3301
COM M
I C A T I Q N S TO THE EDITOR
ate Constant for the Reaction of HOz with
In the absence of CO, the rate expression is
Carbon Monoxide PwbEkation costs assisted by the National Science Foundation
Sir: On the basis of an experimental determination of the relative rate constants, ka/kb = O.OG, for the reactions
which, a t the necessarily low concentration of H202, yields
'H20zI dt
Westenberg and deHaas' suggest that IC, may be as large or larger than the rate constant for the reaction of OH with CO. Because of the importance of these reactions with 60, particularly in the elucidation of photochemical reactions in contaminated atmospheres, we wish to present a contrary conclusion based on results we have obtained. Complete experimental details but not the discussion below are given in a forthcoming paper.2 We have studied the rate of CO, formation at 2.5" under constant illumination in the gaseous system H2O2-C0-O2. The following reactions are pertinent. H20z1- hv(254 nm)
--%20H
(3)
+ Rz02--% HzO + HOz NO2 i- HOZ--% H202 + O2 OH 4- CQ --% COz + H 13 + €12Q2--%HzO + OH H -f- HZOzks, Hz + HOz M i Q, + ;\I --%H O ~+ n4 NOp + CO --%COz + OH
ON
(4)
(5) (6)
(7) (8) (9) (10)
In the presence of sufficient 02,the only reaction of H atom is given by eq 9. This is confirmed since hydrogen is not formed under these conditions. The rate expression for initial conditions is then
1 (dr2) 0 I ' ' ' d[CO
0
=
21,
+~~j~~~o(la/lca)~/
Ik4kz[H20zlo+ IC4[CO]o
+
-
210' [H20z'1
for a fixed illumination geometry with Io' a function only of the incident light i n t e n ~ i t y . ~In earlier work it was found that the photodecomposition of HzOzwas riot affected by added ~ x y g e nand , ~ in the present work the same results were obtained with either H202alone or with added COZ. Consequently, we obtained Bo' from photodecomposition in the system Mz02-CQ2 using the integrated rate expression
The yield of 0 2 was determined and the rate expression was adhered to rigorously. Thus I , in eq II can be sec-'. replaced by Io'[H2Q2]~ with Io'= 8.7 X Actinometry carried out with diethyl ketone yielded the same value of Io'. The value of k , has been estimated4 to be about 3 X 1U-'2 cm3 molecule-' Tee-'. However, the minimum rate derived from eq 11 will be given by the maximum value of IC3. If we assume reacOther tion occurs on every collision, IC, Si 1.0 X 1Q-'O. values used and our experimental conditions are IC, = 9.3 X cm3 molecule-' W C - ~ at 298 K;6 icl = 1.5 X cm3 molecule-' sec-I at 298 K ; o [H202]0 = 3.0 X 10l6, [OzJO = 1.7 X IO", [CQ]o = 3.2 X IO'' molecules cm-3 (0.94, 5.4, and 10.0 Torr, resp~ctively). If k, kd, the calculated initial rate of COz formation from eq 11 using the above values i s 4. while our experimentally determinrd ratc based on COZformation for a time interval during which the initial concentration of any reactant did noi change by more than 1.5yois 2.5 X l O I 3 molecule3 sec-'. (1) A. A. Westenberg and N. deHaas, J . Phys. Chem., 76,1586 (1972). (2) R. A. Gorse and D. H. Volman, J . Photochem., 1, l(1972). (3) ~ D. H. Volman, J . Chem. Phys., 17, 947 (1949). (4) S. N. Foner and R. L. Hudson, Adaan. Chem. Ser., No. 36, 34
(1962). ( 5 ) N. R. Greiner, J . Phys. Chem., 72, 406 (1968). (6) D. L. Baulch, D. D. Drysdale, and A. C. Lloyd, High Temperature Reaction Rate Data Report No. 3, Leeds University, 1969.
The Journal of Physical Chemistry, Vol. 76,N o . $2, 1972
3382 ELirnination of reaction 10 yields the expression
from which we obtain a rate of 3.3 X 1013 molecules see-' in good agreement with our experimental results. The grossest error, say a factor of 10 too low, i n our experimentally determined rate would lead to a calculated value of k4/k, = 1.6 X lo2for eq 11 to give the presumed true rate. Therefore, we conclude that the rate constant for the reaction of HOz with CO is very much less than that for the reaction of OH with
60.
The J o U T n d of Ph,gsysicaE Chemistry, Vol. 76, No. 22, 1972
COMMUNICATIONS TO THE EDITOR The above discussion has been limited to a single experimental value. We should, however, point out that w e have successfully tested eq 15 by plotting lo'/ ( d [ C O ~ ] / d t vs. ) ~ 1/[COj0 for fixed initial O2 and H202 concentrations, which should yield a straight line, with initial CO pressures from 3 t o 30 Torr and with Io' from 6.6 X to 1.9 X see-'. Acknowledgment. Support for this research was received from the National Science Foundation. DEPARTMENT OF CHEMISTRY UNIVERSITY OF CALIFORNIA DAVIS,CALIFORNIA95616
DAVIDH. VOLMAN* ROBERT A. GORSE
RECEIVED JUNE 19, 1972
