22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasmacatalysis: a sustainable and efficient indoor air treatment
K. Van Wesenbeeck, B. Hauchecorne and S. Lenaerts
Research group of Sustainable Energy and Air Purification, Department of Bioscience Engineering, University of
Antwerp, 2020 Antwerpen, Belgium
Abstract: The detrimental impact of polluted indoor air requires the development of an
innovative air purification technology. Plasma catalysis based on corona discharge offers a
sustainable way to remove pollutants from indoor air. This way the disadvantages of the
plasma, as for instance the formation of by-products, are tackled. Plasma catalysis
achieved by applying a photocatalytic coating on the collector electrode is an innovative
and sustainable technology for air purification.
Keywords: corona discharge, photocatalysis, titanium dioxide, plasma catalysis
1.
Introduction
Indoor air quality control is recognised as an important
and challenging 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 [1-3]. These studies concluded
that the indoor air pollutant concentrations are often two
to five times higher than outdoor levels due to a combined
effect of insufficient air exchange and high levels of
indoor emission sources [4-5]. Although people spend
the largest fraction (85%) of their time indoor and despite
the fact that according to WHO 4.3 million people per
year die from the exposure to household air pollution,
poor indoor air quality is still an underestimated problem
[6]. It is thus clear that it forms a significant health risk
and efforts have to be made to improve indoor air quality.
In selecting the most effective air cleaning system, a
number of factors must be taken into account. They
include long-term performance, minimum energy
consumption and minimum amount of unwanted byproduct formation. Furthermore, the capability to work in
indoor conditions is also an important parameter [7]. The
most significant advantage of using a non-thermal plasma
(NTP) in ambient air for air cleaning is the production of
highly reactive oxidising radicals such as O◦, HO◦, and
O 3 . Such species are produced at room temperature and
at a low energy cost compared to any alternative method.
Unfortunately NTP used as stand-alone technology in an
air-cleaning process produces a high amount of byproducts that are sometimes more toxic than the initial
pollutants.
To overcome the deficiencies of the NTP
plasmacatalysis as a combined technology between
catalysis and plasma is proposed. In plasmacatalysis the
presence of a catalyst and the synergetic effect of plasma
and catalyst enhances the removal efficiency and total
oxidation of the components. This synergetic effect
results in a higher removal efficiency compared to the
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sum of plasma and catalyst processes, when they are used
separately [7].
Consequently, the focus of this work lies on the
implementation of an appropriate photocatalytic coating
in a corona discharge unit. Selecting an appropriate
photocatalyst is an important challenge for enhancing the
removal efficiency in a plasmacatalytic process. The
presence of the photocatalyst increases the probability of
surface reactions between the reactants and the reactive
species, which leads to more selective reactions and a
higher removal efficiency. Therefore, the catalyst surface
textural properties including specific surface area, pore
volume, pore size and size distribution, as well as particle
size and crystal phase have an important role on the
plasmacatalyst performance.
Among semiconductor
photocatalysts TiO 2 is the most studied one, due to its
photo-stability, strong oxidising power, non-toxicity,
chemical and biological inertness, stability, as well as its
low cost [8].
Balasubramanian
[9-10]
developed
a
TiO 2
photocatalytic film on stainless steel using a P25-powdermodified-sol-gel method (PPMSGM).
Enhanced
photocatalytic activity and adhesion are achieved in
comparison to conventional sol-gel procedures by using
this methodology. In our previous work [11-13], this
coating is optimised with respect to TTIP:P25, DEA:TTIP
and H 2 O:TTIP molar ratio while a good adhesion to a
metal substrate, a low resistivity and a good
photocatalytic activity in the gas phase are achieved. The
measurements in this study are performed on the coating
with the optimised molar ratios.
2. Material and methods
2.1. The plasma reactor
A schematic diagram of the experimental setup is
shown in Fig. 1. The configuration of the plasma reactor
based on an electrostatic precipitator (ESP) with corona
discharge, is a wire-to-cylinder type with an inner
electrode (SS 316, 140 mm long) and an outer cylinder
electrode (SS 316, 150 mm long). On the discharge
1
electrode, a set of pin pairs (galvanized steel, 1 mm
diameter and 15 mm long) is equally distributed. The gap
between the pin pairs and the outer electrode is equal to
37 mm. In the centre of the inner electrode an UV-lamp
(365 nm) can be placed. A high DC voltage supply
(PHYWE systeme GMBH, type 13671.93) is used in the
experiments.
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.
Fig. 1. Schematic diagram of the experimental set-up.
The dashed line represents the bypass.
The polluted gas flow (2000 cm3 min-1 , 250 ppmv
ethylene) is controlled by four mass flow controllers
(MFC, MKS instruments) and consists of ethylene
(1% ethylene in N 2 , Air Liquide), O 2 (Air Liquide) and
N 2 . The latter can be moisturised by guiding the flow
through a gas wash bottle filled with water, as shown in
Fig. 1. It is always ensured that the oxygen concentration
was 21% in order to mimic the indoor air conditions best.
To have a look at the synergetic effect of the
plasmacatalytic system, the experiments are performed
both with and without coating applied on the collector
electrode.
Each experiment consists of three different steps, all
performed with the high voltage supply turned on. The
first step is in absence of an external UV source. In the
second and the third step an external UV lamp is placed
inside the reactor. In the second step the lamp is turned
on, while in the third step the lamp is turned off.
2.2. The TiO 2 photocatalytic film
The standard procedure for preparing the P25-basedpowder-modified-sol-gel is similar to the method
previously published by our group [12, 13]. For this,
commercial titanium isopropoxide (TTIP, 97 %, Sigma),
isopropanol (i-PrOH, Sigma-Aldrich), diethanolamine
(DEA, Sigma-Aldrich) and Aeroxide TiO 2 P25 (Evonik)
were used.
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
2
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 [13]. To recapitulate,
a negative corona generally gives higher conversion
efficiencies compared to positive corona. 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. A third conclusion was that the influence of
the relative humidity was small. 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 give the
highest conversion efficiency.
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. In this
study, we used ethylene to support our previous results
[13]. The conversion efficiency of ethylene, the CO 2
formation and the formation of ozone in the reactor were
determined before and after applying the coating when
using the predetermined window of operation: 21% O 2 ,
negative corona and 15 kV.
Fig. 2 illustrates that the coating does not have an
adverse effect on the efficiency of the corona discharge
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. As
can be seen on the graph the efficiencies of ethylene
conversion stay almost the same in the 3 steps of the
experiment.
On the other hand, in Fig. 3 it can be seen that when the
external UV lamp is turned on, a higher CO 2 formation is
obtained when a coating is applied. Without the presence
of a coating the effect of the UV lamp is negligible. Since
the ethylene conversion stays more or less the same in
these circumstances (Fig. 2), it can be concluded that
there is more mineralisation of intermediates of ethylene.
Fig. 4 shows that the formation of ozone decreases
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drastically when the UV lamp is turned on while a coating
is applied on the collector electrode. At the same time,
the ethylene conversion is not affected by the UV-lamp,
as shown in Fig. 2.
So, the presence of the coating and the UV lamp
significantly decreases the formation of ozone and at the
same time it promotes the mineralisation of intermediates
of ethylene.
Fig. 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 15 kV. 21% O 2 is applied.
Fig. 3. A comparison of the CO 2 formation (a.u.) by
using corona discharge with uncoated and coated
collector electrode by a negative polarity and a voltage of
15 kV. 21% O 2 is applied.
Fig. 4. A comparison of the formation of ozone (a.u.) by
using corona discharge with uncoated and coated
collector electrode by a negative polarity and a voltage of
15 kV. 21% O 2 is applied.
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4. Conclusion
The purpose of this research is to combine
photocatalysis and corona discharge in order to obtain a
plasmacatalytic system as a sustainable and reliable
indoor air purification technology.
By applying a coating on the collector electrode of the
plasma reactor we studied the influence of the coating on
the performance of the plasma system when operating in
the selected optimal window of operation from previous
research. It was thereby clear that the coating does not
have a detrimental effect on the plasma. Even more,
when a coating is applied on the collector electrode and
when a UV lamp is turned on; more CO 2 and less ozone
formation is observed.
We have illustrated that the implementation of a
photocatalytic coating within a corona discharge reactor,
also referred to as plasmacatalysis, 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.
6. References
[1] E. Rehfuess, C. Corvalan and M. Neira. Bull. World
Health Organ., 84, 508 (2006)
]2] M. Stranger, S.S. Potgieter-Vermaak and
R. Van Grieken.
Sci. Total Environ., 407,
1182-1192 (2009)
[3] J. Van Durme. (Ghent: Ghent University) (2008)
[4] B. Kolarik, P. Wargocki, A. Skorek-Osikowska and
A. Wisthaler. Build. Environ., 45, 1434-1440 ( )
[5] EPA and the C.P.S. Commission. The Inside Story:
A Guide to Indoor Air Quality. (2011)
[6] WHO. Fact Sheet n° 292 (2014)
[7] M. Bahri and F. Haghighat. CLEAN - Soil, Air,
Water, 42, (2014)
[8] H. Fujishima, Y. Ueda, K. Tomimatsu and
T. Yamamoto. J. Electrostat., 62, 291-308 ( )
[9] G. Balasubramanian, D.D. Dionysiou, M.T. Suidan,
Y. Subramanian, I. Baudin and J.M. Laine. J. Mat.
Sci., 38, 823-831 (2003)
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[10] G. Balasubramanian, D.D. Dionysiou, M.T. Suidan,
I. Baudin, B. Audin and J.M. Laine. Appl. Catal. B:
Environ., 47, 73-84 (2004)
[11] A. Van der Maat, K. Demeestere, S. Lenaerts,
H. Van Langenhove and K. Van Wesenbeeck.
MSc-thesis (Antwerp, belgium: University of
Antwerp) (2011)
[12] K. Van Wesenbeeck, B. Hauchecorne and
S. Lenaerts. Commun. Agric. Appl. Biol. Sci., 78,
16-24 (2013)
[13] K. Van Wesenbeeck, B. Hauchecorne and
S. Lenaerts. J. Environ. Solut., 2, 16-24 (2013)
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