Plasma Catalysis as an Alternative Route for Ammonia Production

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Perspective

Plasma catalysis as an alternative route for ammonia production: status, mechanisms, and prospects for progress Jungmi Hong, Steven Prawer, and Anthony B. Murphy ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02381 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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ACS Sustainable Chemistry & Engineering

Plasma catalysis as an alternative route for ammonia production: status, mechanisms, and prospects for progress Jungmi Hong¹,², Steven Prawer² and Anthony B. Murphy¹ ¹ CSIRO Manufacturing, Bradfield Rd, P.O.Box 218, Lindfield NSW 2070, Australia ² School of Physics, David Caro Building, University of Melbourne, Parkville, VIC 3010, Australia

Abstract:

Plasma catalysis has drawn attention from the plasma and

chemical engineering communities in the past few decades as a possible alternative to the long-established Haber–Bosch process for ammonia production. The highly reactive electrons, ions, atoms and radicals in the plasma significantly enhance the chemical kinetics, allowing ammonia to be produced at room temperature and atmospheric pressure. However, despite the promise of plasma catalysis, its performance is still well short of that of the Haber–Bosch process. This is at least in part due to the lack of understanding of the complex mechanisms underlying the plasma–catalyst interactions. Gaining such an understanding is a prerequisite for exploiting the potential of plasma catalysis for ammonia production. In this perspective, we discuss possible benefits and synergies of the combination of plasma and catalyst. The different regimes of plasma discharges and plasma reactor configurations are introduced and their characteristics in ammonia synthesis are compared. Based on detailed kinetic modeling work, practical ideas and suggestions to improve the energy efficiency and yield of ammonia production are presented, setting out future research directions in plasma catalysis for efficient ammonia production.

Keywords:

plasma catalysis, ammonia production, non-equilibrium

atmospheric-pressure plasma, Haber–Bosch process alternatives, plasma chemistry, plasma–surface interactions

Corresponding author: Anthony B. Murphy Email: [email protected]

1

ACS Paragon Plus Environment


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Introduction Lieberman and Lichtenberg stated in the preface of their book that plasma processing is born out of the need to access a parameter space in materials processing unattainable by strictly chemical methods 1. Needless to say, plasma technology has become indispensable in a wide range of manufacturing industries, including semiconductor chip manufacturing, where mind-boggling atomiclevel control is provided by processes such as plasma ALD (atomic layer deposition) (atomic layer etching) 15-16

5-7

, biomedical applications

8-10

, waste management

11-14

2-4

and ALE

and metal processing

. Plasma catalysis is one of the key emerging technologies in the field of low-temperature plasmas.

The many expected synergetic benefits of combining plasma and catalysis processes, together with the huge scale of world-wide use of catalysis in material production, have drawn a great deal of research interest

17

. A particularly successful application has been the removal of volatile organic

chemicals from a gas flow

18

. Ammonia production has been an extensively-investigated plasma-

catalysis application, because of its significance in chemical industries, including production of nitrogen-based fertilizers and a wide range of fine chemicals, as well as its potential to provide safe storage of hydrogen for energy applications 19-25. The conventional thermochemical Haber−Bosch process requires a high temperature and pressure (typically 150 to 200 bar and 500°C), requires a large amount of energy and generates significant CO2 emissions

19, 26

. As a consequence, there has been a sustained effort to seek viable

alternatives including biochemical methods

27-28

, catalytic pyrolysis

in addition to the approach of developing innovative catalysts

33

29-30

and plasma catalysis

20, 31-32

,

for thermal processing. Among the

possible options, plasma catalysis has some significant advantages. The presence of reactive plasma species, such as electrons, ions, atoms and radicals, enables efficient catalysis even at room temperature and atmospheric pressure. This means that plasma catalysis has the potential to provide improved energy efficiency, decreased capital costs and extended catalyst lifetime

34-35

. Further,

based on the well-established example of commercial ozone manufacturing using atmosphericpressure plasma discharge 36-37, there is a clear potential for scale-up. 2


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As a consequence of the potential of plasma processing, a large-scale cooperative project has been initiated in the EU: MAPSYN (Microwave, Ultrasonic and Plasma-assisted Syntheses) focuses on the sustainable process intensification of nitrogen-fixation reactions and selective hydrogenation by plasma, microwave and ultrasound technologies 26, 38. Likewise, in Japan and the USA, interest in large-scale research projects for ammonia production, both in general and using plasma catalysis in particular, is gaining momentum

39-40

. The flexibility of plasma catalytic systems, including the

possibility of reactors of the required capacity sited near a renewable energy source and able to be brought on-line when excess energy is available, is a significant advantage over the Haber–Bosch process. Since Eremin et al.’s

25

pioneering work in the 1970s, extensive studies have been performed

using different types of plasma sources, reactor designs, and catalytic materials. However, the best plasma catalysis results still have over an order of magnitude lower energy efficiency than conventional processes (25-30 g-NH3/kWh 19 compared to around 500 g-NH3/kWh for large-scale Haber–Bosch

41-42

), in both cases excluding the energy required to produce the hydrogen feedstock;

energy efficiency is even lower when high concentrations of ammonia are produced 21. It is not fully understood whether this limit is fundamental, or if it can be overcome by manipulating the surface or plasma parameters. The key to maximizing the benefit and adaptability of plasma catalysis is gaining a better understanding of the underlying mechanisms. For example, how do the chemical pathways differ from those of the thermal equilibrium process? What features will a catalyst optimized for plasma catalysis have that differ from those used for thermal processes? – to date, catalysts have been borrowed directly from those used for thermal catalysis. Based on such understanding, it should be possible to suggest approaches to increase the production of ammonia and minimize the loss mechanisms of the produced ammonia molecules. In this perspectives article, the possible benefits of combining a plasma and a catalytic surface to provide synergies for efficient ammonia production are considered. Specific examples of plasma catalysis are introduced, and the characteristics of the different plasma discharge regimes and plasma 3



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reactor configurations that have been used for ammonia synthesis are compared. After establishing the current state-of-the-art, and analyzing the most important reactions in plasma-catalysis ammonia synthesis, we provide some practical proposals for future research and development directions that have potential to improve the yield and efficiency of ammonia production in plasma catalytic reactors.

Benefits of plasma catalysis Plasma catalysis differs in significant ways from conventional thermochemical catalysis, offering several potential benefits. We summarize in this section the contributions of the plasma to these advantages, and then the contributions of the catalyst and its interactions with the plasma.

The contribution of the plasma – non-equilibrium characteristics of plasma Surface interactions are undoubtedly significant in ammonia production, including in plasma catalysis

43-46

. However, the plasma characteristics critically affect the process, so understanding the

possible reactions involving electrons, ions and reactive plasma species is an important starting point in studying plasma catalysis 47. The most commonly-noted benefits of plasma catalysis arise from the plasma’s highly nonequilibrium characteristics. Under a strong applied electric field, the gas molecules are partially ionized (in typical industrial applications, the ionization degree falls within the range from 10-8 to 10-2) and the discharge is initiated, giving rise to a high density of electrons and ions. In a typical atmospheric-pressure low-temperature discharge, the average electron energy is about 1–2 eV, which corresponds to a temperature corresponding of approximately 12 000–23 000 K. This is a much higher energy than that of the heavy species (molecules, atom and ions) in the plasma volume, but is still small compared to the dissociation and ionization energies of the heavy species. The electron energies have a distribution that includes very high-energy electrons; Figure1 shows an example. The electrons in the high-energy tail of the distribution are the predominant species that drive high-energythreshold processes such as dissociation and ionization. The electron energy, electron density and gas temperature differ depending on the plasma regime, which affects the densities of excited molecules, 4


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atoms and radicals. Number densities and reaction rates typical of low-pressure and atmosphericpressure plasmas are shown in Table 1. The reference values given in Table 1 for the mole fraction and production rate of major gas species were obtained at specific pressure and plasma parameter.22, 46, 48-50

In particular, for an atmospheric-pressure discharge are provided from zero-dimensional chemical

kinetic modelling based on the assumption of spatial and temporal uniformity at a specific plasma condition (reduced electric field strength E/N = 51 Td, electron number density ne = 1.05 × 108 cm-3). Therefore, the results should be treated as indicative, providing only an approximate guide to the difference between atmospheric-pressure and low-pressure discharges. As well as driving ionization or dissociation processes, electrons in the plasma discharge provide highly-efficient excitation of molecules and generation of reactive intermediates, which opens additional chemical reaction pathways. As an example, excitation of a nitrogen molecule to the first vibrationally-excited level N2(X ν=1) requires an input energy of only 0.3 eV, which corresponds to a temperature of 3480 K in thermal equilibrium. For a typical electron-energy distribution, vibrationally-excited molecules are produced in much higher densities than ions, dissociated atoms or electronically excited molecules, and are hence important in sustaining a reactive plasma-catalysis environment, both in the gas phase and on surfaces 50. Reactive radicals, such as NHx in an N2–H2 discharge, which are known to play a significant role in ammonia production

50-52

, can be produced by reactions between dissociated atoms and excited

molecules as shown in Table 1. These NHx radicals can then produce ammonia by three-body reactions in the gas phase or on the catalyst surface. The three-body reactions are often disregarded in low-pressure plasma discharges

44

. However, they play an important role in atmospheric-pressure

discharges particularly in early stage, since the number density of reactants is higher 50. Despite the presence of all these energetic reactive species, the gas temperature is relatively low because the energy transfer to heavy species by elastic collisions with high-energy electrons is weak because of the significant mass difference 1. This low temperature is advantageous for an exothermic process, since it minimizes the loss of the product molecules. It is also favorable from the point of

5



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view of catalyst sustainability, minimizing sintering or coking. Note, however, that higher temperatures can favor important surface reactions, as we will discuss later.

Contribution of the catalytic material and its interactions with the plasma When a dielectric material is used as a catalyst, its presence affects the plasma characteristics because of the modification of the electric field distribution. This may even shift the plasma discharge regime from non-uniform streamers towards a stable glow-like discharge, depending on the specific geometry and material properties, and number densities of electrons, ions, excited species and radicals may be increased 36-37. The enhanced surface reactivity under the influence of the electric field used to generate the plasma, or the UV photons produced by the plasma, can also affect the composition of the plasma discharge by accelerated production of chemically-active species. Exposure to a strong electric field is often regarded as an important factor influencing the surface reactivity. Isolating the contribution of the electric field within the complex plasma-catalysis environment is not a simple task. However, based on the established understanding of a catalytic reaction as the donation and acceptance of an electron between the adsorbate and surface atoms

33, 53

, it is reasonable to infer that the electric field

can be an important factor in perturbing or enhancing the electron exchange process. Different catalytic surfaces respond differently in terms of chemical reactivity, but it has been demonstrated that plasma–surface interactions can result in profound differences in the performance of specific catalytic processes 54. The enhancement of surface reactivity by the effect of the high-energy UV photons produced in a plasma discharge has been proposed as a possible plasma effect, but its importance remains questionable

55

. For photocatalysts such as TiO2, it is known that the catalytic activity can be

enhanced by an increased UV flux 55. However, the plasma-induced UV emission is often estimated to be insufficiently intense to affect the catalytic reactivity of the surface. The typically high specific area and surface morphology of catalysts, which are generally nanostructured

34, 55

, has substantial impact on the activity of the catalyst. Hence, the physical 6


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properties of the surface can affect the reaction rates between surface and gas-phase species, and thereby influence plasma properties. In the presence of a surface, the lifetime of short-lived active species can be extended, allowing them to participate in further reactions and so increasing the concentration of reactive species even further. Due to the contributions of both the plasma and the catalyst surface, abundant active radicals, ions and high-density surface functional groups are provided in the plasma catalysis, making possible a chemically-enriched and reactive environment without the need for high thermal energy input

44

. Needless to say, the plasma and the catalytic

surface reaction are closely interconnected; the plasma characteristics are altered by the chemical and physical properties of catalytic surface and conversely the changed plasma characteristics influence the surface properties and the overall catalytic reactions occurring in the system. We note that real-time in-situ monitoring of the catalytic surface, long used in thermal catalysis, is now beginning to be applied to plasma catalysis

17, 56-59

. Such approaches, for example by using

miniature plasma devices that can be inserted in the measurement cell of a surface characterization diagnostic such as Fourier-transform infrared spectroscopy, will provide much deeper understanding than is currently available from ex-situ measurements of the catalyst surface.

What type of plasma is most suitable for ammonia production? For different power sources, plasma configurations and operating pressure regimes, the plasma kinetics and surface interactions can vary a great deal. To obtain better insight and understanding of these effects and their influence on ammonia production, we review previous publications on plasma synthesis of ammonia in this section. Plasmas of industrial interest can be divided into three main classes: thermal plasmas, low-pressure plasmas and non-equilibrium atmospheric-pressure plasmas; these are considered in separate subsections. Our aim is to demonstrate the advantages and disadvantages of each type of plasma, rather than to present an exhaustive review of the literature.

Thermal plasmas

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Thermal plasmas, which are typically at atmospheric pressure, and which are in thermal equilibrium with high gas and electron temperatures of order 1 eV or more, are widely used in manufacturing processes such as arc welding treatment applications nitrogen oxide

63

14

16, 60

, plasma cutting 15, plasma spraying

61

, and waste

. Such plasma have also been used for chemical synthesis of acetylene

, nitrogen or carbon fluoride compounds

64

, and syngas

14

62

,

from a wide range of

waste products. Due to the intrinsic limitations of high process temperatures, and the need for special arrangements in order to ensure that the plasma–catalyst interactions occur at temperatures below that of the plasma, there have not been many successful attempts at ammonia synthesis using thermal plasma sources. Nakajima and Sekiguchi

65

used an atmospheric-pressure microwave plasma, which

has a high gas temperature, typically well above 1000 K 66, and is considered to be close to thermal equilibrium at the centre of the discharge, with 1 kW power input. The ammonia production rate was very low, about 40 µmol/min with a concentration of 0.003%. However, the experiment provided evidence that ammonia production was directly correlated with the concentration of NH radicals, rather than those of nitrogen atoms, molecular ions and other metastable molecules. It was also observed that the quenching by H2 molecules in the afterglow region, where the external electromagnetic field is zero, enhances ammonia production, while helium quenching did not provide the same effect. The addition of argon in the active plasma volume also increased the N2 conversion rate, which was considered to be through the enhanced NH radical generation initiated by the charge transfer from argon ions as shown below.

Charge transfer from Ar+

N2 + Ar+ → N2+ + Ar

N2H+ ion formation

N2+ + H2 → N2H+ + H

Dissociative recombination

N2H+ + e → NH + N

Of other work on ammonia synthesis using a thermal plasma, of particular note is that of Vankan et al. 67 and van Helden et al.45 , in which a cascaded arc plasma system was used. As shown in Figure 2, this consisted of a high-density plasma source, designed for generating high fluxes of reactive 8


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species using high power (2–8 kW) at high temperature, and a remote processing chamber at low pressure that uses the downstream recombining plasma as the source of species for chemical reactions 68-69

. Van Helden et al. studied ammonia generation in a high-density cascade arc plasma system with

four different gas-supply configurations

45

. It was shown that ammonia production was maximized

(more than 10 vol.% in one reactor design) when both N2 and H2 were fed through the gas inlet of the plasma source (position (I) in Fig. 2), compared to when N2, H2, or both N2 and H2 were supplied to the gas inlet in the remote processing chamber (position (II) in Figure 2). In the cascade arc plasma source, the high temperature (approximately 1 eV) of heavy particles provides almost full dissociation of molecular species in the arc region. However, once the plasma expands through a nozzle into a low-pressure processing chamber (typically at 20 Pa) without a sustaining power input, where the electron temperature is estimated to be only 0.1–0.3 eV, the plasma recombines to a large extent, and further dissociation cannot occur. Supplying an atomic N flux turned out to be crucial, as shown by the observation that the combination of high density atomic H with undissociated N2 molecules did not produce NH3 above the detection limit and in the opposite case, while supplying a high-density atomic N flux and H2 molecules produced some ammonia. Because of the passivation of the surface of the processing chamber by overpopulated atomic N and H, the ammonia synthesis showed no dependence on the presence of a catalytic surface, in contrast to other reported studies. One can conclude that the properties of the outermost surface species dominate the surface reactions; hence the surface modification of the catalyst during plasma catalysis can be a more important factor than the bulk material itself. Based on a series of observations, van Helden et al. concluded that ammonia is produced via plasma−surface interactions by stepwise addition reactions, i.e. the successive hydrogenation of adsorbed nitrogen atoms and the intermediates NH and NH2 at the surface of the plasma reactor. This mechanism is commonly suggested to occur in low-pressure plasma catalysis of ammonia, and reflects the low-pressure environment in which the recombination reactions occur, as distinct from the high-pressure thermal plasma in which the initial dissociation reactions take place. 9



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Direct adsorption of atomic N and H

N → N(s), H → H(s)

Surface adsorbed NH(s) species formation

N(s) + H → NH(s) H(s) + N → NH(s) NH(s) + H → NH2(s)

Successive hydrogenation of NH(s)

NH2(s) + H → NH3 NH(s) + H2 → NH3

The significant dissociation rates obtained, such as over 35% in the N2 plasma, are not attainable in other types of industrial plasmas and are not closely relevant to the case of a non-equilibrium plasma source. However, thermal and other high-temperature plasma sources at high pressure (around 1 atm) can generate H2 efficiently by hydrocarbon reforming

70-72

, which is relevant to the complete

implementation of the Haber−Bosch process and related ammonia production methods.

Non-equilibrium low-pressure plasmas Despite its intrinsic disadvantages of low absolute number density and the consequent low production rate of NH3 molecules, low-pressure plasma processes for ammonia synthesis have been widely investigated in order to gain a fundamental understanding of the mechanisms of plasma catalysis. Low-pressure plasmas typically operate at pressures in the range from several Pa to several hundred Pa, at which high-energy electrons and ions play an important role. The concentration of excited species and atoms, relative to that of ground-state molecules, is high, but the absolute number density is still lower than in an atmospheric-pressure plasma. Of the research work on ammonia catalysis using low-pressure discharges, the work of Uyama and Matsumoto

24

is worthy of mention. Both a microwave (2.45 GHz) and an RF (13.56 MHz)

plasma source were used for ammonia synthesis with the same reactor tube and catalyst arrangement. The investigation revealed that the microwave plasma generated more N and H atoms and NHx radicals but fewer ions, and produced twice the ammonia yield of the RF source. Therefore, it was 10


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suggested that atomic N and H and NHx radicals are more important than ionic species for ammonia production. In their second paper 73, as shown in Figure 3, NH3 production decreased with increasing power, even with increasing NH and H concentrations, which contradicts the previous conclusion. High-density atomic hydrogen was considered to be responsible for the decreased production rate of ammonia, through promotion of dissociation reactions. Although the authors did not comment about the possibility, a generally higher rate of dissociation of NH3 by electron impact

47

could be

associated with the increased electron number density. Venugopalan and co-workers

74-75

investigated the influence of the electrode and catalytic

surfaces. They found that the production yield of ammonia increased proportionally to the electron work function. Metals such as Pt or Ag were most reactive with a high production yield of ammonia in a low-pressure glow discharge. This was attributed to the higher potential gradient near the instantaneous cathodes, which promoted the efficient generation of the ionic and radical precursors of NH3. Due to the significant role of surface reactions in the ammonia synthesis, Ag coated reactor wall provided a production yield twice that of the borosilicate glass wall. Jauberteau et al.

76

provided estimated reaction coefficients for important recombination

processes of atomic and molecular hydrogen with surface-adsorbed NHx species such as NH and NH2; these are important for plasma modeling. Touimi et al. 77 reported detailed measurements of N atoms, NHx radicals, NH3 and N2H2 molecules in a Ar–N2–H2 plasma used for nitriding. Correlations were examined between the plasma chemistry and plasma parameters (electron density and energy electron distribution function) obtained using spatially-resolved Langmuir probe measurements. The results showed the efficient production of electrons and NxHy species using the ternary gas mixture (Ar–N2– H2). Sode et al.43 investigated absolute ion and neutral species densities in an inductively-coupled H2– N2–Ar plasma experimentally and with a zero-dimensional kinetic model. NH3 was found to be produced only on the reactor walls. The concentration of NH3 reached up to 12%. The most abundant ionic species depended on the N2 partial pressure. The calculations, which agreed with the measured ion densities within the experimental uncertainty, revealed the importance of the ion chemistry in the 11



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low-pressure plasma. Carrasco et al.

44

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also focused on the ion and neutral chemistry in H2–N2

mixtures, with experiment and modeling work in a low-pressure hollow cathode discharge. The E−R and L−H surface interaction mechanisms were included and both found to be important in the successive hydrogenation steps. Based on the foundational work of Loureiro and Ricard

78

and Garscadden and Nagpal 79, self-

consistent kinetic modeling results for low-pressure N2–H2 plasmas, taking into account detailed vibrational kinetics and surface reactions, were presented in the 1990s by Gordiets et al.

46-47

. While

this important work has provided insights to many researchers working on N2–H2 plasmas and surface-interaction-related topics, its focus on high-electron energy low-pressure discharges means that it does not consider all aspects of the plasma chemistry that are important for the high-pressure low-electron-energy regime relevant to atmospheric-pressure plasmas. For example, some of the important mechanisms, such as a dissociative adsorption, that are important at atmospheric pressure were not taken into account.

Non-equilibrium atmospheric-pressure plasmas The low processing temperature and high production rates achievable in non-equilibrium atmospheric-pressure plasmas have led to many significant investigations related to plasma catalysis 17-18

. The ammonia synthesis application is no exception. Atmospheric-pressure discharges can have a wide range of electrode and reactor configurations.

However, a high electric field is always required to allow breakdown in high pressure, so the discharge gap and volume are typically smaller than for a low-pressure plasma. The electrons and ions are of lower energy than in low-pressure plasmas, so the chemical reactions involving the highdensity neutral radicals and the excited molecules become dominant. Figure 4 shows the ammonia synthesis system used by Peng et al.

21

, using an atmospheric-

pressure plasma source in a typical DBD (Dielectric Barrier Discharge) configuration for plasma catalysis, with gas recycling. DBD reactor requires at least one dielectric layer between two counter electrodes to prevent a transition to arc discharge 80. In many plasma catalysis applications, a catalyst 12


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or catalytic support functions as a dielectric barrier layer at the same time. In Figure 4, the catalyst is packed into an active plasma discharge zone and the produced NH3 is captured in an absorption bed. The unutilized N2 and H2 gas is recirculated to the plasma zone. Mizushima et al.

81-82

used a porous

alumina membrane on which Ru had been deposited, instead of the more typical packed-pellet or packed-sphere configuration, as shown in Figure 5. Aihara et al.31 showed a useful adaptation of DBD reactor design using large area Cu wool catalyst as a high voltage electrode in Figure 6 unlike typical dielectric material packed design. However, the basic configuration of a coaxial high-voltage electrode, dielectric barrier layer and ground electrode is the same as that of Peng et al., and is very commonly used in catalyst-packed plasma reactors. In Table 2, several references on ammonia synthesis using an atmospheric-pressure discharge are listed. Many different types of plasma sources, such as a pulsed streamer 83, micro-gap discharge 84, packed-bed DBD (dielectric-barrier discharge)

20, 85-86

, and a wide range of catalytic materials have

been reported. At atmospheric pressure, the characteristics of the discharge show significant differences from those typical of a low-pressure plasma. In particular, the much lower electron energies lead to low densities of ions and atomic species. Nevertheless, due to the significant role of surface reactions and high absolute number density of reactants, higher production rates and number densities of NH3 are obtained than in thermal and low-pressure plasmas

23, 86-88

. Hence, the low-

temperature atmospheric-pressure discharge can be regarded as the most attractive plasma regime and configuration for of ammonia production by plasma catalysis. The production rate ranges up to 700 µmol/min

86

, the concentration of produced ammonia

reached 12 vol.% and the most recent energy efficiencies are as high as 25 to 30 g-NH3/kWh

19

.

However, these results are all from different systems, and each approach has weak and strong points. Further, there still exists a considerable gap before the parameters are competitive with the commercial-scale Haber–Bosch process; for which a typical energy efficiency is 36.6 GJ/t-NH3

41-42

(corresponding to 500 g-NH3/kWh) with 15% H2 conversion; this does not include the energy consumption for hydrocarbon reforming to produce H2. Clearly, improved approaches are needed; this will require a detailed examination, both experimental (for example by in-situ surface monitoring of 13



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surface reaction) and theoretical, of the underlying mechanisms of plasma catalysis to solve the problems preventing highly-efficient ammonia production despite the high densities of reactive species.

Mechanisms of ammonia production by plasma catalysis Having presented a general discussion of the influence of the plasma and its interactions with the catalyst, and considered the results reported in the literature for production of ammonia using plasma catalysis, we now discuss in more detail the mechanisms of particular relevance to ammonia production. We first briefly summarize the reaction mechanism of the Haber−Bosch process, to clarify the differences from and similarities with plasma synthesis. Figure 7 shows the potential energy diagram and mechanism of the catalytic process for ammonia production. The catalyst offers an alternate reaction path through which energy barriers can be overcome by the available thermal energy, instead of overcoming the prohibitively high energy barrier for dissociation of the reactant N2 and H2 molecules in the gas phase 34, 89. The surface reaction is initiated by the formation of surface-adsorbed N(s) and H(s), denoted by Nad and Had in Fig. 7, by dissociative adsorption (N2 → 2Nad, H2 → 2Had). Successive hydrogenation by the L−H (Langmuir−Hinshelwood) mechanism subsequently produces NH3(s) , which is then desorbed from the surface 89. In contrast, plasma synthesis provides several different chemical pathways, as shown in Table 3. As well as L−H interactions between NH2(s) and H(s), a number of E−R (Eley−Rideal) interactions can generate NH3. Although most NH3 is produced by surface reactions, it is possible to form NH3 through gas-phase reactions including three-body reactions, as also shown in Table 1, and H2 and NHx radical interactions and electron and ion recombination processes. The dissociation of the stable triple bond of N2 is the slowest step in its dissociative adsorption 89; the mechanism has been extensively researched, with efforts for improvement mainly directed towards finding efficient catalytic materials. On the transition metals that are typical catalysts in

14


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thermal ammonia production, it is understood that N2 donates an electron from its bonding orbitals and accepts a back-donated electron from the surface atom to its antibonding π-orbitals

33, 53

. The

presence of the back-donated electron in antibonding orbitals increases the internal energy of the N2 molecule, which further weakens the molecular nitrogen bonding and results in the dissociation of N2 33-34, 90-91

. This suggests that if the N2 molecule already has high internal energy before adsorption,

this will facilitate its dissociation to form surface-adsorbed atomic nitrogen. Plasmas generate high concentrations of vibrationally- and electronically-excited molecules; i.e. molecules with high internal energy. Molecular-beam experiments with N2 have demonstrated the dramatic enhancement of the dissociative sticking probability as the kinetic energy of incident molecules increases, along with faster dissociation rates of vibrationally-excited molecules

92

.

Quantum-mechanical models have also suggested a significantly-enhanced sticking probability for the dissociative adsorption of vibrationally-excited nitrogen 93-94. Excitation from the ground state (ν = 0) to the vibrational states ν = 1, 3, 10 was predicted to enhance the dissociative sticking probabilities at room temperature by factors of 6, 102 and 104, respectively

94

. Hence, these vibrationally- and

electronically-excited states have significantly higher reaction coefficients than the ground-state N2(X) and H2(X) species as shown Figure 8(a). Therefore, dissociative adsorption of excited molecules is expected to be an additional pathway for production of surface-adsorbed N(s) and H(s) in the plasma environment, particularly in low-electron-energy atmospheric-pressure discharges with low densities of dissociated atomic species. As shown in Figure 8(b), the coefficient of surface adsorption of reactive atoms as intermediate species is always very high (the sticking coefficient is often chosen to be close to 1 on metallic surface in plasma kinetic models

44

). Therefore, in high-electron-energy low-pressure plasmas or

thermal plasmas that provides high densities of radicals and atoms, the direct adsorption of atoms is dominant, and the reaction kinetics can be satisfactorily modeled without considering dissociative adsorption 43-44, 46. The dominant mechanism of formation of surface-adsorbed N(s) and H(s) thus depends on the electron energies and the type of plasma. It is worth stressing that in all cases, despite the highly15



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unfavorable pressures and temperatures, plasmas provide a much higher probability of generating surface-adsorbed N(s) and H(s) than thermal processes, either by direct adsorption of atoms (in lowpressure plasmas) or by efficient dissociative adsorption of molecules (in atmospheric-pressure plasmas). Ions also play an important role in plasma catalysis, particularly in the high-electron-energy plasmas typical of low-pressure conditions. Ions can produce reactive intermediate and dissociated atoms in the gas phase through reactions with electrons such as e + N2+ → 2N and e + NH4+ → NH3 + H

44, 47

. At the same time, when the ions are neutralized by the collisions on the surface, they can be

excited to a metastable state, which take part in either gas or surface chemical reactions with increased reaction coefficients 1. Energetic ions can also dissociate NH3, as do electrons. Therefore, efficient plasma catalytic production of ammonia requires consideration of both the production and loss mechanisms related to a range of species. High-energy electrons play an important role in producing NH3 molecules by increasing production of atoms, ions and radicals. However, electrons are also responsible for the main loss mechanism of the NH3 molecules that are produced 44, 47. Similarly, atomic hydrogen is important for ammonia production, but again it is one of the predominant species to dissociate NH3. The dissociated fragments of NH3 molecule, such as NH2, NH, N and H are efficiently recycled back to generate NH3. However, in a certain range of electron energies and densities, the concentration of ammonia decreases, indicating that high densities of high-energy electrons contribute more to dissociating the ammonia molecule than producing it 50. Therefore, there is an optimum range in electron energies and density for efficient plasma-catalysis processing 44, 50. We present a schematic diagram in Figure 9 to summarize the different characteristics of ammonia synthesis in different types of plasma, in comparison to the conventional Haber–Bosch process. In a low-pressure plasma, when the electron energy and the concentration of atoms is high enough, the surface adsorbed N(s) and H(s) are predominantly produced by a direct adsorption of dissociated N and H atoms and ion chemistry plays an important role as we discussed. As the pressure 16


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increases, the electron energy becomes too low to generate high concentrations of atoms, particularly N atoms, which have a very high dissociation energy, 9.8 eV, as shown in Table 1. Instead of atoms and ions, neutral radicals and vibrationally-excited molecules dominate the plasma chemistry at atmospheric pressure. Under the high-pressure and high-temperature conditions of thermal plasmas, densities of electrons atoms, ions and radicals are high. The gas temperature in the active plasma zone is too high to permit the formation of ammonia molecules. A rapid quenching of the plasma, as in the cascaded arcs discussed in the Thermal plasmas section above, can be used to reduce the dissociation rate of NH3 and thereby increase its production. As has been discussed, the predominant chemical pathway to synthesize NH3 depends strongly on the plasma characteristics. Based on our recent kinetic modeling of non-equilibrium atmosphericpressure plasmas 50, we present a simplified reaction mechanism as an example as below in reactions 1 to 8 and Figure 10. Because of the high reaction rate for the dissociative adsorption of hydrogen molecules (see Figure 3), the surface is preferentially occupied by hydrogen H(s) rather than nitrogen N(s) at low temperature. Despite the low electron energy comparing to a low-pressure plasma, reactions driven by the electron kinetics are still essential in atmospheric-pressure plasmas to supply excited intermediate species, vibrationally- and electronically-excited molecules and dissociated atoms. Between the atoms and excited species, many different reactive species may be generated in the gas phase and on the surface, as shown in reactions 3 to 5 and 7. As we discussed above, the produced ammonia is mainly dissociated by electron interactions (reactions 6 and 8). The NHx intermediates formed are recycled actively to synthesize NH3 again as shown in reactions 4, 5 and 7.

17



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1. H 2 + surf(s) → 2H(s) 2. e + N 2 → e + 2N 3. N + H*2 → H + NH 4. NH + H(s) → NH 2 (s) 5. NH 2 (s) + H(s) → NH3 6. e + NH3 → e + NH 2 + H 7. NH 2 + H(s) → NH3 8. e + NH3 → e + NH + H 2 Figure 10 shows a schematic diagram illustrating these reactions. However, it is important to emphasize that the many other reactions make significant contributions, and reaction rates depend on factors including gas temperature, surface reactivity and the value of the reduced electric field (electric field strength divided by gas-phase number density) E/N, so this should only be treated as an overview of the main reactions.

Possible

pathways

towards

improved

plasma–catalytic

ammonia

production We have presented the current status of ammonia production by plasma catalysis, and discussed the most important gas-phase and surface mechanisms in atmospheric-pressure plasma conditions. As noted, there remains a considerable performance gap between plasma systems and the thermal equilibrium Haber–Bosch process. In this section, we suggest possible means to improve the efficiency of ammonia production by plasma catalysis, based on our understanding of the gas-phase plasma chemistry and plasma–surface interactions.

Optimized electron-temperature and electron-density window The density and energy distributions of electrons are critical in determining the plasma characteristics and ammonia production performance, as discussed previously. While higher electron temperature Te and density ne increase the rate of forward reactions, they do not always increase ammonia production, because the energetic electrons also dissociate NH3 molecules. Above a certain 18


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range of electron temperatures and densities, the ammonia concentration can decrease as Te and ne increase 44, 50. Carrasco et al. presented calculations showing a significant increase in the concentration of NH3 was possible for Te in the 2–3 eV range 44. Below 2 eV or above 3 eV, the predicted density of NH3 was a factor of 2–3 times lower. However, this simulation was targeted at low-pressure (0.8–8 Pa) plasma conditions, with low collisional energy losses and low reactant number densities. In atmospheric-pressure plasmas, the optimum energy range of electrons is lower, around 1.0–1.5 eV, with a temporally- and spatially-averaged electron number density of around 108 cm-3

50

, with a

significantly higher reaction rate because of the considerably increased collisional frequency. To maximize the production yield of ammonia, the low-electron energy atmospheric-pressure discharge has been widely agreed to be the best candidate among other types of plasmas. However, considering the fact that the gas pressure, gas composition, gas and surface temperature and the number density of electrons all affect ammonia synthesis, and that they are often have a complex relation with each other, as well as with the electron energy, there needs to be more careful selection and control of the operating window of the electron temperature and the density for a particular electrode configuration and input power source.

Gas temperature control – to optimize surface reactions In plasma catalysis using an N2–H2 plasma at atmospheric pressure and close to ambient temperature, it was found that surface can be predominantly occupied by surface-adsorbed hydrogen rather than the nitrogen

50

. This is understandable because of a significantly higher reaction

probability of the dissociative adsorption of hydrogen (γ < 0.1) comparing to nitrogen (γ < 1x10-6) 44, 89

, giving considerably a larger reaction coefficient for hydrogen adsorption, as shown in Figure 8(a),

particularly at low temperature. Ertl et al. investigated the interaction between nitrogen and hydrogen on iron surfaces using thermal desorption and Auger spectroscopy. It was found that the surfaceadsorbed hydrogen H(s) can inhibit the adsorption of nitrogen, and ammonia synthesis is favoured by high reaction temperatures ( ≥ 700 K), for which the concentration of H(s) is reduced 95. 19



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This hydrogen-dominated surface chemistry hinders the dissociative adsorption of nitrogen, which is important in increasing the rate of ammonia production; similarly to the hydrogen poisoning that occurs in the thermal equilibrium process using a Ru-loaded catalyst, where the sufficiently lowered activation barrier for the dissociation of N2 can detrimentally increase the adsorption of H2 33. In order to provide available active surface sites for nitrogen and other NHx intermediate species, we suggest simply increasing the surface temperature. This is expected to significantly improve ammonia production by accelerating the recombination of surface-adsorbed H(s). Since the surface temperature will equilibrate with the plasma gas (or heavy-species) temperature, this can be achieved by increasing the gas temperature of the plasma. In low-temperature plasma catalysis, recombination reactions mainly proceed by the E−R (Eley−Rideal) mechanism, such as H + H(s) → H2 or N + N(s) → N2. However, as gas and wall temperatures increase, the role of L−H (Langmuir−Hinshelwood) interactions becomes more important, leading to a significant enhancement in the production of ammonia. As Ertl addressed, surface-adsorbed hydrogen can be desorbed above 200°C (in vacuo); in contrast, surface-adsorbed nitrogen requires over 450°C

89

, indicating that the operating temperature has to be optimized to

maintain a desirable balance between the surface-adsorbed H(s) and N(s). As shown in Figure 11 (a) and (b), which present an extension of our plasma chemistry modeling work

50

performed using ZDPlaskin

96

, the important production and loss mechanism shows a

difference in ammonia generation depending on the temperature condition. As the gas temperature increases, overall gas phase and surface reaction increase except for three-body reaction shown in Figure 11(b). The ammonia production rate through the L−H interaction of NH2(s) and H(s) increased by a factor of 6.5. In particular, Figure 11 (c) and (d) shows a dramatic increase in L−H interaction between H(s) and N(s) to form NH(s) in comparison to low-temperature process characteristics where E−R interaction is dominant. As a result, the predicted concentration of NH3 shows a significant increase in Figure 12, particularly above 500 K. The production of NH radicals is known to be critical in ammonia synthesis,

20


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and the NH concentration is found to have a similar dependence on gas temperature to the NH3 concentration. To provide a more detailed understanding of surface reactions, we have added surface-absorbed NH3(s) to the species considered, and the absorption, desorption and dissociation of NH3(s), to the plasma chemistry model presented in Ref. 50. The activation energy values were obtained from Lin et al.

97

. We find that at 300 K, a large amount of NH3 remains bound to the surface. Increasing the

temperature to 400 K greatly reduces the NH3(s) density, increasing the number of available active surface sites (Surf) and the N(s) density, but also greatly increasing the H(s) density. Increasing the temperature further from 400 K to 600 K decreases the H(s) density by only a small amount, but provides over 30 times increase in available active surface sites in a given condition, which leads to significantly-increased densities of N(s) and NHx(s) on the surface, and a much higher density of gasphase NH3. When the surface temperature reaches 700 K, the surface-adsorbed H(s) rapidly recombines and desorbs to form H2 molecules in the plasma discharge volume and N(s) becomes the predominant surface-adsorbed species. While the model did not give reliable predictions of gas-phase composition at this temperature, it is expected that the NH3 density will increase further. As a summary, we provide the schematic diagram of ammonia synthesis in non-equilibrium atmospheric-pressure discharge in Figure 13. In a less reactive surface condition or low surface and gas temperature, E−R mechanism influences the plasma chemistry more strongly. As temperature increases L−H interaction significantly enhances including L−H recombination of H(s) to H2. Hence, the predominant hydrogen can yield available surface sites to N or NHx radicals and the production yield of ammonia can be significantly improved. There is some experimental evidence of the influence of the gas temperature. Bai et al. measured an exponential growth of ammonia concentration as the gas temperature increases

84

. Kim et al.

reported a similar trend. Below 150°C, thermal catalysis did not occur and at 200°C, when an ammonia concentration of 10 ppm was observed without a plasma, the presence of a plasma provided a significant increase to 810 ppm of NH3. This increased further to approximately 1500 ppm at 250°C 19

. 21



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However, Bai et al. found that higher temperatures can lead to electrode breakdown and reactor damage by severe arcing. The gas temperature can significantly affect the plasma properties, especially at high pressure, and the breakdown voltage can be significantly reduced by increased gas temperature. Therefore, careful reactor design and catalyst selection is required to prevent arcing and minimize the parasitic discharges. This will be discussed in more detail below in relation to suggested improvements in reactor design.

Catalyst optimization for plasma catalysis As mentioned earlier, an ideal catalyst for plasma catalysis can be different from that for a thermal process. Considering the complexities of the plasma–surface interactions and the likely influence of the electric field of the plasma on the catalyst, a catalyst optimized for thermal processing may not perform in exactly the same way in plasma catalysis. So far, the most effective catalyst in plasma catalysis in terms of the concentration of produced ammonia was found to be Cs−Ru supported on MgO 85, with the concentration reaching 12% with gas recirculation. As shown in Figure 14, Peng et al. hypothesized that Cs will readily donate electrons to Ru and Ru will transfer the extra electron to the adsorbate, i.e. surface-adsorbed N2 molecules in the ammonia-production process, and the electron may enter into the anti-bonding state, weakening the bond and finally leading to dissociation 21. Mizushima et al. interpreted the role of Ru differently. As illustrated in Figure 15, they suggested Ru plays an important role in hydrogenation of adsorbed nitrogen through ‘spill-over’ reactions to deliver a hydrogen atom to an adsorbed nitrogen atom 82. Carbon materials such as nanodiamonds or diamond-like carbon coatings are not a conventional choice for ammonia synthesis. However, the interesting combination of sp3/sp2 bonding properties and surface functional groups has been reported to offer new opportunities in some catalysis applications 98

. For the individual dissociative adsorption process, carbon materials may not function in the same

way as transition metals. However, a thin layer of sp2 carbon surrounding the nanodiamond core is understood to be responsible for the remarkable chemical reactivity, because of the delocalized π– electron density

98-99

. In addition, the overall catalytic activity is determined by the balance between 22


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adsorption and desorption reactions. In this context, carbon materials are worthy of investigation as an alternative to conventional catalytic materials, and to closely study the surface chemistry in plasma catalysis 22. It is also important to stress that the surface functionality, and not the bulk material, predominantly governs the plasma–catalyst interaction

45

. The functionality of the carbonyl (C=O)

group is particularly promising to enhance hydrogen adsorption and allow balanced desorption

22

.

However, the inconsistent initial chemical properties of nanodiamonds produced by typical detonation processes and the strong dependence of the reactivity of the carbonyl group on the annealing process remain to be resolved. Methods to sustain the optimized strength of the C=O bond to provide efficient adsorption and desorption reactions have to be developed. Among the remarkable outpouring of new catalysts for ammonia synthesis applications, Ruloaded electride [Ca24Al28O64]4+(e-)4 (Ru/C12A7:e-)

33

appears to be a good candidate to explore for

plasma catalysis. An electride (C12A7:e-) is an efficient electron donor for the Ru catalyst, is chemically stable and allows reversible hydrogen adsorption and desorption. As a result, with nearly half the reaction activation energy, the catalytic activity showed a factor of ten increase over other types of Ru-loaded catalysts in thermal ammonia production. As noted above, however, optimization for thermal catalysis does not necessarily correspond to optimum plasma-catalysis properties.

Pulsed plasma excitation to optimize energy efficiency Applying a pulsed or high-frequency input power with duty-cycle control in order to improve energy efficiency is common in many plasma applications. Our measurements revealed a significant increase in ammonia concentration, from 0.04 vol% to 0.8 vol%, when the input frequency was doubled from 500 Hz to 1 kHz at the same applied voltage of 14 kV, with a 10% increase in reduced electric field strength E/N and a doubled electron density 22. With these input parameters, the model predicted a factor of 6.3 increase in NH3 density, as shown in the top two rows of Table 4, because of the increased input energy and the higher E/N. The results in the bottom two rows suggest that by controlling the duty cycle of the AC input power, efficient NH3 production is possible with lower 23



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energy input; in particular, the NH3 density is predicted to be 2.4 times larger with a lower energy input for 1 kHz excitation with 40% duty cycle than 500 Hz excitation with 100% duty cycle. Using a much higher frequency than in the kHz range with narrower pulse width, such as nanosecond pulsing, is worth investigating for ammonia synthesis to make further improvements in energy efficiency and yield, as in many successful plasma applications 100-102.

High-volume discharges: using specific dielectric materials to overcome limitations of highpressure discharges The breakdown voltage required to initiate a plasma discharge in a given configuration increases proportionally to the product of the pressure (P) and the gap distance (d) between the counter electrodes, following Paschen’s curve, above Pd ∼ 1 Pa m. Typically-available AC power sources in the range 1 kHz – 100 kHz provided zero-to-peak voltages of less than 15 kV. As a rule of thumb, the minimum electric field strength required to cause breakdown in air is approximately 30 kV/cm. The nominal gap distance is then only several mm at atmospheric pressure, so only a limited discharge volume is available, unlike for a low-pressure discharge. This is one of the disadvantages of sustaining plasma discharges at higher pressures. Including ferroelectric materials such as BaTiO3 or PZT, or a less electrically-resistive layer such as a DLC (diamond-like carbon) coating, were demonstrated to enable a large discharge volume with enhanced induced current and power density, leads to higher electron densities

20, 22

. (Note that DLC can be modified to give a wide range of

electrical characteristics 103.) Having a large discharge gap or volume can increase the production rate, since the residence time within the plasma volume for a given gas feed rate increases with the volume.

Reactor design, based on the example of ozone generators Ozone generation is the most successful industrial applications of atmospheric-pressure plasmas 104

. Yamabe et al.

105

achieved an energy efficiency of 400–450 kg-O3 (Wh)-1 in a laboratory-scale

system. Although efficiencies are lower at industrial scales, the production rate can reach several hundred kg/h depending on the feed gas (air or oxygen) and the concentration of ozone produced can 24


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reach 15% in a single pass, and can be controlled and regulated through filtration and recirculation up to 100%. As shown in Figure 16, typical commercial ozone generators use cylindrical discharge tubes of about 20–50 mm diameter and 1–3 m in length. Borosilicate glass is a favored dielectric material, and is mounted inside slightly-wider stainless-steel tubes to form annular discharge gaps of about 0.5– 1 mm width

104

. In ammonia synthesis, in contrast, the space inside the dielectric tube needs to be

filled with catalyst, which is critical to ensure that a stable and uniform discharge fills the gap. The gap between the outside of the dielectric tube and the ground electrode needs to be minimized to avoid unnecessary power dissipation by parasitic discharges, because the active discharge volume is sustained inside the dielectric tube for ammonia production. A metal mesh or sponge-like material would be the best choice of ground electrode to handle thermal expansion while still having high electrical conductivity. An overall schematic is shown in Figure 17. Each individual discharge tube can be electrically connected. Since operation at elevated temperature is an attractive option, as discussed above, and this will lead to thermal expansion and increased likelihood of arcing, a careful approach is needed to determine appropriate insulation materials and separation of the parts at high voltage from the surrounding wall. The wall and the ground electrode of each discharge tube need to be electrically connected at all times. It should be noted that the ammonia production volumes obtainable in such an arrangement are unlikely to match that obtainable in large-scale Haber–Bosch reactors. However, plasma reactors are likely to provide sufficient ammonia production for applications requiring on-site production of ammonia, for example for storage and transport of hydrogen produced by renewable energy.

Conclusion Plasma catalysis, particularly at atmospheric-pressure and close to ambient temperature, is an attractive option for gas treatment and gas production. The yields of ammonia obtained in many experiments are reasonably high, although energy efficiencies are still too low. With catalyst surface temperature control, optimization of catalysts and discharge geometries and applying duration25



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controlled pulsed power, it is expected that substantial improvements will be possible. Plasma processes are likely to be suitable for small-scale on-site production of ammonia, particular with the use of ferroelectric or DLC-coatings to allow large-volume discharges at relatively-low voltages.

26


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10

-3

nex EEDF [cm eV-3/2]

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10 10 10 10 10 10 10 10 10

9 8

Bare Al2O3

7

DLC-coated Al2O3

6 5 4 3 2 1 0

0

5

10

15

Energy[eV] Figure 1. Electron energy distributions for different surface conditions of dielectric packing material in an atmospheric-pressure N2–H2 discharge at 1 kHz, 12 kVp: uncoated Al2O3 and Al2O3 coated with diamond-like carbon 22. [Reprinted from Hong, J. M.; Aramesh, M.; Shimoni, O.; Seo, D. H.; Yick, S.; Greig, A.; Charles, C.; Prawer, S.; Murphy, A. B., Plasma catalytic synthesis of ammonia using functionalized-carbon coatings in an atmospheric-pressure non-equilibrium discharge, Plasma Chem Plasma P 2016, 36, 917-940, with permission of Springer.]

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Figure 2. (a) Schematic of the expanding cascade arc plasma source and (b) a photograph of the discharge in the active plasma region and the remote processing chamber 68-69. [(a) Reprinted from Peerenboom, K. S. C.; van Dijk, J.; Goedheer, W. W. J.; Kroesen, G. M. W., Effects of magnetization on an expanding high-enthalpy plasma jet in argon. Plasma Sources Sci T 2013, 22 (2), 025010, © IOP Publishing. Reproduced with permission. All rights reserved. (b) Reprinted, with permission, from van Helden, J. H.; Zijlmans, R.; Engeln, R.; Schram, D. C., Molecule Formation in N and O Containing Plasmas. IEEE Trans. Plasma Sci., Vol. 33, No. 2, 390 – 391, Apr 2005. ©2005 IEEE.]

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Figure 3. Influence of input power on (a) produced ammonia concentration on zeolite 73, (b) intensities of the band head of NH (A 3Π - X 3Σ-) (△) and the hydrogen atomic line (Hβ)(▲); (c) intensities of the peaks due to NH3 + (▽) and H2 + (▼); and (d) the electron energy kTe (□). [Reprinted from Uyama, H.; Matsumoto, O., Synthesis of ammonia in high-frequency discharges .2. Synthesis of ammonia in a microwave-discharge under various conditions. Plasma Chem Plasma P 1989, 9 (3), 421-432, with permission of Springer.]

29



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Figure 4. Non-thermal plasma ammonia synthesis and absorption system 21; inset is added to show the cross sectional configuration of typical pack-bed DBD (Dielectric Barrier Discharge) reactor. [Adapted from Peng, P.; Li, Y.; Cheng, Y.; Deng, S.; Chen, P.; Ruan, R., Atmospheric pressure ammonia synthesis using non-thermal plasma assisted catalysis. Plasma Chem Plasma P 2016, 36, 1201-1210, with permission of Springer.]

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Figure 5. A schematic of the tubular plasma reactor equipped with the membrane-like catalyst 81-82. [Reprinted from Mizushima, T.; Matsumoto, K.; Sugoh, J.; Ohkita, H.; Kakuta, N., Tubular membrane-like catalyst for reactor with dielectric-barrier-discharge plasma and its performance in ammonia synthesis. Appl Catal a-Gen 2004, 265 (1), 53-59, Copyright 2004, with permission from Elsevier. Reproduced from Mizushima, T.; Matsumoto, K.; Ohkita, H.; Kakuta, N., Catalytic effects of metal-loaded membrane-like alumina tubes on ammonia synthesis in atmospheric pressure plasma by dielectric barrier discharge. Plasma Chem Plasma P 2007, 27 (1), 1-11, with permission of Springer.]

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Figure 6. A schematic and photo of DBD reactor equipped with an outer mesh ground electrode and copper wool high-voltage electrode 31. [Reproduced from Aihara, K.; Akiyama, M.; Deguchi, T.; Tanaka, M.; Hagiwara, R.; Iwamoto, M., Remarkable catalysis of a wool-like copper electrode for NH3 synthesis from N2 and H2 in non-thermal atmospheric plasma. Chem Commun 2016, 52 (93), 13560-13563, with permission of The Royal Society of Chemistry.]

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Figure 7. Potential energy diagram and mechanism of ammonia synthesis on Fe surface 90. The energies are given in kJ/mol. [Reprinted from Ertl, G., Surface science and catalysis - studies on the mechanism of ammonia-synthesis - the Emmett, P.H. Award address. Catal Rev 1980, 21 (2), 201223, by permission of the Taylor & Francis Ltd, http://www.tandfonline.com.]

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Figure 8. (a) Calculated dissociative adsorption coefficients of molecular N2 and H2 as a function of the sticking probability; the sticking probabilities relevant for dissociative surface adsorption of N2 and H2 on Al2O3 and Fe are shown with arrows; (b) calculated reaction coefficients of direct surface adsorption of gas-phase atomic nitrogen and hydrogen as a function of the sticking probability at 400 K 50. [Reprinted from Hong, J.; Pancheshnyi, S.; Tam, E.; Lowke, J., J.; Pawer, S.; Murphy, A., B., Kinetic modelling of NH3 production in N2 –H2 non-equilibrium atmosphericpressure plasma catalysis. Journal of Physics D: Applied Physics 2017, 50 (15), 154005, © IOP Publishing. Reproduced with permission. All rights reserved.]

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Figure 9. Comparison of characteristics of different plasmas: typical electron energy, density and gas temperature range.

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Figure 10. Schematic showing selected mechanisms of ammonia synthesis; 1) formation of adsorbed hydrogen H(s) through the dissociative adsorption of molecular hydrogen, 2) supply of atomic nitrogen from direct excitation by electron, 3) generation of NH radical from atomic N and electronically-excited H2*, 4) generation of surface-adsorbed NH2(s) by the interaction between H(s) and gas phase NH radical, 5) NH3 production by L-H mechanism between NH2(s) and H(s), 6) dissociation of NH3 into NH2 radical and atomic H by electron interaction, 7) recycling of NH2 radicals to reform NH3 through the surface interaction with H(s) and 8) alternative pathway of the dissociation of NH3 into stable H2 molecule and NH radical by electron interaction 50. Note that the contribution from the electronically-excited H2* molecule has been found to be more important for the formation of NH radical than vibrationally-excited H2(X ν), leading to a minor change in the schematic compared to that shown in Ref.50. [Reprinted from Hong, J.; Pancheshnyi, S.; Tam, E.; Lowke, J., J.; Pawer, S.; Murphy, A., B., Kinetic modelling of NH3 production in N2 –H2 nonequilibrium atmospheric-pressure plasma catalysis. Journal of Physics D: Applied Physics 2017, 50 (15), 154005, © IOP Publishing. Reproduced with permission. All rights reserved.]

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Figure 11. Important production and loss mechanisms; NH3 (a) at Tg 400 K, (b) at Tg 600 K, N(s) (c) at Tg 400 K and (d) Tg 600 K; reduced electric field strength E/N = 51 Td, ne 1.05 × 108 cm-3.

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Figure 12. Comparison of densities of (a) gas: NH3 (left y-axis), NH (right y-axis): H(s) (left y-axis), NH3(s), N(s) and Surf(right y-axis) and (b) surface species at different gas/wall temperatures in lowelectron energy atmospheric-pressure discharge. ‘Surf’ indicates the available active sites. Reduced electric field E/N 51.0 Td, electron number density ne 1.05x108 cm-3, N2:H2 input gas composition 1:3 and Fe surface assumed

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Figure 13. Summarized schematic diagram of important surface reaction mechanism in different surface reactivity and temperature condition.

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Figure 14. Enhanced dissociative adsorption of N2 by Cs-Ru catalyst 21. [Reprinted from Peng, P.; Li, Y.; Cheng, Y.; Deng, S.; Chen, P.; Ruan, R., Atmospheric pressure ammonia synthesis using nonthermal plasma assisted catalysis. Plasma Chem Plasma P 2016, 36, 1201-1210, with permission of Springer.]

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Figure 15. Ammonia synthesis mechanism with the assisted hydrogenation by Ru 82. [Reprinted from Mizushima, T.; Matsumoto, K.; Ohkita, H.; Kakuta, N., Catalytic effects of metal-loaded membranelike alumina tubes on ammonia synthesis in atmospheric pressure plasma by dielectric barrier discharge. Plasma Chem Plasma P 2007, 27 (1), 1-11, with permission of Springer.]

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Figure 16. (a) Configuration of discharge tubes in an industrial ozone generator 104; (b) reactor chamber of large ozone generator producing 60 kg-O3/h 104. [Reprinted from Kogelschatz, U., Dielectric-barrier discharges: Their history, discharge physics, and industrial applications. Plasma Chem Plasma P 2003, 23 (1), 1-46, with permission of Springer.]

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Figure 17. Possible configuration for large scale ammonia production adapting the design of commercial ozone generator.

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Table 1. Important gas-phase reactions and typical number densities of important gas phase species in N2–H2 plasmas for ammonia production Gas-phase reactions in N2–H2 plasma

Threshold energy [eV]

Mole fraction x / Rate coefficient k [cm6 s-1] / Production rate d/dt [cm-3 s-1]

Lowpressure plasma 47

Atmosphericpressure plasma 50

Ionization

N2 + e → N2+ + e H2 + e → H2+ + e

15.6 15.4

x[e]

1x10-6

Dissociation

N2 + e → 2N + e

9.8

x[N]

b

1x10-4

1x10-8

H2 + e → 2H + e

4.5

x[H]

b

1x10-3

1x10-6

Vibrational excitation N2(X) → N2(X ν=1)

0.29

x[N2(X ν=1)]

N/A

1x10-4

Electronic excitation N2(X) → N2(A3)

6.17

x[N2(A3)]

1x10-5

1x10-11

1.43 47

x[NH3]

Excitation

Chemical reaction

N + H2* → H + NH NH + H2 + M → NH3 + M (M = N2 and H2) NH2 + H + M → NH3 + M (M = N2 and H2)

c

b

k d[NH3]/dt N/A

e

k d[NH3]/dt

a

e

1x10-4

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1x10-11

d

1x10-2

1x10-30

1x10-32

Plasma Catalysis as an Alternative Route for Ammonia Production

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