22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Abatement of trichloroethylene by combined use of non-thermal plasma and
CeO 2
S. Sultana1, A.M. Vandenbroucke1, M. Mora2, C. Jiménez-Sanchidrián2, F.J. Romero-Salguero2, C. Leys1, N. de Geyter1
and 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 Cordoba, Spain
Abstract: This study is devoted to investigate the opportunities of a plasma-catalytic
system with CeO 2 downstream (i.e., PPC-Post Plasma-catalysis) for the abatement of
trichloroethylene from dry air. A multi-pin-to-plate negative DC corona/glow discharge is
used and showed poor CO x selectivity despite having high abatement efficiency, when
operated alone. Nonetheless, NTP enables catalyst activation at lower temperature. As a
result, complete suppression of unwanted chlorinated by-products as well as high CO x
selectivity at lower energy cost have been achieved.
Keywords: non-thermal plasma, plasma-catalysis, trichloroethylene, cerium oxide
1. Introduction
Air quality issues have become a huge concern of
environmental legislation as a consequence of growing
awareness in our global world. Air pollution is a mixture
of natural and man-made substances in the air because of
industrialization and transportation, which has
repercussions on public’s health and on phenomena like
acidification and eutrophication of the environment.
Among these harmful pollutants, volatile organic
compounds (VOCs) are the cardinal candidates which
cause severe damage on human health due to their
potential toxicity, carcinogenicity and mutagenicity [1].
Furthermore, they are also responsible for odour nuisance,
the creation of tropospheric ozone leading to
photochemical smog, the intensification of global
warming and the depletion of stratospheric ozone layer
[2].
Non-thermal plasma (NTP) technology has attracted
growing interest of scientists over last two decades due to
their distinctive characteristic of being able to provide a
highly chemical reactive environment (e-, O*, HO 2 *,
OH*, N 2 *, O 3 , ....) to decompose VOC at ambient
condition which repudiates the use of expensive vacuum
system [3, 4]. Although other commercial pollution
control techniques (thermal incineration, catalytic
oxidation or adsorption) are very efficient in VOC’s
oxidation, these are energetically expensive and difficult
to operate in case of moderate flow rates with low VOC
concentration. In NTP, energetic electrons (1 - 10 eV) are
produced consuming almost all the electric energy
supplied to the system instead of heating entire gas unlike
thermal and catalytic oxidation. Collision of these highly
energetic electrons with neutral background molecules
close to room temperature, generate active species such as
free radicals, metastables, ions and secondary electrons
through different chemical processes such as dissociation,
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excitation, and ionization. These active species are able
to decompose pollutant molecules to less harmful
Additionally, the
products (CO 2 , H 2 O, HX, X 2 ).
abatement of low concentrated VOC’s (up to 1000 ppm),
feasible to indoor air treatment application, is challenging
for conventional methods because when VOC’s
concentration decreases, the cost per unit pollutant
treatment becomes higher in comparison to NTP.
Unfortunately, industrial implementation of NTP for
VOC abatement is impaired by three main bottlenecks
such as poor product selectivity, formation of undesired
by-products that often increase the overall toxicity of the
treated gas stream and low energy efficiency. In order to
overcome these limitations many attempts have been
made and engendered the development of a hybrid system
using multiple techniques. During the last decade
researchers have persistently been investigating a novel
technique which combines the advantage of rapid ignition
from NTP and high selectivity from catalyst, leading to
increased energy efficiency and supressed unwanted
by-products distribution for VOC decomposition. In such
a hybrid system, the catalyst can be integrated either
inside (IPC-Inside Plasma Catalysis) [5] or downstream
(PPC-Post Plasma Catalysis) [6, 7] of discharge region, in
both cases a synergetic effect has been reported in many
studies [8-10].
The objective of this current work is to abate
trichloroethylene (TCE) from dry air by combined use of
multi-pin-to-plate negative DC corona/glow discharge
with CeO 2 catalyst downstream. In order to minimize the
energy cost of the abatement, combination of low energy
density plasma and moderate catalyst temperature have
been examined.
1
2. Experimental
The experimental set-up was described in detail
elsewhere [11] (Fig. 1). In brief, the plasma source
consists of 10 aligned cathode pins (separated by 28 mm)
and a single anode plate as counter electrode forming a
rectangular duct (40 mm x 9 mm cross section and
400 mm length).
The inter electrode gap in this
configuration is 10 mm. A DC power supply (Technix,
SR40-R-1200) is used to generate glow discharge at
ambient conditions. The discharge voltage and current
were varied between 8.0 - 10.5 kV and 0.04 - 0.20 mA,
respectively.
𝑆𝐶𝐶𝑥 (%) = 𝑆𝐶𝐶 + 𝑆𝐶𝐶2
(4)
where [TCE] in and [TCE] out are influent and effluent
concentration in the gas stream. [CO] and [CO 2 ] are the
concentration of carbon monoxide and carbon dioxide
detected in the effluent gas and [TCE] conv is the
concentration of TCE converted by the plasma.
3.1. TCE abatement with NTP
Fig. 2 shows the TCE abatement and CO and CO 2
selectivity as a function of the energy density. As
expected, with increasing energy density the abatement of
TCE increases. Since higher energy density leads to
higher energetic electrons which trigger the formation of
radicals capable of decomposing TCE. For an energy
density of 240 J/L the abatement and CO x selectivity
reach a maximum at 80 % and 12 %, respectively. This
low selectivity is related to the formation of unwanted and
toxic
by-products
such
as
phosgene
and
dichloroacetylchloride [6].
Fig. 1. Schematic diagram of the experimental set-up.
TCE concentration was controlled by changing feed gas
(dry synthetic air) flow rate through TCE bubbling bottle
by using mass flow controllers (MFC). A total air flow
rate of 500 mL/min containing 500 ppm TCE was
conveyed in all experiments. A FT-IR spectroscope
(Bruker, Vertex 70) and a mass spectrometer (Hiden
HPR-20) was used to identify and quantify by-products
and TCE abatement. Spectra were taken after steady state
condition and consisted of 10 averaged measurements. An
UV ozone detector (Envitec, model 450) was used to
monitor the formation of ozone.
Cerium oxide (commercial catalyst) was calcinated for
4 h at 500 °C under 200 mL/min flow rate of dry
synthetic air. For all tests, 0.5 g of CeO 2 powder was
introduced in a cylindrical Pyrex glass reactor located in a
temperature controlled vertical tubular oven.
The
temperature range was (100 - 500 °C) and (100 - 300 °C)
for catalyst alone test and plasma-catalysis combined test,
respectively. The oven was heated in a period of 60 min
to the desired value. After obtaining thermal balance the
measurements were performed.
3. Results and Discussion
Abatement of TCE is investigated with only NTP, only
heterogeneous catalyst and NTP-catalyst system (PPC).
The examined parameters, TCE abatement (%), CO
selectivity (𝑆𝐶𝐶 , %), CO 2 selectivity (𝑆𝐶𝐶2 , %) and CO x
selectivity (𝑆𝐶𝐶𝑥 , %) are used to evaluate the process and
are defined as follows:
𝑇𝑇𝑇 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 (%) =
𝑆𝐶𝐶 (%) =
𝑆𝐶𝐶2 (%) =
2
[𝐶𝐶]
2×[𝑇𝑇𝑇]𝑐𝑐𝑐𝑐
[𝐶𝐶2 ]
𝑇𝑇𝑇𝑖𝑖 −𝑇𝑇𝑇𝑜𝑜𝑜
× 100
2×[𝑇𝑇𝑇]𝑐𝑐𝑐𝑐
𝑇𝑇𝑇𝑖𝑖
× 100
× 100
(1)
(2)
Fig. 2. TCE abatement and CO and CO 2 selectivity as a
function of the energy density.
Table 1 shows the by-products of NTP treatment
detected with FT-IR and MS. DCAC, phosgene, CO and
CO 2 were observed with both techniques. Hence, the
decomposition of TCE with NTP led to the formation of
phosgene, DCAC and TCAA as incomplete oxidation
products. These results are identical with Vandenbroucke
et al. [6]. According to Kirkpatrick et al. [12], in TCE
oxidation with NTP produces significant amount of
DCAC by the following reaction:
C 2 HCl 3 + ClO* → CHCl 2 COCl + Cl*.
3.2. TCE abatement with CeO 2
CeO 2 has been chosen as catalyst due to its unique
properties, have higher oxygen storage/transport capacity
[13] combined with the ability to shift easily between
reduced and oxidized states (i.e., Ce3+ – Ce4+) which
results in an increase in oxygen vacancies [14, 15]. It is
also known that in association with other catalysts, ceria
(3)
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can effectively reduce NO x emissions as well as convert
harmful carbon monoxide to less harmful carbon dioxide.
Fig. 3 shows the TCE abatement, CO, CO 2 and CO x
selectivity as a function of the catalyst temperature with
CeO 2 . The catalyst activity was evaluated for the
temperature range of 100 - 500 °C.
The plasma-catalytic abatement seems insensitive with
the catalyst temperature. However, CO 2 selectivity
increases more rapidly than CO selectivity, inevitably a
large increase was observed for CO x selectivity (̴ 95%).
Table 1. Detected by-products with NTP.
By-product
Structure
FT-IR
MS
Unwanted
Dichloroacetylchloride
(DCAC)
CHCl 2 COCl
√
√
√
Trichloroacetaldehyde
CCl 3 COH
√
√
Phosgene
COCl 2
√
√
√√
Hydrogen chloride
HCl
√
Chlorine
Cl 2
Carbon monoxide
CO
√
√
Carbon dioxide
CO 2
√
√
Ozone
O3
√
√
√
√
Fig. 3 clearly shows that CeO 2 is not activate below 300
°C. Above this temperature the abatement increases and
reaches maximum value of 51.6% at 500 °C. For a
certain temperature range (300 – 400 °C), CO x selectivity
remains constant ( ̴ 100%) while further increase of the
temperature slightly decreases the selectivity. These
results show that high temperature is essential to achieve
moderate TCE abatement with high selectivity.
Fig. 4. TCE abatement, CO, CO 2 and CO x selectivity as
a function of the catalyst temperature for energy density
80 J/L.
Fig. 5 shows the FT-IR analysis of plasma-catalysis
combined system at energy density 40J/L with different
catalyst temperature (100 - 200 °C). It is evident that
with increasing catalyst temperature, the formation of
chlorinated by-products (DCAC, phosgene) and O 3 are
decreased . Simultaneously CO 2 selectivity increased
proving that plasma-catalysis is an effective method for
the complete oxidation of VOCs.
Fig. 3. TCE abatement, CO, CO 2 and CO x selectivity as
a function of the catalyst temperature.
Fig. 5. FT-IR analysis of plasma-catalysis combined
system at energy density 40J/L with different catalyst
temperature (100 - 200 °C).
3.3. TCE abatement with combined system
In order to reduce the energy cost of the process, we use
low energy density plasma in combination with moderate
catalyst temperature.
Therefore, we operated the
discharge at 40 J/L and 80 J/L and examined the process
for catalyst temperatures between 100 - 300 °C. Fig. 4
shows the TCE abatement, CO, CO 2 and CO x selectivity
as a function of catalyst temperature for energy density
(80 J/L).
4. Conclusions
The plasma-catalytic abatement of TCE showed a great
improvement of the COx selectivity compared to the
plasma alone system. By examining the effect of catalyst
temperature, we found that low energy density plasma in
combination with moderate catalyst temperature
successfully abated dilute TCE in air streams. Future
experiments will include a long term test (100 h) to test
the stability of CeO 2 followed by a full characterization
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of the catalyst to examine if the catalyst morphology has
changed and if catalyst poisoning has occurred.
5. Acknowledgements
M. Mora acknowledges funding from Junta de
Andalucia (FQM-6181), Ministry of Science and
Innovation (MAT 2010-18778) and Fondos Feder.
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