Comprehensive understanding of the thriving electrocatalytic nitrate/nitrite reduction to ammonia under ambient conditions

Ammonia(NH_(3))is a multifunctional compound that is an important feedstock for the agricultural and pharmaceutical industries and attractive energy storage medium.At present,NH_(3)synthesis is highly dependent on the conventional Haber–Bosch process that operates under harsh conditions,which consumes large quantities of fossil fuels and releases a large amount of carbon dioxide.As an alternative,electrosynthesis is a prospective method for producing NH_(3)under normal temperature and pressure conditions.Although electrocatalytic nitrogen reduction to ammonia has attracted considerable attentions,the low solubility of N_(2)and high N≡N cracking energy render the achievements of high NH_(3) yield rate and Faradaic efficiency difficult.Nitrate and nitrite(NO_(x)^(-))are common N-containing pollutants.Due to their high solubilities and low dissociation energy of N=O,NO_(x)^(-)−are ideal raw materials for NH_(3) production.Therefore,electrocatalytic NO_(x)^(-)−reduction to NH_(3)(eNO_(x)RR)is a prospective strategy to simultaneously realise environmental protection and NH_(3) synthesis.This review offers a comprehensive understanding of the thriving eNO_(x)RR under ambient conditions.At first,the popular theory and mechanism of eNO_(x)RR and a summary of the measurement system and evaluation criteria are introduced.Thereafter,various strategies for developing NO_(x)−reduction catalysts are systematically presented and discussed.Finally,the challenges and possible prospects of electrocatalytic NO_(x)^(-1) reduction are outlined to facilitate energy-saving and environmentally friendly large-scale synthesis of NH_(3) in the future.

Modification of Carbon Nanotube Powder Microelectrode and Nitrite Reduction

The properties of the carbon nanotube powder microelectrodes (denoted CNTPME) are remarkably altered by anodic pretreatment and preadsorption of mediators. It seems that anodic pretreatment leads the long and tangled carbon nanotubes to be partially cut shorter, resulting in more openings as shown by TEM. Besides, the anodic pretreatment may adjust the hydrophobicity of nanotubes to match with that of Os(bpy)32+. As a result, the real surface area and the ability of adsorbing mediator Os(bpy)32+ of the nanotubes are markedly increased so as to effectively catalyze NO2- reduction in acidic solution.

Promoting electroreduction of nitrite to ammonia over electron-deficient Pd modulated by rectifying Schottky contacts

Electrochemical nitrite reduction reaction(NO_(2)^(-)RR) is a potential sustainable route for regulating the nitrogen cycle and ambient ammonia(NH_(3)) synthesis.However,it remains a challenge to precisely regulate the reaction pathways and inhibit competing reactions(e.g.hydrogenolysis) for efficient and selective NH_(3) production in an aqueous solution environment.Here,we utilize the Schottky barrier-induced surface electric field to construct high-density electron-deficient Pd nanoparticles by modulating the N content in the carbon carrier to promote the enrichment and immobilization of NO_(2)^(-)on the electrode surface,which ensures the ultimate selectivity for NH_(3).With these properties,Pd@N_(0.14)C with the highest N content achieved excellent catalytic performance for the reduction of NO_(2)^(-)to NH_(3) with the 100% Faraday efficiency at-0.5 and-0.6 V vs,reversible hydrogen electrode(RHE) for NH_(3) production,which was significantly better than Pd/C and Pd@N_(x)C samples with lower N content.This study opens new avenues for rational construction of efficient electrocatalysts for nitrite removal and NH_(3) electrosynthesis.

ITO@TiO_(2)nanoarray:An efficient and robust nitrite reduction reaction electrocatalyst toward NH_(3)production under ambient conditions

Ambient electrochemical nitrite(NO_(2)^(-))reduction is viewed as an effective and sustainable approach for simul-taneously removing NO_(2)^(-)and producing ammonia(NH_(3)).However,the complex multi-electron transfer steps involved in the NO_(2)^(-)reduction reaction(NO_(2)^(-)RR)lead to sluggish kinetics and low product selectivity toward NH_(3),underscoring the need for NH_(3)synthesis electrocatalysts with high activity and durability.Herein,we report amorphous indium-tin oxide sputtered on a TiO_(2)nanobelt array on a Ti plate(ITO@TiO_(2)/TP)as a 3D NH_(3)-producing catalyst for the NO_(2)^(-).In 0.5 M LiClO_(4)with 0.1 M NO_(2)^(-),it shows greatly boosted NO_(2)^(-)RR activity toward NH_(3)production,with excellent selectivity,achieving a large NH_(3)yield of 411.3μmol h^(-1)cm^(-2)and a high Faradaic efficiency of 82.6%.It also shows high durability for continuous electrolysis.A Zn-NO_(2)^(-)battery with ITO@TiO_(2)/TP cathode offers an NH_(3)yield of 23.1μmol h^(-1)cm^(-2)and a peak power density of 1.22 m.

Gliding arc discharge in combination with Cu/Cu_(2)O electrocatalysis for ammonia production

Highly efficient and green ammonia production is an important demand for modern agriculture.In this study,a two-step ammonia production method is developed using a gliding arc discharge in combination with Cu/Cu_(2)O electrocatalysis.In this method,NO_(x)is provided by the gliding arc discharge and then electrolyzed by Cu/Cu_(2)O after alkaline absorption.The electrical characteristics,the optical characteristics and the NO_(x)production are investigated in discharges at different input voltage and the gas flow.The dependence of ammonia production through Cu/Cu_(2)O electrocatalysis on pH value and reduction potential are determined by colorimetric method.In our study,two discharge modes are observed.At high input voltage and low gas flow,the discharge is operated with a stable plasma channel which is called the steady arc gliding discharge mode(A-G mode).As lowering input voltage and raising gas flow,the plasma channel is destroyed and high frequency breakdown occurs instead,which is known as the breakdown gliding discharge mode(B-G mode).The optimal NO_(x)production of 7.34 mmol h^(-1)is obtained in the transition stage of the two discharge modes.The ammonia yield reaches0.402 mmol h^(-1)cm^(-2)at pH value of 12.7 and reduction potential of-1.0 V versus reversible hydrogen electrode(RHE).

Powder Microelectrodes Applied to NO_2 Detection

The reduction of nitrite at Au or carbon electrode in-H_2SO_4 was found to follow achemical-electrochemical (CE) mechanism with a very thin (4×10(-8)cm) preceding reaction zone.It was proposed and experimentally verified that for such kind of electrode processes the totalreaction the could be effectively enhanced by using electrodes with increased true surface area.such as porous electrodes. As a combination of porous electrode and microelectrode, the powdermicroclectrode shows excellent performance for nitrite detection.

Unique Nafion-Os(bpy)3^(2+) Modified Electrodes

It was newly found that the electrodes modified by applying ethanol solutions of Nationcontaining os(bpy)32+ onto the substrate electrode (the one-step method) show two pairs of stableredox peaks of Os(bpy)32+/3+ on cyclic voltammogram near 0.54V and 0.25V, respectively. Thesemoditied electrodes can effectively mediate and catalyze the first and second steps of nitritereduction in acidic media in the potential region 0-0.9V when the loading in the coating (X=F(Os2+) / F(SO3-)) and pH in solution are below 0. 17 and 4, respectively. When X is between 0.33and 0. 17. only the current peak near 0.54V appears regardless of solution pH and only the first stepof NO2 reduction is catalyzed. Thus the modified electrode provides a very useful flexibility thatone can control the reaction pathway and catalytic activity of nitrite reduction by simply changingthe concentration of the mediator in the coating.