22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma catalytic synthesis of ammonia using functionalized-nanodiamond
coatings in an atmospheric-pressure non-equilibrium discharge
J. Hong1,2, O. Shimoni3, D.H. Seo1, M. Aramesh2, S. Prawer2 and A.B. Murphy1
1
CSIRO Manufacturing Flagship, 2070 Lindfield, NSW, Australia
School of Physics, University of Melbourne, 3010 Melbourne, VIC, Australia
3
School of Physics and Advanced Materials, University of Technology Sydney, 2007 Broadway, NSW, Australia
2
Abstract: We investigate the synthesis of ammonia in a non-equilibrium atmosphericpressure plasma using functionalized-nanodiamond coatings on α-Al 2 O 3 spheres as
catalysts. Oxygenated nanodiamonds were found to increase the production yield of
ammonia, while hydrogenated nanodiamonds decreased the yield. Initial investigations of
the role of different functional groups on the catalyst surface in ammonia synthesis are
presented.
Keywords: plasma catalysis, functionalized nanodiamond coating, ammonia production
1. Introduction
Among the many applications of nanodiamonds (NDs),
achievements in the electrochemical/catalytic application
field have been particularly noteworthy, especially when
metals such as Pt, Pd, and Au have been deposited on
NDs. Detonation nanodiamonds have shown catalytic
reactivity and have been successfully applied for
catalysing reactions such as direct dehydrogenation of
ethyl benzene. The unique structure of the ND, which
consists of sp2 and sp3 carbon, results in different
properties from other carbon materials [1-2].
The sp2 carbon atoms on the ND’s surface are normally
functionalized by carboxyl, anhydride, hydroxyl, ketone,
carbonyl and lactone groups after a purification process
that typically uses an oxidizing acid. Unsaturated
carbonyl groups (C=O) in particular have substantial
electron density surrounding the oxygen atom, and thus
can serve as Lewis bases to activate saturated
As
hydrocarbons such as ethylbenzene (C 8 H 10 ).
intermediates, carbonyl groups on the ND surface can be
changed into hydroxyl groups (C-OH) and styrene (C 8 H 8 )
is produced in the dehydrogenation process of ethyl
benzene. It has been proposed that the reaction cycle is
closed by the thermal decomposition of C-OH to C=O
and molecular hydrogen, which is thermodynamically
favourable at high temperatures [2-3]. Based on this
proposed mechanism, we expect NDs, especially
oxygenated NDs (O-NDs), to adsorb hydrogen from the
gas phase effectively and to enable the adsorbed hydrogen
species to diffuse to neighbouring adsorbed nitrogen sites
to generate NH x compounds on the catalyst surface.
In addition, hydrogenated NDs (H-NDs) have been
reported to have interesting electrical properties such as
giant permittivity and remarkable conductivity when
exposed to water [4-5].
The hydrogen-terminated
diamond was also demonstrated to yield facile electron
emission into water, which finally induces reduction of
N 2 to NH 3 at ambient temperature and pressure [6].
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Therefore H-NDs were expected to act as a promoter to
enhance plasma density when used in a gas discharge
environment, in the presence of water vapour.
The standard Haber–Bosch process for commercial
ammonia production requires high pressures and
temperatures (typically 200 bar and 500 °C), as well as a
catalyst, for acceptable production rates and yields,
usually around a 15% conversion of hydrogen to
ammonia. By combining an atmospheric-pressure plasma
with a catalyst, it is, however, possible to produce
ammonia from nitrogen and hydrogen at atmospheric
pressure and room temperature.
In this work, we use a dielectric-barrier-discharge
reactor packed with α-Al 2 O 3 spheres, both uncoated and
coated with functionalized NDs, at atmospheric pressure.
We investigate the influence of the surface functional
group of NDs on ammonia catalysis and the plasma
characteristics. We use electrical measurements to obtain
the discharge power, electron density and average
electron energy, and emission spectroscopy to provide an
indication of atomic hydrogen density. By combining
these data with FT-IR measurements of the concentration
of ammonia produced, we obtain an understanding of the
influence of the different functional groups on the NDs.
2. Experiment
A packed-bed cylindrical dielectric-barrier discharge
reactor is used in this work; a detailed schematic was
given elsewhere [7]. The discharge region between the
high-voltage electrode and the dielectric tube (borosilicate
glass) is 5 mm wide, and was filled with α-Al 2 O 3
(φ 1.6 mm, Christy Catalytics) spheres, either uncoated or
coated with O-NDs or H-NDs. The length of the
discharge region is 200 mm.
A Trek 20/20C high-voltage amplifier, fed by a 1 kHz
sine wave, was used as the power source. The peak
voltage was 14 kV. The ammonia concentration in the
exit gas was monitored in-situ using a Fourier-transform
1
3. Results and discussion
3.1 ND properties
The hydrogenated and oxygenated NDs showed
different size distribution and zeta potential, as in Table 1.
Due to the large number of hydroxyl and carboxylic
groups, the O-NDs have a more negative zeta potential
than the reference NDs, and the H-NDs have a positive
potential owing to the surface-transfer doping
phenomenon in water [9-10]. The oxygenation process
provides a smaller size distribution, presumably because it
etches the graphite-like sp2 carbon binding the
agglomerates of the as-received ND more effectively [8].
Table 1.
Size distribution and zeta potential of
monodisperse ND suspension.
Avg. size
[nm]
Zeta potential
[mV]
ND as received
106.4±21.7
-33.7±4.4
Annealed in H 2
34.5±4.0
45.2±5.3
Annealed in O 2
13.6±1.9
-37.5±4.0
The annealed NDs were characterized by FT-IR, as
shown in Fig. 1. The as-supplied NDs exhibit a C-O peak
(1100~1320 cm-1), OH stretching mode (3000~3650 cm-1)
and another absorption peak originating from O-H bond
vibration at 1630 cm-1. After annealing in hydrogen, the
NDs are confirmed to have a hydrogen termination by the
absorption signal from C-H functional group
(2800~3000 cm-1). In contrast, following the oxygenation
process, a new C=O functional group (1780 cm-1)
appeared in the IR absorbance spectra.
2
Absorbance [a.u.]
infrared (FT-IR) spectrometer (Perkin-Elmer Frontier),
calibrated with a standard mixture of 2% NH 3 diluted in
pure nitrogen (BOC).
The residence time for all
measurements reported was 50 s.
Prior to being coated onto the alumina spheres,
detonation ND (PlasmaChem, purified grade 01) were
annealed in oxygen or hydrogen gas. For hydrogen
termination, NDs were annealed for 4 h at 700 °C in pure
hydrogen at 5 Torr, and for oxygenation, for 4 hrs at
400 °C treated in oxygen (purity 99.999%) at 1 atm [8].
The annealed powders were dispersed in deionized water
at 1 mg/ml dispersion, ultrasonicated for 30 min and
centrifuged for 2 h at 20 000 g to obtain a monodisperse
(mdND) diamond suspension, which is crucial for
improved adhesion and uniformity of the ND coating. To
coat the alumina spheres, 350 g of the spheres were
soaked in 120 ml of the monodisperse ND suspension and
ultrasonicated for 30 min. Finally the coated alumina
spheres were dried in an oven at 120 °C for 2 h.
As-is
H-ND
O-ND
1000 1500 2000 2500 3000 3500 4000
Wavenumber [cm-1]
Fig. 1. FT-IR spectra of functionalized ND powder.
3.2 Electrical characteristics
Based on the measured IV characteristics curve, the
reduced electric field strength E/N is estimated, where E
is the electric field strength across the gas gap and N
denotes the gas number density. With the input parameter
of the reduced electric field strength E/N and gas
composition, the electron energy distribution function
(EEDF) and the electron mobility are obtained using
BOLSIG+ [11]. The electron density is calculated from
the equation for conduction current [12].
We performed experiments with N 2 and N 2 /H 2
mixtures in order to understand the influence of the
different ND coatings on the electrical characteristics.
The H-ND coated alumina catalyst showed unique IV
characteristics, as shown in Fig. 2, and strong and uniform
light emission in a pure nitrogen plasma, corresponding to
a higher density and electron energy distribution function
than were observed for bare alumina. This may be related
to the large permittivity that has been observed for H-NDs.
However, in a N 2 /H 2 plasma, the bare alumina, and
alumina coated with H-NDs and O-NDs, showed very
similar plasma characteristics, as shown in Figs. 3 and 4.
3.3 Ammonia production
To understand the influence of the different surface
functional groups on ammonia synthesis, the ammonia
concentration in the output gas was measured for the bare
alumina, and alumina coated with H-NDs and O-NDs.
The results are shown in Fig. 5, in terms of the production
yield, i.e., the percentage conversion of the input H 2 to
NH 3 . In all cases, the production of ammonia increases
with time, reaching approximate stability after about
80 min for the bare alumina and O-ND case, and much
later for the H-ND case. This trend is thought to be
related to removal of impurities such as water vapour
adsorbed on the alumina after exposure to air, and the
increase in temperature of the reactor.
The conversion is greatest for the O-ND case; coating
the alumina with H-NDs leads to a large decrease in
conversion.
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10
1.0x108
Electron density [/cm3]
H-ND
Charge Q [µC]
8
8.0x10
O-ND
6
2
4.0x107
(a)
0
-20
-15
-10
-5
0
5
Voltage [kV]
100
10
15
2.0x107
20
-6
10
(b)
0
5
10
Energy[eV]
15
20
Fig. 2. (a) QV Lissajous plot, and (b) calculated electron
energy distribution for the different functionalized NDs
and bare alumina in N 2 plasma at room temperature.
100
ref. Al2O3
H-ND
O-ND
-2
EEDF [eV-3/2]
10
10-4
10-6
10-8
10-10
0
5
10
Energy[eV]
15
20
Fig. 3. Calculated electron energy distribution for the
different functionalized NDs and bare alumina in N 2 /H 2
plasma at T g 387 ± 3 K.
For the H-ND case, we observed a large absorption
signal in the FT-IR spectrum of the output gas at
3000~3600 cm-1, associated with the –OH group, which
usually originates from water. It is possible that residual
water from the coating process interferes with the
ammonia production process. Ammonia is highly soluble
in water (18% w/w at 50 °C), so ammonia molecules may
transform into NH 4 + ions and be adsorbed on catalyst
surface.
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Production yield [% H2 conversion]
10-4
10-8
0.0
Al2O3
H-ND
O-ND
Fig. 4. Calculated electron density for the different
functionalized NDs and bare alumina in (a) N 2 plasma at
room temperature, and (b) N 2 /H 2 plasma at T g 387 ± 3 K.
ref. Al2O3
H-ND
O-ND
10-2
EEDF [eV-3/2]
6.0x107
Ref.
Al2O3
4
10-10
N2 14kV
N2H2 14kV
7
2.5
O-ND
2.0
ref. Al2O3
1.5
1.0
H-ND
0.5
0.0
0
30
60
90
120
Time [min]
150
180
Fig. 5. Measured ammonia concentration for the different
functionalized NDs and bare alumina with N 2 /H 2 15/45
sccm.
Fig. 6. shows optical emission spectra. We observed
suppressed H α emission in the H-ND case. This may be
due to quenching by species derived by water. The
emission gradually increased with time, indicating that the
water may be slowly removed by the discharge. However
the correlation between the emission intensity of H α and
ammonia production requires further investigation.
The plasma density and energy distributions are very
similar in the three cases we have investigated (bare
alumina, and alumina coated with H-NDs and O-NDs).
The large differences in ammonia production shown in
Fig. 5 are therefore presumed to arise from different
surface chemical reactions arising from the different
surface functional groups. The C-H functional group on
the H-NDs has very strong covalent bonding, which
means that hydrogen may not be released to either the gas
phase or to surface adsorption sites to produce ammonia
molecules. In contrast, as discussed in Sec. 1, the C=O
functional group on O-NDs adsorbs hydrogen from the
gas phase effectively, and allows the adsorbed hydrogen
species to diffuse to neighbouring adsorbed nitrogen or
NH x sites, leading to an improved production yield in
ammonia synthesis.
3
250
Intensity [a.u.]
200
150
100
t=20min
t=40min
t=60min
t=90min
t=120min
t=150min
t=180min
50
0
655
250
200
Intensity [a.u.]
H-ND
150
656
657
Wavelength[nm]
O-ND
t=20min
t=40min
t=60min
t=80min
100
50
0
655
656
Wavelength[nm]
657
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Fig. 6. Emission spectra of H α for different times after
the discharge turn-on for H-NDs and O-NDs.
4. Conclusion
The influence of monodisperse nanodiamond coating
on heterogeneous ammonia synthesis has been
investigated with a packed-bed dielectric-barrier
discharge reactor.
Hydrogenated and oxygenated
nanodiamond powders were prepared by a thermal
annealing process. The oxygenated nanodiamond coating
gave at least a 20% improvement in production yield over
bare alumina catalyst. The hydrogenated nanodiamond
coating showed unique electrical properties, which could
be useful to enhance plasma density in certain specific
circumstances in the presence of absorbed water, but
produced a reduced amount of ammonia.
The surface reactions on the functionalized
nanodiamond coatings require further investigation
through both surface analysis and plasma diagnostics.
However, it is clear that coating of catalyst materials with
oxygenated nanodiamonds shows promise in increasing
ammonia production in plasma synthesis. There are
likely to be many other gas synthesis and gas cleaning
applications in which nanodiamond coatings will be
useful.
5. References
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