22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Conversion of CH 3 CHO in a cylindrical DBD and role of a MnO 2 based catalyst
O. Koeta, N. Blin-Simiand, L. Magne and S. Pasquiers
Laboratoire de Physique des Gaz et des Plasmas, CNRS (UMR8578), Université Paris-Sud, Bâtiment 210, FR-91405
Orsay Cedex, France
Abstract:
Acetaldehyde conversion kinetics was studied for CH 3 CHO/N 2 and
CH 3 CHO/N 2 /O 2 mixtures with the help of a kinetic model. N 2 metastable states play the
key role in acetaldehyde and by-products dissociation in nitrogen plasma. In oxygenated
mixtures most important species are oxygen atom O(3P) and OH radical which have high
oxidation capacity. Atomic oxygen O(3P) is the one involved in catalytic treatment of
acetaldehyde.
Keywords: pulsed DBD, acetaldehyde, kinetic model, catalyst, MnO 2
1. Introduction
Acetaldehyde is an indoor air pollutant coming from
building materials. It is also emitted in exhausts from
ethanol gasoline combustion or from incinerators. The
abatement of volatile organic compounds (COV) by nonthermal plasmas in polluted gas streams is under
investigation since 20 years [1, 2]. Various types of
pulsed discharges have been used in these studies, such as
dielectric barrier discharges (DBDs) or corona discharges,
working at an ambient temperature, being characterized
by a non-homogeneous reactive medium. Most of them
exhibit promising results but the formation of numerous
by-products suggested catalysts coupling to plasmas.
Few work have been dedicated to acetaldehyde
conversion.
We recently investigated the kinetics of its removal in
N 2 [3] and N 2 -O 2 [4] mixtures in a homogeneous
transient plasma energised by a photo-triggered discharge.
The interpretations of the experimental results, using a
fully self-consistent modelling, led to the determination of
the kinetic scheme for the conversion of CH 3 CHO in such
a discharge. In the present work, we investigate the effect
of the non-homogeneous energy release on such a kinetics
in a DBD discharge.
The effect of a catalyst placed downstream the
discharge zone is also studied.
2. Experimental set-up
The DBD cylindrical reactor was already described [5].
The plasma is created in a Pyrex tube (inner diameter
14 mm, thickness 2 mm). A copper tape (L = 140 mm)
surrounding this tube is grounded and a high voltage
pulse is applied on the central tungsten rod (diameter
2 mm). The corresponding discharge volume obtained is
22 cm3. The electrical energy deposited in the DBD
during one pulse, E pulse , was determined through
measurements of the voltage and current time evolutions
by means of electrical probes connected to a fast digital
oscilloscope (Lecroy LT584, 1 GHz, 4 GS/s). The
repetition frequency of the discharge, F, was up to
300 Hz. A thermo-regulated oven, where the DBD
P-III-9-17
reactor together with the catalyst were placed, allowed
studying thermal effect on the acetaldehyde conversion by
varying the oven temperature T oven , from 20 °C up to
300 °C.
The input discharge energy density, called specific
energy E S is defined as E S = E pulse ×F/Q where Q is the
volumetric flow rate at the oven temperature, regulated at
1.0 L.min−1 in STP conditions. All measurements were
performed for a constant applied voltage of 40 kV on a
purely capacitive load. Values of the specific deposited
energy were varied by changing the frequency up to
300 Hz. The deposited energy per current pulse in the
plasma volume E pulse ranges, depending on temperature,
from 50 up to 85 mJ for mixtures with oxygen, and up to
90 mJ for pure nitrogen.
Moreover, for a given
temperature, E pulse slowly decreases when the repetition
frequency F increases. The specific energy E S is up to
800 J.L-1.
The catalyst used in this work is made of alumina balls
(2 mm diameter) impregnated with MnO 2 doped with 1%
of palladium; its specific area is 179 m2/g with a
volumetric velocity (VVH) of 16 h-1. The catalyst is
placed 5 cm downstream of the plasma zone. At this
place, very few radicals or atoms coming from the
discharge remain. The catalyst then decomposes ozone to
produce atomic oxygen that is able to oxidize residual
pollutant.
Chromatographic and spectroscopic techniques
(GC-TCD, GC-FID and FTIR) were used to measure the
removal of acetaldehyde and to identify by-products and
quantify their concentrations at the exit of the reactor.
The main measured by-products are H 2 , CH 4 , C 2 H 6 ,
HCN, CO and CO 2 in N 2 /CH 3 CHO mixtures plus CH 2 O,
CH 3 OH and CH 3 NO 3 when oxygen is present in the
initial mixture.
3. Modelling of the gaseous kinetics
In order to get insights on the kinetics involved in the
acetaldehyde removal in the gaseous phase, we developed
a modelling of the reactor (without the catalyst).
The DBD itself is a highly inhomogeneous plasma. It is
1
a very complex medium and constitutes therefore a
challenge for modelling. In the present work the
discharge modelling is simplified. The electron energy
transfers to the gas mixture are approximated by a
spatially averaged excitation and dissociation of the initial
mixture components (N 2 , O 2 ) in the whole discharge
volume V d . The coefficients of electrons inelastic
processes are calculated solving the Boltzmann equation
for an assumed value of the reduced electric field E/N.
The density of metastable states N 2 (A) and N 2 (a’, a, w),
radiative states N 2 (C), N 2 (B) and atoms N as well as
O(3P) and O(1D) are then calculated by multiplying these
coefficients by a second parameter Ne×∆t, corresponding
to an assumed product of the averaged electrons density
in the discharge volume V d with the discharge duration.
The so obtained densities of excited states are then used
as initial conditions to solve the balance equations of the
heavy species during the afterglow.
The balance
equations system is then solved during F-1. This system is
elaborated from a kinetic scheme and the most part of the
reaction coefficients are taken from the NIST database.
The effect of temperature is taken into account in the
coefficients of reactions. We assume that the temperature
in the discharge zone is constant and equal to the
temperature of the oven, T oven . The set of reactions used
in this work was determined in previous studies [3, 4].
The gas mixture at the DBD exit has undergone
N p = F.V d /Q successive pulses. The modification of the
gas mixture composition during these N p successive
pulses is calculated by injecting the computed densities at
the end of post-discharge #n as initial mixture
composition for the pulse #n+1. The so calculated
composition of the gas mixture at the outlet of the reactor
is compared to the measured concentration of species.
This simplified model being non self-consistent, the input
parameters such as E/N and Ne×∆t must be determined by
comparing calculation results with of densities of stable
species measured at the reactor outlet.
4. Results
4.1. Conversion of acetaldehyde
The removal was studied in N 2 and N 2 /O 2 (from 1% to
3% of O 2 ) mixtures for 20 °C and 300 °C with 500 ppm
For both
of CH 3 CHO as initial concentration.
temperature and mixture conditions, good agreements
have been found between experiment and model in the
low energy range, assuming a reduced electric field of
200 Td. In the high energy range, the model generally
underestimates the conversion. The heat accumulation in
the discharge when the frequency of discharges is high
may increase reaction constants and accelerates
conversion processes.
Fig. 1 shows a comparison
between experimental and calculation results for the
removal of CH 3 CHO in pure N 2 . For E s < 160 J.L-1. The
model results fit well with the CH 3 CHO measurements.
2
Fig. 1.
Comparison of experimental results with
calculations for 500 ppm CH 3 CHO in N 2 at 20 °C,
E/N = 200 Td; symbols represent experimental results
and lines represent model calculation results.
The kinetic scheme for CH 3 CHO decomposition in
pure nitrogen has been previously studied [3]. The
dissociation of acetaldehyde by direct electrons collisions
can be neglected in front of the dissociation due to the
collisions with nitrogen metastable states. The exit
channels of these dissociative quenching of N 2 * by
acetaldehyde where determined. In the present work we
keep these results in order to test them in such different
conditions. Indeed, the former study was held in a phototriggered discharge which generates homogeneous
plasma, while the present one concerns inhomogeneous
discharges. These processes are:
N 2 * + CH 3 CHO → CH 3 + HCO + N 2
→ CH 4 + CO + N 2
→ CH 2 CO + H 2 + N 2
→ CH 3 CO + H + N 2
(1a)
(1b)
(1c)
(1d)
(1e)
N 2 (a’, a, w) + CH 3 CHO → C 2 H 4 + O(3P) + N 2
→ C 2 H 2 + H 2 + O(3P) + N 2
(1f)
Here, N 2 * denotes both N 2 (A) and N 2 (a’, a, w). The
relative importances are 44.3%, 29%, 16.3% and 8.06%
for (1a), (1b), (1c) and (1d) respectively. The ratios of
reactions (1e) and (1f) are less than 2% each. In pure
nitrogen, the dissociation by quenching of N 2 * represents
more than 80% of the acetaldehyde losses.
At 300°C, no significant change is found regarding to
the relative weights but the conversion yield increases.
The reactions involving atomic nitrogen, atomic oxygen,
atomic hydrogen and CH 3 radical have a minor influence
on the removal in nitrogen.
Because of the quenching of nitrogen metastable states
by molecular oxygen, the addition of O 2 changes the
predominant removal processes. In Fig. 2, experimental
measurements are compared with model calculations for
2% O 2 at 20 °C. For energy higher than 60 J.L-1, the
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model
results
underestimate
the
conversion.
Acetaldehyde removal is better in oxygenated mixture
than in N 2 even for small oxygen percentage [6].
with X = H, HCO, CH 3 CO, C 2 H 3 , C 2 H 5 , CH 3 CHO or
CH 2 O.
Reaction (1c) forms 44% of H 2 ; the other formation
processes are recombination reactions between H atom
and other by-products. CO has numerous sources among
which reaction (1b) is the most important, it produces
42% of CO. HCN is a recombination product between N
or CN and other by-product. The dominant reactions
leading to HCN formation are:
N + CH 3 CHO → HCN + H 2 + HCO
H + H 2 CN → HCN + H 2
N + H 2 CN → HCN + NH
Fig. 2. Comparison of experimental and model results for
500 ppm CH3CHO in 2% O 2 mixture at 20 °C,
E/N = 200 Td.
In such a mixture, dissociation by N 2 * quenching
processes are still efficient, but the major conversion
processes are due to oxidation by O atom and OH radical.
They proceed by hydrogen abstraction:
O + CH 3 CHO → CH 3 CO + OH
OH + CH 3 CHO → CH 3 CO + H 2 O
(2)
(3)
For a mixture with 2% O 2 at ambient temperature, the
contribution of N 2 * quenching to the total removal falls to
around 10% (it is 80% in pure nitrogen) whereas
oxidation processes due to (2) and (3) account for more
than 85% of the whole decomposition processes.
4.2. By-products formation
The addition of O 2 also changes the conversion byproducts.
In N 2 , nitrous compounds and light
hydrocarbons are the most important while in N 2 /O 2 ,
oxygenated by-products are the major formed
compounds.
By-products in N 2 include H 2 , HCN, C 2 H 6 , C 2 H 4 ,
C 2 H 2 , CH 4 , CO, CO 2 , HNCO, NH 3 and number of
nitriles (the most important being CH 3 CN). Fig. 1
displays a comparison between model and experiment for
H 2 , CH 4 , CO, CO 2 , HCN and C 2 H 6 . The calculation fits
correctly the experimental results for the most populated
by-product CH 4 . CO and H 2 are primary products formed
by (1b) and (1c). The main production of CH 4 , 52%, is
due to the reaction (1b), the other processes are CH 3
reaction with primary and secondary by-products:
CH 3 + X → CH 4 + X
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(4)
(5)
(6)
(7)
In mixtures containing 2% of oxygen, CO 2 becomes the
most important by-product. No significant change is
found for CO measurements when transiting from N 2 to
N 2 /O 2 mixture with 2% of O 2 . However the model
underestimates its concentration (a good fit was obtained
in N 2 ) but shows a good agreement with CO 2
experimental measurements (see fig. 2).
The contribution of the oxidation of CO to the creation
of CO 2 is less than 1%. According to the model, the major
contribution comes from reactions (8), (9) and (10)
below:
CH 3 CO 2 → CH 3 + CO 2
O 2 + HCO → CO 2 + OH
O(3P) + CH 3 CO → CO 2 + CH 3
(8)
(9)
(10)
The model gives also good results for the CH 2 O
concentration. As a matter of fact, our model includes a
detailed kinetic scheme for the treatment of
formaldehyde. This by-product is also dissociated by the
quenching of molecular nitrogen metastable states and
oxidised by O atoms and OH radicals.
At 300 °C, oxidation processes are accelerated in N 2 /O 2
mixtures due to the increased coefficient of reaction (2)
especially [6], thus acetaldehyde removal increases
together with the concentrations of CO 2 and CO.
Concentration of H 2 is also increased owing to the
constant deposited energy. At the same time the
concentration of CH 2 O increases for low energies and
decreases when the input energy is higher than 120 J.L-1
at 300 °C for 2% O 2 . In these conditions, CH 2 O is totally
removed for energies higher than 365 J.L-1.
4.3. Plasma-catalyst synergy
MnO 2 catalyst has an ability to decompose ozone
according to several studies [7]. However, in the present
work, it has negligible effect on the removal of
acetaldehyde. Figs. 3 and 4 display by-products of
plasma treatment associated with a catalyst for 500 ppm
acetaldehyde in a mixture with 2% O 2 at 40 °C.
Except H 2 , CO and CO 2 all by-products concentrations
decrease due to catalyst effect. Indeed, H 2 is almost
insensitive to the use of the catalyst while CH 2 O
concentration decreases with the use of the catalyst. The
3
concentrations of CO and CO 2 increase because of the
oxidation processes.
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[3]
[4]
[5]
[6]
[7]
W. Faider, et al. J. Phys. D: Appl. Phys., 46,
105202 (2013)
W. Faider, et al. Plasma Sources Sci. Technol., 22,
065010 (2013)
O. Koeta, et al. Plasma Chem. Plasma Process., 32,
991-1023 (2012)
O. Koeta, et al. Int. J. Plasma Environ. Sci.
Technol., 6, 227-232 (2012)
B. Dhandapani and S.T. Oyama. Appl. Catal. B
Environ., 11, 129-166 (1997)
Fig. 3. Evolution of H 2 and CH 2 O concentrations during
plasma and plasma-catalyst treatment of 500 ppm
CH 3 CHO in N 2 /O 2 with 2% O 2 at 40 °C.
Fig. 4. Evolution of CO and CO 2 concentrations during
plasma and plasma-catalyst treatment of 500 ppm
CH 3 CHO in N 2 /O 2 with 2% O 2 at 40 °C.
In the catalyst zone, the main available species are
molecular compounds. Ozone is decomposed on the
catalyst surface to produce oxygen atoms. The slight
increase of CO and CO 2 concentration denotes the
catalyst activity. To destroy all by-products, catalyst
parameters should be optimised.
5. Acknowledgement
Authors acknowledge the Organisation for the
Prohibition of Chemical Weapons for his financial
support.
6. References
[1] A. Vandenbroucke, et al. J. Hazardous Mat., 195,
30-54 (2011)
[2] N. Sano, et al. Ind. Engng. Chem. Res., 36,
3783-3791 (1997)
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