S16-4
Interaction of Plasma and Catalyst; Current Status and Road to Industrial Use
プラズマと触媒の相互作用の現状と実用化への課題
Hyun-Ha Kim, Yoshiyuki Teramoto, Nobuaki Negishi, and Atsushi Ogata
金賢夏, 寺本慶之，根岸信彰、尾形敦
National Institute of Advanced Industrial Science and Technology (AIST)
16-1 Onogawa, Tsukuba Ibaraki 305-8569, Japan産総研 〒305-8569 茨城県つくば市小野川16-1
Combination of NTP with catalyst is gathering attention due to its various synergistic effects. In thedecomposition of volatile organic compounds (VOCs), for example, the combined system can achieve deepoxidation of VOC to benign CO2 without forming unwanted byproducts (CO, NOx, and aerosols). We
report here some experimental evidence on the interaction of NTP with the active metal nanoparticles
supported catalysts using various measurement techniques, such as a microscope-ICCD camera system,
OES, chemical probe using isotope-labelled oxygen (18O2), and electron-spin resonance ESR and catalyst
characterization (BET surface area, TEM, ICP). Current status of the technology will be presented together
with the future challenges in this field.

1. General
The combination of nonthermal plasma (NTP)
and catalyst is gathering attentions in various
applications such as decomposition of volatileorganic compounds (VOCs), NOx and PM removal
in vehicles, and fuel reforming [1]. One of smart
ways to remove VOCs at moderate operating
conditions (room temperature and atmosphericpressure) is combined system of adsorption andoxygen plasma, which is referred to as cycledsystem. The how and why of the cycled system will
be presented. Adsorption and subsequent O2 plasma
oxidation can oxidize VOC into CO2 without
forming unwanted byproducts such as CO, NOx,and aerosols. It is necessary to investigate thephysical and chemical interactions of NTP andcatalyst which can provide important insights into
the understanding of fundamental aspects inplasma-catalyst process. These interactions werestudied using isotope oxygen (18O2), ICCD camera,ESR, XRD, XPS, and online FTIR.
We report here some recent progress inexperimental evidence for the better understandingof the interaction between NTP and catalyst.
2. Experimental
Plasma was generated using barrier-type reactor
consisting of coil electrode (corona electrode),quartz tube (barrier) and silver paste (groundelectrode). The plasma reactor was energized withan AC high voltage (Trek 20/20B). Discharge
power of the PDC reactor was measured using theautomated V-Q Lissajous program (Insight Co. ver1.71). A condenser (about 100 nF) was connected inseries to the ground line of the plasma reactor. Thecharge Q (i.e. time-integrated current) wasmeasured with a 10:1 voltage probe. Zeolite pellets(1.6 mm in diameter) were packed in the plasmazone, which is also referred to as single-stage
plasma-catalyst reactor. The specific surface areasof 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. Various metal nanoparticles wereimpregnated on the zeolites using various
precursors such as AgNO3 and Cu(NO2)2. The size
and the shape of the loaded metals were measured
by transmission electron microscopy (TEM, TopconCo., Model EM002B).
3. Results and Discussion
Catalyst materials packed in plasma reactors
can change the electrical properties and so does theplasma generation. The authors have been studied
the plasma generation on the surface of various
catalysts using optical lens and ICCD camera
system. Figure 1 shows one example of the plasma
generation on the Ag/HSY zeolite [2]. The ICCD
camera observation provided an important insight
into the understanding of interaction between
discharge plasma and catalyst. Two different modes
have been confirmed in the catalysts packed plasma
reactors. One is partial discharge, which is usuallyobserved vicinity of contact points betweencatalysts. As increasing the applied voltage, plasma
was observed not only at the contact points (partialdischarge) but also on the surface of the catalysts.

Voltage-current characteristics of BaTiO3 pellet
packed reactor showed similar patterns compared toAg/HSY zeolite packed plasma reactor. However,the plasma expansion was not observed on thesurface of BaTiO3. These results clearly indicatethat multidisciplinary analysis is necessary to get abetter understanding on the plasma properties in thecatalyst packed reactor. 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) [3]. The presence of oxygen
decreased the luminescence of surface discharge
plasma [4], which is consistent with the work
reported by Hensel et al for the discharge on porous
ceramic [5]. A positive correlation was foundbetween the plasma generation pattern and thecatalytic activity in the combined process. The areaof discharge plasma expanded over a wide range by
the metal nanoparticles. We also have reported thatthe plasma-induced fixation of oxygen on thesurface of catalysts and their contribution insubsequent chemical reaction using isotope-labelled
oxygen (18O2) [6]Figure 2 summarizes the current understandingof synergy effects in the complementary
combination of plasma with catalyst. The mostcommon finding in the study of synergistic effects
is the better CO2 selectivity (i.e. low CO formation).
It is now believed that surface activation by UV is
negligible in the PDC process due to the low UVflux from the discharge plasma in an air-likemixture. The packing of catalytic materials within
plasma reactors also changes the physical-/
electrical characteristics of plasma generation in thefollowing ways; 1) Decrease in the plasma onset
voltage, 2) Increase in the number ofmicrodischarges, 3) Extension of the plasma area byloading metal nanoparticles on the surface. To makecatalysts being active under plasma activation, it isimportant to form the plasma just near the surfaceof catalyst. Recently, the fluorescence from theplasma-excited Ag clusters on the surface ofzeolites has been observed [7-8], which provides
good evidence for the direct interaction of plasma
with surface.
4. Conclusions
The interaction of plasma and catalyst isinteresting hot topics in the field of plasma
chemistry. Ongoing progress for the fundamental
aspects can accelerate the application ofplasma-catalyst process in various fields in the near
future.

Figure 1. ICCD camera images of plasma over Ag/HSY
catalyst (in N2): (a) without discharge, (b) 16 kV, (c) 19kV, (d) 21 kV. Discharge power was in the range of0.36~1.2 W for the tested applied voltages.
Ineffective
Effective

PlasmaRadicaladsorption

HeatingUV

Plasmaexpansion

Metal NPs

FLProduct

Desorption
Adsorption

Catalyst

Figure 2. Overview of interaction of plasma andcatalyst
Acknowledgments
The authors wish to acknowledge the support
given by the Grant-in-aid for scientific research (C26400539).
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