22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-catalytic reverse water-gas shift reaction in a packed bed dielectric
barrier discharge reactor
Y.X. Zeng, D.H. Mei and X. Tu
Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, L69 3GJ, U.K.
Abstract: A dielectric barrier discharge (DBD) reactor has been used for plasma-catalytic
reverse water-gas shift reaction (RWGS) at low temperatures and atmospheric pressure.
The effect of M/γ-Al 2 O 3 (M = Cu, Mn, MnO x and CuMnO x ) catalysts on the performance
of the process has been investigated. All the catalysts enable RWGS reaction at low
temperature and atmospheric pressure in DBD plasma and show decent performance.
Keywords: plasma-catalysis, CO 2 conversion, reverse water-gas shift reaction
1. Introduction
The reverse water gas shift reaction (RWGS, Eq. 1) has
been regarded as a promising process for carbon dioxide
conversion and utilisation (CCU), which could convert
CO 2 to value-added fuels and chemicals, thus reducing
CO 2 emissions and contributing to the sustainability of
our society.
CO 2 + H 2 → CO + H 2 O
(1)
H 298 0 = 42.1 kJ mol−1
In this process, CO 2 emitted from the combustion of
fossil fuels can react with hydrogen generated from
renewable resources (e.g. water electrolysis using solar or
wind power) to produce CO. CO can be used as a
chemical feedstock to be further converted into
oxygenates or long-chain hydrocarbons. These products
are all important feedstock for chemical and energy
industry. However, reverse water gas shift reaction is a
highly endothermic reaction that favours high
temperatures. At stoichiometric equilibrium conversions
for the RWGS reaction at atmospheric pressure, it
requires over 1000 K in order to achieve the conversion of
50% [1]. In addition, common catalysts (e.g. Cu, Fe, or
Ce based catalysts) are not stable enough at high
temperatures [2]. Because this reaction is reversible, the
conversion of CO 2 and the selectivity of CO are mainly
determined by the reaction temperature, which actually
governs the thermodynamic equilibrium of the reaction.
High temperature process also causes high energy
consumption and running costs. It is a challenge to lower
the reaction temperature for the RWGS reaction, to make
this process more energy-efficient and cost effective.
Non-thermal plasma offers an attractive and promising
alternative to thermal catalytic route for the conversion of
CO 2 and H 2 into CO at atmospheric pressure and low
temperatures [3-4]. In such plasma, the initially generated
electrons collide with the gas molecules to produce
chemically reactive species including free radicals,
excited atoms, ions and molecules [5-6]. Both highly
energetic electrons and reactive species play important
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roles in the initiation and propagation of a variety of
physical and chemical reactions in low temperature
plasma processes [7]. In non-thermal plasmas, the
temperature of electrons and heavy particles (free
radicals, atoms, molecules and ions) are significantly
different. The overall gas kinetic temperature in a plasma
zone can be as low as room temperature, whilst the
electrons are highly energetic and have a typical electron
temperature of 1-10 eV, which can break most chemical
bonds present in inert molecules (e.g. CO 2 ). The nonequilibrium characteristic of non-thermal plasmas could
enable highly endothermic reactions (e.g. RWGS
reaction) to occur at a relatively low temperature. High
reaction rate and fast attainment of steady state allows
rapid start-up and shutdown of plasma processes
compared to other thermal processes, which significantly
reduces the overall energy cost and offers a very
promising route for industrial applications.
The combination of non-thermal plasma and catalysis
therefore can be regarded as a promising and effective
solution to convert CO 2 and renewable H 2 into CO at low
temperatures and atmospheric pressure. However, there
are very limited works of plasma-catalytic reverse watergas shift reaction at low temperatures [1]. In this work,
plasma-catalytic reverse water-gas shift reaction over
supported catalysts M/γ-Al 2 O 3 (M = Cu, Mn and Cu-Mn)
has been investigated in a coaxial packed bed (BaTiO 3 )
dielectric barrier discharge (DBD) reactor at low
temperatures. The influence of the catalyst composition
on the performance of the plasma process has been
evaluated in terms of the conversion of CO 2 , the
selectivity and yield of CO, and the energy efficiency of
the plasma process.
2. Experimental
The experiment was carried out in a coaxial packed bed
DBD reactor, as shown in Fig. 1. A stainless steel mesh
(ground electrode) was wrapped over the outside of a
quartz tube with an outer diameter of 23 mm and wall
thickness of 2 mm, while a stainless steel rod with an
outer diameter of 14 mm was placed in the centre of the
1
quartz tube and used as a high voltage electrode. The
length of the discharge region was 100 mm with a
discharge gap of 2.5 mm. The DBD reactor was supplied
by a high voltage AC power supply with a variable
voltage and a frequency of 8.7 kHz. The applied voltage
was measured by a high voltage probe (Testec, HVP15HF), while the current was recorded by a current
monitor (Bergoz CT-E0.5). The voltage across the
external capacitor (0.47 μF) was also measured. All the
electrical signals were sampled by a four-channel digital
oscilloscope (TDS2014). The Q-U Lissajous method was
used to calculate the discharge power (P) of the DBD
reactor.
transportation and recycle of hydrogen can be more
expensive. Therefore, the RWGS reaction could be less
economic and interesting when the H 2 /CO 2 molar ratio is
larger than 1.
Table. 1. Effect of different H 2 /CO 2 molar ratios on the performance of
the plasma-catalytic RWGS reaction (feed flow GHSV = 2400 h-1, catalyst
Cu/γ-Al 2 O 3 , discharge power 35W, plasma temperature 130-135°C)
H 2 /CO ratio
Con
S
Y
E
Carbon balance (%)
1:1
8.0
80.3
6.4
0.041
99.1
2:1
11.0
81.7
9.0
0.068
99.2
3:1
13.8
85.4
11.8
0.089
99.6
4:1
22.5
86.8
19.5
0.140
98.8
Note: Con is CO 2 conversion (%), S is CO selectivity (%), Y is CO yield
(%), and E is the energy efficiency (mmol CO produced/kJ) of the process.
Fig. 1. Schematic diagram of experimental system
8 wt.% M/γ-Al 2 O 3 (M = Cu, Mn and Cu(4%)Mn(4%)) catalysts were prepared by the method of
incipient wetness impregnation using nitrate salts (Alfa
Aesar, 99.5%) as the metal precursors. The appropriate
weight of support (γ-Al 2 O 3 beads with diameter of 1.52mm) was added to the solution of nitrate salts. The slurry
was continuously stirred at 80 °C for 4 h and then dried at
110 °C overnight, followed by calcination at 600 °C for 6
h. The catalyst pellets were packed with BaTiO 3 beads to
fill the whole plasma region. Prior to the plasma-catalytic
RWGS reaction, two catalysts (denoted as Cu/γ-Al 2 O 3
and Mn/γ-Al 2 O 3 ) were reduced in an argon-hydrogen
discharge at a discharge power of 7.5 W (50 ml min-1, 20
vol. % H 2 ) for 30 mins in the same DBD reactor, while
the other two catalysts were not reduced (denoted as
MnO x /γ-Al 2 O 3 and CuMnO x /γ-Al 2 O 3 ). The reactants
and products were analyzed by a two-channel gas
chromatography (Shimadzu GC-2014) equipped with a
flame ionization detector (FID) and a thermal
conductivity detector (TCD).
3. Results and discussion
Table 1 shows the effect of different H 2 /CO 2 ratios on
the plasma-catalytic RWGS reaction over the Cu/γ-Al 2 O 3
catalyst. Increasing the H 2 /CO 2 molar ratio significantly
increases the CO 2 conversion, CO yield and energy
efficiency. However, the stoichiometric H 2 /CO 2 molar
ratio for RWGS reaction is 1. The production, storage,
2
Table 2 shows the effect of γ-Al 2 O 3 supported metal
and metal oxide catalysts on the reverse water-gas shift
reaction. Compared to the Cu/γ-Al 2 O 3 catalyst, the
combination of plasma with the Mn/γ-Al 2 O 3 and
CuMnO x /γ-Al 2 O 3 catalysts enhances the conversion of
CO 2 by 28% and 26%, respectively. However, the
presence of the MnO x /γ-Al 2 O 3 in the DBD reactor only
improves the CO 2 conversion by 13%. The maximum
CO 2 conversion of 10.2% has been achieved in the
plasma-catalytic RWGS over the Mn/γ-Al 2 O 3 catalyst at
a discharge power of 35 W and a GHSV of 2400 h-1. The
combination of DBD with these catalysts also enhances
the yield of CO. The CuMnO x /γ-Al 2 O 3 catalyst shows
the best catalytic activity for CO production, followed by
the Mn/γ-Al 2 O 3 , MnO x /γ-Al 2 O 3 and Cu/γ-Al 2 O 3 . The
presence of the CuMnO x /γ-Al 2 O 3 catalyst in the plasma
process significantly increases the yield of CO by 41%,
compared to the plasma reaction using the Cu/γ-Al 2 O 3
catalyst.
Table 2. Plasma RWGS using different catalysts (H 2 /CO 2 = 1: 1, feed
flow GHSV = 2400 h-1, discharge power 35W, plasma temperature 135140°C)
Catalyst type
Con
S
Y
E
Carbon balance (%)
Cu
8.0
80.3
6.4
0.041
99.1
MnOx
9.0
68.6
6.2
0.045
97.8
Mn
10.2
77.7
7.9
0.058
98.5
CuMnOx
10.1
89.2
9.0
0.068
99.4
Note: Con is CO 2 conversion (%), S is CO selectivity (%), Y is CO yield
(%), and E is the energy efficiency (mmol CO produced/kJ) of the process.
Table 2 shows that packing the Cu/γ-Al 2 O 3 catalyst in
the DBD reactor results in a lowest energy efficiency of
0.041 mmol/kJ. Introducing the Mn/γ-Al 2 O 3 and
CuMnO x /γ-Al 2 O 3 catalysts into the discharge gap is
found to improve the energy efficiency of the conversion
by 41% and 66%, respectively, while packing the
MnO x /γ-Al 2 O 3 catalyst into the DBD reactor slightly
increases the energy efficiency of the plasma process. The
maximum energy efficiency for the conversion of CO 2
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(0.068 mmol/kJ) is achieved at a discharge power of 34
W and a GHSV of 2400 h-1 when the CuMnO x /γ-Al 2 O 3
catalyst is packed in the plasma.
4. Conclusions
The combination of plasma with the Cu/γ-Al 2 O 3 , Mn/γAl 2 O 3 , MnO x /γ-Al 2 O 3 , and CuMnO x /γ-Al 2 O 3 catalysts
enables the reverse water-gas reaction occur at low
temperature. Compared to the Cu/γ-Al 2 O 3 catalyst, the
combination of plasma with the Mn/γ-Al 2 O 3 and
CuMnO x /γ-Al 2 O 3 catalysts enhances the conversion of
CO 2 by 26-28%, while the CuMnO x /γ-Al 2 O 3 catalyst
shows the best catalytic activity for CO production,
followed by the Mn/γ-Al 2 O 3 , MnO x /γ-Al 2 O 3 and Cu/γAl 2 O 3 . The presence of the CuMnO x /γ-Al 2 O 3 catalyst in
the plasma process significantly increases the yield of CO
by 41%, compared to the plasma reaction using the Cu/γAl 2 O 3 catalyst. In addition, we find the combination of
plasma with the Mn/γ-Al 2 O 3 and CuMnO x /γ-Al 2 O 3
catalysts significantly improves the energy efficiency for
the conversion by 41% and 66%, respectively, while
packing the MnO x /γ-Al 2 O 3 catalyst into the DBD reactor
only slightly increases the energy efficiency of the plasma
process.
5. Acknowledgement
Financial support of this work by the UK Engineering
and Physical Sciences Research Council (EPSRC)
CO2Chem Network and Knowledge Exchange (KE) fund
of the University of Liverpool is also gratefully
acknowledged. Y. X. Zeng and D. H. Mei acknowledge
the PhD fellowship co-funded by the Doctoral Training
Programme (DTP) of the University of Liverpool and the
Chinese Scholarship Council (CSC).
6. References
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Catalysis Today, 232, 27-32 (2014)
[2] W. Wang, S. P. Wang, X. B. Ma, J. L. Gong,
Chemical Society Reviews, 40, 3369-4260 (2011)
[3] X. Tu, J. C. Whitehead, Applied Catalysis B:
Environmental, 125, 439-448 (2012)
[4] E. C. Neyts, A. Bogaerts, Journal of Physics D:
Applied Physics, 47, 224010 (2014)
[5] S. Y. Liu, D. H. Mei, Z. Shen, X. Tu, Journal of
Physical Chemistry C, 118, 10686-10693 (2014).
[6] R. Aerts, X. Tu, C. De Bie, J. C. Whitehead, A.
Bogaerts, Plasma Processes and Polymers, 9, 994-1000
(2012)
[7] X. Tu, B. Verheyde, S. Corthals, S. Paulussen, B. F.
Sels, Physics of Plasmas, 18, 080702 (2011)
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