Kinetics analysis of acetaldehyde removal by non-thermal plasmas of
atmospheric gases
W. Faider, S. Pasquiers, N. Blin-Simiand, P. Jeanney, F. Jorand and L. Magne
LPGP-Laboratoire de Physique de Gaz et des Plasmas, Université Paris Sud 91405 Orsay, France
Abstract: A promising solution to eliminate low concentration of Volatile Organic Compounds in air at low
temperature is association of a catalyst with a non-equilibrium plasma. It is in particular one of the main
combustion products of biofuels. Our work focuses on the removal of CH3CHO for a concentration between
500 ppm and 5000 ppm in nitrogen and N2/O2 mixtures at 460 mbar, using a photo-triggered discharge (60 ns
for duration) producing homogeneous transient plasmas. The homogeneous plasma is found to be very
efficient for the acetaldehyde removal. The main by-products of acetaldehyde removal are hydrogen,
hydrocarbons and several oxygenated compounds detected by chromatography. Moreover, a time resolved
measurement of the hydroxyl radical density, [OH], is made in the discharge afterglow using UV absorption.
Hydroxyl radical is measurable only for N2/O2/CH3CHO mixtures. The maximum of density is measured for
20 % of oxygen in the mixture. The quenching of metastable nitrogen, N2(A3Σ+u), in the phototriggered
discharge is more important than electronic collisions for acetaldehyde removal. We estimate the quenching
coefficient at 5.55×10-11cm-3.s-1.
Keywords: Photo-triggered discharge, Hydroxyl radical, VOC removal, nitrogen metastables state
1. Introduction
The photo-triggered discharge allows producing
transient homogeneous plasma [1, 2]. The present
work deals with the kinetic processes involved in the
removal of acetaldehyde in nitrogen and in
oxygen/nitrogen mixtures. The study is made with the
help of model predictions [3] compared to main byproducts density measured by gas chromatography and
measurement of OH radical by absorption
spectroscopy in the UV. Main parameters are the O2
percentage in the mixture (at 460 mbars), the
concentration of acetaldehyde in the mixture (from
500 to 5000 ppm). Results are discussed with respect
to the existing literature. The removal of acetaldehyde
in nitrogen mixture is examined in order to get
information about the role of N2 metastables. We give
an estimation of acetaldehyde quenching coefficient
by N2(A 3 Σ u+ ) to explain experimental results.
2. Acetaldehyde molecule
In order to study kinetics processes of volatile organic
compounds in non-thermal plasmas, acetaldehyde has
been as a model molecule. The bond energies of
acetaldehyde, ethane and acetone are presented in the
figure 1. Bond energies are obtained following
calculations performed with the 6-311G(d,p) basic set
and using the B3LYP theoretical method [4]. The
difference of bond energies between acetaldehyde and
the others presented is noticeable. Indeed, contrary to
those molecules, acetaldehyde is dissymmetric. This
dissymmetry could be responsible for the decrease of
all the bond energies that makes acetaldehyde to be a
molecule easier to break than symmetric molecules
like ethane.
Figure 1 Acetaldehyde, ethane, and acetone energy bonds.
The major species, produced by the discharge, which
3 +
are able to break CH3CHO bonds are N2(A Σ u ),
nitrogen singlet states, O(1D), O(3P), and electrons. To
the authors’ knowledge, the cross sections for neutral
dissociation processes of acetaldehyde by electron
collisions do not exist, so that it is approximated to the
cross sections of ethane in our model. However the
cross sections for ionization processes were already
measured [5].
3. Experimental Set-up
3.1. The UV510 reactor
A comprehensive description of the photo-triggered
discharge operating mode was previously described in
detail [1, 2] and only a brief description of our device
is given here. Two electrodes, 50 cm long with a
spacing d = 1 cm and a flat profile over 1 cm width,
are directly connected to an energy storage unit of
capacitance C = 17.44 nF charged up to a voltage V0
in a few hundred nanoseconds. Once V0 is reached on
the electrodes, the gas breakdown is achieved through
photo-ionization of the gas mixture by UV photons
that are produced by an auxiliary corona discharge
located at the bottom of the main discharge. The total
pressure of the studied mixtures is fixed at 460 mbar,
and the initially applied reduced electric field between
the gap, (E/N)0, given by (E/N)0 = V0 / (d x N), where
N is the total density of the gas mixture, is 200 Td (V0
= 23 kV). The deposited energy in the discharge is
fixed at 4.6 J (specific energy equal to 92 J l−1 in the
gap volume). The low discharge circuit inductance,
6.5 nH, allows us to obtain a short current pulse
duration of 60 ns. A gas compressor is used to produce
a gas flow through the discharge gap.
Figure 2 - Schematics of the experimental set-up.W1 W2-fused
silica windows, DG-discharge gap, R-UV510 reactor, Ccompressor, G-absolute capacitance gauge, GI-gas mixture inlet,
PU-pumping unit, GCy-Gas cylinder N2 and N2/O2(20%), AcAcetaldehyde in liquid phase.
The discharge frequency, 1.25 Hz, is chosen such that
the whole reactor volume, 500 cm3, is renewed
between two discharges. The volume of the
experimental device, VT = 10.2 L, which corresponds
to the total volume of the gas mixture studied, is much
higher than the discharge one, VD = 50 cm3. The ratio
of the two volumes is 204.
3.2. Time resolved UV absorption
The time resolved UV absorption for the measure of
the OH density has already been described in detail
[6]. The implantation of the absorption diagnostic on
the UV510 reactor is presented in figure 2. The reactor
is closed at each extremity by fused silica windows
(W1, W2) allowing UV radiation transmissions. The
absorption length of the studied medium corresponds
to the electrodes’ length, i.e. 50 cm. It is lighted at one
side by UV radiations emitted by a xenon flash lamp
(FL). This emission is collimated by a fused silica lens
(L1) and passes through the electrode gap. The
transmitted light is then collected by a second lens
(L2) and properly focused on a fused silica optical
fibre (OF). The other extremity of the fibre is imaged
on the entrance slit of an imaging spectrometer (IS)
having a 75 cm focal length and equipped with a 2400
linesmm−1 grating. The detection is achieved using a
gated intensified CCD camera (IC), whose acquisition
gate is synchronized with the xenon flash lamp
emission. The delay between the discharge current
pulse and the acquisition gate together with the flash
lamp emission can be varied in order to scan the time
afterglow, from 0.5µs up to several hundred
microseconds after the current pulse. The temporal
resolution of the measurements is given by the
acquisition gate width, which is fixed to 250 ns. This
gate width allows a good signal over noise ratio to be
obtained for a moderate number of accumulated
transmitted signals (20 shots at the maximum).
3.3. By-products measurement
The concentration value of acetaldehyde is monitored
for a given mixture composition by pumping a gas
mixture sample (1 cm3 at 460 mbar) in a syringe and
introducing this sample in a FID chromatograph. The
following discharge by-products are detected:
methane, acetylene, ethene, ethane, hydrogen cyanide,
acetone, methyl nitrate, nitromethane and acetonitrile.
Only densities of hydrogen cyanide and methyl nitrate
are not quantified because of the difficulty to calibrate
them.
4. Kinetic modelling
The experimental measurements are compared with
predictions of a self-consistent model of the reactor,
linking physics and chemistry of the discharge. This
model was already described in previous publications
in case of a single current pulse [1, 3, 6, 7], together
with kinetic data used for N2 and O2 molecules. For
the present study, it was improved in order to describe
effects of accumulation of current pulses, i.e.
treatment of the various species (in addition to the
acetaldehyde) produced by the discharge pulse after
pulse (locked loop model). We obtain first the time
evolution of species densities up to 1 second after the
initiation of the discharge by the pre-ionisation. At this
time the chemistry of the excited mixture is
completed; there is no more radicals and only different
types of molecules remain in the gas mixture. Then,
the density of the ith specie obtained at 1 second in the
discharge volume, for the first current pulse, XiD(1), is
diluted in the total volume of the device. This new
density values, XiT(1) is simply given by,
XiT(1) = XiD(1).VD / VT
Then, all densities XiT(1) – i.e. for all remaining
molecules at 1 s - are used as initial conditions for the
second pulse, and so on until the Nth pulse for which
densities have been measured by chromatography. For
each pulse, we neglect the effect of electronic
collisions on the by-product diluted in the total
volume, but we take into account quenchings of N2
states by these compounds.
5. Experimental and modelling results
5.1. Acetaldehyde removal in nitrogen
Acetaldehyde removal is realized for different initial
concentrations. The UV 510 reactor appears efficient
for the removal of this compound, as previously
demonstrated for a mixture containing 5 % of oxygen
[2]. However it is very difficult to have a total removal
of the pollutant because the treated mixture is diluted
in the total volume after each pulse; only a part of the
gas mixture is treated by the homogeneous discharge
pulse after pulse. In the followings the number of
discharges, ND, performed on the mixture is converted
in deposited energy, ED, in the total volume. It is given
by,
ED=ND.EP / VT
and the computation results, different exit channels for
reaction (1) can be suggested,
(3) N2(A) + CH3CHO → CH3 +HCO
29%
29%
(4) N2(A) + CH3CHO → CH4 +CO
(5) N2(A) + CH3CHO → CH2CO + H2 28%
(6) N2(A) + CH3CHO → CH3CO + H
8%
(7) N2(A) + CH3CHO → CH2+HCO+H 6%
The main channels are the dissociation of
acetaldehyde in CH3 and HCO, the production of
methane, carbon monoxide and ketene (CH2CO). As
shown in Figure 4, the proposed dissociation channels
are consistent with measurements.
Figure 4 – Composition of the mixture in the total volume of the
device, as function of the number of discharges performed. Lines
are predictions of the self-consistent model.
Figure 3 - Concentration of acetaldehyde measured as a function
of the deposited energy for two initial concentration values.
As shown in Figure 3, the removal law is an
exponential decay, i.e.
C = C0.exp(-ED/β)
where C0 is the initial concentration. The constant β
(the so-called characteristic energy) depends on the
initial concentration: β(CO = 1095 ppm) = 167 J/L, and
β(CO = 4945 ppm) = 383 J/L.
We found that the electronics collisions (excitation
and ionisation processes) are not sufficient to explain
experimental results. In fact, nitrogen metastables play
an important role, as it was previously suggested for
the removal of acetaldehyde in a dielectric barrier
discharge [8]. To fit our experimental measurements
with model results, computation is performed taken
into account the following quenching reactions of
nitrogen metastables by acetaldehyde,
3 +
(1) N2(A Σ u ) + CH3CHO → Products
(2) N2(singlets) + CH3CHO → Products
The appropriate value for rate constants is 5.55 10-11
cm-3.s-1 for reaction (1), and 10-10 cm3s-1 for reaction
(2). Such values are consistent with those of other
+
molecules. Reactivity of N2(A Σ u ) with molecules
introduced in section 2 can be schematized as :
acetone > acetaldehyde > ethane [9]. Thanks to the
comparison between the by-products measurements
3
The UV diagnostic does not detect the OH radical in
the discharge afterglow in case of N2/CH3CHO
mixtures. Moreover previous measurements using LIF
[2], with a much better sensitivity, show the same
effect. That means that the probability to break the
C=O bond in the molecule of acetaldehyde is weak.
However the dissociation of acetaldehyde produces
CH3 and CH2 radicals, which are responsible for the
formation of various hydrocarbons (methane, ethane,
ethene, acetylene), principally CH4 for the methyl
radical,
(8) CH3 + H + N2 → CH4 + N2
(9) CH3 + HCO → CH4 + CO
Acetone comes from the recombination of CH3CO
with CH3. Acetaldehyde can be reformed in the post
discharge by three reactions, including addition of
radicals,
(10) CH3 + HCO → CH3CHO
We have also found that reactions of nitrogen
metastables with by-products [9] are very important
for the model computation, because by-products pump
the N2(A 3 Σ u+ ) and thus it induces a decrease of the
rate of reaction (1).
5.2. Production of OH
The kinetic is more complex in N2/O2/CH3CHO
mixtures than in N2/CH3CHO. Nitrogen metastables
are quenched by the oxygen molecule, and therefore
their importance on the acetaldehyde removal
decreases. However, the oxygen has a positive effect
on the removal of acetaldehyde, as shown in Figure 5.
Figure 5 - Concentration of acetaldehyde measured as a function
of the number of discharges performed in a mixture without and
with 10 % of oxygen, for the same initial concentration value.
In the mixture containing oxygen, O(3P), O(1D), and
OH radical react with acetaldehyde.
Firstly, the electronic collisions dissociate and ionize
acetaldehyde like in the nitrogen based mixture, but
these processes remains not very important. Secondly,
the reactive species produced by the plasma interact
with acetaldehyde; in particular the reaction of
CH3CHO with O(1D) is strongly probable [10]. The
oxidation of CH3CHO produces OH radicals which
later react with CH3CHO and the by-product too.
Important production reactions for the hydroxyl
radical are:
(11) O(3P) + CH3CHO → CH3CO + OH
(12) O(3P) + CH3CO → CH2CO + OH
Probably the atomic recombination reaction,
(13) O+ H + N2 → OH + N2
plays also a role for the studied mixture. It is followed
by:
(14) OH + CH3CHO → CH3CO + H2O
(15) OH + CH3 → Products
(16) OH + HCO → H2O + CO
The time resolved measurement of the OH radical
density was made for different concentration of
oxygen, see figure 6. We found that the effect of
oxygen is not noticeable for concentration from 5 %
up to 20 %. The radical density is in the same order of
magnitude. The maximum density measured is 3.4
1014 cm-3 in mixtures with 10-20% of oxygen.
Figure 6 – Time evolution of the OH radical density measured in
the discharge afterglow, for different values of the oxygen
concentration in the mixture and for 5000 ppm of acetaldehyde.
6. Conclusion
The value of the nitrogen A state quenching rate by
acetaldehyde estimated by this work is 5.5 10 -11 cm3 -1
.s , with a factor 1.5 error.
In oxygen containing mixtures, there are a lot of
parameters and molecules which could play a role in
the kinetics scheme of OH radicals. Currently some
important uncertainties remain. For example, the
addition of the molecular oxygen to the radical
CH3CO produces CH3CO3, which can later give
CH3CO2. However the kinetics of those radicals is not
very well known. Work is in progress to compare
model predictions for the OH radical density to
absorption measurement results, taking into account
the proposed dissociation mechanisms (3)-(7).
REFERENCE:
[1] B. Lacour et al. (2003), Recent Res. Develop.
Appl. Phys. 6, 149-191.
[2] L. Magne et al. (2005), J. Phys.D : Appl. Phys.
38, 3446-3450.
[3] F. Fresnet et al. (2002), Plasma Sources
Sci.Technol. 11, 152-160.
[4] M. Frisch et al 2004 Gaussian 03, Revision C.02,
Gaussian Inc., Wallingford, CT.
[5] J-R. Vacher, N. Blin-Simiand, F. Jorand, S.
Pasquiers, Chem. Phys., 323 (2006) 587-594.
[6] L. Magne et al. (2009), J. Phys. D: Appl. Phys.
42, 165203.
[7] N Moreau et al. 2010 J. Phys. D: Appl. Phys. 43
285201 (14pp).
[8] O. Koeta et al. (2010), Proceedings Hakone
XIIth (12-17 sept., Slovakia).
[9] J. Herron (1999), J. Phys. Chem. Ref. Data 28,
1453.
[10] Jin jin Wang et al. (2003), J. Phys. Chem. A 107,
10834-10844.