Plasma-assisted CH4 reduction of a NiO catalyst – low temperature
synthesis of carbon nanofibres in DBD
Helen J. Gallon1, Xin Tu1, Martyn V. Twigg2 and J. Christopher Whitehead1
1. School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
2. Johnson Matthey Plc, Orchard Laboratories, Orchard Road, Royston, SG8 5HE
Abstract: The low temperature reduction of a NiO catalyst by CH4 was performed in a coaxial double dielectric
barrier discharge (DBD) reactor for the first time. Over the reduced Ni catalyst activation of CH4 to form H2 and
solid carbon nanofibres was found to occur in the plasma at a low gas temperature of 330°C. CH4 conversions of
37 % were achieved in the plasma-catalytic reaction at atmospheric pressure, with 99 % selectivity towards H2
and solid carbon. These results demonstrate a synergistic effect for plasma-catalytic CH4 reforming, where both
the plasma and catalyst are vital for the production of H2 and carbon nanofibres under these conditions. In the
absence of the catalyst stable plasma could not be ignited with a pure CH4 flow and thermal studies showed that in
the absence of the plasma CH4 conversion was < 4 % at 330°C.
Keywords: carbon nanomaterials; hydrogen production; plasma-catalysis
1. Introduction
Supported nickel catalysts are used commercially in
large-scale production of H2 via high temperature
steam methane reforming; the catalysts are usually
supplied in the oxide form and reduced in-situ with
CH4/steam to give the active nano-sized Ni
crystallites. With a growing interest in alternative
routes to H2 for a possible ‘hydrogen economy’,
thermocatalytic decomposition of CH4 to H2 and
solid carbon, according to the moderately
endothermic reaction 1, is being considered.
CΗ4 (g) → 2 Η2 (g) + C (s) ΔH° = 75.6 kJ mol-1 (1)
There has also been much work on non-thermal
plasma-catalysis; CH4 reforming in DBD has been
carried out in the presence of O2 (partial oxidation of
methane) [1], CO2 (dry reforming of methane) [2]
and steam [3] to produce syngas (H2 + CO).
The interactions between plasma and catalyst
become complex when the catalyst is placed directly
in the plasma volume. Subjecting metal catalysts to
plasma discharges can result in a reduction of some
metal oxide precursors to generate small active
metal crystallites without the use of additional
heating. Liu et al. [4] used a H2/N2 atmospheric
pressure glow discharge plasma jet for the reduction
of NiO/γ-Al2O3 and NiO/SiO2 catalysts. Zhang et al.
[5] reported radio frequency H2 plasma reduction of
NiO to Ni at a discharge tube temperature of 65°C.
Reforming of CH4 in non-thermal plasma systems
can lead to carbon deposition of various forms such
as amorphous, filamentous or as carbon nanotubes
(CNT). Carbon nanofibres (CNFs) and CNTs are
highly desirable products in their own right due to
extraordinary mechanical, thermal and electronic
properties. A major challenge at present is the
capability to grow carbon nanomaterials on
substrates whilst preserving features of the substrate
that are inevitably destroyed by the high
temperatures required for growth: typically 600 –
1000°C. The use of plasma-enhanced chemical
vapour deposition (PECVD) for low temperature
synthesis of CNFs and CNTs is being explored.
Microwave PECVD has been used to grow CNTs
using Fe-Si catalysts at 370°C [6]. Kona et al. [7]
used atmospheric pressure rf microplasma to grow
CNTs at a remarkably low temperature of 100°C.
2. Experimental
Reduction of a 33 % wt. NiO/Al2O3 catalyst
(Johnson Matthey Plc.) was carried out in a DBD
reactor consisting of two coaxial quartz tubes with
stainless steel mesh electrodes of 55 mm length and
with a discharge gap of 4.5 mm. Non-uniformly
sized particles of catalyst (18.6 g, 0.85 – 5 mm) were
pre-baked to remove moisture and packed into the
discharge gap, where they were held in place with
quartz wool.
Results and Discussion
AC voltage
CH4
45
Gases exiting the reactor were analysed using a
micro-GC (Agilent 3000A) with Molsieve 5A and
Plot Q columns. A cold trap containing an ice bath
was placed downstream of the reactor to condense
liquid products. Using the molar ratios of each
species in the CH4 feed and exhaust gas, the
consumption of CH4 and selectivities towards H2,
CO, CO2 and C2 – C4 hydrocarbons were calculated.
The carbon balance in the gas stream is defined by
equation 2.
(2)
Carbon Balance(%) =100×
[CH4 ]out + [COx]out + 2 × [C2 ]out + 3 × [C3 ]out
[CH4 ]in
X-ray diffraction (XRD) was performed on catalyst
samples at room temperature using a Philips X’Pert
diffractometer. Images of the catalyst surface were
obtained using environmental scanning electron
microscopy (FEI Quanta 200 ESEM) and
transmission electron microscopy (Philips CM30
TEM). Elemental analysis of the liquid product was
performed using a Carlo Erba 1108 CHNS
instrument.
35
80
30
25
CH4
H2
20
15
CO2
10
CO
5
0
-5
60
40
20
0
0
50
100
150
200
250
Time (mins)
Figure 2. CH4 consumption and selectivity of H2, CO2 and
CO.
50
100
40
90
30
80
20
10
70
0
60
0
50
100
150
200
Carbon Balance (%)
An undiluted CH4 feed was used at 50 ml min-1 and
1 bar. A voltage of 21 kVpk-pk was applied at a
frequency of 35 kHz. A Picoscope ADC-200 was
used to record the waveforms for the high voltage
AC sine wave and the voltage across a capacitor. A
LabVIEW system was used for measurement of the
discharge power by the area calculation of Lissajous
figures. The temperature was measured by attaching
a K-type thermocouple to the outer electrode. No
additional heating was supplied to the reactor.
CH 4 Consumption (%)
Figure 1. Experimental Set-up.
H 2 Concentration (mol. %)
Cold-trap
100
40
Selectivity (%)
Micro-GC
250
Time (mins)
Figure 3. H2 concentration and gas stream carbon balance.
Figure 2 shows the consumption of CH4 and
production of H2, CO2 and CO. Methane
consumption increased to 37.4 % after 65 minutes
plasma treatment and then remained in the range
33.5 – 37.8 %. The overall NiO reduction proceeded
as shown by equation 3 generating the reduction
products CO2 and H2O, followed by the water-gas
shift reaction (4) resulting in the observed evolution
of CO.
4 ΝiΟ + CΗ4 → 4 Νi + CΟ2 + 2 Η2Ο
(3)
CΟ + Η2Ο ↔ Η2 + CΟ2
(4)
The concentrations of both CO2 and CO peaked at
65 minutes, after which the rate of NiO reduction
slowed, due to decreased NiO concentration.
Evolution of both CO2 and CO terminated when
reduction of NiO had gone to completion at 150
minutes. CH4 continued to react and H2 selectivity
The discharge power increased gradually in the
presence of the catalyst, despite the constant applied
voltage, and reached a plateau at 117 W, as shown in
Figure 5. The discharge current increased during the
reduction of NiO to the more conductive Ni phase.
The temperature of the reactor was proportional to
the applied power due to the dissipation of electrical
energy as heat; no additional heating was applied to
C2H2/C2H4
25
C 2H 6
C 3H 6
20
C 3H 8
15
10
5
0
0
50
100
150
200
250
Time (mins)
Figure 4. Higher hydrocarbon selectivities.
400
120
100
300
80
200
60
40
100
20
0
Temperature (°C)
Low levels of C2H2/C2H4 (unseparated), C2H6, C3H6
and C3H8 were also detected as products, as shown
in Figure 4. These species are products of plasmaassisted CH4 reforming generated by radical
coupling reactions in the plasma volume; this has
been demonstrated previously in experimental [8]
and simulated [9] results for CH4 DBDs in the
absence of active metal catalysts. After an initial
increase in C2 – C3 hydrocarbons, the concentrations
decreased to almost zero. A total carbon selectivity
of only 1.0 % can be attributed to all gaseous higher
hydrocarbons over the plasma-reduced Ni catalyst.
Elemental analysis confirmed that the liquid product
(0.78 g) collected in the cold trap over the duration
of the experiments was mainly H2O with traces of
liquid hydrocarbons (≥ C5+).
30
Selectivity (%)
Figure 3 shows the percentage concentration of H2 in
the gas exiting the reactor and the gas stream carbon
balance (2) throughout the plasma treatment of the
catalyst. The drop in carbon balance after the onset
of NiO reduction is attributed to the deposition of
solid carbon. The mirroring feature of these curves
indicates CH4 decomposition (1) catalysed by
metallic Ni is responsible for the generation of both
H2 and carbon. As the reduction proceeded Ni sites
were generated and the rate of CH4 decomposition
increased. When the reduction of NiO was complete
the number of available Ni sites remained constant
and the rate of CH4 decomposition also remained
approximately constant. As a result CH4
consumption, H2 production and the gas stream
carbon balance plateaued.
the reactor and, after the initial increase, the outer
electrode temperature remained below 330°C.
Power (W)
remained approximately constant at > 99 %. The
observed stoichiometry was in agreement with the
endothermic decomposition of CH4 to give H2 and
solid carbon (1).
0
0
50
100
150
200
250
Time (mins)
Figure 5. Power and temperature profiles.
When the same experiment was performed in the
absence of the catalyst and with an applied voltage
of 21 kVpk-pk, a stable discharge could not be ignited
in a pure CH4 flow. This demonstrates that the
presence of the catalyst is vital for the process to
take place under these experimental conditions. In
order to establish the role of the plasma in this
reaction, a purely thermal experiment was carried
out by heating the NiO catalyst in a pure CH4 flow at
50 ml min-1 and 1 bar. The catalyst was heated
gradually to 330°C and held at this temperature for 3
hours to mimic the conditions of the plasma. CH4
conversions were < 4 % and analysis of the exit gas
revealed small quantities of CO2 and H2. These
results demonstrate that both plasma and catalyst
have a vital role, under these experimental
conditions, for the production of H2 and solid carbon.
Counts + Offset
50000
1 = NiO
2 = Ni
40000
11
30000
2
1
2
20000
1 1
10000
0
20
40
11
2
1
60
11
80
a)
2 2
b)
c)
100
Angle (2θ )
Figure 6. XRD patterns of the catalysts after a) no treatment
b) reduction in CH4 DBD and c) heating to 330°C.
a)
b)
from 3.3 – 16.7 %, corresponding to 34 %
conversion of the total CH4 entering the reactor.
Conclusions
Plasma-assisted reduction of a NiO catalyst has been
carried out in a CH4 DBD for the first time, at a low
gas temperature of 330°C. Plasma-assisted
decomposition of CH4 was found to occur over the
reduced Ni catalyst to form H2 and carbon
nanofibres. These results demonstrate a synergistic
effect for CH4 reforming, where both plasma and
catalyst are vital for the process to take place.
Acknowledgements
Support of this work by SUPERGEN XIV –
Delivery of Sustainable Hydrogen (part of the
Energy Programme which is an RCUK cross-council
initiative led by EPSRC and contributed to by ESRC,
NERC, BBSRC and STFC) is gratefully
acknowledged.
References
Figure 7. a) SEM image of the plasma-reduced Ni/Al2O3
catalyst showing CNFs and b) TEM image of Ni-tipped
CNFs that were deposited on the surface of the catalyst.
The structures of the catalyst after plasma treatment
in CH4 and after heating in CH4 were investigated
using X-ray diffraction, and compared to those of
the fresh catalyst. Figure 6 shows that after plasma
treatment NiO had been reduced to Ni, and after
heating to 330 °C in CH4 the catalyst remained in the
oxide form. The SEM image of the plasma-reduced
catalyst in Figure 7a shows an amorphous support
(mainly Al2O3) with highly dispersed nanoparticles
of nickel. The cotton wool-like structures are areas
of filamentous carbon with a mean outer diameter of
55 nm, ranging from 28 – 80 nm. Figure 7b shows a
TEM image of carbon nanofibres that were isolated
from the catalyst surface. The fibres appear to have a
narrow hollow along the centre; however, further
analysis is necessary to determine their structure in
detail. Elemental analysis of the catalyst after plasma
treatment revealed the carbon content had increased
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