- Email: [email protected]
PLP–LIF study of the reactions of chlorine atoms with C2H2, C2H4, and C3H6 in 2–100 Torr of N2 diluent at 295 K
Chemical Physics Letters 494 (2010) 174–178
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
PLP–LIF study of the reactions of chlorine atoms with C2H2, C2H4, and C3H6 in 2–100 Torr of N2 diluent at 295 K Erika Iwasaki a, Hitoshi Chiba a, Tomoki Nakayama a, Yutaka Matsumi a,*, Timothy J. Wallington b,** a b
Solar-Terrestrial Environment Laboratory and Graduate School of Science, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan Systems Analytics and Environmental Sciences Department, Research and Innovation Center, Ford Motor Company, Mail Drop RIC-2122, Dearborn, MI 48121-2053, USA
a r t i c l e
i n f o
Article history: Received 20 April 2010 In final form 9 June 2010 Available online 12 June 2010
a b s t r a c t Pulsed laser photolysis–laser-induced fluorescence (PLP–LIF) techniques were used to study the reactions of Cl(2P3/2) atoms with C2H2 (k1), C2H4 (k2), and C3H6 (k3) in 2–100 Torr of N2 diluent at 295 ± 2 K. The results are in good agreement with those from relative rate studies, improve our understanding of the pressure dependence of the title reactions, and indicate that when timescales for chlorine atom decay and regeneration are not decoupled the results from PLP–RF experiments do not provide reliable kinetic data for reactions of chlorine atoms with organic compounds. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction There is increasing recognition of the importance of chlorine atoms in atmospheric chemistry both in the marine boundary layer and in locations far from coastlines [1–4]. This recognition has led to interest in the reactions of chlorine atoms with organic species. The oxidation of unsaturated hydrocarbons contributes to ozone formation on local and regional scales [5–7]. A detailed understanding of the kinetics and mechanisms of reactions of chlorine atoms with unsaturated organic compounds is needed to assess their importance in atmospheric chemistry. The reactions of chlorine atoms with unsaturated hydrocarbons are of fundamental interest as they proceed via at least two competing mechanisms; addition to the unsaturated bond to give a haloalkyl radical, and hydrogen abstraction to give HCl and an alkyl radical. At sufficiently low pressures the kinetics of the reactions of chlorine atoms with unsaturated compounds are expected to become independent of total pressure reflecting the increasing dominance of the hydrogen abstraction channel(s) at low pressures. Experimental investigations of the fall-off behavior of these reactions are needed to better understand atmospheric chemistry and to provide data to compare with theoretical calculations to gain a better fundamental understanding of these reactions. A large body of relative rate data has been reported for the reactions of chlorine atoms with the three smallest unsaturated compounds (C2H2, C2H4, and C3H6) from several laboratories spanning the pressure range 0.2–5800 Torr of N2, or air, diluent at ambient temperature (Wallington et al. [8,9], Kaiser and Wallington
* Corresponding author. Fax: +81 533 89 5593. ** Corresponding author. E-mail addresses: [email protected] (Y. Matsumi), twalling@ford. com (T.J. Wallington). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.06.034
[10–12], Atkinson and Aschmann [13], Ezell et al. [14], Stutz et al. [15], Glowacki et al. [16], Coquet and Ariya [17], and CeaceroVega et al. [18]). There is excellent agreement between the results from the relative rate studies. In contrast to the situation with the relative rate measurements, there are only two absolute rate studies of these reactions and the results from one of these studies are inconsistent with the previous studies. Stutz et al. [15] used discharge flow–resonance fluorescence (DF–RF) techniques in 1 Torr of He and reported results for k2 and k3 which were consistent with those from relative rate studies. Albaladejo et al. [19] used a pulsed laser photolysis–resonance fluorescence (PLP–RF) system in 20–200 Torr of He to measure k3. In contrast to the results from relative rate experiments, Albaladejo et al. [19] did not observe any effect of total pressure on k3 suggesting that the reaction is at, or near, the high pressure limit in 200 Torr of He. Furthermore, the pressure independent value of k3 reported by Albaladejo et al. [19] is approximately a factor of two lower than the values reported in relative rate studies near atmospheric pressure [10,13–15,17,18]:
C2 H2 þ Clð2 P3=2 Þ ! products
ð1Þ
C2 H4 þ Clð2 P3=2 Þ ! products
ð2Þ
C3 H6 þ Clð2 P3=2 Þ ! products
ð3Þ
To resolve discrepancies in the kinetic database and place our understanding of the fall-off behavior on a firmer basis, we have conducted absolute rate experiments of the reactions of Cl(2P3/2) atoms with acetylene (C2H2), ethene (C2H4), and propene (C3H6) using a pulsed laser photolysis/laser-induced fluorescence (PLP– LIF) technique. Absolute rate coefficients for reactions (1)–(3) were determined in 2–100 Torr of N2 diluent at 295 ± 2 K. Our results
175
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
Pulsed laser photolysis/laser-induced fluorescence (PLP–LIF) techniques were used to determine the absolute rate coefficients for the reactions of Cl(2P3/2) atoms with C2H2, C2H4, and C3H6. Cl(2P3/2) atoms were monitored by one-photon laser-induced fluorescence (LIF) at 134.72 nm corresponding to the 3p5 2P3/2–3p44s 2 P3/2 transition. The experimental methods are discussed in detail elsewhere [20]. Chlorine atoms were generated by the pulsed 351 nm photolysis of molecular chlorine using a single XeF excimer laser (Lambda Physik, COMPex 102):
Cl2 þ hm ! Cl þ Cl
ð4Þ
The photolysis of molecular chlorine at 351 nm generates essentially exclusively spin–orbit ground Cl(2P3/2) atoms [21] and the LIF signal intensity at 135.17 nm (3p5 2P1/2–3p44s 2P1/2 transition) from spin–orbit excited Cl*(2P1/2) is negligible compared to that 134.72 nm from spin–orbit ground state Cl(2P3/2) atoms [20]. From the absorption cross section of molecular chlorine at 351 nm [22] and the power density of photolysis laser, the initial Cl(2P3/2) atom concentration was estimated to be in the range (0.06–1.20) 1011 cm3. Tunable probe vacuum ultraviolet (VUV) light near 135 nm was generated by two-photon resonant four-wave difference frequency mixing in Kr gas (ca. 35 Torr). Using one dye laser (Lambda Physik, Scanmate 2E), 212.56 nm (x1) corresponding to two-photon resonance transition to Kr 5p[1/2]0 level was generated after passing through the b-BaB2O4 crystal. The subsequent fluorescence transition near 503 nm (x2) was generated using the other dye laser (Lambda Physik, FL3002). These tunable laser beams simultaneously pumped by a single XeCl excimer laser (Lambda Physik, COMPex 201) were spatially overlapped, and were focused into a Kr cell with a silica lens (f = 200). After passing through a MgF2 window mounted on the exit side of Kr cell, the coherent VUV light (with frequency xVUV = 2x1–x2) entered the reaction chamber. The fluorescence from Cl(2P3/2) atoms at 134.72 nm was detected by a solar blind photomultiplier (EMR, 542J-09-17), with a KBr photocathode sensitive to 118–150 nm, mounted perpendicular to the direction of probe and photolysis laser beams. The 351 nm photolysis laser beam and the VUV laser beam crossed perpendicularly in the reaction chamber. The velocity components of thermalized Cl atoms were reflected in its Doppler shifts along with the propagation direction of the probe laser beam [23]. It was observed that the translational energy distribution of Cl atoms was collisionally-relaxed by N2 buffer gas within 5 ls. Kinetic data for reactions (1)–(3) were obtained from the decays of the chlorine atom LIF signal at times >5 ls after the pulsed laser photolysis of Cl2. The temporal profiles of Cl(2P3/2)LIF signals were constructed by varying the delay time between the photolysis and probe lasers using a digital delay generator (Stanford Research, DG535), and the time jitter of was controlled within 20 ns. The probe and photolysis lasers were operated at a repetition rate of 10 Hz. The contents of the reaction chamber were replenished continuously by flowing reactant mixtures through the chamber. Total pressure in the reaction chamber was monitored using a capacitance manometer (MKS Baratron, Model 122AA-00010AB for 2– 10 Torr; Model 626A12TAE for 20–100 Torr). All gas flows were controlled using calibrated mass flow controllers (Horiba STEC, SEC-400MARK3). The reactant gas concentrations in the reaction
3. Results Kinetic data for reactions (1)–(3) were obtained by monitoring the decay of Cl(2P3/2)-LIF signal in the presence of an excess of C2H2, C2H4, or C3H6. Fig. 1 shows a typical Cl(2P3/2)-LIF decay obtained following the 351 nm pulsed laser irradiation of a mixture of 0.14 mTorr Cl2 and 35.5 mTorr C3H6 in 4.97 Torr of N2. The insert in Fig. 1 shows the decay in a semi-log format. The lines through the data in Fig. 1 are first-order decay fits to the data. In all experiments the decays of the Cl(2P3/2)-LIF signal followed pseudo-firstorder kinetics. The slopes of lines such as that fitted to the data shown in the insert in Fig. 1 give pseudo-first-order rate coefficients, k0 . Fig. 2 shows typical plots of the pseudo-first-order rate coefficients, k0 , versus the concentration of C2H4 observed following the photolysis of Cl2 in the presence of C2H4 in either 5, 10, or 20 Torr of N2 diluent. The lines through the data in Fig. 2 are linear least-squares fits which give second order rate coefficients of k2 = (2.54 ± 0.40) 1012, (3.96 ± 0.81) 1012, (7.64 ± 1.14) 1012 cm3 molecule1 s1. Quoted uncertainties include two-standard deviations from the linear regression analysis and estimated uncertainty in the reactant concentrations. The linear least-squares fits in Fig. 2 extrapolate through the origin suggesting that secondary loss of chlorine atoms via diffusion and reactions with impurities in the diluent gas are not significant complications in the present work. The rate coefficients obtained for k1, k2, and k3 in this
1.0
Log10(LIF intensity)
2. Experimental
chamber were estimated from the flow rates and the total pressure. Experiments were conducted at 295 ± 2 K, at five pressures (2, 5, 10, 20, 100 Torr) for propene (C3H6), and at three pressures (5, 10, 20 Torr) for ethene (C2H4), and acetylene (C2H2). C2H2, C2H4, and C3H6 reagents diluted with N2 were prepared and stored in a 10-L cylinder. Cl2 was diluted with N2 and stored in 10-L glass-bulbs which were blackened to avoid any photochemistry. Initial reagents concentrations were 0.05–1.05 mTorr Cl2 and either 40.5–233 mTorr C2H2, 41.6–164 mTorr C2H4, or 2.10–163 mTorr C3H6 in 1.96–100 Torr of N2 diluent. The purities of the reagent gases were: Cl2, >99% (Sumitomo Seika Co.); C3H6, >99% (Taiyo Nippon Sanso Co.); C2H4, >99% (Taiyo Nippon Sanso Co.); C2H2, >99% (Taiyo Nippon Sanso Co.); and N2, >99.999% (Nihon Sanso). All gases were used as received without further purification.
100
LIF intensity / arb. units
suggest that the data for k3 reported by Albaladejo et al. [19] are erroneous. The implications for the kinetic database of chlorine atom reactions with organic compounds are discussed.
10-1
10-2 0
0.5
10
20
30
Delay time / µs
40
0.0 0
10
20
30
40
50
Delay time / µs Fig. 1. Cl(2P3/2) decay profile observed in an experiment using a mixture of 0.14 mTorr Cl2 and 35.5 mTorr C3H6 in 4.97 Torr of N2 diluent at 295 ± 2 K. The insert is a logarithmic plot demonstrating the simple exponential decay. The lines through the data are least-squares fits assuming first-order kinetics.
176
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
4
1000
-1
-1
cm molecule s )
5 Torr 10 Torr 20 Torr
3
2
100
1
0
0
1
2
3
4
5
6
-3
[C2H4] (1015 molecule cm ) Fig. 2. Pseudo-first-order loss of Cl(2P3/2) atoms versus [ethene] for experiments in 5 (inverted triangles), 10 (circles) or 20 Torr (diamonds) of nitrogen diluent. The uncertainties indicated for the data points are ±2 standard deviations from firstorder regression analyses such as that shown in Fig. 1. The lines through the data are least-squares fits.
k(Cl+C2H2) (10
-13
k' (104 s-1)
3
10 Wallington et al. [9], N 2 (RR) Kaiser and Wallington [11], N 2 (RR) This work, N2 (PLP-LIF)
1 1
10
100
1000
10000
Pressure (Torr) Fig. 3. Reported rate coefficients for the reaction of chlorine atoms with acetylene at room temperature as a function of total pressure from relative (open symbols) and absolute rate experiments (filled symbols). RR, relative rate and PLP–LIF, pulsed laser photolysis/laser-induced fluorescence.
study are given in Table 1 and are compared with the literature data in Figs. 3–5. The rate constant ratios reported in the relative rate studies given in Figs. 3–5 have been placed on an absolute basis using k(Cl + C2H6) = 5.9 1011 [24], k(Cl + C2H5Cl) = 8.04 1012 [25], k(Cl + CH4) = 1.0 1013 [24], k(Cl + CH3Cl) = 4.8 1013 [26], k(Cl + CH3OCHO) = 1.4 1012, k(Cl + CHCl3) = 1.1 1013 [26], k(Cl + HC(CH3)3 = 1.51 1010 [27], k(Cl + nC4H10) = 2.05 1010 [24], k(Cl + n-C6H14) = 3.3 1010 [27], and k(Cl + n-C7H16) = 3.8 1010 cm3 molecule1 s1 [27].
4. Discussion As seen from inspection of Table 1 and Figs. 3–5, the kinetics of the title reactions showed a significant dependence on total pressure over the range studied. As evident in Figs. 3–5, the values of k1, k2, and k3 measured using an absolute method in the present study are in excellent agreement with the results from the relative rate studies by Wallington et al. [9], Kaiser and Wallington [10–12] and Glowacki et al. [16]. The significant dependence of k3 on pressure observed in the present work is consistent with the results
Table 1 Measured absolute rate coefficients for the reaction of Cl(2P3/2) atoms with C2H2, C2H4, and C3H6 in nitrogen diluent at 295 ± 2 K. Species
a b
Total pressurea
Rate coefficientb
C2H2
5 10 20
(1.02 ± 0.35) 1012 (2.06 ± 0.40) 1012 (3.29 ± 0.60) 1012
C2H4
5 10 20
(2.54 ± 0.40) 1012 (3.96 ± 0.81) 1012 (7.64 ± 1.14) 1012
C3H6
2 5 10 20 100
(5.29 ± 0.82) 1011 (7.19 ± 1.13) 1011 (8.70 ± 1.45) 1011 (1.01 ± 0.17) 1010 (1.63 ± 0.26) 1010
Units of Torr. Units of cm3 molecule1 s1.
Fig. 4. Reported rate coefficients for the reaction of chlorine atoms with ethene at room temperature as a function of total pressure from relative (open symbols) and absolute rate experiments (filled symbols). RR, relative rate; DF–RF, discharge flow– resonance fluorescence; and PLP–LIF, pulsed laser photolysis/laser-induced fluorescence.
from the relative rate studies but is inconsistent with the PLP–RF results from Albaladejo et al. [19]. As discussed by Albaladejo et al. [19], in their PLP–RF experiments the time scales for loss of chlorine atoms by reaction with propene and regeneration of chlorine atoms by reactions of the radical products of reaction (3) with Cl2 were similar. Consequently, Albaladejo et al. [19] observed non-exponential decays of chlorine atoms. Oxygen was added to scavenge the radicals in
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
Fig. 5. Reported rate coefficients for the reaction of chlorine atoms with propene at room temperature as a function of total pressure from relative (open symbols) and absolute rate experiments (filled symbols). RR, relative rate; DF–RF, discharge flow– resonance fluorescence; PLP–RF, pulsed laser photolysis–resonance fluorescence; and PLP–LIF, pulsed laser photolysis/laser-induced fluorescence.
the system and reduce the regeneration of chlorine atoms. Albaladejo et al. reported that after addition of O2 the decays of chlorine atoms followed first-order kinetics and assumed that values of k3 could be calculated without any corrections. The LIF detection system employed in the present work is more sensitive than the RF technique and we are able to use approximately 10 times lower concentrations of Cl2 and approximately 100 times larger concentrations of propene than used by Albaladejo et al. [19]. In our experiments we have essentially complete separation of the time scales for chlorine atom decay and regeneration. Hence, we are able to acquire kinetic data free from complications associated with regeneration. The simplest explanation for the discrepancy between the results of Albaladejo et al. [19] and those from the present work and the relative rate studies shown in Fig. 5 is the presence of complications associated with chlorine atom regeneration and additional consumption of chlorine atoms due to secondary reactions associated with the presence of O2 in the PLP–RF experiments. It is interesting to note that in the study of Albaladejo et al. [19] the reaction of chlorine atoms with isoprene was investigated and a value of k(Cl + isoprene) = (3.64 ± 0.20) 1010 cm3 molecule1 s1 was reported. This result is in excellent agreement with a value of k(Cl + isoprene) = (3.44 ± 0.32) 1010 cm3 molecule1 s1 measured using the PLP–LIF method [28]. Albaladejo et al. [19] noted that for the isoprene experiments ‘the observed chlorine atom decays were truly exponential’. This probably reflects a slower reaction of the resonance stabilized Cl-isoprene adducts with molecular chlorine which decouples the time scales for chlorine atom loss and regeneration. Clearly, in cases where regeneration is not a complication, and where O2 is not added to the reaction mixtures, the PLP–RF method can provide reliable kinetic data. However, in many cases where chlorine atom regeneration has been reported in PLP–RF experiments (e.g. in studies with methanol [29], ketones [20,30], unsaturated alcohols [31], acetates [32], and pentanal [33]) there are significant discrepancies between results from PLP–RF experiments and those from PLP–LIF and relative rate
177
experiments. We conclude that PLP–RF experiments where decay and regeneration occur on the same time scale do not provide reliable kinetic data for reactions of chlorine atoms with organic species. With the exception of the results from Albaladejo et al. [19], there is very good agreement between the results from absolute and relative rate experiments for reactions (1)–(3). Given this agreement we believe the kinetics of the reactions of chlorine atoms with C2H2, C2H4, and C3H6 at ambient temperature over the pressure range 1–1000 Torr are well established. Taking an average of the available data at 700–760 Torr pressure gives k1 = 5.1 1011, k2 = 8.9 1011, and k3 = 2.6 1010 which can be compared to k(OH + C2H2) = 7.8 1013, k(OH + C2H4) = 7.9 1012, and 11 3 k(OH + C3H6) = 2.9 10 cm molecule1 s1 [24]. At atmospheric pressure and 298 K the kCl/kOH rate coefficients ratios are 65, 11, and 9 for C2H2, C2H4, and C3H6, respectively. The atmospheric concentration of OH radicals is approximately 106 cm3 [34], the atmospheric concentration of chlorine atoms is highly variable ranging from <103 cm3 in the free troposphere [35] to 104– 105 cm3 in the marine boundary layer [36]. Hence, the [Cl]/[OH] concentration ratio varies from <0.001 to 0.01–0.1 depending on location. Comparison of the kCl/kOH and [Cl]/[OH] ratios indicates that reaction with chlorine atoms will not be a significant loss mechanism for C3H6 and C2H4 but might contribute to the loss of C2H2 in the atmosphere. Acknowledgements This study was partly supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists, and Grant-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. We thank Bill Kaiser for helpful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
A.A.P. Pszenny et al., Geophys. Res. Lett. 20 (1993) 699. R. Vogt, P.J. Crutzen, R. Sander, Nature 383 (1996) 327. C.T. Chang, T.H. Liu, F.T. Jeng, Environ. Res. 94 (2004) 67. J.A. Thornton et al., Nature 464 (2010) 271. W.P.L. Carter, J. Air Waste Manage. Assoc. 44 (1994) 881. R.G. Derwent, M.E. Jenkin, S.M. Saunders, M.J. Pilling, J. Air Waste Manage. Assoc. 51 (2001) 699. S.M. Saunders, M.E. Jenkin, R.G. Derwent, M.J. Pilling, Atmos. Chem. Phys. 3 (2003) 161. T.J. Wallington, L.M. Skewes, W.O. Siegl, J. Photochem. Photobiol. A: Chem. 45 (1988) 167. T.J. Wallington, J.M. Andino, I.M. Lorkovic, E.W. Kaiser, G. Marston, J. Phys. Chem. 94 (1990) 3644. E.W. Kaiser, T.J. Wallington, J. Phys. Chem. 100 (1996) 9788. E.W. Kaiser, T.J. Wallington, J. Phys. Chem. 100 (1996) 4111. E.W. Kaiser, T.J. Wallington, J. Phys. Chem. 102 (1998) 6054. R. Atkinson, S. Aschmann, Int. J. Chem. Kinet. 17 (1985) 33. M.J. Ezell, W. Weihong, A.A. Ezell, G. Soskin, B.J. Finlayson-Pitts, Phys. Chem. Chem. Phys. 4 (2002) 5813. J. Stutz, M.J. Ezell, A.A. Ezell, B.J. Finlayson-Pitts, J. Phys. Chem. A 102 (1998) 8510. D.R. Glowacki et al., Atmos. Chem. Phys. 7 (2007) 5371. S. Coquet, P.A. Ariya, Int. J. Chem. Kinet. 32 (2000) 478. A.A. Ceacero-Vega, B. Ballesteros, J. Albaladejo, I. Bejan, I. Barnes, Chem. Phys. Lett. 484 (2009) 10. J. Albaladejo, A. Notario, C.A. Cuevas, B. Ballesteros, E. Martinez, J. Atmos. Chem. 45 (2003) 35. K. Takahashi, E. Iwasaki, Y. Matsumi, T.J. Wallington, J. Phys. Chem. A 111 (2007) 1271. Y. Matsumi, K. Tonokura, M. Kawasaki, J. Chem. Phys. 97 (1992) 1065. R. Atkinson et al., Atmos. Chem. Phys. 7 (2007) 987. R.N. Zare, D.R. Herschbach, Proc. IEEE 51 (1963) 173. R. Atkinson et al., Atmos. Chem. Phys. 6 (2006) 3625. P.H. Wine, D.H. Semmes, J. Phys. Chem. 87 (1983) 3572. R. Atkinson et al., Atmos. Chem. Phys. 8 (2008) 4141. J.G. Calvert, R.G. Derwent, J.J. Orlando, G.S. Tyndall, T.J. Wallington, Mechanism of Atmospheric Oxidation of the Alkanes, Oxford University Press, 2008. J.-H. Xing, K. Takahashi, M.D. Hurley, T.J. Wallington, Chem. Phys. Lett. 472 (2009) 39.
178
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
[29] E.W. Kaiser, T.J. Wallington, Int. J. Chem. Kinet. 42 (2009) 113. [30] K. Takahashi, E. Iwasaki, Y. Matsumi, T.J. Wallington, J. Phys. Chem. A 111 (2007) 8044. [31] K. Takahashi, J.-H. Xing, M.D. Hurley, T.J. Wallington, J. Phys. Chem. A 114 (2010) 4224. [32] J.-H. Xing, K. Takahashi, M.D. Hurley, T.J. Wallington, Chem. Phys. Lett. 474 (2009) 268.
[33] E. Iwasaki, T. Nakayama, Y. Matsumi, K. Takahashi, T.J. Wallington, M.D. Hurley, E.W. Kaiser, J. Phys. Chem. A 112 (2008) 1741. [34] R.G. Prinn et al., Science 292 (2001) 1882. [35] J. Rudolph, R. Koppmann, Ch. Plass-Dülmer, Atmos. Environ. 30 (1996) 1887. [36] O.W. Wingenter, B.C. Sive, N.J. Blake, D.R. Blake, F.S. Rowland, J. Geophys. Res. 110 (2005) D20308.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
PLP–LIF study of the reactions of chlorine atoms with C2H2, C2H4, and C3H6 in 2–100 Torr of N2 diluent at 295 K Erika Iwasaki a, Hitoshi Chiba a, Tomoki Nakayama a, Yutaka Matsumi a,*, Timothy J. Wallington b,** a b
Solar-Terrestrial Environment Laboratory and Graduate School of Science, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan Systems Analytics and Environmental Sciences Department, Research and Innovation Center, Ford Motor Company, Mail Drop RIC-2122, Dearborn, MI 48121-2053, USA
a r t i c l e
i n f o
Article history: Received 20 April 2010 In final form 9 June 2010 Available online 12 June 2010
a b s t r a c t Pulsed laser photolysis–laser-induced fluorescence (PLP–LIF) techniques were used to study the reactions of Cl(2P3/2) atoms with C2H2 (k1), C2H4 (k2), and C3H6 (k3) in 2–100 Torr of N2 diluent at 295 ± 2 K. The results are in good agreement with those from relative rate studies, improve our understanding of the pressure dependence of the title reactions, and indicate that when timescales for chlorine atom decay and regeneration are not decoupled the results from PLP–RF experiments do not provide reliable kinetic data for reactions of chlorine atoms with organic compounds. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction There is increasing recognition of the importance of chlorine atoms in atmospheric chemistry both in the marine boundary layer and in locations far from coastlines [1–4]. This recognition has led to interest in the reactions of chlorine atoms with organic species. The oxidation of unsaturated hydrocarbons contributes to ozone formation on local and regional scales [5–7]. A detailed understanding of the kinetics and mechanisms of reactions of chlorine atoms with unsaturated organic compounds is needed to assess their importance in atmospheric chemistry. The reactions of chlorine atoms with unsaturated hydrocarbons are of fundamental interest as they proceed via at least two competing mechanisms; addition to the unsaturated bond to give a haloalkyl radical, and hydrogen abstraction to give HCl and an alkyl radical. At sufficiently low pressures the kinetics of the reactions of chlorine atoms with unsaturated compounds are expected to become independent of total pressure reflecting the increasing dominance of the hydrogen abstraction channel(s) at low pressures. Experimental investigations of the fall-off behavior of these reactions are needed to better understand atmospheric chemistry and to provide data to compare with theoretical calculations to gain a better fundamental understanding of these reactions. A large body of relative rate data has been reported for the reactions of chlorine atoms with the three smallest unsaturated compounds (C2H2, C2H4, and C3H6) from several laboratories spanning the pressure range 0.2–5800 Torr of N2, or air, diluent at ambient temperature (Wallington et al. [8,9], Kaiser and Wallington
* Corresponding author. Fax: +81 533 89 5593. ** Corresponding author. E-mail addresses: [email protected] (Y. Matsumi), twalling@ford. com (T.J. Wallington). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.06.034
[10–12], Atkinson and Aschmann [13], Ezell et al. [14], Stutz et al. [15], Glowacki et al. [16], Coquet and Ariya [17], and CeaceroVega et al. [18]). There is excellent agreement between the results from the relative rate studies. In contrast to the situation with the relative rate measurements, there are only two absolute rate studies of these reactions and the results from one of these studies are inconsistent with the previous studies. Stutz et al. [15] used discharge flow–resonance fluorescence (DF–RF) techniques in 1 Torr of He and reported results for k2 and k3 which were consistent with those from relative rate studies. Albaladejo et al. [19] used a pulsed laser photolysis–resonance fluorescence (PLP–RF) system in 20–200 Torr of He to measure k3. In contrast to the results from relative rate experiments, Albaladejo et al. [19] did not observe any effect of total pressure on k3 suggesting that the reaction is at, or near, the high pressure limit in 200 Torr of He. Furthermore, the pressure independent value of k3 reported by Albaladejo et al. [19] is approximately a factor of two lower than the values reported in relative rate studies near atmospheric pressure [10,13–15,17,18]:
C2 H2 þ Clð2 P3=2 Þ ! products
ð1Þ
C2 H4 þ Clð2 P3=2 Þ ! products
ð2Þ
C3 H6 þ Clð2 P3=2 Þ ! products
ð3Þ
To resolve discrepancies in the kinetic database and place our understanding of the fall-off behavior on a firmer basis, we have conducted absolute rate experiments of the reactions of Cl(2P3/2) atoms with acetylene (C2H2), ethene (C2H4), and propene (C3H6) using a pulsed laser photolysis/laser-induced fluorescence (PLP– LIF) technique. Absolute rate coefficients for reactions (1)–(3) were determined in 2–100 Torr of N2 diluent at 295 ± 2 K. Our results
175
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
Pulsed laser photolysis/laser-induced fluorescence (PLP–LIF) techniques were used to determine the absolute rate coefficients for the reactions of Cl(2P3/2) atoms with C2H2, C2H4, and C3H6. Cl(2P3/2) atoms were monitored by one-photon laser-induced fluorescence (LIF) at 134.72 nm corresponding to the 3p5 2P3/2–3p44s 2 P3/2 transition. The experimental methods are discussed in detail elsewhere [20]. Chlorine atoms were generated by the pulsed 351 nm photolysis of molecular chlorine using a single XeF excimer laser (Lambda Physik, COMPex 102):
Cl2 þ hm ! Cl þ Cl
ð4Þ
The photolysis of molecular chlorine at 351 nm generates essentially exclusively spin–orbit ground Cl(2P3/2) atoms [21] and the LIF signal intensity at 135.17 nm (3p5 2P1/2–3p44s 2P1/2 transition) from spin–orbit excited Cl*(2P1/2) is negligible compared to that 134.72 nm from spin–orbit ground state Cl(2P3/2) atoms [20]. From the absorption cross section of molecular chlorine at 351 nm [22] and the power density of photolysis laser, the initial Cl(2P3/2) atom concentration was estimated to be in the range (0.06–1.20) 1011 cm3. Tunable probe vacuum ultraviolet (VUV) light near 135 nm was generated by two-photon resonant four-wave difference frequency mixing in Kr gas (ca. 35 Torr). Using one dye laser (Lambda Physik, Scanmate 2E), 212.56 nm (x1) corresponding to two-photon resonance transition to Kr 5p[1/2]0 level was generated after passing through the b-BaB2O4 crystal. The subsequent fluorescence transition near 503 nm (x2) was generated using the other dye laser (Lambda Physik, FL3002). These tunable laser beams simultaneously pumped by a single XeCl excimer laser (Lambda Physik, COMPex 201) were spatially overlapped, and were focused into a Kr cell with a silica lens (f = 200). After passing through a MgF2 window mounted on the exit side of Kr cell, the coherent VUV light (with frequency xVUV = 2x1–x2) entered the reaction chamber. The fluorescence from Cl(2P3/2) atoms at 134.72 nm was detected by a solar blind photomultiplier (EMR, 542J-09-17), with a KBr photocathode sensitive to 118–150 nm, mounted perpendicular to the direction of probe and photolysis laser beams. The 351 nm photolysis laser beam and the VUV laser beam crossed perpendicularly in the reaction chamber. The velocity components of thermalized Cl atoms were reflected in its Doppler shifts along with the propagation direction of the probe laser beam [23]. It was observed that the translational energy distribution of Cl atoms was collisionally-relaxed by N2 buffer gas within 5 ls. Kinetic data for reactions (1)–(3) were obtained from the decays of the chlorine atom LIF signal at times >5 ls after the pulsed laser photolysis of Cl2. The temporal profiles of Cl(2P3/2)LIF signals were constructed by varying the delay time between the photolysis and probe lasers using a digital delay generator (Stanford Research, DG535), and the time jitter of was controlled within 20 ns. The probe and photolysis lasers were operated at a repetition rate of 10 Hz. The contents of the reaction chamber were replenished continuously by flowing reactant mixtures through the chamber. Total pressure in the reaction chamber was monitored using a capacitance manometer (MKS Baratron, Model 122AA-00010AB for 2– 10 Torr; Model 626A12TAE for 20–100 Torr). All gas flows were controlled using calibrated mass flow controllers (Horiba STEC, SEC-400MARK3). The reactant gas concentrations in the reaction
3. Results Kinetic data for reactions (1)–(3) were obtained by monitoring the decay of Cl(2P3/2)-LIF signal in the presence of an excess of C2H2, C2H4, or C3H6. Fig. 1 shows a typical Cl(2P3/2)-LIF decay obtained following the 351 nm pulsed laser irradiation of a mixture of 0.14 mTorr Cl2 and 35.5 mTorr C3H6 in 4.97 Torr of N2. The insert in Fig. 1 shows the decay in a semi-log format. The lines through the data in Fig. 1 are first-order decay fits to the data. In all experiments the decays of the Cl(2P3/2)-LIF signal followed pseudo-firstorder kinetics. The slopes of lines such as that fitted to the data shown in the insert in Fig. 1 give pseudo-first-order rate coefficients, k0 . Fig. 2 shows typical plots of the pseudo-first-order rate coefficients, k0 , versus the concentration of C2H4 observed following the photolysis of Cl2 in the presence of C2H4 in either 5, 10, or 20 Torr of N2 diluent. The lines through the data in Fig. 2 are linear least-squares fits which give second order rate coefficients of k2 = (2.54 ± 0.40) 1012, (3.96 ± 0.81) 1012, (7.64 ± 1.14) 1012 cm3 molecule1 s1. Quoted uncertainties include two-standard deviations from the linear regression analysis and estimated uncertainty in the reactant concentrations. The linear least-squares fits in Fig. 2 extrapolate through the origin suggesting that secondary loss of chlorine atoms via diffusion and reactions with impurities in the diluent gas are not significant complications in the present work. The rate coefficients obtained for k1, k2, and k3 in this
1.0
Log10(LIF intensity)
2. Experimental
chamber were estimated from the flow rates and the total pressure. Experiments were conducted at 295 ± 2 K, at five pressures (2, 5, 10, 20, 100 Torr) for propene (C3H6), and at three pressures (5, 10, 20 Torr) for ethene (C2H4), and acetylene (C2H2). C2H2, C2H4, and C3H6 reagents diluted with N2 were prepared and stored in a 10-L cylinder. Cl2 was diluted with N2 and stored in 10-L glass-bulbs which were blackened to avoid any photochemistry. Initial reagents concentrations were 0.05–1.05 mTorr Cl2 and either 40.5–233 mTorr C2H2, 41.6–164 mTorr C2H4, or 2.10–163 mTorr C3H6 in 1.96–100 Torr of N2 diluent. The purities of the reagent gases were: Cl2, >99% (Sumitomo Seika Co.); C3H6, >99% (Taiyo Nippon Sanso Co.); C2H4, >99% (Taiyo Nippon Sanso Co.); C2H2, >99% (Taiyo Nippon Sanso Co.); and N2, >99.999% (Nihon Sanso). All gases were used as received without further purification.
100
LIF intensity / arb. units
suggest that the data for k3 reported by Albaladejo et al. [19] are erroneous. The implications for the kinetic database of chlorine atom reactions with organic compounds are discussed.
10-1
10-2 0
0.5
10
20
30
Delay time / µs
40
0.0 0
10
20
30
40
50
Delay time / µs Fig. 1. Cl(2P3/2) decay profile observed in an experiment using a mixture of 0.14 mTorr Cl2 and 35.5 mTorr C3H6 in 4.97 Torr of N2 diluent at 295 ± 2 K. The insert is a logarithmic plot demonstrating the simple exponential decay. The lines through the data are least-squares fits assuming first-order kinetics.
176
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
4
1000
-1
-1
cm molecule s )
5 Torr 10 Torr 20 Torr
3
2
100
1
0
0
1
2
3
4
5
6
-3
[C2H4] (1015 molecule cm ) Fig. 2. Pseudo-first-order loss of Cl(2P3/2) atoms versus [ethene] for experiments in 5 (inverted triangles), 10 (circles) or 20 Torr (diamonds) of nitrogen diluent. The uncertainties indicated for the data points are ±2 standard deviations from firstorder regression analyses such as that shown in Fig. 1. The lines through the data are least-squares fits.
k(Cl+C2H2) (10
-13
k' (104 s-1)
3
10 Wallington et al. [9], N 2 (RR) Kaiser and Wallington [11], N 2 (RR) This work, N2 (PLP-LIF)
1 1
10
100
1000
10000
Pressure (Torr) Fig. 3. Reported rate coefficients for the reaction of chlorine atoms with acetylene at room temperature as a function of total pressure from relative (open symbols) and absolute rate experiments (filled symbols). RR, relative rate and PLP–LIF, pulsed laser photolysis/laser-induced fluorescence.
study are given in Table 1 and are compared with the literature data in Figs. 3–5. The rate constant ratios reported in the relative rate studies given in Figs. 3–5 have been placed on an absolute basis using k(Cl + C2H6) = 5.9 1011 [24], k(Cl + C2H5Cl) = 8.04 1012 [25], k(Cl + CH4) = 1.0 1013 [24], k(Cl + CH3Cl) = 4.8 1013 [26], k(Cl + CH3OCHO) = 1.4 1012, k(Cl + CHCl3) = 1.1 1013 [26], k(Cl + HC(CH3)3 = 1.51 1010 [27], k(Cl + nC4H10) = 2.05 1010 [24], k(Cl + n-C6H14) = 3.3 1010 [27], and k(Cl + n-C7H16) = 3.8 1010 cm3 molecule1 s1 [27].
4. Discussion As seen from inspection of Table 1 and Figs. 3–5, the kinetics of the title reactions showed a significant dependence on total pressure over the range studied. As evident in Figs. 3–5, the values of k1, k2, and k3 measured using an absolute method in the present study are in excellent agreement with the results from the relative rate studies by Wallington et al. [9], Kaiser and Wallington [10–12] and Glowacki et al. [16]. The significant dependence of k3 on pressure observed in the present work is consistent with the results
Table 1 Measured absolute rate coefficients for the reaction of Cl(2P3/2) atoms with C2H2, C2H4, and C3H6 in nitrogen diluent at 295 ± 2 K. Species
a b
Total pressurea
Rate coefficientb
C2H2
5 10 20
(1.02 ± 0.35) 1012 (2.06 ± 0.40) 1012 (3.29 ± 0.60) 1012
C2H4
5 10 20
(2.54 ± 0.40) 1012 (3.96 ± 0.81) 1012 (7.64 ± 1.14) 1012
C3H6
2 5 10 20 100
(5.29 ± 0.82) 1011 (7.19 ± 1.13) 1011 (8.70 ± 1.45) 1011 (1.01 ± 0.17) 1010 (1.63 ± 0.26) 1010
Units of Torr. Units of cm3 molecule1 s1.
Fig. 4. Reported rate coefficients for the reaction of chlorine atoms with ethene at room temperature as a function of total pressure from relative (open symbols) and absolute rate experiments (filled symbols). RR, relative rate; DF–RF, discharge flow– resonance fluorescence; and PLP–LIF, pulsed laser photolysis/laser-induced fluorescence.
from the relative rate studies but is inconsistent with the PLP–RF results from Albaladejo et al. [19]. As discussed by Albaladejo et al. [19], in their PLP–RF experiments the time scales for loss of chlorine atoms by reaction with propene and regeneration of chlorine atoms by reactions of the radical products of reaction (3) with Cl2 were similar. Consequently, Albaladejo et al. [19] observed non-exponential decays of chlorine atoms. Oxygen was added to scavenge the radicals in
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
Fig. 5. Reported rate coefficients for the reaction of chlorine atoms with propene at room temperature as a function of total pressure from relative (open symbols) and absolute rate experiments (filled symbols). RR, relative rate; DF–RF, discharge flow– resonance fluorescence; PLP–RF, pulsed laser photolysis–resonance fluorescence; and PLP–LIF, pulsed laser photolysis/laser-induced fluorescence.
the system and reduce the regeneration of chlorine atoms. Albaladejo et al. reported that after addition of O2 the decays of chlorine atoms followed first-order kinetics and assumed that values of k3 could be calculated without any corrections. The LIF detection system employed in the present work is more sensitive than the RF technique and we are able to use approximately 10 times lower concentrations of Cl2 and approximately 100 times larger concentrations of propene than used by Albaladejo et al. [19]. In our experiments we have essentially complete separation of the time scales for chlorine atom decay and regeneration. Hence, we are able to acquire kinetic data free from complications associated with regeneration. The simplest explanation for the discrepancy between the results of Albaladejo et al. [19] and those from the present work and the relative rate studies shown in Fig. 5 is the presence of complications associated with chlorine atom regeneration and additional consumption of chlorine atoms due to secondary reactions associated with the presence of O2 in the PLP–RF experiments. It is interesting to note that in the study of Albaladejo et al. [19] the reaction of chlorine atoms with isoprene was investigated and a value of k(Cl + isoprene) = (3.64 ± 0.20) 1010 cm3 molecule1 s1 was reported. This result is in excellent agreement with a value of k(Cl + isoprene) = (3.44 ± 0.32) 1010 cm3 molecule1 s1 measured using the PLP–LIF method [28]. Albaladejo et al. [19] noted that for the isoprene experiments ‘the observed chlorine atom decays were truly exponential’. This probably reflects a slower reaction of the resonance stabilized Cl-isoprene adducts with molecular chlorine which decouples the time scales for chlorine atom loss and regeneration. Clearly, in cases where regeneration is not a complication, and where O2 is not added to the reaction mixtures, the PLP–RF method can provide reliable kinetic data. However, in many cases where chlorine atom regeneration has been reported in PLP–RF experiments (e.g. in studies with methanol [29], ketones [20,30], unsaturated alcohols [31], acetates [32], and pentanal [33]) there are significant discrepancies between results from PLP–RF experiments and those from PLP–LIF and relative rate
177
experiments. We conclude that PLP–RF experiments where decay and regeneration occur on the same time scale do not provide reliable kinetic data for reactions of chlorine atoms with organic species. With the exception of the results from Albaladejo et al. [19], there is very good agreement between the results from absolute and relative rate experiments for reactions (1)–(3). Given this agreement we believe the kinetics of the reactions of chlorine atoms with C2H2, C2H4, and C3H6 at ambient temperature over the pressure range 1–1000 Torr are well established. Taking an average of the available data at 700–760 Torr pressure gives k1 = 5.1 1011, k2 = 8.9 1011, and k3 = 2.6 1010 which can be compared to k(OH + C2H2) = 7.8 1013, k(OH + C2H4) = 7.9 1012, and 11 3 k(OH + C3H6) = 2.9 10 cm molecule1 s1 [24]. At atmospheric pressure and 298 K the kCl/kOH rate coefficients ratios are 65, 11, and 9 for C2H2, C2H4, and C3H6, respectively. The atmospheric concentration of OH radicals is approximately 106 cm3 [34], the atmospheric concentration of chlorine atoms is highly variable ranging from <103 cm3 in the free troposphere [35] to 104– 105 cm3 in the marine boundary layer [36]. Hence, the [Cl]/[OH] concentration ratio varies from <0.001 to 0.01–0.1 depending on location. Comparison of the kCl/kOH and [Cl]/[OH] ratios indicates that reaction with chlorine atoms will not be a significant loss mechanism for C3H6 and C2H4 but might contribute to the loss of C2H2 in the atmosphere. Acknowledgements This study was partly supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists, and Grant-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. We thank Bill Kaiser for helpful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
A.A.P. Pszenny et al., Geophys. Res. Lett. 20 (1993) 699. R. Vogt, P.J. Crutzen, R. Sander, Nature 383 (1996) 327. C.T. Chang, T.H. Liu, F.T. Jeng, Environ. Res. 94 (2004) 67. J.A. Thornton et al., Nature 464 (2010) 271. W.P.L. Carter, J. Air Waste Manage. Assoc. 44 (1994) 881. R.G. Derwent, M.E. Jenkin, S.M. Saunders, M.J. Pilling, J. Air Waste Manage. Assoc. 51 (2001) 699. S.M. Saunders, M.E. Jenkin, R.G. Derwent, M.J. Pilling, Atmos. Chem. Phys. 3 (2003) 161. T.J. Wallington, L.M. Skewes, W.O. Siegl, J. Photochem. Photobiol. A: Chem. 45 (1988) 167. T.J. Wallington, J.M. Andino, I.M. Lorkovic, E.W. Kaiser, G. Marston, J. Phys. Chem. 94 (1990) 3644. E.W. Kaiser, T.J. Wallington, J. Phys. Chem. 100 (1996) 9788. E.W. Kaiser, T.J. Wallington, J. Phys. Chem. 100 (1996) 4111. E.W. Kaiser, T.J. Wallington, J. Phys. Chem. 102 (1998) 6054. R. Atkinson, S. Aschmann, Int. J. Chem. Kinet. 17 (1985) 33. M.J. Ezell, W. Weihong, A.A. Ezell, G. Soskin, B.J. Finlayson-Pitts, Phys. Chem. Chem. Phys. 4 (2002) 5813. J. Stutz, M.J. Ezell, A.A. Ezell, B.J. Finlayson-Pitts, J. Phys. Chem. A 102 (1998) 8510. D.R. Glowacki et al., Atmos. Chem. Phys. 7 (2007) 5371. S. Coquet, P.A. Ariya, Int. J. Chem. Kinet. 32 (2000) 478. A.A. Ceacero-Vega, B. Ballesteros, J. Albaladejo, I. Bejan, I. Barnes, Chem. Phys. Lett. 484 (2009) 10. J. Albaladejo, A. Notario, C.A. Cuevas, B. Ballesteros, E. Martinez, J. Atmos. Chem. 45 (2003) 35. K. Takahashi, E. Iwasaki, Y. Matsumi, T.J. Wallington, J. Phys. Chem. A 111 (2007) 1271. Y. Matsumi, K. Tonokura, M. Kawasaki, J. Chem. Phys. 97 (1992) 1065. R. Atkinson et al., Atmos. Chem. Phys. 7 (2007) 987. R.N. Zare, D.R. Herschbach, Proc. IEEE 51 (1963) 173. R. Atkinson et al., Atmos. Chem. Phys. 6 (2006) 3625. P.H. Wine, D.H. Semmes, J. Phys. Chem. 87 (1983) 3572. R. Atkinson et al., Atmos. Chem. Phys. 8 (2008) 4141. J.G. Calvert, R.G. Derwent, J.J. Orlando, G.S. Tyndall, T.J. Wallington, Mechanism of Atmospheric Oxidation of the Alkanes, Oxford University Press, 2008. J.-H. Xing, K. Takahashi, M.D. Hurley, T.J. Wallington, Chem. Phys. Lett. 472 (2009) 39.
178
E. Iwasaki et al. / Chemical Physics Letters 494 (2010) 174–178
[29] E.W. Kaiser, T.J. Wallington, Int. J. Chem. Kinet. 42 (2009) 113. [30] K. Takahashi, E. Iwasaki, Y. Matsumi, T.J. Wallington, J. Phys. Chem. A 111 (2007) 8044. [31] K. Takahashi, J.-H. Xing, M.D. Hurley, T.J. Wallington, J. Phys. Chem. A 114 (2010) 4224. [32] J.-H. Xing, K. Takahashi, M.D. Hurley, T.J. Wallington, Chem. Phys. Lett. 474 (2009) 268.
[33] E. Iwasaki, T. Nakayama, Y. Matsumi, K. Takahashi, T.J. Wallington, M.D. Hurley, E.W. Kaiser, J. Phys. Chem. A 112 (2008) 1741. [34] R.G. Prinn et al., Science 292 (2001) 1882. [35] J. Rudolph, R. Koppmann, Ch. Plass-Dülmer, Atmos. Environ. 30 (1996) 1887. [36] O.W. Wingenter, B.C. Sive, N.J. Blake, D.R. Blake, F.S. Rowland, J. Geophys. Res. 110 (2005) D20308.