Unveiling Potential Dependence in NO Electroreduction to Ammonia

Controlling the Selectivity of Electrocatalytic NO Reduction through pH and Potential Regulation on Single-Atom Catalysts

Electrocatalytic nitrogen oxide reduction (NOxRR) emerges as an effective way to bring the disrupted nitrogen cycle back into balance. However, efficient and selective NOxRR is still challenging partly due to the complex reaction mechanism, which is influenced by experimental conditions such as pH and electrode potential. Here, we have studied the enzyme-inspired iron single-atom catalysts (Fe-N4-C) and identified that the selectivity roots in the first step of the nitric oxide reduction. Combining the constrained molecular dynamics (MD) simulations with the quasi-equilibrium approximation, the effects of electrode potential and pH on the reaction free energy were considered explicitly and predicted quantitatively. Systematic heat maps for selectivity between single-N and N-N-coupled products in a wide pH-potential space are further developed, which have reproduced the experimental observations of NOxRR. The approach presented in this study allows for a realistic simulation of the electrocatalytic interfaces and a quantitative evaluation of interfacial effects. Our results in this study provide valuable and straightforward guidance for selective NOx reduction toward desired products by precisely designing the experimental conditions.

Selective Electrocatalytic Conversion of Nitric Oxide to High Value-Added Chemicals

The artificial disturbance in the nitrogen cycle has necessitated an urgent need for nitric oxide (NO) removal. Electrochemical technologies for NO conversion have gained increasing attention in recent years. This comprehensive review presents the recent advancements in selective electrocatalytic conversion of NO to high value-added chemicals, with specific emphasis on catalyst design, electrolyte composition, mass diffusion, and adsorption energies of key intermediate species. Furthermore, the review explores the synergistic electrochemical co-electrolysis of NO with specific carbon source molecules, enabling the synthesis of a range of valuable chemicals with C─N bonds. It also provides in-depth insights into the intricate reaction pathways and underlying mechanisms, offering valuable perspectives on the challenges and prospects of selective NO electrolysis. By advancing comprehension and fostering awareness of nitrogen cycle balance, this review contributes to the development of efficient and sustainable electrocatalytic systems for the selective synthesis of valuable chemicals from NO.

Identifying the active sites and intermediates on copper surfaces for electrochemical nitrate reduction to ammonia

Copper (Cu) is a widely used catalyst for the nitrate reduction reaction (NO3RR), but its susceptibility to surface oxidation and complex electrochemical conditions hinders the identification of active sites. Here, we employed electropolished metallic Cu with a predominant (100) surface and compared it to native oxide-covered Cu. The electropolished Cu surface rapidly oxidized after exposure to either air or electrolyte solutions. However, this oxide was reduced below 0.1 V vs. RHE, thus returning to the metallic Cu before NO3RR. It was distinguished from the native oxide on Cu, which remained during NO3RR. Fast NO3- and NO reduction on the metallic Cu delivered 91.5 ± 3.7% faradaic efficiency for NH3 at -0.4 V vs. RHE. In contrast, the native oxide on Cu formed undesired products and low NH3 yield. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis revealed the adsorbed NO3-, NO2, and NO species on the electropolished Cu as the intermediates of NH3. Low overpotential NO3- and NO adsorptions and favorable NO reduction are key to increased NH3 productivity over Cu samples, which was consistent with the DFT calculation on Cu(100).

Activity Trend and Selectivity of Electrochemical Ammonia Synthesis in Reverse Artificial Nitrogen Cycle

Ammonia is important for modern agriculture and food production as it is a major source of fertilizer. Electrochemical ammonia synthesis (EAS) with sustainable energy generated electricity and decentralized reactors has been considered as environmentally friendly process. Several nitrogen sources have been considered and intensively studied in experiments and computations. Recently, it has been proposed and demonstrated that nitrogen oxides (NOx ) electroreduction for selective ammonia production is feasible. Fundamental insights on experimental observation are necessary for more rational design of catalysts and reactors in the future. In this concept, we review the theoretical and computational insights of electrochemical nitrogen oxides reduction, particularly, the activity trend over diverse transition metal catalysts and products selectivity at varying potentials. Finally, we address the opportunities and challenges in the reverse artificial nitrogen cycle, as well as fundamental issues in electrochemical reaction modelling.

Electrochemical NO3- Reduction Catalyzed by Atomically Precise Ag30Pd4 Bimetallic Nanocluster: Synergistic Catalysis or Tandem Catalysis?

Electrochemically converting NO₃^- compounds into ammonia represents a sustainable route to remove industrial pollutants in wastewater and produce valuable chemicals. Bimetallic nanomaterials usually exhibit better catalytic performance than the monometallic counterparts, yet unveiling the reaction mechanism is extremely challenging. Herein, we report an atomically precise [Ag₃₀Pd₄ (C₆H₉)₂₆](BPh₄)₂ (Ag₃₀Pd₄) nanocluster as a model catalyst toward the electrochemical NO₃^- reduction reaction (eNO₃^-RR) to elucidate the different role of the Ag and Pd site and unveil the comprehensive catalytic mechanism. Ag₃₀Pd₄ is the homoleptic alkynyl-protected superatom with 2 free electrons, and it has a Ag₃₀Pd₄ metal core where 4 Pd atoms are located at the subcenter of the metal core. Furthermore, Ag₃₀Pd₄ exhibits excellent performance toward eNO₃^-RR and robust stability for prolonged operation, and it can achieve the highest Faradaic efficiency of NH₃ over 90%. In situ Fourier-transform infrared study revealed that a Ag site plays a more critical role in converting NO₃^- into NO₂^-, while the Pd site makes a major contribution to catalyze NO₂^- into NH₃. The bimetallic nanocluster adopts a tandem catalytic mechanism rather than a synergistic catalytic effect in eNO₃^-RR. Such finding was further confirmed by density functional theory calculations, as they disclosed that Ag is the most preferable binding site for NO₃^-, which then binds a water molecule to release NO₂^-. Subsequently, NO₂^- can transfer to the vicinal exposed Pd site to promote NH₃ formation.

Role of transition metal d-orbitals in single-atom catalysts for nitric oxide electroreduction to ammonia

Recently, surging interests exist in direct electrochemical ammonia (NH₃) synthesis from nitric oxide (NO) due to the dual benefit of NH₃ synthesis and NO removal. However, designing highly efficient catalysts is still challenging. Based on density functional theory, the best ten candidates of transition-metal atoms (TMs) embedded in phosphorus carbide (PC) monolayer is screened out as highly active catalysts for direct NO-to-NH₃ electroreduction. The employment of machine learning-aided theoretical calculations helps to identify the critical role of TM-d orbitals in regulating NO activation. A V-shape tuning rule of TM-d orbitals for the Gibbs free energy change of NO or limiting potentials is further revealed as the design principle of TM embedded PC (TM-PC) for NO-to-NH₃ electroreduction. Moreover, after employing effective screening strategies including surface stability, selectivity, the kinetic barrier of potential-determining step, and thermal stability comprehensively studied for the ten TM-PC candidates, only Pt embedded PC monolayer has been identified as the most promising direct NO-to-NH₃ electroreduction with high feasibility and catalytic performance. This work not only offers a promising catalyst but also sheds light on the active origin and design principle of PC-based single-atom catalysts for NO-to-NH₃ conversion.

Steering from electrochemical denitrification to ammonia synthesis

The removal of nitric oxide is an important environmental issue, as well as a necessary prerequisite for achieving high efficiency of CO₂ electroreduction. To this end, the electrocatalytic denitrification is a sustainable route. Herein, we employ reaction phase diagram to analyze the evolution of reaction mechanisms over varying catalysts and study the potential/pH effects over Pd and Cu. We find the low N₂ selectivity compared to N₂O production, consistent with a set of experiments, is limited fundamentally by two factors. The N₂OH* binding is relatively weak over transition metals, resulting in the low rate of as-produced N₂O* protonation. The strong correlation of OH* and O* binding energies limits the route of N₂O* dissociation. Although the experimental conditions of varying potential, pH and NO pressures can tune the selectivity slightly, which are insufficient to promote N₂ selectivity beyond N₂O and NH₃. A possible solution is to design catalysts with exceptions to break the scaling characters of energies. Alternatively, we propose a reverse route with the target of decentralized ammonia synthesis.

The β-PdBi2 monolayer for efficient electrocatalytic NO reduction to NH3: a computational study

β-PdBi2 was proposed as a novel NORR catalyst for NH3 synthesis with high efficiency and high selectivity, and its catalytic activity can be enhanced by a tensile strain.

Recent Advances on Composition-Microstructure-Properties Relationships of Precipitation Hardening Stainless Steel

Precipitation hardening stainless steels have attracted extensive interest due to their distinguished mechanical properties. However, it is necessary to further uncover the internal quantitative relationship from the traditional standpoint based on the statistical perspective. In this review, we summarize the latest research progress on the relationships among the composition, microstructure, and properties of precipitation hardened stainless steels. First, the influence of general chemical composition and its fluctuation on the microstructure and properties of PHSS are elaborated. Then, the microstructure and properties under a typical heat treatment regime are discussed, including the precipitation of B2-NiAl particles, Cu-rich clusters, Ni3Ti precipitates, and other co-existing precipitates in PHSS and the hierarchical microstructural features are presented. Next, the microstructure and properties after the selective laser melting fabricating process which act as an emerging technology compared to conventional manufacturing techniques are also enlightened. Thereafter, the development of multi-scale simulation and machine learning (ML) in material design is illustrated with typical examples and the great concerns in PHSS research are presented, with a focus on the precipitation techniques, effect of composition, and microstructure. Finally, promising directions for future precipitation hardening stainless steel development combined with multi-scale simulation and ML methods are prospected, offering extensive insight into the innovation of novel precipitation hardening stainless steels.

Emerging p-Block-Element-Based Electrocatalysts for Sustainable Nitrogen Conversion

Artificial nitrogen conversion reactions, such as the production of ammonia via dinitrogen or nitrate reduction and the synthesis of organonitrogen compounds via C-N coupling, play a pivotal role in the modern life. As alternatives to the traditional industrial processes that are energy- and carbon-emission-intensive, electrocatalytic nitrogen conversion reactions under mild conditions have attracted significant research interests. However, the electrosynthesis process still suffers from low product yield and Faradaic efficiency, which highlight the importance of developing efficient catalysts. In contrast to the transition-metal-based catalysts that have been widely studied, the p-block-element-based catalysts have recently shown promising performance because of their intriguing physiochemical properties and intrinsically poor hydrogen adsorption ability. In this Perspective, we summarize the latest breakthroughs in the development of p-block-element-based electrocatalysts toward nitrogen conversion applications, including ammonia electrosynthesis from N₂ reduction and nitrate reduction and urea electrosynthesis using nitrogen-containing feedstocks and carbon dioxide. The catalyst design strategies and the underlying reaction mechanisms are discussed. Finally, major challenges and opportunities in future research directions are also proposed.

Why heterogeneous single-atom catalysts preferentially produce CO in the electrochemical CO2 reduction reaction

Formate and CO are competing products in the two-electron CO₂ reduction reaction (2e CO₂RR), and they are produced via *OCHO and *COOH intermediates, respectively. However, the factors governing CO/formate selectivity remain elusive, especially for metal–carbon–nitrogen (M–N–C) single-atom catalysts (SACs), most of which produce CO as their main product. Herein, we show computationally that the selectivity of M–N–C SACs is intrinsically associated with the CO₂ adsorption mode by using bismuth (Bi) nanosheets and the Bi–N–C SAC as model catalysts. According to our results, the Bi–N–C SAC exhibits a strong thermodynamic preference toward *OCHO, but under working potentials, CO₂ is preferentially chemisorbed first due to a charge accumulation effect, and subsequent protonation of chemisorbed CO₂ to *COOH is kinetically much more favorable than formation of *OCHO. Consequently, the Bi–N–C SAC preferentially produces CO rather than formate. In contrast, the physisorption preference of CO₂ on Bi nanosheets contributes to high formate selectivity. Remarkably, this CO₂ adsorption-based mechanism also applies to other typical M–N–C SACs. This work not only resolves a long-standing puzzle in M–N–C SACs, but also presents simple, solid criteria (i.e., CO₂ adsorption modes) for indicating CO/formate selectivity, which help strategic development of high-performance CO₂RR catalysts.

This report discloses a nontrivial role of the CO₂ adsorption mode in governing the CO/formate selectivity of single-atom catalysts towards two-electron CO₂ reduction.

Amorphous Boron Carbide on Titanium Dioxide Nanobelt Arrays for High-Efficiency Electrocatalytic NO Reduction to NH3

Electrocatalytic NO reduction is regarded as an attractive strategy to degrade the NO contaminant into useful NH₃ , but the lack of efficient and stable electrocatalysts to facilitate such multiple proton-coupled electron-transfer processes impedes its applications. Here, we report on developing amorphous B_2.6 C supported on a TiO₂ nanoarray on a Ti plate (a-B_2.6 C@TiO₂ /Ti) as an NH₃ -producing nanocatalyst with appreciable activity and durability toward the NO electroreduction. It shows a yield of 3678.6 μg h^-1  cm^-2 and a FE of 87.6 %, superior to TiO₂ /Ti (563.5 μg h^-1  cm^-2 , 42.6 %) and a-B_2.6 C/Ti (2499.2 μg h^-1  cm^-2 , 85.6 %). An a-B_2.6 C@TiO₂ /Ti-based Zn-NO battery achieves a power density of 1.7 mW cm^-2 with an NH₃ yield of 1125 μg h^-1  cm^-2 . An in-depth understanding of catalytic mechanisms is gained by theoretical calculations.

Evaluation of Electrocatalytic Activity of Noble Metal Catalysts Toward Nitrogen Reduction Reaction in Aqueous Solutions under Ambient Conditions

Electrochemical nitrogen reduction reaction (NRR) is intensively investigated by researchers for its potential to be the next-generation technology to produce ammonia. Many attempts have been made to explore the possibility of electrochemical ammonia production catalyzed by noble metals. However, the produced ammonia in most reported cases is in ppm level or even lower, which is susceptible to potential contaminants in experiments, leading to fluctuating or even contradictory results. Herein, a rigorous procedure was adopted to systematically evaluated the performance of commercial noble metal nanocatalysts toward NRR. No discernible amount of ammonia was detected in either acidic or alkaline solutions. Further, nitrogen-containing contaminants in catalysts that might cause false positive results were detected and characterized. An effective way to remove pre-existing pollutants by consecutive cyclic voltammetry scan was proposed, helping to obtain reliable and reproducible results.

Efficient nitric oxide electroreduction toward ambient ammonia synthesis catalyzed by a CoP nanoarray

The ever-increasing anthropic NO emission from fossil fuel combustion has resulted in a series of severe environmental issues. Ambient electrocatalytic NO reduction has emerged as a promising route for sustainable...

Advances in Electrochemical Ammonia Synthesis Beyond the Use of Nitrogen Gas as a Source

Electrocatalytic reduction of dinitrogen has emerged as a new strategy for ammonia synthesis. Despite being environmentally benign and energy-saving, it suffers from low conversion efficiency and short yield of ammonia because of the challenges of activating the inert N≡N bond at room temperature and atmospheric pressure. As a result of this, researchers proposed to reduce the nitrogenous species, one category of air and water pollutants, into valuable ammonia. Although remaining largely underexplored, this alternative approach shows promising efficiency for ammonia synthesis, while achieving high catalytic activity and selectivity remains challenging. In this Minireview, we summarize recent electrocatalytic performances of denitrification with selective formation to ammonia in terms of proposed active sites and reaction mechanisms. Additionally, we discuss the common issues in the state-of-the-art experimental tests and highlight the breakthroughs via computational screening of electrode materials. The aim of this is to steer the future research directions in the field, which is aiming for an optimal catalytic system with higher activity and selectivity for electrocatalytic denitrification.