Electrical and Optical Analysis on the Interaction of Nonthermal Plasma
and Catalyst.
Hyun-Ha Kim and Atsushi Ogata
National Institute of Advanced Industrial Science and Technology (AIST),
16-1 Onogawa Tsukuba 305-8569, Japan
Abstract: This work presents interactions of atmospheric-pressure nonthermal
plasma with catalyst. Surface streamers were found to be largely effected by the
presence of metal nanoparticles on the surface of catalyst. This physical influence
of surface plasma expansion was also found to be related with the chemical
property (i.e. catalytic activity) of catalyst. Two different modes of discharge
were observed in the catalyst-packed plasma reactor.
Keywords: Nonthermal Plasma, Catalyst, Plasma-Driven Catalysis (PDC), ICCD
1. Introduction
Pollution control using nonthermal plasma is being
considered from gas, liquid to soil and bio
contamination. Decomposition of VOCs using
atmospheric-pressure nonthermal plasma (NTP)
technology has been the subject of extensive studies
since the early 1990s. Recently, advanced plasma
system, combining NTP with catalyst, offers various
advantages in terms of energy efficiency, product
selectivity and carbon balance [1-6]. The authors
have been studied the single-stage PDC system and
reported a strong influence of oxygen partial
pressure on the performance [2]. The peculiar
behavior has adopted to the cycled system
comprised of adsorption step with plasma and the
decomposition of adsorbed VOC using an oxygen
plasma. The major advantages of the cycled system
include high energy efficiency, high CO2 selectivity,
flexibility to the operational conditions, and free
from the NOx formation. It should be noted that this
unique dependence on the O2 partial pressure can be
observed only with the PDC system.
Understanding of the physical interaction between
nonthermal plasma and catalyst is important for the
further optimization of the system. Recently, the
important physical interaction between nonthermal
plasma and metal nanoparticles supported on various
zeolites (such as MS-13X, mordenite, HY) has been
reported. These nanoparticles assisted plasma to be
spread over wide area [2, 7]. There are a number of
publications on the synergy of the combined process
[8-12]. However, fundamental information on the
interaction of NTP with catalyst is still lack and need
further studies.
In this work, physical interaction of NTP with the
active metal nanoparticles supported zeolites was
investigated both from chemical and physical
aspects. Two important parameters used for the
evaluation are adsorption capacity of VOC and the
enhancement factor. Optical microscope-ICCD
camera system was used for the direct observation of
discharge plasma on the surface of catalysts.
Especially we focused on the interaction between the
supported metal catalyst and the expansion of
plasma (surface streamer) over the zeolites. The
correlation between the catalytic activity and the
physical characteristics in plasma generation on the
surface of catalyst will be also discussed.
2. Experimental
2.1 Optical and electrical measurement
For the plasma observation purpose in open air, a
plane type dielectric barrier discharge (DBD) reactor
was used. Various catalysts were packed in the DBD
plasma reactor. An aluminum tape (60 mm × 50
mm) was attached to the outer sides of the glass
plates as electrodes. The upper part was covered by a
quartz plate, which is mounted just below the
microscope. Nitrogen gas was fed to the reactor with
a flow rate of 1 Lmin-1. A neon transformer (50 Hz)
was used as power supply.
Adsorption mode ( Plasma OFF)
Adsorbent/Catalyst
Clean gas
VOCs
Cycled operation
CO2, H2O
O2
O2 Plasma
Regeneration mode （Plasma ON）
Figure 1. ICCD camera setup for the observation of discharge
plasma on the PDC reactor.
The microscopic observation system consists of
an XY stage, optical microscope, and an intensified
charge coupled device (ICCD) camera (Hamamatsu
Photonics). The DBD reactor was set on a XY stage.
The DBD reactor was energized with a neon
transformer, capable of delivering up to 20 kVmax at
commercial frequency of 50 Hz. The observation
area can be adjusted by changing the optical lens.
For example, the observation area of the 5X lens was
2.0 mm×2.6 mm, which corresponds to the size of
single pellet. This observation system provides rapid
and simple means of visualizing the interaction of
plasma with catalyst. The images were recorded with
a HiPic software (Ver 8.1).
2.2 Catalyst for cycled system
Figure 2 shows the diagram of the cycled system.
Zeolites were in pellet type having 1.6 mm in
diameter and 2~3 mm in length. In those
experiments to screen proper catalyst for the cycled
system, a flow-type PDC reactor was used with
changing oxygen contents in the gas stream. The
potential of the catalysts for the cycled system were
evaluated by the adsorption capacity of VOC and the
catalytic activity. We focused on zeolites having
large surface area, which are advantageous to the
adsorption of VOC.
Fig. 2. Schematic diagram of the cycled system. Plasma was
turned on only in the regeneration mode under oxygen
environment.
An enhancement factor (EF) was used to evaluate
the catalytic activity at higher oxygen content [2, 8].
The EF is defined as the ratio of decomposition
efficiency of VOC with respect to the O2 partial
pressure at the same SIE.
The specific surface areas of zeolites tested in this
work were in the order of HSY ((high-silica Y
zeolite, 690 m2/g), MS-13X (540 m2/g), HY (520
m2/g) and MOR (380 m2/g). The HSY zeolite is
known to have hydrophobic nature, which provides
stable property at humid condition. Active metal
components were supported on the zeolites by
impregnation method. The precursors of the metal
catalysts are as follows; Ag(NO3), ZrO(NO3)2xH2O,
Cu(NO3) 23H2O. The size and the shape of the
loaded metals were measured by transmission
electron microscopy (TEM, Topcon Co., Model
EM002B).
Discharge power dissipated in the reactor was
measured by the automated V-Q Lissajous program
(Insight Co, Ver. 1.72) [3]. The waveforms of the
charge (i.e. integrated discharge current) and the
applied voltage were monitored with a digital
oscilloscope (Tektronix, TDS3032B). Plasma was
applied to the reactor after the zeolites reached
adsorption equilibrium. The concentration of
benzene and the reaction products were monitored at
1 min interval using a FTIR spectrometer
(PerkinElmer, Spectrum One) equipped with a long
optical path-length gas cell.
3. Results and Discussion
Figure 2 shows three random ICCD camera images
of the discharge plasma on the surface of Ag/HSY
zeolite. Exposure time was set at 4 ms.
Luminescence of the discharge increased with the
applied voltage. Discharge power was in the range
of 0.36~1.2 W for the tested applied voltages in this
study. At 16 kV, Fig. 2(b), discharge was observed
vicinity of contact points of zeolite pellets, which is
also referred to as partial discharge. As increasing
the applied voltage, plasma was observed not only at
the contact points (partial discharge) but also on the
surface of the Ag/HSY zeolite. The channel size of
surface streamer, estimated for the ICCD camera
images, ranged from 150 to 180 m. These values
are quite close to those with the gas-phase streamers
in DBD reactor (100-200 m) [7, 13]. The presence
of oxygen decreased the luminescence of surface
discharge plasma (data not given), which is
consistent with the work reported by Hensel et al
[14]. The area of discharge plasma was also
increased with the loading amount of silver. The
influence of metal nanoparticles on the plasma
generation was also supported by the voltage-current
characteristics [7].
Figure 3 shows the discharge images taken by the
ICCD and digital cameras for the BaTiO3-pellet (ɛs =
10,000) packed plasma reactor, which has been used
over the years for VOC removal [15, 16]. Unlike the
zeolites packed cases, surface streamers were not
observed in the BaTiO3-packed reactor. Discharge
plasma was first appeared with several plasma spots
at the contact points of BaTiO3 beads. The number
and size of plasma spots became large as the applied
voltage increased. However, plasma did not
propagate on the surface of BaTiO3 beads. Arai et al
indicated that the plasma area with the BaTiO3 beads
with low dielectric constant produces become larger
compared to those with larger dielectric constant
[17].
Figure 2. ICCD camera images of discharge plasma packed with
Ag/HSY catalyst (in N2 flow). (b) 16 kV, (c) 19 kV (d) 21 kV
(all at 50 Hz).
Figure 3. Plasma images of BaTiO3 pellet-packed plasma
reactor; (a)-(d) ICCD camera, (e), (f) digital camera.
Photos with long exposure time (15 sec) ,(f), was
consistent with the ICCD camera data with short
exposure time (40 ms). Data on EF values are not
given in this work, the plasma generation on the
surface of packing materials is highly related with
the catalytic activity toward VOC oxidation [18].
Among tested active metals, silver showed the most
prominent influence on the plasma generation. The
expansion of plasma area was also influenced by the
loading amount.
4. Summary
The physical interaction of nonthermal plasma and
catalyst has been studied using optical lens and
ICCD camera system. The ICCD camera
observation of the discharge plasma on the surface
of catalyst provided an important insight into the
understanding of discharge plasma and catalyst. A
positive correlation was found between the plasma
generation pattern and the catalytic activity in the
combined process. The area of discharge plasma
expanded over a wide range by the metal
nanoparticles. Two different modes of discharge
plasma were observed according to the applied
voltage.
Acknowledgment
This works has been supported partly by Industrial
Technology Research Grant Program (07A21202a)
from New Energy and Industrial Technology
Development Organization (NEDO), and an
Environmental Technology Development Fund from
Ministry of the Environment of Japan.
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