Inhibiting Action of Hydrocarbons on Ozone Formation by Silent Electrical Discharge EIICHI INOUE and
KIICHIRO
SUGINO
Tokyo Institute of Technology, Tokyo, Japan
The reaction mechanism of ozone formation was studied to clarify the inhibiting action of hydrocar bons. The rate of ozone formation varies linearly with the hydrocarbon concentration, regardless of the type of hydrocarbon.
O z o n e f o r m a t i o n b y s i l e n t e l e c t r i c a l d i s c h a r g e is i n h i b i t e d p e r f e c t l y b y a d m i x i n g h y d r o c a r b o n s i n a i r (1). T h i s w a s t h e m a i n r e a s o n f o r t h e success of t h e s l o w o x i d a t i o n of h y d r o c a r b o n s b y s i l e n t e l e c t r i c a l d i s c h a r g e . I t also caused t h e g r e a t d i f f i c u l t y i n p r e p a r i n g ozone b y t h e same process. A n i n v e s t i g a t i o n w a s m a d e t o c l a r i f y t h e m e c h a n i s m of t h e i n h i b i t i n g a c t i o n . Reaction
Mechanism
O z o n e f o r m a t i o n p r o b a b l y consists of t h e f o l l o w i n g e l e m e n t a r y r e a c t i o n s 0
+ e*
2
4Ο +
Ο* + e
(8).
(8.3 ev.)
(1)
(24.1 kcal.)
(2)
(116.4 kcal.)
(3)
ta Ο + 0
2
+ M -» 0
0 + 0 + Μ-4θ 0
3
3
+ Μ
2
+ e* -> Ο + 0
+ M
2
+ e
(4)
R e a c t i o n 4 c a n b e n e g l e c t e d , as i t s r a t e is t o o s l o w t o c o m p a r e w i t h t h e o t h e r s . T h e s l o w o x i d a t i o n of h y d r o c a r b o n s p r o c e e d s b y t h e f o l l o w i n g c h a i n r e a c t i o n , t h e s o - c a l l e d p e r o x i d e process, as d e s c r i b e d p r e v i o u s l y (1-7) : 0
Ο + Ο* + e
(1)
R H + Ο + M % R- + OH + M
(5)
R H + O H + M —» R · + H 0 + M
(6)
R- + 0 - > R 0 .
(7)
2
+ e*i
2
2
2
R 0 · + R H -+ R 0 H - f R · 2
2
(8)
I n t h e o x y g e n - h y d r o c a r b o n m i x t u r e , R e a c t i o n s 2, 3, a n d 5 a r e m u t u a l l y c o m p e t i t i v e , 313
314
ADVANCES
IN
CHEMISTRY
SERIES
so ozone f o r m a t i o n m a y be i n h i b i t e d w h e n t h e r a t e of R e a c t i o n 5 is g r e a t e r t h a n t h a t of R e a c t i o n 2. I n o u e (2-7) f o u n d t h a t t h e r a t e - d e t e r m i n i n g s t e p of t h e h y d r o c a r b o n o x i d a t i o n is R e a c t i o n 1 a n d t h a t a l l reactions take place i n a s t a t i o n a r y state. Then = 2fc [02][e] 1
fc [0][0 ][M] 2
-
2
fc [0] [M] 3
-
2
fc [RH][0][M] 4
(9)
= 0 Therefore,
[0]
is c o n s t a n t .
T h e n t h e r a t e of ozone f o r m a t i o n is -
fe[0][o,][M]
(io)
F r o m E q u a t i o n s 9 a n d 10 =
2fc [0,][e] 1
/c [0] [M] -
fc [RH][0][M]
2
3
4
(11)
W h e n t h e o x y g e n c o n c e n t r a t i o n is c o n s t a n t , b o t h t h e first a n d s e c o n d t e r m s i n E q u a t i o n 11 b e c o m e c o n s t a n t . T h e r a t e of ozone f o r m a t i o n , a c c o r d i n g l y , c a n be e x p r e s s e d b y t h e first-order f u n c t i o n of t h e h y d r o c a r b o n c o n c e n t r a t i o n . When dm dt
n υ
À; [RH][0][M] = 2/d[0 ][e] 4
Α· [0] [Μ]
2
(12)
2
3
T h e n t h e c r i t i c a l c o n c e n t r a t i o n of h y d r o c a r b o n for t h e p e r f e c t i n h i b i t i o n of ozone f o r m a t i o n is rpTTi Therefore, [ R H ] rent are definite.
Experimental
c r i t i c a l
.
2fc [0 ][e] - / b [ Q ] [ M ] t
2
3
2
becomes constant w h e n b o t h [ 0 ] 2
a n d the discharge c u r
Work
T o i d e n t i f y E q u a t i o n 11 e x p e r i m e n t a l l y , t h e rates of o z o n e f o r m a t i o n f r o m a i r w e r e o b s e r v e d b y c h a n g i n g t h e c o n c e n t r a t i o n of a d m i x e d h y d r o c a r b o n s . The experi m e n t w a s p e r f o r m e d b y u s i n g t h e a p p a r a t u s s h o w n i n F i g u r e 1. U n d e r e x p e r i m e n t a l c o n d i t i o n s i n T a b l e I , t h e m a x i m u m ozone c o n c e n t r a t i o n Table I.
Experimental Conditions
Discharge tube Effective length Space gap Outer diameter of inner tube Effective volume Discharge voltage Discharge current Reaction temp. Total pressure Flow rate Space velocity
12.0 cm. 3.0 mm. 3.80 cm. 42.5 cc. 15.0 kv. 1.45 ma. 200° C. 760.0 mm. of Hg 11.0 l.air/hour 4.15 m i n . -1
r e a c h e d a b o u t 1.0 v o l u m e % , so t h a t t h e v a r i a t i o n of o x y g e n c o n c e n t r a t i o n c o u l d be neglected. T h e a m o u n t s of h y d r o c a r b o n s a d m i x e d were m e a s u r e d b y w e i g h i n g t h e c h a n g e of t h e w e i g h t of e v a p o r a t i o n i n t h e feeder i n F i g u r e 1. T h e q u a n t i t a t i v e a n a l y -
315
INOUE A N D SUGINO-INHIBITING ACTION OF HYDROCARBONS
50ru 100 V.
Figure 1.
Apparatus
1. Blower 2. Silica gel tube 3. Flowmeter 4. Manometer 5. Hydrocarbon evaporator 6. Discharge tube 7. Cooled trap 8. Potassium iodide solution 9. Transformer
sis o f ozone w a s c a r r i e d o u t b y t h e i o d i n e m e t h o d . T h e t o t a l e r r o r of t h e e x p e r i m e n t was w i t h i n ± 3 % . T h e results are shown i n T a b l e I I .
Table II. Hydrocarbon Cyclohexane
n-Hexane
n-Heptane
Experimental Results
G./Hour
Concn. of Hydrocarbon, Vol. %
M m . Hg
Rate of O 3 Formation, Mole/Hour X 10
0.000 0.092 0.227 0.290 0.550 0.850
0.000 0.223 0.549 0.702 1.331 2.051
0.00 1.69 4.18 5.34 10.12 15.64
5.89 4.61 2.67 1.98 0.12 0.09
0.164 0.250 0.304 0.335 0.509
0.388 0.589 0.740 0.792 1.201
2.96 4.50 5.49 6.03 9.23
3.58 2.38 1.82 1.50 0.13
0.056 0.120 0.160 0.264 0.311 0.749
0.114 0.245 0.326 0.538 0.633 1.502
0.87 1.86 2.48 4.10 4.82 11.50
5.22 4.53 4.04 3.05 1.99 0.08
s
I n F i g u r e 2, t h e a m o u n t s of ozone f o r m e d d u r i n g a 1 0 - m i n u t e p e r i o d a r e p l o t t e d a g a i n s t t h e c o n c e n t r a t i o n s of n - h e x a n e , c y c l o h e x a n e , a n d η - h e p t a n e . T h e r a t e of ozone f o r m a t i o n changes l i n e a r l y w i t h t h e h y d r o c a r b o n c o n c e n t r a t i o n ( E q u a t i o n 1 1 ) . And [ R H ] w a s a b o u t 1.1 v o l u m e % (8.5 m m . of m e r c u r y ) f o r a i r . N o m a r k e d difference i n i n h i b i t i n g a c t i o n w a s o b s e r v e d o n c h a n g i n g t h e t y p e o f hydrocarbon. c r i t i c a l
316
A D V A N C E S IN CHEMISTRY SERIES 6,
5
4
\
O CYCLOHEXANE • Π-ΗΕΧΑΝΕ X n-HEPTANE
3
ο
ε
0
0.2
, Ο 0.4 0.6 0.8 1.0 1.2 1.4 CONCENTRATION, HYDROCARBON,VOL.% ' 4 ' 6 ' 8 ' 10 ' PRESSURE, HYDROCARBON, mmH
1.6
t
0
'
2
12
3
Figure 2 .
Literature (1) (2) (3) (4) (5) (6) (7)
Effect of hydrocarbon concentration on ozone formation
Cited
Inoue, E., J. Electrochem. Ibid., 23, 18 (1955). Ibid., p. 76. Ibid., p. 403. Ibid., p. 452. Ibid., p. 574. Ibid., p. 647.
Soc. Japan 22, 668 (1954).
(8) Rideal, Ε. K., "Ozone," Constable, London, 1920. RECEIVED
for review M a y 17, 1957.
Accepted June 19, 1957.
