J . Phys. Chem. 1986, 90, 1900-1906
1900
CHEMICAL KINETICS Kinetic Study of the Elementary Chemical Reaction H(2S1,2) 4- 02(lAg) O(3P) in the Gas Phase
-
OH(211)
+
W. Hack* and H. Kurzke Max-Planck-Institut fur Stromungsforschung, 3400 Gottingen, West Germany (Received: March 25, 1985; In Final Form: December 17. 1985)
-
The reaction of H atoms with electronically excited 02, H(’SII2) + 0 2 ( l A g ) OH(211) + O(3P) (la), was studied in an isothermal flow reactor in the temperature and pressure ranges of 295 5 T / K 5 423 and 70 5 p/Pa 5 1070, respectively, with He as the main carrier gas. The concentration profiles of H and 0 atoms were followed by resonance absorption and those of OH radicals by LIF. The 02(]Ag)molecules which were present in an excess over H atoms were generated in a and their absolute concentrations determined by photoionization. At a pressure of p = microwave discharge in 02(38,-) 130 Pa, an Arrhenius expression of k,,(T) 1 1l: X 10” exp(-(13 & 2 kJ mol-’)/RT) cm3/(mol s) was obtained. It is proved that energy transfer from Oz(’Ag)to H02(X,A) is significant for the understanding of the H + 02(’AB), (32;) system. The result of kl,( T ) and the reaction mechanism including the reactions of 02(’Z;) and the energy-transfer processes 0 2 ( ’ A , ) + H02(g,A) are discussed.
Introduction The chemical reaction of hydrogen atoms with metastable oxygen molecules H(2Sl,2)+ 0 2 ( ’ A g )
-
OH(211) + O(3P)
(la)
is suitable for experimental investigation of the difference in reactivity between electronically excited oxygen molecules and ground-state molecular oxygen. This reaction and the corresponding ground-state reaction proceed on different energy hypersurfaces.’ In addition to this theoretical aspect, the title reaction is also of interest for atmospheric photochemistry, especially of the mesosphere. Reaction l a and its deactivation channel H(’S,/&
+ O2(’Ag)
-+
H(’Si/A
+ 02(3z,-,v)
(1b)
have aroused interest recently, since they are implicated in the consumption of 0 2 ( ] A g ) in the pulsed chemical iodine laser. The first direct investigation of the reaction H(2S,,2) + 02(’Ag)
-
products
(1)
was performed in 1970 by Westenberg et al.,2 who followed the H atom concentration in a flow system by an ESR spectrometer. From their results they concluded that the depletion of H atoms by reaction l a is significantly slower than by the reaction
H(2S1,2)+ 02(3ZJ + M
-
H02&)
+M
(2)
at pressures up to 319 Pa with He or Ar as a third body. Other investigations followed the 0 2 ( ’ A g ) concentration by mass spectrometer3or by ESR4 in a flow system and obtained much higher rate constants for reaction 1. In these experiments the sum of the quenching reaction l b and the chemical reaction l a was observed. In addition, energy transfer from 0 2 ( ’ A g ) to HO,, ( I ) C. F. Melius and R. J. Blint, Chem. Phy. Lett., 64, 183 (1979). (2) A. A. Westenberg, J. M. Roscoe, and N. DeHaas, Chem. Phys. Lett..
7, 597 (1970). ( 3 ) C. Schmidt and H. I. Schiff, Chem. Phys. Lett., 23, 339 (1973). (4) L. T. Cupitt, G. A. Takacs, and G. P. Glass, Int. J . Chem. Kine?., 14,
487 (1982).
0022-3654/86/2090-1900$01.50/0
produced in ground-state reaction 2, has not been taken into account. Until now it has not been possible to find a proper model for the reaction mechanism which is able to explain all the experimental findings concerning reaction 1 and the data available for energy transfer between O,( ]Ag) and H 0 2 radicals. One has to keep in mind, however, that reaction 1 can hardly be separated from the complex reaction scheme given by the interaction of reactants and products including energy-tr_ansfer processes. Since the reaction of O2(lAg) with HO,(X) is a fast p r o c e s ~ , ~ producing H atoms6 it is obvious that it is not sufficient to follow only one of the species H or 02(]Ag). However, by following the concentrations of H and 0 atoms together with OH radicals it should be possible to kinetically model (’Ag) the interactions of all species present in the H + 02(3z,-), system, and thus elucidate this complex reaction mechanism and reconcile the contradictions present in the literature. The aim of this work was to determine the rate constant k,,(V for the chemical channel l a from the [O]profiles.
Experimental Methods The main features of the apparatus have been described in a prevous paper;’ only the differences in experimental technique will be explained. The Flow System. The apparatus consisted of an isothermal flow reactor ending in a black anodized aluminum cell. The inner wall of the flow reactor (id. = 36 mm) was coated with halocarbon wax (15-00, Halocarbon-Product Corp., NJ). This coating reduces the wall’s activity for H atoms as well as for 0 atoms and O H radicals. It was used in the temperature range 295 IT/K 5 353. At temperatures exceeding 353 K we used a HF-washed Pyrex wall. Time resolution was achieved by a movable quartz probe. The difference between the position of the probe tip and the point of detection could be varied between 15 and 65 cm (10 ItR/ms I 90). H atoms were produced in a microwave discharge in a H2/He mixture. Traces of water were removed by passing the gas through ( 5 ) K. J. Holstein, E. H. Fink, J. Wildt, R. Winter, and F. Zabel, J . Phys. Chem., 87, 3943 (1983). (6) W. Hack and H. Kurzke, Chem. Phys. Lett., 104, 93 (1984). (7) W. Hack, 0. Horie, and H. Gg. Wagner, J . Phys. Chem., 86, 765
(1982).
0 1986 American Chemical Society
Kinetic Study of the H
+ O2 Reaction
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1901
two liquid N2 traps. One of these traps was attached directly to the discharge cavity at the top of the p r ~ b e . ~ . ~ For all gases the highest purity commercially available was used. They were all passed through liquid N2 traps to remove water. The gas flow was pumped by a 9.7 X m3/s Rootes pump. The Detection Systems. The concentration of H and 0 atoms was measured with a pulsed resonance absorption device, working at 250 Hz. After passing through the absorption volume with a linear dimension of I = 18 cm, the radiation from the resonance lamp was focussed on the entrance slit of a 20-cm vacuum-UV monochromator (Acton Research VM-502) with a 200-mm MgF2 lens. A solar blind PMT (EM1 G26E315) was attached to the monochromator, feeding a lock-in amplifier. For the H atoms the Lyman a transition 2Pl,2-2Sl/2at X = 121.6 nm and for the 0 atoms the unresovled triplet 3S,-3P2,1,0 transition at X = 130.5 nm was used. The resonance lamp was calibrated in terms of absolute concentration with the fast titration reactions
H
+ NO,+ N
+ NO
+ NO N2 + 0
OH +
Results The title reaction and reaction 2 are much more intermingled than expected due to the fact that the O,(lA,) source yields an 02(lAg), (32;) mixture. Besides the measurements in these mixtures some independent experiments were therefore done in in order to enable a direct consideration of reaction pure 02(32;), 2 in the kinetic modeling of the H + 02(lAg),(32J system. These results for reaction 2 are given before the intrinsic measurements on the title reaction are described. The rate of reaction 2 was measured at room temperature in the pressure range 130 Ip/Pa I530 by monitoring the [HI atom decay profiles. The rate constants under pseudo-first-order conditions [HI,
