Atmospheric chemistry of CF3CF2CHO: Absorption cross sections in the UV and IR regions, photolysis at 308 nm, and gas-phase reaction with OH radicals (T = 263-358 K)

Article abstract of DOI:10.1021/jp410283v

The relative importance in the atmosphere of UV photolysis of perfluoropropionaldehyde, CF₃CF₂CHO, and reaction with hydroxyl (OH) radicals has been investigated in this work. First, the forbidden n → π* transition of the carbonyl chromophore was characterized between 230 and 380 nm as a function of temperature (269-298 K) and UV absorption cross sections, σ_λ, were determined in those ranges. In addition, IR absorption cross sections were determined between 4000 and 500 cm^-1. Pulsed laser photolysis (PLP) of CF₃CF ₂CHO coupled to Fourier transform infrared (FTIR) was employed to determine the overall photolysis quantum yield, Φ_λ, at 308 nm and 298 K. Φ_{λ=308 nm} was pressure dependent, ranging from (0.94 ± 0.14) at 75 Torr to (0.30 ± 0.01) at 760 Torr. This dependence is characterized by the Stern-Volmer parameters Φλ=308 nm0 = (1.19 ± 0.34) and K_SV = (1.22 ± 0.52) × 10^-19 cm³ molecule^-1. End products of the photodissociation of CF₃CF₂CHO were measured and quantified by FTIR spectroscopy. Furthermore, the rate coefficients for the OH + CF₃CF₂CHO reaction, k₁, were determined as a function of temperature (T = 263-358 K) by PLP-LIF. At room temperature the rate coefficient is k₁(T = 298 K) = (5.57 ± 0.07) × 10 ^-13 cm³ molecule^-1 s^-1, whereas the temperature dependence is described by k₁(T) = (2.56 ± 0.32) × 10^-12 exp{-(458 ± 36)/T} cm³ molecule ^-1 s^-1. On the basis of our results, photolysis of CF ₃CF₂CHO in the actinic region could be an important removal process for CF₃CF₂CHO in the atmosphere. The formation of the primary products in the UV photolysis of CF₃CF ₂CHO is also discussed.

Full text of DOI:10.1021/jp410283v

Article
pubs.acs.org/JPCA
Atmospheric Chemistry of CF3CF2CHO: Absorption Cross Sections in
the UV and IR Regions, Photolysis at 308 nm, and Gas-Phase
Reaction with OH Radicals (T = 263−358 K)
María Antinolo,†,§ Elena Jimenez,†,‡ Sergio Gonzalez,† and Jose Albaladejo*,†,‡
́
́
́
̃
†Departamento de Química Física, Facultad de Ciencias y Tecnologías Químicas, Universidad de Castilla-La Mancha, Avda. Camilo
Jose Cela s/n, 13071 Ciudad Real, Spain
‡Instituto de Investigacion
en Combustion
́
́
́ ́ ́
y Contaminacion Atmosferica, Universidad de Castilla-La Mancha, Camino de Moledores
s/n, Edificio Polivalente, 13071 Ciudad Real, Spain
S
* Supporting Information
ABSTRACT: The relative importance in the atmosphere of UV photolysis of
perfluoropropionaldehyde, CF3CF2CHO, and reaction with hydroxyl (OH) radicals has
been investigated in this work. First, the forbidden n → π* transition of the carbonyl
chromophore was characterized between 230 and 380 nm as a function of temperature
(269−298 K) and UV absorption cross sections, σλ, were determined in those ranges. In
addition, IR absorption cross sections were determined between 4000 and 500 cm−1.
Pulsed laser photolysis (PLP) of CF3CF2CHO coupled to Fourier transform infrared
(FTIR) was employed to determine the overall photolysis quantum yield, Φλ, at 308 nm
and 298 K. Φλ=308 nm was pressure dependent, ranging from (0.94 0.14) at 75 Torr to
(0.30
0.01) at 760 Torr. This dependence is characterized by the Stern−Volmer
parameters Φ0λ=308 nm = (1.19 0.34) and KSV = (1.22 0.52) × 10−19 cm3 molecule−1.
End products of the photodissociation of CF3CF2CHO were measured and quantified by
FTIR spectroscopy. Furthermore, the rate coefficients for the OH + CF3CF2CHO
reaction, k1, were determined as a function of temperature (T = 263−358 K) by PLP-LIF. At room temperature the rate
coefficient is k1(T = 298 K) = (5.57 0.07) × 10−13 cm3 molecule−1 s−1, whereas the temperature dependence is described by
k1(T) = (2.56 0.32) × 10−12 exp{−(458 36)/T} cm3 molecule−1 s−1. On the basis of our results, photolysis of CF3CF2CHO
in the actinic region could be an important removal process for CF3CF2CHO in the atmosphere. The formation of the primary
products in the UV photolysis of CF3CF2CHO is also discussed.
degradation of CF3CF2CHCH2 (HFO-1345zfc). As far as we
1. INTRODUCTION
know, no product study is reported for the OH or Cl reactions
Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons
(HFCs) have been used over the last decades as CFC
replacements in several applications, because their ozone
depletion potential (ODP) is much lower or negligible
with HFO-1345zfc.
The OH-reaction with perfluorinated aldehydes (PFALs) has
been reported to possibly be precursors of perfluoroalkyl
carboxylic acids (PFCAs, CxF2x+1COOH),5 which have been
compared to CFCs. Despite the zero ODP of HFCs, in
observed in biota all over the world, and are thought to be
general they have high global warming potentials (GWPs) that
harmful.6 Some aspects of the atmospheric chemistry of some
can contribute to the greenhouse effect and potentially to
climate change.1 For that reason, other alternatives to HFCs
PFALs and fluorinated aldehydes (FALs) have been reported in
the literature.5,7−15 These studies include the kinetic study of
such as hydrofluoroolefins (HFOs) or fluoroalcohols (FAs)
the reactivity toward OH radicals and Cl atoms,3−5,10−13 the
determination of ultraviolet (UV) and infrared (IR) absorption
cross sections,7,8,10,11,14 and the estimation of photolysis
rates.7,11,14,15 The role of UV photolysis of CF3CF2CHO in
the atmosphere is uncertain. In the absence of quantum yield
measurements, Sulbaek Andersen et al.5 reported that
CF3CF2CHO was likely to be removed by photolysis in a
few weeks, while Chiappero et al.7 estimated that the lifetime of
have been considered as potential CFC replacements.2 The
atmospheric oxidation of HFOs and FAs is initiated mainly by
hydroxyl (OH) radicals, producing fluorinated aldehydes as
secondary pollutants. For example, Hurley et al.3 reported that
chlorine (Cl)-initiated oxidation of CF3CF2CH2OH in the
absence of NO yielded the perfluorinated aldehyde
CF3CF2CHO as sole primary product. A similar mechanism
is expected for the corresponding OH-reaction. The oxidation
mechanism for C4F9CHCH2 and C6F13CHCH2 initiated
by Cl atoms yielded C4F9CHO and C6F13CHO, respectively, as
primary products.4 By analogy with this mechanism,
CF3CF2CHO is also expected to be formed in the atmospheric
Received: October 17, 2013
Revised: December 2, 2013
Published: December 3, 2013
© 2013 American Chemical Society
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Article
CF3CF2CHO with respect to photolysis was less than 2 days at
40° latitude, assuming a constant photolysis quantum yield of
0.11 (i.e., independent of pressure and wavelength).
was calculated from the measured pressures and temperature
(1.62 × 1016 to 3.18 × 1017 molecules cm−3). A reference
spectrum was recorded before each measurement. The
determination of the UV absorption cross sections (base e),
The aim of this work is to evaluate the significance of OH
reaction over the photolysis loss of CF3CF2CHO in the
atmosphere. UV and IR absorption cross sections, σλ(230−
σ , was based on the Beer−Lambert Law (A(v) = σ(v)
̃
̃
λ
l[CF3CF2CHO]). σλ was obtained from the slope of the
absorbance at a given wavelength λ versus CF3CF2CHO
pressure. Some examples of these plots are depicted in Figure
S1 of the Supporting Information.
380 nm) and σ(v = 500−4000 cm−1), were determined for
̃
CF3CF2CHO by gas-phase UV and Fourier transform infrared
(FTIR) absorption spectroscopy, respectively. Additionally, the
overall photolysis quantum yield of CF3CF2CHO at 308 nm,
IR absorption cross sections, σ(v), were determined between
̃
500 and 4000 cm−1 with an experimental setup previously
described.13,20,21 A multipass gas cell with a path length of 8 m
coupled to a FTIR spectrometer (Bruker, Tensor 27) with N2-
liquid cooled mercury cadmium telluride (MCT) detector was
used to measure the IR absorption spectrum of the sample. The
resolution of the recorded spectra was 1 cm−1. The measure-
ments were carried out exclusively at room temperature and
under static conditions from diluted mixtures of CF3CF2CHO
in helium. Dilution factors, f, are defined as the ratio of
CF3CF2CHO partial pressure, p(CF3CF2CHO), and the total
pressure in the bulb, pT = p(CF3CF2CHO) + pHe. f values
ranged from 0.01% to 0.09%. Total pressures inside the
absorption cell were varied from 5 to 50 Torr. Initial
concentrations of CF3CF2CHO were determined from the
partial pressure p(CF3CF2CHO) inside the cell
([CF3CF2CHO] = (0.3−7.6) × 1014 molecules cm−3). Optical
saturation was not reached at the concentration level used, and
the Beer−Lambert Law was accomplished in the whole range,
i.e., a linear relationship between the absorbance and the
concentration was observed.
Φ
λ=308 nm, was determined for the first time as a function of
pressure (P = 75−760 Torr of air) by pulsed laser photolysis
coupled to FTIR spectroscopy (PLP−FTIR). Photooxidation
end-products were also detected in situ during the PLP of
CF3CF2CHO samples.
Furthermore, the absolute rate coefficient for the OH-
reaction with CF3CF2CHO (k1) as a function of temperature
(T = 263−358 K) was determined here for the first time by
monitoring OH radicals by the laser-induced fluorescence
(LIF) technique.
OH + CFCF CHO → H2O + CFCF CO
(1)
3
2
3
2
The reported parameters (σλ, Φλ, and k1) allow the
estimation of the relative importance of photolysis of
CF3CF2CHO in the actinic region over its removal by reaction
with OH radicals in the atmosphere. The molar yields of some
primary products and a possible explanation for the formation
of secondary products are also reported.
2. EXPERIMENTAL SECTION
Because different setups have been used in this kinetic and
photochemical study, this section is divided into four
subsections.
2.3. PLP−FTIR System: Φλ=308 nm Determination. The
PLP−FTIR system was described in previous works.14,15
A
cylindrical photolysis cell (L = 10 cm, d = 2 cm) was filled
under static conditions with a diluted mixture of CF3CF2CHO
(f = 0.13−0.25%) in air (pT = 75−760 Torr). An excess of
cyclohexane ([C6H12]0/[CF3CF2CHO]0 = 25−46) was added
to the gas mixture to scavenge OH radicals possibly formed in
the system (see section 3). The sample was photolyzed at 308
nm by a pulsed XeCl excimer laser, operating at a repetition
rate that ranged between 1 and 10 Hz. The number of pulses, n,
was varied between 300 and 2400. A cylindrical lens (−40 mm
focal length) was placed between the laser and the cell to
expand the laser beam and completely irradiate the gas cell.
Spectra of the fresh and photolyzed sample were recorded
through the FTIR spectroscopy setup described in subsection
2.2 and after the expansion of the content of the photolysis cell
into the IR gas cell. The loss of CF3CF2CHO by photo-
dissociation can be described by a first-order rate law. The
2.1. Synthesis of Gaseous CF3CF2CHO. CF3CF2CHO is
easily hydrated and for that reason is commercially available in
the stable hydrated form, CF3CF2CH(OH)2, which is a
crystalline solid (Apollo Scientific Ltd., >90%, mp = 52−53
°C).16 The dehydration procedure was similar to that described
by Braid et al.17 A 10 g amount of molten CF3CF2CH(OH)2
was dropped slowly into a flask containing 25 g of phosphorus
pentoxide, P2O5 (Fluka, >99.9%). The content of the flask was
heated at 60 °C and vigorously stirred. A continuous flow of He
of 20 standard cubic centimeter per minute (sccm) was
introduced through the flask to carry gaseous CF3CF2CHO (bp
= 2 °C at 746 Torr)16 to a trap cooled with liquid nitrogen. A
stainless steel gas bottle was used to store the aldehyde at room
temperature. IR analysis of the crude sample of CF3CF2CHO
collected in the trap did not reveal the presence of a significant
amount of impurities.
2.2. UV and FTIR Absorption Spectroscopy. The UV
absorption spectra were measured between 230 and 380 nm at
different temperatures (269−298 K) using an apparatus
described previously.18,19 It consists of a deuterium lamp as
irradiation source, a Pyrex absorption cell (l = 107 cm), and a
0.5-m spectrograph equipped with a coupled-charge device
(CCD) as a detector. The instrumental resolution of the
spectrograph was 0.17 nm. The wavelengths were daily
calibrated prior use with a pen-ray Hg lamp. The temperature
inside the cell was regulated by ethanol circulating through an
outer jacket by using a thermostatic bath (Huber, Polystat
CC1). The measurements were performed under static
conditions, filling the cell with pure CF3CF2CHO (0.5−9.8
Torr). The range of concentrations inside the absorption cell
carbonyl band of the perfluorinated aldehyde was used to
15
monitor its temporal decay. As described previously,
for
fractional photolysis below 10−4 molecule pulse−1, the
integrated rate equation can be expressed in terms of integrated
absorbances of CF3CF2CHO (An) as:
⎛
⎜
⎝
⎞
⎟
⎠
A0
n
ln
= J
λ v
A
n
rep
(2)
where A0 and An are the integrated absorbances of the fresh (n
= 0) and photolyzed samples, respectively, and vrep is the
repetition rate of the photolysis laser. The photolysis rate, Jλ,
was determined as a function of total pressure from the slope of
the plot of ln(A0/An) versus n. This parameter is defined as:
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Article
J = Φ σ F
thaw cycles prior to use. HNO3 (Panreac, 65%) was used as
supplied. Gases from Praxair were used also as supplied:
synthetic air (99.999%) and He (99.999%).
(3)
λ λ λ
λ
The laser fluence, Fλ, at 308 nm was obtained by chemical
actinometry using acetaldehyde (CH3CHO). At a constant Fλ
(varied from 1.0 × 1016 to 2.1 × 1017 photon cm−2 s−1), the
photolysis quantum yield of CF3CF2CHO at 308 nm can then
be obtained from the following equation:
3. RESULTS AND DISCUSSION
This section has been divided into three subsections for ease of
presentation. The first subsection presents the results and
discussion of the UV and IR absorption cross sections of
CF3CF2CHO. In subsection 3.2, the photodissociation
quantum yield of this fluorinated aldehyde is reported as a
function of total pressure together with the detected photo-
oxidation products. Lastly, the observed temperature depend-
ence of the rate coefficients for reaction 1 is reported. A
comparison with CH3CH2CHO and partially fluorinated
propanals is also included in the discussion on both the
photolysis and the reactive processes.
J (CF CF CHO) × σ
3
2
CH3CHO
λ
Φ
= Φ
CF3CF2CHO
CH3CHO
J (CH3CHO) × σ
CF3CF2CHO
(4)
λ
The photodissociation quantum yield and the absorption cross
section at 308 nm for CH3CHO were taken from the literature
as 0.29 at 1 bar and 3.33 × 10−20 cm2 molecule−1,
respectively.22,23
2.4. PLP−LIF System: Gas-Phase OH Kinetics. As
previously described,24−26 a Pyrex-jacketed reaction cell (V =
200 cm3) was used. Diluted CF3CF2CHO, OH-precursor
(HNO3), and helium were flowed through the cell by means of
calibrated mass flow controllers. Total flow rate ranged between
210 and 550 sccm. Pressure inside the cell (pT = 50−205 Torr)
was controlled by a needle valve and measured by a capacitance
manometer from Leybold Vacuum (CERAVAC CTR90 for
pressures up to 100 Torr and SKY CM 1000 for pressures up to
1000 Torr). Flow rates and total pressure measurements were
used to determine the concentration of CF3CF2CHO inside the
cell ([CF3CF2CHO] = (0.2−7.4) × 1015 molecules cm−3). A
cooling (ethanol) or heating liquid (water) was flowed through
the reactor jacket to vary the temperature inside the cell
between 263 K and 358 K. OH radicals were generated by the
pulsed photolysis of gaseous HNO3 at 248 nm using a KrF
excimer laser at a repetition rate of 10 Hz. Gaseous HNO3 was
obtained by bubbling helium through an aqueous solution of
HNO3 (1−5 sccm).
Unless otherwise stated, uncertainties are at 95% confidence
level ( 2σ) where the standard deviation represents only the
precision of the measurement. Systematic errors have been
estimated to be 6%.
3.1. UV and IR Absorption Cross Sections. Averaged UV
absorption cross sections at temperatures between 269 K and
298 K for CF3CF2CHO are presented in Table S1 of the
Supporting Information and depicted in Figure 1a as well. A
weak absorption band was observed corresponding to the
forbidden n → π* transition of the carbonyl chromophore. An
averaged 5% increase of σλ with temperature was observed
between 230 and 380 nm. Chiappero et al.7 also reported a
negligible variation (<5%) of σλ as a function of temperature
between 249 K and 297 K, as we observed.
At all temperatures, the maximum absorption cross section is
located around 308 nm. The average of the maximum
absorption wavelength is (308.13
0.76) nm. In Table 1 a
Gas-phase concentration of HNO3 ((0.5−1.8) × 1015
molecules cm−3) and initial OH concentrations, [OH]0,
((1.0−4.2) × 1011 radical cm−3) inside the cell were estimated
as described in previous works.18,20 The initial OH radical
concentration, [OH]0, was estimated from the relationship:
summary of the maximum absorption λ and σλ,max is presented
for comparison with literature data. As can be seen, there is a
good agreement, considering the uncertainties, with σλ reported
by Chiappero et al.7 and Hashikawa et al.8 The discrete
absorption cross sections from Borkowski and Ausloos28 and
Pitchard et al.29 were not determined at the maximum
absorption peak, as can be seen in Figure 1b.
[OH]0 = F248nmϕOHσ248nm[HNO3]0
(5)
where σ248 nm is the absorption cross section at 248 nm of
HNO3 (2.03 × 10−20 cm2 molecule−1),27 ϕOH is the OH
quantum yield of the precursor (ϕOH = 0.97),27 and F248 nm is
the photolysis laser fluence at the photolysis wavelength.
The decay of OH radicals was monitored by LIF. A doubled-
frequency dye laser pumped by a Nd:YAG laser was used to
excite OH radicals at 282 nm. LIF signal was detected by a
filtered photomultiplier tube placed orthogonally to both laser
sources (Oriel, band-pass filter with 90% of transmission at 350
nm and fwhm of 150 nm). The LIF signal was transferred to a
computer and analyzed as described previously.24 The pseudo-
first-order conditions were accomplished ([CF3CF2CHO]0/
[OH]0 = 800−52000 and [HNO3]0/[OH]0 = 3670−6270).
The pseudo-first-order rate coefficient (k′) was obtained at
each reactant concentration. k′ is defined as follows:
The averaged IR absorption cross sections, σ(v) (base e), are
̃
presented in Table S2 of the Supporting Information and
depicted in Figure 2 together with those reported by Hashikawa
et al.8 In Figure 2a, the 1500−1000 cm−1 region corresponding
to complex normal modes that include H−C−O bending or
C−F stretching is shown. As can be seen in Figure 2, σ(v)
̃
reported by Hashikawa et al.8 are in good agreement with those
determined in this work. Only for the absorption peak in the
1500−1000 cm−1 region, which is located at 1224.2 cm−1,
Hashikawa et al.8 reported σ(v) approximately 5% higher
̃
max
than ours. The band centered at 1775.8 cm−1 is characteristic of
the carbonyl group in aldehydes. The observed difference in
σ(v) is within the uncertainty of the measurements.
̃
max
The main contribution to the systematic errors in the
reported parameters σ is the uncertainty in the CF3CF2CHO
concentration that was estimated to be less than 5%.
3.2. Pulsed Photolysis of CF3CF2CHO at 308 nm in the
Presence of Air. 3.2.1. Quantum Yields as a Function of
Total Pressure. After absorption of 308 nm radiation, excited
CF3CF2CHO (denoted by an asterisk) can undergo photo-
dissociation via the following three thermodynamically possible
channels:
k′ = k1[CFCF CHO] + k0
(6)
3
2
where k0 is the first-order OH loss in absence of CF3CF2CHO
including the OH loss due to diffusion out of the detection
zone and the reaction with HNO3.
Reagents. Cyclohexane (Aldrich, 99.9%) and CH3CHO
(Sigma-Aldrich, ≥99.5%) were degassed by freeze−pump−
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Figure 2. IR absorption cross sections of CF3CF2CHO (in base e)
recorded (a) between 1500 and 1000 cm−1 and (b) for the carbonyl
band at 1 cm−1 resolution and 298 K. The results reported by
Hashikawa et al.8 are also depicted for comparison purposes.
Figure 1. (a) Average UV absorption cross sections of CF3CF2CHO at
temperatures between 269 K and 298 K. (b) Comparison of the UV
absorption spectrum obtained in this work at room temperature with
28
■
▲
previous studies ( from Borkowski and Ausloos;
from Pritchard
et al.;29 dashed line from Chiappero et al.;7 dotted line from
Hashikawa et al.8).
Table 1. Peak UV Absorption Cross Sections and Maximum
Absorption Wavelength for CF3CF2CHO
σλ,
T, K
λmax, nm
10−20 cm2 molecule−1
reference
this work
269−273
283−298
298
308.3 0.4
308.4 0.6
309
5.29 0.30
5.60 0.22
5.89
this work
Chiappero et al.7
Hashikawa et al.8
298
307−308
6.25
CFCF CHO* → HCO + CFCF
(7a)
(7b)
(7c)
3
2
3
2
→ CO + CFCF H
3
2
Figure 3. FTIR spectra of CF3CF2CHO (2.6 × 1016 molecules cm−3 in
the photolysis cell) in the presence of cyclohexane (1.2 × 1018
molecules cm−3 in the photolysis cell) at 630 Torr of air (a) before
irradiation, and (b) after 1200 laser pulses at 1.4 × 1017 photon cm−2
s−1 (CF3CF2CHO and cyclohexane features were subtracted from the
spectrum).
→ H + CFCF CO
3
2
Figure 3a shows an example of the recorded IR spectrum
before irradiation of a CF3CF2CHO/cyclohexane/air mixture.
After irradiation of the sample with 1200 laser pulses and
subtraction of CF3CF2CHO and cyclohexane features, the
residual IR spectrum obtained is depicted in Figure 3b. The
identified products were CO, CF3CFO, F2CO and small
amounts of HCOOH. The carbonyl band from HCOOH
overlaps with that of CF3CF2CHO, interfering in the
measurements of An. For that reason, at high initial
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CF3CF2CHO concentrations, A0 and An were measured using
the 759−674 cm−1 band (band strength of 2.17 × 10−18 cm2
molecule−1 cm−1), where no interference of HCOOH existed.
In contrast, at low initial CF3CF2CHO concentrations, the
analysis of A0 and An was performed by using the intense
carbonyl band (band strength of 3.85 × 10−18 cm2 molecule−1
cm−1 between 1837 and 1716 cm−1).
Table 2. Photolysis Quantum Yields for CF3CF2CHO in Air
at 308 nm
PT, Torr
[M] × 10−19/molecules cm−3
Φλ=308 nm
75
0.24
0.59
0.91
1.30
1.73
2.04
2.46
0.94 0.14
0.63 0.16
0.64 0.07
0.46 0.06
0.36 0.01
0.35 0.03
0.30 0.01
181
281
402
535
631
760
A plot of ln(A0/An) versus n for CH3CHO at 760 Torr and
for CF3CF2CHO at 280 and 550 Torr are shown in Figure 4a
where KSV is the Stern−Volmer constant and Φ0λ=308 nm is the
quantum yield in the absence of a quencher, i.e., at zero
pressure. In Figure 4b the Stern−Volmer plot is shown for
Φλ=308 nm. The parameters obtained from the linear least-
squares regression of the data were Φ0λ=308 nm = (1.19 0.34)
and KSV = (1.22 0.52) × 10−19 cm3 molecule−1. The Stern−
Volmer parameters were used to derive the photodissociation
quantum yield of CF3CF2CHO at tropospheric pressures (see
section 4). Systematic errors in Φλ are associated to the
accuracy of the photodissociation quantum yield of CH3CHO
( 5%), the precision of the UV absorption cross sections of
CF3CF2CHO at 308 nm ( 1%), and the uncertainty in the
measurement of the IR integrated areas ( 5%). These
systematic uncertainties contribute less than 5% to the total
uncertainty.
At 760 Torr and 298 K, the photolysis quantum yield of
CF3CF2CHO at 308 nm can be compared with that of
CH3CH2CHO, CF3CH2CHO, and CF3CHO. The IUPAC
recommendation for the photolysis quantum yield for
CH3CH2CHO in air at 1 bar and 298 K is around unity and
independent of wavelength between 280 and 310 nm.26 In our
previous work on the photolysis of CF3CH2CHO at 308 nm, it
was concluded that the substitution of a CH3 group by a CF3
provokes a dramatic reduction on the quantum yield (from 1 to
0.023).14 This decrease in Φλ=308 nm is less pronounced for
CF3CHO (0.17)7 with respect to CH3CHO (0.29).25 It is
worth noting that if the CH2 group in CF3CH2CHO is
substituted by a CF2 group, an increase in the photodissociation
quantum yield is observed (from 0.023 to 0.30 at 760 Torr of
air).
3.2.2. Photolysis Products in the Presence of Air. In
contrast to what Chiappero et al.7 observed at 254 nm,
CF3CF2H was not detected in the IR spectra recorded after
photolysis of CF3CF2CHO at 308 nm. These authors reported
that CF3CF2 radicals (channel in eq 7a) and CF3CF2H
(channel in eq 7b) were equally important (ca. 50% of Φλ).
In our study, the photolysis channel eq 7b seems to be
negligible. Therefore, CO is expected to be mainly formed from
the reaction of HCO radicals, from the channel eq 7a, with O2:
Figure 4. (a) Temporal decays of CF3CF2CHO (at 280 and 550 Torr
of air) and the actinometer CH3CHO (at 760 Torr of air) in the
photolysis process at 308 nm. (b) Stern−Volmer plot between 75 and
760 Torr for the photolysis quantum yield of CF3CF2CHO at 308 nm.
(Fλ = 5.2 × 1016 photon cm−2 s−1). An increase of the slope of
these plots for CF3CF2CHO, i.e., Φλ=308 nm according to eq 2
and eq 3, was observed at lower pressures. The averaged values
of Φλ=308 nm at pressures between 75 and 760 Torr and room
temperature are summarized in Table 2. The observed effect of
total pressure on Φλ=308 nm can be explained by a Stern−Volmer
mechanism. Collisional deactivation of CF3CF2CHO* by air is
competing with the photodissociation process. If no other
photophysical process is competing, a Stern−Volmer behavior
HCO + O2 → CO + HO2
(9)
CO has been detected in this work but it was not quantified,
because it can be formed by unavoidable secondary chemistry.
Therefore, in the present study, only CF3CFO and F2CO were
quantified by spectral subtraction of reference spectra of known
concentration. Reference spectra of F2CO and CF3CFO were
provided by I. Barnes30 and T. J. Wallington,31 respectively.
Given the small amounts of HCOOH detected, the
quantification of this species was subject to large uncertainties,
and results were in most cases too scattered to provide a
reliable yield for this minor product. To be conservative, an
is expected for the photolysis quantum yield Φλ=308 nm
:
KSV
Φ0λ=308 nm
1
1
=
+
[M]
Φ0λ=308 nm
Φ
(8)
λ=308 nm
182
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The Journal of Physical Chemistry A
Article
upper limit of 0.07 can be reported for the molar yield of
HCOOH. Figure 5 shows the evolution of CF3CFO and F2CO
The observed molar yield of F2CO is much lower than that
of CF3CFO, which is consistent with the predominance of
reaction 11 over reaction 12.
If CF3CF2CO radicals were formed in the channel shown in
eq 7c, CF3CF2COOH could be formed in a similar way, as
CF3(CH2)xCOOH is formed in the presence of O2 and
HO2:14,15
CFCF CO + O2 + M → CFCF C(O)O2 + M
(14)
(15)
3
2
3
2
CFCF C(O)O2 + HO2 → CFCF COOH + O3
3
2
3
2
No detectable amounts of CF3CF2COOH were found in this
work, maybe because of thermal decomposition of CF3CF2CO
and CF3CF2C(O)O2 radicals:
CFCF CO + M → CFCF + CO + M
(16)
3
2
3
2
CFCF C(O)O2 + M → CFCF O + CO2 + M
(17)
3
2
3
2
Hurley et al.32 estimated that disproportionation reaction of
CF3CF2CO accounts for 52% of its loss under atmospheric
conditions. Moreover, Setokuchi et al.33 calculated at 760 Torr
and 298 K that the thermal decomposition rate for CF3CF2CO
was around 106 s−1 and the yield of CO could range from 23%
to 36%. An upper limit contribution of 4% can be reported for
the CF3CF2COOH formation via the channel eq 7c, by
comparing with a reference IR spectrum. In conclusion, the
main photolysis channel for CF3CF2CHO at 308 nm is shown
in eq 7a, forming HCO and CF3CF2 radicals. The channels
shown in eqs 7b and 7c seem to be negligible at this
wavelength.
3.3. OH-Reaction Rate Coefficients. Examples of
temporal decays of the LIF signal in the absence and presence
of CF3CF2CHO are presented in Figure S2 of the Supporting
Information. From the analysis of the LIF decays at
[CF3CF2CHO] = 0, k0 (99−260 s−1), which includes the
reaction of OH with HNO3 and diffusion out of the detection
zone, was obtained at each temperature. Similarly, k′ values
ranging from 265 to 4027 s−1 were obtained in the presence of
different CF3CF2CHO concentrations. The experimental
conditions employed and the averaged rate coefficients
obtained at all temperatures are tabulated in Table 3.
Figure 5. Plots of CF3CFO and F2CO formation as a function of
CF3CF2CHO loss for all total pressures studied.
formation observed for all experiments carried out, i.e., plots of
[CF3CFO] and [F2CO] versus CF3CF2CHO loss
(Δ[CF3CF2CHO]). The molar yield of CF3CFO and F2CO
was measured to be (0.21
respectively.
0.08) and (0.07
0.04),
Formation of CF3CFO can be due to reaction of CF3CF2
radicals from the channel shown in eq 7a with O2 and further
reaction with HO2 radicals:27
CFCF + O2 + M → CFCF O2 + M
(10)
(11a)
(11b)
3
2
3
2
CFCF O2 + HO2 → CFCF O2H + O2
3
2
3
2
→ CFCFO + HOF + O2
3
Halogenated peroxy radicals formed in reaction 10 can also
undergo self-reaction:
The contribution of k0 in k′ varies between 3% and 55%,
although typically was around 20%. k1 at each temperature was
obtained from the slope of k′ − k0 versus [CF3CF2CHO] plots
according to eq 6. In Figure 6 examples of those plots are
shown for 298 K as a function of total pressure (50−205 Torr)
together with an example at 263 and 358 K. No dependency of
2CFCF O2 → 2CFCF O + O2
(12)
3
2
3
2
Subsequent disproportionation of CF3CF2O can produce
F2CO:
CFCF O + M → CF + F CO + M
(13)
3
2
3
2
Table 3. Experimental Conditions Employed in the Kinetic Study of the Reaction of OH with CF3CF2CHO Together with the
Pseudo-First-Order Ranges and the Absolute Rate Coefficient Obtained at Each Temperature
[CF3CF2CHO],
1015
F248 nm, mJ
pulse−1
cm−2
[HNO3], 1015
[OH]0, 1011
radical cm−3
k1(T) × 1013,
T, K
pT, Torr
FT, sccm
molecules cm−3
molecules cm−3
k0, s−1
k1′, s−1
cm3 molecule−1 s−1
263
270
278
287
298
308
323
338
358
51.7−81.2
49.8−75.0
51.9−90.1
60.2−91.8
50.3−205.1
64.0−91.6
51.7−84.7
61.8−86.9
63.0−95.5
212−338
213−338
212−285
212−336
211−548
212−336
211−314
211−335
211−335
0.4−6.6
0.3−5.9
0.4−6.9
0.4−5.5
0.3−7.4
0.4−6.5
0.2−4.4
0.3−4.6
0.3−5.4
0.5−0.9
0.5−1.0
0.8−1.4
0.7−1.1
0.8−1.6
1.0−1.5
1.1−1.4
1.3−1.5
1.7−1.8
6.7−9.9
8.2−9.8
7.2−10.8
8.4−10.7
5.8−10.6
8.5−11.1
7.5−9.9
6.5−9.3
7.3−9.2
1.3−1.7
1.1−1.9
1.9−2.6
1.4−2.8
1.3−4.2
1.0−1.5
1.1−1.4
1.3−1.5
1.7−1.8
230−121
184−99
97−183
260 −134
173−113
194 −140
162 −135
163−141
178−168
333−4027
305−2746
265−2800
309−3433
369−2817
371−3625
308−2708
359−2977
404−3805
4.55 0.09
4.60 0.08
4.86 0.07
5.11 0.12
5.57 0.07
5.61 0.13
6.28 0.15
6.68 0.17
7.12 0.14
183
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The Journal of Physical Chemistry A
Article
Figure 6. k′ − k0 plots versus [CF3CF2CHO] at different pressures
between 50 and 205 Torr at 298 K. An example of these plots at 263 K
and 358 K are also displayed.
k1 on total pressure is observed at the studied range. Therefore,
the linear least-squares fit of all the room temperature data
0.07) × 10−13 cm3
Figure 7. Arrhenius plot for the OH-rate coefficient of the reaction
with CF3CF2CHO between 263 K and 358 K. Error bars represent
uncertainties at 1σ level.
allowed to obtain k1(298 K) = (5.57
molecule−1 s−1. This value is in a very good agreement with the
only one previous study,5 as can be seen in Table 4. The
Arrhenius plot of these data is depicted in Figure 7. The
Arrhenius equation derived from the exponential fit of the data
from Table 3 is the following:
very similar, because these species contain only one hydrogen
atom from the CHO group susceptible to be abstracted by OH
radicals. As can be seen in Table 4, abstraction of H atom from
CHO group is highly deactivated by the presence of F atoms,
and a decrease in kOH(298 K) was observed as in the series
CH3CH2CHO,26 CF3CH2CHO,13 and CF3CF2CHO. The
activation energies (expressed as Ea/R) for C3 and C4 aldehydes
are similar, within the uncertainties given, and pre-exponential
factor (A) are on the order of 10−12 cm3 molecule−1 s−1. A and
Ea for the OH+CF3CHO reaction have not been reported, as
far as we know, but they are expected to be on the same order
of magnitude as that for CF3CF2CHO, because adding CF2
groups to the molecule has a negligible effect on kOH. The
similar positive temperature dependence observed for these C3
and C4 aldehydes can be indicative of a similar reaction
mechanism, presumably a H-abstraction for the CHO group.34
k1(T) = (2.56 0.32) × 10−12 exp{−(458 36)/T}
cm3 molecule−1 s−1
(18)
Systematic uncertainties in kOH(T) can be associated mainly
to the purity of the sample, which could enhance the secondary
chemistry, and other experimental parameters such as temper-
ature and pressure. Even though, no detectable amounts of
potential sample impurities were observed in the infrared
measurements an uncertainty of 5% has been considered,
taking into account the difference in σ(v) and σ determined in
̃
λ
this work with previous studies. Uncertainties in the pressure
and temperature measurements contribute less than 1% to
the overall uncertainty in CF3CF2CHO concentration. On the
basis of these considerations and being conservative, we
estimate an overall uncertainty in kOH(T) of 10%, and
therefore it must be added to the uncertainties stated in Tables
2 and 3.
4. ATMOSPHERIC IMPLICATIONS
In this work, the tropospheric lifetimes due to photolysis (τhν)
and OH radical reaction (τOH) were estimated to evaluate the
relative importance of both removal pathways for CF3CF2CHO
in the atmosphere. τOH was estimated as in previous works by
using the expression given by eq 19:20
Kinetic parameters for the OH-reaction with CF3CF2CHO
are summarized in Table 4, together with those for other
aldehydes from the literature. At room temperature, the OH-
reactivity of CF3CHO, CF3CF2CHO, and CF3(CF2)2CHO is
Table 4. Comparison of the OH Rate Coefficient for CF3CF2CHO with Other Perfluorinated and Hydrogenated Aldehydes
from the Literature
a
aldehyde
CF3CHO
T, K
298
kOH(298 K) × 1013, cm3 molecule−1 s−1 A × 1012, cm3 molecule−1 s−1 (Ea/R), K technique
reference
Atkinson et al.37
5.70 0.02
5.57 0.07
5.26 0.80
25.9 5.0
200
n.m.
n.m.
CF3CF2CHO
263−358
298
2.56 0.32
n.m.
458 36
n.m.
PLP−LIF
RR−FTIR
PLP−LIF
this work
Sulbaek Andersen et al.5
CF3CH2CHO
CH3CH2CHO
CF3(CF2)2CHO
263−358
240−380
252−273
7.8 2.2
5.1
314 90
405
Antinolo et al.13
̃
Atkinson et al.26
Solignac et al.10
5.80 0.60
2.0 0.6
369 90
PLP−LIF
a
PLP-LIF, pulsed-laser photolysis with laser-induced fluorescence; RR-FTIR, relative rate with FTIR detection; n.m., not measured.
184
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The Journal of Physical Chemistry A
Article
1
might be a very efficient sink for CF3CF2CHO. In contrast, an
efficient photolytical removal of CF3CF2CHO is expected to
occur at relatively high altitudes where the fluorinated aldehyde
could be transported by direct-turbulent transfer.
τOH
=
k
OH(T = 275 K)[OH]avg
(19)
[OH]avg is the global average concentration of OH radicals in
the troposphere and was taken as 1 × 106 molecules cm−3.35 A
temperature of 275 K is preferred in the lifetime estimation in
order to use a tropospheric averaged lifetime. This implies the
use of kOH(T = 275 K), which was derived from the Arrhenius
expression (eq 18) obtained in this work. Globally, τOH was
estimated to be more than 20 days. Sulbaek Andersen et al.5
estimated an atmospheric lifetime for CF3CF2CHO of 40 days
due to reaction with OH using their rate coefficient at 296 K
and relative to CH3CCl3. The difference between our lifetime
estimation and that of Sulbaek Andersen et al.5 arises from the
T-dependences of kOH for CH3CCl3 and CF3CF2CHO. While
5. CONCLUSIONS
The present kinetic and photochemical study reports
information of atmospheric interest for CF3CF2CHO. The
determination of the absorption cross sections in UV and IR
regions (σ and σ(v)) is presented. Photolysis quantum yields
̃
λ
at 308 nm (Φλ=308 nm) as a function of pressure (75−760 Torr)
are reported. Moreover, the stable products formed from the
radicals formed in the photolysis of CF3CF2CHO at this
wavelength (CF3CFO and F2CO) have been quantified and
their molar yields have been determined. Rate coefficients for
the reaction with OH radicals (k1) have been also presented as
a function of temperature.
k
OH for CF3CF2CHO decreases around 15% from 296 K to 275
K (see Table 3), kOH for CH3CCl3 diminishes from 10−14 cm3
molecule−1 s−1 (at 296 K) to 6.5 × 10−15 cm3 molecule−1 s−1
(at 275 K). This difference in both kOH makes that the lifetime
for CF3CF2CHO estimated by Sulbaek Andersen et al.5 be
higher than that reported here at 275 K. For that reason, it is
Degradation of CF3CF2CHO initiated by OH radicals is
expected to occur in several weeks. Photolysis of CF3CF2CHO
in the actinic region could be a major contribution to its
atmospheric removal, even more important than OH radical
reaction. Photochemical degradation of CF3CF2CHO mainly
forms CF3CFO and F2CO, which can undergo heterogeneous
reactions on aqueous surfaces. Thus, the environmental impact
of degradation of the secondary pollutant CF3CF2CHO is not
expected to be important. It will finally depend on the
emissions of its precursors, CF3CF2CHCH2 and
CF3CF2CH2OH.
important to determine the temperature dependence of kOH
.
τhν for CF3CF2CHO was estimated in this work for a given
altitude (z = 3.5 km) and zenith angle (θ = 16° corresponding
to Ciudad Real (Spain) in summer midday) as the reciprocal of
the photolysis rate coefficient, J(z,θ). An estimate of J(z,θ) can
be obtained from eq 20:
J(z, θ) ≈ ∑ Φ σ F
λ λ λ,solar(z, θ)Δλ
(20)
F
λ,solar(z,θ) is the spectral solar actinic flux at the simulated
ASSOCIATED CONTENT
* Supporting Information
Tables S1 and S2 show UV and IR absorption cross sections
determined in this work. Figure S1 shows the Beer−Lambert
Law plots at several wavelengths in the UV region. Figure S2
shows some temporal profiles of the LIF signal from OH
radicals. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
atmospheric conditions and was calculated by using the TUV
radiative transfer model (4.1 version)36 according to the
procedure previously used.14,19
S
The importance of CF3CF2CHO photolysis in the
atmosphere is still uncertain. The lack of wavelength and
pressure dependence studies of Φλ makes difficult to accurately
determine J(z,θ). In the absence of quantum yield measure-
ments, Sulbaek Andersen et al.5,37 reported that τhν for
CF3CF2CHO was likely to be of a few weeks by similarity
with that of C2F5C(O)CF(CF3)2.7,14 This implies that removal
of CF3CF2CHO by reaction with OH radicals is more
important than by UV photolysis. In contrast, Chiappero et
al.7 estimated that τhν was less than 2 days at 40° latitude,
assuming a constant photolysis quantum yield of 0.11 (i.e.,
independent of pressure and wavelength). Up to now,
Chiappero et al.7 reported Φλ=254 nm for perfluoropropanal at
700 Torr of N2/NO to be (0.81 0.09). At this total pressure
(of air, in our case), a Φλ=308 nm of 0.32 was derived from eq 8.
Therefore, Φλ for CF3CF2CHO seems to increase as the
excitation energy increases, similarly to CF3CH2CHO.
In this work, the wavelength dependence of the absorption
cross sections from Table S1 were used in the J(z,θ)
calculation. At 700 Torr, we can assume an exponential
decrease from 254 to 308 nm (Φλ=308 nm = 0.32, this work). It is
expected that Φλ also decreases exponentially at other
wavelengths. Assuming this, J(z,θ) for z = 3.5 km, where T =
275 K (an average temperature for the troposphere) was 7.90 ×
10−5 s−1, i.e., τhν is expected to be 3.5 h. A realistic evaluation of
the OH chemistry and photolysis contribution to the
atmospheric lifetime of CF3CF2CHO requires the use of 2D
photochemical models. Even so, the tropospheric fate of the
CF3CF2CHO depends on where it is formed and/or emitted.
For instance, in the planetary boundary layer the OH chemistry
AUTHOR INFORMATION
Corresponding Author
*Phone: 34 926 29 53 27; fax: 34 926 29 53 18; e-mail: Jose.
Albaladejo@uclm.es.
■
Present Address
§Department of Chemistry, University of Toronto, 80 St.
George, Toronto, ON, Canada M5S 3H6.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Spanish Ministerio de Ciencia e
Innovacio
n (CGL2010-19066 project) and the Junta de
Comunidades de Castilla-La Mancha (PCI08-0123-0381
project) for supporting this work. M. Antinolo thanks the
̃
first institution for providing her a grant. The authors also
thank T. J. Wallington, I. Barnes, and M. Sulbaek Andersen for
providing us a reference spectrum of CF3CFO, COF2, and
CF3CF2COOH, respectively.
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