st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Plasma-Catalyst Hybrid Reaction in Honeycomb Monolith
for Decomposition of Automobile Exhaust Gases
Woo Seok Kang1, Dae Hoon Lee1, Jae-Ok Lee1, Min Hur1, Young-Hoon Song1, and Yong-Ki Park2
1
Korea Institute of Machinery & Materials (KIMM), Daejeon, Republic of Korea
Korea Research Institute of Chemical Technology (KRICT), Daejeon, Republic of Korea
2
Abstract: Plasma-catalyst synergic reaction for environmental control was studied by using a
hybrid reactor that combines plasma with honeycomb-structured catalyst. Developed reactor
generated stable plasmas over the catalyst, and the catalyst reaction became more active at
low temperature / higher plasma power conditions. The mechanism for plasma-catalyst
two-stage reaction was discussed with total HC variation evaluating possible use in cold-start
issues in automobile industry.
Keywords: plasma catalyst hybrid reaction, honeycomb monolith catalyst, dielectric barrier
discharge, plasma-induced desorption and adsorption
1. Introduction
Plasma-catalyst hybrid technology is a promising solution for environmental control to make up for weakness of
catalyst that can be activated only at high temperature.[1]
Effective combination of plasma and catalyst can present
solutions to environmental problems like “cold start issue,” which means most of the exhaust gas from an automobile is emitted to the air without any decomposition
because the catalyst for exhaust after treatment cannot
function properly for a few tens of second until the engine
is heated up.[1-3]
As a practical solution for the cold-start problem, this
work suggests an effective combination of plasma and
catalyst.
2. Experiment
A plasma-catalyst two-stage reactor was designed
which is composed of a commercial three-way catalyst
within two perforated metal electrodes in a quartz tube:
high-voltage electrode is spaced apart from the catalyst by
a few millimeter’s distance; ground electrode is in contact
with bottom of the catalyst. (Figure 1)
As a simulation gas, propylene (C3H6) mixture with nitrogen (N2) and oxygen (O2) is used. Temporal gas decomposition characteristic curves are obtained by measuring total hydrocarbon (HC) concentration using a
real-time gas analyzer.
3. Results
The developed reactor shows typical electrical characteristics of conventional dielectric barrier discharge
(DBD).[4] Dielectric monolith plays a role as a dielectric
barrier and most of external electric potential to be applied within the air-gap between high-voltage electrode
and catalyst, and thereby plasma is effectively produced
within the air-gap by moderate applied voltage. Discharge
power is less than 20 watt.
We operated the system with following procedure: 1)
switch on the heated N2/O2, 2) switch on the HC (hydrocarbon), 3) operates plasmas, 4) switched off plasmas,
and 5) switch on HC and N2/O2. The characteristic curve
of HC variation, as shown in Figure 2, exhibits five distinct reaction phases: catalytic reaction, fast reaction,
heat-combined reaction, transit reaction, and hysteresis
catalytic reaction. The fast reaction and heat-combined
reaction phases are related to instant plasma reaction and
catalyst temperature rise, respectively, and each phase is
explained with the HC concentration and gas temperature
change.
Figure 2. Temporal total HC variations.
Figure 1. A schematic of developed reactor concept.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
During the transition from HC on to plasma on (catalytic reaction phase) in Figure 2, the catalyst alone reacts
with the HC increasing the catalyst temperature by endothermic reaction. When the plasma was turned on, total
HC dropped instantaneously within a second (fast reaction phase), and it gradually decreased and reached
steady-state (heat-combined reaction phase). When the
plasma was turned off, the total HC increased gradually
further reaching a higher value, in comparison to the previous peak (hysteresis catalytic reaction phase). The increase in total HC in this phase indicates that the catalyst
became less active, during which the catalyst temperature
decreased gradually.
In this hybrid reaction, the HC containing gas is decomposed in two-step reactions –by plasma oxidation in
volumetric reaction and by catalytic reaction. Plasma-induced reaction occurs by reacting flowing gas with
atomic oxygen because the generated plasma produces
electrons with low temperature that are adequate to dissociate oxygen.[4] And catalytic reaction occurs continuously over times resulting slow decrease of HC with temperature rise by endothermic reaction. It is noteworthy
that plasma-induced fast reaction accompanies desorption
of HC over the surface of catalyst.[5] Plasma-induced
desorption phenomena may occur by replacement of HC
over a surface site by low-order HC fragments under our
detection limits or generated atomic oxygen that can be
adsorbed on the catalyst easily with longer lifetime.[6]
Figure 3 shows temperature-dependent hydrocarbon
DRE characteristics. In all condition, DRE was increased
as ambient temperature increases. And when plasma is
combined with catalyst, the DRE was enhanced reducing
light-off temperature less than 10°C. Strong synergic effect of plasma-catalyst hybrid reaction was observed
when plasma power was increased. At high temperature,
however, plasma hybrid effect becomes weak because the
catalyst was active enough requiring no external assistance.
Figure 3. DRE by ambient temperature.
4. Summary
Using a reactor combining honeycomb-monolith catalyst and plasma in a practical way, plasma-catalyst hybrid
reaction was studied. Characteristics of generated plasmas
in this reactor resembles that of typical dielectric barrier
discharge, and the dielectric and conductive characteristics of a catalyst make the plasma effectively generated
between catalyst-electrode air-gap. Variations of total HC
shows that plasmas activate catalyst effectively at
low-temperature decreasing light-off temperature compared to catalyst-only one. Plasma-induced desorption
and adsorption phenomena were found along with operation in this reactor. Developed reactor meets practical
criteria for industrial use presenting plasma-catalyst synergic reaction at low temperature with low power and
moderate applied voltage in a widely-used honeycomb
monolith catalyst.
References
[1] H. –H. Kim, Plasma Proc. Polym, 1, 91 (2004).
[2] M. J. Kirkpatric et al., Appl. Catal. B: Environ., 106,
160 (2011).
[3] G. S. Son et al., SAE Technical Paper, 2002-01-2706
(2002).
[4] B. Eliasson et al., IEEE Trans. Plasma Sci., 19, 1063
(1991).
[5] Y. –H. Song et al., J. Electrostat., 55, 189 (2002).
[6] D. H. Lee et al., Plasma Chem. Plasma Proc., 33, 249
(2013)
Acknowledgment
This work was financially supported by the Korea Research Council for Industrial Science & Technology
(ISTK).