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Journal ArticleDOI

Deciphering the synergy between plasma and catalyst support for ammonia synthesis in a packed dielectric barrier discharge reactor

01 Apr 2020-Journal of Physics D (IOP Publishing)-Vol. 53, Iss: 14, pp 144003

Abstract: Plasma-assisted ammonia synthesis in a packed Dielectric Barrier Discharge (DBD) reactor at atmospheric pressure is presented in this work. A broad range of materials (commonly used as catalyst supports) with various chemical properties (acidic α-Al2O3, anatase TiO2 and basic MgO, CaO), surface area and porosity (α-Al2O3 and γ-Al2O3), dielectric properties (quartz wool, TiO2, and BaTiO3), have been investigated for synergetic effects by packing them in the discharge zone of the DBD reactor. All the materials showed a substantial effect on ammonia production, which can be explained solely as a result of the effect of packing on plasma formation and not by a synergy between plasma and surface catalysis. Size and shape of packing material are found to be the key parameters in enhancing the performance. Quartz wool, closely followed by γ-Al2O3, produces the highest concentration of ammonia at 2900 and 2700 ppm, respectively, due to their ability to generate dense filamentary microdischarges. Particles with a diameter of 200 µm yielded a 64% higher concentration of NH3 than 1300 µm particles – because of amplified electric field strength from increased particle-particle contact points. The specific energy input per unit volume also displayed a significant impact on ammonia production. The process parameters such as N2/H2 feed flow ratio, total flow rate and argon dilution were also investigated. In contradiction to catalytic ammonia synthesis, plasma-assisted synthesis favors a N2/H2 feed ratio ≥ 2 instead of the stoichiometric feed ratio of 0.33. At 0.4 L min-1, 3500 ppm of ammonia was produced with an energy efficiency of 1.23 g NH3 kWh-1. Dilution with 2-5 vol% of argon yielded a 2% improvement in the concentration and energy efficiency, which seems insignificant considering the added practical challenges posed by gas separation. To achieve even higher ammonia concentration and energy efficiencies, it is recommended to support transition metal on γ-Al2O3.
Topics: Ammonia production (62%), Dielectric barrier discharge (58%), Ammonia (51%), Specific energy (51%), Catalyst support (51%)

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1
Deciphering the Synergy Between Plasma and Catalyst
Support for Ammonia Synthesis in a Packed DBD Reactor
Bhaskar S. Patil
a*
, Floran J. J. Peeters
b
, Alwin S. R. van Kaathoven
a
, Jürgen Lang
c
, Qi
Wang
a
, Volker Hessel
a*
a
Laboratory of Chemical Reactor Engineering / Micro Flow Chemistry and Process
Technology, Department of Chemical Engineering and Chemistry, Eindhoven University
of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
b
Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 6500 HH
Eindhoven, The Netherlands.
c
Innovation Management, Verfahrenstechnik & Engineering, Evonik Industries AG,
Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany.
*
corresponding authors: Volker Hessel, email: V.Hessel@tue.nl , telephone: +31(0)40 247 2973
and Bhaskar S. Patil at Bhaskar_Patil@outlook.com
ABSTRACT
Plasma-assisted ammonia synthesis in a packed Dielectric Barrier Discharge (DBD)
reactor at atmospheric pressure is presented in this work. A broad range of materials
(commonly used as catalyst supports) with various chemical properties (acidic α-Al
2
O
3
,
TiO
2
and basic MgO, CaO), surface area and porosity -Al
2
O
3
and γ-Al
2
O
3
), dielectric
properties (quartz wool, TiO
2
, and BaTiO
3
), have been investigated for synergetic effects
by packing them in the discharge zone of the DBD reactor. All the materials showed a
substantial effect on ammonia production. Dielectric properties, size and shape of packing
material are found to be the key parameters to enhance plasma. Quartz wool, followed
by γ-Al
2
O
3
, produce the highest concentration of ammonia at 2900 and 2700 ppm,
respectively, due to their ability to generate dense filamentary microdischarges. Particles
200 µm in diameter of yielded a 64% higher concentration of NH
3
than 1300 µm particles
because of amplified electric field strength from increased particle-particle contact
points. The specific energy input per unit volume has a significant impact on the ammonia
production. The process parameters such as a N
2
/H
2
feed flow ratio, total flow rate and
argon dilution have also been investigated. The N
2
/H
2
feed flow ratio above 2 increase
ammonia concentration and energy efficiency compared to the stoichiometric ratio of
0.33. At 0.4 L min
-1
, 3500 ppm of ammonia was produced with an energy efficiency of
1.23 g NH
3
kWh
-1
. Dilution with 2-5 vol% of argon yields 2% improvement in the
concentration and energy efficiency, which seems insignificant considering the added
practical challenges posed by gas separation.
KEYWORDS: Plasma-assisted ammonia synthesis; DBD reactor; catalyst supports;
support size; synergy.

2
1 INTRODUCTION
Ammonia, being the second largest produced chemical compound by volume with vital
for economy and society [1,2]. Ammonia is irreplaceable in fertilizer production which
sustain 40% of the global population [3]. Moreover, ammonia is also used as a starting
compound to produce chemicals such as polyamides, Ɛ-caprolactam, hydrazine and
explosives. All the ammonia is produced by the Haber-Bosch process the process
developed in the beginning of the 20
th
century that combines nitrogen with hydrogen at
450-600
o
C and 150-350 bar in the presence of a catalyst [4]. The process consumes
more than 1% of the world energy and emits around 300 million metric tons of CO
2
[5,6].
Modern technological and ecological demands create new niches for ammonia such as
in NO
x
abatement from diesel engines by selective catalytic reduction [7] and as an
energy storage chemical [8,9]. These new developments demand localized small-scale
ammonia production, for which the Haber-Bosch process is suitable technically or
economically.
One of the most promising possibilities to produce ammonia at a small scale is the
synthesis with non-thermal plasma driven by renewable electricity [1016]. In this plasma,
the temperature of electrons is high (1-10 eV) and the bulk of the gas remains at low
temperature [17,18]. This non-equilibrium nature enables thermodynamically
unfavourable reactions such as ammonia formation at an atmospheric pressure and lower
temperatures.
Non-thermal plasma can be combined with heterogeneous catalysis to increase the
reaction yield and selectivity [1921]. Combining plasma and catalyst often leads to a
synergetic effect the effect where the production of ammonia produced in plasma-
catalysis exceeds the expected sum of effects of plasma and catalysis separately. This
synergy could arise from the enhancement of plasma by the catalyst materials or
chemical interactions between the activated plasma species and catalysts [22,23].
Plasma-assisted synthesis of ammonia has been investigated using various types of non-
thermal plasma: glow [24,25], microwave [2629], radio frequency [26,30], and dielectric
barrier discharge [13,3133]. Some of these studies demonstrated plasma-catalysis
synergy. The basic oxides (MgO and CaO) enhanced ammonia formation, unlike acidic

3
oxides (Al
2
O
3
, WO
3
, and SiO
2
-Al
2
O
3
) [24]. The order of catalytic activity for active metals
was reported as follows: Pt > stainless steel > Ag > Fe > Cu > Al > Zn in glow discharge
plasma [25]. The yield of ammonia was found to be considerably enhanced by iron wires
in radio frequency plasma [30].
While plasma ammonia synthesis has long been investigated, there is little data on the
nature of the synergetic effect in plasma catalysis. Specifically, the influence of its
chemical, physical and electrical properties on the discharge behaviour and its
contribution to ammonia formation rates. In particular, the catalysts used are often
impractical such as platinum metal pieces in contrast to industrially used dispersed
platinum particles supported by refractory oxides. The understanding of the role of such
oxide supports on plasma is a crucial step before further investigating active catalytic
metals added to these support materials.
In this paper, we systematically investigate the plasma-assisted ammonia synthesis in a
packed 1-sided dielectric barrier discharge (DBD) reactor. We compare the materials
widely used catalyst supports for plasma-assisted ammonia syntheses and study the
effect of physical properties of the material and process parameters to establish the
nature of the synergetic effects in ammonia plasma-catalytic synthesis.
2 EXPERIMENTAL
2.1 Materials
Catalyst support materials were chosen based on their dielectric, physical and surface
chemical properties were studied: acidic (α-Al
2
O
3
, TiO
2
) and basic (MgO, CaO), materials
with various surface area, porosity -Al
2
O
3
and γ-Al
2
O
3
), and dielectric properties (quartz
wool, TiO
2
and BaTiO
3
). The following materials were purchased from Mateck GmbH in
the form of ~3 mm: γ-Al
2
O
3
, α-Al
2
O
3
, TiO
2
, and BaTiO
3
. MgO and CaO were purchased
in the form of lumps from Sigma Aldrich and Merck Millipore, respectively. These pellets
and lumps were crushed and sieved into a series of particle fractions using a mechanical
shaker. The particle sizes reported in the work are average and relate to the sieve sizes
of 1600-1000, 850-630, 355-250 and 250-160 µm. The quartz wool (fibers 5-30 µm in
diameter) from Carl Roth GmbH was used as purchased.

4
2.2 Ammonia synthesis in a DBD reactor
The plasma-assisted NH
3
synthesis was performed in a 1-sided DBD plasma reactor at
an atmospheric pressure with 3 mL of the material packed in the discharge zone. The
system is presented in details in reference [34] and shown schematically in
Supplementary Figure S1. Briefly, a stainless steel mesh was wrapped around a quartz
tube with 10 mm inner diameter and 1.5 mm thick. The mesh was a ground electrode,
whereas the quartz tube served as a dielectric barrier. An axial stainless steel rod with 6
mm outer diameter was a high voltage electrode. The discharge gap was 2 mm and length
of the discharge zone was 60 mm. Equations 2-4 show calculation of the total power (
total
P
), specific energy input (
SEI
), and the energy consumption per mole of ammonia (
3NH
E
):
()
total p
onecycle
P f C V t dt
, (1)
/
tot gas
SEI P Q
, (2)
33
/ ( )
NH tot NH gas
E P C Q
, (3)
where, V(t) is the applied voltage, C
p
- capacitance of the capacitor, ƒ - applied voltage
frequency, Q
gas
- volumetric gas flow rate, C
NH3
ammonia concentration in gas.
The gases (N
2
, H
2
, Linde Gases, 99.9%) were used as feed and their flow rate was
controlled using Bronkhorst mass flow controllers. Ammonia was the only reaction
product observed, in agreement with literature [31,33,35,36]. Ammonia concentration was
analyzed with a Fourier transform infrared spectrophotometer (Shimadzu IRTracer-100)
in a gas cell with KBr windows (Specac). The ammonia extinction coefficient was
determined from a series of calibration gas mixtures. The product gases, before leaving
the set-up, were scrubbed in a series of water-filled bottles to capture the ammonia
produced.
The electrical power was supplied to the DBD reactor in the form of microsecond pulses
(Figure S2), set by a waveform generator. The power input to DBD reactor was controlled
by changing the peak to peak voltage of this pulse signal (Figure S2), which is referred to

Figures (7)
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Cites methods from "Deciphering the synergy between pla..."

  • ...The inventory analysis provided for the plasma-assisted process in Section 2.3 has been derived from process simulations incorporating amolar NH3 yield of 1%and an energy efficiency of 1.9 gNH3/kWh (power consumptionA) (Patil et al., 2020)....

    [...]


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