J. Chim. Phys. (1998) 95, 513-522
O EDP Sciences. Les Ulls
Kinetic studies of CI atom reactions with series
of alkanes using the pulsed laser photolysis-resonance
fluorescence method
LCSWCNRS I C , avenue de la Recherche Scientifique,
45071 Orleans cedex 02. France
(Received 15 July 1997; accepted 21 November 1997)
Correspondence and reprints.
Les constantes de vitesse des reactions des atomes C1 avec CH4 (kl), C2& (k2), C3Hs
(k3), et n-C4Hlo (k4) ont Cte determinees par la methode de photolyse laser puldefluorescence resonante (PLP-FR). Ces reactions ont ktd etudiees 2i temperature
ambiante (297 2) K et 60 Torr d'helium. Les constantes de vitesse k2, k3 et b ont kt6
mesurees en presence d'oxygkne. Les valeurs obtenues sont kl = (1.07 0 . 0 3 ) ~ 1 0 ;- ~ ~
k2 = (6.5 + 0.3)x10-" ; k3 = (1.31 0 . 0 3 ) ~ 1 0 - ;' ~b = (2.25 h 0 . 0 6 ) ~ 1 0 '(en
~ ~ cm3
molecule-' dl). Ces rksultats sont compares avec ceux de la litterature et constituent
une validation de la methode PLP-FR rkcernrnent implantke dans notre laboratoire.
*
*
*
Mots-elks : Cinetique, constantes de vitesse, photolyse laser, fluorescence resonante,
atomes de chlore, alcanes.
ABSTRACT
The rate coefficients for the reactions of Cl atoms with CH4 (kl), C2& (k2), C3H8(k3),
and n-C4Hlo( b ) , have been determined using a newly built pulsed laser photolysisresonance fluorescence (PLP-RF) apparatus. The reactions were studied at room
temperature (297 2) K and P = 60 Torr of helium. The rate constants k2, k3 and k4
were measured in the presence of oxygen. The obtained values are: kl = (1.07
0 . 0 3 ) ~ 1 0 -;' ~k2 = (6.5 0.3)x10*" ;k3 = (1.31 0 . 0 3 ) ~ 1 0 -;' ~b = (2.25 0 . 0 6 ) ~ 1 0 - ' ~
(in cm3 molecule-' s"). These results are compared to the literature data and provide a
validation of the newly built PLP-RF apparatus in our laboratory.
*
*
*
*
*
Key words : Kinetics, rate constant, laser photolysis, resonance fluorescence, chlorine
atoms, alkanes.
514
A.
Mellouki
INTRODUCTION
Chlorine and bromine atoms have the potential to contribute significantly to the
chemistry of the atmosphere [I]. In the stratosphere, they are mainly released fiom the
photolysis of halogenated organic compounds and contribute to the chemistry
destroying ozone. In the troposphere, they may be produced fiom heterogeneous
reactions on sea salt [2, 31 and may play a role in the oxidation of organic compounds
and in the production of ozone. Hence, measuring accurately the rate coefficients of
the reactions involving C1 and Br atoms is very important in order to assess the role of
these reactions in atmospheric chemistry at global or regional scales.
We have set up recently in our laboratory a pulsed laser photolysis-resonance
fluorescence (PLP-RF) apparatus. The PLP-RF and the flash photolysis-resonance
fluorescence (FP-RF) methods are widely used to measure reaction rate coefficients of
OH(X~II)radicals [e.g. 4, 51 and atoms such as H(~s)[e.g. 6, 71, o(~P) [e.g. 8, 91,
o('D) [e.g. 10, 1 11, c ~ ( ~ P[e.g.
) 12, 131, B~('P) [e.g. 14, 151. The PLP-RF technique is
also used to measure the quantum yields of the above atomic species production in the
photodissociation of selected molecules (see for example [e.g. 16, 171).
There have been many studies of the reactions of chlorine atoms with methane
and ethane using both absolute and relative methods, while reactions of chlorine atoms
with propane and n-butane have been studied mainly by the relative rate method [18,
191. Only two absolute measurements of the rate coefficient for reaction of CI with
C3H8have been made and they differ by more than 20 % [20,21]. Concerning reaction
of C1 with n-C4Hlo,there have been three absolute measurements, which are in good
agreement [20-221. Besides their atmospheric relevance, the reactions of C1 with the
above alkanes are used as reference in the relative rate studies, therefore, it is
important to have precise values for the rate coefficients of these reactions.
To validate the newly built PLP-RF apparatus and to check its ability, for
measuring reactions rate coefficients in a large range, we have measured the rate
coefficients of the following reactions at 298 K, which extend fiom c.a.
to 2x10-'~
cm3 molecule*' s-' :
J Chim. Phys.
KINETIC STUDIES OF CI ATOM REACTIONS WITH SERIES OF ALKANES
+
515
HCl + CH3
(1)
C1-k c2H6
HCl + C2HS
@I
C1+ C3&
HC1 +C3H7
(3)
C1+ n-C4Hlo
HCl + C4H9
(4)
C1+ C&
The aim of this work was also to improve the kinetic data base for C1
+
hydrocarbon reactions. This paper describes our newly developed PLP-RF system and
the kinetic study of the reactions of chlorine atoms with the four alkanes. The obtained
results are compared to the literature data.
EXPERIMENTAL SECTION
The pulsed laser photolysis - resonance fluorescence (PLP-RF) apparatus used
in the present work is schematically shown in Figure 1. The reaction cell is constructed
-
of Pyrex and has an internal volume of about 200 em3. The radiation (h 135 nm)was
-
obtained fiom a microwave driven lamp, through which He containing 0.2 % of C12
was flowed at P
- 1.8 Tom. This radiation was used to excite resonance fluorescence
from C1 atoms in the cell. The lamp was situated perpendicular to the photolysis laser
beam. A CaF2 window was placed between the lamp and the cell. C1 atoms were
produced by photolyzing C12 at 355 nm. The source of the 355 nrn radiation was a
pulsed, frequency tripled, Nd : YAG laser (Surelite 11, from Continuum), the laser
beam has a pulsewidth of 4-6 ns (fill width at half maximum (fwhrn)) and a linewidth
of 0.002 nm. The resonance fluorescence was collected at 90° to both the resonance
lamp and photolysis laser beams by two CaF2 lenses and imaged onto the
photocathode of a solar blind photomultiplier tube (Hamamatsu R1459P). The regions
between the two lenses and between the last lens and the photomultiplier were
pumped.
Signals were obtained using photon counting techniques in conjunction with
multi-channel scaling. The fluorescence signal t?om the PMT was processed by a
digitizer (7803, Canberra) and sent to an EG&G multi-channel scaler (T914P) to
collect the time resolved signal. The multi-channel scaler was coupled to a
J Chrm. Phys
516
A Mellouki
microcomputer for imaging and analysis of the signals. Data acquisition was started
prior the photolysis pulse to obtain the background level of scattered light. A home
made delay generator pre-triggered the acquisition before triggering the laser pulse.
The background was subtracted &om the post photolysis signal to obtain the temporal
profile of the C1 atoms. The C1 temporal profiles following 5000 to 25000 laser pulses
were coadded to enhance the signal to noise ratio. The detection limit for Cl atoms
defined as S I ~ B= 1, (where S is the signal above the background, B) was around
8x1o9 atoms cm-3for 1 second integration.
Figure 1 : Schematic diagram of the pulsed laser photolysis-resonance fluorescence
experimental set up.
In order to avoid accumulation of photolysis or reaction products, the
experiments were carried out under slow flow conditions. The linear flow velocity
through the cell was in the range 2 to 4 cm s" and the repetition rate of the photolysis
KINETIC STUDIES OF CI ATOM REACTIONS WITH SERIES OF ALKANES
517
laser was 10 Hz. In these conditions, the gas mixture was flushed out from the
interaction zone before the next laser pulse arrived. The change of the repetition rate of
the photolysis laser to 5 Hz did not affect the obtained experimental data. Alkanes and
C12, were flowed from 10 liter bulbs containing dilute mixtures in helium.
Concentrations of the different gases in the cell were calculated from measurements of
the appropriate mass flow rates and the total pressure. The mass flow meters (models
FC 260 and FC 2901) and the pressure gauge (CDLC-31) were from Tylan General.
The main experimental error in the rate constants measurements comes fiom the
uncertainty in the calculated concentration of the alkanes. This later is estimated fiom
the uncertainties in the flow rates and pressure readings which are estimated to be 2%
and 1%, respectively.
The gases had the following purities : He (UHP certified to > 99.9995 %
(Alphagaz)), the carrier gas, was passed fiom tank to cell trough a liquid nitrogen trap,
C12 (99.8 %, UCAR) was degassed several times at 77 K before use ; 0 2 was certified
to > 99.995 % (Alphagaz). The alkanes were : CH4 (> 99.9 %, Alphagaz), C2&
(99.97 %, Alphagaz), C3Hs (> 99.5 %, Alphagaz), and n-C4Hlo (99.98 %, Alphagaz) ;
they were used as supplied.
RESULTS AND DISCUSSION
With a reaction mixture containing C12, the hydrocarbon (RH), helium, and in
the absence of secondary reactions, the temporal profile of C1 is governed by the
following reactions :
Cl+RH+R+HCI
(5)
C1+ loss by diffusion.
(6)
The experiments were carried out under pseudo-first-order conditions with the
concentration of the alkane in large excess over CI concentration. Typically, the initial
~.
these conditions, the
concentration of C1, [Cl], was around 5x10" atom ~ m ' Under
rate of disappearance of Cl atoms follows a simple exponential rate law :
[Cl], = [Cl], exp(-k't)
where k'
=k
[w+k,
A. Mellouki
51 8
where k' is the pseudo first order rate constant, k, the bimolecular rate constant for the
reaction of Cl with alkane, and [RH] the alkane concentration. The first order decay
rate of C1 in the absence of the alkane, k, is the diffusion rate
. . of C1 atoms out of the
detection zone. Under our experimental conditions, k,, was around 35 s-'. The
measured pseudo first order rate constant (k') for a particular experiment was obtained
fiom the slope of ln[CI] vs t plot. Values for k were then obtained fiom the slope of k '
vs [RH] plot.
The initial concentration of C1 used was around 3-5 x 10" molecule cm" in
order to get a sufficient signal to noise ratio. This concentration was obtained with a
C12 concentration of ca 3x10'~molecule cm-3 and 15 to 30 rnJ1pulse as photolysis
fluence at 355 nm. The excess concentration ranges of the alkanes were (in 1012
molecule cmV3) : (200 - 14000), (5 - 33), (4 - 3 I), (3.5 - 25), for C h , C2&, C3Hs,and
n-C4Hlo, respectively. The much lower concentration ranges to be used for C2&,
C3Hs, and n-C&Ilo compared to C h results fiom much higher rate coefficients of the
C1 reaction with C2H6, C3H8, and n - C a l o compared to C1 + C&. Under these
conditions, the C1 decays were exponential for reaction (1) and nonexponential for the
reactions (2), (3), and (4). This behavior for reactions (2-4) was attributed to
regeneration of Cl via the reaction :
R+C12+RCI+C1
kc12
(7)
where R refers to C2HS,C3H7,and C4H9and kI2
to the rate coefficient of the reaction.
This reaction caused secondary chemistry problems. At the limited excess of RH over
C1 used, the C1 production through this reaction was not negligible compared to the
loss rate through reaction C1 + RH. To make this secondary reaction negligible in
reactions (2-4), O2 was added to the reaction mixture.
R+0
2
+ products
k02
(8)
where b2refers to the rate coefficient of the reaction. The above reactions were then
in competition, and avoiding the regeneration of chlorine atoms depends on the ratio
~2[02]k-12[C12],
[02]and [Ch] being the concentrations of 0 2 and C12respectively.
KINETIC STUDIES OF CI ATOM REACTIONS WITH SERIES OF ALKANES
While for ethane only one channel exists (C1 + C2&
519
+ C2HS + HCI),
for
propane and butane, H atoms abstraction by C1 occur at two different sites [22]:
(34
(3b)
(4a)
CH3CH2CH2+ HCl
43 %
+ CH3CHCH3+ HCl
+ CH3CH2CH2CH2+ HCI
+ CH3CH2CHCH3+ HC1
57 %
C1+ CH3CH2CH3
Cl + CH3CH2CH2CH3
(4b)
30 %
70%
But, the formed alkyl radicals show essentially identical relative reactivities towards
CI2 and O2 [22]. The rate coefficients used to calculate the ratios are summarized in
Table I. The rate constant k(C12 + n-C3H7)has never been measured, we assumed that
k(C12 + n-C3H7)/k(02+ n-C3H7)= k(Clz + i-C3H7)/k(Oz+ i-C3H7)as it was reported
for the butyl + 0 2 and C12 reactions. The ratio ko2fl<Cl2was reported to be equal to 0.33
in a recent study by Tyndall et al. [22]. Table I shows that with the [O2]/[Cl2]ratio
used, the rates ratio k02[02]/kC12[C12]
is large enough (2 67) so that R radical reacted
mainly with 0 2 and not with C12.
Table I:Rate coefficients used to calculate the ratio ko2 [02]/ kC12[C~Z].
R
k02
CzHs
n-CjH7
i-CjH7
n-C4H9
i-CsHp
(cm3 molecule-' i ' )
6 . 9 ~ 1 0 . [23]
'~
8x10.'~[19]
1.1x10-" [19]
7.5~10-l2
[24IC
1.7~10'"[24IC
kc12
(cm3 molecule-' i')
8 . 8 ~ 1 0 . [25]
'~
4x10."
5.5x10-" [26]
2.3x10-" [22]
4.8~10'"[22]
262
67
67
109
109
[ 0 2 ]= 1 . 0 ~ 1 and
0 ~ [Ch]
~
= 3x10" molecule ~ t n - ~ .
assuming that k(C12 + n-C3H7)/k(02+ n-C3H7)= k(Ch + i-C3H7)/k(02+ i-C,H,)
taken fiom the only existing measurement between 1 and 4 torr [24].
a calculated for
It has to be mentioned that the
quenching of Cl atoms fluorescence by
0 2
0 2
concentration was limited by the efficient
and also by the absorption of the vacuum
UV radiations of the fluorescence. The intensity of the fluorescence signal was
reduced at least by a factor of 2 in the presence of [02] = 1016molecule cm". Some
experiments were performed with up to 2x10'~molecule cm-3 of
02,
the obtained
A. Mellouki
520
results for k2, k3, and kq were similar to those obtained with 1 0 ' ~molecule cm-' of 0
2
but with larger uncertainties (due to the lack of sensitivity of C1 detection in our system
with such high 0
2 concentration).
The plots of k' vs. the alkane concentrations are shown in Figure 2. The obtained
results for the rate coefficients for the reactions of CI with the studied alkanes are
given in Table I1 together with those &om previous studies.
40
80
120
[Alkane] (molecule cmJ)
Figure 2 : Plots of k'l to kh vs. alkane concentration at room temperature. The lines represent the
linear least-squares fitting.
Agreement between the rate coefficient for reaction of C1 with methane obtained in
this study and that derived from JPL/NASA and IUPAC evaluations [18, 191 is
excellent. The rate coefficients for ethane and propane are also in good agreement with
those derived from the evaluations and those reported recently by two different groups
using different techniques [18, 19, 20 2 11. There is no recommended value for the rate
coefficient for the reaction of C1 with n-butane. However, agreement between the
value obtained in this work and those from the two recent determinations is very good
KINETIC STUDIES OF CI ATOM REACTIONS WITH SERIES OF ALKANES
521
[21, 221. Within the experimental error, the agreement is good with earlier
measurements: kq = 2.23 [20], 1.80 [27], and 1.97 x10-locm3 molecule-' i'[28].
Table I1 : Rate coefficients for the reaction of Cl with alkanes obtained in this work and comparison
to recommended values and recent Literature data.
[ml
Compound
(molecule ~ r n - ~ )
Methane
(2 - 138)x1Ol4
(5 - 33)x10I2
Ethane
Propane
n-Butane
(4 - 3 1)x10i2
(3.5
- 25)x10'~
k'
(s-I)
75 - 1490
397 - 2048
*
k 20
(crn3molecule" s-')
(1.07 0.03)~10-'~
(LO* 0 . 1 ) ~ 1 0 - ~ ~
(1.0 0 . 2 ) ~ l o " ~
(0.94 0.04) x10"~
(6.5 0.3)x10^"
(5.7 *.6 .o.s)xlo-'l
(5.9
a s)x 10-1I
(5.53 rt 0.21)xlo'"
(5.75 0.45)x10-"
(1.31*0.03)~10~'~
(1 -4 *0'4 4 3)~10-"
(1.4 *" 3 31x10-lo
(1.23 0.10)x10-'~
(2.25 0.06)~10'~
(2.1 1 0.18)~10-'~
(2.15 0.15)~10-'~
*
*
*
*
*
553-3857
829 - 5584
*
*
*
*
Reference
This work
1181
1191
[21]
This work
t181
r191
t211
[22]
This work
[I8]
[I91
[21]
This work
[211
[22]
CONCLUSIONS
We have described in this paper the pulsed laser photolysis
-
resonance
fluorescence apparatus recently built in our laboratory, and shown that this equipment
is valid for rate coefficients measurements for chlorine atom reactions with high
sensitivity and good accuracy. The obtained results for the reactions of C1 with the
studied series of alkanes are in good agreement with the literature data. Concerning the
rate coefficients for the reactions of C1 with propane and butane, our absolute
measurements contributes to improve the existing data base.
Acknowledgments Thanks are due to Dr. G. Le Bras for his careful reading of the
manuscript and to J. Sabatier for technical assistance.
J Chim. Phys.
522
A . Mellouki
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