Nitrogen Fixation via Plasma-Assisted Processes: Mechanisms, Applications, and Comparative Analysis—A Comprehensive Review

Abstract

Nitrogen fixation, the conversion of atmospheric nitrogen into biologically useful compounds, is crucial for sustaining biological processes and industrial productivity. Recent advances have explored plasma-assisted processes as an innovative approach to facilitate nitrogen fixation. This review offers a comprehensive summary of the development, current state of the art, and potential future applications of plasma-based nitrogen fixation. The analysis encompasses fundamental principles, mechanisms, advantages, challenges, and prospects associated with plasma-induced nitrogen fixation.

Full text

Citation: Klimek, A.; Piercey, D.G.
Nitrogen Fixation via Plasma-Assisted
Processes: Mechanisms, Applications,
and Comparative Analysis—A
Comprehensive Review. Processes
2024,12, 786. https://doi.org/
10.3390/pr12040786
Academic Editor: Francesco Parrino
Received: 22 February 2024
Revised: 5 April 2024
Accepted: 8 April 2024
Published: 13 April 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
processes
Review
Nitrogen Fixation via Plasma-Assisted Processes:
Mechanisms, Applications, and Comparative Analysis—A
Comprehensive Review
Angelique Klimek 1,2 and Davin G. Piercey 2,3,4,*
1School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47906, USA;
klimek0@purdue.edu
2Purdue Energetics Research Center, Purdue University, 205 Gates Road, West Lafayette, IN 47906, USA
3School of Materials Engineering, Purdue University, 205 Gates Road, West Lafayette, IN 47906, USA
4School of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47906, USA
*Correspondence: dpiercey@purdue.edu
Abstract: Nitrogen fixation, the conversion of atmospheric nitrogen into biologically useful com-
pounds, is crucial for sustaining biological processes and industrial productivity. Recent advances
have explored plasma-assisted processes as an innovative approach to facilitate nitrogen fixation.
This review offers a comprehensive summary of the development, current state of the art, and poten-
tial future applications of plasma-based nitrogen fixation. The analysis encompasses fundamental
principles, mechanisms, advantages, challenges, and prospects associated with plasma-induced
nitrogen fixation.
Keywords: nitrogen fixation; nitric oxide; plasma reactors; plasma catalysis; energy efficiency
1. Introduction
Nitrogen is a crucial element of numerous biomolecules, such as ATP, DNA, RNA,
and amino acids. Nitrogen is essential for synthesizing proteins, photosynthesis, genetic
character determination, and all other vital processes in life [
1
–
3
]. It can be found in a variety
of forms, both organic and inorganic, that can be encountered in nature. It is also required
to synthesize numerous chemicals, including fertilizers, pharmaceuticals, explosives, and
pigments [
4
]. Nitrogen N
2
, which comprises 78% of atmospheric air, contains more than
99% of nitrogen globally [
5
]. However, since N
2
is chemically inert, most organisms cannot
access it. Thus, it must first undergo a process referred to as nitrogen fixation to transform
it into a reactive form (such as ammonia or nitrates). This procedure involves bonding
elements such as O, H, and C to an N atom that originated from breaking the triple bond
of N2.
The most common form of nitrogen fixation worldwide is biological nitrogen fixation
(BNF). However, because of its comparatively slow speed, the process cannot supply
enough fertilizer to support the expanding population. According to a study conducted
by the United Nations Food and Agriculture Organization (FAO), 9 million people remain
hungry as there is a gap between global grain output and demand of 2.216 billion tons
vs. 2.254 billion tons of grain [
6
]. The Haber-Bosch (H-B) process is the most commonly
used commercial nitrogen fixation method. It generates ammonia through a chemical
reaction involving hydrogen and nitrogen at high temperatures and pressures, utilizing
heterogeneous catalysts. Up until 2010, more than 120 million tons of nitrogen per year were
fixed using this process, with approximately 80% employed as fertilizer and the remaining
20% serving as feedstock for the synthesis of other nitrogen-containing compounds [
7
–
9
].
Industrial nitrogen fixation has played a pivotal role in exponentially increasing global
food production to meet the demands of the world’s fast-growing population. Studies
Processes 2024,12, 786. https://doi.org/10.3390/pr12040786 https://www.mdpi.com/journal/processes

Processes 2024,12, 786 2 of 19
by Smil et al. and Erisman et al. reported that, by the end of the 20th century, about 40%
of the world’s population was utilizing fertilizers [
7
,
10
,
11
]. By 2008, this percentage had
risen to 48% of the world’s population, underscoring the substantial impact of industrial
nitrogen fixation. Given the anticipated world population growth, there is an imminent and
growing demand for fertilizers, emphasizing the critical need for continued development
of industrial nitrogen fixation techniques [12].
Numerous attempts have been undertaken throughout history to artificially fix nitro-
gen, including the H-B process, the Frank-Carlo method [
13
–
15
], and the Birkland-Eyde
(B-E) process [
13
,
16
–
18
]. Among these, the H-B process, developed by Fritz Haber and
commercially implemented by Carl Bosch, is largely regarded as one of the largest and
most notable developments of the 20th century. [
10
]. Ammonia is produced through the
reaction of N
2
and H
2
at high temperatures (450–600
◦
C) and pressures (150–350 bar) in the
presence of catalysts, as depicted in Figure 1. The H-B process has undergone extensive
optimization over the past century, leveraging advancements in catalyst technology, the
usage of natural gas as a feedstock instead of coal, and improved heat integration, among
other factors. Consequently, this optimization has led to a reduction in energy usage to 0.48
MJ/mol of generated ammonia [19,20].
Processes 2024, 12, x FOR PEER REVIEW 2 of 19
production to meet the demands of the world’s fast-growing population. Studies by Smil
et al. and Erisman et al. reported that, by the end of the 20th century, about 40% of the
world’s population was utilizing fertilizers [7,10,11]. By 2008, this percentage had risen to
48% of the world’s population, underscoring the substantial impact of industrial nitrogen
fixation. Given the anticipated world population growth, there is an imminent and grow-
ing demand for fertilizers, emphasizing the critical need for continued development of
industrial nitrogen fixation techniques [12].
Numerous aempts have been undertaken throughout history to artificially fix nitro-
gen, including the H-B process, the Frank-Carlo method [13–15], and the Birkland-Eyde
(B-E) process [13,16–18]. Among these, the H-B process, developed by Fri Haber and
commercially implemented by Carl Bosch, is largely regarded as one of the largest and
most notable developments of the 20th century. [10]. Ammonia is produced through the
reaction of N2 and H2 at high temperatures (450–600 °C) and pressures (150–350 bar) in
the presence of catalysts, as depicted in Figure 1. The H-B process has undergone exten-
sive optimization over the past century, leveraging advancements in catalyst technology,
the usage of natural gas as a feedstock instead of coal, and improved heat integration,
among other factors. Consequently, this optimization has led to a reduction in energy us-
age to 0.48 MJ/mol of generated ammonia [19,20].
Figure 1. A flow scheme for the Haber-Bosch process. Reprinted with permission from Ref. [21].
Despite being the primary method for industrial nitrogen fixation, the H-B process is
extremely energy-intensive and has adverse environmental impacts. This process requires
1–2% of the world’s total energy and 2% of the total natural gas, resulting in 300 million
metric tons of CO2 emissions [19,22,23]. The H-B process is currently very close to its the-
oretical energy efficiency limits, with hydrogen generation via the thermal or catalytic
cracking of fossil fuels at high temperatures and pressures over catalysts being the most
energy-intensive aspect of the process. Consequently, improvements in catalyst technol-
ogy are unlikely to significantly enhance energy efficiency [24]. As an alternative to natu-
ral gas as a hydrogen source, electricity-powered water spliing has been proposed. How-
ever, this technique is only viable if renewable electricity is employed, as it would other-
wise result in an overall energy consumption of around 1.5 MJ/mol, which is 3-fold greater
than the H-B process [20]. This is aributed to higher energy demands for H2 production
(360–480 kJ/mol [25]) compared to the steam methane reforming system. Improving the
energy efficiency of nitrogen fixation would be highly advantageous both economically
and environmentally, especially considering the rapidly expanding population and the
depletion of natural resources. Consequently, ongoing research into sustainable nitrogen fix-
ation techniques is evident in the literature, encompassing the advancement of plasma-as-
sisted nitrogen fixation processes and the utilization of metallocomplex homogeneous cata-
lysts.
Figure 1. A flow scheme for the Haber-Bosch process. Reprinted with permission from Ref. [21].
Despite being the primary method for industrial nitrogen fixation, the H-B process is
extremely energy-intensive and has adverse environmental impacts. This process requires
1–2% of the world’s total energy and 2% of the total natural gas, resulting in 300 million
metric tons of CO
2
emissions [
19
,
22
,
23
]. The H-B process is currently very close to its
theoretical energy efficiency limits, with hydrogen generation via the thermal or catalytic
cracking of fossil fuels at high temperatures and pressures over catalysts being the most
energy-intensive aspect of the process. Consequently, improvements in catalyst technology
are unlikely to significantly enhance energy efficiency [
24
]. As an alternative to natural gas
as a hydrogen source, electricity-powered water splitting has been proposed. However,
this technique is only viable if renewable electricity is employed, as it would otherwise
result in an overall energy consumption of around 1.5 MJ/mol, which is 3-fold greater
than the H-B process [
20
]. This is attributed to higher energy demands for H
2
production
(360–480 kJ/mol [
25
]) compared to the steam methane reforming system. Improving the
energy efficiency of nitrogen fixation would be highly advantageous both economically and
environmentally, especially considering the rapidly expanding population and the deple-
tion of natural resources. Consequently, ongoing research into sustainable nitrogen fixation
techniques is evident in the literature, encompassing the advancement of plasma-assisted
nitrogen fixation processes and the utilization of metallocomplex homogeneous catalysts.
Near the turn of the 20th century, as the world’s supply of fixed nitrogen was almost
depleted, Sir William Crookes, the President of the British Association at the time, called

Processes 2024,12, 786 3 of 19
attention to this issue. Sodium and potassium nitrate were the most extensively utilized
agricultural fertilizers in Europe, in the form of bird droppings accumulated and solidified
over millennia [
26
]. Plasma-assisted nitrogen fixation was among the first attempts at
industrial nitrogen fixation. A significant quantity of electrical energy can cause gas to
ionize, generating plasma [
27
–
29
]. A wide variety of pressures, temperatures, electron
densities, and electron temperatures can result in the formation of plasma, making a
universal approach to plasma classification challenging. Plasmas are divided into two
types: high-temperature plasmas and low-temperature plasmas. The temperature of ions
and electrons in high-temperature plasmas is roughly 10
7
K. Thermal and non-thermal
low-temperature plasmas are established [
30
]. Electrons, ions, and background gas are
all present in thermal plasma at a temperature of about 10
4
K. Thermal plasma is used in
arc plasma and plasma torches. However, in non-thermal plasmas, due to their smaller
mass, electrons are often at very high temperatures of the order of 10
5
K. In contrast, ions
and background gas are at room temperature. The first industrial method utilized for
nitrogen fixation was the Birkland-Eyde process, which in 1903 produced thermal plasma
at a high temperature for the generation of NO using an electrical arc discharge [
31
,
32
].
The airflow passed via an arc discharge zone before being quenched with water and going
through a succession of adsorption stages, yielding roughly 1% nitric oxide with an energy
consumption of 3.4–4.1 MJ/mol HNO3, as shown in Figure 2[31,32].
Processes 2024, 12, x FOR PEER REVIEW 3 of 19
Near the turn of the 20th century, as the world’s supply of fixed nitrogen was almost
depleted, Sir William Crookes, the President of the British Association at the time, called
aention to this issue. Sodium and potassium nitrate were the most extensively utilized
agricultural fertilizers in Europe, in the form of bird droppings accumulated and solidi-
fied over millennia [26]. Plasma-assisted nitrogen fixation was among the first aempts at
industrial nitrogen fixation. A significant quantity of electrical energy can cause gas to
ionize, generating plasma [27–29]. A wide variety of pressures, temperatures, electron
densities, and electron temperatures can result in the formation of plasma, making a uni-
versal approach to plasma classification challenging. Plasmas are divided into two types:
high-temperature plasmas and low-temperature plasmas. The temperature of ions and
electrons in high-temperature plasmas is roughly 107 K. Thermal and non-thermal low-
temperature plasmas are established [30]. Electrons, ions, and background gas are all pre-
sent in thermal plasma at a temperature of about 104 K. Thermal plasma is used in arc
plasma and plasma torches. However, in non-thermal plasmas, due to their smaller mass,
electrons are often at very high temperatures of the order of 105 K. In contrast, ions and
background gas are at room temperature. The first industrial method utilized for nitrogen
fixation was the Birkland-Eyde process, which in 1903 produced thermal plasma at a high
temperature for the generation of NO using an electrical arc discharge [31,32]. The airflow
passed via an arc discharge zone before being quenched with water and going through a
succession of adsorption stages, yielding roughly 1% nitric oxide with an energy con-
sumption of 3.4–4.1 MJ/mol HNO3, as shown in Figure 2 [31,32].
Figure 2. Scheme of Birkeland–Eyde arc discharge apparatus. Reprinted with permission from Ref. [20].
Thermodynamic reasoning can be used to rationalize thermal plasma NF by consid-
ering two opposing processes: (a) the creation of NO and (b) the atomization of molecules
of nitrogen and oxygen [29,33]. N2 and O2 molecules predominate at lower temperatures;
however, they dissociate into atoms at higher temperatures. The NO concentration
reaches its maximum at around 3500 K, at 6.5% [33]. This low concentration suggests that,
as opposed to N2 oxidation, most of the energy is used for gas heating.
Regarding environmental concerns, plasma processes hold a significant advantage
over the H-B process as they utilize abundant materials like air and water instead of
Figure 2. Scheme of Birkeland–Eyde arc discharge apparatus. Reprinted with permission from
Ref. [20].
Thermodynamic reasoning can be used to rationalize thermal plasma NF by consider-
ing two opposing processes: (a) the creation of NO and (b) the atomization of molecules
of nitrogen and oxygen [29,33]. N2and O2molecules predominate at lower temperatures;
however, they dissociate into atoms at higher temperatures. The NO concentration reaches
its maximum at around 3500 K, at 6.5% [
33
]. This low concentration suggests that, as
opposed to N2oxidation, most of the energy is used for gas heating.
Regarding environmental concerns, plasma processes hold a significant advantage
over the H-B process as they utilize abundant materials like air and water instead of expen-
sive H
2
. Moreover, there is the potential for plasma processes to be fueled by electricity

Processes 2024,12, 786 4 of 19
produced using renewable resources, such as solar or wind. According to research by the
International Energy Agency, silicon photovoltaics currently provide some of the cheapest
electricity historically as a result of economies of scale, technological advancements in
the production supply chain, increased productivity, and lower basic material costs [
34
].
Additionally, the procedure is sustainable, generating no waste or greenhouse gas emis-
sions. However, as documented in the literature to date, the energy efficiency of thermal
plasma is not competitive with the H-B process. Under speculative conditions of 20–30 bars,
3000–3500 K, and a cooling rate of 10
7
–10
8
K/s, thermal plasma has a theoretical energy
consumption of 0.86 MJ/mol of NO [
35
]. Non-thermal plasma utilization, however, has a
theoretical limit of 0.2 MJ/mol (for NOx synthesis) [
35
,
36
], a value lower than the limits of
the H-B process; however, to date, this limit has not been reached, meaning there is still
room for work in this field to reach the practical application. Furthermore, the non-thermal
plasma technique offers numerous technical benefits, including quick reaction times, rapid
control, and suitability for decentralized and small-scale production using intermittent
energy sources like solar, which traditional H-B cannot use [
29
]. The advancement of
plasma technology in recent decades has significantly influenced nitrogen fixation research,
resulting in numerous discoveries being reported. While there have been other recent
reviews in this field [
37
–
41
], the field of plasma-based nitrogen fixation is quickly growing,
as seen in Figure 3, wherein 2023, almost seven times as many papers were published on
this topic as in 2018. (Search results via Google Scholar using the search term “plasma
based” and “nitrogen fixation”). This review focuses on important work in the field before
the most recent years, as well as significant papers since then.
Processes 2024, 12, x FOR PEER REVIEW 4 of 19
expensive H
2
. Moreover, there is the potential for plasma processes to be fueled by elec-
tricity produced using renewable resources, such as solar or wind. According to research
by the International Energy Agency, silicon photovoltaics currently provide some of the
cheapest electricity historically as a result of economies of scale, technological advance-
ments in the production supply chain, increased productivity, and lower basic material
costs [34]. Additionally, the procedure is sustainable, generating no waste or greenhouse
gas emissions. However, as documented in the literature to date, the energy efficiency of
thermal plasma is not competitive with the H-B process. Under speculative conditions of
20–30 bars, 3000–3500 K, and a cooling rate of 10
7–
10
8
K/s, thermal plasma has a theoretical
energy consumption of 0.86 MJ/mol of NO [35]. Non-thermal plasma utilization, however,
has a theoretical limit of 0.2 MJ/mol (for NOx synthesis) [35,36], a value lower than the
limits of the H-B process; however, to date, this limit has not been reached, meaning there
is still room for work in this field to reach the practical application. Furthermore, the non-
thermal plasma technique offers numerous technical benefits, including quick reaction
times, rapid control, and suitability for decentralized and small-scale production using
intermient energy sources like solar, which traditional H-B cannot use [29]. The advance-
ment of plasma technology in recent decades has significantly influenced nitrogen fixation
research, resulting in numerous discoveries being reported. While there have been other
recent reviews in this field [37–41], the field of plasma-based nitrogen fixation is quickly
growing, as seen in Figure 3, wherein 2023, almost seven times as many papers were pub-
lished on this topic as in 2018. (Search results via Google Scholar using the search term
“plasma based” and “nitrogen fixation”). This review focuses on important work in the
field before the most recent years, as well as significant papers since then.
Figure 3. Number of results obtained via Google Scholar for publications on plasma-based nitrogen fix-
ation by year published. The results had to include phrases “plasma-based” and “nitrogen fixation”.
2. Plasma N
2
Fixation
The general process for plasma nitrogen fixation involves the reaction of nitrogen
with either hydrogen or oxygen, which results in the production of ammonia or nitrogen
oxide (nitric acid). In the case of ammonia synthesis, kinetic reaction rates and nitrogen
dissociation are greater at higher temperatures, whereas higher yields are obtained at
lower temperatures. In addition to easily available nitrogen, costly hydrogen is required
for plasma ammonia production. Air is an abundant and affordable raw material in the
creation of plasma nitric oxide. Because of the high dissociation energy of nitrogen, high-
21
33
48
97
110
146
51
0
20
40
60
80
100
120
140
160
2018 2019 2020 2021 2022 2023 2024
Figure 3. Number of results obtained via Google Scholar for publications on plasma-based nitrogen
fixation by year published. The results had to include phrases “plasma-based” and “nitrogen fixation”.
2. Plasma N2Fixation
The general process for plasma nitrogen fixation involves the reaction of nitrogen
with either hydrogen or oxygen, which results in the production of ammonia or nitrogen
oxide (nitric acid). In the case of ammonia synthesis, kinetic reaction rates and nitrogen
dissociation are greater at higher temperatures, whereas higher yields are obtained at
lower temperatures. In addition to easily available nitrogen, costly hydrogen is required
for plasma ammonia production. Air is an abundant and affordable raw material in the
creation of plasma nitric oxide. Because of the high dissociation energy of nitrogen, high-
temperature processing promotes NO reactions [
42
]. This review will focus on nitrogen
fixation to form nitrogen oxides.

Processes 2024,12, 786 5 of 19
The reaction mechanism of NOx formation follows the Zeldovich mechanism, which
begins with the dissociation of N2in Equation (1) and O2in Equation (2):
N2+ e* ⇌2N* + e−(1)
O2+ e* ⇌2O* + e−(2)
This is followed by the stage of NO generation, as shown in Equations (3) and (4)
below:
O* + N2→NO + N* (3)
N* + O2→NO + O* (4)
NO
2
can be created by oxygen radicals or ozone during continuous oxidation, as
illustrated in Equations (5)–(7):
O* + NO →NO2(5)
O* + O2→O3(6)
O3+ NO →NO2+ O2(7)
Various research groups have published a large number of studies on plasma nitrogen
fixing due to the rapidly advancing developments in our understanding of plasma reaction
kinetics [
43
–
51
]. Researchers have investigated the conversion of atmospheric nitrogen
fixation into NO for laboratory-scale procedures utilizing air plasma [
46
,
49
,
52
,
53
], from
N
2
–O
2
mixtures [
43
,
44
,
51
,
54
], and in argon; argon–nitrogen and nitrogen plasma [
47
,
54
,
55
].
Following earlier studies on thermal plasma (i.e., the electric arc), other plasma types
and plasma reactors have been examined for NO
x
production [
31
,
32
,
56
,
57
]. This includes
(pulsed) spark discharges [
58
–
63
], glow discharges [
59
,
64
], corona discharges [
58
,
65
], laser-
produced discharges [
66
], radio-frequency crossed discharges [
67
], (packed bed) dielectric
barrier discharges (DBD) [
59
,
68
–
70
], (pulsed) (rotating) (gliding) arc discharges [
59
,
71
–
78
],
microwave (MW) discharges [
54
,
79
,
80
], and plasma jets in contact with water [
81
–
91
].
The reported energy consumption for a variety of plasma reactors is listed in Table 1.
Energy consumption and plasma efficiency are difficult to compare between different
authors and publications due to the existence of diverse quantification methods. Energy
consumption is dependent on power and NO
x
concentration. NO
x
concentrations can be
determined via Fourier-transform infrared spectroscopy (FTIR) [
59
,
61
,
68
,
70
,
73
,
74
], mass
spectroscopy [
71
], chemiluminescence [
66
], ion chromatography chromatogram [
65
], as well
as a nondispersive infra-rede sensor with an ultra-violet sensor through a gas analyzer [
56
].
Power can be calculated by the Lissajous method [
59
,
68
], the numerical product of air
density, room temperature, heat capacity of the air, and volume [
66
], numerically integrating
the product of the voltage and current and multiplying by the frequency [
61
,
65
,
70
] where
the current can be measured by the resistor method [
56
,
71
,
73
,
74
] or by a current coil [
59
].
Total power also varies between literature papers as the addition of power consumption to
prepare pure oxygen gas from air to plasma power has been reported [
74
], or the placement
of the measured variables varies from before the plasma vs. right before the power supply.
All of these various methods used by various groups in the field can lead to uncertainty
between results reported by various methods.

Processes 2024,12, 786 6 of 19
Table 1. Comparison of the energy used in plasma reactors to produce NOX.
Plasma Type Energy Consumption
(MJ/mol N) Reference
Electric arc (Birkeland–Eyde) 2.4–3.1 [31,33,57]
Spark discharge 20.27, 40, 1.9–4.4 [58,61,62]
Plate-to-plate ns-pulsed spark discharge 22.18–25.72 [63]
Pin-to-plane ns-pulsed spark discharge 5.0–7.7 [59]
Transient spark discharge 8.6 [60]
(Positive/negative) DC corona discharge 1057/1673 [58]
Pulsed corona discharge 186 [65]
Radio-frequency crossed discharge 24–108 [67]
Laser-produced discharge 8.9 [66]
Pin-to-plane DC glow discharge 7, 2.8–4.8 [59,92]
Pin-to-pin DC glow discharge 2.8 [64]
Dielectric barrier discharge 56–140, 20.7 [59,69]
Packed dielectric barrier discharge 18, 17–33 [68,70]
DC plasma arc jet 3.6 [72]
Propeller arc 4.2 [59]
Pulsed milli-scale gliding arc 2.8–4.8 [73,74]
Gliding arc plasmatron 3.6 [71]
Rotating gliding arc 2.5, 1.8, 0.67, 4.2, 2.1 [68,75–78]
Microwave plasma 3.76 [79]
Microwave plasma with catalyst 0.84 [54]
Electron cyclotron resonance 0.28 [80]
Various gliding arc (GA) reactor layouts have demonstrated potential for gas conver-
sion applications [
71
,
73
–
77
,
93
–
95
]. GA plasmas have reduced electric fields of less than
100 Townsends (Td), resulting in electron energies of roughly 1 eV. Such electron energies
are particularly advantageous for the vibrational excitation of gas molecules [
93
]. Wang
et al. [
74
] researched NO
x
formation mechanisms in a milli-scale (reactor width of 135 mm
and a minimum discharge gap of 1.3 mm) GA reactor with pulsed power. Vervloessem
et al. [
71
] examined the creation of NO
X
in a flow gliding arc plasmatron (GAP) using
a reverse vortex. Schematics of these gliding arc systems can be found in Figure 4. The
results of the chemical kinetics modeling showed that vibrationally excited N
2
molecules
can reduce the nonthermal Zeldovich process’s energy barrier, allowing for more energy-
efficient NO generation. Moreover, the lower N
2
vibrational levels, whose vibrational
distribution function has a Boltzmann shape, undergo considerable thermal dissociation
due to the high gas temperature (>3000 K). The fraction of gas treated by the GA plasma
limits the total amount of N
2
conversion, even though the thermal reactions in GA reactors
are highly efficient at high temperatures. Only 15% of the gas passes through the plasma arc
in the GAP, and the remaining gas passes through the reactor without coming into contact
with the plasma. [
95
,
96
]. Vervloessem et al. [
71
] reported a 1.5% NO
X
yield at 3.6 MJ/mol
N energy consumption. The authors demonstrated that by optimizing the reactor and
avoiding the transmission of vibrational energy from N
2
to O
2
, energy consumption could
be reduced to 0.5 MJ/mol N; however, this finding must be investigated in practice [
71
].
Tsonev et al. [
75
] demonstrated that when pressure increases, the amount of NO
x
produced
increases dramatically, with a record-low EC of 1.8 MJ/mol N, a high production rate of
69 g/h, and a high selectivity (94%) of NO
2
. The authors credit this enhancement to the
enhanced thermal Zeldovich mechanism and a higher rate of NO oxidation relative to
the back reaction of NO with atomic oxygen caused by the elevated pressure [
75
]. Chen
et al. [
77
] found that the rotating gliding arc plasma can efficiently generate NO
x
from a
mixture of N
2
and O
2
or air, with low energy consumption and high processing capacity.
The lowest energy consumption for NO
x
production (4.2 MJ/mol N) is reached with a
voltage of 10 kV, a gas flow rate of 12 L/min, and an O
2
concentration of 20%. The order
of importance for process parameters concerning NO
x
concentration is gas flow rate fol-
lowed by O
2
concentration and applied voltage, while for energy consumption of NO
x

Processes 2024,12, 786 7 of 19
formation, the order is O
2
concentration followed by gas flow rate and applied voltage [
77
].
Muzammil et al. [
76
] showed that in a rotating gliding arc plasma, low specific energy
input (SEI) leads to optimal energy efficiency and high NO selectivity. However, the largest
production rate occurs at a high SEI. Their process has high NO selectivity (up to 95%) and
low energy consumption (~48 GJ per tN) at 0.1 kJ L
−1
SEI, which is equal to
0.67 MJ/mol N
.
Alphen et al. [
78
] focused on improving performance in a rotating gliding arc plasma by
developing an “effusion nozzle”. The nozzle acted as a heat sink, limiting the breakdown
of NO back into N
2
and O
2
due to the fast drop in gas temperature. This system allowed
the achievement of higher NO
x
concentrations of up to 5.9% while keeping the energy
consumption low at 2.1 MJ/mol N.
Processes 2024, 12, x FOR PEER REVIEW 7 of 19
of 10 kV, a gas flow rate of 12 L/min, and an O2 concentration of 20%. The order of im-
portance for process parameters concerning NOx concentration is gas flow rate followed
by O2 concentration and applied voltage, while for energy consumption of NOx formation,
the order is O2 concentration followed by gas flow rate and applied voltage [77]. Muzam-
mil et al. [76] showed that in a rotating gliding arc plasma, low specific energy input (SEI)
leads to optimal energy efficiency and high NO selectivity. However, the largest produc-
tion rate occurs at a high SEI. Their process has high NO selectivity (up to 95%) and low
energy consumption (~48 GJ per tN) at 0.1 kJ L-1 SEI, which is equal to 0.67 MJ/mol N.
Alphen et al. [78] focused on improving performance in a rotating gliding arc plasma by
developing an “effusion nozzle”. The nozzle acted as a heat sink, limiting the breakdown
of NO back into N2 and O2 due to the fast drop in gas temperature. This system allowed
the achievement of higher NOx concentrations of up to 5.9% while keeping the energy
consumption low at 2.1 MJ/mol N.
Figure 4. Schematic of gliding arc systems. (a) Conventional 2D gliding arc plasma. (b) Reverse
vortex flow gliding arc plasma. Reprinted with permission from Ref. [97].
Janda et al. [60] investigated the generation of NOx in a transient spark discharge.
This type of spark discharge starts as a non-thermal plasma known as the streamer phase
and transitions into quick spark current pulses that result in the production of a thermal
plasma [98,99]. Because of the high electron density (approximately 1017 cm3), the spark
phase is marked by significant chemical activity. Excited nitrogen molecules were de-
tected via time-integrated emission spectroscopy in both the streamer and spark phases,
and the energy consumed for NOx generation was 8.6 MJ/mol N [60]. Pavlovich et al. [61]
built a reactor that produced a spark-glow discharge, wherein the plasma discharge had
both a glow (non-thermal plasma) and a spark (thermal plasma) phase in a single cycle.
Schematics for a spark and glow discharge are illustrated in Figure 5. Pavlovich et al.
changed the percentage of the glow phase by adjusting the voltage waveforms. With its
high electron density and energy, the spark phase generated more NO, while the glow
phase made it easier for NO2 to develop. The production of NOX required up to 40 MJ/mol
N of energy. These plasma types often have a limited volume, which means that only a
portion of the N2 gas is exposed to the plasma, and consequently, only a small amount of
NOX is produced. Abdelaziz et al. [62] demonstrated that using a bipolar voltage at high
frequencies in a spark discharge reactor increased NOx production and efficiency by uti-
lizing residual species. Enlarging the plasma zone (reaction channel) by increasing the
electrode gap improves energy efficiency for the desired reaction. The limits of increasing
the gap were overcome by introducing a floating electrode, resulting in increased NOx
generation (1.8%−3.0%) and lower energy consumption (1.9−4.4 MJ/mol N). Zhang et al.
Figure 4. Schematic of gliding arc systems. (a) Conventional 2D gliding arc plasma. (b) Reverse
vortex flow gliding arc plasma. Reprinted with permission from Ref. [97].
Janda et al. [
60
] investigated the generation of NO
x
in a transient spark discharge.
This type of spark discharge starts as a non-thermal plasma known as the streamer phase
and transitions into quick spark current pulses that result in the production of a thermal
plasma [
98
,
99
]. Because of the high electron density (approximately 10
17
cm
3
), the spark
phase is marked by significant chemical activity. Excited nitrogen molecules were detected
via time-integrated emission spectroscopy in both the streamer and spark phases, and
the energy consumed for NO
x
generation was 8.6 MJ/mol N [
60
]. Pavlovich et al. [
61
]
built a reactor that produced a spark-glow discharge, wherein the plasma discharge had
both a glow (non-thermal plasma) and a spark (thermal plasma) phase in a single cycle.
Schematics for a spark and glow discharge are illustrated in Figure 5. Pavlovich et al.
changed the percentage of the glow phase by adjusting the voltage waveforms. With its
high electron density and energy, the spark phase generated more NO, while the glow
phase made it easier for NO
2
to develop. The production of NO
X
required up to 40
MJ/mol N of energy. These plasma types often have a limited volume, which means that
only a portion of the N
2
gas is exposed to the plasma, and consequently, only a small
amount of NO
X
is produced. Abdelaziz et al. [
62
] demonstrated that using a bipolar
voltage at high frequencies in a spark discharge reactor increased NO
x
production and
efficiency by utilizing residual species. Enlarging the plasma zone (reaction channel) by
increasing the electrode gap improves energy efficiency for the desired reaction. The
limits of increasing the gap were overcome by introducing a floating electrode, resulting in
increased NO
x
generation (1.8–3.0%) and lower energy consumption (1.9–4.4 MJ/mol N).
Zhang et al. [
63
] used a plate-to-plate setup to create a nanosecond pulsed spark discharge
for long-term nitrogen fixation under ambient circumstances. The airflow speeds ranged
from 40 to 340 mL min
−1
, resulting in an energy efficiency of 4–11 g kWh
−1
(equal to 22.18

Processes 2024,12, 786 8 of 19
MJ/mol N) and NO
x
production concentrations of 960–10,900 ppm. Using optical emission
spectroscopy and a chemical kinetics model, they discovered that the major intermediate
species in NO
x
reaction pathways are significantly influenced by plasma characteristics
and species residence duration in spark discharges [63].
Processes 2024, 12, x FOR PEER REVIEW 8 of 19
[63] used a plate-to-plate setup to create a nanosecond pulsed spark discharge for long-
term nitrogen fixation under ambient circumstances. The airflow speeds ranged from 40
to 340 mL min−1, resulting in an energy efficiency of 4–11 g kWh−1 (equal to 22.18 MJ/mol
N) and NOx production concentrations of 960–10 900 ppm. Using optical emission spec-
troscopy and a chemical kinetics model, they discovered that the major intermediate spe-
cies in NOx reaction pathways are significantly influenced by plasma characteristics and
species residence duration in spark discharges [63].
Figure 5. Schematic of air discharge in contact with water, (A) spark discharge, (B) glow discharge.
Reprinted with permission from Ref. [100].
Roy et al. [69] introduce a unique DBD system over water in contact with the elec-
trode for nitrogen fixation into NO3, which enables experiments to be carried out with a
minimum electrode separation, overcoming the problem of water surface instability. A
decrease in applied voltage and electrode spacing resulted in a promising energy cost for
nitrogen fixation as low as 20.7 MJ/mol N. Increasing the applied voltage leads to higher
production rates (up to 26 µmol/min) but also increases power absorption, reducing en-
ergy yield. Additionally, lower electrode gaps lead to a diffuse mode, which efficiently
produces NO3. Packed bed DBD reactors, as shown in Figure 6, have also been investi-
gated due to the promise of improving product selectivity and energy efficiency by per-
forming the plasma reaction over a catalyst. The generation of NOx in a DBD filled with
several catalyst support materials (α-Al2O3, γ-Al2O3, TiO2, MgO, TaTiO3, and quar wool)
was studied by Patil et al. [68]. A γ-Al2O3 catalyst with the lowest particle size of 250–160
mm produced the greatest results. However, the obtained energy cost was considerable
(18 MJ/mol N), and the product yield was low (0.5 mol%) in comparison to comparable
atmospheric pressure plasma reactors. Many metal oxides were deposited onto the sup-
port and assessed for their ability to produce NOx with plasma assistance. The maximum
concentration of NOx was formed by the 5% WO3/Al2O3 mixture, which was 10% more
than that of the γ-Al2O3 alone. Selectivity toward NO fell despite an increase in NOx con-
tent; this was explained by oxidation interactions with oxygen species on the catalyst sur-
face. The extremely low electric field, more than 100–200 Td, which produces extremely
energetic electrons and leads mainly to electronic excitation, ionization, and dissociation
instead of vibrational excitation, may account for these subpar DBD results. This prevents
the most energy-efficient NOx formation pathway through the vibrationally induced
Zeldovich mechanism. [93]. Li et al. [70] used needle array electrodes in packed bed DBD
reactors with α-Al2O3 and γ-Al2O3 beads to improve NOx formation at various discharge
parameters and oxygen levels. The unfilled reactor and γ-Al2O3 PBR produced the highest
NOx concentrations (1.1% and 0.97%, respectively) at a low energy cost of 17 MJ/mol N
and 33 MJ/mol N. The authors also demonstrated that increasing pulse width and repeti-
tion rate increases discharge power, resulting in higher electron density.
Figure 5. Schematic of air discharge in contact with water, (A) spark discharge, (B) glow discharge.
Reprinted with permission from Ref. [100].
Roy et al. [
69
] introduce a unique DBD system over water in contact with the electrode
for nitrogen fixation into NO
3
, which enables experiments to be carried out with a minimum
electrode separation, overcoming the problem of water surface instability. A decrease in
applied voltage and electrode spacing resulted in a promising energy cost for nitrogen
fixation as low as 20.7 MJ/mol N. Increasing the applied voltage leads to higher production
rates (up to 26
µ
mol/min) but also increases power absorption, reducing energy yield.
Additionally, lower electrode gaps lead to a diffuse mode, which efficiently produces NO
3
.
Packed bed DBD reactors, as shown in Figure 6, have also been investigated due to the
promise of improving product selectivity and energy efficiency by performing the plasma
reaction over a catalyst. The generation of NO
x
in a DBD filled with several catalyst support
materials (
α
-Al
2
O
3
,
γ
-Al
2
O
3
, TiO
2
, MgO, TaTiO
3
, and quartz wool) was studied by Patil
et al. [
68
]. A
γ
-Al
2
O
3
catalyst with the lowest particle size of 250–160 mm produced the
greatest results. However, the obtained energy cost was considerable (18 MJ/mol N), and
the product yield was low (0.5 mol%) in comparison to comparable atmospheric pressure
plasma reactors. Many metal oxides were deposited onto the support and assessed for
their ability to produce NO
x
with plasma assistance. The maximum concentration of
NO
x
was formed by the 5% WO
3
/Al
2
O
3
mixture, which was 10% more than that of the
γ
-Al
2
O
3
alone. Selectivity toward NO fell despite an increase in NO
x
content; this was
explained by oxidation interactions with oxygen species on the catalyst surface. The
extremely low electric field, more than 100–200 Td, which produces extremely energetic
electrons and leads mainly to electronic excitation, ionization, and dissociation instead
of vibrational excitation, may account for these subpar DBD results. This prevents the
most energy-efficient NO
x
formation pathway through the vibrationally induced Zeldovich
mechanism [
93
]. Li et al. [
70
] used needle array electrodes in packed bed DBD reactors with
α
-Al
2
O
3
and
γ
-Al
2
O
3
beads to improve NO
x
formation at various discharge parameters
and oxygen levels. The unfilled reactor and
γ
-Al
2
O
3
PBR produced the highest NO
x
concentrations (1.1% and 0.97%, respectively) at a low energy cost of 17 MJ/mol N and 33
MJ/mol N. The authors also demonstrated that increasing pulse width and repetition rate
increases discharge power, resulting in higher electron density.

Processes 2024,12, 786 9 of 19
Processes 2024, 12, x FOR PEER REVIEW 9 of 19
Figure 6. Packed DBD plasma reactor with a dielectric material. Reprinted with permission from
Ref. [101].
Low-pressure MW plasmas produced the best outcomes in terms of energy con-
sumption and product yield. For a NO concentration of 6 mol%, it was reported that a
MW plasma with a catalyst used 0.84 MJ/mol N of energy. [54]. An electron cyclotron
resonance plasma, which is a subset of magnetic field plasmas, yielded the highest NO
concentration of 14% at the lowest energy cost of 0.28 MJ/mol N [80]. However, these re-
sults were published in the 1980s and have not been confirmed since. As a result, the
claimed energy yield estimates for the 1980s MW plasma-based NOx generation should
be questioned. Low pressures (66 mbar) during the operation of these MW plasmas pro-
mote vibrational-translational nonequilibrium and, as a result, vibrational-induced Zeldo-
vich mechanisms. This, therefore, contributes to the explanation of their excellent yields
and low energy use. Nevertheless, low energy consumption only takes into consideration
plasma power; they do not take into account the energy needed for the reactor cooling
mechanism and the vacuum apparatus. Consequently, creating NOX in a MW plasma
would require more energy overall. MW reactors may also operate at higher pressures,
but as collision frequency increases, heat losses increase. [102]. Kim et al. [79] reported
3.76 MJ/mol N and 0.6% NOx in 2010 for an MW plasma slightly below atmospheric pres-
sure, an input power between 60 and 90 W, and a fixed flow rate of 6 L/min. Power pulsing
in a MW reactor may limit undesirable vibrational-translational relaxation, raising the vi-
brational temperature and, hence, the vibrational-translational non-equilibrium required
for vibration-induced N2 dissociation [103]. A diagram for a microwave plasma reactor is
illustrated in Figure 7.
Figure 7. A waveguide-based microwave plasma reactor. Reprinted with permission from Ref. [104].
Pei et al. [59] assessed DBD, glow, spark, and arc-type plasmas, among others, and
found an important feature that seemed to link the energy cost of NOx production with a
Figure 6. Packed DBD plasma reactor with a dielectric material. Reprinted with permission from
Ref. [101].
Low-pressure MW plasmas produced the best outcomes in terms of energy consump-
tion and product yield. For a NO concentration of 6 mol%, it was reported that a MW
plasma with a catalyst used 0.84 MJ/mol N of energy. [
54
]. An electron cyclotron resonance
plasma, which is a subset of magnetic field plasmas, yielded the highest NO concentration
of 14% at the lowest energy cost of 0.28 MJ/mol N [
80
]. However, these results were
published in the 1980s and have not been confirmed since. As a result, the claimed energy
yield estimates for the 1980s MW plasma-based NO
x
generation should be questioned.
Low pressures (66 mbar) during the operation of these MW plasmas promote vibrational-
translational nonequilibrium and, as a result, vibrational-induced Zeldovich mechanisms.
This, therefore, contributes to the explanation of their excellent yields and low energy use.
Nevertheless, low energy consumption only takes into consideration plasma power; they
do not take into account the energy needed for the reactor cooling mechanism and the
vacuum apparatus. Consequently, creating NOX in a MW plasma would require more en-
ergy overall. MW reactors may also operate at higher pressures, but as collision frequency
increases, heat losses increase. [
102
]. Kim et al. [
79
] reported 3.76 MJ/mol N and 0.6% NO
x
in 2010 for an MW plasma slightly below atmospheric pressure, an input power between
60 and 90 W, and a fixed flow rate of 6 L/min. Power pulsing in a MW reactor may limit
undesirable vibrational-translational relaxation, raising the vibrational temperature and,
hence, the vibrational-translational non-equilibrium required for vibration-induced N
2
dissociation [103]. A diagram for a microwave plasma reactor is illustrated in Figure 7.
Processes 2024, 12, x FOR PEER REVIEW 9 of 19
Figure 6. Packed DBD plasma reactor with a dielectric material. Reprinted with permission from
Ref. [101].
Low-pressure MW plasmas produced the best outcomes in terms of energy con-
sumption and product yield. For a NO concentration of 6 mol%, it was reported that a
MW plasma with a catalyst used 0.84 MJ/mol N of energy. [54]. An electron cyclotron
resonance plasma, which is a subset of magnetic field plasmas, yielded the highest NO
concentration of 14% at the lowest energy cost of 0.28 MJ/mol N [80]. However, these re-
sults were published in the 1980s and have not been confirmed since. As a result, the
claimed energy yield estimates for the 1980s MW plasma-based NOx generation should
be questioned. Low pressures (66 mbar) during the operation of these MW plasmas pro-
mote vibrational-translational nonequilibrium and, as a result, vibrational-induced Zeldo-
vich mechanisms. This, therefore, contributes to the explanation of their excellent yields
and low energy use. Nevertheless, low energy consumption only takes into consideration
plasma power; they do not take into account the energy needed for the reactor cooling
mechanism and the vacuum apparatus. Consequently, creating NOX in a MW plasma
would require more energy overall. MW reactors may also operate at higher pressures,
but as collision frequency increases, heat losses increase. [102]. Kim et al. [79] reported
3.76 MJ/mol N and 0.6% NOx in 2010 for an MW plasma slightly below atmospheric pres-
sure, an input power between 60 and 90 W, and a fixed flow rate of 6 L/min. Power pulsing
in a MW reactor may limit undesirable vibrational-translational relaxation, raising the vi-
brational temperature and, hence, the vibrational-translational non-equilibrium required
for vibration-induced N2 dissociation [103]. A diagram for a microwave plasma reactor is
illustrated in Figure 7.
Figure 7. A waveguide-based microwave plasma reactor. Reprinted with permission from Ref. [104].
Pei et al. [59] assessed DBD, glow, spark, and arc-type plasmas, among others, and
found an important feature that seemed to link the energy cost of NOx production with a
Figure 7. A waveguide-based microwave plasma reactor. Reprinted with permission from Ref. [
104
].
Pei et al. [
59
] assessed DBD, glow, spark, and arc-type plasmas, among others, and
found an important feature that seemed to link the energy cost of NO
x
production with a

Processes 2024,12, 786 10 of 19
broad range of discharges. As the authors have shown, NO
x
generation efficiency may be
greatly controlled by the average electric field and average gas temperature of the discharge.
The effective electron-impact activation of N
2
molecules by an electric field to promote NO
x
formation and the fast thermal quenching of NO to prevent its conversion back to N
2
and
O
2
molecules when the gas temperature drops more slowly are the two main mechanisms
that control the energy efficiency of NO
X
production in any type of discharge. Strong
electric fields cause N atoms to form, which is a crucial process for NO
x
breakdown [
74
].
The authors proposed many strategies for lowering the average gas temperature, including
cooling the reactor walls with water, utilizing short-duration high-current pulses, and
prolonging the discharge length [
56
]. Vervloessem et al. [
105
] presented a pulsed plasma
jet using dry air with an energy consumption of 0.42 MJ/mol N. The authors illustrated
that pulsing is the main factor of the reactor’s efficiency, as it decreases the temperature
of the gas between the pulses, causing the Zeldovich mechanism’s forward and reverse
reaction rates to be delicately impacted. However, the NO
x
concentrations obtained are
very low at 0.02%, making this system unsuitable for practical applications.
NO
x
formation by plasma jets flowing in (ambient) air (or N
2
atmosphere) and reacting
with water has been observed [
81
–
91
]. While NH
3
/NH
4+
creation was the main focus
of this investigation, the presence of oxygen also led to the detection of NO
2
and NO
3
formation. Water and plasma jets work together to extract the NO
X
product and stop the
plasma from destroying it.
Process parameters, including gas composition, flow rate, and temperature, also play
an important role in NO
x
production, and their influence has been studied extensively. Lee
et al. [
106
] investigated the influence of flow rate and oxygen content on NO
x
generation
by microwave plasma. A reduction in inlet flow rate from 45 standard liters per minute
(slpm) to 25 slpm brought about a sharp rise in NO
x
concentration from 1612 ppm to
9380 ppm when paired with an increase in O
2
content from 1% to 3%. The NO
2
/NO
ratio rose as the flow rate increased (from 4.3% to 14.8%). Na et al. [
107
] generated NO
using a microwave plasma torch, and different N
2
(5–30 slpm) and O
2
(0–250 standard
cubic centimeters per minute (sccm)) flow rates were examined. Abdelaziz et al. [
108
]
investigated gas composition and residence time in a high-frequency spark discharge
plasma as a factor of NO
x
formation. The study has found that O
2
greatly affected the
selectivity of NO
x
products. 95% NO selectivity was reached with 5% O
2
and 43.8% NO
2
selectivity was found at 80% O
2
. Altering the residence time improved the NO selectivity
while maintaining a low energy consumption of 2.1 MJ/mol N. Li et al. [
109
] used a
magnetic field stabilized plasma to investigate the correlation between gas temperature
and electric field versus NO
x
production. They have found that reducing these parameters
increases NO
x
production, resulting in an energy consumption of 2.29 MJ/mol N and a
concentration of 15.925 ppm NOx.
Patil et al. [
73
] conducted a detailed experimental analysis of the process parameters
in gliding arc discharge reactors. In their study, oxygen concentrations ranging from 35
to 48% were determined to be ideal for NO
x
generation, with NO selectivity decreasing
linearly with increasing O
2
content until the O
2
concentration reached 48%. An increase in
O
2
content had no noticeable effect on NO selectivity. It was also discovered in this study
that the NO selectivity reduced as the feed flow rate increased. Lowering the flow rate was
found to increase the specific energy intake and residence time of the reactant, resulting in
a greater conversion of NO to NO
2
. Furthermore, at the lowest flow rate of 0.5 L/min, the
maximum NOxconcentration of 1.4% was achieved.
Temperature affects NO
x
generation, according to Malik et al. [
110
], who used pulsed
sliding discharge to assess the temperature effect on NO
x
production. They found that
when the electrode/dielectric surface in contact with the plasma was heated from 20
◦
C
to 420
◦
C, the generation of NO rose while the production of ozone and NO
2
decreased.
Additionally, it was found that at the same peak voltage, the energy per pulse increased.
High temperatures have the potential to degrade ozone and impede the conversion of NO
to NO
2
. At 420
◦
C, the NO
2
/NO ratio dropped to 0.25, and the energy required to produce

Processes 2024,12, 786 11 of 19
NO was 24–67 MJ/mol. Li et al. [
111
] noticed an opposite trend, with the NO
2
/NO ratio
slightly increasing from 0.67 at 273 K to 0.80 at 373 K in their corona discharge investigation.
It should be emphasized that the gas temperature changes greatly depending on the type
of plasma, reactor, and operation parameters used; therefore, the temperature effect should
be evaluated alongside other aspects rather than independently. Furthermore, additional
heating or cooling will raise the energy cost of NO
x
production as well as the process’s
capital expenses.
Although plasma catalysis has been the subject of extensive research recently, relatively
little has been published about NO
x
production. Cavadias and Amouroux employed WO
3
as catalysts, which produced a nitrogen fixation rate of 19%, which is significantly greater
than the rate reached using plasma alone under low pressure (8%) [
43
]. Their study was
conducted at atmospheric pressure using a fluidized bed reactor, demonstrated in Figure 8,
with a WO3/Al2O3catalyst [112].
Processes 2024, 12, x FOR PEER REVIEW 11 of 19
NO was 24–67 MJ/mol. Li et al. [111] noticed an opposite trend, with the NO2/NO ratio
slightly increasing from 0.67 at 273 K to 0.80 at 373 K in their corona discharge investiga-
tion. It should be emphasized that the gas temperature changes greatly depending on the
type of plasma, reactor, and operation parameters used; therefore, the temperature effect
should be evaluated alongside other aspects rather than independently. Furthermore, ad-
ditional heating or cooling will raise the energy cost of NOx production as well as the
process’s capital expenses.
Although plasma catalysis has been the subject of extensive research recently, rela-
tively lile has been published about NOx production. Cavadias and Amouroux employed
WO3 as catalysts, which produced a nitrogen fixation rate of 19%, which is significantly
greater than the rate reached using plasma alone under low pressure (8%) [43]. Their
study was conducted at atmospheric pressure using a fluidized bed reactor, demonstrated
in Figure 8, with a WO3/Al2O3 catalyst [112].
Figure 8. Representation of the fluid bed reactor. Reprinted with permission from Ref. [113].
Mare, Amouroux, et al. studied the principles of the interaction between the solid
catalyst surface and the chemical species that comprise plasma [43]. WO3 and MoO3, two
transition metal oxides, were used to study the plasma catalytic process. These catalytic
substances were applied to large-surface-area supports, including ZrO2, MgO, and Al2O3.
N-type semiconductors are catalysts such as WO3 and MoO3. These catalysts feature labile
oxygen, which makes it easier to supply the oxygen needed for the oxidation of nitrogen
to NOx. This oxygen’s lability may be crucial to the creation of NOx. The oxidation process
may go considerably more smoothly when a vibrationally stimulated N2 molecule lands
on a surface with labile oxygen [45,54,114]. In their investigations, Mutel et al. employed
MoO3 as catalysts and obtained an energy consumption of 28 MJ/kg of NO, which was
78% greater than that of a plasma jet arc generator [54]. Cu-ZSM-5 and Na-ZSM-5 catalytic
activity was investigated by Sun et al. in a DBD reactor with pellets. Nonetheless, the
study’s primary goal was to investigate ideal processing circumstances for NOx removal.
Cu-ZSM-5 produced significantly more NOx than Na-ZSM-5 when heated to 350 °C [115].
Pei et al. [116] have shed light on how activated Al2O3 catalysts may improve the energy
efficiency of NOx generation in air plasma jet and pin-pin DC glow discharge systems.
Energy consumption can be significantly reduced by up to 45% when the active Al2O3 is
Figure 8. Representation of the fluid bed reactor. Reprinted with permission from Ref. [113].
Mattre, Amouroux, et al. studied the principles of the interaction between the solid
catalyst surface and the chemical species that comprise plasma [
43
]. WO
3
and MoO
3
,
two transition metal oxides, were used to study the plasma catalytic process. These
catalytic substances were applied to large-surface-area supports, including ZrO
2
, MgO,
and Al
2
O
3
. N-type semiconductors are catalysts such as WO
3
and MoO
3
. These catalysts
feature labile oxygen, which makes it easier to supply the oxygen needed for the oxidation
of nitrogen to NO
x
. This oxygen’s lability may be crucial to the creation of NO
x
. The
oxidation process may go considerably more smoothly when a vibrationally stimulated N
2
molecule lands on a surface with labile oxygen [
45
,
54
,
114
]. In their investigations, Mutel
et al. employed MoO
3
as catalysts and obtained an energy consumption of 28 MJ/kg of
NO, which was 78% greater than that of a plasma jet arc generator [
54
]. Cu-ZSM-5 and
Na-ZSM-5 catalytic activity was investigated by Sun et al. in a DBD reactor with pellets.
Nonetheless, the study’s primary goal was to investigate ideal processing circumstances
for NO
x
removal. Cu-ZSM-5 produced significantly more NO
x
than Na-ZSM-5 when
heated to 350
◦
C [
115
]. Pei et al. [
116
] have shed light on how activated Al
2
O
3
catalysts
may improve the energy efficiency of NO
x
generation in air plasma jet and pin-pin DC
glow discharge systems. Energy consumption can be significantly reduced by up to 45%

Processes 2024,12, 786 12 of 19
when the active Al
2
O
3
is used in the DC glow discharge system, especially at 70 mA
discharge current, at lower gas flow rates. The produced NO
2
is significantly increased
by the activated Al
2
O
3
catalyst, even though it is situated outside of the plasma zone.
Additionally, the production of NO
2
by using an air plasma jet infused with floating
activated Al
2
O
3
powder was investigated. The utilization of a neodymium magnet ring
effectively alleviated the occasional destabilization of the plasma discharge caused by the
floating catalyst, which improved the homogeneity and stability of the plasma jet. At larger
discharge currents, the energy efficiency was dramatically increased using the floating
catalytic particle technique. With this method, the lowest energy cost for producing NO
2
was observed at 2.9 MJ/mol [116].
O’Hare [
117
,
118
] patented catalysts to produce NO. As stated by O’Hare, these cat-
alysts also serve as a barrier to prevent the product from dissociating from UV radiation
produced by electric excitation. According to the invented procedure, a more efficient
discharge mechanism and higher/atmospheric pressure are possible. Catalytic materials
described include WO
3
, MoO
3
, Ta
2
O
5
, and other metal oxides adsorbed on Al
2
O
3
, Fe
2
O
3
,
TiO
2
, SiO
2
, Na
γ
type zeolite, Ni
γ
type zeolite, Co
γ
type zeolite, and Mn
χ
and
γ
type
zeolite. O’Hare outlines a technique in which low frequency, high voltage electric arc
discharge and catalyst are combined, with the electric arc forming completely inside the
catalyst bed. As catalysts, WO
3
, MgO, Ta
2
O
5
, MoO
3
, Cr
2
O
3
, Cu
2
Cr
2
O
5
, and porous Al were
employed. The suitability of each of these patented catalytic materials for the generation of
plasma NO
X
has not been properly examined. Using nano-sized TiO
2
photocatalysts in a
three-level, coupled, rotating electrodes plasma was shown by Lei et al. [
119
] to increase
the concentration of NO
x
up to 8335 ppm and lowering the energy consumption from
2.91 MJ/mol N to 1.73 MJ/mol N. The authors claim that the TiO
2
catalyst is activated
by oxygen species, which allows for the conductive band and valence band mechanism
to proceed.
Using Pt, CuO, Fe, and Ag gauzes as catalysts, Belova et al. [
120
] studied the NO
production reaction in a glow-discharge reactor. The 1:1 and 1:4 ratios of nitrogen to oxygen
mixtures were used in the studies. With a NO of 9% for the 1:1 example, Pt was shown
to give the highest nitrogen fixation rate in both circumstances. In ratios of 1:4, Pt may
provide 7.25% of NO. The sequence of catalyst effectiveness for nitric oxide production is
given as Pt > CuO > Cu > Fe > Ag [
120
]. Using seeding material could be an additional
option. Alamaro [
121
] reports using seeding material to increase NO production efficiency.
It was found that the collision rate increases noticeably with seeding, leading to a rise
in the excitation and dissociation rates. Seeding is accomplished by reusing 5% of the
mixture that exits the discharge. By 10% to 20% more NO was produced after seeding,
resulting in a steady state concentration of 6% NO. Bayer et al. [
122
] investigated using a
non-porous Ag wire catalyst in an RF plasma jet to improve NO formation. The authors
found that the non-porous Ag wire catalyst improved the conversion rate of N to NO
through mass transfer processes rather than surface reactions. The use of this type of
catalyst is limited, however, to cases where diffusion transport to the catalyst surface is
faster than the consumption of the reactive gases. Yu et al. [
123
] studied NO production
optimization in a parallel-plate RF plasma with Fe and Pt catalysts over a SiO
2
support.
They have found that the maximum production of NO was achieved at a conversion rate of
0.085%. The study showcased that the catalysts are non-reactive at room temperature, and
NO is oxidized by O
3
to NO
2
and N
2
O
5
; however, at elevated temperatures, O
3
quickly
decomposes, allowing higher NOxyields.
Recent efforts to upscale plasma-based nitrogen fixation have been made. Tsonev
et al. [
92
] investigated this challenge using two pin-to-pin DC plasma reactors—a small one
operating up to 20 L/min and a larger one operating up to 300 L/min. The large reactor was
also tested in torch configuration. For the small reactor, the authors were able to achieve an
energy consumption of 2.8 MJ/mol N with a concentration of 1.72% NO
x
or 4.8 MJ/mol
N with a concentration of 3.51% NO
x
. In the larger reactor, they were able to achieve
2.9 MJ/mol N with a concentration of 0.11% NO
x
or 4.5 MJ/mol N with a concentration

Processes 2024,12, 786 13 of 19
of 0.21% NO
x
in the pin-to-pin configuration; however, in the torch configuration, they
achieved 2.9 MJ/mol N with a concentration of 0.31% NO
x
. This study showcased how
flow rates affect NO
x
concentrations, thus affecting energy consumption, as well as how
important reactor design is in upscaling systems.
3. Prospects of Nitrogen Fixation
When it comes to energy consumption and product production, considering that the
H-B process has been refined for over a century, the plasma-assisted nitrogen fixation
approach is still not comparable to it. As suggested by Patil et al., future studies ought to
concentrate on energy consumption beneath 33–35 GJ/ton of N and production concen-
tration exceeding 15% [
18
]. Additional research on plasma reactors and the relationship
between plasma and catalysis is required to achieve this goal. The choice of catalysts is cru-
cial in plasma catalysis for nitrogen fixation, and further study on catalyst screening will be
required. Furthermore, numerous plasma procedures have demonstrated the advantages
of pulsed energization [
124
–
127
]. The utilization of high-frequency nanosecond pulsing for
plasma production can optimize both the yield and energy efficiency of the plasma-assisted
nitrogen fixation process. The three main components for the energy performance of the
plasma-assisted nitrogen fixation process are, according to Anastasopoulou et al. [
128
], the
integration of renewable energy, the power supply system and reactor, and the process
design at the industrial scale. Few studies have been conducted on topics such as product
separation, absorption, or process design as a whole; much of the research conducted to
date has been on the reactor itself. Moreover, creating large-scale plasma processes is a
difficult task. Due to the complexity of plasma, only a relatively small amount of informa-
tion has been discovered thus far to scale up nitrogen fixation processes that use plasma
assistance. However, the industrialized plasma method, which uses scale-up approaches in
the generation of ozone via multiple parallel reactors, provides valuable knowledge that
could be gained [129,130].
Nonetheless, over the past few decades, technology for producing renewable energy
has improved quickly. The high energy requirements of the plasma process might be
substantially offset by integration with renewable energy, which would also offer a long-
term solution to the environmental issues associated with industrial nitrogen fixation. The
plasma plant might be powered by electricity produced by renewable energy sources like
solar and wind. Furthermore, the idea of decentralized production is well fitted to the
small-scale nature of the plasma-assisted nitrogen fixation process. Plants with container
or modular sizes could be created for the small-scale synthesis of ammonia or NO
x
for
use in fuels and fertilizers. This will facilitate on-site production, give flexibility to meet
fluctuating demand, and significantly save transportation costs and product loss. Even with
the advancements over the last few decades and the increasing interest, plasma-assisted
nitrogen fixation technology has a long way to go before it can compete with the H-B
process on an industrial scale. On the contrary, it is anticipated that small-scale applications
could become a reality soon enough, given certain circumstances. A decentralized method
with plasma assistance can better suit the needs of low capital costs, scale-down economics,
and localized usage of abundant renewable resources rather than large-scale industrial
fertilizer production. Parallel to technological development, an assessment of the techno-
economic viability and sustainability of the plasma-assisted nitrogen fixation process must
be carried out [
128
,
131
,
132
]. This will provide suggestions for further improving process
efficiency toward real applications.
4. Conclusions
The industrial imperative of nitrogen fixation is apparent in the face of global demands
for nitrogen-containing compounds, ranging from fertilizers that sustain agriculture to
pharmaceuticals, explosives, and pigments. However, the chemical inertness of molecular
nitrogen demands a transformative process, nitrogen fixation, to turn it into reactive forms
like ammonia or nitrates. Biological nitrogen fixation represents a natural and sustainable

Processes 2024,12, 786 14 of 19
pathway; its relatively slow speed poses challenges in meeting the escalating fertilizer
requirements. The industrialization of nitrogen fixation, epitomized by the H-B process,
has played a pivotal role in addressing this gap, enabling the exponential growth of
global food production. However, the success of the H-B process comes at a significant
cost, both in terms of energy intensity and environmental impact. As the H-B process
approaches its theoretical energy efficiency limits, the process’s reliance on fossil fuels
and the release of substantial greenhouse gas emissions underscores the need for more
sustainable alternatives. Plasma nitrogen fixation processes present advantages such as
utilizing abundant materials like air and water, as well as the potential for renewable
energy sources. While the energy efficiency of plasma-assisted nitrogen fixation is yet to
match that of the H-B process, ongoing research and technological advancements offer
promising avenues for improvement. The reaction mechanism of NO
x
formation follows
the Zeldovich mechanism. Researchers have investigated and documented the conversion
of atmospheric nitrogen fixation into NO
x
for laboratory-scale procedures utilizing air
plasma. Various plasma reactors have been examined for NO
x
production, including spark
discharges, glow discharges, corona discharges, laser-produced discharges, radio-frequency
crossed discharges, dielectric barrier discharges, arc discharges, microwave discharges, and
plasma jets in contact with water. Research demonstrated that by optimizing the reactor
and avoiding the transmission of vibrational energy from N
2
to O
2
, energy consumption
could be reduced to 0.5 MJ/mol N. Studies have shown that in a variety of discharges, the
average electric field and the average gas temperature of the discharge, the influence of
flow rate, and oxygen content can largely regulate NO
x
generation efficiency. Employing
catalysts to plasma significantly increased the rate of NO
x
generation compared to plasma
alone under low pressure. Catalysts aid in the supply of oxygen needed for the oxidation
of nitrogen to NO
x
, as well as serve as a barrier to prevent the product from dissociating
from UV radiation produced by electric excitation. The integration of renewable energy, the
power supply system and reactor, and the process design at the industrial scale are the three
primary components of the energy performance of the plasma-assisted nitrogen fixation
process. Renewable energy technology has advanced rapidly during the last few decades.
The plasma process’s high energy requirements could be significantly compensated by
integration with renewable energy, which would also provide a long-term solution to the
environmental difficulties connected with industrial nitrogen fixation. Furthermore, the
concept of decentralized production fits the small-scale character of the plasma-assisted
nitrogen fixation process perfectly. Plants with modular sizes could be developed for
small-scale ammonia or NO
x
production for fuel and fertilizer use. This will allow for
on-site production, flexibility to meet variable demand, and a considerable reduction in
transportation costs and product waste.
Author Contributions: Writing—original draft preparation, A.K.; writing—review and editing, A.K.
and D.G.P. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: No new data were created or analyzed in this study. Data sharing does
not apply to this article.
Acknowledgments: We thank the grant from the Army Research Office (ARO), Army Research
Laboratory (ARL), the Office of Naval Research (ONR), and Purdue University for the support of
our lab. Additional thanks to the Army Research Office under Cooperative Agreement Number
W911NF-22-2-0170 for funding Angelique Klimek’s Ph.D. The views and conclusions contained in
this document are those of the authors and should not be interpreted as representing the official
policies, either expressed or implied, of the Army Research Office or the U.S. Government.
Conflicts of Interest: The authors declare no conflicts of interest.

Processes 2024,12, 786 15 of 19
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