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Mechanism of ion formation in high-temperature flames
222
IONS IN FLAMES
REFERENCES 1. PAYNE, K. G., AND WEINBERG, F. J.: Eighth Symposium (International) on Combustion, p. 206. The Williams & Wilkins Company, Baltimore, 1962. 2. LOEB, L. B.: Basic Processes of Gaseous Electronics. University of California Press, Berkeley, 1955.
3. VOGEL, A. L. : A Text Book of Chemical Analysis. Longmans, Green & Company, Inc., London, 1937. 4. PAYNE, K. G., AND WEIN~ERG, F. J.: A preliminary investigation of field induced ion movement in flame gases and its applications. Proc. Roy. Soc. (London), A250, 316 (1959).
17
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES By T A K A Y U K I FUENO, NALIN R. M U K H E R J E E , T A I K Y U E REE AND H E N R Y E Y R I N G
Introduction It is well recognized that free electrons and a variety of positive ions are formed in the hightemperature flames of many fuel-oxidant mixtures. A complete solution of the problem on the mechanism of formation of ions and electrons in flames has not been easy to obtain because of insufficient information on the nature of the chemical changes involved in flames. However, it seems likely that the phenomenon of ionization would be directly related to a few elementary processes occurring in flames. It has been demonstrated mass-spectrometrically by Deckers and van Tiggelen' and by Knewstubb and Sugden2 that the most abundant positive ions produced in flames of hydrocarbons are H30 +. The present paper attempts to calculate the population of this ion by proposing a possible mechanism for its formation mainly in hydrocarbon-air flames. In the first place, the quasi-equilibrium concentrations of H30 + in these flames are calculated on a simple statisticalmechanical basis under the assumption that the total amount of hydrocarbons and oxygen supplied is equilibrated with the system comprising HaO+, CO2 and free electrons at the adiabatic flame temperature. The calculated concentrations of I-IsO+ are found to be too large by a factor of about 10 5 for all of the hydrocarbons studied so far, and this value of 10 5 may be used to predict the ion concentrations in hydrocarbon flames.
Another semitheoretical trial is made for obtaining the ion concentrations in hydrocarbon
flames, if a possible kinetic mechanism for the ion formation is assumed. The essential concept of this treatment lies in supposing that at a particular zone of the flame where the over-M1 rate of combustion is maximum, fuel and oxygen are subjected to a sequence of chemical changes including ionization, thus yielding free electrons and the most abundant positive ions. The formation of these charged species is assumed to satisfy the steady-state condition.
Quasi-Equilibrium Calculations The quasi-equilibrium concentrations of H30 + were calculated for the following systems: C3Hs + Jsa 02 ~--- H30 + + -~ C02 + e ~-C2H2-k
~O2~H30
++3C02+
(i)
e (ii)
a C2H4 q- 2 02 ~ - HalO+ q- } CO2 q- e. (iii) The above quasi-equilibria were formulated in such a way that all the hydrogen atoms present originally in hydrocarbons as fuels go into H30 +, so that no water molecules are formed. The equilibrium constants, K, for these quasi-equilibria at the adiabatic flame temperatures were evaluated by the statistical-mechanical method; partition functions3 for reactants and products were approximated by ignoring the vibrational contributions and the data for the heats of formation were taken from the literature (HsO +, AH ° = 195 kcal/mole;4 CO2, AH ° = -94.1 kcal/mole; 5 C3Hs, AH ° = -24.8 kcal/mole; ~
223
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
C2H2, AH ° = 54.2 kcal/mole; ~ C2H4, AH ° = 12.6 kcal/mole).4 With the use of these evaluated equilibrium constants, the quasi-equilibrium concentrations of H30 + were calculated. The results are given in Table 1 together with such experimental conditions as the equivalence ratio ¢ = (fuel/O2)used/(fuel/O2)stoicMometric, the applied pressure P and the adiabatic flame temperature T a • It is seen in Table 1 that the calculated concentrations of H30 + are extremely large cornpared with the experimental values for the concentrations of positive ions. However, the ratios, p, defined as p = (calculated quasi-equilibrium concentration of H30+)/(experimental concentration of positive ions) and called the recruitment factor, seem to remain almost constant, about 10-5 , for all of the flames studied. This means that the actual concentrations of positive ions in any hydrocarbon flames can be roughly predicted by multiplying the calculated quasiequilibrium concentrations of H30 + by 10-5. General Considerations on the Mechanism of Ion Formation
It is reasonable to assume that a few steps of the chemical changes in the flame are of major importance for the production of the most abundant positive ion, H30 +, and free electrons. According to Deckers and van Tiggelen) the ion C H 0 + is observed in the lower part, i.e., closer to the inner cone, of the hydrocarbon-air flames and the concentration of this ion is very small compared with that of H~O+. They further identified many other less abundant positive ions in acetylene flame and found in particular that C20.2H+ is only observed very near to the quenching position of the burner, i.e., at the relatively earlier stage of combustion. If the ien appearing first is C202H+ in any hydrocarbon flame, then the ion CHO + will be easily formed by the decomposition of C202H+ and the most abundant ion, H30 +, will be produced by the proton transfer between CHO + and H20. The mechanism of the formation ef H30 + via CHO + was suggested by Calcote, 6b who concluded that the rate of proton transfer is very fast so that the lifetime of CHO + is extremely short. Generally, a large number of species of molecules, atoms, radicals as well as ions are observed in hydrocarbons and the reactions in the flames are extremely complex. However, since the most abundant ion in all hydrocarbon flames studied
T A B L E 1. COMPARISON OF THE QUASI-EQUILIBRIUM CONCENTRATIONS OF
H 3 0 + WITH
THE E X -
PERIMENTAL VALUES FOR POSITIVE ION CONCENTRATIONS IN HYDROCARBON-AIR ]q~LAMES*
p (atmos)
Ta
(°K)
Molar Fraction
I
H~O+ X lOa Calculated
Positive o X 10B Ions X [ l0s Ob- ] served
1.25 0.96
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.076 0.076 0.076 0.076 0.043 0.043
I. Pro )ane 1875 0.637 2030 1.178 2140 1.868 2230 2.642 2230 2.674 2200 2.343 2120 1.794 2060 1.414 1980 1.003 2010 1.767 2140 3.050 2200 3.862 2190 2.521 2130 3.691 2450 10.61
13 29
0.7 0.8 0.9 1.0 1.1 1.2 1.4 1.6
0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03
II. Acetylene 2130 8.281 2200 9.392 2280 10.50 2325 11.66 2355 12.42 2380 12.31 2400 11.95 2390 10.49
13 16 19 22 27 29 23 14
0.69 0.89 0.95 1.02 1.18 1.23 1.38
0.043 0.043 0.043 0.043 0.043 0.043 0.043
III. Ethylene 1940 4.398 2130 5.759 2160 7.223 2190 7.697 2190 7.886 2180 7.800 2120 6.131
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.8 0.9 1.0 1.1
1.0 2.9 7.0 13 11 8.5 6.5 4.6 3.3 8.3 12 11
6.7
3.7 6.3 8.3 8.8 9.0 9.1 4.6
1.57 2.46 3.75 4.92 4.12 3.63 3.62 3.25 3.29 4.69 3.93 3.11 2.66 3.52 2.73 1.57
1.70 1.81 1.93 2.17 2.36 1.92 1.33 0.84 1.09 1.15 1.14 1.14 1.17 0.75
* The references for the experimentM data are as follows: propane; 9, ~ acetylene; 9 and ethylene. ~b so far is HsO+, the reaction steps responsible for the formation of this ion must be the same for all hydrocarbons. Thus, it seems probable that the ionization in these flames is the act of a particular hydrocarbon molecule or radical. We assume that saturated hydrocarbons and high-molecular-weight unsaturated hydrocarbons undergo an initial decomposition into acetylene, 7
224
IONS IN FLAMES
followed by ionization. This view appears to be at least partly substantiated by the following evidences: (1) The burning velocity of acetylene-air flame is much greater than that of any other hydrocarbon-air flame, and the activation energy for the acetylene combustion is smaller than that for any other hydrocarbon combustion, s (2) For the same composition of combustible mixture, the concentration of positive ions is greater in the acetylene flame than in any other hydrocarbon flame. 9 (3) Acetylene is observed as an intermediate product in the pyrolyses of hydrocarbons and its concentration is fairly high. I°~ (4) The initial reactions in a combustion process of a saturated hydrocarbon and a highmolecular-weight unsaturated hydrocarbon are mainly those of hydrogen abstraction by oxygen, producing low-molecular-weight, unsaturated hydrocarbons, n In view of the evidences described above it seems probable that the bulk of the scheme for chemical changes in the ionization zone (to be defined later) of hydrocarbon flames is essentially made up of the following sequence of important elementary reactions:
q + o5 ~'~' , ~C5H5
(A)
q- o t h e r o x i d a t i o n p r o d u c t s C5H5 + O5 C5H+O2
C505H+ CHO ++H20
HaO + q - e +
M
' CsH + HO5
(B)
k, , C s O 2 H + + e
(C)
k, , C H O + + CO
(D)
k, ) H a O + + C O
(E)
ko ) H s O q - H q - M
(F)
where Q and M, respectively, denote a hydrocarbon other than acetylene and a third body effective for the ion recombination step, and j5 is the number of moles of C2H2 produced from one mole of Q. Of course, the first reaction should be omitted for acetylene flames. Since Step (C) (ionization step) is an appreciably endothermic reaction, it is possible to consider that kl, k2 >>/ca, i.e., that Step (C) is rate-controlling. This directly leads us to conclude that the formation of C2H radical has essentially been completed before the ionization starts. The concentration of C2H can thus be
approximated as (C2H) = /5(Q), where (Q) stands for the initial concentration of Q at the ionization zone, and /3 is unity in the case of acetylene flames. As already mentioned, Step (E) is considered to be much faster than Step (F) (recombination step). If we assume that Step (D) is also much faster than Step (F) at the flame temperatures so that the HaO + concentration is very large compared with the C202H + concentration as is actually the case, then the rate, va, of formation of HaO+ may be approximated to va = ka (C5H)(O2) = ka fi(Q)(Os).
(1)
The rate, v6, of disappearance of HaO + due to the recombination with electrons is v6 = k6(M)(HaO+)(e).
(2)
Applying the flow method 12 to a flame, the change in the number, n, of HaO+ ions with time in a small volume unit dV near any arbitrary point of the flame can be written as
dn
- (va
dt
-
v6) dV
-
u
dc
(3)
where u is the volume velocity of the flow of the gas mixture passing through the unit volume and dc is the change in the concentration of HaO + in the gas mixture after the flow has passed through the volume unit. If we assume a steady-state with respect to n, then dn/dt = 0 at any point throughout the flame. Under this condition~ Equations (1), (2) and (3) result in /ca 5(Q)(O5) = k6(M)(HaO+)(e)
(4)
at the point of maximum ionization, because dc = 0 at this point. On the assumption that the transition states of ionization and ion-recombination steps are thermally equilibrated with the corresponding reactants, ka/k6 is evaluated according to the theory of absolute reaction rates, a For the ionization step the assumed scheme is H--C--~C +
o
o~
o
I+1 +~ H--C=C
+e H--
--
(transition state)
The specific rate constant, ka, for this step is then written as
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
kT k3 = K3 . - ~ - "
F*(C202H+)F(e) e-au~/Rr. (5)
225
{X(H30)}2 = 2.123
F(C2H)F(O2)
For the ion-recombination step, the specific rate constant, k6, is
× 10 -12 X(Q)X(O2)T 2 P
kT k~ = K6 • - U
5"
(6)
F~ e_aHilRT. F(H30+)F(e)F(M) Here F's appearing in Equations (5) and (6) are the partition functions per unit volume and all other symbols have their usual significance. If it is assumed that M serves only to take off the energy liberated by tile recombination of H30 + with free electrons, without appreciable interaction with ions and electrons, F~ might be approximated as F6t = F(H30)F(M). It is a quite accurate approximation that F(HaO) /F(H30 +) = g(HaO)/g(H30+), where g is the electronic statistical weight. Thus, the ratio of k3 to k6 is written in the form: k3 _ F*(C202H +) {F(e)}2 e- ( ~ ' 3 - ' ~ ' ~ I / , r k6 2F(C2H)F(O2)
(7)
because g(H30)/g(H30 +) = 2 and K3/K~ is probably near unity. Since all the concentrations in Equation (4) are in number of particles per cubic centimeter, they bear a relationship to the corresponding expression in mole-fraction X, e.g., (H30 +) = ( N P ) / ( R T ) ' X ( H 3 0 +)
(8)
where N is Avogadro's number; T, the temperature of flames; P, the total pressure of flames in units of atmospheric pressure; and R is a gas constant (R = 82.06 cm 3 atmos deg-~ mole-'). Equation (4) is then transformed into
X(H30+)X(e) = F : ( C 2 0 2 H +) {F(e)}2 2F(C2H)F(O2) (9)
• R T . ~_ X(Q)X(O2)e_(an~_au~)/nr N P 5" where 3' is the mole-fraction of M. With further reasonable approximation that
F~b(C20~.H)/Fvib(C2H)Fv~b(02) ~ 1 (10) and by evaluating all of the translational and rotational partition functions for C20:H +, C2H, O2 and free electrons as functions of T, we finally obtain
(11)
where X(HaO +) is assumed equal to X(e). The steady-state concentration of H30 + at the maximum ionization point is thus obtainable, in principle, from Equation (11) provided that the magnitudes of AH~ - AH~ and/3/5' are known. Leftover Fraction and Flame Temperature at the Ionization Zone
As already mentioned, we assume that the ionization takes place at a zone in flames where the rate of combustion is maximum. At this zone, defined as the ionization zone, the concentrations of reactants are naturally smaller than in the unburned gas mixture, because most of the fueloxygen mixture has already reacted exothermically, giving the usual combustion products. The maximum rate of reaction is attained by a compromise between concentration and temperature.'a In order to obtain the small fraction (called the leftover fraction) of reactants and the flame temperature at the ionization zone, the following assumptions are introduced: (1) The chemical reaction rate, v, in flames is expressed as
v = A'Tm(Q)(O2)e -E°/Rr.
(12)
That is, the combustion is assumed to be bimolecular, and to be of the first order with respect to each concentration of fuel and oxygen. Here, A ' is the Arrhenius constant modified by excluding temperature dependence, E0 is the over-all activation energy for combustion and m is a constant, being equal to - ~ for the reaction between oxygen and fuel s (2) Specific heats of the reactants and products are approximated to their average values, so that at the combustion zone the temperature of gas mixtures rises linearly with the amount of fuel-oxygen mixture consumed. The fuel and oxygen concentrations at any point of the combustion zone are related to their initial concentrations (Q)0 and (02)o in the unburned gas mixture by (Q)
(02)
(O)o
(02)0
-
x
(13)
226
IONS IN FLAMES
x being t h e leftover fraction a t this point. T h e corresponding flame t e m p e r a t u r e is t h e n expressed as T =
To -k (1 -
X)(Ta -- To)
(14)
where To is t h e initial t e m p e r a t u r e of t h e unb u r n e d gas m i x t u r e (assumed to be 300 ° K) a n d Ta is t h e a d i a b a t i c flame t e m p e r a t u r e . T h e left-over fraction, x i , of t h e r e a c t a n t s a t t h e ionization zone is o b t a i n e d b y maximizing t h e rate, v, with respect to x u n d e r t h e conditions of E q u a t i o n s (13) a n d (14).
Ta x~-
2(Ta--
I To)
2Eo 5+RT~ (15)
--{(5--~ - 2E0~2
16} 1/21
I n T a b l e 2 t h e calculated values of xi for a large v a r i e t y of organic fuels are s u m m a r i z e d t o g e t h e r with t h e values of E0 a n d Ta used. T h e average value of xi is found to b e 0.28 w i t h t h e s t a n d a r d d e v i a t i o n of 0.023. I t i s i n t e r e s t i n g to note t h a t t h e ionization t e m p e r a t u r e , T i = To + 0.72 (Ta - To), derived from t h e a b o v e procedure, is in essential a g r e e m e n t with v a n Tiggelen's m e a n flame t e m p e r a t u r e , T = To + 0.74 (Ta - T0). 14
S t e a d y - S t a t e C o n c e n t r a t i o n s of HaO+ T h e m a x i m u m c o n c e n t r a t i o n s of HaO + in h y d r o c a r b o n flames can be calculated b y E q u a tion (11) provided t h a t t h e values of the p a r a m eters ~/5' a n d A H ~ - A H ~ are known. F o r t h e e v a l u a t i o n of these parameters, E q u a t i o n (11) was applied to flame reactions of propane 9, % acetylene 9, 6b. x~ a n d ethylene ~b premixed w i t h a v a r y i n g a m o u n t of air (or of N2 - 02 mixture). T h e value of xi was t a k e n as 0.28 for all t h e flames considered. T h a t is, b o t h t h e fuels a n d oxygen were a s s u m e d to h a v e reacted u p to 72 per cent giving t h e usual c o m b u s t i o n products, before t h e gas m i x t u r e reaches t h e ionization zone. T h e ionization t e m p e r a t u r e s , T i , were calculated from t h e available a d i a b a t i c flame temperatures, Ta, t h r o u g h
Ti = To +
0.72 (T~ - To).
E q u a t i o n (11) can be t r a n s f o r m e d into L where
109.3 (AH~T~ -
AH~)
+ 2 log~
(16)
1
X(Q)X(O2)T~
L = ~log
{X(HaO+)}2 P
-- 5.8368
(17)
a n d A H ~ - 5 H ~ is in u n i t s of kcal/mole. W i t h t h e use of the evaluated mole-fractions of leftover
TABLE 2, LEFTOVER FRACTION OF ORGANIC FUELS AT THE ZONE OF MAXIMUM REACTION RATE Fuel
E0 Ta(°K) (kcal/ mole)
I. H y d r o c a r b o n s Acetylene . . . . . . . . . . . . . 2580 Benzene . . . . . . . . . . . . . . . 2340 Butadiene-l,3 ......... 2365 n-Butane .............. 2280 Cyclohexane . . . . . . . . . . 2225 Cyclopentane . . . . . . . . . 2235 Cyclopropane . . . . . . . . . 2350 2,2-Dimethylbutane... 2220 2,2-Dimethylpropane... 2220 Ethane ................. 2195 Ethylene ............... 2340 Isopentane ............. 2250 22O0 Methane ............... Methylacetylene ....... 2450 2275 n-Pentane .............. n-Pentene-2 . . . . . . . . . . . . 2320 Propane ................ 2260 Propylene . . . . . . . . . . . . . . 2320 Triptane ............... 2250
27 27 25 28 27 27 26 27 28 26 24 27 26 25 26 26 26 26 27
xl
0.364 0.281 0.299 0.270 0.273 0.274 0.290 0.273 0.266 0.278 0.305 0.275 0.279 0.305 0.284 0.287 0.283 0.287 0.275
Average: 2~ = 0.282 4- 0.021 II. Organic Fuels o t h e r Hydrocarbons Acetaldehyde . . . . . . . . . 2300 Acetone . . . . . . . . . . . . . . . 2210 Acrolein . . . . . . . . . . . . . . 2340 Allyl chloride . . . . . . . . . 2270 n - B u t y l chloride . . . . . . 2225 Diethyl e t h e r . . . . . . . . . 2305 2220 Dimethoxymethane... E t h y l acetate . . . . . . . . . 2125 2425 E t h y l e n e oxide . . . . . . . . Furan ................ 2390 2205 Isopropyl chloride . . . . 2250 Isopropyl e t h e r . . . . . . . Isopropyl m e r c a p t a n . . 2250 M e t h y l ethyl k e t o n e . . 2210 2230 M e t h y l sulfide . . . . . . . . . 2310 Propionaldehyde ...... 2205 n - P r o p y l chloride . . . . . Propylene oxide . . . . . . 2360 Average: 2i = 0.278 -4- 0.018
than 27 27 25 31 28 26 25 27 24 25 30 28 28 26 26 25 30 25
0.279 0.272 0.297 0.250 0.267 0.286 0.289 0.266 0.311 0.301 0.251 0.268 0.268 0.280 0.281 0.295 0.251 0.299
227
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
TABLE 3.
IONIZATION IN HYDROCARBON FLAMES
(atPos)
0.7 0.8 0.9 0.95 1.0 1.05 1.1 1.15 1.2 1.3 1.4 1.5 1.6 1.7 0.8 0.9 0.94 1.25 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 0. 625 0. 757 0. 892 1. 041 1.139 1. 248 1.388 1. 565 0.616 0.69 0. 785 0. 885 0.988 1.12 1.25 1.39 0.69 0.89 0.95 1.02 1.18 1.23 1.38
(¥)
L
Mole F raction of H~O+ X 10* Calculated
T A B L E 4. Mole Fraction of Cation X108 Experimental
I. Propane 1875 1435 3.89091 1.146 1.0 0.3 2030 1545 3.48781 3.310 2.9 0.3 2140 1 6 2 5 3.15101 6. 659 7.0 0.3 2200 1670 3.0018 9.557 10 0.3 2230 1 6 9 0 2.9204 12.30 13 0.3 2250 1 7 0 5 2.9441 12.85 13 0.3 2230 1 6 9 0 3.0041 11.81 11 0.3 2220 1 6 8 0 3.0683 10.42 9.8 0.3 2200 1 6 6 5 3.1346 9.201 8.5 0.3 2120 1610 3. 2523 7.018 6.5 0.3 2060 1565 3.4045 5.056 4.6 0.3 1980 1 5 1 0 3.5467 3. 261 3.3 0.3 1900 1 4 5 0 3.6620 2.208 2.4 0.3 1840 1 4 1 0 3.8832 1.341 1.5 0.3 0.076 I 2010 1 5 3 0 3.3250 6.096 8.3 0.076] 2140 1 6 2 5 3.2150 13.23 12 0.076 2170 1 6 4 5 3.2080 15.71 13 0.043 2130 1615 3. 3680 18.84 13 II. A c e t rlene 0.03 2130 1620 3. 46851 6. 108] 13 0.03 2200 1 6 7 0 3.41711 9. 408[ 16 0.03 2280 1 7 2 5 3.37881 14.60 I 19 0.03 2325 1 7 6 0 3.34371 19.33 I 23 0.03 2355 1780 3. 27701 22.91 27 0.03 2380 1 8 0 0 3.26661 26.98 29 0.03 2390 1805 3. 29731 28.78 [ 28 0.03 2400 1810 3. 39701 29.22 / 23 2095 1 5 9 0 4.48411 0. 799 0.20 1 1 2200 1670 3. 93431 1.553 0.80 1.74 1 2290 1730 3. 62611 2.43' 2350 1 7 7 5 3.42711 3. 241 2.79 1 3.24 2370 1790 3. 37091 3.6Y 1 3.22 2375 1795 3. 37471 3.73a 1 1 2390 1 8 0 5 3.46841 3.82~ 2.61 1.13 2380 1800 3. 83071 3.70~ 1 0.01'. 1980 1510 3. 53211 3.52~ 15 0.01: 2070 1575 3. 44791 6.44( 2O 0.01: 2190 1660 3. 41621 13.17 24 0.013[ 2310 1 7 4 5 3.30941 24.63 34 0.013 I 2370 1 7 9 0 3.32861 35.46 35 0.013 I 2390 1 8 0 5 3.28651 41.11 41 0.013 2350 1775 3. 35541 35.53 36 0.013 2280 1 7 2 5 3.55631 23.70 23 III. E t h rlene 0.0431 1940 1480 3.8560 I 1. 698] 3.7 0.0431 2130 1620 3. 7139 I 6.186 I 6.3 0.043 / 2160 1640 3.6120 I 7.779 I 8.3 0.0431 2190 1660 3.6054 I 8. 880 I 8.8 0.043] 2190 1660 3.6230[ 9. 456] 9.0 0.043 2180 1655 3.6246 9. 277 9.1 0.043 2120 1610 3.9298 6.915 4.6
Flames
AH~ -- AH~
~/t~
kcal/mole
Propane . . . . . . . . . ] Acetylene . . . . . . . . I Ethylene . . . . . . . . I
74 74 74
2.45 X 10-4 4.04 X 10-3 2.86 X 10-3
reactants for X(Q) and X(O=) and the observed mole-fraction of positive ions for X(HaO+), the values of L were calculated from Equation (17) for the three kinds of hydrocarbon flames under the different experimental conditions. In Table 3 the results are summarized together with the experimental conditions. According to Equation (16), plots of L v e r s u s the reciprocal of Ti ought to give a single straight line for a hydrocarbon. Furthermore, the slopes of the straight lines for propane, acetylene and ethylene should be the same, because the kinetics of the ionization process is considered to be unique irrespective of the nature of hydrocarbons. The points were roughly substantiated by plotting the L values against the reciprocals of T i . F r o m the intercepts of these three straight lines, the values of 5'/8 were also evaluated. The results are listed in Table 4. With the use of these empirical parameters, the steady-state concentrations of H30 + were calculated from Equation (11) for the three classes of flames. The calculated values are compared with the experimental positive ion concentrations in the last two columns of Table 3. Discussion
As already stated, the Ha0 + concentrations obtained for hydrocarbon flames by the quasiequilibrium calculations are about 105 times as large as the observed concentrations of positive ions in the flames. This large discrepancy between the calculated and experimental values arises from the omission of the various deactivation processes such as the ion recombination and the reaction between the fuels and oxygen giving the usual combustion products. The observation that the p-factor remains almost constant for all of the hydrocarbon flames studied appears difficult to explain. Yet the finding is quite useful, because the actual concentrations of positive ions in any hydrocarbon flame seem to be roughly predicted by nmltiplying the calculated quasi-equilibrium concentrations of HaO + by 10-5. Steady-state calculations contain two empirical
228
IONS IN FLAMES
parameters, 3'/~ and AH~ - AH~. The value of/3 must be unity for acetylene flames. Providing that B is taken as unity also for hydrocarbons other than acetylene, the mole-fraction, 7, of the third body affecting the ion-recombination step in hydrocarbon flames becomes 'of the order of a few tenths per cent. This might imply that a particular species involved in the flames is incorporated in the recombination between H30 + and free electrons. Although we have no means of specifying this species, OH radical appears to be the most probable because the equilibrium concentration of OH in most hydrocarbon flames has been shown to remain constant over a wide range of equivalence ratios of the flames and to be of the order of 1 per cent. l°b The radical might be of particular importance for the recombination, on account of its high electron affinity (2.7 ev.); TM the attachment of electrons to OH forms the negative ion O H - which may recombine more readily with H~O+. The activation energy of atom recombination is usually very small. The ion recombination may have the same feature. If AH~ is very small compared with A H ~ , then the activation heat for the ionization in hydrocarbon flames will be near 74 kcal/mole. The best rough estimate of the heat of reaction for the third step in our scheme is AH3 = 120 kcal/mole, whereas the activation heat, A H ~ , comes out to be only 74 kcal/mole. To reconcile these results we suppose that either H - - C ~ C or 0~ is excited by a nonequilibrium process which has high probability and a low temperature coefficient. This seems reasonable since the process is occurring in the luminous part of the flame where there are numerous excited species such as CH* and C*. We have not the information to specify this situation more closely at present. On the assumption that various hydrocarbons produce C2H2, a variety of chain mechanisms can be devised; however, the following scheme seems energetically favorable. Q + O2
C ~ H 2 + O2
AH ° = CH+
O
57 k c a l / m o l e
--~CHO ++e; AH ° =
39 k c a l / m o l e
C H O + + H.,O --~ H30 + + CO; AH ° =
24 k c a l / m o l e .
The first two reactions are proposed by Herman, Hornbeck and Laidler, 17 and the last reaction is suggested by Calcote. 6b The calculation of AH ° values are based on recent data. 4 Probably the most important chain terminating reaction is C H O + HO~ --~ CO2 + H20; A H ° = -- 159 k c a l / m o l e . Among various other reactions, a reaction between C~H2 and HO,, parallel to the third step of the chain mechanism is probably C~H2 + HO2--~ C H + CO + H20; AH ° = --41 kcal/mole. Until we are able to make a definite decision about the most important chain mechanism, the calculation principles illustrated in this paper seem sufficient to indicate the procedure. Acknowledgment The authors wish to express their appreciation to the U. S. Air Force for support of this work under Contract No. A F 04(647)-177. REFERENCES 1. I)ECKERS, J., AND VAN TIGGELEN, A. : Nature,
181, 1460 (1958); 182, 863 (1958). 2. KNEWSTUBB, P. F., AND SUGDEN, T. M.:
Seventh Symposium (International) on Combustion, p. 247. Butterworth & Company, Ltd., London, 1959. 3. GLASSTONE, S., LAIDLER, ~4~. J., AND EYRING,
H. : The Theory of Rate Processes, McGrawHill Book Company, Inc., New York, 1941.
--~ C2H2 + o x i d a t i o n p r o d u c t s ( R e a c t i o n s are exothermic)
4. FIELD, F. H., AND FRANKLIN, J. L.: Electron
--~CHO+CHO;
5. MOORE, W. J.: Physical Chemistry, p. 39.
A H ° = -- 55 k c a l / m o l e C H O + O2
C~H~ + H 0 2 - - ~ C H + C H O + O H ;
--~CO+
HO2;
AH ° = --21 k c a l / m o l e
Impact Phenomena, p. 281. Academic Press, Inc., New York, 1957.
2nd ed., Prentice-Hall, New York, 1955. F.: Private communication (Semiannual progress report on the contract AF 04(647)-157, (a) No. 1, June 1958; (b) No. 2, December 1958; (c) No. 3, June 1959.
6. CALCOTE, H.
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
7. LAIDLER, IX. J.: The Chemical Kinetics of Excited States, p. 171. Oxford University Press, London, 1955. 8. FENN, J. B., AND CALCOTE, H. F.: Fourth
Symposium (International)
9. 10.
11.
12.
13. 14. 15. 16. 17.
on Combustion,
p. 231. The Williams & Wilkins Conipany, Baltimore, 1953. CALCOTE,H. F. : Combustion and Flame, 1,385 (1957). (a) GAYDON, A. G., AND WOLFHARD, H. G.: Flames, p. 181. Chapman & Hall, Ltd., London, 1953. (b) ibid., Ch. 12. LEwIs, B., PEASE, R. N., AND TAYLOR, H. S.: Combustion Processes, p. 182. Princeton University Press, Princeton, New Jersey, 1956. FROST, A. A., AND PEARSON, R. G.: Kinetics and Mechanism, p. 183. J o h n Wiley & Sons, Inc., New York, 1953. EYRING, H., GIDDINGS, J. C., AND TENSMEYER, L. G.: J. Chem. Phys., 24, 857 (1956). VAN TIGGELEN, A. : Mem. Acad. Roy. Belg. (C1. Sc.), 27, 1 (1952). PONCELET, J., AND VAN TIGGELEN, A . : Bull. Soc. Chim. Belges, 67, 49 (1958). SmTH, H., AND SUGDEN, W. M.: Proc. Roy. Sou., A211, 31 (1952). HERMAN, R. C., HORNBECK, G. A., AND LAIDLER, K . J. : Science, 112, 497 (1950).
D i s c u s s i o n BY FELIX T. SMITtI There is a possible experiment which should show whether more t h a n one carbon atom is required in the mechanism of ion production. If the ion current is measured as small amounts of methane are added to a hydrogen-air f a m e , a linear relation between current and niethane concentration would imply a mechanism involving only one carbon atom. AUTHORS' REPLY In another paper* presented in this symposium, it has been shown t h a t a radical or molecule containing carbon and hydrogen is directly responsible for the production of the most a b u n d a n t ion in high concentration in flames, and t h a t the CH is such a radical. The formation of the CH from acetylene is energetically favorable and acetylene is found in a b u n d a n t q u a n t i t y even in methaneoxygen flames. Whether the Coil or the CH is the niost important radical responsible for the most a b u n d a n t ion formation, their parent niolecule is acetylene. Studies of ion current versus only methane concentration in a hydrogen-oxygen flame would be beneficial, but may not indicate the mechanism of the most a b u n d a n t ion formation conclusively. However, repetition of these studies with other aliphatic hydrocarbons such as
229
propane, ethylene and acetylene, and then comparison of all these data will be much more valuable in devising the mechanism of the most a b u n d a n t ion formation t h a n the data of m e t h a n e alone. Another point of interest which we proposed is t h a t the OH radical is very i m p o r t a n t for the production of the CH radical and hence for the production of the most a b u n d a n t ion.* Because hydrogen-oxygen flames contain the OH radical in abundance, ion concentrations in hydrogenoxygen-(injected) hydrocarbon f a m e s are expected to be greater t h a n the same hydrocarbonoxygen flames under the same conditions of temperature, pressure and equivalence ratio. DISCUSSION BY A. VAN TIGGELEN In connection with different papers presented at this meeting t h a t deal with the problem of ion formation, it might be interesting to mention some recent and unpublished results which have been obtained in Louvain with the collaboration of J. N. Bertrand. I t has been observed, in the following series of fuels burning with oxygen (methare, methyl alcohol, formaldehyde and formic acid), t h a t ions are formed only in the f a m e s of CH4 and CH3OH. The ions are absent (or in an amount less t h a n l0 s ions/cc) in CH20 and HCOOH flames. This seems to indicate t h a t in the process by which the parent ions are formed, the carbon atom of the particle (molecule or radical) involved in the reaction of chemi-ionization niay not be doubly linked to an oxygen atom. It is presumably the strong exothermicity of the formation of such a bond, with the excitation energy of the hydrocarbon fragment or radical, which renders possible the process of chemi-ionization. To give an example, one could propose processes of the following kind : CH* + O: --* CO + 4- OH + eor CH* + O 2 ~ C O H *
+ OH + e-
C* + O 2 - - * C O + + CO + eand so forth. In order to investigate whether or not the presence of the hydrogen atom linked to the carbon atom is a necessary condition, we expect some good information from a study of a carbon suboxide--oxygen flame (C~O.o + 02). In this ease one of the carbon atoms is neither bound to oxygen nor hydrogen. This work was undertaken in our laboratory a few weeks ago and from the few first experimental results it seems * Eyring, H. : Eighth Symposium (International) on Combustion, p. 222. The Williams & Wilkins Company, Baltimore, 19~2.
230
IONS IN FLAMES
that the C302/Q flames do not contain ions in a quantity comparable to that observed in hydrocarbon flames; however, more experiments are to be made before definite conclusions can be derived. It is also important to note that cyanogen-oxygen flames are ion-poor flames. The only NO + ion has been identified; this should be expected from the fact that NO is formed among the reaction products. AUTHORS' REPLY
Our work indicates that the processes for the formation of possible ions including the H30 + from formMdehyde and formic acid by direct ionization or chemi-ionization are too highly endothermic to produce appreciable ions. It seems that high ionization i~ not expected by a reaction in which one of the reactants contains a carbon atom (or atoms) doubly linked with oxygen. However, the formation of a double bond between a carbon atom and an oxygen atom in one of the products of a chemi-ionization reaction is a favorable process for ionization in flames. As far as the most abundant ion, H~O +, is con-
cerned, the most probable process f o r its formation is a proton-transfer reaction between a suitable proton-donor ion and water molecule. In general, it can be hypothesized that among the similar ionic species containing carbon, hydrogen and oxygen, the ion in the highest oxidized state will be the best proton-donor. In the light of these assumptions it is desirable for the proton-donor ion to be formed with one or two double bonds between carbon and oxygen. Although the ionization potential of nitric oxide is about 9.2 ev., the NO + is not expected to be produced by direct ionization of the NO in flames at temperatures 2000 ° to 2500°C. A probable mechanism of the NO + formation from nitrogen and oxygen molecules are given in the paper "Ions in Flames" of this symposium. The formation of NO + in C2N2--O2 flames is being investigated by us. We agree with Dr. Van Tiggelen that the C* is not directly related to the formation of the most abundant ion. However, in hydrocarbon flames the C* can produce other radicals, such as the CH and CHO, which may be directly responsible for the high ionization.
18 CATALYTIC DISSOCIATIVE REACTIONS IN ELECTRICAL DISCHARGES °
By
F. K A U F M A N AND J. R. K E L S O
Introduction In taking a broad look at the progress of experimental combustion science during the past several years, one discerns some fairly welldefined trends. There is a growing interest in the detailed chemical and physical steps of combustion processes rather than in the complicated total phenomena. One studies energy transfer processes in simple systems; atom and free radical reactions under controlled conditions; the kinetics of simple reactions at lower temperatures in conventional apparatus, or at high temperatures for very short times in shock-tubes. There is also a growing interest in highly energetic species such as electronically excited states, and in the role played by electrically charged species, i.e., elecSupported by the Advanced Research Projects Administration.
trons and positive and negative ions. As a result of these tendencies there has been increasing interaction and overlap with the fields of upperatmosphere science, electron physics and magnetohydrodynamics. We have recently studied reactions of oxygen and nitrogen a t o m s " 2 in flow systems and have used high-frequency electrical discharges to produce the free atoms or other excited species) We have followed the rates of various atom reactions by measuring the intensity of certain glows - - t h e air afterglow and the nitrogen afterglow-whose dependence on atom concentration was known. Moreover, we have been able to utilize simple gas "titrations," NO~ for O-atoms and NO for N-atoms, for the quick determination of atom concentrations. In all of this work, the electrcdeless discharge was used only as a source of free atoms, and no
IONS IN FLAMES
REFERENCES 1. PAYNE, K. G., AND WEINBERG, F. J.: Eighth Symposium (International) on Combustion, p. 206. The Williams & Wilkins Company, Baltimore, 1962. 2. LOEB, L. B.: Basic Processes of Gaseous Electronics. University of California Press, Berkeley, 1955.
3. VOGEL, A. L. : A Text Book of Chemical Analysis. Longmans, Green & Company, Inc., London, 1937. 4. PAYNE, K. G., AND WEIN~ERG, F. J.: A preliminary investigation of field induced ion movement in flame gases and its applications. Proc. Roy. Soc. (London), A250, 316 (1959).
17
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES By T A K A Y U K I FUENO, NALIN R. M U K H E R J E E , T A I K Y U E REE AND H E N R Y E Y R I N G
Introduction It is well recognized that free electrons and a variety of positive ions are formed in the hightemperature flames of many fuel-oxidant mixtures. A complete solution of the problem on the mechanism of formation of ions and electrons in flames has not been easy to obtain because of insufficient information on the nature of the chemical changes involved in flames. However, it seems likely that the phenomenon of ionization would be directly related to a few elementary processes occurring in flames. It has been demonstrated mass-spectrometrically by Deckers and van Tiggelen' and by Knewstubb and Sugden2 that the most abundant positive ions produced in flames of hydrocarbons are H30 +. The present paper attempts to calculate the population of this ion by proposing a possible mechanism for its formation mainly in hydrocarbon-air flames. In the first place, the quasi-equilibrium concentrations of H30 + in these flames are calculated on a simple statisticalmechanical basis under the assumption that the total amount of hydrocarbons and oxygen supplied is equilibrated with the system comprising HaO+, CO2 and free electrons at the adiabatic flame temperature. The calculated concentrations of I-IsO+ are found to be too large by a factor of about 10 5 for all of the hydrocarbons studied so far, and this value of 10 5 may be used to predict the ion concentrations in hydrocarbon flames.
Another semitheoretical trial is made for obtaining the ion concentrations in hydrocarbon
flames, if a possible kinetic mechanism for the ion formation is assumed. The essential concept of this treatment lies in supposing that at a particular zone of the flame where the over-M1 rate of combustion is maximum, fuel and oxygen are subjected to a sequence of chemical changes including ionization, thus yielding free electrons and the most abundant positive ions. The formation of these charged species is assumed to satisfy the steady-state condition.
Quasi-Equilibrium Calculations The quasi-equilibrium concentrations of H30 + were calculated for the following systems: C3Hs + Jsa 02 ~--- H30 + + -~ C02 + e ~-C2H2-k
~O2~H30
++3C02+
(i)
e (ii)
a C2H4 q- 2 02 ~ - HalO+ q- } CO2 q- e. (iii) The above quasi-equilibria were formulated in such a way that all the hydrogen atoms present originally in hydrocarbons as fuels go into H30 +, so that no water molecules are formed. The equilibrium constants, K, for these quasi-equilibria at the adiabatic flame temperatures were evaluated by the statistical-mechanical method; partition functions3 for reactants and products were approximated by ignoring the vibrational contributions and the data for the heats of formation were taken from the literature (HsO +, AH ° = 195 kcal/mole;4 CO2, AH ° = -94.1 kcal/mole; 5 C3Hs, AH ° = -24.8 kcal/mole; ~
223
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
C2H2, AH ° = 54.2 kcal/mole; ~ C2H4, AH ° = 12.6 kcal/mole).4 With the use of these evaluated equilibrium constants, the quasi-equilibrium concentrations of H30 + were calculated. The results are given in Table 1 together with such experimental conditions as the equivalence ratio ¢ = (fuel/O2)used/(fuel/O2)stoicMometric, the applied pressure P and the adiabatic flame temperature T a • It is seen in Table 1 that the calculated concentrations of H30 + are extremely large cornpared with the experimental values for the concentrations of positive ions. However, the ratios, p, defined as p = (calculated quasi-equilibrium concentration of H30+)/(experimental concentration of positive ions) and called the recruitment factor, seem to remain almost constant, about 10-5 , for all of the flames studied. This means that the actual concentrations of positive ions in any hydrocarbon flames can be roughly predicted by multiplying the calculated quasiequilibrium concentrations of H30 + by 10-5. General Considerations on the Mechanism of Ion Formation
It is reasonable to assume that a few steps of the chemical changes in the flame are of major importance for the production of the most abundant positive ion, H30 +, and free electrons. According to Deckers and van Tiggelen) the ion C H 0 + is observed in the lower part, i.e., closer to the inner cone, of the hydrocarbon-air flames and the concentration of this ion is very small compared with that of H~O+. They further identified many other less abundant positive ions in acetylene flame and found in particular that C20.2H+ is only observed very near to the quenching position of the burner, i.e., at the relatively earlier stage of combustion. If the ien appearing first is C202H+ in any hydrocarbon flame, then the ion CHO + will be easily formed by the decomposition of C202H+ and the most abundant ion, H30 +, will be produced by the proton transfer between CHO + and H20. The mechanism of the formation ef H30 + via CHO + was suggested by Calcote, 6b who concluded that the rate of proton transfer is very fast so that the lifetime of CHO + is extremely short. Generally, a large number of species of molecules, atoms, radicals as well as ions are observed in hydrocarbons and the reactions in the flames are extremely complex. However, since the most abundant ion in all hydrocarbon flames studied
T A B L E 1. COMPARISON OF THE QUASI-EQUILIBRIUM CONCENTRATIONS OF
H 3 0 + WITH
THE E X -
PERIMENTAL VALUES FOR POSITIVE ION CONCENTRATIONS IN HYDROCARBON-AIR ]q~LAMES*
p (atmos)
Ta
(°K)
Molar Fraction
I
H~O+ X lOa Calculated
Positive o X 10B Ions X [ l0s Ob- ] served
1.25 0.96
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.076 0.076 0.076 0.076 0.043 0.043
I. Pro )ane 1875 0.637 2030 1.178 2140 1.868 2230 2.642 2230 2.674 2200 2.343 2120 1.794 2060 1.414 1980 1.003 2010 1.767 2140 3.050 2200 3.862 2190 2.521 2130 3.691 2450 10.61
13 29
0.7 0.8 0.9 1.0 1.1 1.2 1.4 1.6
0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03
II. Acetylene 2130 8.281 2200 9.392 2280 10.50 2325 11.66 2355 12.42 2380 12.31 2400 11.95 2390 10.49
13 16 19 22 27 29 23 14
0.69 0.89 0.95 1.02 1.18 1.23 1.38
0.043 0.043 0.043 0.043 0.043 0.043 0.043
III. Ethylene 1940 4.398 2130 5.759 2160 7.223 2190 7.697 2190 7.886 2180 7.800 2120 6.131
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.8 0.9 1.0 1.1
1.0 2.9 7.0 13 11 8.5 6.5 4.6 3.3 8.3 12 11
6.7
3.7 6.3 8.3 8.8 9.0 9.1 4.6
1.57 2.46 3.75 4.92 4.12 3.63 3.62 3.25 3.29 4.69 3.93 3.11 2.66 3.52 2.73 1.57
1.70 1.81 1.93 2.17 2.36 1.92 1.33 0.84 1.09 1.15 1.14 1.14 1.17 0.75
* The references for the experimentM data are as follows: propane; 9, ~ acetylene; 9 and ethylene. ~b so far is HsO+, the reaction steps responsible for the formation of this ion must be the same for all hydrocarbons. Thus, it seems probable that the ionization in these flames is the act of a particular hydrocarbon molecule or radical. We assume that saturated hydrocarbons and high-molecular-weight unsaturated hydrocarbons undergo an initial decomposition into acetylene, 7
224
IONS IN FLAMES
followed by ionization. This view appears to be at least partly substantiated by the following evidences: (1) The burning velocity of acetylene-air flame is much greater than that of any other hydrocarbon-air flame, and the activation energy for the acetylene combustion is smaller than that for any other hydrocarbon combustion, s (2) For the same composition of combustible mixture, the concentration of positive ions is greater in the acetylene flame than in any other hydrocarbon flame. 9 (3) Acetylene is observed as an intermediate product in the pyrolyses of hydrocarbons and its concentration is fairly high. I°~ (4) The initial reactions in a combustion process of a saturated hydrocarbon and a highmolecular-weight unsaturated hydrocarbon are mainly those of hydrogen abstraction by oxygen, producing low-molecular-weight, unsaturated hydrocarbons, n In view of the evidences described above it seems probable that the bulk of the scheme for chemical changes in the ionization zone (to be defined later) of hydrocarbon flames is essentially made up of the following sequence of important elementary reactions:
q + o5 ~'~' , ~C5H5
(A)
q- o t h e r o x i d a t i o n p r o d u c t s C5H5 + O5 C5H+O2
C505H+ CHO ++H20
HaO + q - e +
M
' CsH + HO5
(B)
k, , C s O 2 H + + e
(C)
k, , C H O + + CO
(D)
k, ) H a O + + C O
(E)
ko ) H s O q - H q - M
(F)
where Q and M, respectively, denote a hydrocarbon other than acetylene and a third body effective for the ion recombination step, and j5 is the number of moles of C2H2 produced from one mole of Q. Of course, the first reaction should be omitted for acetylene flames. Since Step (C) (ionization step) is an appreciably endothermic reaction, it is possible to consider that kl, k2 >>/ca, i.e., that Step (C) is rate-controlling. This directly leads us to conclude that the formation of C2H radical has essentially been completed before the ionization starts. The concentration of C2H can thus be
approximated as (C2H) = /5(Q), where (Q) stands for the initial concentration of Q at the ionization zone, and /3 is unity in the case of acetylene flames. As already mentioned, Step (E) is considered to be much faster than Step (F) (recombination step). If we assume that Step (D) is also much faster than Step (F) at the flame temperatures so that the HaO + concentration is very large compared with the C202H + concentration as is actually the case, then the rate, va, of formation of HaO+ may be approximated to va = ka (C5H)(O2) = ka fi(Q)(Os).
(1)
The rate, v6, of disappearance of HaO + due to the recombination with electrons is v6 = k6(M)(HaO+)(e).
(2)
Applying the flow method 12 to a flame, the change in the number, n, of HaO+ ions with time in a small volume unit dV near any arbitrary point of the flame can be written as
dn
- (va
dt
-
v6) dV
-
u
dc
(3)
where u is the volume velocity of the flow of the gas mixture passing through the unit volume and dc is the change in the concentration of HaO + in the gas mixture after the flow has passed through the volume unit. If we assume a steady-state with respect to n, then dn/dt = 0 at any point throughout the flame. Under this condition~ Equations (1), (2) and (3) result in /ca 5(Q)(O5) = k6(M)(HaO+)(e)
(4)
at the point of maximum ionization, because dc = 0 at this point. On the assumption that the transition states of ionization and ion-recombination steps are thermally equilibrated with the corresponding reactants, ka/k6 is evaluated according to the theory of absolute reaction rates, a For the ionization step the assumed scheme is H--C--~C +
o
o~
o
I+1 +~ H--C=C
+e H--
--
(transition state)
The specific rate constant, ka, for this step is then written as
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
kT k3 = K3 . - ~ - "
F*(C202H+)F(e) e-au~/Rr. (5)
225
{X(H30)}2 = 2.123
F(C2H)F(O2)
For the ion-recombination step, the specific rate constant, k6, is
× 10 -12 X(Q)X(O2)T 2 P
kT k~ = K6 • - U
5"
(6)
F~ e_aHilRT. F(H30+)F(e)F(M) Here F's appearing in Equations (5) and (6) are the partition functions per unit volume and all other symbols have their usual significance. If it is assumed that M serves only to take off the energy liberated by tile recombination of H30 + with free electrons, without appreciable interaction with ions and electrons, F~ might be approximated as F6t = F(H30)F(M). It is a quite accurate approximation that F(HaO) /F(H30 +) = g(HaO)/g(H30+), where g is the electronic statistical weight. Thus, the ratio of k3 to k6 is written in the form: k3 _ F*(C202H +) {F(e)}2 e- ( ~ ' 3 - ' ~ ' ~ I / , r k6 2F(C2H)F(O2)
(7)
because g(H30)/g(H30 +) = 2 and K3/K~ is probably near unity. Since all the concentrations in Equation (4) are in number of particles per cubic centimeter, they bear a relationship to the corresponding expression in mole-fraction X, e.g., (H30 +) = ( N P ) / ( R T ) ' X ( H 3 0 +)
(8)
where N is Avogadro's number; T, the temperature of flames; P, the total pressure of flames in units of atmospheric pressure; and R is a gas constant (R = 82.06 cm 3 atmos deg-~ mole-'). Equation (4) is then transformed into
X(H30+)X(e) = F : ( C 2 0 2 H +) {F(e)}2 2F(C2H)F(O2) (9)
• R T . ~_ X(Q)X(O2)e_(an~_au~)/nr N P 5" where 3' is the mole-fraction of M. With further reasonable approximation that
F~b(C20~.H)/Fvib(C2H)Fv~b(02) ~ 1 (10) and by evaluating all of the translational and rotational partition functions for C20:H +, C2H, O2 and free electrons as functions of T, we finally obtain
(11)
where X(HaO +) is assumed equal to X(e). The steady-state concentration of H30 + at the maximum ionization point is thus obtainable, in principle, from Equation (11) provided that the magnitudes of AH~ - AH~ and/3/5' are known. Leftover Fraction and Flame Temperature at the Ionization Zone
As already mentioned, we assume that the ionization takes place at a zone in flames where the rate of combustion is maximum. At this zone, defined as the ionization zone, the concentrations of reactants are naturally smaller than in the unburned gas mixture, because most of the fueloxygen mixture has already reacted exothermically, giving the usual combustion products. The maximum rate of reaction is attained by a compromise between concentration and temperature.'a In order to obtain the small fraction (called the leftover fraction) of reactants and the flame temperature at the ionization zone, the following assumptions are introduced: (1) The chemical reaction rate, v, in flames is expressed as
v = A'Tm(Q)(O2)e -E°/Rr.
(12)
That is, the combustion is assumed to be bimolecular, and to be of the first order with respect to each concentration of fuel and oxygen. Here, A ' is the Arrhenius constant modified by excluding temperature dependence, E0 is the over-all activation energy for combustion and m is a constant, being equal to - ~ for the reaction between oxygen and fuel s (2) Specific heats of the reactants and products are approximated to their average values, so that at the combustion zone the temperature of gas mixtures rises linearly with the amount of fuel-oxygen mixture consumed. The fuel and oxygen concentrations at any point of the combustion zone are related to their initial concentrations (Q)0 and (02)o in the unburned gas mixture by (Q)
(02)
(O)o
(02)0
-
x
(13)
226
IONS IN FLAMES
x being t h e leftover fraction a t this point. T h e corresponding flame t e m p e r a t u r e is t h e n expressed as T =
To -k (1 -
X)(Ta -- To)
(14)
where To is t h e initial t e m p e r a t u r e of t h e unb u r n e d gas m i x t u r e (assumed to be 300 ° K) a n d Ta is t h e a d i a b a t i c flame t e m p e r a t u r e . T h e left-over fraction, x i , of t h e r e a c t a n t s a t t h e ionization zone is o b t a i n e d b y maximizing t h e rate, v, with respect to x u n d e r t h e conditions of E q u a t i o n s (13) a n d (14).
Ta x~-
2(Ta--
I To)
2Eo 5+RT~ (15)
--{(5--~ - 2E0~2
16} 1/21
I n T a b l e 2 t h e calculated values of xi for a large v a r i e t y of organic fuels are s u m m a r i z e d t o g e t h e r with t h e values of E0 a n d Ta used. T h e average value of xi is found to b e 0.28 w i t h t h e s t a n d a r d d e v i a t i o n of 0.023. I t i s i n t e r e s t i n g to note t h a t t h e ionization t e m p e r a t u r e , T i = To + 0.72 (Ta - To), derived from t h e a b o v e procedure, is in essential a g r e e m e n t with v a n Tiggelen's m e a n flame t e m p e r a t u r e , T = To + 0.74 (Ta - T0). 14
S t e a d y - S t a t e C o n c e n t r a t i o n s of HaO+ T h e m a x i m u m c o n c e n t r a t i o n s of HaO + in h y d r o c a r b o n flames can be calculated b y E q u a tion (11) provided t h a t t h e values of the p a r a m eters ~/5' a n d A H ~ - A H ~ are known. F o r t h e e v a l u a t i o n of these parameters, E q u a t i o n (11) was applied to flame reactions of propane 9, % acetylene 9, 6b. x~ a n d ethylene ~b premixed w i t h a v a r y i n g a m o u n t of air (or of N2 - 02 mixture). T h e value of xi was t a k e n as 0.28 for all t h e flames considered. T h a t is, b o t h t h e fuels a n d oxygen were a s s u m e d to h a v e reacted u p to 72 per cent giving t h e usual c o m b u s t i o n products, before t h e gas m i x t u r e reaches t h e ionization zone. T h e ionization t e m p e r a t u r e s , T i , were calculated from t h e available a d i a b a t i c flame temperatures, Ta, t h r o u g h
Ti = To +
0.72 (T~ - To).
E q u a t i o n (11) can be t r a n s f o r m e d into L where
109.3 (AH~T~ -
AH~)
+ 2 log~
(16)
1
X(Q)X(O2)T~
L = ~log
{X(HaO+)}2 P
-- 5.8368
(17)
a n d A H ~ - 5 H ~ is in u n i t s of kcal/mole. W i t h t h e use of the evaluated mole-fractions of leftover
TABLE 2, LEFTOVER FRACTION OF ORGANIC FUELS AT THE ZONE OF MAXIMUM REACTION RATE Fuel
E0 Ta(°K) (kcal/ mole)
I. H y d r o c a r b o n s Acetylene . . . . . . . . . . . . . 2580 Benzene . . . . . . . . . . . . . . . 2340 Butadiene-l,3 ......... 2365 n-Butane .............. 2280 Cyclohexane . . . . . . . . . . 2225 Cyclopentane . . . . . . . . . 2235 Cyclopropane . . . . . . . . . 2350 2,2-Dimethylbutane... 2220 2,2-Dimethylpropane... 2220 Ethane ................. 2195 Ethylene ............... 2340 Isopentane ............. 2250 22O0 Methane ............... Methylacetylene ....... 2450 2275 n-Pentane .............. n-Pentene-2 . . . . . . . . . . . . 2320 Propane ................ 2260 Propylene . . . . . . . . . . . . . . 2320 Triptane ............... 2250
27 27 25 28 27 27 26 27 28 26 24 27 26 25 26 26 26 26 27
xl
0.364 0.281 0.299 0.270 0.273 0.274 0.290 0.273 0.266 0.278 0.305 0.275 0.279 0.305 0.284 0.287 0.283 0.287 0.275
Average: 2~ = 0.282 4- 0.021 II. Organic Fuels o t h e r Hydrocarbons Acetaldehyde . . . . . . . . . 2300 Acetone . . . . . . . . . . . . . . . 2210 Acrolein . . . . . . . . . . . . . . 2340 Allyl chloride . . . . . . . . . 2270 n - B u t y l chloride . . . . . . 2225 Diethyl e t h e r . . . . . . . . . 2305 2220 Dimethoxymethane... E t h y l acetate . . . . . . . . . 2125 2425 E t h y l e n e oxide . . . . . . . . Furan ................ 2390 2205 Isopropyl chloride . . . . 2250 Isopropyl e t h e r . . . . . . . Isopropyl m e r c a p t a n . . 2250 M e t h y l ethyl k e t o n e . . 2210 2230 M e t h y l sulfide . . . . . . . . . 2310 Propionaldehyde ...... 2205 n - P r o p y l chloride . . . . . Propylene oxide . . . . . . 2360 Average: 2i = 0.278 -4- 0.018
than 27 27 25 31 28 26 25 27 24 25 30 28 28 26 26 25 30 25
0.279 0.272 0.297 0.250 0.267 0.286 0.289 0.266 0.311 0.301 0.251 0.268 0.268 0.280 0.281 0.295 0.251 0.299
227
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
TABLE 3.
IONIZATION IN HYDROCARBON FLAMES
(atPos)
0.7 0.8 0.9 0.95 1.0 1.05 1.1 1.15 1.2 1.3 1.4 1.5 1.6 1.7 0.8 0.9 0.94 1.25 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 0. 625 0. 757 0. 892 1. 041 1.139 1. 248 1.388 1. 565 0.616 0.69 0. 785 0. 885 0.988 1.12 1.25 1.39 0.69 0.89 0.95 1.02 1.18 1.23 1.38
(¥)
L
Mole F raction of H~O+ X 10* Calculated
T A B L E 4. Mole Fraction of Cation X108 Experimental
I. Propane 1875 1435 3.89091 1.146 1.0 0.3 2030 1545 3.48781 3.310 2.9 0.3 2140 1 6 2 5 3.15101 6. 659 7.0 0.3 2200 1670 3.0018 9.557 10 0.3 2230 1 6 9 0 2.9204 12.30 13 0.3 2250 1 7 0 5 2.9441 12.85 13 0.3 2230 1 6 9 0 3.0041 11.81 11 0.3 2220 1 6 8 0 3.0683 10.42 9.8 0.3 2200 1 6 6 5 3.1346 9.201 8.5 0.3 2120 1610 3. 2523 7.018 6.5 0.3 2060 1565 3.4045 5.056 4.6 0.3 1980 1 5 1 0 3.5467 3. 261 3.3 0.3 1900 1 4 5 0 3.6620 2.208 2.4 0.3 1840 1 4 1 0 3.8832 1.341 1.5 0.3 0.076 I 2010 1 5 3 0 3.3250 6.096 8.3 0.076] 2140 1 6 2 5 3.2150 13.23 12 0.076 2170 1 6 4 5 3.2080 15.71 13 0.043 2130 1615 3. 3680 18.84 13 II. A c e t rlene 0.03 2130 1620 3. 46851 6. 108] 13 0.03 2200 1 6 7 0 3.41711 9. 408[ 16 0.03 2280 1 7 2 5 3.37881 14.60 I 19 0.03 2325 1 7 6 0 3.34371 19.33 I 23 0.03 2355 1780 3. 27701 22.91 27 0.03 2380 1 8 0 0 3.26661 26.98 29 0.03 2390 1805 3. 29731 28.78 [ 28 0.03 2400 1810 3. 39701 29.22 / 23 2095 1 5 9 0 4.48411 0. 799 0.20 1 1 2200 1670 3. 93431 1.553 0.80 1.74 1 2290 1730 3. 62611 2.43' 2350 1 7 7 5 3.42711 3. 241 2.79 1 3.24 2370 1790 3. 37091 3.6Y 1 3.22 2375 1795 3. 37471 3.73a 1 1 2390 1 8 0 5 3.46841 3.82~ 2.61 1.13 2380 1800 3. 83071 3.70~ 1 0.01'. 1980 1510 3. 53211 3.52~ 15 0.01: 2070 1575 3. 44791 6.44( 2O 0.01: 2190 1660 3. 41621 13.17 24 0.013[ 2310 1 7 4 5 3.30941 24.63 34 0.013 I 2370 1 7 9 0 3.32861 35.46 35 0.013 I 2390 1 8 0 5 3.28651 41.11 41 0.013 2350 1775 3. 35541 35.53 36 0.013 2280 1 7 2 5 3.55631 23.70 23 III. E t h rlene 0.0431 1940 1480 3.8560 I 1. 698] 3.7 0.0431 2130 1620 3. 7139 I 6.186 I 6.3 0.043 / 2160 1640 3.6120 I 7.779 I 8.3 0.0431 2190 1660 3.6054 I 8. 880 I 8.8 0.043] 2190 1660 3.6230[ 9. 456] 9.0 0.043 2180 1655 3.6246 9. 277 9.1 0.043 2120 1610 3.9298 6.915 4.6
Flames
AH~ -- AH~
~/t~
kcal/mole
Propane . . . . . . . . . ] Acetylene . . . . . . . . I Ethylene . . . . . . . . I
74 74 74
2.45 X 10-4 4.04 X 10-3 2.86 X 10-3
reactants for X(Q) and X(O=) and the observed mole-fraction of positive ions for X(HaO+), the values of L were calculated from Equation (17) for the three kinds of hydrocarbon flames under the different experimental conditions. In Table 3 the results are summarized together with the experimental conditions. According to Equation (16), plots of L v e r s u s the reciprocal of Ti ought to give a single straight line for a hydrocarbon. Furthermore, the slopes of the straight lines for propane, acetylene and ethylene should be the same, because the kinetics of the ionization process is considered to be unique irrespective of the nature of hydrocarbons. The points were roughly substantiated by plotting the L values against the reciprocals of T i . F r o m the intercepts of these three straight lines, the values of 5'/8 were also evaluated. The results are listed in Table 4. With the use of these empirical parameters, the steady-state concentrations of H30 + were calculated from Equation (11) for the three classes of flames. The calculated values are compared with the experimental positive ion concentrations in the last two columns of Table 3. Discussion
As already stated, the Ha0 + concentrations obtained for hydrocarbon flames by the quasiequilibrium calculations are about 105 times as large as the observed concentrations of positive ions in the flames. This large discrepancy between the calculated and experimental values arises from the omission of the various deactivation processes such as the ion recombination and the reaction between the fuels and oxygen giving the usual combustion products. The observation that the p-factor remains almost constant for all of the hydrocarbon flames studied appears difficult to explain. Yet the finding is quite useful, because the actual concentrations of positive ions in any hydrocarbon flame seem to be roughly predicted by nmltiplying the calculated quasi-equilibrium concentrations of HaO + by 10-5. Steady-state calculations contain two empirical
228
IONS IN FLAMES
parameters, 3'/~ and AH~ - AH~. The value of/3 must be unity for acetylene flames. Providing that B is taken as unity also for hydrocarbons other than acetylene, the mole-fraction, 7, of the third body affecting the ion-recombination step in hydrocarbon flames becomes 'of the order of a few tenths per cent. This might imply that a particular species involved in the flames is incorporated in the recombination between H30 + and free electrons. Although we have no means of specifying this species, OH radical appears to be the most probable because the equilibrium concentration of OH in most hydrocarbon flames has been shown to remain constant over a wide range of equivalence ratios of the flames and to be of the order of 1 per cent. l°b The radical might be of particular importance for the recombination, on account of its high electron affinity (2.7 ev.); TM the attachment of electrons to OH forms the negative ion O H - which may recombine more readily with H~O+. The activation energy of atom recombination is usually very small. The ion recombination may have the same feature. If AH~ is very small compared with A H ~ , then the activation heat for the ionization in hydrocarbon flames will be near 74 kcal/mole. The best rough estimate of the heat of reaction for the third step in our scheme is AH3 = 120 kcal/mole, whereas the activation heat, A H ~ , comes out to be only 74 kcal/mole. To reconcile these results we suppose that either H - - C ~ C or 0~ is excited by a nonequilibrium process which has high probability and a low temperature coefficient. This seems reasonable since the process is occurring in the luminous part of the flame where there are numerous excited species such as CH* and C*. We have not the information to specify this situation more closely at present. On the assumption that various hydrocarbons produce C2H2, a variety of chain mechanisms can be devised; however, the following scheme seems energetically favorable. Q + O2
C ~ H 2 + O2
AH ° = CH+
O
57 k c a l / m o l e
--~CHO ++e; AH ° =
39 k c a l / m o l e
C H O + + H.,O --~ H30 + + CO; AH ° =
24 k c a l / m o l e .
The first two reactions are proposed by Herman, Hornbeck and Laidler, 17 and the last reaction is suggested by Calcote. 6b The calculation of AH ° values are based on recent data. 4 Probably the most important chain terminating reaction is C H O + HO~ --~ CO2 + H20; A H ° = -- 159 k c a l / m o l e . Among various other reactions, a reaction between C~H2 and HO,, parallel to the third step of the chain mechanism is probably C~H2 + HO2--~ C H + CO + H20; AH ° = --41 kcal/mole. Until we are able to make a definite decision about the most important chain mechanism, the calculation principles illustrated in this paper seem sufficient to indicate the procedure. Acknowledgment The authors wish to express their appreciation to the U. S. Air Force for support of this work under Contract No. A F 04(647)-177. REFERENCES 1. I)ECKERS, J., AND VAN TIGGELEN, A. : Nature,
181, 1460 (1958); 182, 863 (1958). 2. KNEWSTUBB, P. F., AND SUGDEN, T. M.:
Seventh Symposium (International) on Combustion, p. 247. Butterworth & Company, Ltd., London, 1959. 3. GLASSTONE, S., LAIDLER, ~4~. J., AND EYRING,
H. : The Theory of Rate Processes, McGrawHill Book Company, Inc., New York, 1941.
--~ C2H2 + o x i d a t i o n p r o d u c t s ( R e a c t i o n s are exothermic)
4. FIELD, F. H., AND FRANKLIN, J. L.: Electron
--~CHO+CHO;
5. MOORE, W. J.: Physical Chemistry, p. 39.
A H ° = -- 55 k c a l / m o l e C H O + O2
C~H~ + H 0 2 - - ~ C H + C H O + O H ;
--~CO+
HO2;
AH ° = --21 k c a l / m o l e
Impact Phenomena, p. 281. Academic Press, Inc., New York, 1957.
2nd ed., Prentice-Hall, New York, 1955. F.: Private communication (Semiannual progress report on the contract AF 04(647)-157, (a) No. 1, June 1958; (b) No. 2, December 1958; (c) No. 3, June 1959.
6. CALCOTE, H.
MECHANISM OF ION FORMATION IN HIGH-TEMPERATURE FLAMES
7. LAIDLER, IX. J.: The Chemical Kinetics of Excited States, p. 171. Oxford University Press, London, 1955. 8. FENN, J. B., AND CALCOTE, H. F.: Fourth
Symposium (International)
9. 10.
11.
12.
13. 14. 15. 16. 17.
on Combustion,
p. 231. The Williams & Wilkins Conipany, Baltimore, 1953. CALCOTE,H. F. : Combustion and Flame, 1,385 (1957). (a) GAYDON, A. G., AND WOLFHARD, H. G.: Flames, p. 181. Chapman & Hall, Ltd., London, 1953. (b) ibid., Ch. 12. LEwIs, B., PEASE, R. N., AND TAYLOR, H. S.: Combustion Processes, p. 182. Princeton University Press, Princeton, New Jersey, 1956. FROST, A. A., AND PEARSON, R. G.: Kinetics and Mechanism, p. 183. J o h n Wiley & Sons, Inc., New York, 1953. EYRING, H., GIDDINGS, J. C., AND TENSMEYER, L. G.: J. Chem. Phys., 24, 857 (1956). VAN TIGGELEN, A. : Mem. Acad. Roy. Belg. (C1. Sc.), 27, 1 (1952). PONCELET, J., AND VAN TIGGELEN, A . : Bull. Soc. Chim. Belges, 67, 49 (1958). SmTH, H., AND SUGDEN, W. M.: Proc. Roy. Sou., A211, 31 (1952). HERMAN, R. C., HORNBECK, G. A., AND LAIDLER, K . J. : Science, 112, 497 (1950).
D i s c u s s i o n BY FELIX T. SMITtI There is a possible experiment which should show whether more t h a n one carbon atom is required in the mechanism of ion production. If the ion current is measured as small amounts of methane are added to a hydrogen-air f a m e , a linear relation between current and niethane concentration would imply a mechanism involving only one carbon atom. AUTHORS' REPLY In another paper* presented in this symposium, it has been shown t h a t a radical or molecule containing carbon and hydrogen is directly responsible for the production of the most a b u n d a n t ion in high concentration in flames, and t h a t the CH is such a radical. The formation of the CH from acetylene is energetically favorable and acetylene is found in a b u n d a n t q u a n t i t y even in methaneoxygen flames. Whether the Coil or the CH is the niost important radical responsible for the most a b u n d a n t ion formation, their parent niolecule is acetylene. Studies of ion current versus only methane concentration in a hydrogen-oxygen flame would be beneficial, but may not indicate the mechanism of the most a b u n d a n t ion formation conclusively. However, repetition of these studies with other aliphatic hydrocarbons such as
229
propane, ethylene and acetylene, and then comparison of all these data will be much more valuable in devising the mechanism of the most a b u n d a n t ion formation t h a n the data of m e t h a n e alone. Another point of interest which we proposed is t h a t the OH radical is very i m p o r t a n t for the production of the CH radical and hence for the production of the most a b u n d a n t ion.* Because hydrogen-oxygen flames contain the OH radical in abundance, ion concentrations in hydrogenoxygen-(injected) hydrocarbon f a m e s are expected to be greater t h a n the same hydrocarbonoxygen flames under the same conditions of temperature, pressure and equivalence ratio. DISCUSSION BY A. VAN TIGGELEN In connection with different papers presented at this meeting t h a t deal with the problem of ion formation, it might be interesting to mention some recent and unpublished results which have been obtained in Louvain with the collaboration of J. N. Bertrand. I t has been observed, in the following series of fuels burning with oxygen (methare, methyl alcohol, formaldehyde and formic acid), t h a t ions are formed only in the f a m e s of CH4 and CH3OH. The ions are absent (or in an amount less t h a n l0 s ions/cc) in CH20 and HCOOH flames. This seems to indicate t h a t in the process by which the parent ions are formed, the carbon atom of the particle (molecule or radical) involved in the reaction of chemi-ionization niay not be doubly linked to an oxygen atom. It is presumably the strong exothermicity of the formation of such a bond, with the excitation energy of the hydrocarbon fragment or radical, which renders possible the process of chemi-ionization. To give an example, one could propose processes of the following kind : CH* + O: --* CO + 4- OH + eor CH* + O 2 ~ C O H *
+ OH + e-
C* + O 2 - - * C O + + CO + eand so forth. In order to investigate whether or not the presence of the hydrogen atom linked to the carbon atom is a necessary condition, we expect some good information from a study of a carbon suboxide--oxygen flame (C~O.o + 02). In this ease one of the carbon atoms is neither bound to oxygen nor hydrogen. This work was undertaken in our laboratory a few weeks ago and from the few first experimental results it seems * Eyring, H. : Eighth Symposium (International) on Combustion, p. 222. The Williams & Wilkins Company, Baltimore, 19~2.
230
IONS IN FLAMES
that the C302/Q flames do not contain ions in a quantity comparable to that observed in hydrocarbon flames; however, more experiments are to be made before definite conclusions can be derived. It is also important to note that cyanogen-oxygen flames are ion-poor flames. The only NO + ion has been identified; this should be expected from the fact that NO is formed among the reaction products. AUTHORS' REPLY
Our work indicates that the processes for the formation of possible ions including the H30 + from formMdehyde and formic acid by direct ionization or chemi-ionization are too highly endothermic to produce appreciable ions. It seems that high ionization i~ not expected by a reaction in which one of the reactants contains a carbon atom (or atoms) doubly linked with oxygen. However, the formation of a double bond between a carbon atom and an oxygen atom in one of the products of a chemi-ionization reaction is a favorable process for ionization in flames. As far as the most abundant ion, H~O +, is con-
cerned, the most probable process f o r its formation is a proton-transfer reaction between a suitable proton-donor ion and water molecule. In general, it can be hypothesized that among the similar ionic species containing carbon, hydrogen and oxygen, the ion in the highest oxidized state will be the best proton-donor. In the light of these assumptions it is desirable for the proton-donor ion to be formed with one or two double bonds between carbon and oxygen. Although the ionization potential of nitric oxide is about 9.2 ev., the NO + is not expected to be produced by direct ionization of the NO in flames at temperatures 2000 ° to 2500°C. A probable mechanism of the NO + formation from nitrogen and oxygen molecules are given in the paper "Ions in Flames" of this symposium. The formation of NO + in C2N2--O2 flames is being investigated by us. We agree with Dr. Van Tiggelen that the C* is not directly related to the formation of the most abundant ion. However, in hydrocarbon flames the C* can produce other radicals, such as the CH and CHO, which may be directly responsible for the high ionization.
18 CATALYTIC DISSOCIATIVE REACTIONS IN ELECTRICAL DISCHARGES °
By
F. K A U F M A N AND J. R. K E L S O
Introduction In taking a broad look at the progress of experimental combustion science during the past several years, one discerns some fairly welldefined trends. There is a growing interest in the detailed chemical and physical steps of combustion processes rather than in the complicated total phenomena. One studies energy transfer processes in simple systems; atom and free radical reactions under controlled conditions; the kinetics of simple reactions at lower temperatures in conventional apparatus, or at high temperatures for very short times in shock-tubes. There is also a growing interest in highly energetic species such as electronically excited states, and in the role played by electrically charged species, i.e., elecSupported by the Advanced Research Projects Administration.
trons and positive and negative ions. As a result of these tendencies there has been increasing interaction and overlap with the fields of upperatmosphere science, electron physics and magnetohydrodynamics. We have recently studied reactions of oxygen and nitrogen a t o m s " 2 in flow systems and have used high-frequency electrical discharges to produce the free atoms or other excited species) We have followed the rates of various atom reactions by measuring the intensity of certain glows - - t h e air afterglow and the nitrogen afterglow-whose dependence on atom concentration was known. Moreover, we have been able to utilize simple gas "titrations," NO~ for O-atoms and NO for N-atoms, for the quick determination of atom concentrations. In all of this work, the electrcdeless discharge was used only as a source of free atoms, and no