Exploring nitrogen reduction reaction mechanisms in electrocatalytic ammonia synthesis:A comprehensive review

The electrochemical nitrogen reduction reaction(eNRR)holds significant promise as a sustainable alternative to the conventional large-scale Haber Bosch process,offering a carbon footprint-free approach for ammonia synthesis.While the process is thermodynamically feasible at ambient temperature and pressure,challenges such as the competing hydrogen evolution reaction,low nitrogen solubility in electrolytes,and the activation of inert dinitrogen(N_(2))gas adversely affect the performance of ammonia production.These hurdles result in low Faradaic efficiency and low ammonia production rate,which pose obstacles to the commercialisation of the process.Researchers have been actively designing and proposing various electrocatalysts to address these issues,but challenges still need to be resolved.A key strategy in electrocatalyst design lies in understanding the underlying mechanisms that govern the success or failure of the electrocatalyst in driving the electrochemical reaction.Through mechanistic studies,we gain valuable insights into the factors affecting the reaction,enabling us to propose optimised designs to overcome the barriers.This review aims to provide a comprehensive understanding of the various mechanisms involved in eNRR on the electrocatalyst surface.It delves into the various mechanisms such as dissociative,associative,Mars-van Krevelen,lithium-mediated nitrogen reduction and surface hydrogenation mechanisms of nitrogen reduction.By unravelling the intricacies of eNRR mechanisms and exploring promising avenues,we can pave the way for more efficient and commercially viable ammonia synthesis through this sustainable electrochemical process by designing an efficient electrocatalyst.

Overview of emerging catalytic materials for electrochemical green ammonia synthesis and process

The concept of“green-ammonia-zero-carbon emission”is an emerging research topic in the global community and many countries driving toward decarbonizing a diversity of applications dependent on fossil fuels.In light of this,electrochemical nitrogen reduction reaction(ENRR)received great attention at ambient conditions.The low efficiency(%)and ammonia(NH_(3))production rates are two major challenges in making a sustainable future.Besides,hydrogen evolution reaction is another crucial factor for realizing this NH_(3)synthesis to meet the large-scale commercial demand.Herein,the(i)importance of NH_(3)as an energy carrier for the next future,(ii)discussion with ENRR theory and the fundamental mechanism,(iii)device configuration and types of electrolytic systems for NH_(3)synthesis including key metrics,(iv)then moving into rising electrocatalysts for ENRR such as single-atom catalysts(SACs),MXenes,and metal–organic frameworks that were scientifically summarized,and(v)finally,the current technical contests and future perceptions are discussed.Hence,this review aims to give insightful direction and a fresh motivation toward ENRR and the development of advanced electrocatalysts in terms of cost,efficiency,and technologically large scale for the synthesis of green NH_(3).

Li^(+)-ion bound crown ether functionalization enables dual promotion of dynamics and thermodynamics for ambient ammonia synthesis

Electrosynthesis of ammonia from the reduction of nitrogen is still confronted with the limited supply of gas reactant in dynamics as well as high activation barrier in thermodynamics.Unfortunately,despite tremendous efforts devoted to electrocatalysts themselves,they still fail to tackle the above two challenges simultaneously.Herein,we employ a heterogeneous catalyst adlayer-composed of crown ethers associated with Li^(+)ions-to achieve the dual promotion of dynamics and thermodynamics for ambient ammonia synthesis.Dynamically,the bound Li^(+)ions interact with the strong quadrupole moment of nitrogen,and trigger considerable reactant flux toward the catalyst.Thermodynamically,Li^(+)associated with the oxygen of crown ether achieves a higher density of states at the Fermi level for the catalyst,enabling effortless electron transfer from the catalysts to nitrogen and thus greatly reducing the activation barrier.As expected,the proof-of-concept system achieves an ammonia yield rate of 168.5μg h^(-1)mg^(-1)and a Faradaic efficiency of 75.3%at-0.3 V vs.RHE.This system-level approach opens up pathways for tackling the two key challenges that have limited the field of ammonia synthesis.

Effect of the graphitic degree of carbon supports on the catalytic performance of ammonia synthesis over Ba-Ru-K/HSGC catalyst

A series of high surface area graphitic carbon materials （HSGCs） were prepared by ball-milling method. Effect of the graphitic degree of HSGCs on the catalytic performance of Ba-Ru-K/HSGC-x （x is the ball-milling time in hour） catalysts was studied using ammonia synthesis as a probe reaction. The graphitic degree and pore structure of HSGC-x supports could be successfully tuned via the variation of ball-milling time. Ru nanoparticles of different Ba-Ru-K/HSGC-x catalysts are homogeneously distributed on the supports with the particle sizes ranging from 1.6 to 2.0 nm. The graphitic degree of the support is closely related to its facile electron transfer capability and so plays an important role in improving the intrinsic catalytic performance of Ba-Ru-K/HSGC-x catalyst.

Novel Ru - K/Carbon Nanotubes Catalyst for Ammonia Synthesis

A novel ammonia synthesis catalyst, potassium-promoted ruthenium supported on carbon nanotubes, was developed. It was found that the Ru-K/carbon nanotubes catalyst had higher activity for ammonia synthesis (20.85 ml NH3/h/g-cat) than the Ru-K/fullerenes ( 13.3 ml NH3/h/g-cat) at atmospheric pressure and 623 K. The catalyst had activity even at 473 K, and had the highest activity( 23.46 ml NH3/h/g-cat) at 643 K. It was suggested that the multi-walled structure favored the electron transfer, the hydrogen-storage and the hydrogen-spill which were favorable to ammonia synthesis.

Novel Ruthenium Catalyst (K-Ru/C_(60/70)) for Ammonia Synthesis

A novel ammonia-synthesis catalyst. potassium-promoted ruthenium supported on fullerene (K-Ru/C60/70 ). was prepared and evaluated, It was found that K-Ru/C60/70 was the most active catalyst for ammonia synthesis at atmospheric pressure and 623 K compared with other support materials such as silica, activated carbon. zeolite, λ-Al2O3 and rare earth metal oxide.

A novel fused iron catalyst for ammonia synthesis promoted with rare earth gangue

Rare earth gangue, which mainly consists of mixtures of light rare earths such as lanthana, ceria, neodymium oxide and praseodymium oxide, was used as the promoter of fused iron catalysts for ammonia synthesis. The result showed that the activity of the catalyst promoted with rare earth gangue was comparable with those of commercial iron catalysts with high amount of cobalt. The role of rare earths was owed to their advantages for favoring the deep reduction of the main composite in catalyst, i.e., iron oxide. This fmding indicated that the use of rare earth gangue could decrease the content of cobalt or even completely replace cobalt, which was used to be regarded as unsub- stitutable promoters for high performance ammonia catalyst; therefore, the cost of fused iron catalysts would decrease significantly.

Lowering reaction temperature: Electrochemical ammonia synthesis by coupling various electrolytes and catalysts

Ammonia is a vital emerging energy carrier and storage medium in the future hydrogen economy, even presenting relevant advantages compared with methanol due to the higher hydrogen content(17.6 wt% for ammonia versus 12.5 wt% for methanol). The rapidly growing demand for ammonia is still dependent on the conventional high-temperature and high-pressure Haber–Bosch process, which can deliver a conversion rate of about 10%–15%. However, the overall process requires a large amount of fossil fuels,resulting in serious environmental problems. Alternatively, electrochemical routes show the potential to greatly reduce the energy consumption, including sustainable energy sources and simplify the reactor design. Electrolytes perform as indispensable reaction medium during electrochemical processes, which can be further classified into solid oxide electrolytes, molten salt electrolytes, polymer electrolytes, and liquid electrolytes. In this review, recent developments and advances of the electrocatalytic ammonia synthesis catalyzed by a series of functional materials on the basis of aforementioned electrolytes have been summarized and discussed, along with the presentation and evaluation of catalyst preparation, reaction parameters and equipment.

Effect of Activated Carbon as a Support on Metal Dispersion and Activity ofRuthenium Catalyst for Ammonia Synthesis

Ten kinds of activated carbon from different raw materials were used as supports to prepare ruthenium catalysts. N_2 physisorption and CO chemisorption were carried out to investigate the pore size distribution and the ruthenium dispersion of the catalysts. It was found that the Ru dispersion of the catalyst was closely related to not only the texture of carbon support but also the purity of activated carbon. The activities of a series of the carbon-supported barium-promoted Ru catalysts for ammonia synthesis were measured at 425 ℃, 10 0 MPa and 10 000 h -1. The result shows that the same raw material activated carbon, with a high purity, high surface area, large pore volume and reasonable pore size distribution might disperse ruthenium and promoter sufficiently, which activated carbon as support, could be used to manufacture ruthenium catalyst with a high activity for ammonia synthesis. The different raw material activated carbon as the support would greatly influence the catalytic properties of the ruthenium catalyst for ammonia synthesis. For example, with coconut shell carbon(AC1) as the support, the ammonia concentration in the effluent was 13 17% over 4%Ru-BaO/AC1 catalyst, while with the desulfurized coal carbon(AC10) as the support, that in the effluent was only 1 37% over 4%Ru-BaO/AC10 catalyst.

Improving the ammonia synthesis activity of Ru/CeO_(2) through enhancement of the metal–support interaction

The metal–support interactions induced by high-temperature hydrogen reduction have a strong influence on the catalytic performance of ceria-supported Ru catalysts. However, the appearance of the strong metal–support interaction leads to covering of the Ru species by Ce suboxides, which is detrimental to the ammonia synthesis reaction that requires metallic species as active sites. In the present work, the interaction between Ru and ceria in the Ru/CeO_(2) catalyst was induced by NaBH_(4) treatment. NaBH_(4) treatment enhanced the fraction of metallic Ru, proportion of Ce^(3+), content of exposed Ru species, and amount of surface oxygen species. As a result, a larger amount of hydrogen species would desorb by the H_(2)-formation pathway and the strength of hydrogen adsorption would be weaker, weakening the inhibition effect of the hydrogen species on ammonia synthesis. In addition, the strong electronic metal–support interaction aids in nitrogen dissociation. Consequently, Ru/CeO_(2) with NaBH_(4) treatment showed higher ammonia synthesis rates than that with only hydrogen reduction.

A photo-assisted electrochemical-based demonstrator for green ammonia synthesis

Bridging laboratory research and practical utilization is of crucial importance for the development of green ammonia synthetic technologies. A decentralized photo-assisted electrochemical-based demonstrator has been proposed for green ammonia synthesis from renewable electricity, air and water, where well-known defect-laden WO_(3) is used as the working electrode, and a commercially available PV panel supplies renewable electricity. In this demonstrator, defect-laden WO_(3) exhibits the optimum electrochemical NH_(3) formation rate(4.51 × 10^(-12)mol s^(-1)cm^(-2)) in 0.1 M K_(2)SO_(4)in a photovoltaic electrochemical(PV-EC) system. A system-level energy and cost analysis was conducted to investigate its economic viability and a general evaluation tool for system performance and cost estimation was proposed. This advance enables the possibility of integrating the small-scale green ammonia demonstrator into a stand-alone farm system.

Lanthanum-Promoted Ru/Sepiolite in Ammonia Synthesis

A new kind of Ru supported on sepiolite catalyst with La as promoter for ammonia synthesis was prepared. The effects of reaction conditions on catalytic activity were discussed. The result shows that La is an effective promoter for sepiolite-supported Ru based catalyst. When the load of Ru is 5% (mass fraction), and the molar ratio of La/Ru is 1.5, under the condition of 10 MPa 450 ℃ 20000 h-1, the ammonia synthesis rate is 38.5 mmol NH3·g-1·h-1.

Highly efficient subnanometer Ru-based catalyst for ammonia synthesis via an associative mechanism

The industrial manufacture of ammonia(NH_(3))using Fe-based catalyst works under rigorous conditions.For the goal of carbon-neutrality,it is highly desired to develop advanced catalyst for NH_(3)synthesis at mild conditions to reduce energy consumption and CO_(2)emissions.However,the main challenge of NH_(3)synthesis at mild conditions lies in the dissociation of steady N≡N triple bond.In this work,we report the design of subnanometer Ru clusters(0.8 nm)anchored on the hollow N-doped carbon spheres catalyst(Ru-SNCs),which effectively promotes the NH_(3)synthesis at mild conditions via an associative route.The NH_(3)synthesis rate over Ru-SNCs(0.49%(mass)Ru)reaches up to 11.7 mmol NH_(3)·(g cat)^(-1)·h^(-1) at 400℃ and 3 MPa,which is superior to that of 8.3 mmol NH_(3)·(g cat)^(-1)·h^(-1) over Ru nanoparticle catalyst(1.20%(mass)Ru).Various characterizations show that the N_(2)H_(4)species are the main intermediates for NH_(3)synthesis on Ru-SNCs catalyst.It demonstrates that Ru-SNCs catalyst can follow an associative route for N_(2)activation,which circumvents the direct dissociation of N_(2)and results in highly efficient NH_(3)synthesis at mild conditions.

Visualization of atomic scale reaction dynamics of supported nanocatalysts during oxidation and ammonia synthesis using in-situ environmental(scanning) transmission electron microscopy

Reaction dynamics in gases at operating temperatures at the atomic level are the basis of heterogeneous gas-solid catalyst reactions and are crucial to the catalyst function.Supported noble metal nanocatalysts such as platinum are of interest in fuel cells and as diesel oxidation catalysts for pollution control,and practical ruthenium nanocatalysts are explored for ammonia synthesis.Graphite and graphitic carbons are of interest as supports for the nanocatalysts.Despite considerable literature on the catalytic processes on graphite and graphitic supports,reaction dynamics of the nanocatalysts on the supports in different reactive gas environments and operating temperatures at the single atom level are not well understood.Here we present real time in-situ observations and analyses of reaction dynamics of Pt in oxidation,and practical Ru nanocatalysts in ammonia synthesis,on graphite and related supports under controlled reaction environments using a novel in-situ environmental(scanning) transmission electron microscope with single atom resolution.By recording snapshots of the reaction dynamics,the behaviour of the catalysts is imaged.The images reveal single metal atoms,clusters of a few atoms on the graphitic supports and the support function.These all play key roles in the mobility,sintering and growth of the catalysts.The experimental findings provide new structural insights into atomic scale reaction dynamics,morphology and stability of the nanocatalysts.

Target-oriented confinement of Ru-Co nanoparticles inside N-doped carbon spheres via a benzoic acid guided process for high-efficient low-temperature ammonia synthesis

Ru-based heterogeneous catalysts have been used in a wide range of important reactions.However,due to the sintering of Ru nanoparticles their practical applications are somewhat restricted.Herein,for the first time we report a new and facile strategy to confine Ru and/or Co nanoparticles(NPs) in the channels of N-doped carbon using benzoic acid to guide the deposition location of Ru.The developed catalyst with confined RuCo alloy particles exhibits high resistance against Ru sintering and displays excellent activity and long term stability for NH3 synthesis,achieving an NH3 synthesis rate of up to 18.9 mmol NH_(3) gcat^(-1)h^(-1)at 400℃,which is ca.2.25 times that of the catalyst prepared without confinement(with metal deposited on the support surface).In the latter case,there is an increase of nanoparticle size from 2.52 to 4.25 nm together with ca.48% decrease of NH_(3) synthesis rate after 68 h at 400℃.This study provides a new avenue for simple fabrication of precious-metal-based catalysts that are highly resistant against sintering,specifically suitable for low-temperature synthesis of ammonia with outstanding efficiency.

The pitfalls in electrocatalytic nitrogen reduction for ammonia synthesis

Ammonia synthesis by electrocatalytic nitrogen reduction reaction(EC-NRR)has gained momentum in recent years fueled by its potential to operate at ambient conditions,unlike the highly energyintensive yet long-standing Haber-Bosch process.However,the large disparity of the yields and Faradic efficiencies reported for EC-NRR raises serious concerns about the reliability of the experimental results.In this perspective,we elaborate on the potential sources of error when assessing EC-NRR and update the testing protocols to circumvent them,and more importantly,we pose a general call for consensus on ammonia production analysis and reporting to lay the solid foundations that this burgeoning field requires to thrive.

Plasma-assisted ammonia synthesis in a packed-bed dielectric barrier discharge reactor:roles of dielectric constant and thermal conductivity of packing materials

In this article,plasma-assisted NH;synthesis directly from N;and H;over packing materials with different dielectric constants(BaTiO_(2),TiO_(2) and SiO_(2))and thermal conductivities(Be O,Al N and Al_(2)O_(2))at room temperature and atmospheric pressure is reported.The higher dielectric constant and thermal conductivity of packing material are found to be the key parameters in enhancing the NH;synthesis performance.The NH;concentration of 1344 ppm is achieved in the presence of BaTiO_(2),which is 106%higher than that of SiO_(2),at the specific input energy(SIE)of 5.4 k J·l^(-1).The presence of materials with higher dielectric constant,i.e.BaTiO_(2) and TiO_(2)in this work,would contribute to the increase of electron energy and energy injected to plasma,which is conductive to the generation of chemically active species by electron-impact reactions.Therefore,the employment of packing materials with higher dielectric constant has proved to be beneficial for NH;synthesis.Compared to that of Al_(2)O_(3),the presence of Be O and Al N yields 31.0%and 16.9%improvement in NH;concentration,respectively,at the SIE of5.4 k J·l^(-1).The results of IR imaging show that the addition of Be O decreases the surface temperature of the packed region by 20.5%to 70.3℃ and results in an extension of entropy increment compared to that of Al_(2)O_(3),at the SIE of 5.4 k J·l^(-1).The results indicate that the presence of materials with higher thermal conductivity is beneficial for NH;synthesis,which has been confirmed by the lower surface temperature and higher entropy increment of the packed region.In addition,when SIE is higher than the optimal value,further increasing SIE would lead to the decrease of energy efficiency,which would be related to the exacerbation in reverse reaction of NH;formation reactions.

Enhancing Green Ammonia Electrosynthesis Through Tuning Sn Vacancies in Sn‑Based MXene/ MAX Hybrids

Renewable energy driven N_(2) electroreduction with air as nitrogen source holds great promise for realizing scalable green ammonia production.However,relevant out-lab research is still in its infancy.Herein,a novel Sn-based MXene/MAX hybrid with abundant Sn vacancies,Sn@Ti_(2)CTX/Ti_(2)SnC–V,was synthesized by controlled etching Sn@Ti_(2)SnC MAX phase and demonstrated as an efficient electrocatalyst for electrocatalytic N2 reduction.Due to the synergistic effect of MXene/MAX heterostructure,the existence of Sn vacancies and the highly dispersed Sn active sites,the obtained Sn@Ti2CTX/Ti_(2)SnC–V exhibits an optimal NH_(3) yield of 28.4μg h^(−1) mg_(cat)^(−1) with an excellent FE of 15.57% at−0.4 V versus reversible hydrogen electrode in 0.1 M Na_(2)SO_(4),as well as an ultra-long durability.Noticeably,this catalyst represents a satisfactory NH3 yield rate of 10.53μg h^(−1) mg^(−1) in the home-made simulation device,where commercial electrochemical photovoltaic cell was employed as power source,air and ultrapure water as feed stock.The as-proposed strategy represents great potential toward ammonia production in terms of financial cost according to the systematic technical economic analysis.This work is of significance for large-scale green ammonia production.

Recent advances and challenges of nitrogen/nitrate electro catalytic reduction to ammonia synthesis

The Haber-Bosch process is the most widely used synthetic ammonia technology at present.Since its invention,it has provided an important guarantee for global food security.However,the traditional Haber-Bosch ammonia synthesis process consumes a lot of energy and causes serious environmental pollution.Under the serious pressure of energy and environment,a green,clean,and sustainable ammonia synthesis route is urgently needed.Electrochemical synthesis of ammonia is a green and mild new method for preparing ammonia,which can directly convert nitrogen or nitrate into ammonia using electricity driven by solar,wind,or water energy,without greenhouse gas and toxic gas emissions.Herein,the basic mechanism of the nitrogen reduction reaction(NRR)to ammonia and nitrate reduction reaction(NO_(3)^(-))to ammonia were discussed.The representative approaches and major technologies,such as lithium mediated electrolysis and solid oxide electrolysis cell(SOEC)electrolysis for NRR,high activity catalyst and advanced electrochemical device fabrication for(NO_(3)^(-))RR and electrochemical ammonia synthesis were summarized.Based on the above discussion and analysis,the main challenges and development directions for electrochemical ammonia synthesis were further proposed.