sustainable atmospheric ammonia synthesis and nitrogen
TRANSCRIPT
Sustainable atmospheric ammonia synthesis and nitrogen
fixation using non-thermal plasma (NTP)
A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE
SCHOOL OF THE UNIVERSITY OF MINNESOTA BY
Peng Peng
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Advisor: Dr. R. Roger Ruan
Oct, 2018
© Copyright by Peng Peng 2018
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Acknowledgements
Mentorship is what I value the most during my research career. So first of all, I
would like to express my deepest gratitude towards my Ph.D. advisor, Prof. Roger
Ruan. I am very grateful that I have had the opportunity to receive the mentorship
from many excellent individuals throughout my career. I am greatly indebted to
Prof. Paul Chen, Prof. Tom Halbach, and Prof. Brock Lundberg to be the
committee members of my defense and helping me revise my thesis. I want to
thank Dr. Sharon Marsh (Land O’ Lakes Inc.) and Eric Schroeder (Great Plains
Institute) for being my industry mentor, Prof. Friedrich Srienc for giving me the
opportunity to conduct my first research project, and Dr. John Barrett for teaching
me my very first set of academic research skills.
I also would like to thank everyone in Prof. Ruan’s research group for offering
help and assistance whenever I encountered difficulties. I would have never
completed this thesis without them.
Last but not least, I owe my family everything for their unconditional love and
support. They are my lifetime mentors.
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Dedication
This thesis is dedicated to my beloved family, friends, mentors, and collegues.
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Abstract
Ammonia has recently been intensively studied as a clean, sustainable fuel
source and an efficient energy storage medium due to its effectiveness as a
hydrogen carrier molecule. However, the current method of ammonia synthesis,
known as the Haber-Bosch process, requires a large fossil fuel input, high
temperatures and pressures, as well as a significant capital investment. These
volatile conditions and high operating costs prevent decentralized and small-scale
ammonia production at the level of small farms and local communities.
Non-thermal plasma technology represents a promising alternative method of
clean ammonia synthesis, as it circumvents the volatile operating conditions,
fossil fuel use, and high capital costs of the Haber-Bosch process.
In this thesis research, this emerging technology was realized at the bench
scale and was optimized by various efforts, including catalyst improvement,
system development, and absorption enhancement. For the catalyst improvement,
a multi-functional catalyst was introduced and deposited onto various supporting
materials. For absorption enhancement, MgCl2 was implemented for the
absorption enhancement, taking advantage of its capability of forming Mg3N2 and
Mg(NH3)6Cl2 during the process. Meanwhile, the pulse density modulation (PDM)
was introduced to improve the performance of system. The series of efforts
improved the energy efficiency of the system by approximately one fold compared
with previous studies, and achieved the highest value of 20 g/kwh. Furthermore,
an innovative plasma gas-liquid ammonia synthesis approach was explored. It
was found that this approach could produce other nitrogen compounds (nitrate
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and nitrite acids) while generating ammonium, which could potentially add value
to the products of the plasma-assisted ammonia synthesis process.
Based on the results, the challenges and future opportunities of the
plasma-assisted ammonia synthesis approach were discussed. Lastly,
recommendations were made on how this technology could be beneficial to the
ammonia industry, through its potential to promote localized and environmentally
friendly energy production and storage.
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Table of Contents List of tables ........................................................................................................ vii
List of figures ...................................................................................................... viii
CHAPTER 1 Introduction ...................................................................................... 1
CHAPTER 2 Literature review .............................................................................. 6
2.1 Introduction ................................................................................................. 6
2.2 Review methodology ................................................................................... 8
2.2.1 General methodology ........................................................................... 8
2.2.2 Initial Screening .................................................................................... 8
2.2.3 Categorization of literature ................................................................... 9
2.2.4 Data extraction, analysis, and reporting ............................................... 9
2.3 Reaction principles and kinetics ................................................................ 12
2.4 Reactor development ................................................................................ 18
2.5 Catalyst development................................................................................ 29
2.6 Reactant type, composition, and feed rate ................................................ 36
2.6.1 The effect of N2 and H2 composition and feed rate ........................... 36
2.6.2 Reactants other than N2 and H2 ......................................................... 37
2.7 Conclusions .............................................................................................. 40
CHAPTER 3 Atmospheric plasma-assisted ammonia synthesis using Ru-based multifunctional catalyst supported by MgO ......................................................... 42
3.1 Introduction ............................................................................................... 42
3.2 Materials and methods .............................................................................. 45
3.3 Results and discussions ............................................................................ 49
3.3.1 Synergy between NTP, catalysts, and promoters ............................... 49
3.3.2 Feeding gas ratio & flow rate effects .................................................. 50
3.3.4 Frequency & voltage effects ............................................................... 52
3.4 Conclusions .............................................................................................. 54
CHAPTER 4 Atmospheric plasma-assisted ammonia synthesis using Ru-based multifunctional catalyst supported by mesoporous silica MCM-41 ...................... 55
4.1 Introduction ............................................................................................... 55
4.2 Materials and methods .............................................................................. 56
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4.2.1 Improvements ..................................................................................... 56
4.2.2 Catalyst preparation ........................................................................... 56
4.3 Results and discussions ............................................................................ 59
4.3.1 Characteristics of the Catalysts .......................................................... 59
4.3.2 Catalytic synthesis results .................................................................. 67
4.3.3 Synergistic effects between the plasma and catalyst ......................... 71
4.4 Conclusions .............................................................................................. 75
CHAPTER 5 Absorption-enhanced NTP ammonia synthesis with the assistance of magnesium chloride ....................................................................................... 76
5.1 Introduction ............................................................................................... 76
5.2 Materials and methods .............................................................................. 78
5.2.1 System improvements ........................................................................ 78
5.2.2 Absorbent preparation and characterization ....................................... 78
5.3 Results and discussions ............................................................................ 80
5.4 Conclusions .............................................................................................. 93
CHAPTER 6 Atmospheric plasma-assisted nitrogen fixation using water and nitrogen jet plasma ............................................................................................. 94
6.1 Introduction ............................................................................................... 94
6.2 Materials and Methods .............................................................................. 97
6.2.1 Improvements of the system .............................................................. 97
6.2.2 Description of apparatus ..................................................................... 97
6.3 Results and discussion ........................................................................... 100
6.3.1 Synthesis results .............................................................................. 100
6.3.2 Gas phase reactions ......................................................................... 103
6.3.3 Liquid phase reactions ...................................................................... 104
6.4 Conclusions ............................................................................................ 112
CHAPTER 7 Challenges and Opportunities ..................................................... 113
7.1 Challenges .............................................................................................. 113
7.2 Opportunities ........................................................................................... 115
CHAPTER 8 Conclusions and future remarks .................................................. 123
Bibliography ...................................................................................................... 125
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List of tables
Table 1 A summary of the reactor development for NTP ammonia synthesis ..... 23
Table 2 A summary of the catalyst development for NTP ammonia synthesis .... 33
Table 3 A summative comparison between the conventional Haber-Bosch and NTP ammonia synthesis ................................................................................... 120
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List of figures
Figure 1 A flow diagram of the non-thermal plasma ammonia synthesis process ............ 4
Figure 2 A process flow diagram of the systematic and integrated literature review process used in this review. ........................................................................................... 11
Figure 3 A schematic diagram of Dielectric Barrier Discharge plasma generation .......... 19
Figure 4 Dissociation of nitrogen under NTP ................................................................. 44
Figure 5 a) Process flow diagram of the NTP ammonia synthesis approach. b) A simplified diagram of the wiring connection between the invertor and transformer ......... 47
Figure 6 Ammonia synthesis under different conditions. ................................................ 50
Figure 7 Effect of N2 composition on ammonia synthesis efficiency ............................... 51
Figure 8 Effect of gas flow rate on ammonia synthesis efficiency .................................. 52
Figure 9 Effect of discharge frequency on ammonia synthesis efficiency ....................... 53
Figure 10 Effect of applied voltage on ammonia synthesis efficiency ............................. 53
Figure 11 FTIR spectra of the Ru/SI-MCM-41 catalyst ................................................... 60
Figure 12 a) SEM, b) TEM, c) AFM topography, and d) PiFM images of the Ru-promoter/Si-MCM-41 catalyst .................................................................................. 61
Figure 13 EDS results of the Ru-promoter/Si-MCM-41 catalyst ..................................... 62
Figure 14 XPS spectrum of the Ru-promoter/Si-MCM-41 catalyst. a) survey scan. b) high resolution scan for Ru3dC1s. c) high-resolution scan for the Cs .................................... 64
Figure 15 XRD spectrum of the calcined and reduced Ru-promoter/Si-MCM-41 catalyst ...................................................................................................................................... 65
Figure 16 Synthesis efficiency results with different catalyst components ...................... 68
Figure 17 Effects of the N2 feed concentration on the ammonia synthesis effeciency under different voltage conditions .................................................................................. 69
Figure 18 Effects of the inlet gas flow rate on the ammonia synthesis effeciency under different voltage conditions ............................................................................................ 69
Figure 19 Effects of the applied voltage on the ammonia synthesis efficiency and outlet concentration ................................................................................................................. 71
Figure 20 Effects of the applied frequency on the ammonia synthesis efficiency and outlet concentration ................................................................................................................. 71
Figure 21 The proposed mechanism of NTP ammonia synthesis using Ru-promoter/Si-MCM-41 catalyst .................................................................................. 73
Figure 22 Schematic diagrams of the MgCl2 locations relative to the plasma discharge region ............................................................................................................................ 81
Figure 23 Ammonia synthesis results with MgCl2 packed at different locations relative to the plasma discharge region .......................................................................................... 81
Figure 24 Ammonia synthesis results with MgCl2 under different voltage conditions ...... 84
Figure 25 Ammonia synthesis results with MgCl2 under different frequency conditions .. 84
Figure 26 Ammonia synthesis results with MgCl2 under different nitrogen compositions 85
Figure 27 Ammonia synthesis results with MgCl2 under different gas flow rates ............. 85
Figure 28 XRD patterns of the MgCl2 samples under different conditions ...................... 89
Figure 29 SEM image and EDS analysis results of the untreated MgCl2 sample ........... 90
Figure 30 SEM image and EDS analysis results of the MgCl2 sample treated in the plasma discharge region ............................................................................................... 91
Figure 31 SEM image and EDS analysis results of the MgCl2 samples treated below the
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plasma discharge region ............................................................................................... 92
Figure 32 Process flow diagram of the in-situ atmospheric nitrogen fixation process ..... 99
Figure 33 Concentrations of nitrate, nitrite and ammonium at different experimental conditions at 30 C ........................................................................................................ 102
Figure 34 Concentrations of nitrate, nitrite and ammonium of the in-situ reactor at different temperatures ................................................................................................. 106
Figure 35 Synthesis rates of nitrate, nitrite and ammonium at different experimental conditions at 30℃ ........................................................................................................ 108
Figure 36 Synthesis rates at nitrate, nitrite and ammonium of the in-situ reactor at different temperatures ................................................................................................. 109
Figure 37 The propose mechanism for the In-situ plasma jet nitrogen fixation ............. 111
Figure 38 An illustrative diagram of the renewable-to-ammonia approach ................... 118
1
CHAPTER 1 Introduction
Since the 1st half of the 20th century, the synthesis of ammonia was publicly
recognized due to the efforts of Fritz Haber and Carl Bosch, two Nobel Prize
recipients, who developed and industrialized the Haber-Bosch process for
ammonia production. The Haber-Bosch process has shaped our world and
resulted in humanity becoming heavily dependent on ammonia (Erisman, Sutton,
Galloway, Klimont, & Winiwarter, 2008). The most well-known applications of
ammonia include fertilizer, explosives during the 1st world war, and pesticides, etc.
(Galloway et al., 2008; Smil, 1997). In recent years, innovative applications of
ammonia have been intensively studied, including refrigeration, fermentation, and
energy carrier potential (Von Blottnitz & Curran, 2007). For example, ammonia
has been investigated for its potential as an energy source for fuel cells (Cox &
Treyer, 2015; Maffei, Pelletier, Charland, & McFarlan, 2007), transportation (Miura
& Tezuka, 2014), and other off-grid power applications (Davis et al., 2009). Due to
its high hydrogen content, ammonia has been viewed as one of the most efficient
and economical hydrogen carriers (Christensen, Johannessen, Sørensen, &
Nørskov, 2006; Cumaranatunge et al., 2007), as well as derivative species like
ammonia bromine (Smythe & Gordon, 2010). In terms of energy content,
ammonia has a heat of combustion of around 22 MJ/kg and a low heating value
comparable to diesel fuels (Zamfirescu & Dincer, 2009). Furthermore, the high
boiling point of ammonia makes it an ideal material for indirect hydrogen storage
(Demirci & Miele, 2009; Lan, Irvine, & Tao, 2012). Most importantly, the complete
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combustion of ammonia is a sustainable process that does not emit any
greenhouse gases. Therefore, ammonia, or ammonia bromide has also been
introduced with petroleum-based fuels to vehicle engines for cleaner emissions
(Stephens, Baker, Matus, Grant, & Dixon, 2007; Zamfirescu & Dincer, 2009).
Although the Haber-Bosch process is responsible for providing over 130
million tons of ammonia annually to support approximately 40% of the world’s
population, it is also responsible for up to 2% of the global energy consumption (B.
Patil, Hessel, Lang, & Wang, 2016). The reaction conditions of the Haber Bosch
process lie in the range of 200 to 400 atm and 400 to 600 °C, respectively
(Hargreaves, 2014). These intense temperature and pressure conditions are the
main disadvantages of the Haber-Bosch process, as they prevent the possibility
of lowering capital costs (Gilland, 2014; Razon, 2014). Additionally, the high
pressure required for the traditional Haber-Bosch Process is also a limiting factor
in reducing the economies of scale of localized production facilities due to the high
energy (and cost) requirements of compression (Razon, 2014). Life cycle
assessment (LCA) results have shown that the ammonia generation pathways
can greatly impact the for environmental performance of the ammonia-based
power applications (Cox & Treyer, 2015). Therefore, researchers are seeking new
methods of ammonia synthesis, which occur under more moderate conditions
(Vojvodic et al., 2014; Zhang, Chen, & Wen, 2012), require less carbon input
(Gilbert, Alexander, Thornley, & Brammer, 2014), or can be powered by
renewable energy sources (Bardi, El Asmar, & Lavacchi, 2013; H. Liu, 2014).
Since the start of late 1990s, many studies were carried out on non-thermal
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plasma (NTP) based processes for pollution control (H. H. Kim, 2004). Studies
have shown that NTP is capable for the destruction and removal of sulfur dioxide
(H. Ma, Chen, Zhang, Lin, & Ruan, 2002), hydrogen sulfide (H. Ma, Chen, & Ruan,
2001), odors (R. R. Ruan et al., 1999), and other volatile organic compounds (R.
R. Ruan et al., 2000; R. R. Ruan et al., 1999). In recent years, the results from
these studies have been applied to decompose pollutant gases from animal
production to food processing facilities (Schiavon et al., 2015; Schiavon, Schiorlin,
Torretta, Brandenburg, & Ragazzi, 2017). Results show that NTP is more efficient
and less energy intensive than most of the traditional gas treatment technologies
(Stasiulaitiene et al., 2016). (H. Ma et al., 2002) discovered that by injecting SO2
into the odor stream passing through an NTP reactor, solid particles were
produced at the exit of the reactor. These particles were then confirmed to be
ammonia sulfate ((NH4)2SO4). Researchers believed that SO2 was ionized and
reacted under the NTP conditions, resulting in (NH4)2SO4. These studies
confirmed that NTP is capable of ionizing gaseous compounds and may lead to
innovative methods of chemical synthesis reactions. Since then, NTP has been
intensely investigated in the fields of flow control in the past twenty years (Nie et
al., 2013; B. M. Penetrante & Schultheis, 2013).
During this period, NTP was found to be a potential alternative to the high
temperature and pressure method for the synthesis of many chemicals (Petitpas
et al., 2007; R. Ruan et al., 2014), such as benzene and isooctane (Rahemi,
Haghighi, Babaluo, Jafari, & Estifaee, 2013). Additionally the NTP-assisted
nitrogen fixation method has been viewed as an attractive alternatives to the
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Haber-Bosch process (B. Patil, Cherkasov, et al., 2016). In the early years of NTP
nitrogen fixation, emphasis was placed on the synthesis of nitric oxide compounds
instead of ammonia (B. Patil, Cherkasov, et al., 2016). NOx synthesis was favored
over ammonia production because it offered a thermodynamically favorable
method of nitrogen fixation, requiring lower energy input (B. Patil, Wang, Hessel,
& Lang, 2015; W. Wang, Patil, Heijkers, Hessel, & Bogaerts, 2017).
Figure 1 A flow diagram of the non-thermal plasma ammonia synthesis process
A flow diagram of the NTP ammonia synthesis approach is shown in Figure 1.
This process relies on the plasma discharge to dissociate the reactants and form
ammonia with the assistance of catalysts. Various sources such as microwave
and dielectric barrier discharge can be used to generate the plasma required for
the synthesis. Depending on the individual study, the products then go through a
separation process to collect the ammonia product and recycle the unreacted
reactants. The first attempt at using NTP to synthesize ammonia was made at the
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end of 1980s, where the authors tried to synthesize ammonia in high-frequency
discharges (Sugiyama et al., 1986). (Uyama, Nakamura, Tanaka, & Matsumoto,
1993) attempted the NTP synthesis of ammonia using microwave and radio
frequency (RF) plasma. The process was operated at a pressure much lower than
atmospheric pressure, approximately 700 Pa. After the concept of using plasma
to synthesize ammonia was proved in these studies, researchers have started
conducting more thorough investigations into the NTP-assisted ammonia
synthesis process. Further research indicated that the non-equilibrium plasma,
generated under non-thermal conditions, could dissociate diatomic nitrogen and
hydrogen at atmospheric pressure with the assistance of select catalysts. The low
operating pressure of NTP offers the opportunity to circumvent the compression
processes by flowing hydrogen gas directly into the reactor from other renewable
sources, such as gasification of biomass or electrolysis of water. Since
pressurizing the system is not required, the NTP system can be operated
continuously or incorporated directly into a syn-gas production stream. A recent
study proposed that the moderate process conditions could potentially allow for
the economically viable construction and operation of widely distributed NTP
based fertilizer production systems on farms and renewable hydrogen production
sites (Rune Ingels & David B. Graves, 2015).
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CHAPTER 2 Literature review
2.1 Introduction
Along with the NTP methods, there are several recently developed synthesis
approaches that could produce ammonia under low temperature and pressure,
namely the biological and electro-chemical cell synthesis. There have been
review articles published regarding the biological (Hinnemann and Nørskov, 2006)
and electro-chemical cell synthesis (Guo et al., 2017; Kyriakou et al., 2017) of
ammonia. Thus the first goal of this section is to provide a comprehensive review
with an emphasis on the plasma-assisted ammonia synthesis. In this dissertation,
the three crucial factors to the successful development of non-thermal plasma
ammonia synthesis technology were identified as: understanding of relevant
reaction kinetics, reactor development, and catalyst development. The
development of these aspects will significantly improve the energy efficiency and
environmental sustainability of this technology. Additionally, this paper analyzes
the effects of reactant type, composition, and feed rate on ammonia synthesis
efficiency, reactant conversion. Most importantly, this paper recommended how
the improvements of these factors could result in a cleaner ammonia production
process. A cradle to factory gate life cycle assessment shows that nitrogen
fixation by plasma can potentially enhance the global warming potential as
compared to the conventional fixation pathways by 19% (Anastasopoulou et al.,
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2016). However, the published life cycle assessment on nitrogen fixation
technologies focuses primarily on the production of nitric acid using plasma.
Therefore, another goal of this study is to fill in the knowledge gap of between the
published literature and a complete life cycle assessment that targets specifically
on the NTP synthesis of ammonia.
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2.2 Review methodology
2.2.1 General methodology
In order to effectively select the literature that could represent the current
status and help identify the challenges and opportunities of the NTP technology,
this paper incorporates a systematic and integrated literature review methodology
adapted from (Tranfield, Denyer, & Smart, 2003; Whittemore & Knafl, 2005). As
indicated in (Whittemore & Knafl, 2005), the general steps of an integrated
literature review are topic identification, literature search, data evaluation, data
analysis and presentation, as indicated in Figure 2.
2.2.2 Initial Screening
During the literature search stage, the methodology from (Tranfield et al., 2003)
is implemented. The literature search method includes: the identification of
literature, selection of literature, and literature quality assessment. First,
approximately 300 pieces of literature were selected regarding the synthesis of
ammonia via an intensive search from the research databases including: Elsevier,
Web Of Science, and Wiley Online Library, etc. At this stage, the problems of the
conventional ammonia synthesis, such as the high temperature/pressure, and the
large consumption of fossil fuels, were identified. Second, qualities such as the
validity, reliability, and trustworthiness of the studies are used to determine
9
whether they should be analyzed after the initial literature screening (Elo et al.,
2014). In this review, a high-quality study should have a detailed methodology,
complete list of parameters, and a comprehensive and thorough discussion of the
results, etc.
2.2.3 Categorization of literature
Third, the pieces of literature are categorized into different sections. Studies
that involve the life cycle assessment and the environmental issues of the
conventional ammonia production indicate the need of developing an alternative
ammonia synthesis process. These studies also point out the necessity to
improve the accessibility of ammonia production to small industries and farms.
Next, the studies about using NTP to synthesis ammonia under low temperature
and pressure conditions are identified, which function as the building blocks (or
“foundation”) of this review paper. Finally, studies in the related area are selected
to provide potential insight to resolve the challenges of the NTP ammonia
synthesis. Examples of these studies include the plasma nitrogen fixation to
produce nitric oxides and catalyst development of high temperature and pressure
ammonia synthesis.
2.2.4 Data extraction, analysis, and reporting
For the literature included in this review, only the data that effectively
10
represents the consequence of the study and addresses the trend of the studies is
extracted, both qualitatively and quantitatively. The emphasis of data extraction
also depends on the categorization of literature. Representative data such as the
energy efficiency, catalyst information, and conversion are selected as analysis to
the literature on NTP ammonia synthesis. During the data analysis stage, all the
units of the synthesis efficiency and conversion rate are converted for comparison
and analysis. During the reporting stage, two types of comparison are made, one
between the conventional and NTP ammonia synthesis technologies, and the
other within the NTP ammonia synthesis studies. During the reporting phase,
challenges of the NTP ammonia synthesis technology are discussed and the
recommendations to overcome the challenges are proposed.
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Figure 2 A process flow diagram of the systematic and integrated literature review process used in this
review.
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2.3 Reaction principles and kinetics
Since the development of the ammonia production process, intensive
investigations have been made to understand the kinetics of ammonia synthesis
reactions. However, less effort has been demonstrated to study the kinetics of
ammonia synthesis under NTP conditions, compared with the traditional
Haber-Bosch process. The kinetics provides a foundation for manipulating and
modeling the reaction to increase the rate, conversion, and efficiency of an
ammonia synthesis reaction, and is also what guides the design of the NTP
reactors. Overall, the NTP has two major functions for the catalytic conversion of
nitrogen and hydrogen to ammonia. First, it directly causes N2 and H2 to
dissociate and form NH3 with or without catalyst. Second, it provides the ionization
energy necessary to produce electrons for the catalysis system to function (Neyts,
Ostrikov, Sunkara, & Bogaerts, 2015). Therefore, modeling of the synthetic
process under NTP conditions will not only allow more efficient reaction, but will
benefit significantly to the reactor and process design, which will further contribute
to a cleaner production of ammonia using this technology. On a macroscopic level,
the ammonia synthesis using N2 and H2 as the reactants were centered on
equation (2.1), and its kinetics could be described via the following power
equation (2.2) (Jennings, 2013).
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N2 (g) + 3H2 (g) ↔ 2NH3 (g); ∆H= -92.44 kJ/mol (2.1)
𝑟 = 𝑁2𝛼𝐻2
𝛽 (2.2)
The reported 𝛼 and 𝛽 values for the power kinetic equation in the NTP
ammonia synthesis were 𝛼 = 0.77 and 𝛽 = 1.16 for a wool-like copper catalyst
(Aihara et al., 2016). Comparing with the reported coefficients under high
temperature and pressure conditions using transitional metal based catalysts (0.8
to1.2 for 𝛼 and 1.5 to 2.2 for 𝛽), the 𝛼 and 𝛽 values from the NTP synthesis
were lower, which corresponded to the slower synthesis rate of NTP over the high
temperature and pressure process (Hagen et al., 2003). Furthermore, it was
reasonable to propose that the kinetics of the NTP ammonia synthesis is limited
more by the dissociation of N2 than H2 since the 𝛼 value is lower than the 𝛽
value. However, due to the complex and non-equilibrium nature of the NTP, a
more detailed kinetics analysis must be carried out to provide a better
understanding of the intermediate species and chemical pathways in the NTP
plasma synthesis (Hong et al., 2017).
Previous studies hypothesized that the synthesis of ammonia from nitrogen
and hydrogen gas involved the following three mechanisms: decomposition,
structural rearrangement, and fragment elimination (B. Penetrante et al., 1997;
Van Durme, Dewulf, Leys, & Van Langenhove, 2008). It has been widely
understood that the first steps of plasma ammonia synthesis are the dissociation
and ionization of the inlet nitrogen and hydrogen gases (Mindong Bai, Zhang, Bai,
14
Bai, & Ning, 2003; Matsumoto, 1981). Furthermore, the formation of radical
species by the elemental impacts in the plasma region is the essential step in the
reaction (Mizushima, Matsumoto, Sugoh, Ohkita, & Kakuta, 2004). Previous
studies proposed a detailed mechanism that can be described by the following
reaction, with the dissociation of the injected N2 gas being the rate-limiting step
(Baldur Eliasson & Kogelschatz, 1991).
N2 → 2N∗ (2.3)
H2 → 2H∗ (2.4)
N∗ + H∗ → NH (2.5)
NH + H∗ → NH2∗ (2.6)
NH2∗ + H∗ → NH3 (2.7)
Amongst the reactive plasma species shown above, it is believed that the NH
radical is the precursor of NH3 formation and its concentration has a strong effect
on the ammonia synthesis rate (Uyama & Matsumoto, 1989). In a recent study
performed by (Hong et al., 2017), the authors analyzed the kinetics of the NTP
ammonia synthesis by taking into account both electron and vibrational kinetics
and surface reactions of the adsorbed species. The authors confirmed that the
reactions between radicals and vibrationally-excited molecules had a more
significant affect on the reaction kinetics of ammonia synthesis than the ion-ion
reactions. Additionally, the authors identified several other important factors in the
NTP-mechanism including: electron excitation and vibrational excitation reactions
15
in the gas phase, and surface adsorbed radicals (important in the later stages of
forming NH3). The general mechanisms described by equations (2.3) to (2.7) can
then be broken down into more detailed gas-phase (Carrasco, Jiménez-Redondo,
Tanarro, & Herrero, 2011) and surface adsorption (Hong et al., 2017) reactions.
Some examples of the reactions in the gas phase and surface adsorption phase
are specified in equations (2.8) to (2.9) and (2.10) to (2.11), respectively. In these
equations, surf stands for the catalyst adsorption surface; (s) stands for the
surface-adsorbed species; and (υ) represents the vibrationally-excited molecules.
Examples of the synergistic effect between the gas phase and the surface
adsorbed species are represented by equations (2.12) and (2.13).
e + N2 → e + 2N (2.8)
N + H2(υ) → H + NH (2.9)
N + surf → N (s) (2.10)
H + surf → H (s) (2.11)
NH + H(s) → NH2(s) (2.12)
NH2(s) + H(s) → NH3 (2.13)
In the gas-phase, to weaken and break the triple bond of diatomic nitrogen,
electrons must pass into the anti-bonding orbital of N2 through the d orbital
(Henrici‐Olivé & Olive, 1969). The energy provided by the NTP could then break
the weakened nitrogen triple bond. However, NTP alone could not provide
sufficient energy for efficient nitrogen fixation. Based on the specific type of
16
discharge, the energy provided by NTP ranges from 3 to 6eV, which is insufficient
to ionize the Ru catalyst (ionization energy of 7.36eV) (Alexander Fridman &
Kennedy, 2004; Kerpal, Harding, Lyon, Meijer, & Fielicke, 2013). For the surface
adsorption reactions, they are the primary mechanism for NH3 (or NOx) synthesis
since the previously discussed gas-phase reactions are capable of providing the
excited molecules necessary for surface reactions. Under the NTP discharge
conditions, the nitrogen and hydrogen gas form N, H, and NHx radicals. Within the
discharge volume, these radicals will react with the surface of the catalysts via a
direct adsorption process and stick to the surface of the catalysts (Carrasco et al.,
2011). In the gas phase, the atomic nitrogen formed by the electron-excited
diatomic N2 reacts with the vibrationally-excited hydrogen molecules to form NH
radicals. These NH radicals can then form NH3 via reaction with the atomic
nitrogen molecules in the gas-phase or the ones adsorbed by the catalyst surface
(Hong et al., 2017). This general mechanism agrees with the previously discussed
importance of specific energy input and active surface area (i.e. specific energy
input provides high-energy electrons and active surface area dictates surface
reactions). Therefore, whenever a ruthenium-based catalyst is selected as an
ammonia synthesis catalyst without the presence of intense temperature and
pressure, promoters must be present to facilitate the electron transfer and
enhance the catalytic activity. In summary, it is the interactive reactions between
these two phases that form the synergistic synthesis of NH3 by non-thermal
plasma (Neyts et al., 2015). Therefore, the development of the catalyst is crucial
in two aspects, to promote the dissociation of diatomic nitrogen in the gas-phase
17
and to facilitate the surface adsorption reactions.
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2.4 Reactor development
As mentioned in section 1, one of the advantages of the NTP synthesis of
ammonia is that it could be operated under low temperature and pressure. The
proper reactor design could significantly improve the efficiency of the ammonia
production system and lead to a higher production to energy input ratio, which
further represents a cleaner production of ammonia. Previous studies have
explored a wide array of NTP reactors as means of ammonia synthesis. When the
technology first started to emerge, Uyama et al. (Uyama et al., 1993) attempted
the NTP synthesis of ammonia using microwave and RF plasma. These
processes were achieved at approximately 700 Pa, much lower than atmospheric
pressure. Due to the vacuum condition required to carry out the reactions, the
experiments were carried out in a batch reactor. To eliminate the vacuum
requirements and operates the system in a continuous fashion, the studies
published in recent years have been largely focused on the synthesis of ammonia
using dielectric barrier discharge (DBD) plasma except those conducted by
(Nakajima & Sekiguchi, 2008).
Figure 3 shows a schematic diagram of a typical DBD plasma reactor, also
known as a silent discharge reactor. The structure of the DBD reactor is
composed of two electrodes and a dielectric barrier. As shown in the figure below,
the dielectric barrier is placed between the high voltage electrode and the ground
electrode. For DBD to occur, a strong electric field is required. The distance
19
between the electrodes and the applied voltage dictate the strength of the electric
field. The optimum voltage frequency to generate DBD is between 1 kHz and 10
MHz (Kogelschatz, 2003).
The discharge occurs and the plasma is generated in the small gap between
the two electrodes. Among the plasma treatment technologies, DBD plasma
offers the advantages of low operational/maintenance cost (Conrads & Schmidt,
2000; C. Liu, Brown, & Meenan, 2006), and has the broadest range of operational
pressures, ranging from 5 to 105 Pa (Schütze et al., 1998; Q. Wang, Chen, Jia,
Chen, & Li, 2011). For example, a recent comparison between DBD plasma and
microwave-induced argon plasma showed that DBD plasma generates higher
electron density and atomic oxygen concentration with less temperature increase
(Florian, Merbahi, Wattieaux, Plewa, & Yousfi, 2015).
Figure 3 A schematic diagram of Dielectric Barrier Discharge plasma generation
20
By using DBD plasma, most studies on NTP ammonia synthesis after the 20th
century were able to achieve continuous operation under atmospheric pressure.
(Mingdong Bai, Bai, Zhang, & Bai, 2000) used a planner structured DBD reactor
to synthesize ammonia. The study at atmospheric pressure enhanced the
conversion of ammonia and most importantly, greatly shortened the gas residence
time from the scale of hours to the scale of milliseconds. The authors further
made an improvement to the planner structured system by reducing the discharge
gap to the level of 10-1 millimeters (Mindong Bai, Zhang, Bai, Bai, & Gao, 2008). In
two more recent studies, Gómez et al. (2015, 2016) introduced a ferroelectric
packed bed reactor for the NTP synthesis of ammonia. The reactor consisted of
two circular plain electrodes with catalysts packed between them. The authors
pointed out that by using this design; the effect of the electrode gap or the size
and shape of the inner-electrode could be investigated more flexibly than the
planner or tubular electrodes. The results from this study showed a conversion of
nitrogen to ammonia at approximately 8%. Note that in this study, the ferroelectric
material acts both as the catalyst and electrode, which is later implemented in a
more recent study that uses a wool-like copper wires as both the catalyst and
electrode in a tubular NTP reactor (Aihara et al., 2016). Since the current N2
conversion of the continuous NTP ammonia synthesis is low (less than 10% of N2
conversion), Due to the low conversion rate of most NTP ammonia synthesis
technologies, a method of recycling the gas streams could contribute to material
saving and lowering the capital cost of this technology, which further promotes the
clean production of ammonia.
21
Another significant part of the reactor development was the selection and
design of the electrical discharge systems and operating conditions. Proper
matching between the electrical discharge power supply and the reactor was
essential to reduce the energy input of the system by lowering the specific energy
input to reach the same plasma intensity. For the electrical discharge conditions,
the alternate current (AC) and direct current (DC) discharges have been used for
the NTP synthesis of ammonia. In general, AC was used more commonly for DBD
plasma, as DC cannot pass the insulated dielectric layer (Kogelschatz, 2003).
However, it was worthwhile to point out that the studies carried out by (Mindong
Bai et al., 2008; Mindong Bai et al., 2003) used DC as their power supply. Table 2
further illustrated AC power’s superiority to DC power as the greatest energy
efficiencies and conversions were achieved by studies carried out using AC
power as opposed to DC power. Previous research indicated that the current of
the DC DBD plasma spikes when the voltage reached a certain value, usually 5kV
to 8kV depending on the specific dielectric layer (Ishida, Hirasawa, Dozen, & Tada,
2011). The dramatic increase in the operating current led to a much greater
energy consumption. For this reason, the NTP ammonia synthesis studies that
used AC power were conducted at higher voltages than their DC counterparts.
Besides the energy efficiency, the different voltage ranges used in NTP studies
are also associated with the plasma intensity, electron density, and ionization
potentials (Keller, Rajasekaran, Bibinov, & Awakowicz, 2012). For DBD plasmas,
the main collision of the ionized gas is with neutrals (Paliwoda, 2016). The
collision intensity and frequency of the plasma was closely related to the electron
22
temperature, which was proportional with the driving voltage (Naz, Ghaffar,
Rehman, Shahid, & Shukrullah, 2012). Research also showed that for AC power
at higher frequencies (over 10 kHz), the N2 breakdown voltage and the power
consumed per cycle of the DBD plasma was significantly lower than at low
frequency regions (less than 100 Hz) (Khatun, Sharma, & Barhai, 2010). The
plasma intensity was directly related to the effects of the feeding gas
compositions in the DBD ammonia synthesis reactor, which would be discussed in
detail in Section 6.
23
Table 1 A summary of the reactor development for NTP ammonia synthesis
Plasma
type
Plasma
parameter
Pressure Temperatur
e
Reactor
time
Greatest
reported
energy
efficiency
Greatest
Conversion
Comments Referenc
e
Glow
dischar
ge
DC, 6 mA 5-10 torr Room
temperature*
Batch N/A 77.4% for
MgO & CaO;
no ammonia
detected for
Al2O3,
WO3, and
SiO2-Al2O3
Batch
process, 30
min
(Sugiyam
a et al.,
1986)
Microw
ave
plasma
13.56 MHz to
2540 MHz,
150W
5 Torr 600-700 K Batch 0.078
g/kWh**
N/A Batch
process. 2
hrs
(Tanaka,
Uyama, &
Matsumot
o, 1994;
24
Uyama et
al., 1993)
DBD
dischar
ge
DC,
140kV/cm,
1us pulse
width,
5000Hz
750 Torr Room
temperature*
Continuous N/A 0.5 % Continuous
process,
residence
time 1.90s
(Mingdon
g Bai et
al., 2000)
DBD
dischar
ge
DC, 80us
pulse width,
1800V to
2150V,
10kHz
750 Torr
78 to 155 C Continuous 1.83 g/kWh N/A Continuous
process,
residence
time 0.20s
(Mindong
Bai et al.,
2003)
DBD
dischar
ge
AC, 2.5 to
4.5kV,
21.5kHz
750 Torr Room
temperature*
Continuous 0.34 g/kWh**
(5.5E-9
mol/J)
2.3 % Continuous
process,
residence
time 0.95s to
(Mizushim
a et al.,
2004)
25
3.82s
DBD
dischar
ge
DC, 240 W,
10 kHz; the
pulse width,
40 μs.
750 Torr 293K to
473K
Continuous N/A 9.1 % Continuous
process,
residence
time 1.6 s
(Mindong
Bai et al.,
2008)
Microw
ave
plasma
2.45GHz,
1.3kW
750 Torr 790K to
1240K
Continuous 0.78 g/kWh** 2.5E-2% Continuous
process,
residence
time 0.0065s
to 0.27s
(Nakajima
&
Sekiguchi
, 2008)
DBD
plasma
jet
DC
pulse,16kHz,
300W
750 Torr Room
temperature*
Simi-continu
ous
(ammonia
absorbed
after
1.16 g/kWh** 1.7E-6% at
45 min**
Semi-continu
ous
absorption
process,
Residence
(Kubota,
Masatomi
, & Tamio,
2010)
26
production) time 45 min
DBD
dischar
ge
AC, 1kHz, 11
to 13kV
750 Torr 450K to
1750K
Continuous N/A 4.2% Continuous
process,
residence
time 27s
(Hong,
Prawer, &
Murphy,
2014)
DBD
dischar
ge
AC, 0 to
7.5kV,
750 Torr 323K Continuous 0.9 g/kWh 2.7% Continuous
process,
residence
time 8.8 to
17.5s
(Gómez-
Ramírez,
Cotrino,
Lambert,
&
González-
Elipe,
2015)
DBD
dischar
ge
AC, 50kHz,
5kV
750 Torr Room
temperature*
Continuous 3.3 g/kWh
(H2:N2=0.5:
1)
3.5%
(H2:N2=3:1)
Continuous
process,
residence
(Aihara et
al., 2016)
27
time 9.7s
DBD
dischar
ge
AC, 500 to
5000Hz, 2.5
to 5.5kV,
750 Torr 333K Continuous 0.7 g/kWh 7% Continuous
process,
residence
time 9 to
121s
(Gómez‐
Ramírez,
Montoro
‐Damas,
Cotrino,
Lambert,
& Gonzá
lez‐
Elipe,
2016)
DBD
dischar
ge
AC, 500 and
1kHz, 12 to
17.5kV
750 Torr 387 K Continuous 0.16 g/kWh 2.3% Continuous
process,
residence
time 50s
(Hong et
al., 2016)
DBD AC, 7 to 14 750 Torr Room Continuous 0.68 g/kWh N/A Continuous (Xie et al.,
28
dischar
ge
kV, 11 to
14.5 kHZ
temperature process,
residence
time
2016)
DBD
dischar
ge
AC, 20 kV,
20 kHz
750 Torr 413 K Continuous N/A 12% Continuous
process,
residence
time 4 min
(Akay &
Zhang,
2017)
DBD
dischar
ge
AC, 5 kV, 50
kHz
750 Torr Room
temperature
Continuous N/A N/A Continuous
process,
residence
time 9.7 s,
(M.
Iwamoto,
Akiyama,
Aihara, &
Deguchi,
2017)
* Only the gas temperature before the plasma treatment is reported.
** Unit conversion is required from the original result to g/kWh
29
2.5 Catalyst development
Catalyst development is another critical factor to improve the NTP ammonia
synthesis process and achieve a cleaner production. To facilitate the NTP
ammonia synthesis reaction and enhance its efficiency, various efforts have been
made to the catalyst development for this technology. The first catalysts used to
synthesize ammonia under plasma conditions were iron and molybdenum wires
(Tanaka et al., 1994; Uyama et al., 1993). Other studies have examined the
performance of catalysts loaded on metal oxide supports, Al2O3 (Mizushima et
al., 2004)and MgO (Mingdong Bai et al., 2000; Mindong Bai et al., 2003), in
non-thermal plasma reactors for ammonia synthesis. Table 3 summarizes the
catalyst used for the NTP synthesis and their maximum energy efficiency or
conversion rate reached. It can be seen from the table that there are several key
factors for the catalyst selection of the NTP synthesis of ammonia.
The first factor is the presence of catalyst. Although many would believe that
catalyst is a critical factor in the NTP synthesis of ammonia, there are three
studies that successfully generated ammonia without the presence of any catalyst.
For the first study without catalyst, (Nakajima & Sekiguchi, 2008) relied on using
the quenching gas including Ar and He. This study was able to generate ammonia
at an energy efficiency of 0.78 g/kWh (best reported value). The authors also
stated the importance of using quenching gas to improve the synthesis of
ammonia. For the second study, (Mindong Bai et al., 2008) successfully
30
synthesized ammonia at a relatively high conversion rate (9% H2 equivalent)
using CH4 and N2. For these two studies, the reactant species and reactant
composition played another key role and the details of the gas reactants will be
discussed in the later section. For the other study without catalyst, the authors
used a liquid solution containing water and ethanol to immediately absorb the
ammonia after reaction in the plasma jet and achieved a relatively comparable
energy efficiency (1.16 g/kWh) (Kubota, Masatomi, et al., 2010).
It has been understood that the decomposition of ammonia, which is very
likely to happen under NTP conditions, is one of the limiting factors of the NTP
synthesis (Lu et al., 2014). Therefore, by injecting the reactant and product
mixture right into the solution immediately after the reduction, the authors were
able to separate and collect ammonia and prevent it from decomposing. If this
idea is adopted in future studies with more effective catalysts, it is reasonable to
achieve a breakthrough on the synthesis efficiency.
The second factor is the type of catalysts. So far, for all the studies of plasma
synthesis of ammonia, metallic catalysts have been used except several studies
that did not introduce any catalyst. The metallic catalyst can be divided into
several types as well. The first is a mono catalyst system, where only one type of
metal is used, which in these cases are MgO (Mingdong Bai et al., 2000; Mindong
Bai et al., 2003), Cu wires (Aihara et al., 2016; Tanaka et al., 1994), and Lead
zironate titanate (Gómez-Ramírez et al., 2015; Gómez‐Ramírez et al., 2016).
Among the studies that used mono catalysts, the copper wires with wool-like
shape presented the greatest energy efficiency (Aihara et al., 2016). Note that in
31
this study, the copper wools act not only as the catalyst, but an electrode of the
NTP discharge system as well. The authors further performed a comparison
analysis between different wool-like metallic catalysts including Au, Pt, Pd, etc.
This study shows that for wool-like metallic catalysts, Au has the highest activity
(M. Iwamoto et al., 2017). The second type is the catalyst-support system, which
coat catalyst (ruthenium and carbon) on to supporting materials including MgO
and Al2O3 (Hong et al., 2016; Mizushima et al., 2004; Xie et al., 2016). Another
study performed by (Akay & Zhang, 2017) uses microporous silica to support
nickel catalyst to enhance the surface area. For this catalytic system, promoters
are used to enhance the synthesis efficiency and conversion rate.
The third factor is the shape and structure of the catalyst. As discussed in the
previous section, the reactors used in the plasma synthesis of ammonia were
fixed-bed reactors with gas-phase reactions. Therefore, the shape and structure
of the catalyst was crucial as they could influence the flow channels, reaction time,
surface area, number of active sites, etc., which were all critical parameters for
the plasma synthesis of ammonia. More specifically for DBD plasmas, shielding
effects and discharge with between the catalyst and electrodes should also be
taken into consideration. From table 3, it can be seen that the shape of the
catalyst used in the previous studies were powders, wires, and pellets. Prior to
2014, the study on NTP synthesis of ammonia mainly used powder and wire
catalysts. Pellet-shaped catalyst were introduced and used in most studies after
2014. As (H. H. Kim, Teramoto, Ogata, Takagi, & Nanba, 2016) mentioned,
powder catalysts are not suitable for plasma process because they tend to spread
32
under plasma conditions. According to other literature, the spreading of the
powdered catalyst could be due to the charging and static electricity during the
plasma treatment (Mazumder et al., 2006). In a later study, carried out by (Aihara
et al., 2016) mentioned in the previous paragraph, the authors claimed that the
wool-like copper catalyst could increase the surface area of the electrode,
allowing for more discharge, and enabling more effective ammonia synthesis over
the electrode discharge.
33
Table 2 A summary of the catalyst development for NTP ammonia synthesis
Catalyst type Catalyst Catalyst shape Reference
Mono catalyst MgO &
CaO Al2O3;
WO3, and
SiO2-Al2O3
Disks (Sugiyama et al.,
1986)
Mono catalyst Iron and
molybdenum
Wires (Tanaka et al., 1994;
Uyama et al., 1993)
Mono catalyst MgO Powders (Mingdong Bai et al.,
2000)
Mono catalyst MgO Powders (Mindong Bai et al.,
2003)
Catalyst-support Ru/ Alumina Powders (Mizushima et al.,
2004)
N/A No catalyst N/A (Mindong Bai et al.,
34
2008)
N/A No catalyst N/A (Nakajima &
Sekiguchi, 2008)
N/A No catalyst N/A (Kubota, Masatomi,
et al., 2010)
Mono catalyst MgO and glass
spheres
Pallets (Hong et al., 2014)
Mono catalyst Lead zirconate
titanate, BaTiO3
Pellets (Gómez-Ramírez et
al., 2015)
Electrode catalyst Copper Small wool-like wires (Aihara et al., 2016)
Mono catalyst Lead zirconate
titanate (pallets)
Pallets (Gómez‐Ramírez
et al., 2016)
Catalyst-support Nanodiamonds and
diamond-like carbon
coated Al2O3
Sphere powders (Hong et al., 2016)
Catalyst-support Ru on Al2O3 Powders (Xie et al., 2016)
35
Catalyst-support Ni on microporous
silica
Pallets (Akay & Zhang,
2017)
Mono catalyst Au, Pt, Pd, Ag, or Cu Small wool-like wires (M. Iwamoto et al.,
2017)
36
2.6 Reactant type, composition, and feed rate
2.6.1 The effect of N2 and H2 composition and feed rate
From table 2, it can be seen that the studies with the highest conversion do
not correlate with the highest energy efficiencies. Several studies demonstrated
that the greatest ammonia synthesis occurs with a feed-gas composition equal to
(Mindong Bai et al., 2003; Gómez‐Ramírez et al., 2016), or nearly equal to, the
stoichiometric ratio in equation (2.1) (Gómez-Ramírez et al., 2015; Mizushima et
al., 2004). The increased hydrogen content promotes an increase in both electron
density and temperature, which leads to a positive shift, or increase, in the mean
electron energy distribution. This shift favors the formation of the NH species in
equations 2.3 to 2.7. For these studies, the plasma discharge conditions were
relatively less intense, utilizing lower voltages and frequencies (below 5 kHz and
500 kV/m). However, there are several studies that reported increased ammonia
synthesis under high N2 compositions (Aihara et al., 2016; Mingdong Bai et al.,
2000; Hong et al., 2016; Nakajima & Sekiguchi, 2008). For these studies, the
gas-phase discharge conditions are relatively high, above 500 kV/m and 10 kHz.
According to the recent kinetic modeling study, the effects of the N2 and H2
composition on the continuous NTP synthesis of ammonia depend on several
factors, including the electron temperature and density (Hong et al., 2017). It is
mentioned in this study that N2 compositions higher than the stoichiometric value
in equation (2.1) are favored under high-density plasma conditions with
37
high-energy electrons. The same trend was observed in the study using thermal
plasma to synthesize ammonia as well (Van Helden et al., 2007).
The second factor affected by the reactant feed rate is the residence time.
Residence time also has a significant impact on ammonia synthesis, especially for
the continuous NTP packed-bed reactors. From table 2, it can be observed that
the range of residence time in the NTP ammonia synthesis studies varied from the
scale of milliseconds to a minute. The study from (Gómez-Ramírez et al., 2015)
indicated that longer residence time would result in less ammonia production. This
concept is supported by several other studies that specifically explored the
dissociation of ammonia under plasma conditions (Lu et al., 2014; L. Wang, Zhao,
Liu, Gong, & Guo, 2013). These studies revealed that non-thermal plasma could
also promote the decomposition of ammonia back to hydrogen and nitrogen gas,
especially with the presence of iron-based catalysts (L. Wang et al., 2013).
Therefore, decreasing the residence time of the feed-gas within the NTP region
helps prevent efficiency diminishing back reactions, such as ammonia
decomposition.
2.6.2 Reactants other than N2 and H2
Similar to the conventional Haber-Bosch process, most of the NTP
ammonia synthesis studies used N2 and H2 gas as the reactants. However, there
are two exceptions. Overall, the attempts of investigating in reactants other than
N2 and H2 is to explore the possibilities of improving the efficiency for the NTP
ammonia synthesis approach and enhance its environmental sustainability.
38
However, there is one study that has the potential of leading to a promising
alternative to combine the NTP process with other waste treatment technologies
for a cleaner production of ammonia. The study carried out by (Mindong Bai et al.,
2008) used CH4 and N2 as feed gas for NTP ammonia synthesis. Although there
were no catalysts used in this study, the authors managed to produce ammonia
with a maximum conversion of 0.8%. The authors did not report the synthesis
efficiency data. But it is notable that in this study, many CxHy species were formed
during the reaction from the dissociation of CH4, such as C2H6, C2H4, C3H8, etc.
Although this study is only at preliminary stage, its results provide a potential
alternative for the clean production of ammonia. As CH4 is a greenhouse gas
emission during anaerobic digestion, used at many waste water treatment
facilities (Z.-Y. Ma et al., 2015), its use for synthesizing ammonia could
significantly improve the sustainability of the waste treatment industry. The
second study used N2 gas with water and ethanol solution. Note that in this study,
the water and ethanol solution used act not only as the reactants, but also
absorbents for the synthesized ammonia. Therefore, the ammonia produced in
this study was in the form of aqueous NH4+. Like the previous study that used N2
and CH4 as the reactants, this study also led to the formation of side products,
which were aqueous NO2- and NO3
-.
Although N2 and H2 were used as the reactants, the two studies carried out
by (Nakajima & Sekiguchi, 2008) and (Hong et al., 2014) incorporated the idea of
introducing a “quenching gas” into the system. The authors suggested that due to
the exothermic nature of ammonia synthesis, injection of a quenching gas could
39
effectively lower the temperature of the reactor, which could further enhance the
ammonia synthesis by preventing the decomposition of ammonia. This study
demonstrated an increase in ammonia production with the introduction of a
quenching gas. As a result, the study by (Hong et al., 2014) managed to achieve a
4.2% conversion to ammonia, which is one of the highest conversions yet
reported amongst NTP ammonia synthesis studies. With the exception of the
three studies just discussed, a review of literature demonstrated an almost
unanimous use of N2 and H2 as reactant feedstock. However, despite the
continuity in choice of feedstock, these studies exhibited remarkably different
results when testing gas feed rates. Of note, an additional study that utilized
expanding thermal plasma (ETP) in the synthesis of ammonia from N2 and H2
also confirmed the positive effects of introducing Ar as a quenching gas (Van
Helden et al., 2007).
40
2.7 Conclusions
In summary, the current development of this technology is mainly focused on
the kinetic study, reactor/catalyst development, and reactant composition studies.
For the kinetics studies, most of the efforts have been made trying to identify
the rate-limiting step of the plasma ammonia synthesis and classify the complex
plasma reactions. For reactor development, reactors from rectangular,
ferroelectric, to tubular DBD discharge reactors, etc. have been explored. System
parameters such as discharge conditions and residence time are the crucial factor
to develop and design efficient reactors for this technology and realize cleaner
production of ammonia with less energy input. For catalyst development,
non-catalyst, mono catalyst, catalyst-support, and catalyst-support with promoters
were investigated. During the investigation of the reactant composition, the effects
of changing the N2 to H2 ratio were studied. However, several preliminary studies
revealed the possibility of using CH4 or a liquid-gas reaction to synthesis ammonia
under NTP conditions. These studies provided a potential avenue of combining
the NTP technology with other waste treatment processes for a cleaner and more
environmental sustainable ammonia production industry.
From the results obtained in previous studies, the main barriers faced by this
technology were identified as the fixation of nitrogen gas and prevention of
unwanted back reactions (ammonia decomposition). To overcome these
obstacles, reactors with instantaneous product absorption and separation are
41
recommended, along with catalysts of stronger plasma synergistic activities.
Furthermore, development of a catalyst with higher activities and introduction of a
fast-separation reactor, using solid or liquid sorbents, could potentially solve the
challenges and ensure these technologies competitiveness. Despite these
challenges, this technology’s remarkably low theoretical energy floor provides an
enticing opportunity for future use in industrial process. Its other advantages, such
as low capital cost, less land use, small-scale accessibility, offer this technology
many opportunities to reshape the ammonia production industry towards a
greener and cleaner process.
42
CHAPTER 3 Atmospheric plasma-assisted
ammonia synthesis using Ru-based
multifunctional catalyst supported by MgO
3.1 Introduction
The concept of synthesizing ammonia at atmospheric pressure through the
non-thermal plasma (NTP) assisted catalytic reactions was proven (Mingdong Bai
et al., 2000; Mindong Bai et al., 2003; Lan, Irvine, & Tao, 2013; Mizushima et al.,
2004). NTP is electrically energized matter in a gaseous state, which is not in
thermodynamic equilibrium, and can be generated through electric discharge in a
gaseous volume. The NH radicals formed in the nitrogen-hydrogen plasma and
the NH radicals would be quenched by hydrogen molecules to form ammonia
(Chauvet, Thérèse, Caillier, & Guillot, 2014; H. Ma et al., 2002). The
non-thermal plasma species have an energy level in the range of 2 to 10 eV at
temperature close to ambient condition. It is known that these kinds of species
can alter organic or inorganic compounds through at least three mechanisms: (1)
decomposition, (2) structural rearrangement, and (3) fragment elimination (B.
Penetrante et al., 1997; Van Durme et al., 2008). One or all of these
mechanisms may be responsible for promoting the disassociation of hydrogen
and nitrogen, which is necessary for the synthesis of ammonia through catalytic
43
reactions. Ru based catalysts have been widely used and proved efficient in
conventional ammonia synthesis, especially under the presence of MgO or
carbon nanotube support and Cs promoter (Larichev, 2010; Y. Yan et al., 2015).
Figure 4 provides a general illustration of an electron passing onto the
anti-bonding orbital of N2 through the d orbital of Ru in order to weaken the triple
bond of nitrogen. The weakened triple bond can then be broken with additional
energy, in this case, energy provided by NTP. However, the energy provided by
NTP is approximately 3 to 6eV according to the specific type of discharge, which
is insufficient to ionize Ru, which has ionization energy of 7.36eV (Alexander
Fridman & Kennedy, 2004; Kerpal et al., 2013; B. Penetrante et al., 1996; B. M.
Penetrante, Hsiao, Merritt, Vogtlin, & Wallman, 1995). The situation changes
when promoter Cs is attached to Ru. Cs can easily be ionized, producing
electrons that are passed onto Ru, and then to nitrogen (J. Iwamoto, Itoh, Kajita,
Saito, & Machida, 2007; Kitano et al., 2012). Therefore NTP on one hand directly
causes N2 and H2 to dissociate and form NH3 with or without catalyst, and on the
other hand, provides ionization energy necessary to produce electrons for the
catalysis system to function (Neyts et al., 2015). The promoters used in these
studies are alkaline metals, typically Cs, Ba, and K. Studies have shown that for
these promoters are able to increase the catalytic activity of the catalytic system
by effectively decreasing the energy barrier for the dissociation of N2, and facilitate
the destabilization of the NHx species in equations generated during the plasma
synthesis of NH3 (Aika, 2017; Z. Ma, Zhao, Pei, Xiong, & Hu, 2017). Attaching
promoters such as Cs and Ba to Ru can enhance the catalytic conversion process.
44
Since Cs can easily be ionized, it can pass electrons onto Ru and nitrogen, which
will aid the dissociation of nitrogen gas (J. Iwamoto et al., 2007). The high applied
frequency and the metallic cation promoters added could enhance the electron
transfer within the catalyst systems to aid the dissociation and ionization of
nitrogen, which requires the highest ionization potential among the reactions
(Mindong Bai et al., 2003; Larichev, 2010). The fact that the Ru catalysts with Cs
promoters has much higher dissociative catalytic activity on N2 than catalysts
without C was proved by a study that used an isotropic analysis on the kinetic of
the ammonia synthesis (Hinrichsen, Rosowski, Hornung, Muhler, & Ertl, 1997). A
relatively more recent kinetic study pointed out that in this catalyst-promoter
ammonia synthesis system, the cesium and barium each acted as an electronic
promoter and structural promoter that controls the concentration of active sites
(Szmigiel et al., 2002).
Figure 4 Dissociation of nitrogen under NTP
45
3.2 Materials and methods
The catalysts used for synthesis study were loaded with Ru (10 wt%) as
the main catalyst. Cs, K, and Ba were used as promoters. After impregnation, the
catalysts were dried under infrared light for 4 hr and then calcined at 500 °C in a
muffle furnace for 4 h. Then the catalysts were reduced at 500 °C using hydrogen
gas for 4 h prior to application. The NTP synthesis reactor consisted of two
co-axial quartz tubes and a stainless steel tube, which act as the dielectric barrier
and the discharging electrodes, respectively. Figure 5 shows the process flow
diagram of the NTP ammonia synthesis system used in this study. The AC high
voltage to the system was supplied using an SSD-110 inverter and a coupled
transformer manufactured by Plasma Technics Inc. (Wisconsin, USA). The space
between the dielectric barrier and the electrode was 1.5 mm and the length of the
reactor is 30 cm. Catalysts were packed to the same length matched with the
length of the electrode between the two dielectric barriers (20 mm). When a
certain voltage is applied to the electrode, electrical discharge takes place
between the dielectric barriers and on the surface of the catalyst particles. The
voltage and frequency of the power supply could be adjusted during synthesis
process and was measured using a Tektronix oscilloscope (Oregon, USA)
connected to the high voltage electrode of the reactor. The temperatures were
measured using an inferred thermometer (Fluke, USA). The synthesis efficiency
was determined by dividing the ammonia synthesis rate (g/hr) over the power
consumption (kW) at the DBD region. The power consumption was calculated by
46
multiplying the voltage and current at the discharge region. The current was
determined using coil coefficient provided of the transformer (provided by the
manufacture), and the current from the inverter side, which was measured using a
Sperry DSA-540A current meter (USA). Nitrogen and hydrogen gases were fed
from compressed gas cylinders at approximately 26 C and pressure 60 psi. The
gas flow rate is controlled using BROOKS Smart Mass Flow gas flow meters. The
ammonia was qualitatively detected using FTIR. The product gas was collected
using dilute sulfuric acid solution and the quantitative measurements were carried
out using a DR 5000™ UV-Vis Spectrophotometer (Hach Technology Inc. USA).
47
Figure 5 a) Process flow diagram of the NTP ammonia synthesis approach. b) A simplified diagram of the wiring connection between the invertor and transformer
The energy efficiencies listed in Chapter 2 was referring to using the “plasma
power” measured at the point of plasma discharge. The most common
measurements of “plasma power” were the analytical approach, current and
48
voltage based methods, the Lissajous approach, and a comparative approach.
The descriptions of these methods was reported in previous literature (Hołub,
2012). The method used in this thesis research to characterize energy efficiency
was a derivative of the current and voltage based methods. Figure 5 b) showed a
simplified wiring diagram of between the invertor and the transformer.
The measured parameters used to determine the power consumption were
the voltage on the secondary side and current on the primary side of the invertor.
The secondary voltage, V2 was measured using the Trek high voltage probe and
the primary current, A1 was measured using the Sperry DSA-540A current meter.
The energy efficiency in g/kWh was estimated using the following equation
𝜂(𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦) = �̇�∙�̅�
𝐴1𝑅𝑀𝑆∙𝑁1𝑁2∙𝑉2𝑅𝑀𝑆
(3.1)
Where �̇� was the synthesis rate in mol/hr, �̅� was the molecular mass in g/mol,
N1 and N2 were the coil numbers on the primary and secondary sides of the
transformer, and RMS was the room mean square value of the current or the
voltage.
49
3.3 Results and discussions
3.3.1 Synergy between NTP, catalysts, and promoters
Figure 6 shows the ammonia yields using different catalysts under NTP
environment. It is indicated that NTP alone was able to promote ammonia
synthesis, suggesting that NTP species provided energy to dissociate N2 and H2
and allowed the formation of NH3 under atmospheric conditions. Additionally,
ammonia yield is improved with the presence of the MgO support and increased
dramatically when both promoter Cs and MgO support are introduced. Therefore,
it is reasonable to conclude that NTP, Ru/MgO and Cs promoters are critical
components in this NTP reaction system. Previous studies shows that the
presence of MgO particles can act as supporting material for Ru based catalysts
and is able to promote intensive surface discharges, which are expected to favor
dissociation of N2 and H2 and promote ammonia formation (Mingdong Bai et al.,
2000; Sugiyama et al., 1986). However, the catalyst packing affects the
characteristics of the discharge. Since the high dielectric constant of MgO could
intensify the discharge, it is possible that the yield increase with Ru/MgO is
caused by the change within the plasma discharge. Similarly for the yield increase
when the system includes both MgO support and Cs promoter under the same Ru
load. Since the addition of Cs promoter causes the formation of an electric double
layer between Cs+ cations and Ru atoms, the work function of ruthenium is
decrease and discharge conditions could be altered (Larichev, 2010; Larichev et
50
al., 2007).
Figure 6 Ammonia synthesis under different conditions.
3.3.2 Feeding gas ratio & flow rate effects
N2/H2 ratio of 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3 were tested on the pilot system
for ammonia synthesis under NTP conditions. Results show that N2/H2 ratio=3:1
produces the highest amount of ammonia. This confirms that ammonia synthesis
has higher efficiency under a N2 rich environment. This also indicates
residual/unreacted nitrogen after the gas exited the reaction volume. This
indicates that reaction under the condition of higher N2/H2 ratio was more
desirable for ammonia concentration. The possible reason is that nitrogen has a
higher chemical bond energy that requires higher energy to break the bond, and
the high density of nitrogen gas in the plasma system could lead to higher density
of active nitrogen molecular. The possibility of active nitrogen molecule reacting
0
0.5
1
1.5
2
2.5
3
3.5
4
NTP NTP + MgO NTP + Ru /MgO NTP + Cs-Ru
/MgO
Am
mo
nia
yie
ld (
%)
51
with hydrogen would be increased. Results in Figure 8 show that higher flow rate
produced ammonia much more efficiently in the current NTP plasma catalytic
synthesis system. Tests on the pilot system show that higher gas flow rate is able
to improve energy efficiency. Note that when the synthesis efficiency reaches a
plateau at gas flow rate of 5 to 8 liters/min.
Figure 7 Effect of N2 composition on ammonia synthesis efficiency
0
0.3
0.6
0.9
1.2
0
20
40
60
80
0.25 0.33 0.50 0.67 0.75 0.80 0.83
Eff
icie
ncy
(g/
kW
h)
Rat
e (μ
mo
l∙m
in-1
)
N2 composition
Rate
Efficiency
52
Figure 8 Effect of gas flow rate on ammonia synthesis efficiency
3.3.4 Frequency & voltage effects
The maximum frequency and voltage that can be achieved by the power
supply are 25,000 Hz and 30,000 V respectively. Tests show that higher applied
voltage resulted in a higher ammonia concentration, which is reasonable greater
plasma energy favors the ionization of Ru catalyst, which further leads to
breakage of N2 bond. However, the trade off for voltage increase is energy
consumption. The increase in applied power requires larger energy input from the
power supply. By calculating the energy consumption associated with ammonia
synthesis rate under different voltages, results in figure 10 conclude that the
overall ammonia synthesis efficiency decreases with increasing voltage. On the
0.00
0.30
0.60
0.90
1.20
0
20
40
60
80
1 2 3 4 5
Eff
icie
ncy
(g/
kW
h)
Rat
e (μ
mo
l∙m
in-1
)
Gas flow rate (L/min)
Rate
Efficiency
53
other hand, frequency shows less significant effect on ammonia synthesis
efficiency. Based on experimental results of voltage and frequency analysis, we
determined that 10,000Hz and 6000v are found to provide the highest energy
efficiency for NPT ammonia synthesis.
Figure 9 Effect of discharge frequency on ammonia synthesis efficiency
Figure 10 Effect of applied voltage on ammonia synthesis efficiency
-0.10
0.10
0.30
0.50
0.70
0.90
1.10
0
20
40
60
80
100
8000 10,000 12,000 14,000 16,000
Eff
icie
ncy
(g/
kW
h)
Rat
e (μ
mo
l∙m
in-1
)
Frequency (Hz)
Rate
Efficiency
0.00
0.30
0.60
0.90
1.20
0
20
40
60
80
100
120
5000 6000 7000 8000 9000
Eff
icie
ncy
(g/
kW
h)
Rat
e (u
mo
l/m
in)
Voltage (V)
Rate
Efficiency
54
3.4 Conclusions
This study proved the feasibility of the synthesis of ammonia using
non-thermal plasma above micro-gap discharge at low temperature. Results also
show that N2/H2 ratio=3:1 produced the highest amount of ammonia, which further
confirms that ammonia synthesis has higher efficiency under a N2 rich
environment due to the relative difficulty in disassociation of nitrogen. Achieve
further enhancement of ammonia generation efficiency under NTP condition is
needed to match the industrial requirements of ammonia production, which could
be potentially by improving reactor configuration and new catalyst preparation.
55
CHAPTER 4 Atmospheric plasma-assisted
ammonia synthesis using Ru-based
multifunctional catalyst supported by
mesoporous silica MCM-41
4.1 Introduction
To enhance the ammonia synthesis efficiency of the NTP approach, an
effective catalytic-promoter system and mesoporous supporting material were
used. In this study, a heterogeneous multifunctional catalytic system is used,
which consists of different solid surface catalytic sites (Dixit, Mishra, Joshi, &
Shah, 2013). Ruthenium, one of the catalysts with the highest catalytic activity for
ammonia synthesis, is selected to be the main catalyst. Additionally, the Cs and
Ba promoters were also used, which facilitates the NH3 synthesis by being ionized
under relatively low energy and transporting electrons onto Ru and N. The Ru
catalyst and promoters were deposited onto a Si-MCM-41 support to form the
Ru-promoter/Si-MCM-41 catalyst system.
56
4.2 Materials and methods
4.2.1 Improvements
The mesoporous-structured Si-MCM-41 was chosen in this study for
several reasons. First, mesoporous materials are notable for having a large
surface area to volume ratio when compared with traditional metal oxide supports
(Erjavec, Kaplan, Djinović, & Pintar, 2013). Second, the highly ordered pores with
diameters from 10 nm to 50 nm make the Si-MCM-41 material an ideal host for
the Ru and promoters (Gallezot, Chaumet, Perrard, & Isnard, 1997; Mondal,
Mondal, & Roy, 2016). Third, the inter-connected pores can increase the
residence time of the reactant gases in the plasma region and enhance the
plasma catalytic interactions (Mondal et al., 2016). Lastly, due to its low
conductivity, Si-MCM-41 can maintain a stable discharge environment for the
dielectric barrier discharge (DBD) plasma. On the other hand, mesoporous
catalysts, such as iron or carbon-based or nickel-based materials; tend to form
micro discharges within the pores, or between the surface of the support and
electrodes, which will lower the conversion and energy efficiency of the ammonia
synthesis (Akay & Zhang, 2016).
4.2.2 Catalyst preparation
As mentioned previously, the catalytic system used in this study was
ruthenium-based catalysts with the Si-MCM-41 supports. Liquid impregnation was
57
used to deposit the catalysts and promoters onto the support. The primary recipe
and preparation steps were determined from previous literature, which was also
described in the previous chapter (Kowalczyk, Krukowski, Raróg-Pilecka,
Szmigiel, & Zielinski, 2003; Saadatjou, Jafari, & Sahebdelfar, 2015). All the
chemicals used in this study were purchased at Sigma Aldrich (USA). For each
gram of Si-MCM-41 support, 0.17g of Ru(NO)(NO3)3, 0.12g of CsNO3, 0.05g of
KNO3, and 0.036g of Ba(NO3)2 were used. During liquid impregnation, 10 minutes
of ultrasulnication was used followed by 20 minutes of stirring. After impregnation,
the catalysts were dried under infrared light for 4 hr with constant stirring and then
calcined at 500°C in a muffle furnace for 4 hr. Before being used for the ammonia
synthesis reactions, the catalysts were reduced at 500°C for 4 hr under hydrogen
environment.
The calcined catalysts in this study were characterized using Fourier
transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM),
scanning electron microscope (SEM), Atomic-force microscopy (AFM), Infrared
Photo induced Force Microscopy (IR PIFM), Energy-dispersive X-ray
spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray
diffraction (XRD). The reduced catalyst was also compared analyzed using XRD.
The FTIR spectrum was obtained under the Attenuated Total Reflection (ATR)
mode of a Nicolet™ iS™ 50 FT-IR Spectrometer. The TEM and SEM image are
taken using FEI Tecnai T12 and Ultra-high Resolution SEM SU8200 (Hitachi,
Japan). AFM and PiFM images were taken using the Vistascope Platform from
Molecular Vista (California, USA). X-ray diffraction (XRD) patterns were obtained
58
on a Siemens D5005 X-ray Diffractometer with sealed Cu source and the XPS
analysis was performed using the SSX-100 XPS device (Surface Science
Instruments, USA).
59
4.3 Results and discussions
4.3.1 Characteristics of the Catalysts
Figure 11 shows the FTIR spectra of the Ru-promoter/Si-MCM41 catalyst.
Three high intensity peaks are observed at 1100 cm-1, 460 cm-1, and 750 cm-1.
The Si-O-Si peak is observed at wavenumber 1100 cm-1 (Chaudhary, Ghatak,
Bhatta, & Khushalani, 2006). The other peak that denotes the Si-MCM-41 support
is the one at approximately 460 cm-1 (Nyquist & Kagel, 2012). The band at 600
cm-1 to 890 cm-1 represents the Ruthenium complex (Nakamoto, 1986). Figure 12
shows the results from the microscopy analysis. The mesoporous structure of the
silica Si-MCM-41 can be revealed from the SEM image. Although the catalytic
system is presented in an agglomerated form, pores are still visible in the SEM
image. As shown in the image, the micro and meso pores in the material are
mainly from 10 to 50 nm. The results found from the TEM image agree with
previous findings for the Ru-MCM complex with mesoporous structures (Vanama,
Kumar, Ginjupalli, & Komandur, 2015). The size of the ruthenium particles ranges
from approximately 10 to 20 nm, with agglomeration shown in the surface of the
Si-MCM-41 support. From the size distribution of the ruthenium particles (2 to 6
mm) (Joo et al., 2010) and the pores of the support, it could be found that the
Ruthenium particles could be deposited both inside and outside of the pores of
the Si-MCM-41 support. In fact, some of the Ru nanoparticles are present on the
outside of pores as well. These dispersion results partially indicate the strong
60
interactions between the Ru nanoparticles and the support, which agree with
previous findings (Makgwane & Ray, 2013; Vanama et al., 2015). As observed in
previous studies, the Si-MCM-41 structured mesoporous materials often exist in
the form of agglomerated spherical particles (Costa et al., 2014; B. Yan, Li, &
Zhou, 2009). Therefore, the AFM and PiFM images were taken, focusing on the
one agglomerated spherical particle. From both the AFM topography and PiFM
image, the particle size is approximately 500 nm in diameter, which also agree
with previous findings (Costa et al., 2014). Since the wavenumber of the PiFM is
selected based on the FTIR spectra, the different color bands in the PiFM image
correspond to different atomic interactions in the catalyst system. As observed in
the TEM image, the PiFM image further indicates the non-uniformed distribution
of the ruthenium particles. However, the microscopy images are not capable of
determining the detailed distribution of each catalytic compound. Therefore, an
elemental mapping was performed to further study the deposition pattern of the
catalyst.
Figure 11 FTIR spectra of the Ru/SI-MCM-41 catalyst
0
0.04
0.08
0.12
0.16
0.2
250 550 850 1150 1450 1750
Lo
g (
1/T
)
Wavenumber (cm-1)
61
Figure 12 a) SEM, b) TEM, c) AFM topography, and d) PiFM images of the Ru-promoter/Si-MCM-41 catalyst
62
Figure 13 EDS results of the Ru-promoter/Si-MCM-41 catalyst
The elemental mapping results represented in Figure 13 show that the Si, Ru
elements are deposited less evenly than the Cs and K elements. This difference in
the distribution is caused by how the species were differently deposited onto the
supports during the liquid impregnation process. Note that due to its small fraction,
63
the Ba element was not detected during several repeated analysis. This could also
be due to the fact that it has been deposited into the porous structures for the
cluster. For the other inorganic salts of KNO3 and CsNO3, they were easily
dissolved into the liquids and were deposited onto the supports during the drying
and calcine stage. Therefore, they are more likely to be deposited on the support
area in the form of the oxidized state. On the other hand, the Si-MCM-41 support
and Ru(NO)(NO3)3 forms dispersion instead of dissolving in the solution during the
liquid impregnation process. Since dispersion was formed instead of a stable
solution, the Ruthenium particles are more likely to agglomerate than the
promoters, which leads to a less uniform distribution of the element. This specific
distribution of catalysts and promoters could induce localized promotion when the
catalyst system is in service.
0
500
1000
1500
2000
2500
3000
05001000
Co
un
ts
Binding Energy (eV)
64
Figure 14 XPS spectrum of the Ru-promoter/Si-MCM-41 catalyst. a) survey scan. b) high resolution scan for
Ru3dC1s. c) high-resolution scan for the Cs
The XPS data shows the oxidation states of different elements within the
calcined catalyst-promoter system. Results of the survey scan and the
high-resolution scan are shown in Figure 14. The binding energy for Si2p is shown
at 90 eV. The two bands between 265 eV and 277 eV correspond to the band for
Ru3d, formed by the split spin-orbit components of Ru (K. Kim & Winograd, 1974).
Furthermore, the binding energy of the However, the two peaks of the Ru3d shown
at 267 eV and 271 eV should have an area ratio of approximately 2 to 3, while the
peaks determined by the high-resolution scan fail to match with the theoretical
values (Rochefort, Dabo, Guay, & Sherwood, 2003). The slightly lower binding
energy of Ru3d in this study could be due to the influence of the promoters on the
Ru oxide clusters, which have been reported in previous literature (Larichev, 2010).
Furthermore, one peak is found at approximately 284 eV, which corresponds to
the peak of C1s. From the peaks fitting data shown in the high-resolution scan in
Figure 14, some of the Ru3d peaks overlap with the C1s peak are observed
100
250
400
260270280290
Co
un
ts
Binding Energy (eV)
200
350
500
650
703713723733
Co
un
ts
Binding Energy (eV)
65
(Berthoud et al., 2008). This overlap is formed due to the carbon contamination
from the carbonation effect generated from the excessive heating rate during the
calcination process in the Si-MCM-41 structure (Dong, Fei, Zhang, & Yu, 2015;
Mun, Ehrhardt, Lambert, & Madic, 2007). Lastly, the binding energy of the Si2p
peak at approximately 100 eV is lower than the reported values (Vanama et al.,
2015; Wagner & Muilenberg, 1979). This could happen due to the fact that the
originally surrounded oxygen atoms are replaced by nitrogen, which are more
electronegative (Xia & Mokaya, 2004). Due to the high nitrogen contents of the
compounds used in the liquid impregnation process, the SiN4 environment is
more dominant compared with other Si-MCM-41 environments reported in
previous literature.
Figure 15 XRD spectrum of the calcined and reduced Ru-promoter/Si-MCM-41 catalyst
degrees 2-theta2 4 6 8 10
inte
nsity (
a.u
.)
0
50
100
150
200X R D calcined
degrees 2-theta10 20 30 40 50 60
inte
nsity (
a.u
.)
0
20
40
60
80
100
120
degrees 2-theta2 4 6 8 10
inte
nsity (
a.u
.)
0
50
100
150
200X R D reduced
degrees 2-theta10 20 30 40 50 60
inte
nsity (
a.u
.)
0
20
40
60
80
100
120
66
The XRD patterns shown in Figure 15 indicate the overall amorphous nature
of the catalyst system after loading Ru and the other promoters. The sharp Bragg
peak at 2.2° shows the hexagonal pore structure of the unloaded Si-MCM-41
support (Beck et al., 1992; W. Zhu et al., 2013). The results shows a slight
decrease in intensity in this peak of calcined and reduced catalysts compared with
the previously reported raw Si-MCM-41 (Gokulakrishnan, Pandurangan,
Somanathan, & Sinha, 2010). This result further indicates that although slightly
destroyed and deformed from the Ru complex, the pore shape within the
Si-MCM-41 structure is largely retained after calcination and H2 reduction
(Deshmukh, Kinage, Kumar, & Meijboom, 2010). The broad peak shown at
around 2θ of 23° corresponds to the amorphous Si-MCM-41 structure
(Dündar-Tekkaya & Yürüm, 2015). Furthermore, the peak of Ru in the reduced
catalyst at 2θ of approximately 43° replaces the peak for Ru2O3 in the calcined
catalyst. This new peak indicates the appearance of a complex after the H2
reduction process. Since the Ru-Si complex is a fairly stable complex that could
be formed under high temperature conditions, it is likely to be formed during the
reduction process (Gasser, Ruiz, Kolawa, & Nicolet, 1999; Y. Wang, Zhang, Cao,
Liu, & Shao, 2011). The peak corresponding to Ru appears after the reduction.
Interestingly, the appearance of the band at 2θ of 43° indicates the formation of
RuSi complex after the H2 reduction process (Perring, Bussy, Gachon, &
Feschotte, 1999).
67
4.3.2 Catalytic synthesis results
The improvement on the ammonia synthesis efficiency is shown in Figure 16
when different components of the catalytic system are introduced. The results
indicate that using Ru catalyst and promoters can lead to approximately 3 times
increase of the synthesis efficiency to the process. Figure 17 shows the effect of
N2 concentration in the feed gas to the NH3 synthesis efficiency. For Si-MCM-41
catalysts, the greatest synthesis efficiency occurs at around 50 vol% of N2 feed
concentration. This result holds for all the conditions under the applied voltages
from 5 kV to 7 kV. The temperature range of the experiments performed in section
3.2 were between 100 °C to 150 °C, which was far less than the temperature
required for the Haber Bosch process. Previous literature results show that the
temperature of the electrode increases linearly with the discharge voltage and
frequency, and agrees with the range of the temperature measurements in this
study (Jidenko, Bourgeois, & Borra, 2010). Meanwhile, the narrow temperature
range in this study further indicated the dominance of catalytic effects of the
plasma and catalysts used in this study. The optimum feeding concentration
obtained for the Si-MCM-41 catalysts differs from the 33 vol% obtained from
previous chapter, which used MgO supports with the Cs promoters but operated
under frequencies lower than 1.6 kHz. Due to the higher surface area of the
mesoporous structure, the residence time and interaction mechanism of the two
reactants will increase in the catalytic system, which could allow more NH radicals
formed in the nitrogen-hydrogen plasma. The results presented in this section are
68
obtained using the catalyst prepared according to the steps described in section
2.1.
Figure 16 Synthesis efficiency results with different catalyst components
Results from Figure 17 also suggest that the NH3 synthesis efficiency is less
dependent on the N2 feed concentration at greater voltage conditions. The output
concentration and concentration of NH3 increases with the applied voltage, the
synthesis efficiency is lower at higher voltage conditions. The effect of gas flow
rate at the optimum feeding concentration of 50 vol% N2 at different voltage
conditions is shown in Figure 18. The ammonia synthesis efficiency increases with
the flow rate. But the trend becomes less obvious after the flow rate is above 4
L/min for the three voltage conditions. Since the plasma synthesis reaction is a
rapid process, it is reasonable that the higher flow rate leads to greater amount of
ammonia produced, and therefore greater synthesis efficiency. Results shown in
Figures 17 and 18 indicate a decrease pattern of NH3 synthesis efficiency with
voltage, and are further confirmed in Figure 19.
0.30
0.50
0.70
0.90
NTP only SupportwithoutRu and
promoters
Supportwithout
promoters
With Ruand
promoters
g N
H3
/kW
h)
69
Figure 17 Effects of the N2 feed concentration on the ammonia synthesis effeciency under different voltage
conditions
Figure 18 Effects of the inlet gas flow rate on the ammonia synthesis effeciency under different voltage
conditions
Figure 19 shows that although the concentration of NH3 increases with
voltage, the synthesis efficiency decreases. The results further indicate that the
reaction equilibrium shift towards the NH3 synthesis at higher voltage. However,
the excess energy from the dielectric discharge at higher voltage leads to the
lower efficiency. As mentioned in section 2.2, the synthesis efficiency is
determined using the ammonia synthesis rate divided by the power. Therefore,
0.20
0.40
0.60
0.80
1.00
0.2 0.4 0.6 0.8
g N
H3
/kW
h
N2 feed concentration
5 kV
6 kV
7 kV
0
0.2
0.4
0.6
0.8
1
0.5 1.5 2.5 3.5 4.5 5.5 6.5
g N
H3/k
Wh
Gas flow rate (L/min)
5 kV
6 kV
7 kV
70
the compensation between the increase in ammonia production and the decrease
in energy efficiency at higher voltage conditions could be the reason for the
synthesis efficiency plateau observed at intervals 5.5-6 kV and 6.5-7kV in Figure
19. On the other hand, in the interval of 6-6.5 kV, the increased power dissipated
into the synthesis reaction might not be enough to increase the synthesis rate of
ammonia, which causes the plateau in ammonia ppm and a relatively dramatic
decrease in synthesis efficiency. Figure 20 shows the effect of the applied
frequency on the synthesis rate and synthesis efficiency at 50 vol% N2 feed
concentration and 5 kV of the applied voltage. Results show that the synthesis
rate of NH3 is less dependent on the frequency than the synthesis efficiency. The
rate reaches the maximum at around 22000 Hz while the synthesis efficiency
increases more dramatically. The greatest synthesis efficiency achieved was 1.7
g/kWh of ammonia at the frequency of 2.6 kHz. It is expected that the resonance
effect of the dielectric barrier discharge can contribute to the homogeneity of the
discharge, which can further increase synthesis efficiency at higher frequency
conditions (B Eliasson & Gellert, 1990; A Fridman, Chirokov, & Gutsol, 2005).
71
Figure 19 Effects of the applied voltage on the ammonia synthesis efficiency and outlet concentration
Figure 20 Effects of the applied frequency on the ammonia synthesis efficiency and outlet concentration
4.3.3 Synergistic effects between the plasma and catalyst
Although the ammonia synthesis process can overall be described by the
general reaction shown in equation (2.1) in section 2, the detailed reaction
0.00
0.30
0.60
0.90
1.20
0.00
20.00
40.00
60.00
80.00
100.00
5 5.5 6 6.6 7
Eff
icie
ncy
(g/
kW
h)
Rat
e (u
mo
l/m
in)
Voltage (kV)
Rate
Efficiency
0.00
0.40
0.80
1.20
1.60
2.00
0.00
10.00
20.00
30.00
40.00
50.00
60.00
20000 22000 24000 26000
Eff
icie
ncy
(g/
kW
h)
Rat
e (μ
mo
l∙m
in-1
)
Frequency (Hz)
Rate
Efficiency
72
mechanism under the plasma conditions needs further explanation. It has been
understood that the synthesis of ammonia from nitrogen and hydrogen gas
involves the following three mechanisms: decomposition, structural
rearrangement, and fragment elimination (B. Penetrante et al., 1997; Van Durme
et al., 2008). For the plasma synthesis of ammonia, the first decomposition step
includes the dissociation and ionization of the inlet nitrogen and hydrogen gases
(Mindong Bai et al., 2003; Matsumoto, 1981). Furthermore, the formation of radical
species generated by the elemental impacts in the plasma region is the essential
part of the reaction (Mizushima et al., 2004).
Within the reactive plasma species shown above, it is believed that NH radical
is the precursor of NH3 formation and its concentration has a strong effect on the
synthesis rate of ammonia (Uyama & Matsumoto, 1989). It is believed that in the
last step of the mechanism, the NH radicals formed in the nitrogen-hydrogen
plasma and the H radicals would be combined with the hydrogen molecules and
produce ammonia (Chauvet et al., 2014; H. Ma et al., 2002).
The high applied frequency combined with a metallic cation promoter could
enhance the electron transfer within the catalyst systems to aid the dissociation
and ionization of nitrogen, which requires highest ionization potential among the
reactions (Mindong Bai et al., 2003; Larichev, 2010). Some most recent
literature have reported that the Cs, K, and Ba are the exclusive promoters that
can increase the catalytic activity of the catalytic system, effectively lower the N2
dissociation barrier, and facilitate the destabilization of the NHx species in
equations (2.4) to (2.6) (Aika, 2017; Z. Ma et al., 2017). Therefore, the synthesis
73
efficiency in this shows improvement compared with previous reported results,
which did not exceed 1.3 g/kWh (Gómez‐Ramírez et al., 2016). However, the
efficiency still requires significant enhancement for this process to be comparable
with the high-temperature and high-pressure synthesis.
Figure 21 The proposed mechanism of NTP ammonia synthesis using Ru-promoter/Si-MCM-41 catalyst
To explain the lack of conversion rate and synthesis efficiency, discussions on the
synergistic effect and interdependence between the plasma and catalyst are
required. Figure 21 shows the proposed mechanism of the NTP ammonia
synthesis using Ru-promoter/Si-MCM-41 catalyst. With the use of multifunctional
catalyst, the transformation of the multi-step reactions could be realized within this
one catalytic system (Dixit et al., 2013). In this process, the plasma formed from
the dielectric discharges result in the chemical and electronic changes in the
catalyst to aid the formation of ammonia (Mizushima, Matsumoto, Ohkita, &
Kakuta, 2007). Electron transfer occurs between the Ru catalyst and the
promoters once the plasma ionizes the catalytic system. This process provides
N2H2
Plasma
NH*
HH
H2
e-PromoterNH2*
H*
NH3
NH3
NH*
Ru Cs
Cs
Si-MCM-41 support
74
dissociation energy to the inlet gases and forms the precursor for a series of
reactions described above. Therefore, ammonia is predominantly formed due to
the increase in concentration of the NH* precursor (Neyts et al., 2015). Although
the large surface area is introduced using Si-MCM-41 as a catalyst support,
catalyst-shielding effects remain in this process due to the surface-treatment
nature of plasma (Amama, Ogebule, Maschmann, Sands, & Fisher, 2006; Bao et
al., 2014). Experimental results indicate that the high frequency of dielectric
barrier discharge promotes the synergistic effects between the plasma and
catalysts. Furthermore, due to the high porosity of the Si-MCM-41 structure
comparing with metal oxide catalysts, less H2 is required in this study compared to
MgO (Mindong Bai et al., 2003). Results have reported that the surface area of Ru
deposited Si-MCM-41 are in the range of 600 to 1000 m2/g, while the Ru
deposited MgO catalyst are approximately 100 m2/g. The high porosity of the
MCM41 supports allows more frequent attachment of the hydrogen radical on the
catalyst surface, therefore the faster mechanism shown in Figure 21 is more likely
to happen. While the shielding effects could be one of the limitations for ammonia
synthesis efficiency, the other limitations are the dissociation of product and
reverse reaction under plasma conditions. Studies have reported the likelihood of
ammonia dissociation when subjected to dielectric plasma discharge. Therefore,
future improvements of the plasma catalytic ammonia synthesis approach could
be made targeting to solve these two specific problems.
75
4.4 Conclusions
In this study, the non-thermal synthesis of ammonia using the Ru-based
multifunctional catalyst was studied, along with the effects of different
experimental parameters of the non-thermal plasma. It is found that the high
voltage inhibits while the high frequency favors the synthesis of ammonia. The
most efficient synthesis achieved was 1.7 g/kWh, which occurred under the
condition of 5000V and 26,000 Hz. The high surface area, low conductivity, the
structured and interconnected pores of the Si-MCM-41 structure enhanced the
efficiency of ammonia synthesis comparing with metal oxide supports. Lastly, a
new concept that includes a two-step method, separating dissociation and
synthesis parts of the process designs is proposed for future investigations. Future
studies could be made on scale-up simulations and to explore other catalyst and
plasma generation options, which could potentially lower the capital cost and
promote the commercialization of this technology.
76
CHAPTER 5 Absorption-enhanced NTP ammonia
synthesis with the assistance of magnesium
chloride
5.1 Introduction
It is mentioned in section 4.3 that one of the most critical challenges of using
NTP to synthesize ammonia was the dissociation of the product within the plasma
region. Therefore, the work in the chapter aims to separate the produced
ammonia from the NTP process immediately after it is synthesized. Various
sorbents have been investigated for the separation of ammonia from gas streams,
such as MgCl2, CaCl2, and activated carbon, etc (Huang, Li, & Chen, 2008;
Sharonov & Aristov, 2005b; Sharonov, Veselovskaya, & Aristov, 2006; H. Zhu, Gu,
Yao, Gao, & Chen, 2009).
In several recent studies, magnesium chloride has been used as an efficient
absorbent that could lower the operating pressure and enhance the performance
of the Haber-Bosch process (Malmali, Wei, McCormick, & Cussler, 2016). The
absorption of ammonia onto MgCl2 usually relies on the formation of Mg(NH3)6Cl2,
which has a strong ammonia-holding capacity (H. Zhu et al., 2009). Other
advantages of MgCl2 are its compatible cost and easy regeneration.
Furthermore, a recent study showed that magnesium-based catalysts can
77
promote the ammonia synthesis under plasma conditions by forming the
intermediate compound of Magnesium nitride (Mg3N2) (Zen, Abe, & Teramoto,
2018). The authors propose Mg3N2 to be an ideal solid-state ammonia carrier
under atmospheric conditions. This study also shows that the Mg3N2 intermediate
could be used to form additional NH3 molecules by reacting with water after the
plasma treatment under nitrogen environments. Compared with MgO, MgCl2 is
easier to react with N2 to form Mg2N3 due to its lower lattice energy. Since the
lattice energy of MgCl2 is significantly lower than MgO, it is reasonable that the
formation of Mg2N3 from MgCl2 much easier than MgO.Due to all the advantages
mentioned above, MgCl2 is identified as the absorbent used for the study in this
chapter.
78
5.2 Materials and methods
5.2.1 System improvements
The experimental apparatus, setup, and procedures were the same as the
previous chapter with the addition of a pulse density modulation (PDM) added to
the SSD 110 inverter. Introducing the PDM to the inverter is critical to the system
improvement in various ways. Recent literature showed that adding the PDM
control of high-frequency inverters could lead to high efficiency (by greatly
reducing the switching loss), small size, and the ability of self power factor
correction (Sandali & Chériti, 2017). In other words, the PDM not only allowed a
linear control of plasma according to the command signals, but also capable of
reducing the power delivered to output of the inverter, which was the load, while
maintaining the high frequency and voltage. The PDM was installed to the system
by connecting it to the first, third, and sixth nodes on the invertors control panel.
The nodes, along the horizontal position of the PDM was adjusted until a stable
waveform on the oscilloscope was formed before starting the system.
5.2.2 Absorbent preparation and characterization
In this chapter, MgCl2 acted as one of the reactants, and also the absorbent
for the process. Anhydrous MgCl2 powder was purchased from Sigma-Aldrich
(USA). The absorbents were packed in the “catalyst region” showed in Figure 5.
79
The system was dried in nitrogen environment under 130 ℃ before starting each
run. The unreacted and reacted MgCl2 samples were characterized using X-ray
diffraction (XRD). Data analysis was performed with the assistance of the
Materials Data Incorporated's JADE 8.0 software package with whole pattern
fitting.
80
5.3 Results and discussions
Figure 22 showed a schematic diagram of how the MgCl2 was placed at
different positions relative to the plasma discharge region. The corresponding
ammonia synthesis rate and energy efficiency of the different placements were
reported in Figure 23. The results showed that the ammonia synthesis rate and
synthesis efficiency both increased when MgCl2 was placed in the plasma region
and was the highest among the different placements. This showed that the
formation of nitrogen-based intermediate could be a critical factor to the
enhancement caused by MgCl2. The results also showed around 6% of ammonia
was absorbed by MgCl2 and was not dissociated in the plasma region. For the
other two circumstances, when MgCl2 was placed immediately below the plasma
discharge region or 5 cm below the discharge region, the total ammonia synthesis
rates were increased compared with synthesis without MgCl2 as well. However,
with a closer look to the rate composition, the enhancement of these two
displacements were caused mainly by the absorption, as the synthesis rates
(shown in red) were approximately the same as the plasma only synthesis. Due to
the short residence time of the reactant and product gases in the reactor, part of
the synthesized ammonia was absorbed by MgCl2 immediately after passing the
plasma discharge region, before forming back to N2 and H2.
81
Figure 22 Schematic diagrams of the MgCl2 locations relative to the plasma discharge region
Figure 23 Ammonia synthesis results with MgCl2 packed at different locations relative to the plasma
discharge region
The dielectric effect was what contributed to the catalytic functions of the
0
2
4
6
8
10
0.00
10.00
20.00
30.00
40.00
50.00
Plasmaonly
Indischarge
Belowdischarge
5 cm belowdischarge
Eff
icie
ncy
(g/
kW
h)
Rat
e (μ
mo
l∙m
in-1
)
Total rate
Rate absorbed
Efficiency
82
MgCl2. Within the plasma discharge region, the magnesium is capable of bonding
at its surface with the free radicals (N*, H*) formed by the electron excitation and
vibrational excitation in the gas phase, providing additional reaction sites with the
NHx* free radicals to form ammonia 8, 14, 27. To further study how the change in
discharge characteristics due to dielectric effect may contribute to improvement of
efficiency, Figures 24 to 27 showed the effects of different plasma parameters to
the NH3 synthesis rate. absorption rate, and energy efficiency. From the results,
the effects of the different parameters could be divided into two categories. For
the first category, the changes in the parameters, which were the discharge
voltage and N2 composition, affected both the synthesis rate and the efficiency.
Figure 24 showed both the maximum NH3 synthesis rate and the energy
efficiency occurred at 6.44 kV. For the synthesis rate, the absorbed rate and the
total rate shared the same relation to the discharge voltage. Both rates increased
with voltage until 6.44 kV and decreased between 6.44 kV to 7.14 kV. This trend
agreed with the findings in the previous chapters that the increase in the
discharge voltage could promote the forward reaction to form NH3, until the
dissociation affect started to dominate. This “double-sided sword” effect was more
obvious in the study using MgCl2, as the absorbed rate started to decrease after
the voltage increased beyond 6.44 kV. Similarly for the N2 composition, at lower
N2 contents, NH3 was synthesized less efficiently and at lower rates. As the N2
composition started to increase within the reactant stream, both the rate and the
efficiency started to rise. However, the performance was less ideal during the N2
content of 0.75. The most efficient synthesis was achieved at the N2 composition
83
of 0.5, which represented a volumetric flow rate ratio of 1:1 for N2 and H2. On the
other hand, for parameters including the discharge frequency and flow rate, the
affects of them to the synthesis rate was smaller than to the efficiency. The
influence of these two parameters on the discharge current from the invertor to
the transformer was obvious. When the discharge frequency was adjusted to 23
kHz, the current was significantly smaller than other conditions. For the gas flow
rates, the high gas flow rates contributed to the increase in energy efficiency and
the maximum was reached at 4 L/min, although the influence on the synthesis
rate was not as obvious. However, the rates for ammonia absorption by the MgCl2
decreased at higher flow rates due to the decrease in the residence time. As
discussed in Chapter 4, due to the fast and rapid nature of plasma-related
reactions, higher flow rates could lead to more products being formed in the region,
which could further cause greater synthesis efficiency.
84
Figure 24 Ammonia synthesis results with MgCl2 under different voltage conditions
Figure 25 Ammonia synthesis results with MgCl2 under different frequency conditions
0
2
4
6
8
10
0
10
20
30
40
50
5.04 5.74 6.44 7.14
Eff
icie
ncy (
g/k
Wh
)
Ra
te (
μm
ol∙m
in-1
)
Voltage (kV)
Total rate
Rate absorbed
Efficiency
0
4
8
12
16
20
0
10
20
30
40
50
60
20 21.5 23 24.5 26
Eff
icie
ncy (
g/k
Wh)
Ra
te (
μm
ol∙m
in-1
)
Frequency (kHz)
85
Figure 26 Ammonia synthesis results with MgCl2 under different nitrogen compositions
Figure 27 Ammonia synthesis results with MgCl2 under different gas flow rates
0
5
10
15
20
0
10
20
30
40
50
60
70
0.25 0.375 0.5 0.625 0.75
Eff
icie
ncy (
g/k
Wh
)
Ra
te (
μm
ol∙m
in-1
)
N2 composition
Total rate
Rate absorbed
Efficiency
0
5
10
15
20
0
10
20
30
40
50
60
70
1 2 3 4 5
Eff
icie
ncy (
g/k
Wh
)
Ra
te (
μm
ol∙m
in-1
)
Reactant flow rate (L/min)
Total rate
Rate absorbed
Efficiency
86
The XRD results of the untreated MgCl2 and the treated MgCl2 in and below
the discharge region were shown in Figure 28. Based on the proposed
mechanisms of this process, three compounds were identified from the XRD
patterns, which were MgCl2, Mg3N2, and Mg(NH3)6Cl2. The peaks associated with
each compound were labeled, along with their intensities. To improve the clarity
and readability of the figure, the peaks associated with MgCl2 were labeled on the
untreated pattern and the peaks for Mg3N2 and Mg(NH3)6Cl2 were labeled on the
in discharge and below discharge patterns, respectively. The XRD pattern for the
untreated sample matched closely with the ones for MgCl2 in the Jade software
database, and with the findings reported in previous literature (Stavrou et al.,
2016). The diffraction peaks at 21,32, and 34 2-degree theta were associated to
Mg3N2 (Luo, Kang, Fang, & Wang, 2011). And the peaks at 16 and 22 2-degree
theta corresponded to Mg(NH3)6Cl2 (Guangming, Peihua, Zhiming, Mingzhen, &
Minxong, 2004; Y. Liu et al., 2011). The results from the figure showed that both
Mg3N2 and Mg(NH3)6Cl2 were identified in the in-plasma and below-plasma
treated samples. Intensity wise, the intensities of the MgCl2 associated peaks
decreased after being treated by the plasma, and further decreased when placed
below the plasma discharge region. This result corresponds with the findings from
Figure 28. When MgCl2 was placed below the plasma discharge region, the ratio
of the absorbed ammonia was higher than when it was placed in the plasma
region. Therefore, it is reasonable that the concentration of pure MgCl2 would
decrease compared with the untreated and in-discharge samples.
87
The SEM images from Figures 29 to 31 indicated that the effects of the
plasma treatment on the shape and size of the MgCl2 were negligible. However,
the EDS analysis results demonstrated difference between the MgCl2 samples
treated under different conditions. For the MgCl2 samples treated inside and
below the plasma discharge regions, nitrogen was identified during the EDS
analysis. The nitrogen concentration in the treated sample reported by the EDS
analysis was 0.16 wt%, which showed relatively good correspondence with the
theoretical nitrogen concentration based on the amount of ammonia absorbed
during the process.
From the analysis above, two critical reaction mechanisms of how MgCl2
promoted the ammonia synthesis could be developed. The first mechanism
supported the hypothesis from in Section 5.1. Similar to other Mg-based catalysts,
such as MgO, the MgCl2 could form the critical intermediate compound Mg3N2
(magnesium nitride) during the plasma synthesis via the nitridation mechanism
(Zen et al., 2018). Compared with MgO, MgCl2 had smaller lattice energy so that it
was easier to carry out the nitridation mechanism for MgCl2 than MgO. As a result,
the ammonia synthesis rate in this chapter was higher than those achieved for
MgO (Zen et al., 2018). The second mechanism was the rapid absorption of the
produced ammonia by MgCl2 in the plasma region. The fast absorption was
achieved via the formation of Mg(NH3)6Cl2. Past literature repeatedly reported the
strong ability of MgCl2 for absorbing and holding ammonia from this reaction
(Sharonov & Aristov, 2005a; H. Zhu et al., 2009). Compared with the absorption
result from these studies, which was about 33 mg NH3/g MgCl2 at 100 ℃, the
88
amount absorbed by MgCl2 was larger (over 100 mg NH3/g MgCl2) due to the first
absorption mechanism that involved the formation of Mg3N2. This could indicate
within the synergistic absorption effects, the formation of Mg3N2 was favored over
the Mg(NH3)6Cl2. Another possibility was that Mg(NH3)6Cl2 could be dissociated
under the treatment of plasma. While the ability of retaining the Mg3N2 compound
within plasma was reported in previous literature, no research managed to
determine the fate of Mg(NH3)6Cl2 under plasma treatment. Therefore, this could
be a direction for future studies on exploring ammonia holding capabilities of
different absorbents under plasma conditions. Past literature reported the ability
of MgCl2 for absorbing and holding ammonia of forming Mg(NH3)6Cl2 (Sharonov &
Aristov, 2005a; H. Zhu et al., 2009), which was about 33 mg NH3/g MgCl2 at 100
℃. The fact that the amount absorbed in the solid-phase in this study was larger
(over 100 mg NH3/g MgCl2) indicated that within the synergistic absorption effects,
the formation of Mg3N2 was favored over the Mg(NH3)6Cl2.
89
Figure 28 XRD patterns of the MgCl2 samples under different conditions
90
Figure 29 SEM image and EDS analysis results of the untreated MgCl2 sample
91
Figure 30 SEM image and EDS analysis results of the MgCl2 sample treated in the plasma discharge region
92
Figure 31 SEM image and EDS analysis results of the MgCl2 samples treated below the plasma discharge
region
93
5.4 Conclusions
In conclusion, two major improvements were carried out in this chapter. First,
the pulse density modulation (PDM) added to the SSD 110 inverter improved the
efficiency of the invertor-transformer system by reducing the current and switching
energy loss. Second, magnesium chloride used in this chapter could enhance the
synthesis rate and efficiency through forming two critical compounds for the
process, Mg3N2 (reaction with N2 in the plasma) and Mg(NH3)6Cl2 (absorption of
the NH3 product). These improvements led to a greatest energy efficiency of 20.5
g/kwh, with the corresponding synthesis rate being 50 mmol/min.
94
CHAPTER 6 Atmospheric plasma-assisted nitrogen
fixation using water and nitrogen jet plasma
6.1 Introduction
The fixation of nitrogen gas, one of the most abundant resources in the world,
has been historically affecting the human society for over centuries.
Nitrogen-derived chemicals such as nitric acid and ammonium nitrate has been
used largely in the field of agriculture, paints and dyes, and explosives. In recent
years, ammonia has been intensively investigated as a green fuel and sustainable
hydrogen storage material. However, the conventional industrial nitrogen fixation
process that the Haber-Bosch method to generate ammonia, which is further used
to synthesize other nitrogen-based chemicals, is an energy-intensive process that
consumes a large amount of fossil fuels. This process itself is responsible for
approximately 1 to 2% of the global electrical usage and emits significant
greenhouse gases (Galloway et al., 2008; Razon, 2014). Furthermore, the high
temperature (400 to 600 C) and pressure requirements (400 to 600 atm) of this
process make the synthesis of ammonia centralized and prevent local farms and
small industries to perform on-site ammonia generation (Hargreaves, 2014;
Tallaksen, Bauer, Hulteberg, Reese, & Ahlgren, 2015).
Plasma-based nitrogen fixation provides an alternative nitrogen fixation
process that is sustainable, accessible to distributed production, and most
95
importantly under atmospheric temperature and pressure. The NOx – compounds
formed during the fixation process using nitrogen plasma and water have been
reported responsible for bacterial inactivation (Shen et al., 2016), and can
promote the increase plants’ growth rates(Park et al., 2013). There have been
several attempts on the synthesis of NOx – compounds using plasma-assisted
processes. In these studies, different plasma discharge methods have been used
to fix nitrogen gas into water and produced nitric and nitrous acids, including
gliding arc discharge plasma (Bian, Shi, & Yin, 2009; Burlica, Kirkpatrick, & Locke,
2006; Park et al., 2013; Yang, Li, Zhong, Guan, & Hu, 2016), nozzle-type plasma
jet (Shen et al., 2016), and dielectric barrier discharge plasma (Kovačević et al.,
2017). While the emphasis of these studies has been focusing on the production
of NOx – in the liquid phase, the generation of NOx gas during the process is rarely
considered and the ammonium content in the liquid phase has not been reported.
On the other hand, a majority of the studies on the plasma-assisted synthesis
of ammonia and NOx gas under atmospheric temperature and pressure have
been using catalytic plasma discharge of nitrogen and other related gases, such
as hydrogen (Gómez‐Ramírez, Montoro‐Damas, Cotrino, Lambert, & Gonzá
lez‐Elipe, 2017) and oxygen (B. Patil, Cherkasov, et al., 2016; B. S. Patil et al.,
2017). The process that uses nitrogen plasma and water circumvents the use of
hydrogen gas, which can reduce the capital cost and safety issues of the process.
However, this is a novel process that has been sparsely investigated. The idea
was first introduced by Kubota et al., which used a nozzle-type plasma jet for the
96
production of ammonia in water (Kubota, Koga, Ohno, & Hara, 2010). Later, two
most recent studies reported the synthesis of ammonia by purging the nitrogen
plasma gas into water under the irradiation of UV light (Haruyama et al., 2016;
Sakakura et al., 2017a). The authors pointed out that the UV irradiation could
improve the synthesis of ammonia by promoting the surface dissociation of water
molecules as hydrogen donors in the reaction. The findings from the literature
also indicated that the synthesis of ammonia is a reaction that primarily happens
at the liquid surface and is highly dependent on the reaction area (Haruyama et al.,
2016). Therefore, increasing the surface area and reducing the distance between
the plasma generation and the water surface are critical to improve the
performance of the process, Furthermore, since previous literature focus on the
separate synthesis of ammonium, nitrite, and nitrate, it would be ideal to have a
process that aims to co-synthesis all three compounds during the fixation.
97
6.2 Materials and Methods
6.2.1 Improvements of the system
In this process, two significant improvements are introduced to the
process to enhance the performance of the process. First, a spray-type plasma jet
is used, which greatly increases the reaction area at the plasma-water interface.
Second, this study uses an innovative in-situ synthesis approach to facilitate the
plasma-liquid interface reactions. This approach largely shortens the distance
between the discharge region and the liquid interface, therefore significantly
reduced the back reaction of the nitrogen plasma species. Furthermore,
understanding towards all of the nitrogen-based products formed during the
process is critical for improving and optimization the plasma-based nitrogen
fixation technology. Thus, this paper first takes into consideration of both the
ammonium and NOx – generated in the liquid phase, as well as the NOx produced
in the gas phase. In this paper, a comprehensive reaction mechanism of forming
the different nitrogen fixation products is proposed, including the synergistic
integrations between the gas phase and liquid surface products.
6.2.2 Description of apparatus
The in-situ synthesis of ammonia was carried out in the 10-liter constant
stirring tank reactor shown in Figure 32 filled with 2 L of distilled water. The
plasma discharge head was installed inside the reactor. The distance between the
98
discharge head and the liquid surface was 3 to 4 cm. The plasma was formed and
dispersed onto the liquid surface by an atmospheric corona discharge plasma jet
generator (Enercon Industries, USA). The discharge head consisted of two
electrodes separated by a distance of 3 cm. Nitrogen gas was supplied to the
system at a flow rate of 57 standard liter per minute (SLPM). The high flow rate of
the gas allowed the plasma to spread out through the nozzle and disperse onto
the liquid surface. A well-insulated electric cord connected the high voltage
electrode to a step-up transformer located in the main console outside of the
reactor, which amplified the 120 V, 60 Hz utilities alternate current (AC) power
supply to 4.4 kV. The UV irradiation was produced using a 15 W UV lamp supplied
by Technical Precision Inc. (USA). Ammonia in the gas outlet was detected using
a colorimetric gas detection system from RAE Systems, Inc. (USA). The NOx
concentration was determined by an IMR gas analyzer (IMR Inc. USA) and the
concentration of N2 was verified using a Varian CP-4900 gas chromatography
(Agilent, USA). The liquid samples collected from the reactor were analyzed using
a DR 5000™ UV-Vis Spectrophotometer (Hach Technology Inc. USA).
99
Figure 32 Process flow diagram of the in-situ atmospheric nitrogen fixation process
100
6.3 Results and discussion
6.3.1 Synthesis results
During the experiments, the concentration of the products in the liquid phase
built up as time increased, as shown in Figures 33 and 34. On the other hand, the
NOx concentration built up instantly to the steady state value shown in Figures 35
d) and 36 d), and remained throughout each run. In general, the concentrations
shown in Figures 33 and 34 increased at approximately constant rates with time at
various experimental conditions. Due to the short life spam of the active plasma
species, the distance between the plasma discharge and the liquid surface is an
important factor in this experiment. The in-situ setup led to twice as much as
products than the ex-situ setup, where the plasma discharge was placed outside of
the reactor. From Figure 32, it was obvious that the in-situ operation, and the UV
irradiation improved the nitrogen fixation significantly, as well as the final
concentrations of nitrate, nitrite and ammonium at the end of the 20-minute
reaction. Furthermore, based on the results in Figure 35, the total nitrogen fixation
rate increased from 22.3 μmol min-1 (ex situ) to 51.1 μmol min-1 (in situ), which
also showed improvements compared with other reported ex-situ synthesis with a
similar reaction area (Haruyama et al., 2016; Sakakura et al., 2017b). For each
nitrogen fixation product, the increase in both final concentration and synthesis
rate was also dramatic (greater than 2 times increase). In general, one of the main
challenges for the plasma-assisted ammonia synthesis was the tendency for the
101
dissociated nitrogen plasma species to reform the diatomic nitrogen. The in-situ
plasma generation (shown in Figure 32) greatly shortened the distance between
the plasma discharge point and the liquid surface. Therefore, a region with
concentrated nitrogen plasma species was formed, which enabled the dissociated
nitrogen to react directly with the hydroxyl or hydrogen radicals to form NOx or
ammonia. This increased the possibility for the formation of ammonia at the liquid
gas interface. Furthermore, the spray-type plasma jet used in this study increased
the surface area of the reaction. The reaction surface area of the spray-type
plasma jet was approximately 5 cm2, which was significantly larger compared with
a nozzle type plasma jet of similar power (usually less than 1 cm2). Second, since
high flow rate is used, the plasma species.
102
Figure 33 Concentrations of nitrate, nitrite and ammonium at different experimental conditions at 30 C
0
40
80
120
160
0 5 10 15 20
μm
ol∙
L-1
Time (min)
a) NO3-
Ex-situ w/ UV
In-situ w/ UV
In-situ w/o UV
0
5
10
15
20
25
0 5 10 15 20
μm
ol∙
L-1
Time (min)
b) NH4+
Ex-situ w/ UV
In-situ w/ UV
In-situ w/o UV
0
100
200
300
400
0 5 10 15 20
μm
ol∙
L-1
Time (min)
c) NO2-
Ex-situ w/ UV
In-situ w/ UV
In-situ w/o UV
103
6.3.2 Gas phase reactions
The co-synthesis of the nitrate, nitrite, ammonium, and NOx demonstrated in
this study was the result from both the gas phase and liquid phase reactions, as
well as the interactions between these two phases. Figure 37 described the
proposed synergistic mechanism involved in this study. In the gas phase, the
electron excitation of the nitrogen gas led to the formation of free nitrogen radicals
in the gas phase (equation 6.1) (Carrasco et al., 2011). At the mean time, the
hydroxyl radicals were formed by the electron excitation of the vaporized water via
the mechanism shown in equation (6.2) (Itikawa & Mason, 2005; D.-X. Liu,
Bruggeman, Iza, Rong, & Kong, 2010). The excited free nitrogen and hydrogen
radicals formed the NH radical, which acted as the main intermediate to form
ammonia. As pointed out by Sakakura et al. (Sakakura et al., 2017a), there should
also be a negligible amount of hydrogen gas formed by the hydrogen radicals in
the gas phase (equation 6.3), which would immediately react with the hydroxyl
radicals in the liquid phase and could promote the formation of NH4+ in the solution.
However, without the presence of catalysts, this series of reactions in the gas
phase (equations (6.4) to (6.6)) was minor. And due to the close distance between
the discharge head and the liquid surface, the NH3 particles produced in the gas
phase were immediately absorbed in the liquid. Therefore, the ammonia
concentration in the outlet gas was below 1-ppm detection limit. Under the effect
of UV irradiation, the reaction of the nitrogen and hydroxyl radicals represented by
104
equations (6.7) and (6.8) would occur, leading to the production of the NOx
species in the gas phase (Sharma, Hosoi, Mori, & Kuroda, 2013).
N2(𝑣) + e → 2N(𝑣) + e (6.1)
H2O (𝑣 ) + e → OH (𝑣) + H(𝑣) + e (6.2)
2H(𝑣) → H2(𝑣) (6.3)
N(𝑣) + H(𝑣) → NH(𝑣) (6.4)
NH(𝑣) + H(𝑣) → NH2(𝑣) (6.5)
NH2(𝑣) + H(𝑣) → NH3(𝑣) (6.6)
N(𝑣) + OH(𝑣) → NO(𝑣) + H(𝑣) (6.7)
NO(𝑣) + OH(𝑣) → NO2(𝑣) + H (𝑣) (6.8)
6.3.3 Liquid phase reactions
In the liquid phase, a two-step mechanism was demonstrated in previous
literature for the ammonia synthesis (Sakakura et al., 2017b). At the liquid-gas
interface, the free radicals formed by the electron-excited nitrogen gas, and its
related radicals, reacted with the water molecules excited by UV light (equation 6.9)
to form ammonia at the surface of the liquid, which was then dissolved into the
solution. On the other hand, the main nitrogen-based products in the aqueous
105
phase formed during the experiments were the NO3 −
and NO2 −, with NO2
– being
the predominant compound. In the liquid phase, the plasma species in the gas
phase formed by the dissociated nitrogen gas have the tendency to form more
stable compounds in the liquid, which were the nitrate and nitrite products
according to reactions mechanisms and demonstrated in an earlier publications
(Szili, Hong, Oh, Gaur, & Short, 2017), (R Atkinson et al., 1989).
H2O (𝑙 )UV irradiation→ OH (𝑙) + H(𝑙) (6.9)
H2(𝑣) + OH(𝑙) → H2O + H (𝑙) (6.10)
N(𝑣) + H(𝑙) → NH(𝑙) (6.11)
NH(𝑣 𝑜𝑟 𝑙) + H(𝑙) → NH2(𝑙) (6.12)
NH2(𝑣 𝑜𝑟 𝑙) + H(𝑙) → NH3(𝑙) (6.13)
NO(𝑣) + OH(𝑙) → HNO2(𝑙) (6.14)
NO2(𝑣) + OH(𝑙) → HNO3(𝑙) (6.15)
106
Figure 34 Concentrations of nitrate, nitrite and ammonium of the in-situ reactor at different temperatures
Literature documented the formation of the hydroxyl and hydrogen radicals
0
40
80
120
160
0 5 10 15 20
μm
ol∙
L-1
Time (min)
a) NO3-
40C
30C
50C
70C
60C
0
10
20
30
40
0 5 10 15 20
μm
ol∙
L-1
Time (min)
b) NH4+
40C
30C
70C
50C
60C
0
100
200
300
400
0 5 10 15 20
μm
ol∙
L-1
Time (min)
c) NO2-
40C
30C
50C
70C
60C
107
from water with the assistance of UV light, which was represented by equation
(6.9) (Chai, Zheng, Zhao, & Pollack, 2008). This reaction could further react with
the plasma-excited nitrogen radicals at the liquid-gas interface, in either the gas or
liquid phase, to promote the formation of nitrite acid, ammonium, and nitrate
(equations 6.10 to 6.15). The promotional effect of UV irradiation towards the
liquid surface dissociation plays a critical role in nitrogen fixation. Although the
promotion effect of the UV light on plasma-assisted ammonia synthesis was
reported (Haruyama et al., 2016; Matsuo et al., 2015), results from this study
showed that the promotion of UV irradiation could promote the formation of other
nitrogen species in the liquid as well. More specifically, the promotional effect of
UV irradiation was selectively and more effective towards the NO2 – and NH4
+,
respectively. As shown in Figure 35, introducing the UV light into the system led to
approximately 3 and 2 times increase in the synthesis rate of NO2 – and NH4
+,
whereas the rate for NO2 – increased only around 1.3 times. To explain this, it was
noteworthy to point out that the rate constant of reaction (6.7) is the orders of 10-11
cm6 ∙ molecule−1s−1, while the rate constant for reaction (6.8) is in the order of 10-32
cm6 ∙ molecule−1s−1, which explained the predominant improvement of the UV
irradiation on the synthesis rate of NO2 –
in the aqueous phase (R. Atkinson et al.,
2004). On the other hand, UV irradiation had minor effect on the production of
NOx.
108
Figure 35 Synthesis rates of nitrate, nitrite and ammonium at different experimental conditions at 30℃
0
2
4
6
8
10
12
14
16
18
Ex-situ w/
UV
In-situ w/o
UV
In-situ w/
UV
Ra
te (
μm
ol∙
min
-1)
a) NO3−
0
0.5
1
1.5
2
2.5
Ex-situ w/ UV In-situ w/o UV In-situ w/ UV
Ra
te (
μm
ol∙
min
-1)
b) NH4+
0
5
10
15
20
25
30
35
40
Ex-situ w/
UV
In-situ w/o
UV
In-situ w/
UV
Ra
te (
μm
ol∙
min
-1)
c) NO2−
0
20
40
60
Ex-situ w/
UV 30C
In-situ w/o
UV
In-situ w/
UV
pp
m
d) NOx
109
Figure 36 Synthesis rates at nitrate, nitrite and ammonium of the in-situ reactor at different temperatures
Overall, the temperature had minor effects on the production of the
ammonium and the other nitrogen-based products. The influences of temperature
on the synthesis of nitrate and nitrate were more dramatic than ammonium.
Previous literature showed that the hydroxyl radicals formed by the dissociation of
water under UV irradiation was the highest at around 40 °C (Matsuo et al., 2015),
which corresponded with the finding from this study that the highest synthesis
rates for NO3 – (15.1 μmol∙min-1) and NO2
– (40.3 μmol∙min-1) was detected at
0
4
8
12
16
20
20 40 60 80
Rate
(μ
mol∙
min
-1)
Temperature (℃)
a) NO3−
0
0.6
1.2
1.8
2.4
3
20 40 60 80
Ra
te (
μm
ol∙
min
-1)
Temperature (℃)
b) NH4+
0
15
30
45
60
20 40 60 80
Ra
te (
μm
ol∙
min
-1)
Temperature (℃)
c) NO2−
0
30
60
90
120
150
20 40 60 80
pp
m
Temperature (℃)
d) NOx
110
around 40 °C, shown in Figure 36. On the other hand, the NOx concentration in
the gas phase increased with temperature. As mentioned previously, the NOx in
the outlet gas was formed by reactions between the vaporized H2O and the
nitrogen plasma. At higher temperatures, the vapor content of water in the gas
phase increased dramatically, therefore favored the formation of NOx gases
represented by equations (6.7) and (6.8). As the byproducts of this process, the
exhaust NOx could be absorbed to form nitrate and nitrite solution in the
downstream steps. Based on the experiments from this study, the optimum
operation temperature was determined to be around 40 °C, where the nitrogen
fixation rate was the highest and the formation of NOx byproducts was low.
111
Figure 37 The propose mechanism for the In-situ plasma jet nitrogen fixation
112
6.4 Conclusions
In conclusion, the co-synthesis of nitrate, nitrite, and ammonium during a
plasma-assisted nitrogen fixation process from water and nitrogen was
investigated. The in-situ spray-type plasma used in this study could significantly
enhance the nitrogen fixation rate. The UV irradiation also promoted the fixation
via the excitation of vapor and liquid water molecules. The fixation mechanism
determined in this study was a synergistic interaction between the gas and liquid
phase. The highest nitrate, nitrite, and ammonium synthesis rate achieved in this
study are 15.1 μmol∙min-1, 40.3 μmol∙min-1, and 2.5 μmol∙min-1, respectively.
113
CHAPTER 7 Challenges and Opportunities
7.1 Challenges
Based on the relevant studies discussed in the previous chapters, the main
challenges of the NTP synthesis of ammonia are two-fold. The first challenge is
the nitrogen fixation. The energy required to break the nitrogen triple bond is
approximately 945 kJ/mol (9.79 eV), which is the largest activation energy
required in both conventional and NTP ammonia synthesis (Misra, Schlüter, &
Cullen, 2016). The specific energy plays a critical role in facilitating the reaction to
synthesize ammonia. Unfortunately, the effects of increased specific energy input
are a bit of a double-edged sword with regards to ammonia synthesis. Increased
specific energy input leads to an increase in the number of high-energy electrons
and important reaction intermediates (such as NH*), which are reportedly
responsible for both product formation and undesired back reactions (the
decomposition of the product) (Van Helden et al., 2007). This leads to the second
challenge of the NTP ammonia synthesis, which is the immediate dissociation of
ammonia after it is produced. The decomposition effect of NTP on ammonia was
first revealed by studies related to air pollution and off-gas treatment (Oda, 2003;
Urashima & Chang, 2000). (L. Wang et al., 2013) demonstrated that NTP could
decompose ammonia with the assistance of Fe catalysts and avoid nitrogen
poisoning during the process. Furthermore, other studies also indicate that
114
transitional metals including Fe, Co, Cu and Ni demonstrated relatively high
activities during plasma-assisted ammonia decomposition (Ji et al., 2013; L.
Wang et al., 2015). Therefore, to minimize the dissociation of ammonia during its
plasma-assisted synthesis process, it is encouraged that transition metals should
be avoided during the catalyst selection.
115
7.2 Opportunities
This section is a discussion of some of the opportunities to overcome several
of the major difficulties in developing an efficient method NTP ammonia synthesis,
as well as future opportunities in the application of this technology as a
competitive and environmentally friendly alternative to the traditional method of
ammonia synthesis. Since NTP ammonia synthesis is still considered a
cutting-edge research topic, it is very possible that the opportunities for
breakthroughs could be inspired from the most recent and innovative ammonia
synthesis studies under intense conditions. Some studies that synthesize
ammonia under high temperature and pressures have been depositing ruthenium
onto carbon nanotubes, or developed innovative structures to enhance the
catalytic binding efficiencies (Wu et al., 2003). Another creative catalyst structure
that could potentially benefit NTP ammonia synthesis is the use of electrode
support within the catalyst matrix. Several studies, although performed under high
temperature and pressure, used a new catalyst structure that loads ruthenium
onto a new electrode support 12CaO∙7Al2O3 to prepare a unique catalytic
structure of Ru/C12A7:e- (Kanbara et al., 2015; Kitano et al., 2012; Kitano et al.,
2015). The three studies pointed out that the stable electrode material can
perform both as electron donor and as a reversible hydrogen storage site, while
simultaneously enhancing ammonia synthesis by shifting the rate limiting reaction
from nitrogen dissociation to hydrogen bonding. With regards to the previously
discussed reactor development, the rapid absorption of ammonia after its
116
synthesis is critically important to preventing back reactions. A recent study
pointed out a potential opportunity to overcome this obstacle, showing that the
introduction of Lewis acidic and redox-active sites to porous sorbents could
enhance the ammonia absorption (Tan et al., 2015), which could be another
worthwhile topic of investigation related to NTP ammonia synthesis. Since
ammonia can be easily decomposed by NTP, it is greatly beneficial if the
ammonia can be absorbed and removed from the plasma discharge region
immediately after it is synthesized. In this way, this technology can be significantly
more environmental sustainable due to the large increase in conversion and
efficiency.
Despite the current challenges, NTP is still considered a promising technology
for the future of ammonia production. The largest opportunity of this technology is
the elimination of fossil fuels from the ammonia synthesis process. Due to its
“green” nature, this technology is recognized as one potential method of realizing
sustainable agriculture (Pfromm, 2017). Furthermore, this technology could act as
an essential part of the wind-to-ammonia concept, where the hydrogen from
renewable sources can be fed the NTP reactor to sustainably generate ammonia
to be used as a fertilizer and clean fuel (Tallaksen et al., 2015). The new version
of this renewable concept with the implementation of NTP ammonia synthesis
technology is demonstrated in Figure 38. On farm sites or renewable energy
plants, the renewable energy sources such as wind, solar or hydro can be used to
provide energy for the electrolysis of water, which would provide the hydrogen gas
required for ammonia synthesis. The hydrogen gas, or other hydrogen-rich gases
117
such as methane, can also be converted from biomass agricultural waste through
gasification or anaerobic digestion. Lastly, the NTP plasma synthesis can be
powered by the renewable energy technologies. This renewable energy solution
provides a promising alternative for farm sites and small industries to become
accessible to sustainable energy and fertilizer via a renewable pathway.
Furthermore, this pathway has promising opportunities due to the fact that it fully
utilizes the advantages of the NTP ammonia synthesis such as less land use, low
capital cost, and less intense reaction conditions, and incorporates these
advantages with other renewable technology processes. Due to ammonia’s
excellent performance as an energy carrier and storage material, one of the
advantages of using wind energy to power the NTP ammonia synthesis is that the
ammonia could be considered as an excellent off-peak energy storage material
(Matzen, Alhajji, & Demirel, 2015). Furthermore, the combination of using wind
and electrolyzes with the NTP ammonia synthesis will reduce fuel transportation
costs and enable the synthesis fuel and fertilizers on site. For small industries and
local farms, this is critical since the distributed energy and fertilizer synthesis will
free them from market price fluctuations (Morgan, Manwell, & McGowan, 2014).
Additionally, using gasification to convert biomass into hydrogen gas can utilize
the waste produced by farms and generate sustainable fuel and energy (Lozano
& Lozano, 2017). However, the main challenges for the wind and biomass
gasification technologies are their inconsistency (Karltorp, 2016). For wind energy,
the power produced for the plasma is highly dependent on the wind resource at
the specific time frame. On the other hand, the large variance of the biomass
118
would result in the change of syn-gas components from the gasification process
(Sansaniwal, Rosen, & Tyagi, 2017). Therefore, the hydrogen separation process
needs to be adjusted according to the syngas composition, which would introduce
additional undesired cost.
Figure 38 An illustrative diagram of the renewable-to-ammonia approach
In terms of energy conversion, non-thermal plasma is a technology that could
theoretically fix nitrogen more efficiently than the conventional thermal fixation
methods (B. Patil, Hessel, et al., 2016). (Cherkasov, Ibhadon, & Fitzpatrick, 2015)
Pressure swing adsorption (PSA)
Renewable energy source (wind, solar, hydro etc.)
Electrolysis of water
Biomass conversion
Hydrogen NTP synthesis
Ammonia Nitrogen
119
reported that NTP N-fixation has a theoretical efficiency floor of 0.2MJ/mol, which
is more efficient than the Haber-Bosch method of 0.48 MJ/mol. Additionally,
(Azizov et al., 1980) allegedly reported an energy consumption of 0.29 MJ/mol
through the oxidation of diatomic Nitrogen in a microwave-induced plasma
generator. Other factors that make NTP one of the most attractive alternatives to
the Haber-Bosch method are the low land requirement and mobile capability.
Being able to produce ammonia in a continuous manner at a scale accessible to
the small industries and local farms, the NTP ammonia synthesis is well suitable
for the world’s trend of distributed and renewable energy production (Rune Ingels
& David B Graves, 2015).
A summative comparison between the NTP synthesis and the conventional
Haber-Bosch process is shown in Table 1. To achieve this, discussions and
analysis needs to surpass the summarization of published literature on process
developments. Therefore, this paper addresses the correlations between the
state-of-art of this technology with cleaner productions and how its development
could improve the sustainability in the ammonia production industry.
120
Table 3 A summative comparison between the conventional Haber-Bosch and NTP ammonia synthesis
Advantages Drawbacks
Conventional
Haber-Bosch
synthesis
Large scale available;
High production rate
Large energy input;
High temperature and
pressure Requires fossil
fuels;
Emission of green house
gases;
Centralized production;
High capital cost
Non-thermal
plasma
synthesis
Clean, carbon-free
production;
Low temperature and
pressure;
Accessible to small-scale,
on-site production;
Conversion and efficiency
need to be improved;
Available only at small scale
As mentioned in Section 1, the two other relatively new technologies that
synthesis ammonia under low temperature and pressure conditions are the
biological and electro-chemical synthesis. Among the three processes, NTP is
reported to have greater future improvement prospects of a variety of N-fixation
121
methods including Haber-Bosch, Biological, etc. (Cherkasov et al., 2015). The
biological ammonia synthesis utilizes nitrogenase enzyme in microorganisms to
fix the nitrogen from nitrogen gas or nitrate/nitrite solutions and convert them into
ammonia (Hinnemann & Nørskov, 2006). For this technology, the growth media
provides the energy sources of the microorganisms. Therefore, electricity, or other
energy input is not required. However, the drawbacks of this technology are that
the system could only be operated in the liquid phase with slow production rates,
and produces byproducts such as nitrate and nitrite species. Furthermore, like
other biological conversion processes, the improvements of this technology relies
heavily on the engineering of complex enzymes, which is a more challenging
obstacle compared with the development of inorganic catalysts. Instability in the
market of enzyme producers and lack of fixed cost is also a large hindrance to
expanding enzyme-based technologies. For the electro-chemical synthesis
approach, it uses electrolytes to facilitate hydrogen transfer between the reactants
and produce ammonia (Kyriakou, Garagounis, Vasileiou, Vourros, & Stoukides,
2017). The advantages of this technology is that it can be operated with both
liquid and gas reactants. However, the major limitations of this technology are the
high cost for the coated mesoporous electrolytes (Montoya, Tsai, Vojvodic, &
Nørskov, 2015). Furthermore, the production rate and the efficiency of this
technology could only be enhanced via the development of new electrolytes and
coated catalysts, which restricts its room for improvements. On the other hand,
the wide selection of plasma discharge types offers more opportunities for the
NTP synthesis of ammonia. According to the plasma type, the operational
122
pressure could be from vacuum to atmospheric pressure. Furthermore, the
different discharge type, as listed in table 2, has different discharge principles and
leads to a larger room of improvement for this technology beyond the catalyst
development.
123
CHAPTER 8 Conclusions and future remarks
In conclusion, the NTP synthesis of ammonia is a potential alternative
technology to the Haber-Bosch ammonia synthesis process that could lead to
cleaner ammonia production. It is a promising technology for the future of
ammonia production and is suitable for distributed hydrogen energy production
and storage. Overall, this technology could be beneficial to the ammonia industry,
through its potential to promote localized and environmentally friendly energy
production and storage. Throughout this research, the energy efficiency of the
plasma-assisted approach was increased to approximately one-fold. Although the
efficiency for the plasma approach (20 g/kWh achieved in this research) still
requires improvement to be competitive compared with the Haber-Bosch
approach. Future researcher could work to further develop catalysts with stronger
plasma synergistic activities, and optimizing the reactor to be more compatible
with the electric circuit within invertor and transformer. Additionally, exploring
absorbents with strong ammonia-holding capacity under plasma region could be
another direction to improve the energy efficiency of this process. Finally, the
opportunities of the non-thermal plasma technology lie with providing an avenue
towards a cleaner ammonia industry, including a renewable pathway that
incorporates this technology with other renewable energy approaches. So future
research regarding the compatibility of the plasma ammonia synthesis process
124
with renewable energy sources are significantly beneficial to its industrial
adoption.
125
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Acknowledgements and references to reproduced/adapted journal publications
The author has published parts of this dissertation in the peer-reviewed journals listed below. Parts of the published contents are adapted/reproduced in this dissertation, in chapters included in parentheses, with permissions from the following publishers.
1. Peng, P., Chen, P., Schiappacasse, C., Zhou, N., Anderson, E., Chen, D., ... & Ruan, R. (2018). A review on the non-thermal plasma-assisted ammonia synthesis technologies. Journal of Cleaner Production, 177, 597-609. DOI: 10.1016/j.jclepro.2017.12.229. Reprinted/adapted in parts with permission from Elsevier. (Chapters 1, 2 and 7)
2. Reprinted/adapted by permission from Copy Right Clearance Center Inc. Springer Nature, Plasma Chemistry and Plasma Processing. Peng, P., Li, Y., Cheng, Y., Deng, S., Chen, P., & Ruan, R. (2016). Atmospheric pressure ammonia synthesis using non-thermal plasma assisted catalysis. Plasma Chemistry and Plasma Processing, 36(5), 1201-1210. DOI: 10.1007/s11090-016-9713-6. Copyright 2016. (Chapter 3, except for the contents listed in source 4)
3. Peng, P., Cheng, Y., Hatzenbeller, R., Addy, M., Zhou, N., Schiappacasse, C., ... & Ruan, R. (2017). Ru-based multifunctional mesoporous catalyst for low-pressure and non-thermal plasma synthesis of ammonia. International Journal of Hydrogen Energy, 42(30), 19056-19066. DOI: 10.1016/j.ijhydene.2017.06.118. Reprinted/adapted with permission from Hydrogen Energy Publications and Elsevier. (Chapter 4)
4. Reprinted/adapted in parts with permission from Peng, P., Chen, P., Addy, M., Cheng, Y., Anderson, E., Zhou, N., ... & Ruan, R. (2018). Atmospheric plasma-assisted ammonia synthesis enhanced via synergistic catalytic absorption. ACS Sustainable Chemistry & Engineering. In press. DOI: 10.1021/acssuschemeng.8b03887. Copyright (2018) American Chemical Society. (Figure 5, page 48 and Chapter 5)
5. Peng, P., Chen, P., Addy, M., Cheng, Y., Zhang, Y., Anderson, E., ... & Ruan, R. (2018). In situ plasma-assisted atmospheric nitrogen fixation using water and spray-type jet plasma. Chemical Communications, 54(23), 2886-2889. DOI: 10.1039/c8cc00697k. Reprinted/adapted with permission from The Royal Society of Chemistry. (Chapter 6)
Erratum In Chapter 5.4, page 93, the corresponding synthesis rate should be 50 µmol/min.