Atmospheric Chemistry of HFC-236cb
J. Phys. Chem. A, Vol. 101, No. 15, 1997 2829
Aqueous uptake may also be a possible sink for CF3CF2-
CFHOOH.5
volumes into the cell. Reference spectra of HC(O)F and CF3-
CF2C(O)F were taken from the library at Ford Motor Company.4
HC(O)F concentrations were quantified using an absorption
cross section of 1.0 × 10-18 cm2 molecule-1 at 1849 cm-1 at
room temperature. The absorption cross section for HC(O)F
was corrected for temperature dependence by adding 5% at 269
and 250 K and 10% at 238 and 228 K by analogy with the
behavior of HC(O)Cl.8 For CF3CF2C(O)F the integrated
absorption over the range 1850-1920 cm-1, σint ) 2.47 × 10-17
cm2 molecule-1, was used assuming the integrated cross section
is independent of temperature.
To understand the ultimate products and, hence, the envi-
ronmental impact of HFC-236cb, data concerning the atmo-
spheric chemistry of the CF3CF2CFHO radical are needed. A
previous study has shown that reactions 7 and 8 compete for
CF3CF2CFHO radicals under atmospheric conditions:
CF3CF2CFHO + O2 f CF3CF2C(O)F + HO2
(7)
CF3CF2CFHO + M f CF3CF2 + HC(O)F + M (8)
Two chemical sources of CF3CF2CFHO radicals were used:
either the CF3CF2CFHO2 self-reaction 9a or the CF3CF2CFHO2
+ NO reaction 3a. CF3CF2CFHO2 radicals were prepared by
reaction of Cl atoms with HFC-236cb in the presence of O2.
Molecular chlorine was used as the source of Cl atoms.
At 700 Torr total pressure and 296 K, the rate constant ratio
k8/k7 ) kd/kO2 was found to be 3 × 1020 molecule cm-3 4
.
However, the effect of temperature and total pressure on kd/kO2
is unknown.
In the previous study of reactions 7 and 8 we employed the
self-reaction of CF3CF2CFHO2 radicals as a source for CF3-
CF2CFHO radicals. The self-reaction of peroxy radicals is a
convenient source of alkoxy radicals for laboratory studies.
However, in the atmosphere the main source of CF3CF2CFHO
radicals is not the self-reaction of CF3CF2CFHO2 radicals but
rather the reaction of CF3CF2CFHO2 radicals with NO.
Cl2 + hν f 2Cl
(10)
(11)
CF3CF2CFH2 + Cl f CF3CF2CFH + HCl
CF3CF2CFH + O2 + M f CF3CF2CFHO2 + M (2)
Mixtures of CF3CF2CFH2/O2/Cl2/N2 and CF3CF2CFH2/O2/
Cl2/N2/NO were irradiated with UV light, and the yields of
HC(O)F and CF3CF2C(O)F were followed by FTIR spectros-
copy. At 295 K, Cl atoms react with CF3CF2CFH2 with a rate
constant of k11 ) (1.5 ( 0.3) × 10-15 cm3 molecule-1 s-1 4
and with HC(O)F with a rate constant of 2.0 × 10-15 cm3
CF3CF2CFHO2 + CF3CF2CFHO2 f
CF3CF2CFHO + CF3CF2CFHO + O2 (9a)
CF3CF2CFHO2 + CF3CF2CFHO2 f
CF3CF2C(O)F + CF3CF2CFHOH + O2 (9b)
molecule-1 s-1 9
Loss of HC(O)F via reaction with Cl is
.
important at large conversions of HFC-236cb. Therefore, only
small conversions of HFC-236cb (<10%) were used. The
temperature dependence of the Cl + HC(O)F reaction was
recently measured by Bednarek et al.10 They find the activation
temperature for Cl + HC(O)F to be Ea/R ) 1130 K. Since we
expect the activation energy for the Cl + HFC-236cb to be
similar to that for the Cl + HFC-134a reaction (Ea/R ) 1700)
and since we use small conversions of HFC-236cb, we expect
that loss of HC(O)F via reaction with Cl will be neglible in the
present work at low temperatures as well. In addition, no
evidence for any heterogeneous loss or photolysis of HC(O)F
was observed. Two factors complicate the use of absolute HC-
(O)F and CF3CF2C(O)F yields to determine the fate of CF3-
CF2CFHO radicals. First, the use of small conversions of HFC-
236cb imposes large uncertainties in the measurements of the
loss of CF3CF2CFH2 compared to the uncertainties in the
quantification of the formation of HC(O)F and CF3CF2C(O)F
yields. Second, the combined yields of HC(O)F and CF3CF2C-
(O)F are expected to fall below 100% compared to the HFC-
236cb loss, due to the formation of trioxides and peroxynitrates,
as discussed in ref 11. Therefore, instead of measuring the
absolute yields of CF3CF2C(O)F and HC(O)F as a function of
loss of HFC-236cb, we chose to monitor the relative yields of
HC(O)F and CF3CF2C(O)F normalized to a combined yield of
HC(O)F + CF3CF2C(O)F ) 100%.
CF3CF2CFHO2 + NO f CF3CF2CFHO + NO2 (3a)
The aim of the present work is twofold: first, to extend our
previous study of kd/kO2 to temperatures other than 295K and
pressures other than 700 Torr; second, to investigate the
possibility of a “hot alkoxy radical” effect in the presence of
NO in the oxidation of HFC-236cb.
2. Experimental Section
The system consists of a Fourier transform infrared (FTIR)
spectrometer coupled to a temperature-regulated reaction vessel.
The system has been described previously6,7 and will only be
described briefly here. The reaction vessel, which has a volume
of 47 L, was operated at temperatures between 228 and 296 K
and pressures from 7.8 to 1000 Torr. Hanst type optics to
multipass the IR beam were mounted inside the cell.
The spectrometer, a BOMEM DA3.01, was coupled to the
cell via evacuable transfer optics. The total path length of the
analyzing IR beam was 32.6 m, and a spectral resolution of 1
cm-1 was used. For each spectrum 100-500 interferograms
were acquired. The UV photolysis light was provided by a
xenon arc lamp situated at the end of the reaction cell. The
light was filtered by a Corning 7-54 glass filter that transmits
light between 240 and 400 nm.
Gases were expanded from known volumes and flushed into
the reaction chamber with N2 or O2. The gas mixtures were
allowed to stand for 5 min to reach temperature equilibrium
prior to each experiment. Gas mixtures were subjected to three
to five successive UV irradiations each of a duration of 5-20
min, and IR spectra were acquired after each irradiation. The
loss of reactants and the formation of products were quantified
by comparison to calibrated reference spectra. Reference spectra
of C(O)F2 and CF3CF2CFH2 were acquired by expanding known
Three sets of experiments were performed. First, to determine
the effect of temperature on the rate constant ratio kd/kO2 for
CF3CF2CFHO radicals from the self-reaction of CF3CF2CFHO2
radicals, mixtures of CF3CF2CFH2/O2/Cl2 with N2 added to a
total pressure of 800 Torr were irradiated at 296, 278, 269, 250,
238, and 228 K. Second, to determine the effect of total
pressure on kd/kO2 for CF3CF2CFHO radicals from the self-
reaction of CF3CF2CFHO2 radicals at 238 K, mixtures of CF3-
CF2CFH2/Cl2/O2/N2 with the total pressure varied between 7.8
and 1000 Torr were subject to UV irradiation. Finally, to