incorporated SnO₂ toward acetone has been carefully discussed.

Enabling the electrocatalytic fixation of N2 to NH3 by C-doped TiO2 nanoparticles under ambient conditions

The conventional Haber-Bosch process for industrial NH₃ production from N₂ and H₂ is highly energy-intensive with a large amount of CO₂ emissions and finding a more suitable method for NH₃ synthesis under mild conditions is a very attractive topic. The electrocatalytic N₂ reduction reaction (NRR) offers us an environmentally benign and sustainable route. In this communication, we report that C-doped TiO₂ nanoparticles act as an efficient electrocatalyst for the NRR with excellent selectivity. In 0.1 M Na₂SO₄, it achieves an NH₃ yield of 16.22 μg h^-1 mg_cat. ^-1 and a faradaic efficiency of 1.84% at -0.7 V vs. the reversible hydrogen electrode. Furthermore, this catalyst also shows good stability during electrolysis and recycling tests.