st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Plasma catalysis: integration of a photocatalytic coating in a corona discharge
unit.
1
K. Van Wesenbeeck1, B. Hauchecorne1, S. Lenaerts1
Research group of Sustainable Energy and Air Purification, Department of Bio-Science Engineering, University of
Antwerp, Belgium
Abstract: The use of a corona discharge in an electrostatic precipitator offers an unique way
to remove pollutants from indoor air. There are however, still some disadvantages, like the
formation of by-products and the occurrence of irreversible deposition on the collector electrode, resulting in a declined removal efficiency. Applying a photocatalytic coating on the
collector electrode, to obtain plasma catalysis, can resolve the disadvantages.
Keywords: Corona discharge, Photocatalysis, Titanium dioxide, Plasma catalysis
1. Introduction
The environmental issue is recognised as an important
problem, both nationally and worldwide. Nowadays air
pollution, both indoors and outdoors, is a serious problem
for human health as well as for the environment in general.
Numerous studies report the occurrence of surprisingly
high amounts of pollutants in enclosed environments
[1,2,3]. These studies concluded that the indoor air pollutant concentrations are often 2 to 5 times higher than
outdoor levels due to a combined effect of insufficient air
exchange and high levels of indoor emission sources [4,5].
This forms a significant health risk to the inhabitants. Although people spend the largest fraction of their time
(85%) indoor and despite the fact that worldwide 1.5 million people per year die due to the inhalation of indoor air
pollutants, poor indoor air quality is still an underestimated problem [6,3]. It is thus clear that it forms a significant health risk and efforts have to be made to improve the indoor air quality (IAQ).
These efforts can be identified in 3 different categories:
(1) controlling the emission of pollutants from indoor
sources through the selection of low-emitting materials,
(2) diluting pollutants via (natural) ventilating indoor
spaces and/or (3) removing harmful pollutants from the
air. Each of these techniques has its specific drawback, so
it is not sufficient to use only one single method. Better
would be to combine different methods in order to
achieve a better IAQ. Traditionally, effective infiltration
and natural ventilation have been used to affect a controlled exchange of indoor air in order to abate bad IAQ.
However, in the past few years, energy efficiency considerations have resulted in a incline in the gas tightness of
buildings adversely impacting infiltration and discourages
the use of natural ventilation [6]. As a result, infiltration
and ventilation do not improve indoor air quality sufficiently. It is thus clear that more innovative methods
should be taken into consideration. One of the best op-
tions to improve the IAQ, is to actually purify the air to
remove indoor air pollutants.
Many studies have shown that the incorporation of
non-thermal plasma (NTP), e.g. corona discharge, in an
electrostatic precipitator (ESP) offers a unique way to
induce gas phase reactions and remove pollutants, but
there are, however, still some disadvantages [3,7]. Primarily, there is a incomplete oxidation with the formation
of harmful by-products, like ozone that is a powerful oxidans and which upon inhaling could react with the body’s
internal tissues. Secondly, the occurrence of irreversible
deposition on the collector surface which results in the
decline of removal efficiency. This irreversible deposition
can be avoided by improving the discharge mode, including the lay-out of the reactor, by decreasing the frequency
and the voltage of the power supply or by the combination
of the discharge unit with a catalyst [8].
The latter option will be used in this research, where a
photocatalyst is combined with the discharge unit by applying a photocatalytic coating on the collector electrode.
Concerning the combination with a photocatalytic coating
on the collector electrode, one can speak of catalysis assisted plasma, where the creation of a plasma is improved
by the photocatalyst, or of plasma assisted catalysis,
where the photocatalyst is activated by means of the light
produced from the plasma itself [9,10]. The combination
of both techniques implies several advantages. First of all,
the aforementioned irreversible deposition can be avoided,
which implicates that there is no longer a need for additional cleaning of the collector electrode because the
catalyst is able to remove the adsorbed species. Secondly,
the by-products of the corona discharge will be converted
into harmless products. It is thus clear that the use of
plasma catalysis has high potential to improve the NTP
process [9,3].
Consequently, the focus of this work lies on the imple-
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
mentation of an appropriate photocatalytic coating in an
corona discharge unit. Among semiconductor photocatalysts, TiO2 is the most studied one, due to its
photo-stability, strong oxidising power, non-toxicity,
chemical and biological inertness, stability, as well as the
low cost [11]. Balasubramanian [12,13] developed a TiO2
photocatalytic film on stainless steel using a
P25-powder-modified-sol-gel method (PPMSGM). By
using this method, enhanced photocatalytic activity and
adhesion is achieved compared with conventional sol-gel
procedures. In our previous work [14,15], this coating is
optimised with respect to TTIP:P25 molar ratio while a
good adhesion to a metal substrate, a low resistivity and a
good photocatalytic activity in the gas phase are achieved.
We concluded that a P25-powder-modified-sol-gel with a
TTIP:P25 molar ratio of 1 has promising properties for a
sustainable application in air purification. As a result, the
tests in this study are performed on the coating with a
TTIP:P25 molar ratio of 1.
The polluted gas flow (100 ppmv ethylene; 2000 cm³
min-1) was controlled by four mass flow controllers (MFC,
MKS instruments) and consisted of ethylene (1% ethylene
in N2, Air Liquide), O2 (Air Liquide) and N2. The latter
could be moisturised by guiding the flow through a gas
wash bottle filled with water, as shown in Figure 1. It was
always ensured that the oxygen concentration was 21% in
order to mimic the indoor air conditions best.
Each of the performed experiments is performed in 5
phases as described in our earlier work [16,17].
2.2. The TiO2 photocatalytic film
The
standard
procedure
for
preparing
the
P25-based-powder-modified-sol-gel is similar to the
method previously published by our group [14,15]. For
this, commercial titanium isopropoxide (TTIP, 97 %, Aldrich), isopropanol (i-PrOH, Sigma- Aldrich), diethanolamine (DEA, Sigma-Aldrich) and Aeroxide TiO2 P25
(Evonik) were used.
2. Materials and methods
2.1 The plasma reactor
A schematic diagram of the experimental setup is shown
in Figure 1. The configuration of the plasma reactor,
which is an ESP based on corona discharge, is a conventional wire-to-cylinder type with a wire electrode (SS 316,
7 mm diameter and 140 mm long) and an outer cylinder
electrode (SS 316, 80 mm diameter and 150 mm long).
On the discharge electrode, a set of pin pairs (galvanized
steel, 1 mm diameter and 15 mm long) was equally distributed over the wire. A high DC voltage supply
(PHYWE systeme GMBH, type 13671.93) was used in
the experiments.
The uncoated SS 316 cylinder was pretreated with ethanol
(96%, Royal Nedalco) after which it was dried at 105 °C
for 24 h prior to coating. Afterwards, 15 mL of the sol
was applied on the inner wall by unrolling the cylindrical
electrode to a flat surface so that a homogeneous coating
is obtained. Thereafter, the electrode was vertically hung
up in order to let the excess of sol run off the wall. After
this step, the cylinder was dried for 24 h at room temperature. Subsequently, the coated substrate was heated in
air with a gradient of 3 °C min-1 until a temperature of
100 °C was reached. This temperature was held for 1 h.
Afterwards, the temperature was further increased with
3 °C min-1 until 500 °C was reached. The temperature was
again kept for 1 h. Finally, the coating was cooled to room
temperature by natural convection. The complete cooling
process took approximately 12 h. As a result, a deposition
of 0.45 mg cm-2 was obtained on the electrode.
3. Results
In our previous work an optimal window of operation
for our plasma reactor is determined by varying several
characteristics, namely polarity, applied voltage, relative
humidity and reactor configuration [17]. The conversion
of NO is used as case study to confirm the activity of the
plasma in the gas phase. By combining the conclusions of
each parameter, it was possible to define the optimal
window of operation of the plasma reactor for the mineralisation of pollutants.
Figure 1: Schematic diagram of the experimental set-up. The
dashed line represents the bypass.
To recapitulate, a negative corona generally gives
higher conversion efficiencies compared to positive co-
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
rona. Secondly, it became clear that with a higher applied
voltage, the conversion efficiency increases. Thus, working with a negative polarity and a voltage higher than 15
kV is preferable. In the experiment with ethylene, the
highest conversion efficiency was obtained by working
with 20 kV. A third conclusion was that the influence of
the relative humidity was small. Therefore, the humidity
is varied by sending 0%, 12.5% and 25% of the N2 gas
stream through the gas wash bottle in order to obtain a
relative humidity of 0%, 10.4% and 20.3%, respectively.
The last parameter that was changed in our previous set of
experiments, was the configuration of the plasma reactor
and more specifically, the amount of pin pairs that are
attached to the discharge electrode. It could be concluded
that 10 pin pairs gives the highest conversion efficiencies.
This optimal window of operation is also used in the
final stage of the study, where the coating was applied on
the collector electrode of the plasma reactor. The risk of
implementing a coating on the collector electrode involves that the charged particles are not attracted to the
collector electrode anymore since the coating gives a loss
in conductivity of the electrode. It is thus required that the
coating does not have an adverse effect on the efficiency
of the corona discharge reactor.
In this study, we used ethylene to support our previous
results [17]. The conversion efficiency of ethylene (Figure
2) in the reactor was determined before and after applying
the coating when using the predetermined window of operation. This means 21% O2, negative corona and 20 kV.
The purpose of this research is to combine photocatalysis
and corona discharge in order to obtain a plasma catalytic
system as a sustainable and reliable indoor air purification
technology.
Therefore, an optimal window of operation is determined
by varying several parameters, namely polarity, applied
voltage, reactor configuration and relative humidity.
Combining the conclusions of each parameter leads to the
definition of an optimal window of operation for our
plasma reactor in order to achieve the mineralisation of
pollutants by plasma catalysis.
By applying a coating on the collector electrode of the
plasma reactor, it was possible to study the influence of
the coating on the performance of the plasma system
when operating in the selected optimal window of operation. It was thereby clear that the coating does not have a
detrimental effect on the plasma.
With this study, we have illustrated that the implementation of a photocatalytic coating within an corona discharge reactor, also referred to as plasma catalysis, has
high potential as an integrated and sustainable indoor air
purification technology. Further research about the implementation of the coating into an ESP is ongoing in
order to investigate a possible synergy between the
plasma and the photocatalytic activity of the coating.
5. Acknowledgments
The authors wish to thank the University of Antwerp for
supporting and funding this research. Tom Tytgat and
Hilde Vanderstappen are greatly acknowledged for their
help during the experiments.
Figure 2: A comparison of the conversion efficiency of ethylene
(%) by using corona discharge with uncoated and coated collector electrode by a negative polarity and a voltage of 20 kV. 21%
O2 is applied
As can be seen in Figure 2, there is no detrimental effect
on the conversion efficiency by applying the coating. This
means it does not inhibit the working of the discharge
corona itself.
4. Conclusion
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
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