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Electrolysis Processes in D.C. Corona Discharges in Humid Air
J. Lelièvre, N. Dubreuil, J.-L. Brisset
To cite this version:
J. Lelièvre, N. Dubreuil, J.-L. Brisset. Electrolysis Processes in D.C. Corona Discharges in Humid Air.
Journal de Physique III, EDP Sciences, 1995, 5 (4), pp.447-457. �10.1051/jp3:1995139�. �jpa-00249322�
Classification Physics Abstracts
52.40 52.80
Electrolysis Processes in D.C. Corona Discharges in Hundd Air
J. Lelibvre(~), N. Dubreuil(~) and J.-L. Brisset(~,*)
(~) S-I-R-C-O-B-, Universitd de Versailles-Saint-Quentin, Bitiment Lavoisier, 45 Avenue des Etats-Unis, 78000 Versailles, France
(~) L-A-S-T-M- L-E-I-C-A-, UFR Sciences de l'Universitd de Rouen, 76821 Mont-Saint-Aignan Cedex, France
(Received 2 November1994, revised 19 December 1994, accepted 3 January1995)
Rksumd. L'exposition d'une solution aqueuse aux neutres d'une ddcharge couronne pointe-
plan continue dtablie darts l'air humide fait apparaitre en solution des ions nitrites et nitrates
qui dquilibrent la formation de protons. La concentration en nitrates croit continfiment tan- dis que celle des nitrites prdsente un maximum. Un mdcanisme d'oxydations successives est
proposd; il implique des rdactions dlectrochimiques h chaque dlectrode et rend compte que la
ddcharge ndgative engendre des concentrations
en nitrite supdrieures h la ddcharge positive. Un d4veloppement du modkle concourt h expliquer la diff4rence d'effets observds pour des ddcharges positives ou n4gatives selon la nature du gaz plasmagbne.
Abstract. Aqueous solutions exposed to the flux of the neutrals emitted in a d-c- point-to- plane corona discharge in air are enriched with NOp and NOp anions
as the matching counter- ions of the protons. The nitrate concentration continuously increases with the treatment time while that of the nitrites presents a maximum. Both concentrations
are increasing functions of
the current intensity and the exposure time. These results are examined in terms of successive
electrochemical reactions and involve oxidation and reduction reactions at each electrode.
1. Introduction
In their study [1,2] of the pitting corrosion of aluminum exposed to a d-c- corona treatment in humid air, the authors developed their interpretation up to suggest certain similarities between the chemical transformations involved in and by the plasma treatment and those occurring in
an electrochemical cell. In this paper we plan to go on with this approach and state its validity
on the basis of new experiments performed with a d-c- point-to-plane corona discharge in humid air. We intend to demonstrate that classical electrochemistry holds and provides a worthwhile complementary tool for understanding and predicting a determinant part of the (*) to whom correspondance should be addressed
© Les Editions de Physique 1995
448 JOURNAL DE PHYSIQUE III N°4
chemical reactions involved in a d.c. corona discharge, although it remains unsuitable to explain
the formation of the activated species.
When a d-c- point-to-plane corona discharge is ignited, a series of complex reactions takes
place at the active electrode (I.e. the electrode with the smaller radius of curvature) and involves the occurrence of both charged and chargeless species in its close neighbourhood, I-e- in the ionization volume. In the drift zone lying between the electrodes, the most numerous
heavy species found are the so-called "neutrals", I-e- atoms, molecules and radicals, which are carried away by the electric wind and the ions which are attracted by the plane electrode of opposite polarity (e.g. the anions for a negative discharge created by a HV negative point).
In addition, all these heavy species are present in their fundamental state or raised to some
electronically and for vibrationally excited state.
The corona discharge in air has been thoroughly considered by numerous authors [1-11] and
some of them focused on the relevant chemical properties. The main ions generated in the discharge in humid air were those listed [4-11] according to the polarity of the active electrode.
Anions such as nitrites and nitrates, carbonates and oxygen anions are the prominent ions in
~ negative discharge while protons, oxygen and (NO~) cations are the major cationic species created in a positive discharge.
Since we are interested in the chemical properties induced by the neutral species, we first trapped the ions and exposed an aqueous solution to the flux of the neutrals to give evidence of their reactivity.
2. Experimental
The device suggested by Goldman [12] has been thoroughly used [13-16] and revealed conve- nient for ion trapping. The stainless steel point electrode is raised for most of the time to the
negative HV; the plane electrode is replaced by a grounded plane stainless steel grid acting as
an ion collector but remainp transparent to the chargeless species. The point electrode is set
vertically and perpendicularly to the grid. An electrically insulated aqueous solution placed
under the grid is exposed to the flux of the chargeless activated species and is subsequently analyzed. The proton and the nitrate [17] concentrations are measured by means of specific
electrodes (Tacussel); the nitrites are determined spectrophotometrically according to standard methods and confirmeJ by ion chromatography measurements [18,19].
The pH measurements were performed according to the following procedure which allows
the target solution to remain at the same distance from the point for the whole discharge duration. The discharge was operated for about 5 minutes and then switched off; the solution
was then stirred until the pH stabilizes. Then a new run was initiated on the same solution by stopping the stirrer and switching on the discharge for another 5 minutes. Such a procedure
was found to favour the diffusion into the solution of the species formed at the surface and to
provide more reliable results than the mere exposure of a liquid target to the neutral flux with
or without (vigorous) stirring.
The experimental setup used for the (NOp ions measurements involved a 10 Mfl resistance instead of 25 Mfl for the acidity study.
3. Results and Discussion
The acidity increase of the solution after exposure to the flux of the chargeless activated species was demonstrated by the use of acid-base chemical indicators [13]. It appears as a
general feature of plasma treatment in humid atmosphere since Waldie et al. [20] pointed out a marqued enhance of the local acidity on their underwater plasma cutting experiments. We
quantified [14] the pH variations by means of electrode measurements: the pH (I.e. pH m
logic C(H+)) decrease is a linear function of both the current intensity and the exposure time, I.e. of the quantity of electricity involved in the discharge. This suggests that electrolysis
processes are involved.
However, the electroneutrality principle holds for the aqueous solution and leads to question
about the associated anions. Conductometric measurements [15] achieved in connection with the acidity measurements confimed the occurrence of anionic species but were unsuitable to determine their nature. Since the formation of nitrates and nitrites was reported [1,11] we
focused on this couple of ions.
The evolution of the concentrations of NOp and NOp ions with the current intensity has been reported [7-11] for both discharge polarities: the nitrate concentration was found to continuously increase for a negative corona, while the nitrite concentration presents a maximum and tends to zero for high intensities. Also, for positive coronas, the general behaviours of the
relevant concentrations versus the current intensity are similar, but the concentration values
are markedly enhanced for NOp. In fact, the experimental setup used for some of these
experiments [11] is unsuitable to give clear evidence of the chemical effect of the neutral species:
the plane electrode is immersed in the solution which implies that oxidation-reduction reactions must take place at the electrode in the liquid phase. In addition, the discharge duration is not specified but we can reasonably assume that it was standardized. In the relevant paper
measurements of "water evaporation" versus the current discharge are mentioned, but the mere
electrolysis of water was not considered at this juncture.
Resuming the formation of nitrites and nitrates ions in the insulated aqueous target on ex-
posing the aqueous solution only to the flux of the neutrals, we report here the variations of the relevant ions concentrations with the exposure time t for intensity- stabilized discharges.
The nitrite concentration (Fig. 1) presents a maximum while the nitrate concentration con-
tinuously increases with t and exhibits an inflexion point. Besides, the characteristic points
on both plots (I.e. the maximum for the NOp plot and the the inflexion for the NOp plot)
for both polarities of the active electrode take place for the same value of t. Such a feature reminds the chemist the classical scheme for two lst- order consecutive reactions.
Kinetic experiments performed with different current intensities I lead to similar results for
a negative discharge: the C(NOp) versus t plot presents a maximum and the C(NOp) versus
t plot increases continuously (Figs. 2-4). Another illustration is provided by Figures 5 and 6 which are relevant to the variations of nitrate and nitrite concentrations with the current
intensity for given exposure times. Therefore, the concentrations obey similar variation laws with t and I: they must then depend on the product I t, I-e- on the quantity of electricity
involved in the discharge.
The puzzling maxima exhibited in Figure 6 already suggest some remarks. For high current intensities which are expected to lead to the higher nitrate concentrations the concentration of the active species at the gas / solution interface is limited by the flow of the electric wind
which blows them away since this flow increases with the current intensity. Consequently less
numerous NOp ions formed at the surface are able to diffuse into the solution, which can lower
the nitrate concentration in the liquid phase.
We then varied the discharge polarity. For a given current intensity and for fixed geometrical parameters of the reactor, a negative discharge was found more efficient than a positive one
since the NOp yield was marquedly superior for a negative discharge than for a positive one
(Figs. 7-8). Such a feature was already observed for the proton concentration in a solution exposed to the neutrals. However, the values of the nitrate concentrations (Figs. 7-8) suggest
opposite conclusions. This may be connected [11] to the occurrence of streamers, which form
more easily in a positive discharge than in a negative one.
450 JOURNAL DE PHYSIQUE III N°4
~NOi ~
N03 ,
o'
o-o-o
~' / °
100 /
~ o
/
~
_/ o
~/° °o"°fi
%' /
° 2
~
u /°
u o
u oo
coo~o
tm,n
t m"n
Fig. I ° ~°° Fig. 2 ~°°
Fig. I. Typical plots for the evolution with the exposure time t (minutes) of the nitrite (empty squares) and nitrate (filled dots) concentrations in a negative discharge. Current intensity: 90 pA;
electrode gap: d = 11.4 mm; point to target distance: D
= 20 mm.
Fig. 2. Nitrite production (mg. L~~)
versus exposure time (minutes) as a function of the current
intensity for a negative discharge [dots: 1
= 90 pA; stars: 1
= 50 pA; squares: 1
= 15 pA]. d
= llA
mm; D
= 20 mm.
,l'~~~
/~~~~~.~'
/°/ /~ /~
s ~
c /°
l'
~ cl %/ .
c ~
tm;n ° tm~n
o o
Fig. 3 Fig. 4
Fig. 3. Variations of the Nitrate concentration C~~- (mg.L~~) versus exposure time t (minutes)
for various current intensities [dots: 1 3
= 50 pA; squares: 1 = 30 pA; triangles: 1= 15 pA] in a
negative discharge. Id
= llA mm; D
= 20 mm].
Fig. 4. Variations of the Nitrate concentration C~~- (mg.L~~) versus exposure time t (minutes)
for various current intensities [dots: 1 3
= 90 pA; empty squares: 1
= 70 pA; stars: 1= 50 pA] in a
negative discharge. Id = llA mm; D = 20 mm].
4. A Tentative Interpretation
We plan now to consider the experimental results reported above and to interpret most of them
on the basis of electrolytic processes in water, in agreement with the relevant fundamental laws. The following assumptions which are basically mere truisms from an electrochemist's point of view appear as complementary to the reactions usually considered: I) the release and
°2 ,~~.-.~
~~
~~
~
./" ~u~~°~ ~/
o
°
/
.
.~.-.-.~
-~_
o
452 JOURNAL DE PIIYSIQUE III N°4
fable I. Selected values of standard electrode potentials (Volts, versus NHE). From the mere
thermodynamical point of view, nitrogen should exist only at the oxidation degrees -III, 0 or + V; all the intermediate can be oxidized into various nitrogen oxides and nitrates, and NO into
nitrites and nitrates.
031 02 [2.07] N03-/ N2 [1.24] H02-/OH- [0.88]
N2 NH4+ [0.27] HN3/NH4+ [1.96] 021H20 [1.23]
N03-lN204 [0.81] 02/ H02" [-0,08] H2021H20 [1.77]
N03"1 N20 il. I1] N03"1 N02 [0.81] C021CO [-0.12]
N201N2 [1.77] N2041HN02 [1.07] NOIH2N202 [0.71]
C021HC02H [-0.2 NOIN20 [1.59] HN021NO [0.98]
021H202 [0.69] N21N2H5+ [-0.23] NO+INO [1.46]
N03-INO [0.96] N03"lN02- [0.49] C021H2C204 [-0.49]
H3NOH+lN2H5+ [1.42] N03-IHN02 [0.94] 0210H" [0.41]
H2Ole~q [-2.07] N2H51NH4+ [1.27]
considered in solution. This is a reasonable assumption since the discharges take place in
a water-saturated air so that any solid surface in atmospheric air is covered with adsorbed
water molecules. Hence the standard electrode potentials (Table I) can be used and provide
an interesting tool for predicting purposes. According to Table I, the various nitrogen oxides
should not thermodynamically exist. This is due to a series of dismutation reactions and the only system to be considered should be N2/NOp but actually, most of the NO~ species
exist and react slowly in the standard conditions [21-23]. This allows us to consider as a first
approach the following kinetic system of consecutive oxidation reactions:
N2 - NOp - NOp.
Direct consequences of the considered electrolysis processes in humid air will be first with- drawn from a "static" point of view corresponding to the reactions at the electrodes. We shall later extend to transport phenomena and consider the diffusion of the created species from the electrode where they are generated to the other. We shall then focus on the particular case
of a point-to-plane corona discharge since the device induces an unsymmetrical transport of
matter from the point electrode to the plane one due to the electric wind and prevents the
diffusion of species in the opposite direction.
The reduction reactions at the cathode involve the electrode material M (I.e. the reduction of oxide layers if any) and the gaseous environment (e.g. oxygen may be reduced into oxygen
anions or into hydrogen peroxide since water vapour is present). So:
02 (oxidation number 0 for O] yields O(-I) like in O(~, H202 or HOP and O(-II) like in
O~~, OH~ and H20.
H20: the protons in the water molecule may be reduced into H2.
N21 N(0) may formally yield small quantities of hydroxylamine [N(-I)], hydrazine [N(-II)]
and ammonia [N(-III)] due to the occurrence of hydrogen.
Some of the relevant electrochemical reactions
are given as follows:
02 + 4H+ + 4e~
= 2H20 (E°
= 1.23 V) (1)
02 + 2H+ + 2e~
= H202 (E°
= 0.68 V) (2)
02 + 2H20 + 4e~
= 40H~ (E°
= 0.40 V) (3)
2H20 + 2e~
= H2 + 20H~ (4)
N2 + 4H+ + 4e~
= 1(H2 .N4 (5)
N2 + 6H+ + 6e~
= 2NH3 (6)
N2 + 8H+ + 6e~
= 2NH( (E°
= 0.27 V) (7)
The hydrogen peroxide formed may be reduced into water:
H202 + 2H+ + 2e~
= 2H20 (E°
= 1.77 V) (8)
In fact, only some of these reactions are known not to occur in solution due to kinetic reasons and must then be discarded (e.g. the reduction of N2).
The oxidation reactions at the anode concern the electrode material and the ambient gases.
The metal of the electrode may be oxidized into metal oxide. Since air is a mixture of oxygen and nitrogen both components must be considered, as well as water vapour.
Oxygen yields ozone in the presence of water vapour:
02 + H20 = 03 + 2H+ + 2e~ (E°
= 2.07 V) (9)
Oxygen O(-II) and O(-I) respectively in H20 and H202 is oxidized into H202 and 02.
2H20 = H202 + 2H+ + 2e~ (E°
= 1.77 V) (10)
H202 " 02 + 2H+ + 2e~ (E° = 0.68 V) (11)
2H20 = 02 + 4H+ + 4e~ (E°
= 1.23 V) (12)
Nitrogen N(0) is not easily oxidized in solution and is said to be inert. However, when activated by an electric discharge or at elevated temperature, it dissociates and reacts with molecular oxygen to give NO. This oxide is further oxidized by air into N02 and yields NOp by reacting with water according to the overall reactions [21]
N2 + 02 = 2NO (13)
2NO + 02
" 2N02 (14)
3N02 + H20
= 2HN03 + NO (15)
The tables report the,>following electrochemical reactions at the electrode:
NO + H20
= HN02 + H+ + e~ (E° = 0.99 V) (16)
N02 + H20
= NOp + 2H+ + e~ (E°
= 0.81 V) (17)
454 JOURNAL DE PHYSIQUE III N°4
HN02 + H20 = NOp + 3H+ + 2e~ (E°
= 0.94 V) (18)
NO + 2H20
= NOp + 4H+ + 3e~ (E°
= 0.96 V) (19)
N20 + 5H20 = 2NO[ +10H+ + 8e~ (E°
= 1.11 V) (20)
N2 + 6H20
= 2NOp +12H+ +10e~ (E°
= 1.24 V) (21)
One-way transport of matter: effects of the electric wind. We have just reviewed the electro- chemical reactions which can take place at each electrode and involve the electrode material
and/or the ambient gas. We have now to take into account the one-way transport of matter from the point electrode to the plane which is induced by the electric wind.
The species electrochemically formed at the point electrode are carried away to the plane electrode. They can there electrochemically react according to their reverse formation reaction
or combine with products created at the plane electrode. Preparation of ozone is clearly
illustrative at this juncture, since satisfactory yields of ozonizers put on the use of a separator between the electrodes to prevent the mixing of the fluxes, and consequently the reductive destruction of the product. It's noticeable that for the same purpose separated compartments
are of common occurrence in solution electrolysis devices.
Direct consequences proceed from these considerations. Discharges burning in the same conditions should lead to quantitatively different results according to their polarity: the closer is the target solution to the anode, the higher are the NO~ concentrations in the solution,
in agreement with the pH decrease. This feature is due to the ability of the oxidized (or reduced) neutral molecules to react with the reduced (respectively oxidized) species in the cathodic (respectively anodic) compartment. It may then explain the different yields in the NO~ concentrations in the aqueous target, for a negative and a positive discharge, additionally
to the filtering effect of the grid which is more efficient towards the NO~ anions for a positive discharge than for a negative one.
A development of this approach also leads to propose [12] that in certain cases the neutrals should be of different natures depending on the discharge polarity. The resulting treatment of
a target by the neutrals should be then directly related to the discharge polarity. This may
occur when the plasma gas undergoes oxidation and reduction reactions.
General feature of the "electrolysis approach". We have now to check whether this approach holds or not unreservedly for the chemical effects observed in a d-c- point-to-plane corona discharge burning in humid air. Our starting assumptions were that oxidation and reduction reactions take respectively place at the anode and at the cathode, and also that the standard oxidation-reduction potentials were to be considered as the determining terms to control the reactions which are formally the same as those which occur in solution.
The first main difference between the corona treatments and the electrolysis processes lies on the magnitude of the potential falls. The corona discharge is put on if the voltage fall is superior
to a threshold of several kilovolts while electrolysis are performed in solution with much lower
potential falls (a few volts only). The second difference is that the potential scales used in the two processes are referred to different origins: the high voltage for the gas discharge is referred to ground while both the anode and the cathode potentials in electrolysis are referred to that of the H+ /H2 system in standard conditions (I.e. the NHE).
Despite the descrepancies mentioned above, numerous arguments are available in favour of
the occurrence of oxidation-reduction reactions in the d-c- corona discharge treatment. They
are presented as the following themes: I) oxidation reactions take place at the anode and reduction reactions at the cathode, and they involve the electrode material or the ambient gas.
