22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-Pd/γ-Al 2 0 3 catalytic system for methane, toluene and propene oxidation:
effect of temperature and plasma input power
T. Pham Huu1, 2, P. Da Costa3, S. Loganathan1 and A. Khacef1
1
GREMI-UMR 6744, CNRS-Université d'Orléans, 14 rue d’Issoudun, P.O. Box 6744, FR-45067 Orléans Cedex 02, France
Institute of Applied Material Science, Vietnam Academy of Science and Technology, 1 Mac Dinh Chi, HCMC, Vietnam
3
Sorbonne Universités, UPMC Paris 6, Institut Jean Le Rond d’Alembert, CNRS UMR 7190, 2 place de la gare de
ceinture, FR-78210 Saint Cyr l’école, France
2
Abstract: A pulsed non-thermal plasma and 1 wt% Pd/γ-Al 2 O 3 catalyst was used to
investigate the CH 4 , C 3 H 6 , and C 7 H 8 oxidation in air. Effects of temperature and specific
input energy on the VOCs conversion were studied. The plasma-catalyst interaction
revealed the benefit effect on the VOCs oxidation even at low temperature leading to high
CO 2 selectivity. The synergistic effect of combining plasma with catalyst in one-stage
configuration is observed only for toluene.
Keywords: non-thermal plasma, Pd/γ-Al 2 0 3 , synergistic effect, methane, propene, toluene,
oxidation
1. Introduction
Volatile organic compounds (VOCs) emitted from
various industrial and domestic processes are important
sources of air pollution and therefore, become a serious
problem for damaging the human health and the
environment.
The well-established technologies for
VOCs abatement, thermal and catalytic oxidation [1],
require a thermal energy that makes them unsuitable and
energetically expensive for the treatment of moderate gas
flow rates containing low VOC concentrations. As an
alternative to the catalytic oxidation, there have been
extensive researches on using non-thermal plasmas (NTP)
to remove various types of gas-phase hazardous pollutants
over the last two decades [2–5].
However, NTP
technology has a disadvantage like undesirable
by-products formation such as ozone, aldehyde, and NO x .
More recently, effective use of NTP on air pollution
control is possible by exploiting its inherent synergistic
potential through coupling with heterogeneous catalyst
[6-11]. The combination of NTP with catalyst can be
made in either single stage (plasma driven-catalyst) or
two-stage configuration (plasma-assisted catalyst).
Compared to conventional NTP alone, the effectiveness
of these systems have been demonstrated in terms of
energy efficiency, products selectivity and carbon
balance.
In this work, the oxidation of methane, propene and
toluene in air at atmospheric pressure was investigated
using a Dielectric Barrier Discharge (DBD) reactor
combined or not with γ-Al 2 O 3 and 1 wt% Pd/γ-Al 2 O 3
catalysts.
Results are reported as a function of
temperature, specific input energy (SIE), and position of
the catalyst in the reactor (IPC, In Plasma catalysis and
PPC Post-Plasma Catalysis).
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2. Experimental
The plasma reactor is a cylindrical DBD gives the
possibility to combine the catalyst with plasma reactor in
single stage (IPC) or two-stage (PPC) configuration as
shown in Fig. 1.
Fig. 1. Schematic overview of the plasma-catalysis
hybrid reactor: (a) In-Plasma Catalysis (IPC) and (b) PostPlasma Catalysis (PPC).
The plasma reactor was powered by a pulsed
sub-microsecond high voltage generator delivering an
output HV up to 20 kV into 0.5 µs pulses (FWHM) at a
maximum frequency of about 200 Hz.
Electrical
characterization of plasma was performed by currentvoltage measurements using a Tektronix P6015A HV
probe (attenuation ratio 1000:1) and Pearson 4001current
probe (10 ns rise time), respectively. The energy
consumption of the plasma reactor was evaluated through
the specific input energy (J/L) which is the energy
deposited per unit volume of gas in the reactor and is
given by SIE = (E p .f)/Q) (E p is the discharge pulse
energy, f the pulse frequency, and Q the total gas flow
rate). Experiments were conducted by maintaining E p
1
constant as 13 mJ, the corresponding maximum SIE is
148 J/L.
Palladium supported catalysts (0.5 and 1 wt%) were
prepared by wet impregnation method of γ-Al 2 O 3 beads
(1.8 mm; Sasol Germany GmbH). The catalysts crystal
structure was examined by XRD pattern (Bruker D5005
diffractometer, Cu Kα radiation). The atoms chemical
states in the catalyst surface were investigated by XPS
(AXIS Ultra DLD spectrometer, Kratos Analytical). The
surface area/porosity measurements were evaluated by the
multipoint BET and BJH methods. Finally, Pd metal
loading was determined by ICP-OES using an ACTIVA
spectrometer (Horiba Jobin Yvon).
Methane, propene, and toluene oxidation was
performed in a continuous flow gas fixed to 1 L/min
corresponding to a VVH of about 15 000 h-1. Initial
concentrations of the VOCs have been fixed as 1000 ppm.
End-products detection and quantification were carried
out using a FTIR (Nicolet 6700, Thermo-Scientific)
equipped with a liquid nitrogen cooled MCT detector and
10 m gas path cell.
Steady-state activity of the catalyst, methane (CH 4 ),
propene (C 2 H 6 ) and toluene (C 7 H 8 ) conversion rates,
were measured in the presence and in the absence of
catalyst for the temperature between 22 and 500 °C. The
uncertainty was determined by repeating each experiment
four times. After reproducibility, we can conclude that
the experimental relative uncertainty error is less than 5%
for all cases studied. The CH 4 , C 3 H 6 and C 7 H 8
conversion rate was defined by Eq. 1, where VOC design
the considered molecule and the brackets refer to the
concentrations.
𝜏𝑉𝑉𝑉 =
[𝑉𝑉𝑉]𝑖𝑖 − [𝑉𝑉𝑉]𝑜𝑜𝑜
[𝑉𝑉𝑉]𝑖𝑖
Fig. 3 shows the propene conversion for thermal,
plasma, and plasma-catalysis (IPC and PPC) systems. It
can be seen that the conversion of propene by thermal
catalysis has a threshold temperature of~130 °C and
increases steeply with increasing the temperature and
reaches complete conversion at 250 °C.
Plasma
processing of propene, with and without catalyst, exhibits
much lower threshold temperature and the conversion
take place from room temperature. Plasma-catalysis, both
IPC and PPC, have shown more propene conversions than
the thermal catalysis for all temperature studied. The
most striking difference is at 150 °C, both IPC and PPC
have shown 62% propene conversion. Under the similar
operating conditions conventional thermal catalysis has
shown only 9% propene conversion.
(1)
3. Results and discussion
Experiments were designed so that the effect of thermal
catalysis, plasma-catalysis, and plasma conversion alone
could be compared. As reported in Fig. 2, the methane
conversion by plasma reached a maximum of 67%. In
that case, the observed main products were CO, CO 2 , O 3 ,
When plasma is combined with
and HNO 3 .
1 wt% Pd/γ-Al 2 O 3 catalyst, it exhibits higher catalytic
activity at low-temperature for both IPC and PPC
configurations. CH 4 conversion improvements by a
factor 3 at 300 °C and 1.4 at 350 °C were obtained in
plasma-catalytic system compared to thermal catalytic
experiments for the highest specific input energy used.
Although the difference is weak, the IPC configuration
seems to be more efficient compared with PPC. In all
cases, the reaction using Pd/γ-Al 2 O 3 catalyst becomes
more selective in CO 2 formation than the reaction in
plasma alone and, at high temperature O 3 and HNO 3
disappears in favour of NO x . At high temperature, the
catalyst itself leads to 100% conversion and the plasma is
no more needed and can be switched-off. Moreover,
performance was slightly better in IPC configuration.
2
Fig. 2. CH 4 conversion as a function of temperature (IPC
versus PPC): SIE = 148 J/L.
Fig. 3. Plasma, catalytic, and plasma-catalytic (IPC and
PPC) conversion of propene as a function of temperature
(SIE = 23 J/L).
At temperature lower than 150 °C, IPC exhibits higher
propene conversion efficiency as compared to PPC
configuration. Whilst the conversion efficiencies are
quite different, the nature of by-products observed are
similar (CO, CH 2 O, CH 2 O 2 , CH 3 NO 3 , O 3 , and NOx).
For SIE = 87 J/L, the total C 3 H 6 conversion was achieved
at 100 °C as compared to 250 °C for conventional thermal
catalysis. The increase in energy deposition enhances the
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conversion efficiency and decreases the difference
between IPC and PPC configuration.
Fig. 4 shows the CO concentration produced by plasma
and plasma-catalysis as a function of temperature. At low
temperature (< 100 °C), CO concentrations are quiet
similar for IPC and PPC configurations. At higher
temperature, CO concentration linearly increased in case
of plasma-alone and slightly decreased when plasma was
combined to a catalyst. We observed that at 150 °C,
compared to thermal catalysis, the addition of the catalyst
to the plasma increased the CO 2 selectivity to about
85-90%. At a given temperature, the amounts of byproducts such as CH 2 O and CH 2 O 2 produced from the
partial oxidation of C 3 H 6 decrease in both IPC and PPC
systems and could be drastically reduced by increasing
the specific input energy.
Fig 4. CO concentration from oxidation of C 3 H 6 as a
function of temperature (SIE = 23 J/L).
In the case of toluene oxidation, the situation is quite
different. We observed huge toluene desorption from the
catalyst surface (porous γ-Al 2 O 3 support) at low
temperature (< 100 °C) when the catalyst was placed in
the plasma discharge (IPC), while for the PPC
configuration toluene desorption was not observed. Fig. 5
shows a typical FTIR spectra illustrating the toluene
desorption from the catalyst surface in IPC configuration
during plasma discharge at room temperature. This
phenomenon, which is much more pronounced at highenergy deposition, is presented by negative conversion
(Eq. 1) reported in Fig. 6 (the outlet toluene concentration
is higher than the initial one).
In IPC system above 100 °C, compared to PPC, higher
toluene conversion is observed. Depending on SIE, about
10-30% more toluene conversion is evidenced.
Therefore, it can be suggested that, in IPC configuration;
the catalyst could be activated by high-energy species
produced by the plasma and the in-situ decomposition of
ozone could increase the toluene conversion. The
increase in SIE not only increases the toluene conversion,
but also leads to the better CO 2 selectivity. The
improvement of the toluene conversion in the IPC system
by the increasing the specific input energy also leads to a
better CO 2 selectivity, as compared to PPC system.
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Fig. 5. FTIR spectra for catalytic and plasma-catalysis
processing of air-toluene mixture at room temperature for
SIE = 148 J/L (1 wt% Pd/γ-Al 2 O 3 catalyst).
Fig. 6. Plasma, catalytic, and plasma-catalytic (IPC and
PPC) conversion of toluene as a function of temperature
(SIE = 23 J/L).
The synergistic effect of combining plasma with
catalyst in one-stage configuration (IPC) is shown in
Fig. 7 for toluene removal in air at 150 and 200 °C. From
left to right, the contributions to the toluene conversion
are from plasma alone and from thermal catalysis alone.
These are combined to give the resultant effect that one
might predict if the two effects were additive (~37% at
150 °C, and 83% at 200 °C). The actual toluene removal
achieved by plasma-catalytic reactor is ~59% at 150 °C
and ~93% at 200 °C show a synergistic gain of a factor of
~2 and ~1.2, respectively. At higher temperature, there is
a much less marked difference between the conversion
achieved thermally and by plasma catalysis indicating that
synergism is dependent on the operating temperature.
Under over experimental conditions, no synergistic
effects were observed for methane and propene removal.
These results are in agreement with the literature [8, 9].
As reported by Whitehead [11], for dichloromethane,
synergetic effects have not been observed. Authors
concluded that the synergistic effects do not unanimously
occur in plasma catalysis processes. It depends on several
factors such as nature of catalysts, types of plasma
reactors, operating conditions and nature of the pollutants.
3
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Fig. 7. Synergistic effect of plasma-catalytic hybrid
system for toluene conversion.
4. Conclusions
Methane, propene and toluene oxidation in air by
thermal-catalytic, plasma, and plasma-catalytic systems
have been investigated in wide range of temperature and
plasma input energy. Whatever the molecule studied, the
plasma significantly enhanced the catalytic activity even
at low temperature leading to high CO 2 /CO selectivity.
Performance of the system was slightly better when the
catalyst was located in the discharge zone, i.e., IPC
configuration. For instance, to achieve 100% conversion,
IPC requires 100 °C whereas thermal catalysis process
requires 250 °C. At room temperature and for given input
energy, the catalyst was helpful in minimizing the byproducts produced during plasma discharge (CO, CH 2 O,
CH 2 O 2 , HNO 2 ).
It is evidenced that 1 wt% Pd/γ-Al 2 O 3 catalyst shows
the synergistic effect on only toluene conversion at low
temperature in in-plasma catalysis hybrid system.
However, post-plasma catalysis system did not show
synergistic effect.
5. Acknowledgments
This work was performed in the framework of
international research network (GDRI) "Catalysis for
polluting emissions aftertreatment, and production of
renewable energies". The authors greatly acknowledge
S. Gil and A. Giroir-Fendler from IRCELYON for
assistance in catalyst analysis and fruitful discussions.
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