FORMATION OF COS IN PHOTOLYSIS OF 0
3 IN
2935
LIQUID COS
Formation of Carbon Trioxide in the Photolysis of Ozone in Liquid Carbon Dioxide'" by W. B. DeMore and C. W. Jacobsenlb Jet Propulsion Laboratorv, California Institute of Technology, Pasadena, California 91 108 (Received Februarv 4, 1969)
The 2537-A photolysis of 03 in liquid COz and in liquid SFe has been investigated at about -45". In liquid COZ the quantum yield of O3 disappearance was 0.65 & 0.05, independent of the 02/03 ratio. By contrast, the 03 quantum yield in liquid SFawas much lower and declined with increasing Oz/03 ratios. The results of the photolysis of 03 in COZcan only be explained in terms of a COZmechanism. Relative rate measurementsindicate that COSis formed in the O(1D)-C02 reaction at a rate that is comparable to the reaction rate of O(lD) with propane and isobutane. cos reacts with neither O1nor Oz but apparently disappears by the reaction cos COa + 2c02 0 2 .
+
+
Introduction I n recent years there has been considerable interest in the possible formation of a new oxide of carbon, the C03 molecule. The most direct evidence has come from infrared studies of solid matrices a t low temperatures, in which absorption bands attributable to C 0 3 have been observede2 I n addition, results of the isotopic exchange of singlet atomic oxygen with COZ have suggested a C03 intermediateSa-6 The intervention of a long-lived COS complex has frequently been suggested to explain a loss of oxygen in Cot photolyand in related systems.'O I n the present experiments we have investigated COa formation by the photolysis of 0 8 in liquid COZand in liquid SFs containing COS at temperatures near -45". Under these circumstances the formation of COO can be detected through its effect on the quantum yields of O3 disappearance. I n this manner it has been possible to obtain information on the rate of COS formation relative to other reactions of O(lD).
The solutions were irradiated with a low-pressure Hg lamp through a methanol filter, so that the radiation absorbed by 0 3 was almost entirely at 2537 8. Quantum yield measurements were based on a liquid Oa-Nz actinometer, as previously described.12 The 0 3 concentrations were monitored by spectrophotometric measurements in the Hartley band. The 0 3 extinction coefficients were assumed to be the same as in liquid Ar, an assumption which is probably valid in view of the fact that the Hartley band extinction coefficients are nearly the same in the gas phase and in several low-temperature solvents.la
Results Photolysis of O3 in Liquid CO,. I n these experiments and in most of the others, the initial OSconcentrations M . The quantum yield of 0 3 photolywere about was 0.65 f 0.05 in liquid COz, and remained sis, 90,, constant throughout the course of the photolysis. The quantum yields were measured for approximately 15 intervals during the photolysis, and the total 0s de-
Experimental Section The methods and apparatus used in this work were similar to those of previous work on the photolysis of OSin liquid Ar.ll I n the present case the coolant was liquid COz, maintained at a pressure of about 115 psig, corresponding to a temperature of about -45". The COZ employed as a solvent for the O3 photolysis was Matheson Instrument grade, of 99.99% stated purity. Gas chromatographic analysis showed that the major impurities were air and CH,, both of which could be partially removed by freezing and pumping. It was shown in several ways (see Results and Discussion) that the rate of photolysis was in fact quite insensitive to impurities. The SFG was Matheson Tank grade, of 98% minimum purity. The SFG was further purified by passing through Ascarite to remove C 0 2 and by pumping off a large fraction of the condensed material to remove volatiles.
(1) (a) This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration. (b) Summer employee at the Jet Propulsion Laboratory, 1968. (2) (a) N . G. Moll, D . R. Clutter, and W. E. Thompson, J. Chem. Phys., 45,4479 (1966); (b) E. Weissberger, W. H.Breckenridge, and H. Taube, ibid., 47, 1764 (1967). (3) D . Katakis and H . Taube, ibid., 36,416 (1962). (4) H. Yamazaki and R.J. Cvetanovib, ibid., 40, 582 (1964). (5) D . L. Baulch and W. H. Breckenridge, Trans. Faraday SOC.,62, 2768 (1966). (6) P. Warneck, J. Chem. Phys., 41,3435 (1964). (7) A. Y. Ung and H. I. Schiff, Can. J . Chem., 44, 1981 (1966). (8) T . G. Slanger, J. Chem. Phys., 45,4127 (1966). (9) R . A. Young and A. Y. Ung, ibid., 44, 3038 (1966). (10) M. Clerc and A. Reiffsteck, ibid., 48,2799 (1968). (11) W. B . DeMore, ibid., 47, 2777 (1967), and previous references quoted therein. (12) 0. F. Raper and W. B. DeMore, ibid., 40,1053 (1964). (13) W. B. DeMore and 0. F. Raper, J. Phvs. Chem., 68,412 (1964).
Volume 78, Number 9
September 1969
W. B. DEMOREAND C. W. JACOBBEN
2936 composed was usually about 85%. Saturation of the solution with O2 at a partial pressure of about 3 atm did not affect @o$, Approximate solubility measurements showed that under these latter conditions the 0, concentration was at least 50 times greater than the 0s concentration. It was also found that GO, was unchanged when a large excess of 0 3 (about 0.1 M ) was photolyzed in the C02, and Goawas then measured near the end of the photolysis. The dark rate of 0 8 decomposition was zero. I n none of these experiments could any products be detected by chromatographic analysis of the cell contents following separation of the C02 solvent and any residual 03. Photolgsis of O3 in Liquid 8Fs. The main purpose of these experiments was to determine whether or not O3 photolysis in SFe could be explained in terms of an atomic oxygen mechanism in order to establish a basis of comparison for the GO2 experiments. The Os quantum yields in SF6were much lower than in COz and declined as photolysis proceeded. Table I ~
~
~~
Table I : Quantum Yields of OsPhotolysis in Liquid SFs a t -45"
Initial Ox, M x 104
11.1
8.8 6.0 3.1 2.4 a
Total photolyais time," seo
Oa decomposed,
%
700
6.5 18.4 29.5
980 1260 1743 2205 2643 3100 3476 140 140 140 200
39.0 47.2 59.1 68.1 75.1 81.6 86.9 8.7 8.5 11.3 16.7
140 420
Average *oa during photolysis interval
0.17 0.16 0.15 0.13 0.11 0.09 0.07
0.06 0.05 0.04
Relative Rates of O(lD) Reaction with COZand Alkanes. It has been shown in earlier work14 that relative rates of O(lD) reaction with different substrates can be measured in inert solvents by means of Os quantum yield measurements. Provided that spurious losses of Osdue to reactions of O(aP) or radicals are suppressed by 02,the O8quantum yields are given by
I n eq 1 k,, kd, and IC, represent the rate constants for O(lD) reaction with the substrate, O(lD> deactivation by the substrate, and O(lD>deactivation by the solvent, respectively. The quantity r#~ is the initial quantum yield of O(lD) formation for Os photolysis at 2537 8 in the solution. The reciprocal of the slope k,/k, thus provides a measure of the reactivity of a given substrate. I n the present experiments the solvent was SFO, and about O.l-l% 08 was added to scavenge O(aP). Under these circumstances, the first-order decay constant k, should reflect the deactivating properties of both the SFs and the added 0 2 . However, we have observed here arid in previous work in liquid argon14 that k, is not sensitive to the presence of Oz in this concentration range. Thus, despite the fact that SFe is a poor deactivator of O(lD) in the gas phase, the quenching of reactive O(lD) in the liquid must be attributed mainly to the SFs and not to the 02. The method of eq 1 was used to measure the rate of O(lD)-C02 reaction relative to the reactions of O(lD) with propane and isobutane. The results, shown in Figure 1, indicate that the reaction of O(lD) with C02 is about half as fast as reaction with the alkanes. Since there is evidence that O(lD) reacts with C3Hs at nearly the collision rate,15
0.18 0.12
0.09 0.09
15
Corrected where necessary for incomplete light absorption by
0s.
illustrates the change in Goaduring one experiment and also shows the initial values of Qjo, for experiments with different initial 03 concentrations. I n general, the initial quantum yields are highest for the highest initial O3 concentrations. One problem in these experiments is that the 0 2 concentration present as an impurity in the sF6 solvent is not negligible compared to the typical O3 concentrations used. For that reason it is imposratios with accuracy. sible to establish the initial 02/03 Pressurization of the solutions with 0 2 , which gives OZ/03 >> 1, reduced @os to a value of 0.004. Thus, in contrast to the results in liquid COZ,the 0 3 quantum yields in SF6show a dependence on the 0 2 / 0 3 ratio. The Journal of Physical Chemistry
01
0
I
I
5
10
[SUBSTRATE]-',
I
15
M-l
Figure 1. 0 3 quantum yield measurements used to measure the relative rates of O(Q) reaction with COz, C3H8, and i-CiH10 in liquid SFe. The solutions were saturated with 0%to suppress reactions of O(3P). (14) W. B. DeMore, J . Phys. Chem., 73, 391 (1969).
2937
FORMATION OF COBIN PHOTOLYSIS OF O8 IN LIQUIDCOZ these results imply a high collision efficiency for COS formation. It may be noted that the relative rates for propane and isobutane are consistent with the earlier observation14 that the rates of O(lD) reaction with alkanes in the condensed phase are proportional to the number of C-H bonds present. Spectrophotometric Observations in the Uv, Visible, and Infrared Regions. The absorption spectra of the Oa-C02 solutions in the 2200-8000-A region were scanned following partial and complete 0 8 decomposition. In no caEie was there evidence of new absorption. With sapphire windows on the cell and with SFe as the solvent, it was a,lso possible to search the infrared region from 2 to 5.5 p . Again, no new absorption was present following the photolysis of several O3-CO2 mixtures. These observations lead us to conclude that no longlived (greater than about 1 min) C03is produced in the solution.
Discussion The results have shown that 03 photolysis in liquid C02 differs strikingly from that in liquid SFe in two major respects: (1) the O3 quantum yield in COZ is much higher than in SF6; (2) there is no dependence of @oson the ratio 02/03 in COz, It can be further shown that the results in SFs are consistent with the expected atomic oxygen mechanism, involving both O(lD) and O(3P), but that the results on 08 photolysis in CO2 require the intermediacy of the COa molecule. Photolysis of pure ozone can be quite complex because of the participation of excited intermediates, decomposiwhich give rise to high quantum yields of 0% tion.16 However, in the limiting case of high dilution with inert gases, the excited particles are deactivatedle and the following simple mechanism is adequate.
+ hv +0 o + o2+ M -+ + n4 0 + os +202 9
03
--j
0 2
(2)
0 3
(3) (4)
This mechanism ignores any initial electronic excitation in the primary products 0 and O2because it is assumed that all excited particles are quenched. Under these circumstances, the O3 quantum yield is given by @ 0 3=
1
+
that the value 4 = 0.65 is indicated for the present experiments in C02. A priori estimation of the ratio k3/k4 for the liquid phase is difficult for several reasons. First, reaction 3 is almost certainly diffusion controlled, and also is a three-body process. I n regard to possible third-body effects, it is probable that the reaction is at or very near the limiting second-order region, where kd[M] >> k, (M E c02 or SFa).
0
+
kr 0 2 k.
kd
[MI
os*+0s
(6)
Klein and Herronl8 reported a value of 0.22 M-I for the ratio kd/kr, so that the condition k d [ A I ] = k , should be met a t M concentrations of about 4.5 M . I n support of this, Sauer and Dorfman19 noted a fall-off in the effective third-order rate constant for this reaction a t M (where M = Ar) concentrations of about 4 M . The C02 concentration in the liquid is about 27 M , so that the reaction should be fairly well into the second-order region. A second difficulty arises from the fact that the temperature coefficient of reaction 4 is not well established, with reported activation energies ranging from about 3 to 6 kcal/mol.*o At -45", this corresponds to an uncertainty of about lo3 in the estimated rate. Despite these problems, there is no reason to doubt that the condition k3 2 k4 will hold in the liquid phase, and therefore from eq 5 a dependence of @ 0 3on the 02/03ratio is expected for an atomic oxygen mechanism. As shown in the Results, this expectation was verified by the experiments in liquid SFe. The contrasting results in liquid CO2 thus argue strongly against any rate-determining role of O(3P) in that solvent. The rapid rate of 0s photolysis in COZ could, in principle, be due to the reaction
O(lD)
+ 03
--f
202
(7)
However, this would imply an unexpectedly long lifetime for O(lD) in the COZsolvent, compared to what has been observed in liquid Ar17 and liquid SFe in the present work. Also, gas-phase experiments1sshow that COZdeactivates O(lD) more rapidly than does SFe. The foregoing discussion seems to rule out any significant role of either reaction 4 or 7 in the 03-C02 photoly-
29 k3[02]/k4[031
I n this case 4 represents the total initial quantum yield of atomic oxygen production in reaction 2, and k3 and kq are the effective second-order rate constants for reactions 3 and 4, respectively. Equation 5 should be applicable to the photolysis of 0 3 in a truly inert solvent. The photolysis of O3 in liquid Ar" indicated a value of 9 near unity for that solvent. It will be shown in later paragraphs, however,
(15) K.F. Preston and R. J. Cvetanovi6, J . Chem. Phys., 45, 2888 (1966). (16) R. G.W.Norrish and R. P. Wayne, Proc. Roy. Soc., Ser. A , 288, 200 (1965). (17) W. B. DeMore and 0. F. Raper, J . Chem. Phys., 44, 1780 (1966). (18) F. S. Klein and J. T. Herron, ibid., 44, 3645 (1966). (19) M.C. Sauer and L. M. Dorfman, J . Amer. Chem. Sac., 87,3801 (1965). (20) H.I. Schiff, Abstracts of the Symposium on Laboratory Measurements of Aeronometric Interest, York University, Toronto, Canada, Sept 3-4, 1968,p 201. Volume 78, Number 9
September I969
2938
W. B, DEMOREAND C. W. JACOBSEN
sis and suggests that atomic oxygen disappears from the system by the reaction
O('D)
+ COz(+M) +COa(+M)
(8)
Perhaps the most convincing evidence for this hypothesis is found in the experiments of Figure 1, in which small concentrations of C02 were added to the SFe solvent. These experiments show that COz is about half as effective as C3HBand i-CeHlain removing O(ID) from the reaction mixture in liquid SF6. No possible impurity in the COZ,or any mechanism involving excited 0 8 as the reactive agent, could account for the high Os quantum yields in those experiments. This is particularly true in view of the fact that high 02 concentrations were present which would tend to suppress any possible chain decomposition of 0 3 . 1 1 The following mechanism accounts for the photolysis of 0 3 in liquid COz.
+ hv(2537 A) --+ + O(*D) O(lD) + COz + & * 'Icos + M Cos + cos 2c02 + oz 0 3
0 2
---3
(9) (8) (10)
Reaction 10 is the only path for cos loss which is consistent with all the experimental results. The observed independence of @08on either the 0 2 or 03 concentration argues against the participation of the following reactions.
cos + cos +
0 3
---+
0 2
coz + 202
(11)
c02
(12)
----f
+
0 3
Failure to observe reaction 12 is in conflict with the results of Weissberger, Brecltenridge, and Taube,zbwho found evidence for this reaction in the warm-up of solid matrices containing COS and 02. The reason for this discrepancy is not clear, but it may be pointed out that this reaction is spin-forbidden unless one of the products appears in an excited triplet state. I n earlier work, Raper and DeMoreZ1proposed the reaction
co3* + 0 2
----f
c02
+ os
(13)
as a contributing step in the photolysis of liquid 0 2 4 0 mixtures. In that case, however, there was reason to believe that the C03* was in a triplet excited state, so that a higher reactivity might be expected. The present observation that the reaction does not occur for C 0 3 formed by the O(lD) C02 reaction supports our contention that different electronic states of cos are involved in the two cases.
+
The Journal of Physical Chemistry
There is no evidence that COSremained stable in solution, and in particular the infrared observations did not show any absorption at or near the 2053-cm-l region where strong co3 absorption is believed to occur.2 The mechanism presented above for OS photolysis in Cog leads to the conclusion that Cp = +oa = 0.65. This value of #I is lower than that previously observed in liquid Ar17 and indicates that CO:! is somewhat more effective in quenching the initially photoexcited 0 2 . It may be noted in this connection that the earlier result: of TaubeZ2on the photolysis of 02 in water at 2537 A were also consistent with an initial 0 3 quantum yield near 0.6. Although we have written reaction 8 as an ordinary three-body reaction, a more general mechanism would include predissociation of the initially formed COa* as a competing step. Br
O(lD)
+ COz JICOa* -% COz + O(3P) (14) kr
COS*
+ nil -% cos+ M
(15)
The rate of Cog formation is
From the present results it is evident that in the liquid phase the term kd[M] is dominant in the denominator of eq 16, so that the effective rate constant is kf. This contrasts with the previous results on the formation of N2OZ3and C0212by the reactions of O(lD) in liquid Nzand liquid CO, respectively. I n both of these latter cases, it was found that predissociation of the initially formed collision complex was the major reaction. Equation 16 is based on a strong collision model, in which a single deactivating collision removes sufficient energy so that both ks and k , are effectively reduced to zero. I n the intermediate case of weaker collisions, k , may remain competitive with the term kd [IS1J following one or more collisions, in which case a more complex pressure dependence for the over-all COO formation would be found. This type of reaction has been discussed in more detail in a previous paper.12 (21) 0. F. Raper and W. B. DeMore, J. Chem. Phys., 40, 1047 (1964). (22) H. Taube, Trans. Faraday Soc., 53, 656 (1957). (23) W. B. DeMore and 0 . F. Raper, J. Chem. Phys., 37, 2048 (1962).
