22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
The application of plasma-catalysis hybrid systems in the hydrocarbon selective
catalytic reduction of NO x at ambient temperature
C.E. Stere1, W. Adress2, R. Burch1, S. Chansai1, A. Goguet, W. G. Graham2 and C. Hardacre1
1
Centre for the Theory and Application of Catalysis, CenTACat, School of Chemistry and Chemical Engineering,
Queen’s University Belfast, Belfast, N. Ireland, U.K.
2
Centre for Plasma Physics, School of Mathematics and Physics, Queen’s University Belfast, N. Ireland, U.K.
Abstract: The development of an atmospheric pressure non-thermal plasma system led to
remarkable enhancement in the activity of a Ag/Al 2 O 3 catalyst for the SCR reaction. Up to
70% NO x conversion and >90% hydrocarbon reduction was achieved at temperatures <200 °C
but only when the plasma glow region was in contact with the catalyst. The DRIFTS
measurement provides a real insight into the mechanism by which NTP can promote the
reaction.
Keywords: Ag/Al 2 O 3 , HC-SCR, NTP, NO x reduction, toluene, octane, DRIFTS
1. Introduction
Close coupling of heterogeneous catalysis with plasma
constitutes a significant advance in catalytic reaction
engineering. A considerable number of studies have
examined the use of non-thermal plasmas (NTP)-catalyst
systems in VOC oxidations, automotive catalysis,
reforming and hydrogenation reactions [1-5] based on the
combined advantages of fast and low temperature reaction
from atmospheric NTP and high selectivity from
heterogeneous catalysis.
In the present work, atmospheric pressure non-thermal
plasma activated catalysis for the removal of NO x using
hydrocarbon selective catalytic reduction (Fig. 1) has
been studied utilising toluene and octane as the
hydrocarbon reductants. A remarkable enhancement in
activity was observed, even at room temperature, but only
when the plasma glow region was in contact with the
catalyst. A newly designed diffuse in situ reflectance
infra-red (DRIFTS) system coupled with the plasma was
designed to investigate the reaction mechanism.
∆
NOx+ HC + O2 → N2 + CO2 + H2O + ?
Reduction of
NOx to N2
Oxidation of HC
to CO2 and H2O
Fig. 1. The removal of NOx using HC-SCR.
2. Experimental
Two different electrode configurations were used to
generate the atmospheric pressure helium, non-thermal
plasma. For the catalytic activity tests, performed at
250 °C and ambient temperatures, two external circular
copper electrodes were used to form dielectric barrier
discharge in a 4 mm diameter quartz tube, while for the
DRIFTS experiments, a quartz covered tungsten wire with
O-3-5
3034
3075
2936
2884
Plasma Off
2143
Plasma
On
Time on Stream
Competitive Reactions
a pencil-shaped tapered end was inserted into a quartz
tube to act as a powered electrode. The capillary tube,
with a 50 cm3 min-1 helium flow, was one arm of a
T-shaped cylindrical quartz tube. The generated plasma
was in direct contact with the catalyst bed, as shown in
Fig. 2. A high voltage probe (Tektronix, P6015) and a
calibrated Rogowski coil (Pearson) connected to a digital
oscilloscope (LeCroy WavePro 7300A) were used to
measure the time-dependent applied voltage and current
(between 5 - 7 kV, with repetition rate of 18 to 25 kHz).
The reaction mixture consisted of 720 ppm NO,
620 ppm toluene or 542 ppm C 8 H 18 , 7.2% CO 2 , 7.2%
H 2 O (when added), 5% O 2 , and with the balance He. The
catalyst was prepared by the impregnation of γ-Al 2 O 3
with a silver nitrate solution followed by drying and
calcination to give a sample with an Ag metal loading of
2 wt%. The analysis was carried out with a NO x analyser
and a Tensor 27 Bruker FTIR spectrometer fitted with a
gas cell with a volume of 190 cm3. The diffuse
reflectance FT-IR (DRIFTS) measurements were carried
out at ambient temperature in situ in a high temperature
cell fitted with ZnSe windows.
Wavenumber / cm-1
Fig. 2. FTIR spectra of gas phase species in the absence
and presence of NTP under toluene-SCR of NO x reaction
conditions over 2% Ag/Al 2 O 3 catalyst at 250 °C.
3. Results and discussion
It was observed that when combining the NTP with a
Ag/Al 2 O 3 catalyst the activity was strongly enhanced
1
when compared with conventional thermal activation.
FTIR spectra of the gas phase outlet as a function of
switching the plasma on and off is presented in Fig. 2.
With an applied electrode voltage of 7 kV, it was found
that the IR bands between 3100 and 2800 cm-1, assigned
to the vibrational C-H stretching of gas phase toluene,
disappeared. This indicated that toluene had been
activated for NO x reduction and at least partially
converted to form gas phase CO 2 and CO as shown by the
bands at 2143 cm-1 [6]. No significant lag was observed
between the gas phase spectra and presence or absence of
the plasma indicating that the species formed are short
lived and only affect the activity when the plasma is
applied.
Of major importance is the assessment of the synergetic
effect of the NTP+catalyst in the enhancement of the SCR
activity at low temperatures. A comparison of the
conversion and selectivity for reactions performed using
the NTP in the absence and presence of the catalyst within
the discharge area is shown in Fig. 3.
C=24.5%
C=71.9% C=89.1%
C=42.4%
N2O (%)
Unknown (%)
N2 (%)
100%
100%
90%
90%
Selectivity /%
Selectivity /%
Unknown (%)
80%
70%
60%
50%
40%
30%
20%
CO2 (%)
CO (%)
80%
70%
60%
50%
40%
30%
20%
10%
10%
0%
0%
Blank reactor+
thermocouple
2% Ag/Al2O3
NOx
Blank reactor+ 2% Ag/Al2O3
thermocouple
Toluene
Fig. 3. The effect of NTP-catalyst system on the NO x and
hydrocarbon conversion and products’ selectivity during
the HC-SCR of NO x reaction over 2 wt% Ag/Al 2 O 3 .
Feed composition: 720 ppm NO, 4340 ppm (as C1) HC,
4.3% O 2 , 7.2% CO 2 , 7.2% H 2 O and He balance. The
total flow rate and space velocity was 276 cm3 min-1 and
165600 cm3 g-1 h-1 , respectively.
The presence of the catalyst in the discharge area had a
major effect on the conversion of the hydrocarbons and
NO x . For the toluene-SCR, in the presence of the NTP,
the total NO x conversion was found to be <5% in the
empty reactor with an increase to ~25% on introducing
the thermocouple into the reactor. In the presence of the
thermocouple and the 2% Ag/Al 2 O 3 catalyst within the
plasma discharge area, the total NO x conversion with
toluene in the feed was ~42%. A similar trend was
observed for the toluene conversion with ~8% found
using the empty reactor with the NTP compared with
~72% in the presence of the thermocouple and ~89% in
the presence of the catalyst and thermocouple. A 20%
decrease in the N 2 formation was observed in the absence
of the catalyst with the thermocouple acting as a second
2
ground electrode, while insignificant quantities of N 2
were formed when the thermocouple was removed. The
selectivity towards CO 2 was also improved in the
presence of the catalyst and thermocouple compared with
the empty reactor in the presence of the thermocouple. In
the former case 90% selectivity to CO 2 was observed
compared with ~74% in the latter arrangement.
Importantly, the change in the CO 2 selectivity is not due
to a significant change in the CO selectivity which only
decreases from ~13 to 11% on addition of the catalyst.
The presence of the catalyst in the NTP eliminates the
formation of other C-containing by-products, reported as
“unknown products” in Fig. 3. In the absence of the
catalyst, these represent 13% of the carbon balance.
In situ DRIFTS spectra of surface species adsorbed on
the Ag/Al 2 O 3 catalyst during the octane-SCR reaction
with plasma at ambient temperature showed the formation
of reaction intermediates formed during thermal
activation but at much higher reaction temperatures. This
indicates that the plasma could help to activate both NO
and hydrocarbon.
4. Significance
When the plasma was combined with an Ag/Al 2 O 3
catalyst a strong enhancement in activity was observed
when compared with conventional thermal activation with
high conversions of both NO x and hydrocarbons obtained
at temperature ≤ 250 °C, where the silver catalyst is
normally inactive. Importantly, significant activity was
obtained at 25 °C. This low temperature activity provides
the basis for increased exhaust gas treatment during cold
start conditions which remains an important issue for
mobile and stationary applications.
5. References
[1] J.V. Durme, J. Dewulf, C. Leys and H.V.
Langenhove. Appl. Catal. B, 78, 324-333 (2008)
[2] H. Than Quoc An, T. Pam Huu, T. Le Van, J.M.
Cormier, and A. Khacel. Catal. Today, 176,
474-477 (2011)
[3] C. Shi, Z. Zhang, M. Crocker, L. Xu, C. Wang,
C. Au and A. Zhu. Catal. Today, 211, 96-103
(2013)
[4] D.H. Lee, J.-O. Lee, K.-T. Kim, Y.-H. Song, E. Kim
and H.-K. Han. Int. J. Hydrogen Energy, 37,
3225-3233 (2012)
[5] H. Wang, X. Li, M. Chen and X. Zheng. Catal.
Today, 211, 66-71 (2013)
[6] S. Chansai, R. Burch, C. Hardacre, J.P. Breen and
F.C. Meunier. J. Catal., 276, 49 (2010)
O-3-5