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(A

3Σ

+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 by-products 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+Σ u

3 ) 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

are able to break CH3CHO bonds are N2(A+Σ u

3 ),

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, C-compressor, G-absolute capacitance gauge, GI-gas mixture inlet, PU-pumping unit, GCy-Gas cylinder N2 and N2/O2(20%), Ac-Acetaldehyde 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, Xi

D(1), is diluted in the total volume of the device. This new density values, Xi

T(1) is simply given by, X i

T(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

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,

(1) N2(A+Σ u

3 ) + 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

3 ) with molecules

introduced in section 2 can be schematized as : acetone > acetaldehyde > ethane [9]. Thanks to the comparison between the by-products measurements

and the computation results, different exit channels for reaction (1) can be suggested,

(3) N2(A) + CH3CHO → CH3 +HCO 29% (4) N2(A) + CH3CHO → CH4 +CO 29% (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. 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 +Σ u

3 ) 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

cm-

3.s-1, 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.