st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
TCE abatement with a plasma-catalysis combined system
using MnO2 as catalyst
A.M. Vandenbroucke1, M. Mora2, C. Jiménez-Sanchidrián2, F.J. Romero-Salguero2, N. de Geyter1, C. Leys1, R.
Morent1
1
Research Unit Plasma Technology (RUPT), Department of Applied Physics, Faculty of Engineering and
Architecture, Ghent University, Belgium
2
Department of Organic Chemistry, Faculty of Sciences, University of Córdoba, Spain
Abstract: In this study, the abatement of trichloroethylene (TCE) with a plasmacatalysis combined system using MnO2 as catalyst was investigated. The combined
system greatly improved the selectivity of the process towards total oxidation.
Arrhenius plots were used to prove that the plasma provides an extra activation of the
catalyst. A plausible reaction scheme for the plasma-catalytic TCE abatement is given.
Keywords: Non-thermal plasma, plasma-catalysis, trichloroethylene, manganese oxide
1. Introduction
The condition of our environment is increasingly
endangered by air pollution which poses serious risks to
ecological life and public health. In this regard, volatile
organic compounds (VOCs) are a large group of
chemical compounds that significantly contribute to poor
air quality in many populated areas. Therefore, air
quality regulations are tightened and new remediation
technologies are consequently being explored in order to
overcome these issues.
As an alternative to existing air remediation methods,
non-thermal plasma (NTP) technology has been
investigated for the removal of dilute VOCs (< 1000
ppm) from waste gases and indoor air since the last 2
decades [1, 2]. In a NTP, highly energetic electrons (1-10
eV) trigger multiple chemical processes such as
ionization, excitation and dissociation through collisions
with neutral background molecules (N2, O2, H2O).
Hence, pollutants are being exposed to a reactive
chemical environment containing ions, radicals, excited
species and metastable states by which they are
converted to less harmful products.
The combination of NTP with heterogeneous catalysis
has attracted increased attention since the last decade.
The introduction of a catalyst, either inside or
downstream of a NTP reactor, leads to an enhancement
of the energy efficiency, carbon balance and CO2selectivity compared to plasma alone systems. Recently,
Vandenbroucke et al. have extensively reviewed the
recent achievements and current status of this hybrid
technology for VOC abatement [3].
For the removal of VOCs with NTP, the formation and
emission of large amounts of ozone is a serious
drawback because O3 is responsible for respiratory
diseases and is implied in the formation of
photochemical smog. Several catalytic formulations have
been tested to minimize the emission of ozone. Among
these materials, MnO2 has shown to possess the most
interesting properties for use as catalyst, such as ease of
synthesizing crystalline phases [4] and the mobility of
oxygen in the crystal lattice, creating vacancies, which
promote the formation of oxygen groups in the catalyst
surface [5], making them appropriate materials for VOC
oxidation reactions [6].
The aim of this study is to investigate the opportunities
of a plasma-catalytic system with MnO2 downstream for
the abatement of a typical chlorinated VOC,
trichloroethylene (TCE), in terms of conversion and
COx-selectivity. The effect of catalyst temperature has
been studied in detail. Finally, a degradation scheme is
proposed for the destruction of TCE in the gas-phase and
on the catalyst surface.
2. Experimental
Fig. 1 shows the experimental set-up. A bottle of dry
synthetic air (Alphagas 1, Air Liquide) was connected to
two mass flow controllers (MFC). Next, air was fed to a
TCE bubbling bottle located in an ice-water bath. The
initial TCE concentration was controlled by the feed gas
flow rate. All experiments were carried out with a total
air flow of 500 mL/min containing 500 ppm TCE. The
TCE conversion and the identification of by-products
were determined with a FT-IR spectrometer (Bruker,
Vertex 70). The optical length of the gas cell and the
resolution of the spectrometer were set at 20 cm and 4
cm-1, respectively. Spectra were taken after steady state
condition and consisted of 10 averaged measurements.
The formation of ozone was analyzed with an UV ozone
monitor (Envitec, model 450).
Ozone Monitor
Plasma Source
MFC
Ov en with
cataly st
14 2 p pm
0
1
-
TCE Bubbler
E
FT-IR
MFC
DC Power Supply
Pressurized
Air
Fig.1: Schematic diagram of the experimental set-up.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
The plasma source is based on the concept of a
multi-pin-to-plate negative DC corona/glow discharge
[7]. The rectangular duct of the reactor has a cross
section of 40 mm × 9 mm and a length of 400 mm. The
plasma source consists of 10 aligned cathode pins
which were positioned 28 mm from each other. The
distance between the 10 cathode pins and the single
anode plate is 10 mm. The discharge is powered with a
DC power supply and generated at atmospheric
pressure and room temperature. During experiments,
the energy density of the plasma (i.e. the ratio of the
plasma input power to gas flow rate) is changed by
varying the plasma voltage. A high voltage probe
(Fluke 80 K-40, division ratio 1/1000) measures the
voltage applied to the inner electrode. The discharge
current is determined by recording the voltage signal
across a 100 Ω resistor placed in series between the
counter electrode and ground. The spherical surface
segments in the anode plate have a radius of curvature
of 17.5 mm and a depth of 5 mm. Stable and uniform
discharge operation is ensured by the gas flow and by
ballasting each cathode pin with a 1.5 MΩ resistor. The
fraction of the total electrical power that is dissipated in
these resistors amounts to 10 % at most.
Manganese oxide was purchased from Panreac.
Before its use in the catalytic test, it was calcinated for
4 hours at 500 °C under a stream of dry synthetic air
with a flow of 200 mL/min. For all tests, 1 g of MnO 2
was introduced in a glass reactor located in a vertical
tubular oven operating in the temperature range of 300500 °C.
3. Results and discussion
To investigate the performance of the combined
plasma-catalytic system, both parts are initially
considered separately, i.e. destruction of TCE through
the use of NTP and via catalytic oxidation. To evaluate
the process the following parameters are employed:
The TCE abatement is calculated from:
TCE abatement (%) =
TCE in - TCE out
x 100
[CO]100
x 100
)
(2 x [TCE]
3.1 TCE abatement using non-thermal plasma
The abatement of TCE with the plasma alone system
was investigated for a gas flow rate of 500 mL/min dry
air containing 500 ppm TCE. Fig. 2 shows the
abatement of TCE and the COx-selectivity for energy
densities ranging from 40 to 240 J/L.
Fig. 2: TCE abatement using non-thermal plasma.
As expected, an increase of the energy density in the
plasma led to an enhanced TCE abatement. For a
maximum energy density of 240 J/L the TCE
abatement reached 85 %. This conversion was however
related to the formation of undesirable chlorinated byproducts such as phosgene (COCl2) and dichloroacetyl
chloride (DCAC) [8]. Also, an increase of the energy
density gave a slight increase of the selectivity to COx,
reaching 18 % at 240 J/L. According to Kirkpatrick et
al. [9], DCAC is the major by-product of TCE
decomposition with NTP due to reaction of TCE with
ClO radicals (generated in situ by decomposition of the
same VOC in the plasma) according to the following
reaction:
C2HCl3 + ClO → CHCl2COCl + Cl
(1)
TCE in
where [TCE]in is the concentration introduced in the
reactor and [TCE]out is the concentration in the effluent
gas.
The selectivity’s of CO, CO2 and COx are defined as:
SCO (%) =
where [CO] and [CO2] are the concentrations of carbon
monoxide and carbon dioxide detected in the effluent
gas as a result of TCE oxidation and [TCE]conv is the
concentration of TCE converted by the plasma.
3.2 TCE abatement using MnO2
As the performance of a catalyst depends on the
operating temperature, we chose a wide range of
temperatures (300-500 °C) in order to monitor both the
TCE abatement and the selectivity to COx (Fig. 3).
(2)
conv
[CO 2 ]
SCO (%) =
(2 x [TCE]
2
conv
SCO (%) = SCO + S CO
x
x 100
)
2
(3)
(4)
Fig. 3: TCE abatement using MnO2.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
As shown in Fig. 3, a higher catalyst temperature
favors the TCE abatement, reaching a maximum of
42.6 % at 500 °C. Also, the COx-selectivity
significantly increased when comparing both the NTP
and catalytic reactor, reaching 98 % at higher catalyst
temperature, thereby minimizing the formation of
unwanted chlorinated byproducts. This result is quite
expected considering that MnO2 can induce the
following total oxidation reactions for TCE [10]:
C2HCl3 + 2 O2 → 2 CO2 + HCl + Cl2
C2HCl3 + O2 → 2 CO + HCl + Cl2
The catalytic activity of the oxide is related to several
factors. One of the most important factors is related to
the reaction temperature (catalytic furnace temperature)
and the oxygen mobility in the crystal lattice of the
solid, i.e. the presence of a large number of crystal
defects [11].
3.3 TCE abatement using the plasma-catalytic
combined system
In order to improve the energy efficiency of the entire
system, we investigated the plasma-catalytic abatement
of TCE using the same temperature range (300-500 °C)
but applying low energy density (40 and 80 J/L).
The TCE abatement increased with respect to the
catalyst temperature, obtaining the best value (78 %)
for an energy density of 80 J/L and maximum
temperature of 500 °C. For these conditions, the CO2selectivity reached 40 % while the formation of
unwanted chlorinated byproducts was greatly
suppressed. Therefore, we can conclude that the
plasma-catalytic system is an effective method for the
abatement of TCE.
Our previous results of the combined system, prove
that the use of plasma has a beneficial effect on the
performance of the catalyst. In order to examine if the
plasma provides an extra activation of the catalyst,
Arrhenius plots were used to calculate the activation
energy (Ea) of the three systems (Fig. 4).
total conversion. The calculated activation energy of
8.67 kcal/mol is in agreement with earlier results
obtained for catalytic VOC oxidation [12]. However, in
case of plasma-catalysis, there is only a small
temperature dependence on the total conversion. From
these results it seems clear that the plasma participates
in the transformation of TCE. Therefore, it is
reasonable to suggest that there is a positive interaction
of intermediate species with the catalyst surface.
Although the mechanism is not yet elucidated, it is
reasonable to consider that the oxygenated
intermediates produced by the plasma (phosgene,
DCAC) reach the catalyst surface. Because these
molecules are more susceptible to catalytic oxidation
by MnO2, the activation energy is significantly reduced
when compared to the catalyst alone system.
Previous research has shown that the combination of
NTP with heterogeneous catalysis often induces a
synergetic effect on the removal efficiency of the entire
process [13]. To evaluate the synergy in our process,
we introduce a synergy factor for the TCE abatement,
as well as for CO and CO2, which are calculated as
followed:
(TCE abatament) plasma-catalysis
fT CE =
[(TCE abatament) plasma + (TCE abatament)
fCO =
catalysis]
(YCO)plasma-cat alysis
(6)
[(Y CO)plasma + (Y CO)catalysis]
(YCO ) plasma-catalysis
fCO =
2
2
(7)
[(Y CO ) plasma + (Y CO ) catalysis ]
2
(5)
2
and where
and
are the yields to CO and CO2,
respectively, which are defined as:
YCO (%) =
YCO (%) =
2
[CO] out
2 x [TCE]
[CO2] out
2 x [TCE]
x 100
(8)
x 100
(9)
in
in
Thus, the synergy factor gives the relation of the
studied parameter for plasma-catalysis with respect to
the sum of its individual values for plasma and catalyst
alone conditions. If this value exceeds 1, a synergetic
effect is observed.
Table 1: Synergy factors for TCE abatement
using plasma-catalysis.
Fig. 4: Arrhenius plot to calculate the activation energy.
For the case of MnO2 without plasma, a large
temperature dependence is observed with respect to the
Temp
(°C)
300
350
400
450
500
40 J/L
80 J/L
40 J/L
80 J/L
40 J/L
80 J/L
0,84
0,99
0,92
0,89
0,83
1,01
0,97
0,91
0,87
0,78
8
5,9
3,29
2,26
1,53
6,47
5,2
3,56
2,63
1,72
3,72
3,57
1,96
1,56
1,29
4,22
4,06
2,28
1,78
1,44
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Table 1 illustrates that the synergy factor for TCE
abatement is in the range of 0.78 – 1, which indicates
that the plasma-catalytic system does not offer any
synergy with respect to the TCE abatement. Moreover,
this factor decreases with increasing catalyst
temperature. This is understandable because at higher
temperatures, the catalyst efficiency for VOC removal
increases. In contrast, the synergy factors for the yields
to CO and CO2 range from 1.3 to 8, indicating that the
plasma-catalytic system greatly improves the
selectivity of the process towards total oxidation.
4. Conclusions
In this study the abatement of trichloroethylene
(TCE) with a plasma-catalysis combined system using
MnO2 as a catalyst was investigated and compared with
the plasma and catalyst alone systems.
Our results prove that the combined system greatly
improve the selectivity of the process towards total
oxidation. This can be attributed to the fact that the
plasma provides extra activation of the catalyst. The
plasma converts TCE to a mixture of oxygenated
intermediates and final oxidation products. The ozone
produced by the plasma dissociates on the catalyst
surface to form peroxide groups. These surface species
promote the complete oxidation of TCE.
5. References
[1] C. M. Nunez, G.H. Ramsey, W.H. Ponder, J.H.
Abbott, L.E. Hamel, P.H. Kariher, Air and Waste,
43 (1993).
[2] D. Evans, L. Rosocha, G. Anderson, J. Coogan,
M. Kushner, Journal of Applied Physics, 74
(1993).
[3] A. M. Vandenbroucke, R. Morent, N. De Geyter,
C Leys, Journal of Hazardous Materials, 195
(2011).
[4] M. A. Peluso, L.A. Gambaro, E. Pronsato, D.
Gazzoli, H.J. Thomas, J.E. Sambeth, Catalysis
Today, 133 (2008).
[5] P. Ruetschi, Journal of the Electrochemical
Society, 131 (1984).
[6] C. Lahousse, A. Bernier, P. Grange, B. Delmon,
P. Papaefthimiou, T. Ioannides, X. Verykios,
Journal of Catalysis, 178 (1998).
[7] Y. Akishev, O. Goossens, T. Callebaut, C. Leys,
A. Napartovich, N. Trushkin, Journal of Physics
D: Applied Physics, 34, (2001).
[8] A. M. Vandenbroucke, M.T. Nguyen, Dinh, J.M.
Giraudon, R. Morent, N. De Geyter, J.F.
Lamonier, C. Leys, Plasma Chemistry and Plasma
Processing, 31 (2011).
[9] M. J. Kirkpatrick, W.C. Finney, B.R. Locke,
Plasmas and Polymers, 8 (2003).
[10] E. Finocchio, G. Sapienza, M. Baldi, G. Busca
Applied Catalysis B-Environmental, 51
(2004).
[11] V. P. Santos, M.F. Pereira, J. Orfao, J.L.
Figueiredo, Applied Catalysis B-Environmental,
99 (2010).
[12] C. M. Su, R. W. Puls, Environmental Science &
Technology, 33 (1999).
[13] J. Van Durme, J. Dewulf, C. Leys, H. Van
Langenhove,
Applied
Catalysis
B:
Environmental, 78 (2008).