1526
T. L. Osif and J. Heicklen
Combust., [Proc.], 13th. 705 (1971). (4) R. N. Butlin and R. F. Simmons, Combust. Flame, 12, 447 (1968). (5) W. E. Wilson, Jr., J. T. O'Donovan, and R. M. Fristrom, Symp. (Int.) Combust., [Proc.] 12th, 929 (1969). (6) D. R. Blackmore, G. O'Donnei and R. F. Simmons, Symp. (Int.) Combust., [ Proc.], loth, 303 (1965). (7) W. A. Rosser, H. Wise, and J. Miller, Symp. (lnt.) Combust., [Proc.], 7th, 175 (1959). (8) H. Eyring and M. Poianyi, 2.Phys. Chem., 812, 279 (1931). (9) A. Wheeler, B. Topiey, and H. Eyring, J. Chem. Phys., 4, 178 (1936). (IO) S.W. Mayer and L. Schieler, J. Phys. Chem., 72, 236 (1968). (11) R. 8. Timmons and R. E. Weston, Jr., J. Chem. Phys., 41, 1654 (1964). (12) C. A. Parr and D. G. Truhiar, J. Phys. Chem., 75,1844 (1971). This review reports new experimental measurements by C. A. Parr and A. Kuppermann. (13) J. M. White, J. Chem. Phys., 58,4482 (1973). (14) J. M. WhiteandD. L. Thompson, J. Chem. Phys., 61, 719 (1974). (15) L. M. Raff, L. Stivers, R. N. Porter, D. L. Thompson, and L. Sims, J. Chem. Phys., 52,3449 (1 970). (16) R. D. Levlne and R. B. Bernstein, Chem. Phys. Left., 29, 1 (1974).
(20) (21) (22) (23) (24) (25) (26) (27)
(a) G. A. Takacs and G. P. Glass, J. Phys. Chem., 77, 1060 (1973); (b) J. E. Breen and G. P. Glass, J. Chem. Phys., 52, 1082 (1970). A. Persky and A. Kuppermann, J. Chem. Phys., 81, 5035 (1974). Reported in ref 20 of this paper as ref 26. H. Y. Su, J. M. White, L. M. Raff, and D. L. Thompson, J. Chem. Phys., 62, 1435 (1975). J. D. McDonald and 0.R. Herschbach, J. Chem. Phys., 62,4740 (1975). M. A. A. Clyne and H. W. Cruse, J. Chem. SOC., Faraday Trans. 2,68, 1281 (1972). A. A. Westenberg, Prog. React. Kinet., 7,24 (1973). A. A. Westenberg and N. deHaas, J. Chem. Phys., 48,490 (1967). P. G. Dickens, R. D.Gould, L. W. Linnett, and A. Richmond, Nature (London), 187,686 (1960). B. Khouw, J. E. Morgan, and H. I. Schiff, J. Chem. Phys., 50,66 (1969). P. B. Davies, B. A. Thrush, A. J. Stone, and F. D. Wayne, Chem. Phys. Lett., 17. 19 . - flR771 \.-.3
(28) R: J. DonovaAand D. Husain, Chem. Rev., 70, 509 (1970). (29) C. F. Bender, B. J. Garrison, and H. F. Schaefer, lii, J. Chem. Phys., 62, 1186 (1975).
Oxidation of HCO Radicals T. L. Osif and Julian Heicklen" Department of Chemistry and Ionosphere Research Laboratory, Pennsylvania State University, University Park, Pennsylvania 16802 (Received August 1 1, 1975: Revised Manuscript Received February 4, 1976)
Mixtures of Clp, 0 2 , CHz0, and sometimes Nz or He at room temperature (-23 "C) or -7 "C were irradiated at 3660 A to photodecompose the Clz. The chlorine atoms abstract a hydrogen atom from CH2O to produce HCO radicals which can react with 0 2 : HCO 0 2 HC03 (la); HCO 0 2 CO HOz (lb); HCO 0 2 COz HO (IC). The HC03 radical ultimately becomes HCOOH, so that HCOOH, CO, and COz become measures of the relative importance of the three reaction paths. I t was found that k&lb = 5 f l and h&lb _< 0.19 at -23 OC (total pressure = 62-704 Torr) and -7 "C (total pressure = 344-688 Torr). As the [Clz]/[O2] ratio increases, the HCO radical can also react with Clz: HCO C12 C1+ HCClO HCl CO Cl(3a); HCO Cl2 HCOCl2 termination (3b). At both temperatures h3a/h3b 7.5 and h3b/hlb = 6 (4-7, -2).
+
+
+
-
+
+
+0 2
-
HC03
+ HOz * C02 + HO -+
+
-
Introduction The reactions of HCO with 0 2 are important in flames and combustion, in the chemistry of the upper atmosphere, and in the production of oxidants in photochemical smog. Therefore a complete characterization of HCO 0 2 chemistry is important for understanding these processes. The three possible reactions of HCO with 0 2 are: HCO
-
CO
(la) (lb) (IC)
The HC03 ultimately becomes HCOOH, either by proceeding through HC03H or HCOl as an intermediate. Thus the amounts of HCOOH, CO, and C02 produced are measures of the relative efficiencies of reactions la, Ib, and IC,respectively. The photooxidation of CH20, which almost certainly involves the oxidation of HCO radicals, was studied by Carruthers and Norrishl who found CO, H2, HCOOH, and smaller amounts of COz as products, but no peroxides or peracid. They reported quantum yields of CHzO removal of 8.5-12.6 at 100 OC. Horner et a1.2 studied the photochemical oxidation of CH2O from 100 to 275 OC and the thermal oxidation to 375 OC and found the same products. The Journal of Physical Chemistry, VoI. 80, No. 14, 1976
-
-
-
-
+
+
-+
+
+
Rate coefficients have been reported by three groups, but they are not in good agreement. Peeters et al.3 studied lowpressure methane-oxygen flames and reported hlb 5 X cm3/s for 1400 K < T < 1800 K. Demerjian et al.4 selected the following rate constant values such that simulated data were optimized to fit experimental data.
-
hlb
-
1.7 X
cm3/s; h i a
-
6.8 X
cm3/s; kla/klb
-
0.4
Washida et al.5 measured HCO radical concentrations with a photoionization mass spectrometer and reported h l b = (5.7 f 1.2) X cm3/s at 297 K and for total pressures of 1.5-5 Torr. In this study mixtures of Cl2,02, CH20, and sometimes Nz or He were irradiated with the 3660-A line of a mediumpressure mercury arc at -23 and -7 OC. The chlorine photodissociates and the chlorine atoms abstract an H atom from CH20 to produce HCO radicals. The products measured were HCOOH, CO, and COz so that the above branching ratios for reaction 1 could be obtained.
Experimental Section Mixtures of Cl2, CHz0,02, and in some cases an inert gas were irradiated at 3660 A and at -23 and -7 "C. The products
Oxidation of HCO Radicals
1527
TABLE I: Extinction Coefficients at 3660 d to Base 10, Torr-' cm-' Temp, "C
CH3NpCH3
c12
25 -7
1.87 x 10-4 1.85 x 10-4
1.39 x 10-3 1.53 x 10-3
CO, CO2, and HCOOH were measured. The reactions were carried out in a cylindrical quartz cell 5 cm in diameter and 10 cm long. Irradiation was from a Hanovia utility ultraviolet quartz lamp which passed through a Corning 7-37 filter. Since this filter passes light from -3800 to -3300 A, the effective radiation was at 3660 A. A conventional high-vacuum line utilizing Teflon stopcocks with Viton "0"rings and glass stopcocks greased with Apiezon N was used to handle the gases. Pressures were measured with a silicone oil manometer in conjunction with a cathetometer, a 0-800 Torr Wallace and Tiernan absolute pressure indicator, and a Veeco thermocouple gauge. For the studies done at -7 "C, the reaction cell was housed in a box constructed of Dyfoam (made by Zonolite). Two evacuated cylindrical Pyrex cells with quartz windows were placed through the box wall, one through each of two opposite sides. In this way light entered the box from one side via one Pyrex cell, passed through the photolysis cell, and passed out of the other side of the box through the second Pyrex cell to impinge on a RCA 935 phototube. The temperature inside the box was lowered by passing nitrogen gas through a copper coil immersed in liquid nitrogen and flushing this cold nitrogen through the box. The temperature was measured with an iron vs. constantan thermocouple and was controlled by changing the flow rate. Actinometry was done by photolyzing C H ~ N Z C for H ~which @{N2]= 1.0.6The light intensity after passing through the cell, I , was always monitored with the RCA 935 phototube. From the N2 produced, measured by gas chromatography, the measured extinction coefficient of azomethane (see Table I), the light intensity, Io (mTorr/min), was calculated using Beer's law. Since IO is proportional to the phototube signal when the cell is empty, the now calibrated phototube signal gave a measure of IO before each run with chlorine. Thus the absorbed light intensity, I,, for the runs with chlorine was calculated from Beer's law, knowing IO,the measured extinction coefficient of Cl2, and the average Clz pressure (computed as the initial chlorine pressure minus 1/4 the CO produced, since each C1 atom consumed produces one CO molecule). This was done instead of matching absorbances because IJIO was always less than 16% and was usually 4-5%. The formaldehyde was prepared from Fisher Scientific paraformaldehyde by a procedure patterned after that of Spence and Wild,7 and was stored at -196 "C. The chlorine was obtained from the Matheson Co. It was degassed a t -160 "C and distilled from -130 "C (n-CsH12 slush) to -160 "C (i-CsH12 slush). After purification it was stored at room temperature in a darkened storage bulb. The azomethane was prepared as described elsewhere.s The oxygen and nitrogen were also obtained from the Matheson Co. The nitrogen was always passed through a trap containing glass wool immersed in liquid nitrogen. The oxygen was treated the same, except the liquid nitrogen was replaced with liquid argon when the total pressure was greater than about 130 Torr. The oxygen and nitrogen were then analyzed by gas chromatography and found to be free of Cop. The oxygen was also free of CO, but the nitrogen contained 0.035% CO. The CO yields were appropriately corrected in the runs
with nitrogen. Runs were also done with helium instead of nitrogen to avoid this problem. This helium was taken from the carrier gas stream of the gas chromatograph and was used without purification. After photolysis, the noncondensables at -196 "C (or -186 "C, depending on the oxygen pressure) were analyzed for CO by gas chromatography using a 12 ft. X 0.25 in. 0.d. copper column packed with 13X molecular sieves, operated at 0 "C. The condensables were distilled from -130 to -196 "C.The gases condensed by liquid nitrogen were analyzed for C02 by gas chromatography using a 10 ft. X 0.25 in. 0.d. copper column packed with 50-80 mesh Porapak Q. The condensables at -130 "C were then transferred to an ir cell and analyzed for HCOOH. Most, if not all, the CHzO was removed at -130 "C. If small amounts of CH2O remained, they would not interfere with the HCOOH analysis, because the HCOOH spectrum is much stronger than that of CHz0. Since formic acid exists as a mixture of dimer and monomer, the ir was calibrated for HCOOH by placing known "total" pressures of formic acid (measured on a manometer) in the ir cell and measuring the absorbance at 2940,1740, and 1213 cm-l. These were plotted in the usual manner. The equilibrium constant, K = 2.85 Torr at 300 "C,9 was used to convert the "total" formic acid pressure in a run to the pressure it would be if it were all monomer. All formic acid numbers reported here have been thus corrected. Results When Cl2 was photolyzed at 3660 in the presence of CH2O and 0 2 the products measured were CO, CO2, and HCOOH. The HCOOH exists as a mixture of the dimer and monomer. However from the known equilibrium constant, K = 2.85 Torr a t 300 0C,9the total HCOOH yield (assuming all monomer) could be obtained and these values are listed in Tables I1 and 111,along with quantum yields of CO and COz. The parameters were varied as follows: Room temperature: [ 0 2 ] from 696 to 1.88 Torr, [Cl,] from 5.41 to 1.04 Torr, [CHzO] from 11.2 to 2.32 Torr, [C12]/[02] from 0,807 to 0.0020, I , from 75.4 to 8.4 mTorr/min, the irradiation time from 17.5 to 0.25 min, and total pressure from 704 to 62 Torr. -7 f 1 "C: [ 0 2 ] from 645 to 1.53 Torr, [Clz] from 4.59 to 1.13 Torr, [CHzO] from 10.1 to 2.84 Torr, [C12]/[02] from 0.91 to 0.0022, I , from 30.0 to 7.18 mTorr/min, the irradiation time from 60 to 1.5 min, and total pressure from 688 to 344 Torr. The product yields were independent of the variation in all parameters except the ratio [C12]/[02].As this ratio increased @(CO)increased from a lower limiting value of 2.0 to an upper limiting value of 15 at both temperatures. +{HCOOH)however was measured only at low [C12]/[02].At high [C12]/[02]ratios, it would have been difficult to obtain enough HCOOH to measure accurately, without decomposing an excessive amount of CH20. At low [C12]/[Op] ratios, scatter is not surprising, considering the difficulties in measuring HCOOH. One problem is that some of the acid may be lost to the walls. In the room temperature runs, the HCOOH pressure could usually be measured at 2940,1740, and 1213 cm-l. The three values obtained usually differed by only a few percent. However, at -7 O C , a different ir instrument was used and the 2940-cm'l peak was usually impossible to measure accurately, and the 1213-cm-l peak was sometimes obscurred by an unknown peak. Nevertheless, when the 1213-cm-l peak could be used the pressure calculated from it was usually less than 10% different from the pressure calculated from the 1740cm-l peak. The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
1528
T. L. Osif and J. Heicklen
TABLE 11: Product Quantum Yields in the Photolysis in Chlorine at 3660 A in the Presence of 02 and CHzO a t Room Temperature (-23 "C) [Clz]/ [Oz]
[Clz], Torra
Torr
0.0020 0.0021 0.0021 0.0021 0.0024 0.0024 0.0031 0.0039 0.0040 0.0040 0.0040 0.0042 0.0044 0,0045 0.0046 0.014 0.015 0.016 0.016 0.017 0.021 0.023 0.024 0.028 0.035 0.037 0.050 0.070 0.081 0.083 0.100 0.167 0.172 0.222 0.226 0.234 0.382 0.396 0.697 0.714 0.725 0.740 0.807
1.29 1.37 1.39 1.38 1.47 1.38 2.15 1.36 1.34 1.36 1.38 1.40 1.49 1.50 1.46 1.30 1.53 1.54 5.41 1.57 1.04 1.20 2.35 1.47 1.08 2.04 2.44 1.13 1.07 2.23 2.63 2.15 2.23 1.25 2.62 2.78 3.90 2.02 1.31 1.47 1.50 2.47 4.63
659 655 655 649 609 570 696 345 339 346 349 332 337 334 319 90 105 95 331 95.0 48.8 52.8 100 52.5 30.6 55.4 48.8 16.1 13.2 27.0 26.2 12.9 13.0 5.62 11.6 11.9 10.2 5.1 1.88 2.06 2.07 3.34 5.74
[02],
[CHzO], Torr
[N2], Torr
3.71 3.50 3.47 10.0 3.83 6.44 6.27 3.30 4.22 3.77 4.18 8.93 3.18 4.82 9.90 9.06 5.65 6.90 8.90 3.28 5.66 5.94 5.96 7.90 5.11 4.51 10.8 5.35 5.19 11.0
0 0 0 0 0 0 0 0 0 0 0 0 0 0
11.2
7.30 7.11 3.86 7.41 7.41 7.42 4.66 6.20 9.74 2.32 11.0 2.49
316 0 0 0 0 0 0 0
524 0 0 0
[He], I,, mTorr/ Torr min 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
590 590 0 0
608 588
584
0
0
631 0 0 0
625 613 615 627 627
0 0
0
641
638 618
0 0
0 0 0 0 0
597 638 610 605 606 0 605
639 0
The values of @'(C02) are widely scattered, probably because of the low COz yields in general, and the fact that there may be several sources of C02. a(C02)varies from 0.36 to 0.13 a t room temperature and from 0.37 to 0.02 a t -7 OC. It was observed that the CHzO tended to polymerize on the walls of the reaction vessel. Thus irradiation done in the absence of added CH2O also gave some products. In one run @{COz)= 0.49 and @{CO)= 1.55. However in the presence of gaseous CH20, all the chlorine atoms produced in the photolysis should react with gaseous CH20 and the wall reaction cm3/d5 and is ignored ( h for C1 CH4 = (1.5 f 0.1) X the reaction of C1 with CH2O should be faster). Some products were also found in dark runs. The yields presented in Tables I1 and I11 are corrected accordingly. Usually the time necessary to add the gases to the cell and analyze the products was comparable or greater than the irradiation time. Therefore no attempt was made to measure the dark yield per unit time.
+
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
20.8 22.2 22.0 21.0 24.4 21.8 18.7 20.2 20.4 21.2 20.8 23.2 22.4 23.3 23.6 19.7 21.6 23.2 75.4 24.4 17.4 20.1 22.8 60.4 18.0 33.0 26.0 19.2 8.4 20.5 21.8 22.2 22.3 20.6 25.8 27.5 51.4 34.0 17.9 20.2 20.5 26.9 62.9
Irradiation time, min 2.0 4.0 6.5 3.0 9.45 5.0 17.5 5.0 4.0
4.0 4.0 3.5 11.0 6.25 3.5 4.0 2.0 4.0 1.0
2.0 10.0 10.0
17.5 2.75 4.0 1.5 8.0 2.0 4.0 7.0 2.0 12.5 4.0 2.0 2.0 0.75 0.25 2.0 1.75 0.75 0.75 5.0 0.25
+{CO)
O{HCOOH)
+{COz)
10.8 1.63 1.87 1.58 1.86 1.91
9.0 10.3 8.50 11.1
0.16 2.15 2.10 2.20 2.08 2.10 2.08 2.24 1.34 2.98 3.27 2.87 2.20 2.51 3.17 3.04 3.33 3.18 4.50 3.83 5.40 5.53 9.92 8.75 9.92 7.96 10.1
8.92 11.8 11.8 12.9 12.0 12.9 16.4 14.6 14.8 11.4
8.67 8.92 10.3 8.50 9.67 11.8 11.8
9.42 11.2 0.27 10.3 0.17 0.13 0.13 0.36 0.15 0.15 0.14 0.33
At room temperature the dark correction for CO was 40 mTorr when the 0 2 pressure was -600 Torr, 20 mTorr when the 0 2 pressure was -300 Torr, and no correction when the 0 2 pressure was I100 Torr. The maximum correction was 30%, but the vast majority of the corrections were less than 10%. The dark production of C02 a t room temperature was -2 mTorr. The COz measured after irradiation was usually between 10 and 20 mTorr, but the values ranged from 6.7 to 103 mTorr. No products from the dark could be seen by ir at room temperature. At -7 O C , the dark production of CO when the Cl2 pressure was -1.5 Torr and the 0 2 pressure was -600 Torr was -20 mTorr. At other 0 2 pressures, the dark production was from 5 to 10 mTorr. The maximum correction was 11% and most were -5%. However, when the Cl2 was -4.5 Torr, -50 to 70% of the CO found in the light runs was found in the dark
1529
Oxidation of HCO Radicals
TABLE 111: Product Quantum Yields in the Photolysis of Chlorine at 3660 A in the Presence of 02 and CHzO at -7 f 1°C
I,, mTorr/ Irradiation time, [Oz],Torr [CHzO],Torr [He], Torr
[Clz]/[Oz] [Clz],Torr
3.37 1.43 645 . 0.0022 9.80 609 1.33 0.0022 3.46 1.42 558 0.0023 2.94 628 1.45 0.0023 3.07 625 1.46 0.0023 9.80 1.41 0.0023 603 3.18 622 0.0024 1.48 7.65 338 1.16 0.0034 10.1 330 4.24 0.013 6.94 65.2 1.13 0.017 6.17 65.4 1.32 0.020 9.50 67.6 1.37 0.020 6.34 68.9 1.47 0.021 9.72 66.2 1.51 0.023 4.90 29.3 1.46 0.050 9.18 16.2 1.51 0.093 5.46 14.2 1.32 0.093 9.30 49.1 0.094 4.59 4.54 14.6 1.40 0.096 14.6 4.00 1.44 0.099 9.96 7.44 0.202 1.50 7.72 6.99 1.45 0.208 2.84 2.13 1.49 0.70 8.87 1.84 1.34 0.73 8.70 1.53 1.39 0.91 a Pressures reported are at room temperature.
0 0 0 0 0
0 0 0 0 275 276 350 269 262 309 314 314 288 321 240 326 324 329 324 323
runs. These CO yields were disregarded and are not reported here. The COSdark production at -7 “C was -2 mTorr. The COz measured after irradiation was usually less than 10 mTorr, but ranged from 3 to 63 mTorr. No HCOOH was ever produced in the dark. However, there was another product which was sometimes seen in both light and dark runs at -7 “C. Its largest absorption was at 1185and 1175 cm-l, and it also absorbs at 2865,2855,2800,2790,2780, 980,970,965,960, and 950 cm-l. (There may be more than one compound.) No correlation could be found between this “unknown” and the experimental parameters or product quantum yields. Discussion At 3660 A only the Clz absorbs and it photodecomposes to give chlorine atoms which can then react with CHzO Cl2
+ hu (3660 A)
-
C1+ CHzO
HC1+ HCO
The HCO can react with HCO
0 2
+0 2
+
(2)
+
+
8.85 8.33 8.24 9.28 8.66 9.37 9.64 7.19 26.5 7.18 8.35 9.37 13.3 9.69 8.86 9.90 7.90 30.0 8.95 8.75 9.50 9.50 9.40 9.04 9.06
8.0 8.0 12.0 8.0 8.0 8.0 8.0 16.0 3.0 60.0 17.1
HC03H
+
HC03
HO
11.5 9.33 8.25 8.66 9.71 11.8
11.2 6.77 10.8 13.7 9.46
3.17 3.75 4.12 3.4 3.94 4.18 6.30 6.22
5.0
11.2
+ HzCO
0.17 8.95 13.4 11.8
9.12
HC03H
HCOOH
+ CO
(1b)
+ COz
(14
+
0.11 0.13 0.03 0.08 0.04 0.03 0.15 0.08 0.06 0.09 0.07
9.9
6.2 6.46 9.37 9.46 8.83 13.4 15.1
- +
HC03
0.21
2.30 3.27 2.22 2.84 2.37 3.46 2.82 3.48
10.0 16.0 8.0 3.0 11.25 3.0 7.0 3.0 3.0 3.0 2.0 1.5 2.0
+ HCO
(1/2)02 on the wall
0.02 0.14 0.27 0.16 0.37 0.11
(4) (5)
There is no direct evidence for reaction 4. Performic acid was never observed in this study. However it was always a t least 45 min after the run that the ir spectrum was taken. I t was assumed that all the HC03H decomposed in that time. I t has been shown that for the analogous oxidation of CHsCHO, the sole initial acid is peracetic acid and it decomposes quantitatively to CH3COOH.10 We would expect the same behavior for HCO oxidation with the conversion of HCO3H to HCOOH being even faster. Performic acid has been observed to be much less stable than peracetic acid, its decomposition being appreciable even at 0 “C.ll Bone et a1.,12 however, in the thermal oxidation of CHzO reported observing peroxides, “one which behaved as though it were the performic acid”, and formic acid. Formic acid has also been seen by many other workers in the photooxidation of CH20.1,2J3J4 The HOz radical is removed by 2H02
HOz
@(CO) @(HCOOH) @{COz)
-
H2Oz
+0 2
which proceeds with a rate coefficient of cm3/s.15 The possible competing reaction
HCO Cl2 HClCO C1- HC1+ CO C1 (3a) Reaction l a should be pressure dependent at sufficiently low pressures, but a t the pressures used in our studies, there is no indication of a total pressure effect so we can consider reaction l a to be in the second-order region. The fates of the species produced from the HCO reactions are as follows: HC03 ultimately gives HCOOH. The route we favor is +
min
or Cl2
+
+
2C1 rate = I ,
min
HO2
+ CHzO
-
H2Oz
(6) k6
= 3.3 X 10-12
+ HCO
is so slow ( k = 1.7 X exp(-8000/RTJ cm3/s15) that it never makes more than an 8% contribution to @(CO]and usually much less. Since this is within the experimental uncertainty of the CO measurements, this reaction can be ignored. The OH radical reacts rapidly with HzCO OH + HzCO
+
HzO
+ HCO
(7)
The rate coefficient for the reaction of C1 with CH4 is k = (1.5 The Journal of Physical Chemistry, Vol. SO,No. 14, 1976
1530
T. L. Osif and J. Heicklen IO
l
~
l
~
l
l
l
.
l
l
~
l
)
,
I
I
(
-
90
8-
0 0
I
-
98-
f 0.1) X cm3/s.15 The rate coefficient k2 is probably cm3/s.15These rate faster. The rate coefficient k , = 1.4 X coefficients are so large that competing reactions can be ignored. Even the reaction of C1 with 0 2 plays no role since it is highly reversible
c1+ 0 2
7 s
ClO2
Finally the HClCO species is known to decompose readily to HC1 and C0.l6 To check that all the HClCO was decomposed before analysis at room temperature, the gases of two runs were allowed to remain in the photolysis cell for 1h after photolysis before the analysis was started. Normally, the time between the end of photolysis and the start of the analysis was less than 1min. If all the HClCO were not decomposed, these two runs should contain an excess of CO relative to the others. They did not. At -7 "C, the gases were permitted to warm for -25 min after photolysis before the analysis was started. The above mechanism predicts that @{CO)should continually increase with increasing [Cl2]/[02].Figures 1and 2 show The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
plots of @(C0)/2vs. [Cl2]/[0~]at room temperature and -7 "C, respectively, and it is clear that an upper limiting value is reached at each temperature. Consequently an additional terminating step is needed which is important a t high [Cl2]/ [O,]ratios but which is independent of I , or any of the individual reactant pressures. The indicated step is the analogous one to reaction la. HCO
+ Cl2
-
HCOCl2
-
termination
(3b)
The production of formyl chloride from the photolysis of Cl2 in the presence of formaldehyde was shown by Krauskopf and Ro1lefson.l' They reported quantum yields of -104 at total pressures of 30-60 Torr at 80 OC. Presumably, the product of the HCO Cl2 termination reaction postulated here is unstable under their conditions and thus would not be an effective termination, allowing larger quantum yields to be obtained in their system. The mechanism now predicts that at high values of [Clz]/ [Oz], the upper limiting value of @(C0)/2should be equal to
+
Oxidation of
io0
HCO Radicals
1531
c
TABLE IV: Summary of Rate Coefficient Ratios a t Room Temperature and -7 "C, and for Total Pressures of 704-62 and 688-344 Torr, respectively ksb/kib k 1alk Ib k I c l k Ib k3alk3b
6(+7, -2) 5 f l
10.19 -7.5
Figure 3. Log-log plot of (@(CO] 2)/(15 @{CO))vs. [Cl,]/[O,] in the photolysis of Cln in the presence of 02 and CHPO at room temperature (-23 O C ) and -7 OC.
All of the data obtained for @(HCOOH}were at low values of [C12]/[02], where the last term in eq I1 is negligible. The values for @{HCOOH)generally lie between 8 and 12 at -7 "C and room temperature, so that kla/klb = 5 f 1.The pressure ranged from 704 to 62 Torr at room temperature and from 688 to 344 Torr at -7 "C. The data for COz were badly scattered but at both temperatures they never exceeded 0.38, so that the upper limiting value for k l,/k 1b is 0.19 at both temperatures. The low yield of C02 is in agreement with work done by Carruthers and Norrishl and Horner et al.2who found low yields of C02 in the photochemical oxidation of formaldehyde. The rate coefficient ratios obtained in this study are summarized in Table IV. Finally we comment on the lack of pressure dependence on the ratio kI,/klb. If the pressure is sufficiently reduced, k l , must fall off and ultimately into the third-order regime. The fall off might be expected to start at -50 Torr, so that, because of the scatter in our data, we would not see it. At 1.5-5 Torr, where Washida et a1.6 worked, reaction l a would make a 15-47% contribution to reaction 1.Thus the value of 5.7 f 1.2 X cm3/s they report for klb would include a 31 f 16% contribution from reaction la. The regular trend of reaction l a may not have been seen.
k3a/k3b. The data in Figure 1indicate an upper limiting value of -7.5 for @(C0)/2a t high values of [Cl2]/[02]. In Figure 2, the data are much more limited, but the same upper limiting value for @{C0)/2is not unreasonable, and this is assumed to be the case. Application of the steady-state hypothesis to the mechanism gives
Acknowledgment. We wish to thank R. Simonaitis for useful discussions. This work was supported by the Atmospheric Sciences Section of the National Science Foundation through Grants No. GA-33446X and GA-42856 and by the Environmental Protection Agency through Grant No. R800874 for which we are grateful.
-
[HCO] = 21a/(klb[02]
-
+ k3b[C12])
and the quantum yields of each of the measured products should follow the rate laws:
Figure 3 shows log-log plots of the left-hand side of eq I vs. [C12]/[02] at room temperatures and -7 OC, where the values of ksa/k3b 7.5 is used to evaluate the ordinate. In Figure 3 the points at low and high values of [C12]/[02] are inaccurate because they are computed from small differences of similar numbers. Since the slope of the log-log plot is forced to unity, the dashed lines represent outside reasonable limits for the points a t [C12]/[02] between 0.02 and 0.6. From these lines outside limits to ksb/klb are found to be 4 and 13. The solid line represents the best fit to the data and gives k3b/klb = 6. Thus k3b/kib = 6 (+7, -2).
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References a n d Notes ( 1 ) J. E. Carruthers and R. G. W. Norrish, J. Chem. SOC., 1036 (1936). (2)E. C. A. Horner, D. W. G. Style, and D. Summers, Trans. faraday SOC.,50, 1201 (1954). (3)J. Peeters and G. Mahnen, Symp. (int.) Combust., [froc.], 14th, 1972, 133 (1973). (4)K. L. Demerjian,J. A. Kerr, and J. G. Calvert, Adv. Environ. Sci. Technoi., 4, 189 (1974). (5) N. Washida, R. I. Martinez, and K. D. Bayes, Z.Naturforsch. A, 29, 251 (1974). (6)J. G. Calvert and J. N. Pitts, Jr., "Photochemistry", Wiley, New York, N.Y., 1966,p 463. (7)R. Spence and W. Wild, J. Chem. SOC.,338 (1935). (8) R. Renaud and L. C. Leitch, Can. J. Chem., 32, 545 (1954). (9)A. S.Coolidge, J. Am. Chem. SOC.,50, 2166 (1928). (IO) J. Weaver, J. F. Meagher, R. Shortridge, and J. Heicklen, J. Photochem., 4,341 (1 975). (11) P. A . Giguereand A. W. Olmos, Can. J. Chem., 30, 821 (1952). (12)W. A. Bone and J. B. Gardner, Roc. R. SOC.London, Ser. A, 154, 297 (1936). (13)E. C. A. Horner and D. W. G. Style, Trans. faraday SOC., 50, 1197 (1954). (14)D. W. G. Style and D. Summers, Trans. Faraday SOC.,42, 338 (1946). (15) D. Garvin and R. F. Hampson, National Bureau of Standards Report NBSiR
74-430(1974). (16) E. Sanhueza, i. C. Hisatsune, and J. Heicklen, Presented at the 169th National Meeting of the American Chemical Society, Philadelphia, Pa., April,
1975. (17)K. B. Krauskopf and G. K. Rollefson, J. Am. Chem. SOC.,56, 2542 (1934).
The Journal offhysicai Chemistry, Voi. 80, No. 14, 1976
