OZONE CHEMISTRY AND TECHNOLOGY

Mechanism of Gas Phase Decomposition of Ozone. Thermal and Photochemical Reactions ARTHUR E. AXWORTHY, Jr.,

1

Downloaded by STANFORD UNIV GREEN LIBR on May 31, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0021.ch052

Department

and SIDNEY W. BENSON

of Chemistry, University of Southern California,

Los Angeles 7, Calif.

Most of the known data on the thermal decomposi tion of ozone can be explained quantitatively in terms of the simple atomic mechanism

in which Reactions 1 and 2 are at their low pressure limit. There is no evidence for a direct bimolecular reaction or an energy chain mechanism. Surface reactions do not a p p e a r to be important in this system. The relative efficiencies of various gases as M in Reactions 1 a n d 2 are: O = 1.00, O = 0.44, N = 0.41, CO = 1.06, and He = 0.34. Quantitatively reproducible results are difficult to achieve. The energy chain mechanism proposed previously to account for the high quantum yields in the far-ultraviolet photolysis is inconsistent with both the photochemical results obtained with red light a n d the postulated thermal mechanism. 3

2

2

2

T h e r a t e of t h e gas phase d e c o m p o s i t i o n of ozone has b e e n s t u d i e d b y m a n y i n v e s t i g a t o r s u n d e r a w i d e r a n g e of c o n d i t i o n s , y e t n o c o m p l e t e l y s a t i s f a c t o r y m e c h a n i s m has e v e r b e e n p r o p o s e d f o r e i t h e r t h e t h e r m a l o r p h o t o c h e m i c a l d e c o m p o s i t i o n s . Thermal

Decomposition

T h e e a r l y w o r k e r s (19) i n v e s t i g a t e d t h e p y r o l y s i s of d i l u t e ozone a n d a t t e m p t e d to e x p l a i n t h e i r r e s u l t s i n t e r m s of t h e s i m p l e J a h n m e c h a n i s m ι 0 ;=± 0 3

+ Ο

2

2

Ο + 0 —» 2 0 3

Rapid equilibrium Rate determining

2

(1,2) (3)

i n w h i c h i t is assumed t h a t the oxygen atoms are a t t h e i r e q u i l i b r i u m concentration w i t h respect t o R e a c t i o n s 1 a n d 2. T h e r e p r o d u c i b i l i t y of these d i l u t e ozone e x p e r i 1

P r e s e n t address, S h e l l O i l C o . , M a r t i n e z , C a l i f . 388

In OZONE CHEMISTRY AND TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

AXWORTHY A N D B E N S O N — G A S PHASE

389

DECOMPOSITION


m e n t s w a s v e r y p o o r , a n d n o c o n c l u s i v e evidence w a s ever p r e s e n t e d t h a t t h e r a t e i s e x a c t l y i n v e r s e l y p r o p o r t i o n a l t o t h e o x y g e n c o n c e n t r a t i o n , as r e q u i r e d b y t h i s m e c h a n i s m . G a r v i n , h o w e v e r , r e i n v e s t i g a t e d t h e t h e r m a l d e c o m p o s i t i o n of d i l u t e ozone, e m p l o y i n g a f l o w t e c h n i q u e (6). H e f o u n d t h e r a t e t o be n e a r l y i n v e r s e l y p r o p o r t i o n a l t o t h e o x y g e n c o n c e n t r a t i o n a n d i n d e p e n d e n t of t h e c o n c e n t r a t i o n of a d d e d n i t r o g e n . S e v e r a l i n v e s t i g a t i o n s h a v e been c a r r i e d o u t u s i n g c o n c e n t r a t e d ozone o b t a i n e d b y t h e f r a c t i o n a l d i s t i l l a t i o n of d i l u t e ozone (8, 12-14, 17). M o s t of t h e results were r e p r e s e n t e d e m p i r i c a l l y i n t e r m s of s i m u l t a n e o u s f i r s t - a n d s e c o n d - o r d e r r e a c t i o n s . T h e m o s t s a t i s f a c t o r y s t u d y , w i t h respect t o b o t h t h e e x p e r i m e n t a l m e t h o d s e m p l o y e d a n d t h e w i d e r a n g e of c o n d i t i o n s c o v e r e d , a p p e a r s t o b e t h a t of G l i s s m a n n a n d S c h u m a c h e r (8). T h e y s t u d i e d t h e r e a c t i o n b e t w e e n 7 0 ° a n d 110° C . i n vessels v a r y i n g i n size f r o m 0.5 t o 12 l i t e r s i n t h e presence of a d d e d o x y g e n , n i t r o g e n , h e l i u m , a n d carbon dioxide. T h e following mechanism i n v o l v i n g simultaneous bimolecular a n d a t o m i c r e a c t i o n s as w e l l as a n e n e r g y c h a i n w a s p r o p o s e d : 0

3

+ 0 -> 3 0 + 69 kcal.

Bimolecular

(4)

0

2

+ 0 —> 0

Atomic

(5)

Energy chain

(3)

3

2

+ 0

2

Μ + Ο + 0 -> M + 0

3

3

2

2

+ Ο "J V

Ο + 0 —» 20% + 93 kcal. J 3

0% + 0 - + 0

2

+ 0

Ο* + M —> 0

2

+ M

3

2

+ 0

(2)

J

(6) (7)

T h e r e i s , h o w e v e r , a n e r r o r i n t h i s scheme, because i t m a y b e s h o w n (4) t h a t R e a c t i o n 5 m u s t h a v e t h e same f o r m of p r e s s u r e d e p e n d e n c e as R e a c t i o n 2. T h i s e r r o r does n o t s h o w u p m a r k e d l y i n t h e i r e x p e r i m e n t s o n o z o n e - o x y g e n m i x t u r e s , because, a c c o r d i n g t o t h e i r m e c h a n i s m , t h e a t o m i c r e a c t i o n is i m p o r t a n t o n l y u n d e r c o n d i t i o n s w h e r e t h e ozone c o n c e n t r a t i o n i s s m a l l . T h i s m e c h a n i s m does n o t a c c o u n t f o r t h e m a r k e d a c c e l e r a t i o n w h i c h is o b s e r v e d w h e n i n e r t gases, e s p e c i a l l y c a r b o n d i o x i d e , a r e a d d e d . S c h u m a c h e r (15) p o i n t e d o u t t h a t t h i s m u s t b e d u e t o t h e effect of t h e i n e r t gases o n t h e r a t e of R e a c t i o n 5, b u t h e w a s a t a loss t o e x p l a i n w h y t h e a c c e l e r a t i o n is t h e greatest u n d e r c o n d i t i o n s w h e r e R e a c t i o n 4 s h o u l d p r e d o m i n a t e . A n o t h e r d i f f i c u l t y w i t h t h i s m e c h a n i s m i s , as discussed b y G e i b ( 7 ) , t h a t t h e v a l u e assigned {15) t o t h e r a t e c o n s t a n t of R e a c t i o n 4 y i e l d s a steric f a c t o r of close t o 100, w h i c h is h i g h f o r t h i s t y p e of b i m o l e c u l a r r e a c t i o n . S u t p h e n (18) m e a s u r e d t h e r a t e of t h e t h e r m a l d e c o m p o s i t i o n b e t w e e n 2 5 ° a n d 115° C . o v e r a w i d e r a n g e of t o t a l pressures f r o m 29 m m . of m e r c u r y t o 6 a t m . H e c o n c l u d e d t h a t a l l t h e steps of t h e c o m p l e x m e c h a n i s m p r o p o s e d b y G l i s s m a n n a n d Schumacher are required to account for his results. T h i s includes the reactions w h i c h were p r o p o s e d t o b e i n i t i a t e d b y a c t i v a t e d o x y g e n molecules f o r m e d i n t h e r e a c t i o n . S u t p h e n also c o n c l u d e s t h a t R e a c t i o n 2 does n o t r e a c h i t s l o w p r e s s u r e l i m i t o v e r t h i s r a n g e of pressures. A r e i n v e s t i g a t i o n o f t h e t h e r m a l d e c o m p o s i t i o n w a s u n d e r t a k e n i n a n effort t o d i s c o v e r t h e exact m e c h a n i s m of t h i s i n t e r e s t i n g r e a c t i o n (2). T h e experimental p r o c e d u r e p r e s e n t e d here is s i m i l a r t o t h a t of G l i s s m a n n a n d S c h u m a c h e r (8). Experimental. T w o sources of o x y g e n were e m p l o y e d : t a n k o x y g e n passed o v e r h o t c o p p e r oxide a n d o x y g e n f o r m e d b y t h e p y r o l y s i s of p o t a s s i u m p e r m a n g a n a t e . Both sources g a v e e q u i v a l e n t results a f t e r t h e l a t t e r h a d b e e n d r i e d t o r e m o v e t h e l a r g e q u a n t i t y of w a t e r t h a t w a s p r e s e n t . T h e o x y g e n w a s condensed a t t h e t e m p e r a t u r e of l i q u i d n i t r o g e n , s l o w l y d i s t i l l e d t h r o u g h a g l a s s - w o o l t r a p cooled i n l i q u i d o x y g e n , a n d s l o w l y d i s t i l l e d t h r o u g h a b o r o s i l i c a t e glass o z o n i z e r . T h e o z o n e - o x y g e n m i x t u r e w a s c o n d e n s e d a t t h e t e m p e r a t u r e of l i q u i d n i t r o g e n a n d o p e n e d t o t h e p u m p s u n t i l n e a r l y a l l of t h e o x y g e n w a s r e m o v e d . T h e ozone w a s t w i c e t r a p - t o - t r a p v a c u u m - d i s t i l l e d , t h e f i n a l p o r t i o n of t h e second d i s t i l l a t i o n b e i n g d i s c a r d e d . A f t e r t h e r e a c t i o n vessel r e a c h e d r e a c t i o n t e m p e r a t u r e , the ozone w a s a l l o w e d t o e v a p o r a t e i n t o i t . O x y g e n w a s t h e n a d d e d . A t t h e l a t t e r


A D V A N C E S IN CHEMISTRY SERIES


390

t w o p o i n t s t h e d a n g e r of e x p l o s i o n i s h i g h . T h e d e a d space w a s flushed w i t h o x y g e n a n d t h e p r e s s u r e r e a d i n g s were b e g u n . T h e i n i t i a l ozone pressures r a n g e d f r o m 9 0 t o 230 m m . of m e r c u r y , a n d t h e d e c o m p o s i t i o n w a s u s u a l l y f o l l o w e d t o a final ozone p r e s sure of a b o u t 10 m m . of m e r c u r y . T h e p r e s s u r e w a s m e a s u r e d u s i n g a b o r o s i l i c a t e glass B o u r d o n gage w h i c h w a s s e n s i t i v e t o 0.1 m m . of m e r c u r y as a n u l l i n s t r u m e n t t o a m e r c u r y m a n o m e t e r . T h e final p r e s s u r e w a s m e a s u r e d a f t e r t h e r e a c t i o n vessel h a d b e e n h e a t e d a b o v e 200° C . f o r 8 h o u r s o r l o n g e r . T h e m a x i m u m d e a d space of 14 cc. w a s efficiently flushed w i t h o x y g e n a t t h e s t a r t of e a c h e x p e r i m e n t a n d w a s d u l y c o r r e c t e d f o r i n t h e c a l c u l a t i o n (2) of t h e ozone c o n c e n t r a t i o n s . T h e h o l l o w - b o r e v a c u u m s t o p c o c k s u s e d i n t h e s y s t e m were l u b r i c a t e d w i t h H a l o c a r b o n ( h i g h t e m p e r a t u r e g r a d e ) s t o p c o c k l u b r i c a n t . T h e u s u a l p r e c a u t i o n s w e r e t a k e n t o m i n i m i z e t h e a m o u n t of m e r c u r y v a p o r i n t h e system. Quantitative D a t a . R E S U L T S . F o u r i n i t i a l e x p e r i m e n t s w e r e c a r r i e d o u t a t t e m p e r a t u r e s close t o 100° C . i n t w o d i f f e r e n t r e a c t i o n vessels. T w o of t h e e x p e r i m e n t s , one w i t h a b o u t 500 m m . of m e r c u r y of o x y g e n a d d e d i n i t i a l l y , w e r e c a r r i e d o u t i n a 537-cc. b o r o s i l i c a t e glass s p h e r e a n d t h e o t h e r t w o i n a n i r r e g u l a r l y s h a p e d , 4 6 1 - c c . b o r o s i l i c a t e glass vessel w i t h a b o u t 4 5 % g r e a t e r s u r f a c e . T h e r e s u l t s of these f o u r e x p e r i m e n t s a r e t a b u l a t e d i n T a b l e I . T a b l e I.

Comparison of Results of Experiments with Proposed Mechanism

Experiment Vessel Temperature, ° C. Pressure after total decomposition k„ Χ 10 at 95 mm. of ozone 3

2 I 99.4 301.3 131 (115) 144 (135) 153 (150) 183 (180) 231 (243)

1 I 99.3 620.2

8

b

k, Χ 10 at 70 mm. of ozone 3

3


b

b

kg Χ 10 at 20 mm. of ozone 3

(124) 159 (135) 165 (143) 172 (153) 190 (169)

b

k Χ 10 at 55 mm. of ozone t

T h e r e s u l t s of s i x

b

Values listed are second-order rate constants in liter per mole-second. predicted by mechanism and rate constants developed here. b

4 II 100.0 208.1 104 (101) 126 (122) 152 (141) 178 (173) 280 (258)

3 II 99.9 207.2 110 ( 99) 126 (120) 148 (139) 172 (170) 258 (255)

Values in parentheses are

s i m i l a r e x p e r i m e n t s of G l i s s m a n n a n d S c h u m a c h e r a t n e a r l y t h e same t e m p e r a t u r e a r e t a b u l a t e d i n T a b l e I I . I n one of t h e i r e x p e r i m e n t s 610 m m . of n i t r o g e n w a s a d d e d . T h e results are i n qualitative agreement b u t t h e y cannot be c o m p a r e d exactly, except i n t e r m s of a specific m e c h a n i s m , because t h e t o t a l pressures w e r e n o t t h e s a m e . T a b l e II.

Comparison of Results of Glissmann and Schumacher (8) with Proposed Mechanism

Experiment Vessel Temperature, ° C . Pressure after total decomposition k» Χ 10 at 95 mm. of ozone 3





4

46 He 99.8 767.8

b

(122) 127 (131) 136 (135) 144 (142) 153 Q52)

45 II 99.8 408.8 131 (131) 145 (149) 160 (165) 178 (188)

43 II 99.8 254.6 113 (110) 126 (131) 144 (151) 166 (181)

40 II 99.8 120.6

('87) 101 (101) 132 (126) 197 (203)

(229) (257) * M m . of mercury. Values listed are second-order rate constants in liter per mole-second. theses are predicted by mechanism and rate constants developed here. One-liter spherical vessel. 610 mm. of nitrogen present. b

39 II 99.8 50.7

119 (114)

66^ II 99.8 125.9 (oxygen)

(354) 415 (406) 475 (478) 605 (641)

Values in paren

0

d


AXWORTHY

391

A N D B E N S O N - G A S PHASE DECOMPOSITION

E x p e r i m e n t s 3 and 4 ( T a b l e I ) represent n e a r l y identical conditions a n d , therefore, i n d i c a t e t h e o r d e r of r e p r o d u c i b i l i t y of p r e s e n t r e s u l t s . T h e s a m e g e n e r a l t r e n d s o c c u r i n b o t h sets of e x p e r i m e n t s . T h e s e c o n d - o r d e r r a t e c o n s t a n t s increase m a r k e d l y d u r i n g a g i v e n r u n , t h e m a g n i t u d e of t h i s effect d e c r e a s i n g w i t h increasing oxygen pressure. These second-order rate constants were obtained b y t a k i n g t a n g e n t s a t v a r i o u s p o i n t s o n a n i n v e r s e 0 c o n c e n t r a t i o n vs. t i m e p l o t . A l l of t h e d a t a of G l i s s m a n n a n d S c h u m a c h e r were p l o t t e d i n t h i s m a n n e r i n o r d e r t o o b t a i n t h e best r a t e c o n s t a n t s . A t a g i v e n ozone p r e s s u r e , t h e a d d i t i o n of o x y g e n accelerates t h e r a t e a t h i g h ozone pressures a n d i n h i b i t s i t a t l o w e r pressures. 3


D I S C U S S I O N . T h e s i m p l e a t o m i c m e c h a n i s m ( R e a c t i o n s 1 t o 3 ) a p p e a r s t o be c o m p a t i b l e w i t h t h e q u a l i t a t i v e f e a t u r e s of t h e t h e r m a l d e c o m p o s i t i o n of ozone. A n a t t e m p t w a s m a d e , t h e r e f o r e , t o test q u a n t i t a t i v e l y . A l t h o u g h t h e r a t e l a w d e r i v e d f r o m t h i s m e c h a n i s m is c o m p l e t e l y i n t e g r a b l e , i t w a s s i m p l e r t o test t h e d a t a i n t e r m s of t h e d i f f e r e n t i a l r a t e e x p r e s s i o n . T h i s e x p r e s s i o n , a s s u m i n g R e a c t i o n s 1 a n d 2 to be a t t h e l o w pressure l i m i t , m a y be p u t i n t h e f o l l o w i n g form: (M)/*.(0,) = (k/2fc,fc,)(M)(0 )/(0,) + l/2/f!

(8)

2

where (M) = (0 ) + α ( 0 ) + α ( Ν ) + aco (C0 ) + a (He) 3

θ 2

2

Ν2

2

2

2

(9)

He

k = -[d(0 )/dt]/(0,)«.

and

8

(10)

8

T o test t h i s p r o p o s e d m e c h a n i s m , (M)/k (0 ) T h e b e s t fit w a s o b t a i n e d u s i n g s

3

was p l o t t e d against ( M ) ( 0 ) / ( 0 ) . 2

(M) = (0 ) + 0.44(O ) + 0.41(N ) + 1.06(CO ) + 0.34(He) 3

2

2

2

3

(11)

A l l 4 7 e x p e r i m e n t s of G l i s s m a n n a n d S c h u m a c h e r (8) were p l o t t e d i n t h i s m a n n e r except experiments 31, 37, a n d 94, w h i c h h a d n u m e r i c a l or t y p o g r a p h i c a l errors i n the t a b u l a t e d d a t a . T h e i r r e s u l t s y i e l d e d s t r a i g h t l i n e s (2, 5) f o r a l l e x p e r i m e n t s , i n c l u d i n g those w i t h a d d e d i n e r t gases w i t h a n a v e r a g e e r r o r of less t h a n 5 % , e x c e p t a t v e r y h i g h ozone c o n c e n t r a t i o n s a t t h e h i g h e r t e m p e r a t u r e s s t u d i e d . T h i s is excellent a g r e e m e n t , c o n s i d e r i n g t h a t k m a y v a r y b y a f a c t o r of m o r e t h a n 6 a t a g i v e n t e m p e r a t u r e d e p e n d i n g u p o n t h e c o n d i t i o n s (see T a b l e s I a n d I I ) . T a b l e I s h o w s t h a t a l l f o u r e x p e r i m e n t s fit t h i s same m e c h a n i s m w i t h a n a v e r a g e e r r o r of less t h a n 1 0 % . T h e v a l u e s of k a n d k k /k o b t a i n e d f r o m t h e i n t e r c e p t s a n d slopes of these g r a p h s y i e l d e d excellent A r r h e n i u s p l o t s . B e c a u s e r e s u l t s a r o u n d 99.8° C . w i t h o u t a d d e d i n e r t gases agree w e l l w i t h t h e results of G l i s s m a n n a n d S c h u m a c h e r u n d e r t h e same c o n d i t i o n s i n t e r m s of t h e p r o p o s e d m e c h a n i s m , it a p p e a r s t h a t t h e l a r g e b o d y of d a t a p r e s e n t e d b y t h e m is r e l i a b l e . T h e v a l u e s of t h e r a t e c o n s t a n t s , a c t i v a t i o n energies, a n d a's p r e s e n t e d i n t h i s p a p e r w e r e o b t a i n e d b y r e i n t e r p r e t a t i o n of t h e r e s u l t s of G l i s s m a n n a n d S c h u m a c h e r . F r o m these are o b t a i n e d t h e v a l u e s (for M e q u a l t o 0 ): s

x

1

s

2

8

fa = 4.61 ± 0.25 Χ 10 fafa/k

= 2.28 ± 0.16 X 10

2

12

exp ( - 2 4 , 0 0 0 / # Τ )

Liter/mole-second

15

exp (-30,600/#T)

Second"

1

T h e v a l u e s o b t a i n e d b y G a r v i n (6) f o r t h e q u a n t i t y k k /k at m u c h t e m p e r a t u r e s a r e n e a r l y i d e n t i c a l w i t h those o b t a i n e d f r o m t h e a b o v e e x p r e s s i o n . k n o w n t h e r m a l d a t a (11) 1

K

= h/k

e(l

2

= 7.7 X 10 exp (-24,600/#T)

3

2

Moles/liter

4

k = 6.00 X 10 exp ( + 6 0 0 / Λ 7 )

Liter /mole -second

7?3 = 2.96 X 10 exp ( - 6 0 0 0 / O T )

Liter/mole-second

2

7

7

10

higher From

2

2

T h e rates o b s e r v e d b y G l i s s m a n n a n d S c h u m a c h e r a r e a p p r e c i a b l y h i g h e r t h a n those p r e d i c t e d b y t h e a b o v e m e c h a n i s m w h e n t h e a b s o l u t e r a t e of t h e r e a c t i o n is g r e a t e r t h a n a b o u t 1 X 1 0 ~ m o l e p e r l i t e r - s e c o n d . T h r e e possible e x p l a n a t i o n s of 6


392

A D V A N C E S IN

CHEMISTRY SERIES

these a b n o r m a l l y h i g h rates h a v e b e e n c o n s i d e r e d : first, s e c o n d a r y a c t i v a t i o n of ozone b y the a c t i v a t e d oxygen molecules formed i n R e a c t i o n 3 ; secondly, a direct bimolecular r e a c t i o n ; a n d , f i n a l l y , t h e possible existence of t e m p e r a t u r e g r a d i e n t s i n t h e s y s t e m . T h e i m p o r t a n c e of s e c o n d a r y a c t i v a t i o n ( e n e r g y c h a i n s ) w o u l d d e p e n d o n t h e r a t i o of t h e ozone c o n c e n t r a t i o n t o t h e r a t e of d e a c t i v a t i o n of t h e a c t i v a t e d o x y g e n molecules. B e c a u s e t h i s r a t e of d e a c t i v a t i o n s h o u l d be n e a r l y t e m p e r a t u r e i n d e p e n d e n t , t h e effect of e n e r g y c h a i n s s h o u l d be e q u a l l y i m p o r t a n t a t 70° a n d 90° C . f o r t h e same ozone a n d o x y g e n c o n c e n t r a t i o n s . H o w e v e r , t h e o b s e r v e d rates w e r e n o r m a l a t 70° C . a n d h i g h a t 90° C . a t s i m i l a r c o n c e n t r a t i o n s . If a direct bimolecular reaction is t o c o m p e t e w i t h R e a c t i o n 1, i t m u s t h a v e a n a c t i v a t i o n e n e r g y of less t h a n 24 k c a l . because t h e f r e q u e n c y f a c t o r of & is h i g h e r t h a n w o u l d be e x p e c t e d f o r a n o r d i n a r y b i m o l e c u l a r r e a c t i o n . O n t h e o t h e r h a n d , unless t h i s possible d i r e c t b i m o l e c u l a r r e a c t i o n t o f o r m o x y g e n has a n a c t i v a t i o n e n e r g y a p p r e c i a b l y g r e a t e r t h a n R e a c t i o n 1, i t s h o u l d c o m p e t e e q u a l l y as w e l l a t 70° C . as a t 90° C . A g a i n t h i s is c o n t r a r y t o t h e observed results.


x

T h e possible existence of t e m p e r a t u r e g r a d i e n t s i n r e a c t i n g s y s t e m s has been t r e a t e d t h e o r e t i c a l l y (3), a n d i t is p r e d i c t e d t h a t t h e i r m a g n i t u d e s h o u l d be p r o p o r t i o n a l t o t h e a b s o l u t e r a t e of t h e r e a c t i o n — i . e . , t o t h e r a t e of heat e v o l u t i o n i n t h i s case. H a r t e c k a n d D o n d e s (9) h a v e o b s e r v e d a t e m p e r a t u r e rise of a few degrees i n t h e c e n t e r of a vessel of r a p i d l y d e c o m p o s i n g ozone, a n d i t w o u l d seem l i k e l y t h a t t e m p e r a t u r e g r a d i e n t s also s h o u l d exist u n d e r some of t h e c o n d i t i o n s s t u d i e d b y G l i s s m a n n a n d S c h u m a c h e r . A s i m i l a r effect has b e e n o b s e r v e d b y A l l e n a n d R i c e (1) i n t h e d e c o m p o s i t i o n of a z o m e t h a n e n e a r t h e e x p l o s i o n l i m i t . B e c a u s e t h e m a g n i t u d e of t h i s r a t e increase o v e r t h e p r e d i c t e d r a t e is a p p a r e n t l y d e p e n d e n t o n l y o n t h e a b s o l u t e r a t e of t h e r e a c t i o n ( a l t h o u g h these increases a r e i n g e n e r a l n o t l a r g e c o m p a r e d w i t h t h e a v e r a g e e x p e r i m e n t a l e r r o r , m a k i n g t h e m difficult t o t r e a t q u a n t i t a t i v e l y ) , it w o u l d seem t h a t these a b n o r m a l i t i e s i n t h e d a t a c a n be best e x p l a i n e d i n t e r m s of t h e existence of s m a l l t e m p e r a t u r e g r a d i e n t s . T h e o b s e r v e d rates are a b o u t 2 5 % h i g h , c o r r e s p o n d i n g t o a n a v e r a g e t e m p e r a t u r e increase of 2.3° C . , w h e n t h e r a t e is 6.8 X 10 mole per liter-second. - 6

T h e v a l u e of α ( E q u a t i o n 9) is k n o w n o n l y w i t h i n ± 2 0 % . W h e n sufficient o x y g e n is p r e s e n t t o c o n t r i b u t e a p p r e c i a b l y t o M , t h e reverse r e a c t i o n , R e a c t i o n 2, is fast e n o u g h t h a t t h e effect of M o n t h e o v e r - a l l r a t e is l i m i t e d . F o r t h i s r e a s o n α c a n n o t be d e t e r m i n e d m o r e a c c u r a t e l y w i t h t h e d a t a a v a i l a b l e . T h e v a l u e s o b t a i n e d f o r k a n d , t h e r e f o r e , k are n e a r l y i n d e p e n d e n t of t h e v a l u e chosen f o r α . T h e value for k varies b y ± 1 0 % when α is v a r i e d b y ± 2 0 % . I f a c c u r a t e d a t a were a v a i l a b l e i n t h e v e r y l o w ozone r e g i o n w h e r e t h e J a h n m e c h a n i s m p r e v a i l s , k% c o u l d be o b t a i n e d i n d e p e n d e n t l y of a T h e v a l u e s of a f o u n d f o r t h e i n e r t gases are a c c u r a t e t o a b o u t ± 1 0 % , because t h e y m a y be m e a s u r e d u n d e r c o n d i t i o n s w h e r e R e a c t i o n 1 is n e a r l y r a t e - d e t e r m i n i n g . T h e u n c e r t a i n t y i n t h e a c t i v a t i o n energies of k a n d k is 0.5 k c a l . , a n d of k i t is a p p r o x i m a t e l y 0.7 k c a l . θ 2

θ 2

x

2

s

θ 2

θ 2

0 r

x

2

s

N o t r e n d is o b s e r v e d i n t h e d a t a w h i c h c a n be a t t r i b u t e d t o s u r f a c e effects e v e n t h o u g h t h e s u r f a c e - t o - v o l u m e r a t i o w a s v a r i e d b y a f a c t o r of n e a r l y 6. ( T h e vessels used b y G l i s s m a n n a n d Schumacher h a d the following volume-to-surface ratios i n c m . : I = 0.85, I I = 2.1, a n d I V = 4.74.) T h i s indicates t h a t surface reactions, if present a t a l l , m u s t c o n t r i b u t e t o less t h a n 1 0 % of t h e r a t e . H a r t e c k a n d D o n d e s (9) f o u n d t h a t i n c r e a s i n g t h e s u r f a c e b y a f a c t o r of 100 i n c r e a s e d t h e r a t e b y a f a c t o r of o n l y 4 t o 6. T h i s also seems t o i n d i c a t e t h a t s u r f a c e effects are s m a l l i n a n u n p a c k e d vessel. T h i s a t o m i c m e c h a n i s m r e q u i r e s , of course, t h a t t h e Ο a t o m c o n c e n t r a t i o n be less t h a n t h a t w h i c h w o u l d be p r e s e n t i f e q u i l i b r i u m p r e v a i l e d w i t h respect t o R e a c t i o n s 1 a n d 2. T h e r a t i o of t h i s s t e a d y - s t a t e Ο a t o m c o n c e n t r a t i o n t o t h a t c a l c u l a t e d f r o m t h e e q u i l i b r i u m of R e a c t i o n s 1 a n d 2 d e p e n d s o n t h e r e l a t i v e rates of R e a c t i o n s 2 a n d 3 a n d , therefore, on the ratio ( M ) ( 0 ) / ( 0 ) . T h e p r e d i c t e d v a l u e s f o r t h e r a t i o of Ο a t o m c o n c e n t r a t i o n s a r e g i v e n i n T a b l e I I I as a f u n c t i o n of ( M ) ( 0 ) / ( 0 ) a n d of 2

3

2


3

AXWORTHY A N D B E N S O N - G A S

Table III.


(M) (00/(00. M m . Hg 1,000 2,000 3,000 6,000 9,000 12,000 15,000 18,000 21,000 24,000 27,000 30,000 60,000 120,000 240,000 480,000

393

PHASE DECOMPOSITION

( 0 ) . . / ( 0 ) eq as a Function of Temperature and ( Μ ) ( θ 2 ) / ( θ 3 )

200 7.5» 13.9 19.5 32.6 42.1 49.2 54.8 59.2 62.9 65.9 68.5 70.7 82.8 90.5 95.0 97.5

130 23.4 37.9 47.8 64.6 73.1 78.5 82.0 84.5 86.5 87.9 89.0 90.0 94.9 97.3 98.6 99.2

* Values listed are ( 0 ) „ / ( 0 )

—

90 45.4 62.4 71.3 83.2 88.1 90.8 92.5 93.7 94.6 95.2 95.7 96.1 98.1 99.0 99.5 99.7

110 33.0 49.6 59.6 74.6 81.4 85.4 88.0 89.7 91.0 92.0 93.0 93.5 96.8 98.4 99.2 99.5 e q

70 60.0 74.9 81.7 90.0 93.0 94.6 95.8 96.5 96.9 97.3 97.5 97.8 98.9 99.4 99.8 99.9

25 88.0 93.5 95.6 97.7 98.5 98.9 99.1 99.3 99.4 99.4 99.5 99.6 99.8 99.9

0 95.6 97.9 95.5 99.3 99.5 99.6 99.7 99.8 99.8 99.8 99.8 99.9

X 100.

t e m p e r a t u r e . T h i s t a b l e shows t h a t u n d e r t h e t y p i c a l d i l u t e ozone c o n d i t i o n s o f 3 % ozone i n a n a t m o s p h e r e of o x y g e n [ ( M ) ( 0 ) / ( 0 ) = 1 1 , 1 0 0 m m . of m e r c u r y ] t h e o x y g e n a t o m c o n c e n t r a t i o n w i l l b e o n l y a b o u t 7 7 % of i t s e q u i l i b r i u m v a l u e a t 1 3 0 ° C . a n d a b o u t 8 7 % a t 1 0 0 ° C . T h e r e f o r e , e v e n u n d e r these d i l u t e c o n d i t i o n s , t h e r a t e s h o u l d n o t be e x a c t l y s e c o n d - o r d e r n o r s h o u l d i t b e e x a c t l y i n v e r s e l y p r o p o r t i o n a l t o t h e o x y g e n c o n c e n t r a t i o n . U n d e r t h e c o n d i t i o n s u s e d b y G a r v i n (6) t h e r a t e s h o u l d b e a b o u t one h a l f of t h a t p r e d i c t e d b y t h e J a h n m e c h a n i s m . 2

3

D e t a i l e d c a l c u l a t i o n s h a v e b e e n m a d e o n t h e r e s u l t s of a l l p r e v i o u s w o r k e r s t o c o m p a r e these results w i t h t h e p r o p o s e d m e c h a n i s m a n d r a t e c o n s t a n t s (2). T h e r e p r o d u c i b i l i t y w a s v e r y p o o r i n m o s t of these e a r l i e r i n v e s t i g a t i o n s b u t i n n e a r l y e v e r y case t h e l o w e s t rates o b s e r v e d a p p r o a c h e d closely t o those p r e d i c t e d b y t h i s s i m p l e a t o m i c m e c h a n i s m , e v e n i n vessels of v e r y l a r g e s u r f a c e - t o - v o l u m e r a t i o s . T h e s e c a l c u l a t i o n s seem t o i n d i c a t e t h a t a t r a c e c a t a l y s t w a s p r e s e n t i n m o s t of t h e s t u d i e s a n d when this was eliminated, the simple atomic mechanism prevailed. A p p a r e n t l y G l i s s m a n n a n d Schumacher were t h e o n l y early investigators w h o were able to eliminate t h i s t r a c e c a t a l y s t c o m p l e t e l y . N o r e l i a b l e evidence w a s ever p r e s e n t e d t o i n d i c a t e t h a t t h e surface p l a y e d a n i m p o r t a n t role e v e n i n those cases w h e r e c a t a l y s i s w a s o c c u r r i n g — e . g . , p a c k i n g t h e vessel w i t h glass w o o l (9) c o u l d e a s i l y i n t r o d u c e a t r a c e catalyst w h i c h was a c t u a l l y homogeneous i n its action. T h e r e s u l t s of S u t p h e n (18) s u p p o r t t h e c o m p l e x m e c h a n i s m p r o p o s e d b y G l i s s m a n n a n d S c h u m a c h e r a n d i n d i c a t e t h a t t h e u n i m o l e c u l a r d e c o m p o s i t i o n of o z o n e i s n o t a t i t s l o w p r e s s u r e l i m i t . R e s u l t s of a l l 6 7 of these e x p e r i m e n t s h a v e b e e n c o m p a r e d w i t h t h e i n t e g r a t e d f o r m of E q u a t i o n 8 ; t y p i c a l d a t a a r e t a b u l a t e d i n T a b l e I V , w h e r e £ is c o m p a r e d w i t h £ from this integrated equation. E x c e p t for the experiments i n w h i c h 6 a t m . of oxygen were present, Sutphen's results fit t h e proposed m e c h a n i s m w i t h a n a v e r a g e e r r o r of 1 0 t o 1 5 % w i t h o n l y a f e w e x c e p t i o n s . Experiments 228, 2 3 3 , a n d 2 3 4 , i n w h i c h e n e r g y c h a i n s a r e p r o p o s e d t o b e i m p o r t a n t , fit t h e s i m p l e a t o m i c mechanism well. ( T h e p e r c e n t a g e d e c o m p o s i t i o n w a s so g r e a t i n E x p e r i m e n t 2 2 8 t h a t a n y e r r o r i n t h e f i n a l ozone p r e s s u r e w o u l d b e g r e a t l y m a g n i f i e d i n i c a i c d . ) E x p e r i m e n t s 2 6 2 a n d 2 7 5 , w h i c h w e r e c i t e d as e v i d e n c e a g a i n s t t h e s i m p l e a t o m i c m e c h a n i s m , a r e a c t u a l l y i n a g r e e m e n t w i t h i t . T h e rates o b s e r v e d i n t h e presence o f 6 a t m . of oxygen were m u c h faster t h a n t h e predicted rates. H o w e v e r , this cannot be due t o a h o m o g e n e o u s b i m o l e c u l a r r e a c t i o n as p r o p o s e d b y S u t p h e n , because t h e authors have observed a l o w t e m p e r a t u r e rate slower b y a factor of 5 t h a n t h e rate p r e d i c t e d b y S u t p h e n ' s b i m o l e c u l a r r a t e c o n s t a n t . T h e m o s t o b v i o u s e x p l a n a t i o n of these h i g h r a t e s is t h e i n t r o d u c t i o n o f a t r a c e c a t a l y s t w i t h t h e o x y g e n o r t h e f o r m a t i o n of s u c h a c a t a l y s t d u r i n g t h e d i s c h a r g e w h i c h t o o k p l a c e i n t h e r e a c t i o n vessel. T h e first of these m i g h t also e x p l a i n t h e h i g h rates i n e x p e r i m e n t s 2 0 5 a n d 2 1 5 . T h e f a s t rates a t v e r y h i g h o x y g e n pressures a p p e a r e d t o b e e x a c t l y s e c o n d - o r d e r . o b s d

c a l c d



394


T a b l e IV.

Comparison of Sutphen's (78) Results with Proposed Mechanism

99.5 99.5 115.5 115.5 99.5 115.5 99.5 115.5 99.5 99.5 99.5 99.5 99.5 115.5 115.5 115.5 115.5 99.5 99.5 115.5 85.5

P°o Mm. Hg 62.4 63.1 62.0 63.0 15.11 31.12 62.37 28.13 20.47 10.09 28.49 88.26 86.07 9.20 29.00 87.86 85.79 28.60 28.76 29.20 62.81

P'o,, M m . Hg 42.1 28.4 11.3 9.9 6.75 7.48 32.59 8.51 15.22 5.06 20.43 46.94 43.88 6.16 22.47 47.86 34.19 16.12 7.22 3.02 44.02

P°o , Mm. Hg 191.8 194.1 190.7 192.6 209.0 228.7 387.9 390.4 572.1 19.07

115.5 115.5 99.5 99.5 85.5 70.0 70.0 25.0 25.0

15.12 63.61 62.53 14.69 61.08 89.08 61.36 88.45 124.9

7.74 24.24 48.28 10.00 47.81 53.33 39.25 76.71 108.4

4612 4483 4079 4322 4680 4564 4564 4540 4598

Temp.,

Expt. 178 183 186 189 210 202 206 213 205A 269 232 275 273 258 226 262 276 233 234 228 215

°C.

212 197 198 208 216 287 289 280 290

v

2

Pco , Atm.

Sec. 900 900 900 900 3000 960 900 1200 1560 9000 2400 1860 1800 1500 600 600 720 600 600 600 900

6.16 1.00 6.13

é.'oo

6.00

755.4 18.68 743.1

199.3

0.262 3.88 4.05

loalod

toalod,

lobed,

2

4800 1800 1080 7200 2700 0.15» 0.18» 1.73» 2.48»

Sec. 956 882 890 780 3423 918 1154 1184 2071 9807 2948 1942 2006 1530 693 486 630 632 557 377 1450

1.06 0.98 0.99 0.87 1.14 0.96 1.29 0.99 1.33 1.09 1.23 1.04 1.12 1.02 1.16 0.81 0.87 1.05 0.93 0.63 1.61

10180 4054 3730 26437 20495 2.33» 2.31» 456» 324»

2.12 2.25 3.46 3.66 7.58 15.7 15.5 264 262

tobad

» X 10».

Reproducibility.

RESULTS.

T O test t h e r e p r o d u c i b i l i t y of r e s u l t s , a series of 17

experiments was carried out o n ozone-oxygen mixtures a t temperatures between 80° a n d 100° C .

These a l l gave rates 0 t o 1 5 % above t h e p r e d i c t e d values.

e x p e r i m e n t a l p r o c e d u r e w a s c o n t i n u a l l y i m p r o v e d , these d e v i a t i o n s

Although the

(which

amounted

t o less t h a n 1 0 % b e t w e e n m o s t of t h e r u n s ) c o u l d n e i t h e r b e e l i m i n a t e d n o r e x p l a i n e d . D u r i n g a n y g i v e n e x p e r i m e n t t h e r a t i o of t h e a c t u a l r a t e t o t h e p r e d i c t e d r a t e r e mained approximately constant. of o x y g e n

w h i c h were used.

N o difference w a s o b s e r v e d b e t w e e n t h e t w o sources

H i g h i n i t i a l rates were

observed

i n those

cases

where

t e m p e r a t u r e gradients w o u l d have been predicted f r o m i n t e r p r e t a t i o n of t h e results of G l i s s m a n n a n d S c h u m a c h e r .

I n i t i a l s t i r r i n g of t h e o z o n e - o x y g e n m i x t u r e s h a d n o

effect o n t h e r a t e o r t h e o r d e r of r e p r o d u c i b i l i t y . F o u r e x p e r i m e n t s w e r e c a r r i e d o u t i n w h i c h 8 t o 2 0 m m . of w a t e r v a p o r w a s p r e s e n t i n t h e o z o n e - o x y g e n m i x t u r e s a t 9 0 ° a n d 100° C . T h e s e g a v e r a t e s f r o m 12 t o 1 9 % above the predicted rates, a n d there was little correlation between t h e a m o u n t of w a t e r p r e s e n t a n d t h e m a g n i t u d e of t h e r a t e i n c r e a s e . T h e s e r a t e s a r e c o n s i d e r e d t o b e s i g n i f i c a n t l y f a s t e r t h a n those o b t a i n e d w i t h t h e p u r e o z o n e - o x y g e n m i x t u r e s . A n a t t e m p t w a s m a d e t o i n t r o d u c e 0.7 m m . o f h y d r o g e n p e r o x i d e v a p o r i n t o a vessel c o n t a i n i n g a p p r o x i m a t e l y 175 m m . o f ozone a t 9 0 ° C . T h i s r e s u l t e d i n a n i m m e d i a t e explosion. H y d r o g e n p e r o x i d e (0.2 m m . ) w a s s u c c e s s f u l l y i n t r o d u c e d i n a s i m i l a r e x p e r i m e n t i n w h i c h o n l y a b o u t 100 m m . of o z o n e w a s p r e s e n t . T h i s g a v e a n a v e r a g e r a t e o n l y 1 1 % f a s t e r t h a n t h e r a t e p r e d i c t e d i n t h e absence of t h e p e r o x i d e . T h e system was t h e n completely rebuilt a n d a l l stopcocks were replaced w i t h p a c k l e s s m e t a l v a l v e s i n a n effort t o e l i m i n a t e a n y source of c a t a l y s i s w h i c h m i g h t b e caused b y the stopcock l u b r i c a n t . H o w e v e r , there was n o evidence t h a t H a l o c a r b o n grease is n o t e n t i r e l y i n e r t t o ozone. O z o n e ( 1 9 4 m m . ) a n d o x y g e n ( 2 8 4 m m . ) w e r e sealed off i n t h e o p a q u e r e a c t i o n vessel w h i c h w a s t h e r m o s t a t e d a t 29.8° C . A f t e r 2 w e e k s , 111 m m . of o z o n e r e m a i n e d i n t h e v e s s e l ; t h i s w a s h e a t e d t o 7 0 ° C . so t h a t d a t a a t t w o d i f f e r e n t t e m p e r a t u r e s c o u l d b e o b t a i n e d f r o m t h e s a m e s a m p l e . A t 29.8° C . a r a p i d loss of o z o n e w a s o b s e r v e d d u r i n g t h e first 5 t o 10 h o u r s , a n d t h e n t h e r a t e became almost exactly second-order i n ozone over t h e entire 2-week p e r i o d . T h e s e c o n d - o r d e r r a t e c o n s t a n t w a s 5.81 X 1 0 — l i t e r p e r m o l e - s e c o n d w h i c h m a y b e c o m p a r e d w i t h t h e v a l u e of 28.9 X 1 0 ~ w h i c h w o u l d b e p r e d i c t e d b y S u t p h e n ' s (18) p r o p o s e d b i m o l e c u l a r r a t e c o n s t a n t a n d t h e v a l u e of 1.7 X 10 w h i c h w o u l d be p r e d i c t e d 5

5

5



AXWORTHY A N D B E N S O N - G A S

PHASE

DECOMPOSITION

395

b y t h e proposed atomic m e c h a n i s m . T h e rate a t 70° C . w i t h this same sample was n e a r l y 5 0 % f a s t e r t h a n t h e p r e d i c t e d r a t e suggesting t h a t some i m p u r i t y h a d b e e n included. A control r u n was made at 90° C . on a similarly prepared sample, a n d this g a v e rates 7 0 t o 3 0 0 t i m e s t h e r a t e p r e d i c t e d b y o u r a t o m i c m e c h a n i s m . I t w a s c o n c l u d e d t h a t m e t a l oxides h a d b e e n i n t r o d u c e d i n t o t h e r e a c t i o n v e s s e l . T h e r e f o r e , t h i s system was abandoned. A series of e x p e r i m e n t s was i n i t i a t e d i n w h i c h t h e ozone w a s f o r m e d i n t h e r e a c t i o n vessel b y a m e t h o d s i m i l a r t o t h a t d e v e l o p e d b y S u t p h e n (18). P u r i f i e d t a n k o x y g e n (400 m m . ) w a s sealed off i n t h e r e a c t i o n vessel a n d t h i s w a s p a r t i a l l y c o n d e n s e d b y c o o l i n g t h e l o w e r p o r t i o n of t h e vessel i n l i q u i d n i t r o g e n . A T e s l a c o i l d i s c h a r g e w a s p a s s e d t h r o u g h t h e vessel a b o v e t h e s u r f a c e of t h e l i q u i d a n d n e a r l y a l l o f t h e o x y g e n was c o n v e r t e d t o ozone i n t h i s m a n n e r . A f t e r t h e ozone h a d e v a p o r a t e d a t r o o m t e m p e r a t u r e , t h e vessel w a s h e a t e d t o 7 0 ° C . a n d t h e r a t e w a s f o l l o w e d as u s u a l . T h e i n i t i a l r u n g a v e rates f r o m 2 t o 4 t i m e s f a s t e r t h a n t h e p r e d i c t e d r a t e s , a n d a s e c o n d r u n w i t h t h e same s a m p l e of o x y g e n g a v e r a t e s w h i c h w e r e e v e n f a s t e r b y a n o t h e r f a c t o r of 1.5 t o 2. B e c a u s e i t a p p e a r e d t h a t a c a t a l y s t w a s b e i n g f o r m e d i n t h e d i s c h a r g e , t h i s p r o c e d u r e w a s also a b a n d o n e d . D I S C U S S I O N . T h e cause of t h e 1 0 % d e v i a t i o n s a m o n g t h e v a r i o u s e x p e r i m e n t s u n d e r s i m i l a r c o n d i t i o n s w a s n o t d i s c o v e r e d , b u t a possible e x p l a n a t i o n m i g h t b e t h e i n c l u s i o n of a t r a c e c a t a l y s t i n v a r y i n g a m o u n t s . T h e r e s u l t s of m o s t of t h e o t h e r w o r k e r s w e r e n o t so r e p r o d u c i b l e , a l t h o u g h o n l y i n a v e r y f e w cases w e r e e x p e r i m e n t s r e r u n u n d e r n e a r l y i d e n t i c a l c o n d i t i o n s . O n l y t h e r e s u l t s of G l i s s m a n n a n d S c h u m a c h e r a p p e a r t o b e free of a n y s u c h u n c e r t a i n t y i n t h e r e p r o d u c i b i l i t y ( w h e n c o m p a r e d i n terms of proposed m e c h a n i s m ) , b u t even i n their thorough investigation control e x p e r i m e n t s w e r e n o t c a r r i e d o u t . R e p r o d u c i b i l i t y r e s u l t s w e r e close e n o u g h t o t h o s e p r e dicted b y t h e proposed atomic m e c h a n i s m over a wide enough range of conditions t o l e n d c o n s i d e r a b l e s u p p o r t t o t h e c l a i m t h a t these a r e t h e o n l y s i g n i f i c a n t r e a c t i o n s w h i c h occur i n this system. T h e r e s u l t s o b t a i n e d w i t h a d d e d w a t e r v a p o r set a n u p p e r l i m i t o n t h e effect o f w a t e r , b u t i t w o u l d b e d i f f i c u l t t o p r o v e t h a t t h e o b s e r v e d increase i n t h e r a t e w a s n o t due e n t i r e l y t o a n i m p u r i t y i n t r o d u c e d w i t h t h e w a t e r v a p o r . T h e r e s u l t s w i t h a d d e d h y d r o g e n p e r o x i d e were s o m e w h a t i n c o n c l u s i v e , because i t d e c o m p o s e s r a p i d l y o n t h e s u r f a c e of a b o r o s i l i c a t e glass vessel a t 9 0 ° C . D u r i n g t h e f e w m i n u t e s w h i c h e l a p s e d b e t w e e n t h e i n t r o d u c t i o n o f t h e p e r o x i d e a n d t h e t a k i n g of t h e first p r e s s u r e r e a d i n g , m o s t of t h e p e r o x i d e m a y h a v e d e c o m p o s e d . T h u s t h e l a c k of a n y l a r g e effect of h y d r o g e n p e r o x i d e ( o t h e r t h a n t h e e x p l o s i o n w h i c h i t a p p a r e n t l y caused) does n o t p r o v e t h a t i t m a y n o t h a v e been a n i m p o r t a n t t r a c e c a t a l y s t i n some o f t h e e a r l i e r w o r k or even i n t h e present investigation. T h e rates o b t a i n e d a t 29.8° C . w e r e s l o w e r b y a f a c t o r of 5 t h a n t h e r a t e s w h i c h w o u l d b e e x p e c t e d f r o m S u t p h e n ' s (18) b i m o l e c u l a r m e c h a n i s m , e v e n t h o u g h t h e r e w a s some t y p e of c a t a l y s i s t a k i n g p l a c e as s h o w n b y t h e a n o m a l o u s r e s u l t s o b t a i n e d a t 70° C . w i t h t h e s a m e s a m p l e . T h e r e f o r e , t h e l o w t e m p e r a t u r e rates o b t a i n e d b y S u t p h e n c a n n o t b e e x p l a i n e d b y a h o m o g e n e o u s b i m o l e c u l a r r e a c t i o n . T h e s e d a t a set a n u p p e r l i m i t o n t h e r a t e of a p o s s i b l e d i r e c t b i m o l e c u l a r r e a c t i o n , b u t d o n o t r u l e o u t t h e s i m p l e a t o m i c m e c h a n i s m as t h e o n l y m e c h a n i s m e v e n a t l o w t e m p e r a t u r e s , b e cause some c a t a l y s i s w a s o b v i o u s l y o c c u r r i n g . T h i s d u a l t e m p e r a t u r e t e c h n i q u e w o u l d appear to be a n interesting m e t h o d of s t u d y i n g t h e role of accidental catalysis i n this s y s t e m . T h e r e s u l t s of t h i s l o w t e m p e r a t u r e r u n p o i n t o u t t h e h i g h s t a b i l i t y o f ozone to t h e r m a l decomposition at r o o m t e m p e r a t u r e . E v e n i n this system where i m p u r i t i e s are s u s p e c t e d , e x t r a p o l a t i o n o f t h e o b s e r v e d r e s u l t s w o u l d i n d i c a t e t h a t a s a m p l e c o n t a i n i n g 5 % ozone i n a n a t m o s p h e r e of o x y g e n c o u l d b e s t o r e d a t r o o m t e m p e r a t u r e f o r n e a r l y 2 m o n t h s b e f o r e t h e ozone c o n c e n t r a t i o n w o u l d f a l l b e l o w 4 % . I n a s y s t e m where o n l y t h e homogeneous atomic m e c h a n i s m is occurring (if this c a n be a t t a i n e d ) , t h i s storage t i m e c o u l d b e o v e r 6 m o n t h s . T h e t w o e x p e r i m e n t s i n w h i c h t h e ozone w a s f o r m e d i n t h e r e a c t i o n vessel p o i n t o u t some of t h e p r o b l e m s i n v o l v e d i n s u c h a t e c h n i q u e . T h e o x y g e n source a n d e x p e r i -



396

mental techniques were identical to most of the previous experiments, i n which the highest rate ever obtained was only 1.25 times the predicted rate. Therefore, some catalyst must have been formed i n the discharge. M o s t of Sutphen's results are i n accord with the proposed mechanism, even though he used this technique i n all his experiments. However, i n his system the ozone preparation took less than 10 minutes, while in the experiments reported here it required 2 to 3 hours.


Photochemical

Decomposition

The photochemical decomposition of ozone has long been cited as an example of one of the few systems in which it is necessary to propose an energy chain mechanism. This came about from the observation that at very short wave lengths quantum yields as high as 6 were observed (10), whereas the primary photochemical process 0

3

+ Κμ

> 0

2

+ Ο

followed b y Reactions 2 and 3 would give a maximum quantum yield of 2 . Therefore, Reaction 6 is usually included to account for these high quantum yields. This previously proposed photochemical mechanism is not consistent with the present interpretation of the thermal decomposition, because there is no evidence that Reaction 6 occurs even at very high ozone concentrations. T h e results obtained with visible light (16) also tend to rule out any such energy chain, because they m a y be explained very well in terms of a simple mechanism involving Reactions 12, 2, and 3 even under conditions where the high quantum yields were observed with ultraviolet light. T h e value of k /k obtained (2) from this simple mechanism was 0.0104 mole per liter and may be compared with the value of 0.0053 calculated at 17° C . from the proposed rate constants. This difference should not be considered too seriously until further data are available, because this original work was rather crude and there was some uncertainty i n the earlier work as to whether the quantum yield was based on ozone or oxygen. A t any rate, the observed quantum yields are definitely a function of wave length, and this would not be the case if an energy chain involving Reactions 3 and 6 were involved. s

2

T o arrive at a photochemical mechanism which is consistent with all of the observed photochemical as well as thermal results, the various possible energy levels or excited states of the primary photochemical products should be considered i n detail. Such a mechanism has been proposed ( 5 ) , which can account for the high quantum yields observed i n the far ultraviolet. However, this is somewhat speculative, because the experimental difficulties encountered in the photochemical studies appear to be even more insurmountable than those encountered in the thermal decomposition. More quantitative data are needed on the photochemical system before its mechanism can be definitely established. Literature

Cited

(1) Allen, A. O., Rice, Ο. K., J. Am. Chem. Soc. 5 7 , 310 (1935). (2) Axworthy, A. E., Jr., Ph.D. thesis, University of Southern California, Los Angeles, Calif., 1958. (3) Benson, S. W., J. Chem. Phys. 2 2 , 46 (1954). (4) Benson, S. W., Axworthy, A. E., Jr., Ibid., 2 1 , 428 (1953). (5) Ibid., 2 6 , 1718 (1957). (6) Garvin, D., J. Am. Chem. Soc. 7 6 , 1523 (1954). (7) Geib, Κ . Η., Z. Elektrochem. 4 7 , 761 (1941). (8) Glissmann, Α., Schumacher, H. J., Z. physik. Chem. Abt. B, 21, 323 (1933). (9) Harteck, P., Dondes, P., J. Chem. Phys. 2 1 , 2240-1 (1953). (10) Heidt, L. J., J. Am. Chem. Soc. 5 7 , 1710 (1935). (11) Natl. Bur. Standards (U. S.), Circ. 500 (1952). (12) Riesenfeld, E. H., Bohnholtzer, W., Z. physik. Chem. 1 3 0 , 241 (1927). (13) Riesenfeld, Ε. H., Schumacher, H. J., Ibid., 1 3 8 , 268 (1928). (14) Riesenfeld, E. H., Wassmuth, E., Ibid., A b t . A, 143, 397 (1929).


AXWORTHY

AND

B E N S O N - G A S PHASE DECOMPOSITION

397

Schumacher, H. J., "Chemische Gasreaktion," p. 433, T. Steinkopff, Dresden, 1938. Schumacher, H. J., J. Am. Chem. Soc. 5 2 , 2377 (1930). Schumacher, H. J., Sprenger, G., Z. physik. Chem. Abt. B, 6, 446 (1930). Sutphen, W. T., Publ. 13,288, University Microfilms, A n n Arbor, M i c h . ; Dissertation Abstr., November 1957. (19) Wulf, O. R., Tolman, R. C., J. Am. Chem. Soc. 4 9 , 1183, 1202 (1927). (15) (16) (17) (18)


RECEIVED June 19, 1957. Accepted June 19, 1957. Work supported by the Office of Ordnance Research under Contract N o . DA-04-495-Ord-345 with the University of Southern California.


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