22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-photocatalytic conversion of carbon dioxide into value-added chemicals
D.H. Mei, J.D. Yan and X. Tu
Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, U.K.
Abstract: Plasma-catalytic conversion of pure CO 2 into CO and O 2 over BaTiO 3 and
TiO 2 photocatalysts has been investigated in a coaxial dielectric barrier discharge (DBD)
reactor. The synergistic effect from the combination of DBD with photocatalysts (BaTiO 3
and TiO 2 ) contributes to a significant enhancement of both CO 2 conversion and energy
efficiency by up to 250%.
Keywords: plasma-catalysis, photocatalysis, CO 2 conversion, synergistic effect
1. Introduction
The carbon dioxide issue has recently become the focus
of global attention because of the position of CO 2 as the
primary greenhouse gas and the implication of its
emissions on the problems of climate change and global
warming. In the past decade, strategies to address the
challenge of global climate change have largely focused
on the development of different technologies for CO 2
capture and storage (CCS). The idea is rather than treating
CO 2 as a waste, it can be regarded as a low value raw
chemical for the production of value-added fuels and
chemicals, finding beneficial ways to “use” in addition to
permanently storing the emitted CO 2 .
From the point of view of thermodynamics, it is a great
challenge to direct convert CO 2 into CO and O 2 due to
the high stability of CO 2 molecules. Non-thermal plasma
provides an attractive alternative to the conventional
catalytic route for the conversion of greenhouse gas into
valuable fuels and chemicals because of its nonequilibrium properties, low power requirement and its
unique capacity to induce both physical and chemical
reactions at low temperatures. In non-thermal plasma, the
overall gas temperature remains low, while the electrons
are highly energetic with a typical electron temperature of
1-10 eV, which is sufficient to break most chemical bonds
of inert molecules and produce highly reactive species for
the initiation of plasma chemical reactions [1]. Moreover,
the use of non-thermal plasma in combination with solid
catalysts has great potential to enhance the conversion of
feed gases, improve the selectivity towards the desirable
products and to reduce the operating temperature of the
catalyst which both increases the energy efficiency of the
process and improves the stability of the catalyst by
reducing poisoning, coking and sintering, therefore has
attracted increasing interest for environmental and energy
applications [2-7].
2. Experimental setup and analysis
In this study, the plasma-catalytic conversion of pure
CO 2 into higher value chemicals is performed in a coaxial
DBD reactor. An aluminium foil is wrapped over the
outside of a quartz tube with an external diameter of 22
mm and an inner diameter of 19 mm. A stainless steel
O-15-5
tube with an outer diameter of 14 mm is used as the inner
electrode. The discharge gap was fixed at 2.5 mm, while
the discharge length can be varied from 90 to 150 mm.
CO 2 was used as the feed gas without dilution at a flow
rate of 15-60 ml/min. The DBD reactor is supplied by an
AC high voltage power supply with a peak-to-peak
voltage of 10 kV and a frequency of 50 Hz. All the
electrical signals are sampled by a four-channel digital
oscilloscope. To understand the interactions between
plasma and catalyst, catalysts (BaTiO 3 , TiO 2 ) are packed
into the discharge gap along the bottom of the quartz tube.
This partially packing method provides a large gas
volume and a small volume of the catalyst with a high
void fraction in the plasma gap [2]. Our previous work
demonstrated that this packing method induces effective
plasma-catalyst interactions, which might generate a
synergistic effect promote plasma-catalytic chemical
reactions [8]. The gas products are analysed by a twochannel gas chromatography (Shimadzu 2014) equipped
with a flame ionisation detector (FID) and a thermal
conductivity detector (TCD). The concentration of ozone
was measured by an ozone monitor (2B, Model 106-M).
The temperature (< 150 oC) in the DBD reactor was
measured by a fibre optical thermometer (Omega,
FOB102).
3. Results and discussion
Fig.1 shows the effect of specific energy density (SED)
on the conversion of CO 2 and energy efficiency of the
plasma reaction in the absence of a catalyst. Clearly,
increasing the SED is found to significantly enhance CO 2
conversion due to the increase in the energy input to the
discharge. The conversion of CO 2 is increased by a factor
of 3 (from 6.65% to 21.72%) as the SED rises from 8
kJ/L to 80 kJ/L. Similar conversion trends have been
reported using either cold plasma alone or plasmacatalysis for chemical reactions [9-10]. Our previous
works have shown that increasing discharge power at a
constant frequency could effectively enhance the electric
field, electron density and gas temperature in the
discharge [11-13], all of which may contribute in different
ways to the improvement in conversion. Moreover,
increasing the discharge power produces more chemically
1
reactive species (e.g. O atoms), which can further induce
CO 2 dissociation to enhance its conversion. A lower feed
gas flow rate was reported to be beneficial to improving
the conversion of reactants due to longer residence time
of the reactants in the plasma. In contrast, the specific
energy density shows an opposite effect on the energy
efficiency of the plasma process. Increasing the SED from
8 kJ/L to 80 kJ/L leads to a decrease of the energy
efficiency from 0.37 mmol/kJ to 0.12 mmol/kJ, which is
consistent with previous results [14]. In the plasma
process without a catalyst, the maximum energy
efficiency of 0.37 mmol/kJ is achieved at the lowest
specific energy density of 8 kJ/L with a discharge power
of 8 W, a CO 2 feed flow rate of 60 ml/min and a
discharge length of 150 mm.
Fig.1. CO 2 conversion and energy efficiency as a
function of SED.
Fig. 2 presents the effect of BaTiO 3 and TiO 2
photocatalysts on the conversion of CO 2 . Clearly, the
presence of both BaTiO 3 and TiO 2 in the discharge
significantly enhances the CO 2 conversion and energy
efficiency. Packing BaTiO 3 pellets into the discharge gap
exhibits the exceptional performance with a remarkable
enhancement of both CO 2 conversion (from 15.23% to
38.30%) and energy efficiency (from 0.24 mmol/kJ to
0.60 mmol/kJ) by a factor of 2.5 at a SED of 28 kJ/L. To
understand the role of plasma in the reaction, a purely
thermal experiment has been carried out by heating both
photocatalysts in a pure CO 2 flow at 150 oC. No
conversion and adsorption of CO 2 is observed. The
thermodynamic equilibrium calculation of CO 2
conversion has also confirmed that the conversion of CO 2
is almost zero at low temperatures (e.g. 150 ◦C),
suggesting that low CO 2 conversion is expected using
thermal catalytic reduction of CO 2 at the same
temperature used in the plasma reaction. The results
clearly show that the exception reaction performance has
been achieved by using plasma-catalysis, which is much
higher than the sum of plasma-alone and catalysis alone,
indicating the formation of a synergistic effect of
combining plasma with photocatalysts at low temperature.
Previous results suggested that the presence of the
catalyst pellets in the plasma gap play a crucial role in
inducing physical effects, such as enhancement of the
2
Fig. 2. Demonstration of the synergistic effect of plasmacatalysis for the conversion of CO 2 (SED = 28 kJ/L).
electric field and production of more energetic electrons
and reactive species, which in turn leads to chemical
effects and contributes to the conversion of CO 2 . In this
study, the electric field is increased by 9.0% and 10.9%
with the presence of TiO 2 and BaTiO 3 in the discharge
gap, respectively, which contributes to the enhancement
of the CO 2 conversion. It is interesting to note that the
enhancement of the reaction performance in terms of CO 2
conversion and energy efficiency is more significant than
the change in the electric field, which suggests that in
addition to the plasma physical effect and resulted gas
phase reactions, the contribution of plasma-activated
catalytic reaction to the synergy of plasma-catalysis
cannot be ruled out.
TiO 2 is a widely used photocatalyst with a wide band
gap of 3.2 eV for anatase phase, while BaTiO 3 is a
perovskite semiconductor photocatalyst with a band gap
of 2.8-3.0 eV for tetragonal phase. It is well known that
photocatalysts can be activated through the formation of
electron-hole (e––h+) pairs with the aid of sufficient
photonic energy (hv) with appropriate wavelength to
overcome the band-gap between the valence band and
conductive band.
TiO 2 + hv → e − + h +
−
BaTiO3 + hv → e + h
(1)
+
(2)
It is believed that UV radiation can be generated in the
process of plasma discharge without using extra UV
sources (e.g., UV lamps). This can be confirmed by the
dominated N 2 (C–B) bands (between 300 nm and 400 nm)
due to an impurity in the gas. Nevertheless, it has been
reported that UV radiation generated by plasma
discharges is not always the controlling factor to activate
photocatalysts. However, Whitehead has suggested that
electron-hole pairs can be created by electron impact upon
the photocatalyst surface since the electrons generated by
the DBD are of very similar mean energy (3 - 4 eV) to the
photons [15]. In this study, the high energetic electron
generated by plasma is considered as the main driving
force of activating photocatalysts for CO 2 conversion.
O-15-5
Oxygen vacancies (Vo), as one of the defect disorders
in photocatalysts, has been considered as the active site
for the adsorption and activation of reactants in the
photocatalytic reaction. XPS measurement in this study
has demonstrated the existence of Vo in both BaTiO 3 and
TiO 2 , as evidenced by the presence of Ti3+. Moreover,
more Vo are contained in BaTiO 3 , resulting in the higher
CO 2 conversion.
The process of the plasma-assisted photocatalytic
conversion of CO 2 is shown in Fig. 3. In the plasmacatalyst system, electron (e–) -hole (h+) pairs are generated
by the highly energetic electrons from the gas discharge,
and move towards the opposite directions by the strong
electric field, which can reduce the recombination rate of
electron/hole pairs. Through the electron transfer process,
CO 2 adsorbed onto the Vo can be reduced to anion radical
CO 2 ∙ – by electrons, followed by the decomposition of
CO 2 ∙ – into CO and the occupation of one oxygen atom
into the Vo site [16]. The overall reaction is expressed as:
CO 2 + [ Photocatalyst + Vo ] → CO + [ Photocatalyst ] (3)
where [Photocatalyst + Vo] and [Photcatalyst] represent
the defective and defect-free photocatalyst, respectively.
However, the V 0 can be regenerated by oxidising the
surface O2- anions using holes, followed by the releasing
of O 2 . In order to balance the charge, the Ti4+ cations in
the vicinity of the regenerated Vo are reduced to Ti3+ by
the electrons. This cyclic healed-regeneration of the
oxygen vacancies maintain the equilibrium of the active
sites in the photocatalysts and accelerate the
decomposition of CO 2 , which can be confirmed by our
experimental results that the CO 2 conversion is almost
constant when the plasma CO 2 conversion runs for two
hours.
Fig. 3. Schematic of the plasma-assisted photocatalytic
CO 2 conversion process.
4. Conclusions
In this study, plasma-photocatalytic conversion of pure
CO 2 into CO and O 2 has been investigated using a DBD
reactor combined with BaTiO 3 and TiO 2 . The
combination of plasma with BaTiO 3 and TiO 2 has
generated a synergistic effect, which significantly
enhance the conversion of CO 2 and energy efficiency by
a factor of 2.5 compared to the plasma reaction in the
O-15-5
absence of a catalyst. The overall synergistic effect
resulting from the integration of DBD and photocatalysis
for CO 2 conversion at low temperatures (without extra
heating) can be attributed to both the physical effect
induced by the presence of catalyst in the discharge and
photocatalytic surface reaction driven by the discharge.
5. Acknowledgements
Support of this work by the UK EPSRC CO2Chem
Network is gratefully acknowledged. D.H. Mei
acknowledges the PhD studentship co-funded by the
Chinese Scholarship Council (CSC) and the Doctoral
Training Programme (DTP) of the University of
Liverpool.
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