Electrochemical Ammonia Generation Directly from Nitrogen and Air Using an Iron-Oxide/Titania-Based Catalyst at Ambient Conditions

Electrocatalytic nitrogen reduction to ammonia at low potential using a phenalenyl-based iron(III) complex

In recent years, electrochemical nitrogen reduction reaction (ENRR) has emerged as a promising alternative for ammonia production in a clean and energy-efficient manner. We reported the remarkable performance of a transition metal-based electrocatalyst, Fe(III)(PLY)3 (where PLY-H = 9-hydroxyphenalenone), for electrochemical NRR. In an acidic electrolyte, Fe(PLY)3 catalyst demonstrates remarkable performance, achieving a high faradaic efficiency (FE) of 43.4% and an impressive ammonia (NH3) yield rate of 99.7 μg h-1 mgcat-1 at -0.2 V compared to a reversible hydrogen electrode (RHE).

Recent progress on Ti-based catalysts in the electrochemical synthesis of ammonia

Electrochemical ammonia synthesis presents a sustainable alternative, offering the potential for enhanced energy efficiency and environmental benefits compared to the conventional Haber-Bosch process. In recent years, the electrocatalytic reduction of nitrate to ammonia (NO3-RR) has emerged as a crucial approach for achieving sustainable NH3 production. To enhance energy efficiency and successfully convert NO3- to NH3, it is essential to investigate cost-effective electrocatalysts that provide high Faraday efficiency and demonstrate adequate stability. Ti-based materials are considered ideal candidates as catalysts due to their environmental friendliness and robust stability. This review initially summarizes the nitrate reduction reaction pathway and concisely discusses the impact of the potential intermediates and reaction steps on the overall reaction efficiency and product selectivity. Subsequently, an overview of the fundamental characteristics of Ti and TiO2 is presented. Additionally, the research process on Ti-based electrocatalysts in the electrochemical reduction of nitrate for ammonia synthesis is summarized. Finally, the design strategies, such as heteroatom doping and the introduction of oxygen vacancies, to enhance catalytic efficiency and selectivity are presented. Through this comprehensive review, we endeavor to furnish researchers with the most recent insights into the application of titanium-based electrocatalysts in nitrate reduction reactions and to stimulate innovative thought processes on the electrocatalytic synthesis of ammonia.

Strategic design of VO2 encased in N-doped carbon as an efficient electrocatalyst for the nitrogen reduction reaction in neutral and acidic media

Electrocatalytic nitrogen fixation to ammonia (NH3), a precursor for fertilizer production and a promising energy carrier, has garnered widespread interest as an environment-friendly and sustainable alternative to the energy-intensive fossil-feedstock-dependent Haber-Bosch process. The large-scale deployment of this process is contingent on the identification of inexpensive, Earth-abundant systems that can operate efficiently, irrespective of the electrolyte pH for the selective production of NH3. In this regard, we discuss the scalable synthesis of VO2 anchored on N-doped carbon (VO2@CN), and its applicability as a robust electrocatalyst for the nitrogen reduction reaction (NRR). Benefitting from the presence of exposed VO2, which presumably acts as the active site for nitrogen reduction, and its activity over a broad pH range (from acidic to neutral), VO2@CN exhibits a high NH3 yield of 0.31 and 0.52 μmol h-1 mgcat-1 and a maximum Faradaic efficiency (FE) of 67.9% and 61.9% at -0.1 V vs. RHE, under neutral and acidic conditions, respectively. The obscured reaction intermediates of the NRR were identified from in situ ATR-IR studies under both electrolyte conditions. Additionally, the high selectivity of the catalyst was ascertained from the absence of hydrazine production and the competing hydrogen evolution reaction (HER). However, ammonia production underwent a reduction over 12 h of continuous operation presumably owing to the leaching of catalyst under these electrolysis conditions, which was more pronounced in electrolytes with acidic pH. Overall, the present report unveils the performance of an earth-abundant vanadium oxide-based system as an efficient electrocatalyst for the NRR under acidic and neutral pH conditions.

One-Pot Synthesis of Ruthenium-Based Nanocatalyst Using Reduced Graphene Oxide as Matrix for Electrochemical Synthesis of Ammonia

Conventional ammonia production consumes significant energy and causes enormous carbon dioxide (CO₂) emissions globally. To lower energy consumption and mitigate CO₂ emissions, a facile, environmentally friendly, and cost-effective one-pot method for the synthesis of a ruthenium-based nitrogen reduction nanocatalyst has been developed using reduced graphene oxide (rGO) as a matrix. The nanocatalyst synthesis was based on a single-step simultaneous reduction of RuCl₃ into ruthenium-based nanoparticles (Ru-based NPs) and graphene oxide (GO) into rGO using glucose as the reducing agent and stabilizer. The obtained ruthenium-based nanocatalyst with rGO as a matrix (Ru_nano-based/rGO) has shown much higher catalytic activity at lower temperatures and pressures for ammonia synthesis than conventional iron catalysts. The rGO worked as a promising promoter for the electrochemical synthesis of ammonia due to its excellent electrical and thermal conductivity. The developed Ru_nano-based/rGO nanocatalyst was characterized using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), ultraviolet-visible (UV-vis) absorption spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the size of the Ru-based NPs on the surface of rGO was 1.9 ± 0.2 nm and the ruthenium content was 25.03 wt %. Bulk electrolysis measurements were conducted on thin-layer electrodes at various cathodic potentials in a N₂-saturated 0.1 M H₂SO₄ electrolyte at room temperature. From the chronoamperometric measurements, the maximum faradic efficiency (F.E.) of 2.1% for ammonia production on the nanostructured Ru_nano-based/rGO electrocatalyst was achieved at a potential of -0.20 V vs reversible hydrogen electrode (RHE). This electrocatalyst has attained a superior ammonia production rate of 9.14 μg·h^-1·mg_cat.^-1. The results demonstrate the feasibility of reducing N₂ into ammonia under ambient conditions and warrant further exploration of the nanostructured Ru_nano-based/rGO for electrochemical ammonia synthesis.

Metal ferrites-based nanocomposites and nanohybrids for photocatalytic water treatment and electrocatalytic water splitting

Photocatalytic degradation is one of the most promising technologies available for removing a variety of synthetic and organic pollutants from the environmental matrices because of its high catalytic activity, reduced energy consumption, and low total cost. Due to its acceptable bandgap, broad light-harvesting efficiency, significant renewability, and stability, Fe₂O₃ has emerged as a fascinating material for the degradation of organic contaminants as well as numerous dyes. This study thoroughly reviewed the efficiency of Fe₂O₃-based nanocomposite and nanomaterials for water remediation. Iron oxide structure and various synthetic methods are briefly discussed. Additionally, the electrocatalytic application of Fe₂O₃-based nanocomposites, including oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, and overall water splitting efficiency, was also highlighted to illustrate the great promise of these composites. Finally, the ongoing issues and future prospects are directed to fully reveal the standards of Fe₂O₃-based catalysts. This review is intended to disseminate knowledge for further research on the possible applications of Fe₂O₃ as a photocatalyst and electrocatalyst.

Polymeric Hydronitrogen N4H: A Promising High-Energy-Density Material and High-Temperature Superconductor

Solid nitrogen-rich compounds are potential high-energy-density materials (HEDMs). The enormous challenge in this area is to synthesize and stabilize these energetic materials at moderate pressure and better under near-ambient conditions. Here, we perform an extensive theoretical study on hydronitrogens by the reverse design method considering both energies and energy densities. Four hydronitrogens with different stoichiometries, that is, N4H, N3H, N2H, and NH, are found to be stable at pressures of about 80-300 GPa and metastable with pressure releasing to ambient pressure. The energy densities of these hydronitrogens are about 5.6-6.5 kJ/g and 1.3-1.5 times larger than that of trinitrotoluene (TNT). Most importantly, the Pbam phase of the N4H compound is an excellent high-temperature superconductor with a Tc of 37.7 K at 72 GPa. The present findings enrich new phases of hydronitrogens under high pressure and characterize their structural and energetic properties and superconductivity, which offer crucial insights for further design and synthesis of exceptional materials with high energy density and high-temperature superconductivity.

A Rearrangement Reaction to Yield a NH4 + Ion Driven by Polyoxometalate Formation

Triethylamine is a volatile liquid and exists in the atmosphere in the gas phase. It is a hazardous air pollutant and identified as a toxic air contaminant. Thus, producing ammonia (a vital chemical for fertilizer production) from the vapor state of this toxic substance is a challenging task. Diffusion of the vapor of triethylamine, (C₂H₅)₃N, into an acidified aqueous solution of sodium molybdate results in the formation of single crystals of compound [(C₂H₅)₃NH]₂[(C₂H₅)₄N][NaMo₈O₂₆] (1). Notably, compound 1 includes a [(C₂H₅)₄N]⁺ cation, even though the concerned reaction mixture was not treated with any tetraethylammonium salt. The formation of the [(C₂H₅)₄N]⁺ cation from (C₂H₅)₃N in an acidic aqueous medium is logically possible only when an ammonium cation (NH₄ ⁺) is formed in the overall reaction: 4(C₂H₅)₃N + 4H⁺ = 3[(C₂H₅)₄N]⁺ + [NH₄]⁺. Although the resulting NH₄ ⁺ cation (identified by Nessler's reagent test) is not included in the crystals of compound 1 as a cation, it can be made associated with a crown ether in the isolation of single crystals of compound [NH₄⊂B15C5]₃[PMo₁₂O₄₀]·B15C5 (2), (B15C5 = benzo-15-crown-5). Crystal structure analysis and ¹H NMR studies of compound 2 have established the presence of an H-bonded NH₄ ⁺ ion in compound 2, thereby established the rearrangement reaction.

Amorphous Chromium Oxide with Hollow Morphology for Nitrogen Electrochemical Reduction under Ambient Conditions

The electrocatalytic nitrogen reduction reaction (NRR), an alternative method of nitrogen fixation and conversion under ambient conditions, represents a promising strategy for tackling the energy-intensive issue. The design of high-performance electrocatalysts is one of the key issues to realizing the application of NRR, but most of the current catalysts rely on the use of crystalline materials, and shortcomings such as a limited number of catalytic active sites and sluggish reaction kinetics arise. Herein, an amorphous metal oxide catalyst H-CrO_x/C-550 with hierarchically porous structure is constructed, which shows superior electrocatalytic performance toward NRR under ambient conditions (yield of 19.10 μg h^-1 mg_cat^-1 and Faradaic efficiency of 1.4% at -0.7 V vs a reversible hydrogen electrode, higher than that of crystalline Cr₂O₃ and solid counterparts). Notably, the amorphous metal oxide obtained by controlled pyrolysis of metal-organic frameworks (MOFs) possess abundant unsaturated catalytic sites and optimized conductivity due to the controllable degree of metal-oxygen bond reconstruction and the doping of carbon materials derived from organic ligands. This work demonstrates MOF-derived porous amorphous materials as a viable alternative to current electrocatalysts for NH₃ synthesis at ambient conditions.

Electrocatalytic Reduction of Nitrogen to Ammonia: the Roles of Lattice O and N in Reduction at Vanadium Oxynitride Surfaces

Vanadium oxynitride and other earth-abundant oxynitrides are of growing interest for the electrocatalytic reduction of nitrogen to NH₃. A major unresolved issue, however, concerns the roles of lattice N and lattice O in this process. Electrochemistry and photoemission data reported here demonstrate that both lattice N and dissolved N₂ are reduced to NH₃ by cathodic polarization of vanadium oxynitride films at pH 7. These data also show that ammonia production from lattice N occurs in the presence or absence of N₂ and involves the formation of V≡N: intermediates or similar unsaturated VN surface states on a thin vanadium oxide overlayer. In contrast, N₂ reduction proceeds in the presence or absence of lattice N and without N incorporation into a vanadium oxide lattice. Thus, both lattice N and N₂ reduction mechanisms involve oxide-supported V surface sites ([V]_O) in preference to N-supported sites ([V]_N). This result is supported by density functional theory-based calculations showing that the formation of V≡N:, V-N═N-H, and a few other plausible reaction intermediates is consistently energetically favored at [V]_O rather than at [V]_N surface sites. Similar effects are predicted for the oxynitrides of other oxophilic metals, such as Ti.

Bi2S3 quantum dots in situ grown on MoS2 nanoflowers: An efficient electron-rich interface for photoelectrochemical N2 reduction

Photoelectrocatalysis is considered a green, environmentally friendly, sustainable technology for NH₃ synthesis. However, the low efficiency of ammonia synthesis is currently the primary problem in photoelectrochemical nitrogen reduction reactions (PEC NRR). Herein, a nanocomposite BQD/MS developed through the in-situ growth of Bi₂S₃ quantum dots (BQD) on MoS₂ (MS) nanoflowers was demonstrated as an efficient PEC NRR catalyst. Experimental results showed that the strong interaction between BQD and MS modulated the interfacial charge distribution and increased the electron density on the MS side. Meanwhile, the excellent structure of BQD/MS promoted the effective migration of photogenerated electrons from excited BQD to the MS surface. The electron-rich MS reaction interface was conducive to cleaving the stable NN bond and improving the N₂ reduction performance. As a result, the prepared BQD/15MS photocathode obtained an excellent Faradaic efficiency of 33.2% and an NH₃ yield of 18.5 μg h^-1 mg^-1, which was about three times that of bare MS.

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.

Ambient Electrosynthesis of Ammonia Using Core-Shell Structured Au@C Catalyst Fabricated by One-Step Laser Ablation Technique

This work reports the synthesis of core-shell structured Au@C composite through a simple one-step laser ablation technique. The results demonstrate that the Au@C with a mean nanosphere size of ∼8.0 nm is composed of a spherical shaped Au core and 1-2 layered graphitic carbon shell with abundant defects. As a nitrogen reduction reaction (NRR) electrocatalyst, the Au@C gives a large NH₃ yield rate of 241.9 μg h^-1 mg_cat.^-1 with a high faradaic efficiency of 40.5% at -0.45 V versus reversible hydrogen electrode in a 0.1 M Na₂SO₄ electrolyte (pH = 6.3) under ambient conditions, surpassing the performances of most aqueous-based NRR electrocatalysts recently reported. The ¹⁵N labeling experimental results demonstrate that the produced NH₃ is undoubtedly originated from the NRR process catalyzed by Au@C. The superior NRR performance of Au@C can be ascribed to the ultrathin carbon layer, effectively inhibiting the aggregation of Au nanospheres during the NRR, and the abundant defects such as carbon vacancies existed in the ultrathin carbon layer, providing additional NRR catalytic active sites. Our theoretical calculation results further confirm the role of carbon vacancies in the electrocatalytic NRR.

Efficient Photoelectrochemical Route for the Ambient Reduction of N2 to NH3 Based on Nanojunctions Assembled from MoS2 Nanosheets and TiO2

Efficient nitrogen fixation under ambient conditions is an exigent task in both basic research and industrial applications. Recently, reduction of N₂ to NH₃ based on photocatalysis and/or electrocatalysis offers a possible route to the typical Haber-Bosch process. However, achieving a high yield of N₂ reduction reaction (NRR) is still a challenging goal because of the limitations of efficient catalysts. Herein, we propose a photoelectrochemical NRR route based on the rational design of MoS₂@TiO₂ semiconductor nanojunction catalysts through a facile hydrothermal synthetic method. The developed MoS₂@TiO₂ photocathode attains a high NH₃ yield rate (1.42 × 10^-6 mol h^-1 cm^-2) and a superhigh faradaic efficiency (65.52%), which is the highest record to the best of our knowledge. Moreover, MoS₂@TiO₂ exhibits high stability over 10 consecutive reaction cycles. Therefore, this work demonstrates an effective NRR photoelectrocatalyst and results in a breakthrough in the low faradaic efficiency because of the interfacial electronic coupling and synergistic effects between the MoS₂ and TiO₂ components.

Ambient electrochemical N2 reduction to NH3 under alkaline conditions enabled by a layered K2Ti4O9 nanobelt

A K₂Ti₄O₉ nanobelt effectively electrocatalyzes ambient N₂-to-NH₃ fixation. In 0.1 M KOH, a NH₃ yield of 22.88 μg h⁻¹ mg⁻¹_cat. and a faradaic efficiency of 5.87% are attained, with good selectivity and high electrochemical stability.