22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Catalyst screening for plasma-catalytic removal of acetone
X. Zhu1,2, X. Tu2, X. Yu1, C. Zheng1 and X. Gao1
1
State Key Laboratory of Clean Energy Utilization, Zhejiang University, CN-310027 Hangzhou, P.R. China
2
Department of Electric Engineering and Electronics, University of Liverpool, Liverpool L69 3GJ, U.K.
Abstract: In this work, plasma-catalytic removal of acetone over a series of MOx/γ-Al 2 O 3
(M = Ce, Co, Cu, Mn and Ni) catalysts were investigated. The best removal efficiency of
94.2% and CO 2 selectivity of 80.1% were achieved when using the CuOx/γ-Al 2 O 3 catalyst.
Meanwhile, the formation of by-products (HCHO and HCOOH) were substantially reduced
in the presence of tested catalysts.
Keywords: plasma-catalysis; dielectric barrier discharge; acetone; metal oxide catalysts
1. Introduction
The emission of volatile organic compounds (VOCs) in
the atmosphere is associated with the formation of fine
particle matter, such as haze and photochemical smog,
and consequently cause air pollution [1, 2]. VOCs have
also been found to impose significant negative effects on
human health. Acetone, as one of the most abundant
oxygenated VOCs in the atmosphere, is mainly released
from automobile exhaust, tobacco smoke and incineration
of waste materials. The increasingly stringent emission
regulations and growing public concern for the
environment is the main drive to develop new and costeffective gas cleaning processes for environmental
pollution control.
A variety of different pollution control technologies
have been developed including adsorption, thermal
processes, vapour condensation, and catalytic oxidation
and combustion. Conventional methods for the removal
of VOCs in high volume waste gas streams are not costeffective because of the large flow rates to be treated and
high operating costs due to the energy consumption of
these methods.
Non-thermal plasma technology offers an attractive and
promising alternative as means for the removal of
environmental pollutants such as VOCs in high volume
waste gas streams. In non-thermal plasma, the overall
plasma gas temperature can be as low as room
temperature, while the electrons are highly energetic with
a typical electron temperature of 1-10 eV. The initially
generated electrons collide with the gas molecules (e.g.,
carrier gas, pollutant molecules) to produce chemically
reactive species including free radicals, excited atoms,
ions and molecules for plasma reactions. The use of a
plasma discharge has shown that a wide range of VOCs
can be effectively decomposed and oxidized at low
temperatures [3]. However, the main drawback of plasma
processes for pollution remediation is the formation of
unwanted and hazardous by-products, especially organic
by-products. For example, Lyulyukin et al. [4] found
acetone (205 ppm) in air can be effectively oxidized in a
negative corona discharge at a discharge power of 60 W,
with the main products being HCHO, CO and CO 2 with a
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maximum CO 2 selectivity of 67.6%. Narengerile et al.
[5] reported the formation of 0.2-0.9% CH 4 , HCOOH and
HCHO in the effluent using a direct current (DC) water
plasma torch for the decomposition of acetone. The
combination of plasma and heterogeneous catalysis has
been regarded as a promising and effective solution to
overcome the drawback of plasma processes [6].
Introducing a catalyst into a plasma reactor could change
the electron energy distribution function (EEDF) and
generate more highly energetic and reactive species,
which favour the plasma-induced reactions towards deep
oxidation of VOCs. The combination of plasma with
catalysis has great potential to lower the activation
temperature of the catalysts, enhance the removal of
pollutants, and minimize the formation of unwanted byproducts: All of which may contribute in different ways
to the enhancement of the energy efficiency of the plasma
process.
Different metal catalysts have been investigated in
single-stage plasma-catalytic gas cleaning processes for
the oxidation of acetone, including supported precious
metal catalysts (Ag, Pd and Pt) due to their excellent
activity even with low metal loading [7-9]. However, the
high cost of precious metals is a major barrier which
limits their use in plasma systems for industrial
applications. Transition metal oxide catalysts have gained
growing interest in combination with plasma for pollution
abatement because of their comparable performance and
low cost. Trinh et al. reported that introducing ceramic
supported MnO 2 catalysts into a dielectric barrier
discharge (DBD) reactor significantly enhanced the
removal efficiency of acetone, while the presence of
ceramic supported ZnO in the discharge weakly affected
acetone removal efficiency [10]. In our previous work,
we found that the combination of plasma with γ-Al 2 O 3
supported Cu, Co and Mn catalysts improved the removal
efficiency of acetone and drove the reaction towards deep
oxidation in humid conditions [11]. Similar improvement
in acetone removal efficiency was achieved using a
LaCoO 3 catalyst in a packed-bed plasma reactor [12]. To
the best of our knowledge, far less has been done to
investigate the effect of a range of supported metal oxide
1
catalysts on the plasma-catalytic removal of low
concentration acetone. The fundamental understanding of
catalyst properties in the plasma-catalytic oxidation of
acetone is still very limited. It is of primary interest to
screen a range of supported metal oxide catalysts to get
new insights into the roles of different metal oxides and
related structures in the plasma-catalytic removal of
acetone and to establish a relationship between the
process performance and catalyst properties.
In this work, the plasma-catalytic removal of acetone
over MOx/γ-Al 2 O 3 (M= Ce, Co, Cu, Mn and Ni)
catalysts has been investigated in a coaxial DBD plasma
reactor. The effect of these catalysts on the plasmacatalytic removal of acetone as a function of specific
energy density (SED) has been evaluated in terms of
acetone removal efficiency and CO 2 . Based on the
experimental results, a steady-state approximation method
has been employed to represent the reaction kinetics in
the plasma-catalytic process for acetone decomposition.
2. Experimental
The experiment was carried out in a coaxial DBD
reactor, as shown in Fig. 1. A 60 mm-long aluminum foil
(ground electrode) was wrapped around a quartz tube with
an outer diameter of 10 mm and wall thickness of 1 mm.
A stainless steel rod with an outer diameter of 4 mm was
placed inside the quartz tube to act as a high voltage
electrode. The reactor was supplied by an AC power
supply with a maximum peak voltage of 30 kV and a
frequency of 10 kHz. Zero grade air (BOC) was used as
carrier gas, while acetone (99.5%, Fisher Scientific) was
introduced into the DBD reactor by passing a dry air flow
through a water bubbler kept in a water bath (20 °C). All
gas streams were premixed prior to the DBD reactor.
Catalyst samples (100 mg) were placed at the end of the
discharge region and were held there using quartz wool.
The total flow rate was kept at 1 l min-1, resulting in a gas
hourly space velocity of 600,000 ml g-1 h-1, with the initial
concentration of acetone fixed at 1184 ppm.
Fig. 1. Schematic diagram of the experimental setup.
The applied voltage was measured by a high voltage
probe (Testec, HVP-15F, 1000:1), while the voltage on
the external capacitor (0.47 µF) was measured by a
Tektronix P5100 probe. All the electrical signals were
2
sampled by a four-channel digital oscilloscope (Tektronix
3034B). The discharge power (P) of the DBD was
calculated using the Q-U Lissajous method. An online
power measurement system was used to monitor and
control the discharge power of the DBD reactor in real
time. The gas temperature in the center of the discharge
area was measured using a fiber optical thermometer
(Omega, FOB102). The maximum gas temperature in the
center of the discharge region was less than 150 °C.
Gas products were analyzed online using a Fourier
transform infra-red (FTIR) spectrometer (FTIR-4200,
Jasco) with a resolution of 2 cm-1. The spectrometer was
equipped with a 1-16 m variable gas cell (PIKE
Technologies), while the effective length in this study was
5.3 m.
The quantitative analysis of acetone was
calibrated with known concentrations, while the
quantitative analysis of other gaseous products was
conducted by comparing the obtained spectra with the
standard FTIR spectra from the Pacific Northwest
National Laboratories (PNNL) database [21]. Each
measurement was obtained by averaging 128 scans and
was repeated three times. The estimated error of the
results, taking into consideration the measurement and
quantification errors, was less than 5%.
The MOx/γ-Al 2 O 3 (M= Ce, Co, Cu, Mn and Ni)
catalysts with metal loading of 10 wt.% were prepared by
incipient wetness impregnation using nitrate salts (Alfa
Aesar, 99.5%) as the metal precursor. Initially, the metal
nitrates were dissolved in deionized water and stirred for
1 h at room temperature to get 0.1 M solutions. The
appropriate weight of support (γ-Al 2 O 3 ) was added to the
solution. The slurry was continuously stirred at 80 °C for
4 h and then dried at 110 °C overnight, followed by the
calcination at 500 °C for 5 h. All the catalysts were
sieved in 35-60 meshes prior to the plasma reaction. Pure
γ-Al 2 O 3 support was also used for comparison in this
study.
3. Results and discussions
Fig. 2 presents the effect of different catalysts on the
performance of the plasma-catalytic process in terms of
the removal efficiency of acetone and the CO 2 selectivity.
Clearly, both acetone removal efficiency and CO 2
selectivity increase with SED regardless of the catalyst
used. Increasing the SED effectively enhances the
electric field of the discharge and generates more highly
energetic electrons and chemically active species which
are capable of initiating a variety of different reactions,
hence leading to an increase in the acetone removal rate.
The increase in SED also raises the gas temperature of the
plasma. Taking the CuOx/γ-Al 2 O 3 catalyst for example,
the measured gas temperature in the DBD reactor
increases from 92.0 °C at a SED of 670 J l-1 to 143.1 °C at
1012 J l-1. Among these samples, the combination of
plasma with pure γ-Al 2 O 3 shows the lowest acetone
removal efficiency (64.1%) and CO2 selectivity (73.0%),
even at the highest SED of 1287 J l-1, whilst the
MOx/γ-Al 2 O 3 catalysts significantly improve the removal
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efficiency of acetone and CO 2 selectivity over the tested
SED range, even though these MOx/γ-Al 2 O 3 catalysts
have a much smaller specific surface area compared to
that of the pure γ-Al 2 O 3 support. This indicates the role
of the active metal oxides in the MOx/γ-Al 2 O 3 catalysts
for acetone oxidation in the plasma environment. The
combination of plasma and the CuOx/γ-Al 2 O 3 catalyst
exhibits the best reaction performance. The removal
efficiency of acetone is increased by a factor of 3, from
34.2% to 94.3%, as the SED varies from 670 J l-1 to
1012 J l-1, while the maximum selectivity of CO 2 (80.1%)
is achieved at a SED of 1012 J l-1. After the reaction for
5h, the removal efficiency of acetone was almost constant
at a SED of 1012 J l-1 when using this catalyst.
onto the catalyst surface, followed by the oxidation with
surface oxygen species generated by the catalysts to form
CO, CO 2 , H 2 O and by-products. The consumed surface
oxygen species can be replenished by reactive O atoms
generated in the plasma near the surface of the catalyst.
a)
a)
b)
Fig. 3. Effect of different catalysts on the formation of
organic by-products as a function of SED in plasmacatalytic oxidation of acetone: (a) HCHO; (b) HCOOH.
b)
Fig. 2. Effect of different catalysts on plasma-catalytic
process as a function of SED: (a) removal efficiency of
acetone; (b) CO 2 selectivity.
It is clear that the performance of the plasma-catalytic
removal of acetone mainly depends on the properties of
the different metal oxide catalysts and the interactions
In this
between them and the γ-Al 2 O 3 support.
single-stage plasma-catalytic process, where the catalysts
are directly in contact with the plasma, plasma-assisted
surface reactions also play a crucial role in the oxidation
of acetone besides the plasma gas phase reactions. Both
acetone molecules and intermediates can be adsorbed
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The formation of organic by-products is undesirable in
the plasma gas cleaning process for the removal of low
concentration VOCs. In this study, both HCHO and
HCOOH were detected by the FTIR for all the catalysts.
The formation of HCHO and HCOOH decreases with
increasing SED, as plotted in Fig. 3, which agrees with
the evolution of CO 2 selectivity as a function of SED.
Increasing the energy input produces more chemically
reactive species, especially oxidative species, all of which
contribute to the oxidation of by-products and
intermediates. In this study, the combination of plasma
with the MOx/γ-Al 2 O 3 catalysts significantly reduces the
formation of HCHO and HCOOH.
The lowest
concentration of HCHO (28.7 ppm) and HCOOH
(57 ppm) was achieved using the CuOx/γ-Al 2 O 3 catalyst
at a SED of 1012 J l-1, followed by MnOx/γ-Al 2 O 3 ,
CoOx/γ-Al 2 O 3 , CeOx/γ-Al 2 O 3 then NiOx/γ-Al 2 O 3 . In
the presence of the MOx/γ-Al 2 O 3 catalysts in the DBD
reactor, plasma-assisted surface reactions also play an
3
important role in the oxidation of acetone, intermediates
and by-products, whilst the participation of surface
oxygen species in the plasma-catalytic process may be
what drives the reaction towards deep oxidation and
substantially inhibits the formation of undesirable
by-products. In addition, the carbon balance of the
plasma-catalytic process over these catalysts is between
85.4% and 95%. The combination of plasma with the
CuOx/γ-Al 2 O 3 catalyst gives the highest carbon balance
of 95%.
[9]
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4. Conclusions
In this work, catalyst screening of various MOx/γAl 2 O 3 (M = Ce, Co, Cu, Mn and Ni) catalysts for the
plasma-catalytic removal of acetone has been
investigated. The combination of plasma with MOx/γAl 2 O 3 catalysts significantly enhances the removal
efficiency of acetone and the CO 2 selectivity of the
plasma process, whilst substantially reducing the
formation of organic by-products (HCHO and HCOOH).
The maximum acetone removal efficiency of 94.2 % and
CO 2 selectivity of 80.1% was achieved when the CuOx/γAl 2 O 3 catalyst was directly packed into the DBD reactor.
Introducing the CuOx/γ-Al 2 O 3 catalyst into the plasma
system also shows the lowest formation of by-products.
These results show that the presence of MOx/γ-Al 2 O 3
catalysts could accelerate the oxidation of acetone in
plasma-catalysis system.
5. Acknowledgements
Support of this work bythe National Natural Science
Foundation of China (No. 51076140 & No. 51206143),
the National Science Fund for Distinguished Young
Scholars (No. 51125025) and the Royal Society of the UK
(RG120591) is gratefully acknowledged.
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