Superior decomposition of NO over plasma-assisted catalytic system induced by microwave

Article abstract of DOI:10.1246/cl.2000.578

More than 99% of NO was directly decomposed into N₂ and O₂ without formation of N₂O at space velocity of 45360 h^-1 and apparent temperature of 280 °C over plasma-assisted Co₃O₄/Cu-ZSM-5 cat

Full text of DOI:10.1246/cl.2000.578

578
Chemistry Letters 2000
Superior Decomposition of NO over Plasma-Assisted Catalytic System Induced by Microwave
Hyun-Seog Roh, Yong-Ki Park, and Sang-Eon Park*
Catalysis Center for Molecular Engineeing, Korea Research Institute of Chemical Technology (KRICT), Taejon 305-600, Korea
(Received February 14, 2000; CL-000153)
More than 99% of NO was directly decomposed into N2
and O2 without formation of N2O at space velocity of 45360 h-1
and apparent temperature of 280 °C over plasma-assisted
Co3O4/Cu-ZSM-5 catalyst.
Nitrogen oxides in exhaust gases are major sources of air
pollution, and new abatement technologies are being developed
to overcome current NH3-SCR or SNCR. Direct decomposition
of NO has been proposed as one of candidates for NOx abate-
ment. Since Iwamoto et al. pioneered direct decomposition of
NO over Cu-exchanged zeolite catalyst,1,2 other types of zeo-
lites or mixed metal oxides have been suggested but their cat-
alytic performances are still too low to satisfy practical applica-
tion. As an alternative for the decomposition of NO, plasma
could be considered induced by microwave or electrical dis-
charge. However, due to the difficulties in generating and con-
trolling the quality at atmospheric condition, its application also
has been limited. That is, high electric field energy at atmos-
pheric condition or microwave at low pressure are required for
its generation.3,4 To circumvent limit of plasma itself for the
abatement of NOx, several types of plasma processes combined
with catalysts or reductants have been proposed. These were
effective for the decomposition of NOx at atmospheric pressure
and low-temperature but their activities decreased rapidly at
high space velocity (> 5000 h-1).4,5 So, we tried to overcome the
limits exposed in plasma-assisted NO decomposition by rein-
forcing catalytic function to the plasma. It was successful and
superior NO decomposition activity was obtained over this
plasma-assisted catalytic system even at high space velocity.
The catalysts were designed to introduce functions of NO
decomposition and microwave absorbing for plasma generation.
As a microwave absorbent, two types of dielectric materials
having high microwave loss factor were chosen; one is catalyti-
cally inert SiC and the other is Co3O4 having NO decomposi-
tion activity. These microwave absorbents were dispersed com-
binatorially on two types of porous supports using impregnation
method; one is catalytically inert silica and the other is Cu-
ZSM-5 having high NO decomposition activity. Cu-ZSM-5
was prepared by ion-exchange method. All catalysts were dried
and calcined at 120 °C for 2 h and 550 °C for 6 h. These four
types of catalysts could be heated up to 1000 °C by increasing
the loading of microwave absorbents. To reach 500 °C for plas-
ma generation, 25 wt% of Co3O4 and 95 wt% of SiC were
required in 300 W microwave input power, respectively. Prior
to plasma generation the catalysts were treated in He flow at
150 °C for 1 h. Plasma was triggered by controlling input power
of microwave in He flow. The input power was increased grad-
ually up to 300 W/g-cat. to reach 500 °C. At a certain point
around this temperature, reflected microwave power dropped
rapidly to almost zero and light blue-colored plasma was
formed (Table 1). Stable and blue-colored plasma was obtained
over all of the prepared catalysts by the irradiation of continu-
ous microwave in atmospheric condition without vacuum.
However, the quality of plasma was strongly influenced by the
loading of microwave absorbents. At lower loading, the
microwave could not be absorbed enough to trigger plasma
while at too high loading most of the absorbed microwave ener-
gy was dissipated as radiation heat instead of plasma generation
due to the enhanced heating (> 600 °C). Thus, to utilize
microwave effectively for NO decomposition, the loading
should be controlled strictly in the region of plasma formation.
To see the catalytic functions of plasma-assisted catalytic sys-
tem, NO decomposition was carried out in a microwave reac-
tion apparatus similar to Wan et al.’s6 equipped with specially
designed fixed-bed quartz reactor and 2.45 GHz microwave
generator (ASTEX, Model AX 3120) (Table 1). NO decompo-
sition was carried out in the plasma-catalytic reaction system
while changing NO concentration from 1000 to 10000 ppm
NO/He and flow rate from 120 to 1200 ml/min over 1g catalyst.
The quartz reactor was specifically designed to locate catalyst
cylindrically inside of the reactor for uniform microwave
absorbing and heat distribution. That is, because the microwave
is mainly absorbed by the catalyst located outer part of reactor
the inner part of quartz reactor was blanked such as empty
torus. Products were monitored by a chemiluminescent NOx
analyzer (42H, Thermo. Env. Inc.) and GC (HP-5890) equipped
with molecular sieve 13X column. Temperature was measured
by IR pyrothermometer (MIKRON M90-ZB).
For all of the catalysts, plasma was indispensable to
achieve high decomposition activity. The catalysts unable to
produce plasma did not show any activity even though the
apparent temperature was higher than 500 °C. The catalytically
inert SiC/silica did not show any decomposition activity even in
the presence of stable blue-colored plasma. However, 5 wt%
Cu-ZSM-5 loaded on SiC showed 95% NO conversion at
45360 h-1. The decomposed NO was converted into N2 and O2
without detectable formation of N2O. This superior decomposi-
tion activity over SiC/Cu-ZSM-5 may be resulted from the cat-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
579
alytic function of Cu-ZSM-5 assisted by plasma. It is a good
contrast to the AC, DC or Corona discharge plasma systems
where only limited conversion and selectivity were obtained at
high space velocity.4 Contrast to the SiC, the Co3O4 showed
quite different behavior as a microwave absorbent. It has high
decomposition activity itself as well as microwave absorbing
ability and NO could be decomposed effectively without the
help of NO decomposition catalyst. The Co3O4 has high NO
sorption ability as one of main component in NO decomposi-
tion catalyst,7,8 which may be the reason for high activity in
plasma-assisted NO decomposition. When the Co3O4 was mod-
ified further with catalytically active Cu-ZSM-5 (75 wt%), it
was strengthened enough to decompose NO at space velocity
higher than 45000 h-1 (Table 2). This improved catalytic per-
formance was not observed in other combination of microwave
absorbents and supports, which makes it clear that both plasma
and catalytic function are necessary for superior decomposition
activity of NO. To clarify the importance of catalyst in plasma-
assisted catalytic system, decomposition activities of this study
were compared with those of pure plasma and catalyst itself
(Table 2). In our system, we changed the Cu ion-exchange
level from 100 % to 122 %, and Si/Al ratio from 15 to 40, but
there was not any significant change. Different from the plas-
ma-assisted catalytic system, the decomposition activities are
strongly influenced by the space velocity in catalytic or plasma
reactions. At the space velocity higher than 10000 h-1, conver-
sion decreased to the half of their initial activity. In the case of
AC plasma, even high input voltage of 2.75 kV is applied it is
impossible to decomposed more than 20% of NO at 18000 h-1.
However, the limits exposed in catalytic or plasma systems
could be overcome easily in our plasma-assisted catalytic sys-
tem induced by microwave. More than 95% of conversion of
NO was achieved even at 45360 h-1, which is superior to the
catalyst or plasma itself.
ma is formed then it can provide enough energy to the NO mol-
ecules to be activated to the transition state and the activated
NO molecules will recombine into NO again or decompose into
N2 and O2. In pure plasma system, this process will strongly
depend on the probability of collisions and only limited
amounts of NO will decompose. However, by locating decom-
position catalyst in the plasma, the recombination process will
be influenced through the preferential interaction of activated
NO molecules with catalyst. That is, the decay of activated NO
will convert catalytically into thermodynamically stable N2 and
O2 through selective recombination of N-N and O-O bonds
over decomposition catalyst proposed as follows. Co and Cu
species could be suggested as active sites for NO decomposi-
tion. However, it is a suggestion and further spectroscopy study
should be done to verify it more clearly.
Plasma: NO → NO*
Catalyst: 2NO* → N2 + O2
In conclusion, plasma-assisted Co3O4/Cu-ZSM-5 catalyst
shows remarkably high activity (99%) for the direct decomposi-
tion of NO into N2 and O2 at GHSV > 45000 h-1 and apparent
temperature of 280 °C.
References
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J. Huang and S.L. Suib, J. Phys. Chem., 97, 9403 (1993).
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The dependence of microwave power was also checked in
decomposition activity (Figure 1). According to the types of
catalysts, decomposition activities showed different behaviors.
In the case of catalytically reinforced Co3O4/Cu-ZSM-5, activi-
ty was maintained constantly regardless of input microwave
power until the plasma broke down. However, in other catalyst
activities decreased as the input power decreased. This means
that both plasma and catalytic function are important but the
catalytic function is more important than plasma. Once the plas-
J. K. S. Wan, G. Bamwenda, and M. C. Depew, Res.
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