472
HUYBRECHTS ET AL.
Y. Hubin wishes to thank IWONL for a grant.
radicals (Step 5) is the fastest chain termination (see
Fig. 5) and that this step has produced about 0.005%
C2Cl6 at the end ofthe experiment. The calculated ini-
tial chain length (rate ofStep 3/rate ofStep 5) is as
high as 105. The simulations show finally that a surface
controlled removal ofCl atoms would play a role if
BIBLIOGRAPHY
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1964, 60, 1608.
5
its efficiency (Cl)* is not lower than about 3 ϫ 10Ϫ .
The lowest limit value of (Cl) measured for “poi-
2. Amorebieta, V. T.; Colussi, A. J. Int J Chem Kinet 1985,
17, 849.
5
soned” walls is ca. 10Ϫ [21,22]. It therefore seems
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likely that the experiments were carried out in a well-
conditioned reaction vessel.
Taylor and Hanson [3] and Aver’yanov et al. [5]
suggested that the heterogeneous wall-initiated disso-
ciation ofCl may occur much faster than the homo-
2
geneous initiation step 1 (see Table I). Nevertheless,
Taylor and Hanson [3] observed an increase ofonly
10% of the overall chlorination rate for a seven-fold
increase ofthe surafce-to-volume ratio ofthe reactor
(similar to our observations). If, however, the chain
termination should occur also entirely heterogene-
ously on the wall ofthe reaction cell, a heterogeneous
increase ofthe rate constant ofthe reaction 1 in the
packed cell might be obscured by a greater chain ter-
mination efficiency. The overall reaction rates re-
ported in [3] and [5] are, however, at least 10 times
higher than observed in this work. This may suggest
a difference in the conditioning of the reaction cell or
an effect of impurities. Indeed, we observed that when
a reaction cell has been exposed to O2, the next runs
proceeded much faster. The conditioning of the reac-
tion cell then had to be renewed. Note that simulations
show that the reaction rates observed in [1,3–7] re-
quire a chain initiation step ofthe type Cl : 2Cl,
2
which would be at least 104 times faster than the ho-
mogeneous step 1 and are therefore most probably cat-
alyzed by the wall.
In conclusion, the thermal chlorination ofCHCl
3
studied in this work can be simulated with a kinetic
model that is entirely homogeneous. This model is an
additional check ofthe validity ofa good model ofr
the pyrolyses ofCCl [8] and C2Cl6 [8,10] between
4
651 and 718 K. Indeed, almost all ofits reaction steps
and parameters are the same and where used at lower
temperatures ranging from 573 to 635 K.
* (Cl) ϭ rate ofremoval ofCl atoms per unit surface/rate ofcol-
lision ofCl atoms per unit surface given by kinetic theory ofgases.