11 Removal of Trace Contaminants from the Air Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 04/08/19. For personal use only.
The Reaction Between Ozone and Hydrogen Sulfide: Kinetics and Effect of Added Gases SOTIRIOS GLAVAS and SIDNEY TOBY School of Chemistry, Rutgers University, New Brunswick, N. J. 08903
Abstract. The r e a c t i o n between O and H S was s t u d i e d from 20 t o 70°C over a pressure range of 0.005 - 0.1 torr O and 0.2 - 5 torr H S and the r a t e constant i s g i v e n by l o g (k/M / sec ) = 5.0 ± 0.5 - (5000 ±700)/2.30 RT. Up t o 1.7 t o r r o f added O was found t o have no e f f e c t on the r e a c t i o n . However, 95 torr of added CO reduced the observed r a t e constant slightly and 68 torr of C F reduced the observed r a t e constant by a f a c t o r of 2. The r e s u l t s are e x p l a i n e d in terms of a f r e e radical mechanism which accounts f o r the observed product ratios found by us (3) and f o r the observed r a t e l a w . The e f f e c t of added gases on the r e a c t i o n i s a l s o accounted f o r by the proposed mechanism. We use the data t o e x t r a p o l a t e an approximate v a l u e f o r the initial bimolecular r e a c t i o n between O and H S. I m p l i c a t i o n s f o r air pollution are considered. 3
2
3
2
-1 2
-1
2
2
2
3
6
2
Introduction. Because of their importance as pollutants in the lower atmosphere, many reactants of ozone and of hydrogen sulfide are of great interest. The kinetics of their reaction with each other has been l i t t l e studied and there has been no attempt to explain the observed rate law in terms of a mechanism.. Cadle and Ledford (l_) reported a rate law of -d[0 ]/dt = k [ O 3 ] ' , !.·£.· zero order in H S. Hales, Wilkes and York (2) followed the rate by measuring the rate of production of S0 and found d[S0 ]/dt = k [ 0 ] / [ H ^ ] ' . Both studies used flow systems with ozonized air or oxygen as a reactant and the stoichiometry was assumed to be 0 + H S -> S0 + H 0. A concerted molecular rearrangement is extremely unlikely and would not give rise to the observed rate laws. It is not clear whether the two reported rate laws are compatible with each other, and in any case no explanation has been given for the laws. 3
2
3
2
3
2
1
2
2
3
2
3
2
2
2
122
11.
GLAVAS
Ozone-Hydrogen
AND TOBY
Sulfide
Reaction
123
We h a v e r e c e n t l y (3) 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 O 3 a n d H 2 S u s i n g p u r e O 3 a s a r e a c t a n t . We f o u n d t h e m o s t a b u n d a n t p r o d u c t was o x y g e n a n d t h a t t h e ( O 2 f o r m e d ) / ( O 3 u s e d ) r a t i o a p p r o a c h e d 1.5 a s [ θ 3 ] Q i n c r e a s e d . In addition H 2 O / S O 2 was f o u n d t o v a r y c o n s i d e r a b l y f r o m u n i t y . The r a t e o f O 3 d i s a p p e a r a n c e was f o u n d t o obey a t h r e e - h a l v e s o r d e r r a t e l a w a n d t w o mechanisms were s u g g e s t e d w h i c h a c c o u n t e d f o r t h e o b s e r v e d k i n e t i c s and t h e observed product r a t i o s . I n t h i s p a p e r t h e mechanism w i l l be c o n s i d e r e d i n more d e t a i l a n d t h e e f f e c t s o f a d d e d i n e r t g a s e s w i l l be d i s c u s s e d . P r o d u c t measurements a n d mass b a l a n c e s h a v e b e e n g i v e n e l s e w h e r e (3) a n d w i l l n o t b e r e p o r t e d h e r e . Experimental
Section.
A c o n v e n t i o n a l h i g h vacuum s y s t e m was u s e d w i t h t r a p s c o o l e d w i t h d r y i c e t o exclude mercury vapor from the quartz r e a c t i o n vessel. The c y l i n d r i c a l r e a c t i o n v e s s e l w h i c h was o f l e n g t h 21.9 cm a n d v o l u m e k60 c m , was c o n n e c t e d t o t h e r e a c t a n t i n l e t a n d r e s t o f t h e s y s t e m b y T e f l o n h i g h vacuum s t o p c o c k s . An i n l e t t u b e c o n s i s t i n g o f a " c o l d f i n g e r " o f a p p r o x i m a t e l y 5 cm volume was a t t a c h e d v i a a s t o p c o c k t o t h e r e a c t i o n c e l l . The d e s i r e d a mount o f H 2 S was f r o z e n i n t o t h i s i n l e t t u b e a n d a l l o w e d t o warm before r e a c t i n g . The r e a c t i o n v e s s e l was m o u n t e d h o r i z o n t a l l y i n a t h e r m o d a t t e d o v e n ( - . 8 ° ) w i t h q u a r t z windows a t e a c h end. The ozone c o n c e n t r a t i o n was m o n i t o r e d b y a b s o r p t i o n o f 2^k nm r a d i a t i o n o b t a i n e d f r o m a n Osram l o w - p r e s s u r e m e r c u r y lamp a n d t w o i n t e r f e r ence f i l t e r s . The d e c a d i c a b s o r p t i o n c o e f f i c i e n t o f o z o n e was t a k e n {h) a s 3010 M- cm- a n d c h e c k e d b y m e a s u r i n g t h e a b s o r b ance o f O 3 w h i c h was t h e n decomposed t o 0 a n d t h e p r e s s u r e meas u r e d on a McLeod gauge. B e e r p l o t s w e r e l i n e a r u p t o a b o u t 0 8 5 t o r r o f O 3 . O 3 p r e s s u r e s w e r e k e p t b e l o w 0.1 t o r r b e c a u s e t h e r e a c t i o n r a t e o t h e r w i s e became t o o f a s t t o m e a s u r e w i t h o u r s y s tem. The t r a n s m i t t e d l i g h t beam was m e a s u r e d w i t h a 1P28 p h o t o m u l t i p l i e r c o n n e c t e d t o a K e i t h l e y 6l0A e l e c t r o m e t e r a n d a r e corder. Ozone was g e n e r a t e d b y p a s s i n g o x y g e n ( M a t h e s o n U l t r a p u r e grade) through a discharge from a T e s l a c o i l . The 0 was c o n d e n s e d a t - I 9 6 a n d t h e 0 pumped away. H y d r o g e n s u l f i d e , h e x a f l u o r o e t h a n e a n d c a r b o n d i o x i d e were e a c h d i s t i l l e d b e f o r e u s e , r e j e c t i n g h e a d a n d t a i l f r a c t i o n s . The i n i t i a l p r e s s u r e s o f r e a c t a n t s r a n g e d f r o m 0 . 0 0 5 - 0 . 1 t o r r f o r 0 a n d p.2-5 t o r r f o r H S. 3
3
1
1
2
#
3
0
2
3
2
Results. The r e a c t i o n was r a p i d w i t h a t y p i c a l h a l f - l i f e o f ~2 s e e s , under t h e c o n d i t i o n s t u d i e d . Assuming a r a t e l a w o f t h e f o r m - d [ 0 ] / d t = k [ 0 ] [ H S ] we s t u d i e d t h e k i n e t i c s i n t h e m
3
3
n
2
124
R E M O V A L OF
TRACE CONTAMINANTS F R O M THE
AIR
p r e s e n c e o f e x c e s s H S f o r a p p r o x i m a t e l y 95% o f t h e r e a c t i o n . L o g r a t e vs_ l o g [ O s l o p l o t s g a v e s l o p e s o f b e t w e e n 1.5 and 2 . We t h e r e f o r e u s e d t h e i n t e g r a t e d e q u a t i o n s and made p l o t s c o r r e s p o n d i n g t o o r d e r s o f 1, 1.5, 2 and 2.5. The f i r s t and 2.5 o r d e r p l o t s were d i s t i n c t l y c u r v e d b u t t h e 1.5 a n d s e c o n d o r d e r p l o t s were b o t h s t r a i g h t w i t h s t a t i s t i c a l c o r r e l a t i o n f a c t o r s o f more t h a n 0.99 i n most c a s e s . However, t h e o v e r a l l l i n e a r i t y o f t h e 1.5 o r d e r p l o t s was s l i g h t l y b e t t e r t h a n t h a t o f t h e s e c o n d o r d e r plots. The 1.5 o r d e r r a t e c o n s t a n t s w e r e m e a s u r e d o v e r a 2 0 - f o l d r a n g e o f [ O ] a t 2 0 , 50 a n d T0°C and t h e r e s u l t s were p l o t t e d i n F i g u r e 1. The r a t e c o n s t a n t s show r e a s o n a b l y good i n d e p e n d e n c e of [ 0 ] . The e f f e c t o f i n i t i a l [ H S ] on t h e 1.5 o r d e r r a t e c o n s t a n t a t 20° i s shown i n F i g u r e 2 and a l t h o u g h t h e r e i s s c a t t e r , t h e r e i s no d i s t i n c t t r e n d . On t h e o t h e r h a n d , p l o t s o f r a t e c o n s t a n t s o f o r d e r 0.5 o r h i g h e r i n H S a g a i n s t [ H S ] showed a c l e a r d e p e n d e n c e . We c o n c l u d e t h a t t h e r a t e l a w i s z e r o o r d e r i n H^S and i s g i v e n b y 2
3
3
0
Q
2
0
2
-d [ 0 ] / d t 3
= k [Ο3] · 1
2
(1)
5
R a t e c o n s t a n t s m e a s u r e d a t 2 0 , 50 and 70° w e r e p l o t t e d a s a n A r r h e n i u s p l o t w h i c h g a v e l o g ( k / M - / s e c - ) = 5.0 ± 0.5 -(5000 Î 700)/2.3RT. The e f f e c t o f a d d e d 0 2 , C 0 2 and C2F6 was s t u d i e d a t 25° w i t h i n i t i a l o z o n e p r e s s u r e k e p t a t c a 0,03 t o r r and i n i t i a l H S p r e s s u r e k e p t a t a p p r o x i m a t e l y 0.35 t o r r . These: r e s u l t s a r e g i v e n i n Table I . 1
2
1
2
TABLE I .
EFFECT OF ADDED GASES ON 1.5
I n i t i a l O3, μ moles
Initial H S, μ moles
ORDER RATE CONSTANT
2
Torr
k a t 25° M-l/2 s"
0.94 0.65 0.60 0.51 0.74 0.68 0.60 0.75 0.66 0.64 0.82 0.79 0.65
7.66 8.64 8.68 8.70 8.62 7.96 8.60 8.58 8.60 8.53 8.67 8.52 8.10
0.12 0.14 0.65 1.14 2.00 2.04 10.6 28.1 41.5 49.8 69.0 87.0 95.4
40.4 59.0 59.9 45.2 45.5 62.5 63.3 55.9 52.0 49.9 43 . 2 43.0 41.6
0.65 0.68 0.51 0.74
9.00 7.86 8.15 8.69
!02 0.35 0.90 1.30 1.68
46.6 41.4 41.4 51.3
11.
GLAVAS
Ozone-Hydrogen
AND TOBY
Sulfide
Reaction
125
TABLE I ( c o n t ' d . ! Initial
Initial H S,
\
μ moles
Torr
8.70 8.65 8.70 8.63 8.63 8.65 Q.6k
5.TO 11.38 15.80 21.70 31.0
2
μ moles 0.89 0.82 0.66 0.66 0.76 0.70 0.66
F
6 M~l/
s e c -1
2
hh.h
55.3 31.6 30.9 30.8 2k.Q 23.5
h9.9
68.6
DISCUSSION I n o r d e r t o e x p l a i n t h e v a r i o u s p r o d u c t r a t i o s f o u n d (3_)_ a n d a c c o u n t f o r t h e o b s e r v e d r a t e l a w , we p o s t u l a t e t w o p o s s i b l e mechanisms d i f f e r i n g i n t h e i n i t i a t i n g s t e p s . Mechanism A H S
+
HSO
+
HO
+
202
H 0 2
+
HS
1* ->
HSO
+
H S
OS
i
EO2
+
03
2
HO
+
H S
3 ->
HS
+
°3
2
2
2HS
+
M
5
HSO
+
°3
6 6a -> 7 ->
+
03
SO
+
°3
2
+
°2 M
HS
+
20
HO
+
SO
so
+
2
HSO
H0
2
2
2
°2
T a k i n g s t e a d y s t a t e s i n [ H O ] , [ H 0 ] , [ H S ] , [SO] a n d [HSO] a n d putting M = H S leads t o £
2
-a[o ]/at = (3+Κ)^[Η 8][Ο ] + ( 2 + κ ) ^ / 3
1
2
2
-d[H S]/dt = (2+K)k [H S][0 ] +
κ k^/
•d[H 0]/dt = (1+Κ)^[Η 3][0 ] +
Κ kj /
2
1
3
2
2
2
d[S0 ]/dt= 2
2
6
a
).
1
2
1
[Sp2]
Κ + kf / w
2
1
5
1
/
5
2
3
3
2
2
3
3
1
/
2
2
2
3
ki k ~' ^ p ] / t
Κ [0 ]!/ 3
2
Κ [o ]^ 3
2
5
3
2
[Η^Γ [H S] 2
1
- 1
2
(3) (It)
3
L
(2)
2
3
[ 0 ] /
a n d (5) g i v e
1 + Κ + k f / V^- / 1
^ "
K kj /
3
E q s . (h)
[H 0] 2
2
l t
2
k - / [0 ] /
k
1
Κ ^[Η ε][0 ] +
w h e r e Κ = k6a/(kg + k
l t
2
1
3
k- / [o ] / 1
k
3
2
(5)
126
R E M O V A L OF
Eqs.
(3)
a n d (5)
TRACE CONTAMINANTS F R O M T H E
give
[H S used]
2 + Κ + k^ /
[H 0 formed]
1 + Κ + ki" /
1
2
1
2
AIR
2
1
k^" / 2
1
2
ki+ks" /
2
Κ [O3] /
2
Κ [O3] /
2
1
1
[H S]^"
1
2
l
[R S]~ 2
These p r o d u c t r a t i o s h a v e b e e n f o u n d t o be i n q u a l i t a t i v e agreement w i t h experiment (3,). S t e p s 1 a n d 2 i n t h e mechanism c a n be r e p l a c e d by a s e q u e n c e i n v o l v i n g HSO2 r a t h e r t h a n HO2. I n a d d i t o n , HSO2 may a l s o be f o r m e d b y t h e r e a c t i o n b e t w e e n HSO a n d O3. We p o s t u l a t e , a s a n alternative possibility: Mechanism Β HS HS0 HSO 2
2
0 0 0
+ + +
l a
3 8 3
6 b
3
> HS0 > HSO ^ HS0
2
2
+ + +
HO 20 0
2
2
t o g e t h e r w i t h s t e p s 3 , h 5 6, 6a a n d 7 o f M e c h a n i s m A. M e c h a n i s m Β g i v e s i d e n t i c a l r a t e l a w s f o r e q s . ( 3 ) , (h) a n d (5) w i t h k]_ r e p l a c e d b y k . I n t h e c a s e o f e q . (2) t h e r a t e l a w i s f o r m a l l y u n c h a n g e d b u t Κ i s now r e d e f i n e d a s Kj_ = ( k + 2k^) / (k + 6a)· cLata do n o t d i s t i n g u i s h b e t w e e n Mechanisms A and B. At t h e h i g h e r v a l u e s of i n i t i a l 0 used a c a t a l y t i c d e s t r u c t i o n o f O3 o c c u r r e d . T h i s r e q u i r e s a c h a i n component o f t h e mechanism. A l i k e l y p o s s i b i l i t y i s s t e p 2 a l o n g w i t h HO + 0 -+ H0 + 0 . Evidence f o r the chain decomposition o f 0 resulting f r o m s t e p s 2 and 9 has r e c e n t l y b e e n o b t a i n e d by DeMore and T s c h u i k o w - R o u x ( 5 ) . However, o t h e r c h a i n s e q u e n c e s a r e p o s s i b l e , s u c h a s s t e p s 6b a n d 8. We d i d n o t d e t e c t H S i n o u r GC a n a l y s e s (3) b u t G u n n i n g et_ a l (6) h a v e r e p o r t e d t h a t t h i s s u b s t a n c e r e a d i l y decomposes: H S •> H S + S. T h i s w o u l d a c c o u n t f o r t h e s u l f u r we f o u n d d e p o s i t e d and t h e s l i g h t l y l o w mass b a l a n c e f o r S. A c c o r d i n g t o Rommel a n d S c h i f f (7) a t v e r y l o w p r e s s u r e s t h e l i k e l y f a t e f o r HS r a d i c a l s i s 2HS -> H S + S. However, i n t h e h i g h e r p r e s s u r e s o f o u r s y s tem s t e p 5 i s t h e most l i k e l y r e a c t i o n . As T a b l e I shows, a d d e d O2 up t o a 6 0 - f o l d e x c e s s r e l a t i v e t o O 3 , h a d no e f f e c t on t h e r a t e . T h i s c o n t r a s t s w i t h what has b e e n f o u n d f o r t h e r e a c t i o n b e t w e e n O3 a n d a l k e n e s , where t h e r a d i c a l i n t e r m e d i a t e s a r e e a s i l y s c a v e n g e d by O2 l e a v i n g t h e r e s i d u a l i n i t i a t i n g step (Q). I n t h e p r e s e n c e o f s u f f i c i e n t a d d e d d e a c t i v a t i n g gas M, e q . (2) becomes 9
9
l a
6 a
0 u r
k
6
3
g
3
2
2
3
2
2
2
2
2
2
3
2
1
2
1
-d[0 ]/dt=k .[H S][Qi]+k [0 ] / tH S] / /(iH2S]+B[M]) / 3
l b
2
1 1
3
2
2
(6)
11.
GLAVAS
Ozone-Hydrogen
A N D TOBY
Sulfide
127
Reaction 200
ο ο
ο
100 Ι
70° Ο
οο
100
c ο
50
ο
50° 0 50
ο - ο °
°°
25 1/2
3
2
I
0
οο
Ο
-ο
Ο
Ο
ο
25
ι
ι
I
40 60 Ν I ΤΙ ALH S,/
