Plasma-catalytic dry reforming of methane over γ-Al2O3 supported Ni
catalysts
Xin Tu, Helen J. Gallon and J. Christopher Whitehead
School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
Abstract: A coaxial dielectric barrier discharge (DBD) reactor has been developed for plasma catalytic dry
reforming of CH4 into syngas over Ni/γ-Al2O3 catalysts. Partially packing the Ni catalyst in flake form into the
gap has a weak effect on the discharge behaviour and shows a strong plasma-catalyst interaction. Catalyst
screening using different Ni/γ-Al2O3 catalysts is carried out to get the optimal design of the Ni catalyst for
plasma-catalytic dry reforming reaction. We find both the conversion rate of CH4 (56.4 %) and H2 yield (17.5 %)
are almost doubled when a 10 wt% Ni/γ-Al2O3 catalyst in flake form (1 g) calcined at 300 oC is packed in the
plasma. This synergistic effect is attributed to both strong plasma-catalyst interactions and high activity of the
Ni/γ-Al2O3 catalyst calcined at low temperature.
Keywords: Dielectric barrier discharge, Plasma-catalysis, Dry reforming, Synergistic effect
1. Introduction
The reforming of methane with carbon dioxide,
also known as dry reforming, has recently attracted
considerable interest due to simultaneous utilization
and reduction of two major greenhouse gases, CH4
and CO2.
CH4 + CO2 → 2H2 + 2CO
This process generates synthesis gas (syngas)
with a suitable H2/CO molar ratio, which is preferred
for the synthesis of valuable oxygenated chemicals
and long-chain hydrocarbons. Nevertheless, dry
reforming of methane using conventional catalytic
methods still faces two major challenges that limit
the use of this process on a commercial scale: firstly,
high reaction temperatures (>700oC) are required to
obtain reasonable yields of syngas due to the very
endothermic reaction and the strength of the C-H
bond in CH4, incurring high energy cost; secondly,
the formation of severe coke deposition and a
subsequent blocking of active metal sites on the
catalyst surface, causing rapid deactivation of the
catalysts, especially for non-noble metal catalysts [1].
Non-thermal plasma technology is considered as
an attractive alternative for converting greenhouse
gases into syngas and other valuable chemicals at
lower temperatures. Highly active species generated
in such a plasma are favorable for both initiation and
propagation of chemical reactions [2]. Recently, the
combination of plasma and heterogeneous catalysis
for fuel production from CH4 reforming has attracted
increasing interest. The interactions between plasma
and catalyst become complex when the catalyst is
placed directly in the plasma. Both chemical and
physical properties of the plasma and catalyst can be
modified by the presence of each other. Meanwhile,
this interaction could generate a synergistic effect,
which might provide a unique way to separate the
activation steps from the selective reactions. Our
previous work suggested that the synergistic effect
of the combination of plasma with catalysis for CH4
reforming depends on the balance between the
change in discharge behaviour induced by the
catalyst and the plasma generated activity of the
catalyst [3].
In this work, a coaxial DBD reactor is developed
for the plasma-catalytic dry reforming reaction. The
influence of Ni/γ-Al2O3 catalyst packed in the gas
gap on the electrical characteristics of the discharge
has been investigated. Catalyst screening using
different Ni/γ-Al2O3 catalysts is carried out to get the
optimal design of the Ni catalyst for plasma-catalytic
dry reforming reaction.
2. Experimental
The experiment is carried out in a cylindrical
DBD reactor, as described in detail in our previous
work [3].The DBD reactor consists of two coaxial
quartz tubes, both of which are covered by a
stainless steel mesh electrode. The inner electrode is
connected to a high voltage output and the outer
electrode is grounded via an external capacitor (22
nF). The discharge length is 55 mm and discharge
gap is 3 mm. CH4 and CO2 are used as feed gas with
a constant total mass flow rate of 50 ml min-1 and a
molar ration of 1:1. The DBD reactor is supplied by
an ac high voltage power supply. The applied
voltage is measured by a high voltage probe, while
the total current is recorded by a Rogowski-type
current monitor. The voltage on the external
capacitor is measured to obtain the charge generated
in the discharge. All the electrical signals are
sampled by a four-channel digital oscilloscope. A
LABVIEW control system is used for the online
measurement of discharge power by the area
calculation of Q-U Lissajous figure. An equivalent
electrical circuit of the DBD reactor can be found in
Ref [3].
3. Results and discussion
3.1 Effect of catalyst on discharge behaviour
Fig. 2 presents the electrical signals of the
discharge in the mixing of CH4 and CO2 with and
without Ni/γ-Al2O3 catalyst at a fixed discharge
power of 50 W. Similar electrical signals suggest
that partially packing the catalyst in flake form does
not significantly change the discharge behaviour and
still show strong filamentary microdischarges. This
behaviour is completely different to our previous
study in which fully packing the catalyst pellets into
the gap leads to a transition in discharge behaviour
from a typical filamentary microdischarge to a
combination of a spatially limited microdischarge
and a predominant surface discharge on the catalyst
surface [3]. This packing method could maintain a
strong plasma-catalyst interaction, which is believed
to be favorable for chemical reactions. When the
catalyst pellets are fully packed into the gap, the
plasma breakdown voltage significantly decreases
from 3.3 to 0.75 kV [3]. However, the breakdown
voltage of the DBD does not change when the
catalyst in flake form is packed into the plasma
region.
Figure 1. Schematic diagram of the experimental setup
The Ni/γ-Al2O3 catalysts were prepared by
incipient wetness impregnation using the γ-Al2O3
support (AAC Eurovent) and aqueous solution of
nickel nitrate (FSA Laboratory). The impregnated
catalysts were first dried at 100 oC overnight and
then calcined at different temperatures (300-800 oC)
for 4 h. Prior to the dry reforming reaction, the
catalyst is reduced in an argon-hydrogen discharge
(100 ml min-1, 20 % H2) in the same DBD reactor.
The feed and product gases are analyzed by a twochannel micro gas chromatography (Agilent 3000A)
equipped with two thermal conductivity detections
(TCD). The definition of conversion rate, selectivity
and yield of products for the dry reforming reaction
can be found in Ref [3].
Figure 2. Electrical signals of the CH4/CO2 DBD: (a) without
packing; (b) partially packed with 1 g Ni/γ-Al2O3 catalyst in
flake form along the discharge gap.
3.2 Plasma-catalytic dry reforming reaction
Plasma-catalytic dry reforming of CH4 is carried
out using a mixing ratio of CH4/CO2 = 1 and a total
flow rate of 50 ml min-1. H2 and CO are the major
reaction products, while smaller amounts of
acetylene, ethylene, ethane and propane are also
formed. Conversions of both CH4 and CO2 increase
with increasing discharge power as shown in Fig.3.
The conversion rates of CH4 and CO2 increase up to
33 % and 21.4 % at a discharge power of 60 W.
Previous modelling and experimental studies have
shown that increasing plasma power at a constant
excitation frequency effectively enhances the
electric field, electron density and gas temperature in
the discharge [4], all of which may contribute in
different ways to the improvement in conversion for
both gases. In addition, an increase in plasma power
produces more active species, such as O and OH,
and can also dissociate CH4 producing more methyl
radicals. The selectivity of products is almost
independent of the discharge power between 30 and
60 W. The maximum H2 yield is 9.6 % at a discharge
power of 60 W.
When the Ni/γ-Al2O3 catalyst pellets (0.85-1.7
mm, 1g) are partially packed in the gas gap and held
by quartz wool, the CH4 conversion rate increases to
38 %, as presented in Table 1. Small size catalyst
(0.5-0.85 mm) shows a higher selectivity of CO. It is
interesting to note that quartz wool which is
generally used to hold catalysts in the reactor plays
an important role in the plasma-catalytic reaction.
We can see that the presence of quartz wool in the
discharge also leads to an increase of CH4
conversion (40.2 %) and H2 yield due to strong
interaction between plasma and quartz wool. These
findings lead us to suggest that the improvement of
the reaction performance in the presence of Ni
catalyst pellets (held with quartz wool) is probably
attributable to the effect of plasma-quartz wool
interaction rather than the activity of the Ni/γ-Al2O3
catalyst.
Table 1. Conversion rate, selectivity and yield in plasmacatalytic dry reforming reaction at 50 W (1 g catalyst pellets are
partially packed in the gap and held by quartz wool)
Conversion
(%)
Selectivity
(%)
Yield
(%)
CH4
CO2
H2
CO
H2
30
19.2
29
44.7
8.7
10% Ni/Al2O3 a
38
21.2
27.6
45.3
10.5
b
32
19.4
32.8
54.9
10.5
40.2
24.8
30.1
51.4
12.2
No catalyst
10% Ni/Al2O3
Quartz wool
Calcination temperature 500 oC
a. 0.85-1.7 mm; b. 0.5-0.85 mm (pellet size)
Figure 3. Conversion and selectivity in plasma dry reforming
reaction without catalyst.
Figure 4. Selectivity of hydrocarbons in plasma-catalytic dry
reforming at 50 W (1 g catalyst pellets are partially packed in
the gap and held by quartz wool).
Table 2 shows the performance of plasma dry
reforming over different Ni/γ-Al2O3 catalysts. In this
experiment, catalyst (1 g) in flake form is packed in
the discharge gap and no quartz wool is required to
hold the catalyst. Compared to the plasma reaction
without catalyst, the conversion rate of CH4 and CO2
significantly increases from 30 % to 56.4 % and
from 19.2 % to 30.2 % when the Ni/γ-Al2O3 catalyst
calcined at 300 oC is packed in the plasma region.
The yield of H2 and C2H2/C2H4 is also doubled.
However, the presence of the Ni/γ-Al2O3 catalyst
calcined at high temperature (800 oC) increases the
selectivity of C2H6 and C3H8 (Fig. 5), but had little
effect on the conversions and selectivity of H2 and
CO. We also find that the H2 yield decreases with
the increase of the calcination temperature for the
catalyst.
Table 2. Conversion rate, selectivity and yield in plasmacatalytic dry reforming reaction at 50 W (1 g catalyst in flake
form is partially packed in the gap without holding by quartz
wool)
Conversion
(%)
Selectivity
(%)
Yield
(%)
CH4
CO2
H2
CO
H2
No catalyst
30
19.2
29
44.7
8.7
5%Ni/Al2O3300
34.2
24.8
28.1
38.3
9.6
10%Ni/Al2O3 300
56.4
30.2
31
52.4
17.5
10%Ni/Al2O3 500
31.4
14.8
35.4
54.1
11.1
30
15.8
29.2
46.3
8.8
10%Ni/Al2O3
800
High calcination temperature leads to the
increase in the intensity of NiO-γAl2O3 interactions
and the formation of NiAl2O4 spinel, which is found
to be unfavorable to the reduction of the catalyst,
and thus cause a decrease in the activity for the low
temperature dry reforming reaction. In this study, the
synergistic effect of plasma-catalysis is attributed to
two effects: strong plasma-catalyst interactions and
high activity of the Ni/γAl2O3 catalyst calcined at
low temperature.
4. Conclusion
In this study, plasma-catalytic dry reforming of
CH4 is investigated using a DBD reactor combined
with different Ni/γ-Al2O3 catalysts. A synergistic
effect of the plasma-catalysis is observed, which
shows both the conversion rate of CH4 (56.4 %) and
hydrogen yield (17.5 %) are almost doubled when a
10 wt% Ni/γ-Al2O3 catalyst in flake form calcined at
300 oC (1 g) is partially packed in the DBD. This
synergistic effect is attributed to both strong plasmacatalyst interactions and high activity of the catalyst
calcined at low temperature.
Acknowledgement
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
[1] Y. H. Hu and E. Ruckenstein, Adv. Catal. 48 297
(2004)
[2] A. M. Harling, D. J. Glover, J. C. Whitehead and
K. Zhang, Environ. Sci. Technol. 42 4546 (2008).
[3] X. Tu, H. J. Gallon, M. V. Twigg, P. A. Gorry
and J. C. Whitehead, J. Phys. D: Appl. Phys. (2011)
in press.
Figure 5. Selectivity of hydrocarbons in plasma-catalytic dry
reforming at 50 W (1 g catalyst in flake form is partially packed
into the gap without holding by quartz wool)
[4] D. Petrovic, T. Martens, J. Van Dijk, W. J. M
Brok and A. Bogaerts, J. Phys. D: Appl. Phys. 42
205206 (2009)