Ammonia synthesis by nonthermal plasma catalysis: a review on recent research progress

Ammonia is one of the most important industrial chemicals which is commonly used for producing fertilizers and cleaning solutions, as the refrigerant gas, and as the precursors for making various chemicals. With the goal of sustainable development, ammonia is also proposed as the clean fuel for decarbonized transportation. The current the Haber–Bosch process for ammonia synthesis has large footprint and operates under harsh conditions using fossil fuels as the feedstock, being recognized as the major carbon emission source. Accordingly, call for sustainable production of green ammonia using renewable energies is proposed. Ammonia synthesis assisted by nonthermal plasmas has emerged in recent years as a novel and mild electrified technology, which can potentially be coupled with intermittent renewable energies and green hydrogen. Although being promising, significant development is still needed to advance the technology towards practical applications at scales. Hence, this review comments the progression of key aspects of the plasma-assisted ammonia synthesis such as catalyst and reactor design, mechanistic understanding, and process parameters. The snapshot of the current developments and proposed perspectives hope to provide guidance for the future research efforts to drive the technology towards higher technology readiness levels.


Introduction to ammonia synthesis
Ammonia (NH 3 ) is a colorless gas with a distinct pungent odor which holds great importance in various industrial processes, such as production of nitrogen-based fertilizers and other relevant chemicals (e.g.nitric acid and amines).Haber-Bosch process is the principal method for ammonia synthesis, which involves several steps including (i) nitrogen (N 2 ) separation from air, (ii) hydrogen (H 2 ) production from fossil fuels via steam reforming processes and (iii) catalytic conversion of N 2 and H 2 over iron-based catalysts to form ammonia under harsh conditions of 450 • C −550 • C and 200−300 bar [1,2].The conventional NH 3 synthesis processes are energy demanding with huge energy consumption (30 GJ/t NH3 ) and significant carbon emissions (2 kg CO2 /kg NH3 ) [3].Hence, to answer the challenges of sustainable development towards low-carbon industry and society, the development of energy-efficient and environmentally friendly alternatives to the conventional Haber-Bosch processes is urgently needed to achieve renewable ammonia synthesis, and this is exemplified by electrochemical synthesis [4], photochemical synthesis [5], and plasma-assisted synthesis [6,7].Among these techniques, plasma-assisted synthesis is the electrified process with the key features of mild conditions (theoretically ambient temperature and pressure) and short induction time, which can be coupled with renewable energy and green hydrogen, being a promising technical solution for distributed ammonia synthesis.
Plasma is a highly reactive medium that can be generated by applying a high electric field in gas.Depending on the thermal equilibrium between electrons and heavy particles, plasmas can be roughly categorized as thermal plasma (where the electron temperature equals the temperature of heavy particles) and nonthermal plasma (NTP, where the electron temperature is much higher than that of heavy particles at room temperature).The non-equilibrium nature of NTP offers unique opportunities for energy and gas conversions.Molecules like H 2 and N 2 that require high temperatures and/or high pressures to activate through thermochemical processes can be activated in NTP through collisions with high-energy species (such as free electrons), leading to excitation and/or dissociation [8].Short-lived reaction intermediates, including ions, free radicals, electronically and vibrationally excited species, can achieve chemical transformations and pathways that are unattainable by conventional thermal catalysis when they collide with catalytic surfaces [9].Therefore, NTP catalysis can facilitate numerous energy-intensive heterogeneous catalytic reactions which are kinetically and/or thermodynamically limited, such as CO 2 hydrogenation and reforming [10][11][12], pollutant decomposition [13,14], and ammonia synthesis [15,16].
In the context of ammonia synthesis, although plasma discharges adeptly activate and dissociate molecular N 2 and H 2 , the effective steering of the conversions towards NH 3 formation remains an imperative challenge.The inclusion of relevant catalysts in NTP discharge is beneficial to improving NH 3 synthesis rate, as evidenced by both experimental [16,17] and simulation [18][19][20] studies.The catalysts, through their intrinsic physical and chemical features, can modulate the interplay between plasma discharges and surface reactions, effectively improving the concentration of critical active NH x intermediates at the catalytic interface and within the gas discharge.In other words, the NTP catalysis assists NH 3 formation effectively combining the effective activation of reactants by plasmas with the high selectivity of surface reactions inherent in catalysts.The design and development of effective catalysts suitable for the NTP catalysis necessitate a thorough understanding of the mechanisms involved, including discharge characteristics, transport of reactive species, and surface reactions.Consequently, computational simulations and in-situ and/or operando techniques are needed probe and reveal these mechanisms, being beneficial to establish rational design perspectives which can enhance catalytic activity.To improve the efficiency of the NTP catalysis regarding both energy and economy, reactor engineering and process optimization are necessary as well.Currently, the energy efficiency of the Haber-Bosch process for ammonia synthesis is about 90-100 g NH3 kWh −1 , which is much higher than that achieved by the current alternative technologies.Regarding the plasma-assisted processes, according to its attributes, it can be suitable for small-scale, distributed production, and to make the technology technically competitive, the energy efficiency needs to be >45 g NH3 kWh −1 [21], thus significant technical developments are needed.
Here this review comments on the state-of-the-art development in the catalysts used for the NTP-assisted ammonia synthesis and discuss the relevant catalyst design features based on the current understanding of the hybrid system.To facilitate the latter, current studies into the reaction mechanisms, such as the reaction pathways for ammonia synthesis under NTP conditions, were also visited.Additionally, the process and reactor engineering efforts to improve the NTP-assisted ammonia synthesis were also discussed critically.The review provides a timely and critical snapshot of the current research in relevant fields, which can help the further progression of the hybrid technology towards applications in reality.

Metal catalysts
Metals, often serving as active sites, are loaded onto carriers through different methods such as incipient wetness impregnation [22] and ball milling [23] for promoting ammonia synthesis.In the industrial context, thermal catalytic synthesis of ammonia commonly employs iron (Fe)-based catalysts.However, experimental evidence suggests that Fe is not the optimal metal for the NTP-catalytic processes since the rate-limiting steps and reaction mechanisms in the NTPcatalysis differ from that in thermal catalytic counterparts.Specifically, in thermal catalysis, N 2 decomposition on active sites is believed to be the rate-limiting step, whilst in NTP catalysis, N 2 can be activated in gas discharge, and consensus  on the rate-limiting step in such hybrid systems is not yet reached [18][19][20]24].The precious metal ruthenium (Ru) was considered to be one of the most active catalysts for ammonia synthesis regardless the activation means [20,25].Under NTP conditions, Ru catalysts were one of the major candidates (along with Ni catalysts) to be explored for catalyst design to improve ammonia synthesis, as shown in table 1, possibly due to their higher activity at low temperatures (being relevant to the milder NTP conditions).Also, previous studies also suggested that Ru can promote hydrogen spillover to provide the dissociated hydrogen to the adsorbed N species on the support, and thus the hydrogenation reactions for NH 3 formation [16,26].In addition, Zhang et al also reported that hydrogen poisoning on Ru catalysts could be mitigated in the NTP-catalytic ammonia synthesis [16].
Patil et al conducted a comparative study of 16 different metal catalysts (all supported on γ-Al 2 O 3 ) for NTP-catalytic ammonia synthesis in a dielectric barrier discharge (DBD) reactor (as shown in figure 1) [27] for Co/Al 2 O 3 ) [28].This could be attributed to the changed reaction mechanisms between thermal and NTP systems [29].
However, a mechanistic understanding on the activity difference of various supported metal catalysts is still lacking till now.In addition, findings by Herrera et al also suggest that the variation in the supported metal catalysts does not alter the intrinsic characteristics of bulk plasma discharge.Hence, the design of ammonia catalysts with the intrinsic high activity under the conditions relevant to NTP conditions (i.e. at <200 • C and ambient pressure) is necessary.As shown in table 1, Ni based catalysts are also the popular choices for promoting the NTP-catalytic ammonia synthesis.The selection of Ni can be rational.A recent study by Carreon et al (based on pure bulk metals) has suggested that Ni is a good hydrogen sink catalyst, which can stabilize the atomic H (produced by gas discharge) for the hydrogenation of N species towards ammonia synthesis under NTP conditions [29], providing the key rationale for catalyst design based on Ni (as shown in table 1).
Secondary metal doping is an effective strategy to tune the intrinsic property of metal catalysts in thermal catalysis (i.e.bimetallic catalysts) since it may improve the physicochemical properties (and hence the activity and selectivity) of the catalysts significantly due to the relevant synergistic effects.For the NTP catalysis under discussion, Liu et al reported the enhanced activity of the bimetallic Co-Ni/Al 2 O 3 catalyst (with 5 wt.% metal loading and the molar ratio of 1:1) in the NTP-catalytic ammonia synthesis compared to the 5 wt.% monometallic Co/Al 2 O 3 and Ni/Al 2 O 3 catalysts [30], and attributed the activity enhancement to (i) the reduction in surface acidity on the bimetallic catalyst (which was believed to benefit NH 3 desorption) and (ii) the plasma discharge (e.g. the increased average electric field).However, the conclusions are not very convincing since thorough understanding of the properties of the metallic phases is lacking, neither the surface reactions on them in situ.Regarding the second claim, it is contradictory to the findings by Herrera et al [28], necessitating further detailed investigation.The activity of Ru can also be altered by secondary metal doping.For instance, Liu et al compared the activity of different bimetallic Ru-M/BaTiO 3 catalysts (where M = Fe, Co, and Ni, with 3 wt.%metal loading and the molar ratio of 1:1), and the findings show that Ni doping improved the energy yield of NH 3 under the NTP conditions (i.e.0.39 g NH3 kWh −1 for Ru-Ni/BaTiO 3 ), whilst Fe and Co doping showed the detrimental effect [31].The authors hypothesized that the Ni doing affected both the Ru metal and the BaTiO 3 support by electron donation to Ru sites and partial substitution of Ti 4+ .The former effect was believed to enhance the electron transfer from the d orbital of Ru to the anti-bonding orbital of N 2 (to weaken the triple bond of the adsorbed N 2 ), as proposed previously by Peng et al.The latter was found to increase the surface concentration of oxygen vacancy (OV) which could improve the interaction between N 2 and the support [32].However, these aspects need further studies since the roles of the adsorbed N 2 and that activated in gas discharge in the NTP-catalysis are not yet fully understood.Additionally, the findings also show that the Ni doping also increased the basicity of the Ru-Ni/BaTiO 3 catalyst (54.2 µmol g −1 by CO 2 -TPD) compared to that of the monometallic Ru/BaTiO 3 (31.9µmol g −1 ) and reduced the strong metal-support interaction between Ru and BaTiO 3 which were proposed to benefit the ammonia synthesis efficiency [31].Similar to the thermal catalytic ammonia synthesis [33], the use of promoters (such as alkali and rare earth metal compounds) was also reported as the effective means to enhance the catalytic activity of metal catalysts in NTP-catalytic ammonia synthesis.For Ru/γ-Al 2 O 3 catalysts, Mg was found as the most effective promoter (in comparison with K and Cs) to improve the activity likely due to electronic promotion of N 2 dissociation on catalyst surface (for the plasma-induced vibrationally excited N 2 ) [34,35].
Overall, in the NTP-catalysis, the intrinsic activity of the metal phases (relevant to the NTP conditions) is an important aspect needs to be carefully investigated to advance the development.For example, the size of metal nanoparticles was found play a crucial role in the plasma-assisted ammonia synthesis.Li et al compared Co single-atom catalysts with different nitrogen coordination numbers [36].They discovered that a lower Co-N coordination number is favorable for the adsorption and dissociation of N 2 , which enhances the reduction activity of N 2 and promotes an increase in ammonia yield.Gorky et al compared the activity of the supported Ni nanoparticles (of 5.6 nm and 13.5 nm) on SiO 2 in a DBD system (at 13.75 ± 1.25 kVpk-pk with a total flow rate of 50 ml min −1 , N 2 /H 2 = 1:), showing that the smaller Ni nanoparticles exhibited an ammonia synthesis rate at least twice as high as that of the larger Ni nanoparticles [37].

Catalyst supports
Previous discussions indicate the importance of the role of catalyst supports in the NTP catalysis.This is reasonable since, compared to the metals, the supports are the major bulk components in the gas discharge (especially DBD NTPs), and thus their physicochemical properties can affect the plasma discharge more significantly.The dielectric properties of the supports is known to play a pivotal role in plasma discharge in DBD, affecting the key features such as discharge current, electron energy in NTP and the uniformity of filamentary discharges [49,50].The effect of the dielectric constant of inter packing materials (of BaTiO 3 , TiO 2 and SiO 2 with the dielectric constant of 7200, 85 and 3.8, respectively) on NTP-assisted ammonia synthesis was investigated by Liu et al and the findings showed that packing materials with higher dielectric constants could enhance plasma discharge in the DBD reactor, thereby improving ammonia synthesis in the gas phase [51].Similar results were achieved by ferroelectric materials (such as lead zirconate titanate, PZT, with a dielectric constant of 1900) as well [52].Additionally, findings by Liu et al also suggest that the inclusion of packing materials of higher thermal conductivity in the DBD discharge benefited NH 3 formation due to the improved heat transfer (with lower bulk system temperatures) [51].In addition to the selection of supports with the appropriate intrinsic properties, the packed bed with physical mixtures is also the option, which is demonstrated by Akay and Zhang in a DBD system with the highly porous Ni/SiO 2 catalyst and BaTiO 3 spheres [46].
Variation in the basicity of the catalysts can affect ammonia synthesis under NTP conditions [31].The effect of alkaline and acidic properties of alumina supports on NTP-assisted ammonia synthesis was studied by Zhu et al [53], in which acidic, alkaline and neutral γ-Al 2 O 3 pellets (with similar porous properties) were employed.The findings show the positive correlation between the ammonia synthesis rate and the basicity of the supports with the highest energy efficiency of 6.58 g kWh −1 achieved by the alkaline γ-Al 2 O 3 .A similar conclusion was drawn from the research conducted by Xie et al showing that acidic sites of the support enhanced NH 3 decomposition, consequently leading to a reduction in the synthesis rate [54].
In addition to the dielectric and alkaline properties of the supports, porous properties of the supports were proved to be important factor affecting ammonia synthesis in NTP significantly by many studies.Compared to non-porous and less porous supports, highly porous supports often favor the process, and the reason is multiple, such as the increased surface microdischarges and adsorption-enhanced reactions [55].Chen et al compared two types of packing beads (with same diameter of ∼1.5 mm) in a DBD reactor, i.e. porous silica (with the average pore size of ∼8 nm) and smooth nonporous soda lime glass beads, and the porous silica beads enhanced the system energy efficiency significantly [56].Framework materials, especially zeolites, are therefore rational candidates for designing relevant catalysts for the NTP-catalysis.Shah et al compared the NTP alone system with that packed with different zeolite powders having different intrinsic pore sizes (including AIPO-18, 3A, 4A, 5A and 13X), showing that (i) the inclusion of zeolites in the NTP system enhanced ammonia synthesis and (ii) the pore size effect on the process efficiency was most significant at pore sizes of ⩾∼0.5 nm [57].Such improvement was reported for mesoporous silica (i.e.MCM-41 with the pore size of about 2.5 nm) as well in comparison to the corresponding NTP alone system by Wang et al [47].
Recent studies also suggested the possible reactive role of relevant catalyst supports with surface defects, such as OV [31], which might be the active sites for N 2 activation as proposed by Li et al and Liu et al [31,45].Zhang et al investigated this aspect specifically in NTP-assisted ammonia synthesis using the catalyst support of layered double hydroxides (LDHs), showing that OV on the LDH carrier is highly beneficial to NH 3 formation under NTP conditions [16].In addition, the work proved that with the loading of active metals (e.g.Ni and Ru) on the LDH, the NTP-catalytic processes were enhanced further due to presence of multiple reaction pathways and the synergy between the surface OV and metal sites.It is worth noting that in thermal catalytic ammonia synthesis the role of nitrogen vacancy (NV) was recently suggested as the functional sites for activating N 2 and promoting NH 3 formation [58].However, relevant research on this aspect in NTP-catalytic ammonia synthesis is not yet attempted.

Ammonia decomposition under NTP conditions
Ammonia synthesis is a known reversible reaction in thermal catalysis, and, under NTP conditions, the plasma discharge can not only enable ammonia synthesis but also facilitate ammonia decomposition, and the latter can limit the overall rate of ammonia synthesis.Hence, during plasma-assisted ammonia synthesis, the simultaneous NH 3 decomposition deserves a thorough consideration and investigation.Iwamoto et al studied ammonia decomposition during NTP-catalytic ammonia synthesis in a DBD reactor using wool-like metals as both catalysts and electrodes [59].They found that (i) the wool-like gold electrode was the most active catalyst for promoting the forward reaction and (ii) the calculated NH 3 decomposition constants were similar, being independent on the type of metal electrodes.The latter suggest NH 3 decomposition during the NTP-catalytic ammonia synthesis might occur mainly in the gas discharge.The same group then systematically investigated NH 3 decomposition in the same DBD system [60].The findings confirm that under the NTP conditions the decomposition process is primarily dependent on discharge power and gas residence time, and complete NH 3 decomposition could be achieved in the NTP-catalytic system (i.e.applied power of ∼50 W, applied voltage of 5 kV, frequency of 50 kHz, residence time of 1.2 s, and NH 3 partial pressure of 4.87% with N 2 balance).It is worth noting that the NTP systems employed by the two studies [59,60] were based on bulk metal wools which are different from the supported metal nanoparticles, and hence further investigation of these aspects in conventional NTP-catalytic systems is needed.A similar conclusion was also obtained by detailed studies (of NTPassisted NH 3 decomposition) conducted by Anderson et al using both experimental and simulation methods [61,62].The experiments (at 21 W and 75 Nml min −1 NH 3 ) were conducted employing different packing materials of catalyst supports with different dielectric constants of 4-30 (such as ZSM-5, TiO 2 and MgAl 2 O 4 ), which might affect the formation of micro-discharges in the NTP system.The results established a linear correlation between the number of micro-discharges and NH 3 conversion (figure 2(a)), suggesting NH 3 decomposition mainly in gas discharge due to collisions between NH 3 and electrons [61].The further kinetic modeling study revealed the possible reaction networks in the discharge and afterglows (figure 2(b)), and proposed that the packing materials (exemplified by MgAl 2 O 4 ) enabled the formation of surface-bound hydrogen atoms, which promoted the Eley-Rideal (E-R) pathway for ammonia re-formation (via the reaction with the gas phase NH 2 ) [62].The findings above suggest that catalyst design which can suppress the gas-phase NH 3 decomposition during the NTP catalysis is necessary to improve the rate of ammonia synthesis, and this is demonstrated recently by the work by Wang et al (as shown in figure 2(c)) [47].In specific, the Ni catalyst supported on the external surface of mesoporous MCM-41 was developed to enable the diffusion of the formed NH 3 molecules (on the catalyst surface) into the inner pore regime where discharge is absent, and hence being protected from being decomposed by gas discharge (which was coined as the 'shielding protection' effect).

Mechanistic understanding of NTP-assisted ammonia synthesis
Further development of the NTP system requires a deeper understanding on the interplay between plasma, catalyst, plasma-induced species and reactions (both on catalyst surface and in gas discharge), as well as the corresponding reaction mechanisms.This can be achieved by various means including comprehensive characterization of the catalysts, plasma diagnostics and in situ characterization of the catalyst surface to know the variation in the physicochemical properties of the catalysts and relevant species (as illustrated in figure 3), and various phenomena in the system including adsorption, desorption, surface reactions, photocatalytic activation, surface modification, Joule heating, charging, formation of surface discharges, and electric field enhancement [63].

In situ characterization of the mechanisms of NTP-catalytic ammonia synthesis
Quantifying and qualitatively information on the relevant species in the NTP systems are key to establish the reaction mechanisms.Optical emission spectroscopy (OES) is an effective technique for providing the information on the emitted species in gas discharges, being able to analyze the plasma electron density (Stark broadening), blackbody (heavy particles in plasma) temperature (blackbody radiation) and reactive group properties (note that OES does not provide the information of all excited chemical species in NTP systems).OES , especially noticeable at a wavelength of 391.4 nm, suggests the possible presence of N 2 + (0,0) [23].Based on OES characterization of the NTP systems (with SBA-15 and Ag/SBA-15 catalyst packing), Gershman et al employed Stark broadening of the H α line to determine electron density and N 2 molecular bands to assess N 2 vibrational excitation to quantify N 2 decomposition in gas discharge [48].Also, the comparative analysis shows that the presence of Ag facilitated the NTP-assisted ammonia synthesis at lower plasma power (i.e.11 W), indicating the presence of two regimes of (i) a metal driven regime and a surface-plasma driven regime.Wang et al performed a comparative OES characterization of the NTP systems (with Al 2 O 3 and Ni/Al 2 O 3 catalyst packing) to establish the correlation between NH (in gas discharge at 336 nm), surface NH 2 (s) and NH 3 formation rate, which confirms the crucial role of NH radicals, contributing to the surface reactions towards NH 3 synthesis under the NTP conditions (via both E-R and Langmuir-Hinshelwood, L-H, mechanisms) [65].
Compared to ionization and/or dissociation, the energy threshold for vibrational excitation of molecules is lower, making it predominant in catalytic reactions.However, due to the lack of direct spectroscopic evidence regarding vibrationally excited molecules during catalytic processes, the understanding of the underlying mechanisms behind the NTPassisted catalysis remains limited.Given the significant challenges associated with elucidating the interactions between catalytic surfaces and NTP-induced species, the development of spectroscopic techniques that facilitate in-situ analysis of catalytic surface reactions in NTP environments is essential.In-situ Fourier transform infrared (FTIR) spectroscopy is one of the most effective spectroscopic tools for exploring reaction mechanisms by real-time monitoring of dynamics of catalytic surface reactions, providing valuable insights into the vibrationally excited states of molecules.Winter et al designed a custom-designed high-vacuum NTP-catalysis batch reactor coupled with in situ FTIR (figure 4(b)), allowing the experimental vibrational measurements of NTP-activated intermediates under relevant atmospheric conditions [18].The development helped to reveal the different reaction pathways on Ni/γ-Al 2 O 3 and Fe/γ-Al 2 O 3 catalysts and the effect of temperature on NH 3 formation under NTP conditions.Wang et al designed another in situ FTIR system (figure 4(c)) for the simultaneous spectral analysis of the catalytic surface reactions during the NTP-catalytic ammonia synthesis, which helped to corroborate the significance of the metallic Ni sites in N 2 dissociation, leading to the formation of NH intermediates.Compared with the vibrational excitation and dissociation, electron excitation on Ni catalysts is paramount for the activation of N 2 and H 2 under NTP conditions [47].

Mechanistic understanding by relevant modeling studies
Relevant modeling studies are also helpful to gain the mechanistic understanding of the NTP-catalytic ammonia systems, being helpful to design the specific catalysts and systems.This was first demonstrated by Mehta et al who used a densityfunctional-theory (DFT)-based microkinetic model to include the vibrationally excited N 2 states (produced by DBD plasma) in the reaction network to reveal the effect plasma on the conversion (by reducing the activation barrier), which could help the screening of metal catalysts for the NTP-catalysis [20].Given the significance of N 2 activation on metals during NTPcatalytic ammonia synthesis, Chen et al developed a quantum chemical model based on DFT to explore the effect of electric fields and surface electrons on N 2 adsorption and dissociation on Ru and Ni catalysts (with different surface morphologies) in the simplified plasma environment [66].They proposed that the local redistribution of electrons on the metal surface (e.g. the local accumulation of more electrons on the upper surface of the catalyst), induced by the electric field, is key to promote N 2 adsorption/dissociation.The considerable effect of electric field on the surface reactions (on Fe(110) structure) in NTP-catalytic ammonia synthesis was further confirmed by Shao and Mesbah using a first-principles framework coupling DFT calculations and microkinetic modeling [67].The integrated modeling framework was also used to understand the trade -off between NH 3 production and energy efficiency by exploring the parameter space of the NTP system.However, the DFT-based studies employed ground state N 2 and overlook the roles by the plasma-induced species which might not be appropriate for the NTP-catalysis.For example, during the adsorption and dissociation of N 2 on metal surfaces, electron transfer and energy losses may occur between the surface and the molecules, which could impact the predicted outcomes of the NTP catalysis [68,69].

Effect of process parameters on NTP-assisted ammonia synthesis
In addition to the exploration in catalyst design and mechanisms, process optimization is also important aspect for progressing the NTP catalysis to improve the key performance indicators such as NH 3 productivity and energy efficiency.
Here relevant process parameters affecting the NTP-catalysis was commented.

N 2 -to-H 2 (N 2 /H 2 ) ratio and flowrate
Conventional thermal catalytic ammonia synthesis commonly employs the stoichiometric N 2 /H 2 ratio (1:3), whilst under NTP conditions relevant studies showed that the N 2 -rich environment is beneficial to promote the ammonia production rate [27,39,54,57,[70][71][72]. For example, Patil et al conducted a comparative investigation of the N 2 /H 2 ratio on NH 3 formation over various metals supported on Al 2 O 3 , showing the optimal N 2 /H 2 ratio in the NTP-catalysis was either 1 or 2 depending the type of metals [27].This is reasonable since hydrogen can be easily dissociated than nitrogen by discharge (dissociation energy: 4.48 eV vs. 9.76 eV), and hence excess hydrogen in the feed could reduce the possibility of N 2 activation in gas discharge.In addition, the activated H species is prone to facilitate decomposition of ammonia formed [73].Conversely, some experimental findings show that the stoichiometric N 2 /H 2 ratio was optimal, exemplified by the NTP system employing PZT, which showed the efficiency peaked at 0.44 g NH3 kWh −1 at the N 2 /H 2 ratio of 1:3 then decreased by increasing N 2 in the feed [52].
Flowrate (and hence residence time) is known to affect NTP-catalytic processes [74].In NTP-assisted ammonia synthesis, different results were measured.For example, in the NTP system employing PZT (at 500 Hz and 3 kV and N 2 /H 2 = 1:3), a reduction in residence time (from 17.5 s to 8.8 s) increased the energy efficiency from 0.44 to 0.9 g NH 3 kWh −1 [52].Patil et al studied the effect of flowrate in the system over the 2 wt.%Rh/γ-Al 2 O 3 and 5 wt.%Ni/γ-Al 2 O 3 catalysts on ammonia synthesis under NTP conditions (at 21 kHz and the pulse width 25 µs and N 2 /H 2 = 2).Specifically, energy input was also considered during the analysis, as shown in figure 5.At higher flowrates under a comparable energy input, variation in the flowrate from 0.6 l min −1 to 1 l min −1 caused insignificant changes in ammonia formation.Conversely, at lower flowrates, a decrease in the flowrate from 0.18 l min −1 to 0.1 l min −1 led to the decrease in ammonia synthesis at similar energy inputs [27].Interestingly, studies based on M/γ-Al 2 O 3 (M = Ru, Co, and Ni) catalysts (with different pellet sizes and proportions of γ-Al 2 O 3 dilution) showed that the ammonia production rate either increased or decreased by increasing the residence time [44].The findings above suggest that the effect of flowrate on the NTP-catalysis is much more complex than that in the thermal counterpart with many aspects to be considered such as variation in residence time, thermal effect (due to Joule heating and heat dissipation) and discharge/ activation.

Discharge properties
DBD is the most common plasma type used for the NTPassisted ammonia synthesis with other plasma types of corona discharge, glow discharge and gliding arc were explored as well [75][76][77][78][79].The preference for DBD plasmas is attributed to their distinctive advantages, including the generation of nonequilibrium plasmas allows for efficient chemical reactions without significant heating, making it ideal for synergistically working with heat-sensitive materials.Furthermore, its operation at low temperatures suits reactions requiring precise temperature control.Importantly, DBD operates over a wide pressure range, from atmospheric pressure to several pascals, offering substantial flexibility in the design of industrial reactors.The presence of microdischarge pulses within the dielectric medium between electrodes ensures spatial uniformity of plasma, promoting uniform reactions throughout the process.Moreover, DBD systems are recognized for their simplicity and cost-effectiveness, operating safely and sustainably under atmospheric conditions without necessitating complex vacuum systems or high-power supplies.Given these significant advantages, this review primarily concentrates on exploring DBD plasma assisted ammonia synthesis.
In DBD plasma systems, high-voltage pulsed power supplies are the key equipment driving the discharge process, capable of adjusting voltage, pulse width, and frequency.The voltage ensures that the applied electric field between electrodes reaches the necessary breakdown strength.At a specific electrode gap and gas pressure, according to Paschen's Law, there is a threshold range for breakdown voltage; once the voltage reaches this range, plasma can form.The pulse width directly influences the lifetime of the plasma, while the impact of discharge frequency on the plasma is relatively complex.The plasma consists of lighter electrons, heavier ions, and neutral particles.The applied voltage rapidly accelerates the electrons, whereas the ions, due to their larger mass, accelerate more slowly.If the voltage is continuously applied in DC form, the velocity of ions will gradually increase as well.The energy exchange between electrons, ions, and neutral particles eventually leads to thermal equilibrium of the plasma.In this state, the rise in gas temperature may cause excessive decomposition of materials and rapid corrosion of electrodes.To prevent this in NTP systems, it is necessary to periodically stop applying voltage to avoid further temperature increase, thereby maintaining a stable NTP discharge.Thus, frequency, which controls the duration and intervals of voltage application, is key to achieving this objective.
Commonly used energy sources in NTP-assisted processes in DBD plasmas are continuous radio frequency powers with discharge frequencies ranging from 3 kHz to 300 MHz, and relevant efforts are dedicated into the effect of discharge frequency on various aspects of NTP-assisted processes.In the study by Ozkan et al at constant total current, the conversion and energy efficiency (using CO 2 splitting as the model reaction by varying the frequency from 16.2 to 28.6 kHz) were favored at lower frequencies (where glow regime was possibly exist), whilst higher frequencies led to the increased gas temperature and decreased effective plasma voltage in the filamentary regime [80].Also, in NTP-catalytic ammonia synthesis over Ru/γ-Al 2 O 3 catalysts, an increase in the discharge frequency from 8.2 to 11 kHz caused the decrease in the output voltage and power, and hence the a decline in the ammonia synthesis rate and energy efficiency [81].In contrast, Peng et al used a high-frequency power source and found that an increase in frequency from 22 to 26 kHz enhanced the energy efficiency of ammonia synthesis (over a Ru on MCM-41 catalyst) rather significantly (from 0.9 to 1.7 g NH3 kWh −1 ) [22].They attributed the measured enhancement to the possible resonance effect (between the high-voltage ac generator and DBD reactor) and the resulting homogeneous plasma discharge, which benefited the conversions and energy efficiency in the systems, and such phenomena were observed in many studies [22,32,81].
Microwave (MW) discharge is generated by exposing gas to high-frequency electromagnetic waves (at frequencies of 300 MHz-10 GHz) with power levels varying from a few watts to several hundred kilowatts.Compared to DBD plasma, MW discharges require much higher input power and can be operated with higher flowrates (e.g. at 1.1 kW and 15 l min −1 in the work by Nakajima and Sekiguchi for a catalyst-free system [82]).However, MW may induce dielectric heating, especially when MW-absorbing catalysts are used, causing a significant temperature rise of the system which may result in lower energy efficiency [82,83].Hence, systems employing MW discharges need to be investigated thoroughly regarding both throughput and energy efficiency, as well as the associated thermal effects, for ammonia synthesis.
In recent years, radio frequency-driven pulsed discharge gained significant attention due to the theoretical high energy efficiency.Zeng et al have demonstrated that by adjusting conditions such as the discharge gap and pulse waveform (as shown in figure 6), an energy efficiency of 1.2 g NH3 kWh −1 was achieved at a voltage of 7 kV and a nanosecond pulse waveform of 50 ns/0 ns/50 ns without a catalyst [84].Importantly, the work identified that during the NTP process, the low energy efficiency was due to the excessive energy consumption by gas dissipation and dielectric materials.Hence, finding the means to reduce the energy consumption by the two aspects can be critical to improve the process energy efficiency, which can be potentially achieved by reactor engineering and process optimization.Kim et al also employed pulsed discharge (e.g. 2 ms) and a conical copper mesh as a catalyst to achieve an energy efficiency as high as 7.7 g NH3 kWh −1 for ammonia synthesis [85].Compared to continuous operation, in the pulsed operation, the plasma discharge is extinguished before a significant fraction of the formed ammonia diffuses back into the gas phase, where it could be rapidly dissociated by the discharge.Although the pulsed discharges show the promises, the reported energy efficiencies are still lower than the conventional DBD systems, necessitating further development and optimization.
The selection of electrode materials can alter the distribution of the electric field inside a plasma reactor as well, and hence the effect of electrode materials on the NTP process efficiency is worthy of investigation.Jahanmiri et al compared plasma-assisted hydrocarbon cracking reactions using steel and copper electrodes, showing that the steel electrode increased the energy efficiency by up to 123% [86].For NTPassisted ammonia synthesis, Ma et al studied the tangled wire electrodes of different materials (i.e.NiFe, SS, Ti, and Cu) in comparison to the conventional rod electrodes (for DBD plasma) with the aim to enhance ammonia yield [87], showing that the Cu electrode tangled wire was the best candidate, which indicates the possible reaction functionality in addition to the changed discharge properties.However, relevant studies on the effect of electrode materials on NTP catalytic ammonia synthesis are not yet attempted, and, based on conclusions drawn from relevant research [88,89], it can be hypothesized that appropriate selection of electrode materials might enhance the efficiency of ammonia synthesis.

Aspects of reactor configuration and process intensification
Plasma and catalytic chemistry operate on different time scales, with plasma reactions typically occurring on the order of 10 −10 to 10 −6 s, whilst the time scale of catalytic turnover is in the range of 10 −3 -1 s [90,91].Furthermore, plasma chemistry operates on a volumetric scale, whereas catalytic chemistry predominantly occurs on the catalyst surface.This implies that effective coupling between plasma and surface catalytic chemistry can only be achieved when the ratio of surface to volume is large enough to allow the substantial transfer of the short-lived plasma-induced species to the catalyst surface.Therefore, achieving a high surface-to-volume ratio is key to coupling the fast-volume plasma chemistry with the slower but highly selective surface catalytic reactions.Hence, relevant aspects of reactor configuration need to be studied to enable the efficient coupling between rapid plasma chemistry and selective surface catalysis.Additional, in situ separation of NH 3 from the processes could be the effective process intensification method to shift the equilibrium towards the improved ammonia synthesis.

Structured catalysts
Packed bed configuration was commonly employed to include catalysts in a plasma reactor, especially DBD, since it is very convenient to translate the conventional catalyst design (e.g.active site engineering) to the NTP-catalysis.In addition to packed bed, structured materials were also proposed as catalysts and/or electrodes.Mizushima et al fabricated Ru supported on tubular alumina membrane-like catalysts (figure 7(a)) and compared its performance in NTP-assisted ammonia synthesis with the plasma alone and the system with the bare tube (without Ru loading) under various conditions [92].Although the ammonia yield was enhanced by the structured catalyst under different conditions, the relevant mechanism was not revealed and comparison with the packed bed system was not performed, which needs further thorough investigation.Recent work on a similar NTP system (i.e. a stainlesssteel membrane distributor-type DBD) demonstrated that such membrane design could improve mixing and selective activation of reactants for the reaction with multiple reactants [10].And these aspects were later confirmed by Veng et al in their study based on DBD reactors with membrane dielectrics (noncatalytic systems) for ammonia synthesis [93].Iwamoto et al proposed the use of wool-like metals (figure 7(b)) as packing materials, which also served as the inner electrodes [59].The DBD reactors were used for ammonia decomposition, and some of the metal wires (such as copper) were claimed to have catalytic function as well.However, such structured materials may have intrinsic drawbacks since plasma discharge can modify the metal wires over time and bulk metal are not efficient catalysts.

Reactor configurations
DBD plasma is commonly employed by most NTP-catalytic systems including ammonia synthesis.However, the energy efficiency of DBD systems is relatively low, and many aspects of DBD configurations such as electrode spacing (or gas gap) electrode configuration and reactor configuration (e.g.tubular, planar or fluidized) need to be thoroughly investigated and optimized for optimizing ammonia yield and energy efficiency under NTP conditions.Bai et al designed a planar DBD reactor with the micro discharge gap (of 0.47 mm) to improve the NTP-assisted ammonia synthesis (without catalysts and packing materials) due to the intensified gas discharge (e.g. higher concentration of free electrons) [94].In a DBD reactor, Akay and Zhang demonstrated the effect of electrode configurations on NTP-catalytic ammonia synthesis, showing that exposing one of the two electrodes to the reactant gas and catalyst (Ni/SiO 2 ) could reduce the specific energy consumption by ∼60% (with the same N 2 conversion and ammonia production) [46].Water electrodes were also proposed [47,65] to mitigate the Joule heating effect to maintain a constant system temperature (as shown in figure 8(a)).However, the temperature effect on a specific NTP-catalytic system needs to be fully understood to enable the appropriate selection of the bulk system temperature.Recent reactor innovation was demonstrated by Zen et al who designed a fluidized bed DBD (FB-DBD) reactor with the Ru/Al 2 O 3 catalyst for NTP-catalytic ammonia synthesis, showing that the FB-DBD plasma achieved an energy efficiency of 5.9 g NH3 kWh −1 , being higher than some of conventional DBD systems [95].It is worth noting that most current relevant developments in the field are at the laboratory scale.As shown in figure 8(b), for potential industrial adoption of NTP-catalytic ammonia synthesis, high throughput reactors need to be developed, which can be built on relevant developments such as the scalable large-flow DBD reactor designed for NTP-assisted toluene decomposition (with the flowrate up to 110 l min −1 ) [96].

Process configurations
To improve process performance, process intensification via ammonia separation may be necessary.Since ammonia synthesis can be limited by equilibrium, in situ separation of ammonia from the process can be one of the means of achieving process intensification.In conventional processes, separation by condensation is used due to the comparatively higher condensation point of ammonia (compared to other gases in the streams).This approach is also sought in NTP-assisted systems.For instance, Riotto et al proposed an integrated process design for NTP-assisted ammonia synthesis (figure 9(a)) and assessed the process using life cycle analysis regarding the corresponding environmental impacts [97].The results suggested that the energy consumption for generating plasma is significantly high, necessitating substantial improvements in the efficiency of the NTP process to improve its economic competitiveness and reduced the associated environmental impacts.During the NTP-assisted ammonia synthesis, in situ separation of ammonia can shifts the reaction equilibrium effectively to improve ammonia production and energy efficiency.
Peng et al demonstrated that magnesium chloride (MgCl 2 ) could serve the purpose for NTP-assisted the ammonia synthesis (figure 9(b)), claiming the bifunctionality of MgCl 2 as both catalyst and absorbent which could be misleading since MgCl 2 is not the catalyst for ammonia synthesis.Also the NH 3 sorption was chemisorption (with Mg 3 N 2 and Mg(NH 3 ) 6 Cl 2 as the intermediates), and hence regeneration of the material can be issue [32].Rouwenhorst et al employed 4A zeolite as the adsorbent in a DBD reactor (non-catalytic) to improve the energy efficiency of a NTP-assisted ammonia synthesis system  by removing ammonia in situ from the gas discharge [98].The storage of the formed NH 3 in the zeolite was claimed to inhibit the plasma-induced NH 3 decomposition during the process, thereby enhancing the energy efficiency up to 2.3 g NH3 kWh −1 .However, regeneration after saturation needs to be considered for practical operation.

Conclusions and perspectives
Compared to conventional thermal-catalytic ammonia synthesis, the electrified NTP-assisted processes, especially that employing efficient catalysts, have the advantages of short induction time, mild reaction conditions, ability to couple with green electricity/hydrogen and distributed production, holding the promise for being one of the low carbon technical solutions for various sectors such as chemical and energy.However, the energy efficiency and production scale of NTP-assisted ammonia synthesis are far from satisfactory for making the technology realistic in industrial settings.Hence, more research and development efforts are needed to progress the technology, as well as assessing its suitability for the targeted applications.
The use of appropriate catalysts in NTP-assisted ammonia synthesis has shown very promising results in improving the system energy efficiency.However, detailed studies into the catalyst design lack significantly.This is particularly important since the activation mechanism under plasma and thermal conditions are quite different, and hence different design strategies are needed, such as engineering metallic phases/sites/nanoparticles to have the intrinsically high activity and selectivity under the plasma conditions and design of appropriate carriers with the required physicochemical features to accommodate the active phases and maximize the discharge.To enable the rationale design of catalytic materials for the technology, a thorough understanding of the mechanisms within (regarding discharges, activation, species transport, gas phase reactions and surface reactions) is highly desired, which demand advanced in situ techniques and modelling studies.
Previous studies have shown that majority of the system energy input of the NTP processes are consumed by the electrode, gap, and dielectric instead of the chemical conversions.Hence, in addition to the rational design of highly efficient NTP catalysts for ammonia synthesis (which could maximize the chemical conversions), reactor engineering and process optimization regarding the reduction in energy consumption by the electrode/gap/dielectric can be vital.Also, if the proportion of this cannot be reduced, then relevant strategies are needed to enable energy recovery and reuse.At the end, the development of scaling-up strategies (along with reactor engineering and process optimization) with the consideration of product separation, feed recycling and energy recovery need to be explored to improve the productivity and efficiency of the technology.

Figure 1 .
Figure 1.(a)-(b) The effect of the N 2 to H 2 volumetric feed ratio on ammonia formation in a DBD reactor packed with different metal supported on γ-Al 2 O 3 catalysts and the bare γ-Al 2 O 3 support.(a)-(b) Reprinted from [27], Copyright (2021), with permission from Elsevier.
. The results show the activity order of Ru > Rh > Ni > Pt > Co > Fe > Pd > Mo, and Ru/γ-Al 2 O 3 exhibited the highest energy efficiency of 1.51 g NH3 kWh −1 .Contrary to the high activity of Fe catalysts in thermal catalysis, Fe/γ-Al 2 O 3 was not active under the plasma conditions (with the applied voltage at a frequency of 21 kHz and the pulse width 25 µs).Similar results were obtained by Herrera et al as well, showing that Fe/γ-Al 2 O 3 exhibited a much lower activity than the Co and Ni catalysts (e.g.site-time yield: 0.5 min −1 for Fe/Al 2 O 3 vs.4.25 min −1

Figure 2 .
Figure 2. (a) Linear correlation between the NH 3 conversion and number of micro-discharges (regulated by packing materials) occurring per half period in NTP-assisted NH 3 decomposition.Reproduced from [61].CC BY 4.0.(b) Proposed reaction pathways for NH 3 decomposition under NTP conditions in a micro-discharge and its afterglow.Reproduced from [62].CC BY 4.0.(c) Schematic to illustrate the proposed mechanism of the 'shielding protection' effect of mesoporous MCM-41 in NTP-catalytic ammonia synthesis.Reprinted with permission from [47].Copyright (2022) American Chemical Society.

Figure 3 .
Figure 3. Relevant species and possible major reaction pathways in NTP-catalytic ammonia synthesis systems.

Figure 4 .
Figure 4. (a) Full OES spectrum of the NTP alone system with N 2 and H 2 .Reprinted with permission from [23].Copyright (2022) American Chemical Society.(b) Design of the high-vacuum NTP-catalysis batch reactor coupled with in situ FTIR.Reprinted with permission from [18].Copyright (2020) American Chemical Society.(c) Schematic of the in situ FTIR gas cell for the plasma-catalytic reaction.Reproduced from [47].CC BY 4.0.

Figure 6 .
Figure 6.Diagram illustrates the energy pathway during pulse voltage applied by the nanosecond power supply.(a) Waveforms of current and voltage at the reactor; (b)-(d) the energy absorption within the electrode, gap, and dielectric during the pulse voltage application, respectively; (e) the potential routes of energy transfer in plasma-assisted ammonia synthesis.Reprinted with permission from [84].Copyright (2023) American Chemical Society.

Figure 7 .
Figure 7. (a) Schematic of the BDB reactor with the structured catalyst based on alumina tubular membranes.Reprinted from [92], Copyright (2004), with permission from Elsevier.(b) Schematic and photo of the DBD reactor with the wool-like metals as one of the electrode.Reprinted with permission from [59].Copyright (2017) American Chemical Society.

Table 1 .
Comparison of relevant NTP-catalytic systems over different catalysts investigated for ammonia synthesis.