T H E J O U R N A L O F
PHYSICAL CHEMISTRY Registered i n U.S. Patent Office
0 Copyright, 1979, by t h e American Chemical Society ~
VOLUME 83, NUMBER 16
Rate Constant Measurements for the Reaction CI Regarding the Removal of Stratospheric Chlorine
+ CH,O
-
~
~
~~~~~~~
AUGUST 9,1979
HCI 4- CHO. Implications
Philip C. Anderson and Michael J. Kurylo” Center for Thermodynamics and Molecular Science, National Bureau of Standards, Washington,D.C. 20234 (Received January 17, 1979; Revised Manuscript Received March 20, 1979) Publication costs assisted by the National Bureau of Standards
-
The flash photolysis resonance fluorescence technique was employed to investigate the rate constant for the reaction C1+ CHzO HCI + CHO from 223 to 323 K. An Arrhenius fit of the data gives kl = (1.09 f 0.40) X exp[-(131 f 98)/T] in units of cm3molecule-1 The results are compared to two very recent kinetic studies and are assessed in view of the reaction’s role in disrupting the C1-C10 stratospheric ozone depletion chain.
Introduction The effects of the anthropogenic release of chlorofluoromethanes on stratospheric ozone1 has fostered an interest in a large body of chemical reactions pertinent to the quantification of the chlorine atom catalytic cycle for ozone destruction.2 An important subset of these reactions deals with the disruption of the C10, chain by conversion of C1 into less reactive HC1. In the lower stratosphere, the major role in such chain termination has been assigned to organic hydrides (principally CHJ and rate constants for the reactions of C1 atoms with several candidates have been r e p ~ r t e d .Above ~ 40 km, H o p plays the major role in this conversion. Recently the similar roles of non-methane hydrocarbons in general4 and acetylene in particular5have been examined for their stratospheric significance. Little attention, however, has been paid to the possible importance of other hydrogenous species in the conversion of C1 to the HCl “holding reservoir”. One potentially active class of compounds are the products of alkane (and haloalkane) photooxidation. The most important member of this class is formaldehyde, for which there are no measured atmospheric mixing ratios, although significant stratospheric concentrations are predictable. We report herein the results of a flash photolysis resonance fluorescence (FPRF) investigation of the reaction This article not subject to
C1+ C H 2 0
2HC1+ CHO
(1)
over the temperature range 223-323 K. Since the inception of these studies, two measured values for hl have been r e p ~ r t e d . ~We , ~ are pleased to report close agreement among the results of all three studies.
Experimental Section Experiments were conducted with a variable temperature FPRF apparatus operated in a flow modeS3s8Procedural descriptions and details concerning the flash lamp, resonance lamp, and reaction cell are given in these earlier publications. C1 atoms were produced by the flash photolysis of 20-60 mtorr of CC14 (1mtorr = 0.133 Pa = 9.6587 X 1015/T (K) molecules ~ m - a~t )wavelengths above the Suprasil cutoff (A > 165 nm). The C1 time history after the flash was monitored by detecting (without wavelength resolution) the vacuum UV atomic fluorescence resonantly scattered by the microwave discharge resonance lamp (0.1% Clz in 1torr of Ar). Variation of flash lamp intensity from 2 X 10l2to 8 X quanta cm-’ per flash coupled with changes in CC14concentration permitted production of initial chlorine atom concentrations, [Cl],, of 1 x 1O1O to 1 X loll ~ m - ~The . absence of any perceptible dependence of the atom decay rate on initial atom concentration was viewed as evidence for the lack of inter-
U.S. Copyright. Published 1979 by the
American Chemical Society
2056
The Journal of Physical Chemistry, Vol. 83, No. 16,
1979
P. C. Anderson and M. J. Kurylo
TABLE I: Rate Constants for the Reaction C1 t CH,O .+ HCl i CHO
d‘
I
I
10’’k , cm3 molecule- s- ’
T,K
7.07
323 293 273 250 223
i
0.74a
7.18 t 0 . 6 1 6.76 i. 0.98 6.50 t 0.92 5.98 t 0.96
a Error limits are i 20. from a linear least-squares analysis of the C1 decay rate vs. [CH,O] data plus an additional 510%uncertainty t o account for experimental variability due t o mixture composition, flow rate, reactant loss, etc.
I
I
i ‘
“i 1%
3
f , , , 3‘ 33 34 05
33
32
x
v
[CH2C], mTorr
Figure 1. First-order CI decay rates at 298 K as a function of [CH,O]. The solid line is a linear least-squares fit of the data. Error bars are
20.
ference from secondary atom-radical reaction^.^ The majority of experiments were performed with [Cl], x 3 X 1O1O ~ m - ~ . Reaction mixtures containing CC14,CH20, and Ar were prepared manometrically in glass storage bulbs prior to their being admitted to the reaction cell. Such mixtures were flowed through the cell a t 10 torr total pressure with flow rates between 70 and 280 cm3 s-l. Such a procedure minimized any product buildup or reactant depletion during the course of an experiment. Initial studies exhibited a tendency for the atom decay rates to increase with increasing flow rate and eventually level off a t values approximately 20% higher. Such changes were not observed for atom decays in the absence of formaldehyde and were attributed to a loss of CHzO on the walls of the cell by interaction with the solvent used to clean the cell. Thus the early experiments were performed a t the higher (leveled off) flow rates. Subsequent runs conducted a t the slower flow rates failed to exhibit any changes in rate presumedly due to a gradual removal of the active wall species. The majority of the experiments reported here were performed under these latter conditions. Similar “conditioning” of a teflon-coated stainless steel absorption cell (very much like the present reaction cell) resulted in long term (several hours) stability of static CH20 fillings as measured by UV spectroscopy.1° The atom decay curves always demonstrated good exponential behavior (since pseudo-first-order conditions were employed; [CH20]>> [Cl],). These were analyzed by a non-linear least-squares routine and the value of hl a t each temperature was determined from the slope of a plot of the first-order decay rate vs. formaldehyde concentration. Ultra high purity Ar (99.999%) was used directly from the cylinder. Spectro grade CC14was degassed via several freeze-pump-thaw cycles prior to use. Formaldehyde was prepared by heat-evaporating mixtures of paraformaldehyde and phosphorus pentoxide. Samples thus obtained were purified by low temperature distillation and stored a t 77 K. IR and UV analysis of several samples indicated no measurable impurities.
r
J , 3.0
-121
2x102,5
,
,
3.5
,
4.0 4.3
,
5.0
ICI~/T(KI Figure 2. Arrhenius plot of k , . Error bars are 20,
Results and Discussion The first-order atom decay rates (kist) measured a t 293 K are plotted vs. [CH20] in Figure 1. The solid line is a linear least-squares fit of the data (the slope of which equals the value for hl at this temperature). Similar plots with comparable data scatter and uncertainties were obtained at four additional temperatures between 223 and 323 K. The kl values a t the five temperatures are given in Table I and are plotted in Arrhenius form in Figure 2. The Arrhenius fit to the data in Figure 2 takes into account the error bars of each value and is given by hl = (1.09 f 0.40) X exp[-(131 f %)/TI cm3 molecule-l s-l. The high reaction rate and near zero activation energy are consistent with the 16 kcal mol-l exothermicity of reaction 1. There are two very recent studies with which we can compare our present results. Using a Fourier transform IR spectroscopic method for product analyses, Niki e t ale6 report a rate constant role of h1/k2 = 1.3 f 0.1 a t 298 K, where h2 is the rate constant for the reaction C1
+ CzH6
ka --+
HC1 + C2Hb
(2)
Using the most recent evaluation for hZ,l1h2(298K) = 5.7 f 0.8 x cm3 molecule-l s?, one obtains hl = (7.4 f 1.3) x to be compared with our measured value of a t 293 K. 7.18 X An absolute determination by Stief et al.7 gives values (7.46 f 0.86) X for (k, f 2 0 ) of (7.45 f 0.96) X and (7.52 f 1.18) X cm3 molecule-l s-l a t 500, 298, and 200 K, respectively. The absence of a perceptible temperature dependence for hl (as indicated by these results) may be contrasted with the small value of E determined in the present study. However, over the temperature region of measurement, all results overlap within the quoted 2g error bars. It is interesting to note
The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
Reactlon of CH, with 0, 02,and NO
that the Stief study dealt with the same initial H2C0 wall-loss problems that we experienced (and in fact solved them in precisely the same manner). Thus if any significance is to be placed on the difference in calculated temperature dependencies from the two studies, it would have to be attributed to difficulties in handling such minute quantities of reactant necessitated by the rapidity of the reaction. The conclusions drawn by Stief e t al. regarding the stratospheric implications of this fast reaction rate are strongly supported by the results of the present study. At 230 K (-30 km), for example, the value we obtain for kl is 2.3 X lo3 times larger than that for k3. C1+ CHI
2HC1+ CH3
(3)
Since CH4has an approximate mixing ratio (V/V) of lo4 at midstratospheric altitudes, formaldehyde could be significant (210% as effective) in the removal of active Similar chlorine for mixing ratios in excess of 4 X conclusion can be drawn from comparison with removal of C1 by HOz above 40 km. I t is clear from the present results that further studies on the reactivity of other photooxidation products, as well as direct stratospheric measurements of their concentrations, are necessary to accurately assess the effect of stratospheric C1X injection on the ozone budget.
Summary Rate constants reported herein for the C1 + C H 2 0 reaction by the FPRF technique permit calculation of a simple Arrhenius expression over the temperature range 223-323 K. Results indicate a very rapid reaction with a near-zero activation energy. The role of this reaction in
2057
converting active C1 into the HC1 stratospheric sink could be significant for CH20concentrations presently calculated for the midstratosphere. The role of formaldehyde, as well as other hydrocarbon oxidation products in moderating the C10, catalytic cycle for O3 destruction, demands further study. Acknowledgment. This work was supported in part by the Upper Atmospheric Research Office of the National Aeronautics and Space Administration and the Fluorocarbon Research Program of the Manufacturing Chemists Association. Supplementary Material Available: Supplementary Table I contains the first-order decay rate data from which the kl values are calculated (3 pages). Ordering informations is available on any current masthead page.
References and Notes "Halocarbons: Effects on Stratospheric Ozone", Natlonal Academy of Sciences, 1976, and references contained therein. "Chlorofluoromethanes and the Stratosphere", NASA RP 1010 (1977). R. G. Manning and M. J. Kurylo, J . Phys. Chem. 81, 291 (1977). W. L. Chameides and R. J. Cicerone, J . Geophys. Res., 83, 947 (1978). (a) F. S.Lee and F. S.Rowland, J . Phys. Chem., 81, 684 (1977); (b) G. Poulet, G. LeBras, and J. Combourieu, ibid., 81, 2303 (1977). H. Niki, P. D. Maker, L. P. Breitenbach, and C. M. Savage, Chem. Phys. Leff., 57, 596 (1978). (a) L. J. Stief, J. V. Michael, W. A. Payne, D. F. Nava, D. M. Butler, and R. S.Stolarski, Geophys. Res. Lett., 5, 829 (1978); (b) J. V. Michael, D. F. Nava, W. A. Payne, and L. J. Stief, NASA Technical Memorandum 79675 (Nov 1978). M. J. Kurylo and W. Braun, Chem. Phys. Leff., 37, 232 (1976). M. J. Kurylo, N. C. Peterson, and W. Braun, J . Chem. Phys., 53, 2776 (1970). A. M. Bass, private communication. NASA Laboratory Measurements Committee Report (1978) updating recommendations in NASA RP 1010.
Temperature Dependence of the Reactions of Methylene with Oxygen Atoms, Oxygen, and Nitric Oxide C. Vlnckier" and W. Debruyn Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan ZOOF, 3030 Heverlee, Belgium (Received December 29, 1978; Revised Manuscript Received March 13, 1979) Publication costs assisted by the Nationaal Fonds voor Wetenschappelijk Onderzoek Belgium
Molecular beam sampling and subsequent mass spectrometric analysis have been used as detection techniques for methylene radicals produced in the oxidation process of acetylene with oxygen atoms in a fast flow reactor. Reactions of CH2with oxygen atoms, molecular oxygen, and nitric oxide are investigated in the temperature region between 295 and 600 K. For reaction 2, CH2 + 0, an activation energy of about 0 kcal mol-l has been found while for reaction 3, CH2 + 02,the value E3 = 1.5 f 0.3 kcal mol-' is derived. Reaction 4, CH2 + NO, shows non-Arrhenius behavior and has a negative activation energy, E4 = -1.1 f 0.4 kcal mol-'. In addition, complete Arrhenius expressions for the rate constants of reactions 3 and 4 are given and an attempt is made to determine the possible reaction products of these reactions.
Introduction The demand for accurate kinetic data of elementary reactions in the gas phase increased considerably since computer modeling techniques have been introduced in the field of air and (or) stratospheric pollution, combustion, etc. While extensive compilation and data evaluation programs of many elementary reactions exist,l only a limited number of studies on the methylene radical have been carried out. Since this radical is a major intermediate 0022-3654/79/2083-2057$0 1.OO/O
in the oxidation process of acetylene and other hydroc a r b o n ~ , ~its- ~kinetic behavior is important primarily because it may lead to the formation of highly unsaturated hydrocarbons in flames. In the past, reactions of methylene with various hydrocarbons have been studied and, in general, it undergoes an addition reaction to double bonds5 and an insertion into or an abstraction from single bonds.6 While most of the kinetic parameters were deduced from the formation rate of stable products, only in @ 1979 American Chemical Society
