Sustainable ammonia synthesis:An in-depth review of non-thermal plasma technologies

Ammonia serves both as a widely used fertilizer and environmentally friendly energy source due to its high energy density,rich hydrogen content,and emissions-free combustion.Additionally,it offers convenient transportation and storage as a hydrogen carrier.The dominant method used for large-scale ammonia production is the Haber-Bosch process,which requires high temperatures and pressures and is energy-intensive.However,non-thermal plasma offers an eco-friendly alternative for ammonia synthesis,gaining significant attention.It enables ammonia production at lower temperatures and pressures using plasma technology.This review provides insights into the catalyst and reactor developments,which are pivotal for promoting ammonia efficiency and addressing existing challenges.At first,the reaction kinetics and mechanisms are introduced to gain a comprehensive understanding of the reaction pathways involved in plasma-assisted ammonia synthesis.Thereafter,the enhancement of ammonia synthesis efficiency is discussed by developing and optimizing plasma reactors and effective catalysts.The effect of other feeding sources,such as water and methane,instead of hydrogen is also presented.Finally,the challenges and possible solutions are outlined to facilitate energy-saving and enhance ammonia efficiency in the future.

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.

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.

A highly stable and active mesoporous ruthenium catalyst for ammonia synthesis prepared by a RuCl_3/SiO_2-templated approach

Molecular nitrogen is relatively inert and the activation of its triple bond is full of challenges and of significance.Hence,searching for an efficiently heterogeneous catalyst with high stability and dispersion is one of the important targets of chemical technology.Here,we report a Ba‐K/Ru‐MC catalyst with Ru particle size of 1.5–2.5 nm semi‐embedded in a mesoporous C matrix and with dual promoters of Ba and K that exhibits a higher activity than the supported Ba‐Ru‐K/MC catalyst,although both have similar metal particle sizes for ammonia synthesis.Further,the Ba‐K/Ru‐MC catalyst is more active than commercial fused Fe catalysts and supported Ru catalysts.Characterization techniques such as high‐resolution transmission electron microscopy,N2 physisorption,CO chemisorption,and temperature‐programmed reduction suggest that the Ru nanoparticles have strong interactions with the C matrix in Ba‐K/Ru‐MC,which may facilitate electron transport better than supported nanoparticles.

Study on Chemisorption and Desorption of Hydrogen and Nitrogen on Ru-based Ammonia Synthesis Catalyst

The effects of promoters K, Ba, Sm on the chemisorption and desorption of hydrogen and nitrogen, dispersion of metallic Ru. and catalytic activity of active carbon (AC) supported ruthenium catalyst for ammonia synthesis have been studied by means of pulse chromatography, temperature-programmed desorption, and activity test. Promoters K, Ba and Sm increased the activity of Ru/AC catalysts for ammonia synthesis significantly, and particularly, potassium exhibited the best promotion on the activity because of the strong electronic donation to metallic Ru. Much higher activity can be obtained for Ru/AC catalyst with binary or triple promoters. The activity of Ru/AC catalyst is dependent on the adsorption of hydrogen and nitrogen. The high activity of catalyst could be ascribed to strong dissociation of nitrogen on the catalyst surface. Strong adsorption of hydrogen would inhibit the adsorption of nitrogen, resulted in decrease of the catalytic activity. Ru/AC catalyst promoted by Sm2O3 shows the best dispersion of metallic Ru, since the partly reduced SmOx on the surface modifies the morphology of active sites and favors the dispersion of metallic Ru. The activity of Ru/AC catalysts is in accordance to the corresponding amount of nitrogen chemisorption and the desorption activation energy of nitrogen. The desorption activation energy for nitrogen decreases in the order of Ru>Ru-Ba>Ru-Sm>Ru-Ba-Sm>Ru-K>Ru-K-Sm>Ru-K-Ba>Ru-K-Ba-Sm, just opposite to the order of catalytic activity, suggesting that the ammonia synthesis over Ru-based catalyst is controlled by the step of dissociation of nitrogen.

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.

Role of surface defects of carbon nanotubes on catalytic performance of barium promoted ruthenium catalyst for ammonia synthesis

Carbon nanotubes(CNTs) with abundant surface defects are prepared by a liquid oxidation and thermal annealing method. The defective CNTs-D supported Ba–Ru/CNTs-D catalysts exhibit superior catalytic performance in ammonia synthesis with a TOF be increased up to 0.30 s-1, which is 2.5 times of oxidized CNTs-O supported Ba–Ru/CNTs-O catalysts and 5 times of the Ba–Ru/CNTs. The characterizations by CO chemisorption, transmission electron microscope, Raman, and X-ray photoelectron spectroscopy revealed that the uniformly well dispersed Ru NPs can be stabilized on the defective sites of CNTs-D. The great improvement of the catalytic performance and stability of the Ba–Ru/CNTs-D is contributed to the strong interaction between Ru NPs and surface defect of the CNTs.

Effects of Reaction Conditions on Performance of Ru Catalyst and Iron Catalyst for Ammonia Synthesis

Activated carbon-supported Ru-based catalyst and A301 iron catalyst were prepared,and the influences of reaction temperature,space velocity,pressure,and H2/N2 ratio on performance of iron catalyst coupled with Ru catalyst in series for ammonia synthesis were investigated.The activity tests were also performed on the single Ru and Fe catalysts as comparison.Results showed that the activity of the Ru catalyst for ammonia synthesis was higher than that of the iron catalyst by 33.5%-37.6% under the reaction conditions：375-400 °C,10 MPa,10000 h-1,H2︰N2 3,and the Ru catalyst also had better thermal stability when treated at 475 °C for 20 h.The outlet ammonia concentration using Fe-Ru catalyst was increased by 45.6%-63.5% than that of the single-iron catalyst at low tem-perature （375-400 °C）,and the outlet ammonia concentration increased with increasing Ru catalyst loading.

Ru surface density effect on ammonia synthesis activity and hydrogen poisoning of ceria-supported Ru catalysts

Evaluating the effect of metal surface density on catalytic performance is critical for designing high-activity metal-based catalysts.In this study,a series of ceria(CeCO_(2))-supported Ru catalysts(Ru/CeCO_(2))were prepared to analyze the effect of Ru surface density on the catalytic performance of Ru/CeCO_(2) for ammonia synthesis.For the Ru/CeCO_(2) catalysts with Ru surface densities lower than 0.68 Ru nm^(-2),the Ru layers were in close contact with CeCO_(2),and electrons were transferred directly from the CeCO_(2) defect sites to the Ru species.In such cases,the adsorption of hydrogen species on the Ru sites in the vicinity of 0 atoms was high,leading to a high ammonia synthesis activity and strong hydrogen poisoning.In contrast,the preferential aggregation of Ru species into large particles on top of the Ru overlayer resulted in the coexistence of Ru clusters and particles,for catalysts with a Ru surface density higher than 1.4 Ru nm^(-2),for which Ru particles were isolated from the direct electronic influence of CeCO_(2).Consequently,the Ru-Ceth interactions were weak,and hydrogen poisoning can be significantly alleviated.Overall,electron transfer and hydrogen adsorption synergistically affected the synthesis of ammonia over Ru/CeCO_(2) catalysts,and catalyst samples with a Ru surface density lower than 0.31 Ru nm^(-2) or exactly 2.1 Ru nm^(-2) exhibited high catalytic activity for ammonia synthesis.

Characterization and performance of Cu/ZnO/Al_2O_3 catalysts prepared via decomposition of M(Cu,Zn)-ammonia complexes under sub-atmospheric pressure for methanol synthesis from H_2 and CO_2

Methanol synthesis from hydrogenation of CO2 is investigated over Cu/ZnO/Al2O3 catalysts prepared by decomposition of M（Cu,Zn）-ammonia complexes （DMAC） at various temperatures.The catalysts were characterized in detail,including X-ray diffraction,N2 adsorption-desorption,N2O chemisorption,temperature-programmed reduction and evolved gas analyses.The influences of DMAC temperature,reaction temperature and specific Cu surface area on catalytic performance are investigated.It is considered that the aurichalcite phase in the precursor plays a key role in improving the physiochemical properties and activities of the final catalysts.The catalyst from rich-aurichalcite precursor exhibits large specific Cu surface area and high space time yield of methanol （212 g/（Lcat·h）;T=513 K,p=3MPa,SV=12000 h-1）.

Analysis on Ammonia Synthesis over Wustite-Based Iron Catalyst

Wustite-based catalyst for ammonia synthesis exhibits extremely high activity and easy to reduction under a wide range of conditions. The reaction kinetics of ammonia synthesis can be illustrated perfectly by both the classical Temkin-Pyzhev and modified Temkin equations with optimized a of 0.5. The pre-exponent factors and activation energies at the pressures of 8.0 and 15.0MPa are respectively k0 = 1.09 x 1015, 7.35 X 1014Pa0.5.s-1, and E = 156.6, 155.5kJ-mol-1 derived from the classical Temkin-Phyzhev equation, as well as k0 = 2.45 X 1014, 1.83 X 1014Pa0.5s-1, and E = 147.7, 147.2kJ-mol-1 derived from the modified Temkin equation. Although the degree of reduction under isothermal condition is primarily dependent upon temperature, low pressure seems to be imperative for reduction under high temperature and low space velocity to be considered as a high activity catalyst. The reduction behavior with dry feed gas can be illustrated perfectly by the shrinking-sphere-particle model, by which the reduction-rate constants of 4248exp (-71680/KT) and 644exp (-87260/RT) were obtained for the powder (0.045-0.054mm) and irregular shape (nominal diameter 3.17 mm) catalysts respectively. The significant effect of particle size on reduction rate was observed, therefore, it is important to take into account the influence of particle size on reduction for the optimization of reduction process in industry.

RAMAN SPECTRA OF HYDROGEN ADSPECIES ON AMMONIA SYNTHESIS IRON CATALYST

Raman peaks at 1951 and 2165 cm^(-1) can be confirmed further by H_2/D_2 isotope exchange as H-adspecies on the doubly promoted iron catalyst for ammonia synthesis and are probably ascribed to two terminally adsorbed H-species.

Study on the Carbon-Methanation and Catalytic Activity of Ru/AC for Ammonia Synthesis

The effects of promoters K, Ba, Sm on the resistance to carbon-methanation and catalytic activity of ruthenium supported on active carbon (Ru/AC) for ammonia synthesis have been studied by means of TG-DTG (thermalgravity-differential thermalgravity), temperature-programmed desorption, and activity test. Promoters Ba,K, and Sm increased the activity of Ru/AC catalysts for ammonia synthesis significantly. Much higher activity can be reached for Ru/AC catalyst with bi- or tri-promoters. Indeed, the triply promoted catalyst showed the highest activity, coupled to a surprisingly high resistance to methanation. The ability of resistance of promoter to methanation of Ru/AC catalyst is dependent on the adsorption intensity of hydrogen. The strong adsorption of hydrogen would enhance methanation and impact the adsorption of nitrogen, which results in the decrease of catalytic activity.

A theoretical study of electrocatalytic ammonia synthesis on single metal atom/MXene

Electrocatalytic ammonia synthesis under mild conditions is an attractive and challenging process in the earth’s nitrogen cycle,which requires efficient and stable catalysts to reduce the overpotential.The N2 activation and reduction overpotential of different Ti3C2O2-supported transition metal(TM)(Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Mo,Ru,Rh,Pd,Ag,Cd,and Au)single-atom catalysts have been analyzed in terms of the Gibbs free energies calculated using the density functional theory(DFT).The end-on N2 adsorption was more energetically favorable,and the negative free energies represented good N2 activation performance,especially in the presence Fe/Ti3C2O2(﹣0.75 eV).The overpotentials of Fe/Ti3C2O2,Co/Ti3C2O2,Ru/Ti3C2O2,and Rh/Ti3C2O2 were 0.92,0.89,1.16,and 0.84 eV,respectively.The potential required for ammonia synthesis was different for different TMs and ranged from 0.68 to 2.33 eV.Two possible potential-limiting steps may be involved in the process:(i)hydrogenation of N2 to*NNH and(ii)hydrogenation of*NH2 to ammonia.These catalysts can change the reaction pathway and avoid the traditional N–N bond-breaking barrier.It also simplifies the understanding of the relationship between the Gibbs free energy and overpotential,which is a significant factor in the rational designing and large-scale screening of catalysts for the electrocatalytic ammonia synthesis.

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.

Progress in electrocatalytic nitrate reduction for green energy:Catalyst engineering,mechanisms,and techno-economic feasibility

Ammonia(NH_(3))is an irreplaceable chemical that has been widely demanded to keep the sustainable development of modern society.However,its industrial production consumes a huge amount of energy and releases extraordinary greenhouse gases(GHGs),leading to various environmental issues.Achieving the green production of ammonia is a great challenge,which has been extensively pursued in the last decade.In this review,the most promising strategy,electrochemical nitrate reduction reaction(e-NO_(3)RR),is comprehensively investigated to give a complete understanding of its development and mechanism and provide guidance for future directions.However,owing to the complex reactions and limited selectivity,a comprehensive understanding of the mechanisms is crucial to further development and commercialization.Moreover,NO_(3)^(-)RR is a promising strategy for simultaneous water treatment and NH_(3)production.A detailed overview of the recent progress in NO_(3)^(-)RR for NH_(3)production with nontransition and transition metal based electrocatalysts is summarized.In addition,critical advanced techniques,future challenges,and prospects are discussed to guide future research on transition metal-based catalysts for commercial NH_(3)synthesis by NO_(3)^(-)reduction.

Plasma-assisted ammonia synthesis under mild conditions for hydrogen and electricity storage:Mechanisms,pathways,and application prospects

Ammonia,with its high hydrogen storage density of 17.7 wt.%(mass fraction),cleanliness,efficiency,and renewability,presents itself as a promising zero-carbon fuel.However,the traditional Haber−Bosch(H−B)process for ammonia synthesis necessitates high temperature and pressure,resulting in over 420 million tons of carbon dioxide emissions annually,and relies on fossil fuel consumption.In contrast,dielectric barrier discharge(DBD)plasma-assisted ammonia synthesis operates at low temperatures and atmospheric pressures,utilizing nitrogen and hydrogen radicals excited by energetic electrons,offering a potential alternative to the H−B process.This method can be effectively coupled with renewable energy sources(such as solar and wind)for environmentally friendly,distributed,and efficient ammonia production.This review delves into a comprehensive analysis of the low-temperature DBD plasma-assisted ammonia synthesis technology at atmospheric pressure,covering the reaction pathway,mechanism,and catalyst system involved in plasma nitrogen fixation.Drawing from current research,it evaluates the economic feasibility of the DBD plasmaassisted ammonia synthesis technology,analyzes existing dilemmas and challenges,and provides insights and recommendations for the future of nonthermal plasma ammonia processes.

The precursor phase composition of iron catalyst and discovery of FeO based catalyst for ammonia synthesis

The relationship between the activity and the precursor phase composition of the molten iron catalyst for ammonia synthesis has been studied with high pressure testing equipment and XRD. A humped curve between the activity and Fe2+/Fe3+ has been obtained. It is found that the unicity of the iron oxidate phase in precursor is an essential condition of the high activity of the iron catalyst and that the uniform distribution of the adominant phase and the promoters is the key to preparing a catalyst with better performance The humped curve is interpreted using the ratio f of the phase compositions in precursor. A new idea has been obtained that the activity change of the molten iron catalyst depends essentially on the molecule ratio of the different iron oxidates in precursor under the certain promoters, and it is found that the FeO based catalyst for ammonia synthesis with Wustite phase structure (Fe1-xO, 0.04≤x≤0.10) has the highest activity of all the molten iron catalysts for ammonia synthesis.

Effect of barium and potassium promoter on Co/CeO_2 catalysts in ammonia synthesis

The Co/CeO2 catalysts promoted with Ba or K were prepared to study the effect of promoter on the catalytic performance of ammonia synthesis. The results show that the presence of Ba or K promoter changes the properties of CeO2-supported Co catalysts including the surface area, the crystallite size and the morphology of CeO2, the reduction degree of cobalt species and the adsorption performance of hydrogen and nitrogen. As a consequence, the samples promoted with an appropriate amount of Ba show higher ammonia synthesis rates, while the catalysts with high Ba loading or K promoter all exhibit low catalytic activities.

Anchoring Mo on C_(9)N_(4) monolayers as an efficient single atom catalyst for nitrogen fixation

Electrochemical nitrogen fixation via a convenient and sustainable manner,exhibits an intriguing prospect for ammonia generation under ambient conditions.Currently,the design and development of high-efficiency and low-cost electrocatalysts remains the major challenge confronting nitrogen reduction reaction(NRR).Herein,anchoring the single Mo atom on the C_(9)N_(4) substrate(Mo@C_(9)N_(4)) to form an efficient single-atom catalyst(SAC) is proposed for the conversion of N2 to NH3.By employing density functional theory(DFT) calculations,we demonstrated that gas phase N2 can be sufficiently activated and efficiently reduced to NH3 on the surface of Mo@C_(9)N_(4).Meanwhile,we found that the NRR dominantly occurred on the Mo center via a preferred distal pathway with favorable limiting potential of 0.40 V.Importantly,the as-established Mo@C_(9)N_(4) catalyst exhibits an outstanding structural stability and good selectivity toward NRR.These findings provide a promising platform for designing Mo-based SACs for electrochemical N2 fixation.