report

EUROCHAMP-2 (GA 228335)
Transnational Access Activity:
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PI:
Papagiannakopoulos Panos,
Department of Chemistry, University of Crete, GR
Host Chamber:
CNRS-ICARE, CNRS Orleans, FR
Period:
2012.02.06 – 2012.03.09
Campaign name:
Coworkers:
Vassileios C. Papadimitriou, Department of Chemistry,
University of Crete, GR;
Mathieu Cazaunau, Institut de Combustion, Aérothermie
Réactivité et Environnement, CNRS, Orléans, FR;
Maria Lendar, Institut de Combustion, Aérothermie Réactivité
et Environnement, CNRS, Orléans, FR;
Abdelwahid Mellouki, Institut de Combustion, Aérothermie
Réactivité et Environnement, CNRS, Orléans, FR.
TA description (max. 5 pages) includes1:
TA Title: Atmospheric Chemistry of hexafluoroisobutene (CF3)2C=CH2
Reason for Choosing the ICARE-CNRS Installation
The atmospheric impact evaluation of the new-proposed CFC substitutes requires their detailed
kinetic and mechanistic investigation, in order to determine their atmospheric lifetimes, atmospheric
degradation products and the strength of their interaction with the IR radiation in the critical range
of the atmospheric window (800 – 1200 cm-1). Unsaturated hydrofluorocarbons, such as
(CF3)2C=CH2 (HFIB), have been recently considered as potent CFC alternatives with several
commercial and industrial application (refrigerants, fire suppressing, cleaning, and etching agents);
thus, their atmospheric life cycle assessment is necessary before their mass-scale production. The
ICARE chamber constitutes a very powerful set-up to study the reactivity of HFIB against the
dominant atmospheric oxidation species, e.g., OH radicals, Cl atoms and O3, under ambient
atmospheric conditions. Moreover, the white-cell FTIR spectroscopy and the PTR/TOF-MS
spectrometry that the chamber is equipped with, to in-situ monitoring the reaction products, for OH
and O3 initiated reactions, provides with the advantage of obtaining useful information,
simultaneously, both about the reaction mechanism and the end-products of the atmospheric
oxidation scheme. Finally, the experience and the expertise of the host co-workers in Atmospheric
Chemistry and kinetics assured the scientific and technological support for the project to be carried
out producing reliable and of high quality results.
Experimental Set-ups
Two experimental set-ups were used during the course of the title experiments: a. The Big-Chamber
of ICARE-CNRS to determine the kinetics and the end-products for the reactions of OH radicals
and O3 with HFIB, at T =298 K and P =760 Torr. and b. The Small-Chamber of ICARE-CNRS to
measure the rate coefficients for the reaction of Cl atoms with HFIB, at T =298 K and P =760 Torr.
1
The author(s) decide which details are publishable. By sending this document the author(s) agree with its publication
on the EUROCHAMP web site.
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a. ICARE-CNRS Big-Chamber
The kinetics for the reactions of OH radicals and O3 were studied using the ICARE-CNRS BigChamber facility. The chamber is made of Teflon with a total volume of 7.3 m3 and surface to
volume ratio 3.2 m-1, hosted in a specially designed dark-housing. Reactants’ well-mixing was
accomplished by using two Teflon fans, positioned at opposite sides. O3 was introduced in the
chamber using a Trailigaz O3 generator and was continuously monitored using a Horiba (APOA370) ultra-violet ozone analyzer and Infrared spectroscopy. OH radicals were produced by H2O2
photolysis using up to 24 UV lamps centered at λ = 254 nm that are located outside the two
opposite sides of the Teflon Chamber. (CF3)2C=CH2 and the reference compounds used to measure
the relative rate coefficients for OH initiated reactions were introduced into the chamber by a
separate inlet, via a 0.9 dm3 titrated buffer volume. Temperature, relative humidity and NOx levels
were continuously monitored using one dedicated sensor and a NOx analyzer (HORIBA, APNA360 and Thermo Environment 42i TL), respectively. Reactants and products were measured using a
white-cell multi-pass FTIR spectrophotometer (Nicolet Magna 5700) of total optical length l =143
m, by co-adding 130 scans at resolution of R =1 cm-1. PTR/TOF-MS (IONICON PTR/TOF-MS
8000) that is assorted with the chamber was also used for reactants and products monitoring. SF6
was used as a marker to estimate diffusion and first order losses of the reactants, e.g. wall loss.
Photoreactor total pressure was balanced with the continuous admission of ultrapure synthetic air at
760 Torr.
b. ICARE-CNRS Small-Chamber
Cl kinetics were studied using the ICARE-CNRS Small-Chamber facility employing the relative
rate method. Reactor Teflon-bag volume was 140 dm3 surrounded by six black lamps (Philips, TL
20W/05) with maximum intensity centered at 365 nm. The chamber and and the UV lamps were
enclosed in a wooden box with the internal faces covered with aluminium foil. Reagents were
admitted into the photoreactor using calibrated bulbs and the total pressure was balanced at
atmospheric pressure with ultrapure synthetic air. Cl atoms were generated by Cl2 photolysis and
Gas Chromatography - Flame Ionisation Detector (GC-FID, Star 3800 CX, Varian) was used for the
quantitative analysis of reactants and reaction products. Chromatographic separation was achieved
by using a DB-1 capillary column (J&W Scientific, 30 m, 0.25 mm id, 5 µm film). The columns
were operated over the temperature range 313 – 343 K and helium was used as the carrier gas.
Time Table and Initial Objectives
The initial objectives of the four-weeks project was to measure the rate coefficients for the OH
radicals and O3 with (CF3)2C=CH2 in the gas phase, at T = 298 K and P = 760 Torr, as well as to
determine the end-products of the title reactions based on the following time-table schedule:
1st week: Calibration Experiments
2nd and 3rd week: OH kinetics and Product Studies
4th week: O3 kinetics and Product Studies
The targets of the project were entirely fulfilled without any deviations from the initial project. In
addition to that, Cl kinetics were also studied (during the 3rd week).
Calibration Experiments
Prior to rate coefficients determination and end-product studies, several calibration experiments and
tests were carried out. In particular, IR spectra for (CF3)2C=CH2, C2H6, C3H8, SF6, H2O2, and O3
were recorded in order a. to check for possible bands interferences and the appropriateness of using
the selected references for the kinetic measurements and b. to optimise the conditions for the
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quantitative kinetic measurements based on the band-strength of the selected for the kinetic
measurements IR regions, and to improve the precision of the rate coefficient determinations.
Moreover, possible photolysis, for all the compounds used, from UV light irradiation was tested in
absence of OH radicals’ precursor, H2O2. In addition, diffusion out of the chamber and wall and
hydrolysis loss was measured to be negligible under dark conditions (lamps off) as it was verified
by comparing with the observed loss of SF6 that was always less than 2% after ~ 5 hours of test
measurements. It is worth to note that SF6 was also used as marker during UV light irradiation of
the reaction mixture, since it is not photolabile at the applied wavelengths as it was verified during
the course of this project.
OH Kinetics and Product Studies
The rate coefficient for the reaction of OH radicals with (CF3)2C=CH2 (1) was measured in the gas
phase, at T = 298 ±3 K and P =760 Torr (in synthetic air), employing the relative rate method. OH
radicals were produced by UV photolysis of H2O2 at λ = 254 nm that was introduced in the reaction
chamber along with (CF3)2C=CH2 (HFIB), reference compound and SF6. Two different reference
reactions were used, namely OH + C2H6, k1ref(298 K) = 2.50 ×10-13 cm3 molecule-1 s-1 and OH +
C3H8, k2ref(298 K) =1.07 ×10-12 cm3 molecule-1 s-1, in order to suppress and better estimate the
systematic uncertainties of the measurements. It can be proved that under our experimental
conditions, at which HFIB and the selected reference compound are only lost due to their reaction
with OH radicals, the following expression is valid:
ln([HFIB]0/[HFIB]t) = k1/kref × ln([Ref]0/[Ref]t)
(1)
where, [HFIB]0, [HFIB]t, [Ref]0 and [Ref]t are the concentrations of (CF3)2C=CH2 and the
corresponding references, with indexes 0 to corresponding to the measured concentrations before
the UV photolysis of the reaction mixture (in absence of OH radicals) and t at discrete time
intervals after UV irradiation (OH radicals present). k1 and kref correspond to the rate coefficients
for the reaction of OH radicals with HFIB and references, respectively. Therefore, the least square
analysis of the experimental data based on expression 1 lead to k1 determination as it is depicted in
figure 1a. The rate coefficients determined using each of the reference were, k1(298 K) = (7.96
±0.15) ×10-13 cm3 molecule-1 s-1 and k1(298 K) = (7.54 ±0.11) ×10-13 cm3 molecule-1 s-1 using C2H6
and C3H8 as references, respectively. The quoted uncertainties are the 2σ (95 % level of confidence)
precision of the measurements.
A series of separate measurements were carried out for end-products analysis. In this course,
reference compounds and SF6 were not introduced in the chamber to avoid interferences and the
reaction progress was monitored until at least of 60% conversion of (CF3)2C=CH2 due to its
reaction with OH radicals keeping identical to kinetic measurements conditions. CO formation was
observed that was attributed to HCHO formation followed by its reaction with OH radicals (figure
1b1). Moreover, fluorinated products were also observed that possibly correspond to fluorinated
aldehydes and/or acids (figure 1b2). Further analysis and interpretation of the obtained data is under
consideration.
O3 Kinetics and Product Studies
The reaction kinetics for the reaction of O3 with (CF3)2C=CH2 (2) were studied under pseudo-first
order conditions in (CF3)2C=CH2 ([(CF3)2C=CH2]>>[O3]), at T = 298 K and P = 760 Torr. During
the whole course of measurements the loss of (CF3)2C=CH2 due to its reaction with O3 after at least
of 5 hour measurements was negligible, within experimental uncertainty, and in the same order with
the measured loss of diffusion out of the reactor (kd =(5.78 ±0.10) ×10-6 s-1), as it was determined
monitoring both SF6 and (CF3)2C=CH2. Therefore, only an upper kinetic limit for the title reaction
was able to be determined that was k2 <3 ×10-21 cm3 molecule-1 s-1 (figure 2).
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a.
b1.
b2.
Figure 1. a. Relative rate coefficients determination for the reaction of OH + (CF3)2C=CH2 at T =
298 K and P = 760 Torr. Symbols and data analysis are given as insets, with the dashed lines to
corresponding to least-square analysis of the experimental data, based on expression 1. b. IR
regions were reaction products were observed. Red and blue lines correspond to the IR spectra
recorded in absence of OH radicals and after ~ 60 % conversion of HFIB after subtracting the initial
reactants spextra, respectively. b1 shows the CO formation and b2 depicts products formation in CF IR region.
Figure 2. Rate coefficient determination for the reaction of O3 with (CF3)2C=CH2 under pseudofirst order conditions in HFIB.
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Cl Kinetic Studies
Similar to OH kinetics measurements, the rate coefficient for the reaction of Cl with (CF3)2C=CH2
(3) was determined employing the relative rate method using the ICARE-CNRS Small Chamber
facility equipped with GC-FID detection technique. Cl atoms were produced by Cl2 photolysis by
irradiating the reaction mixture with light centered at λ =365 nm.
C2H6 (1ref) and
CH3CH2C(O)CH3 (2ref) were used as reference compounds with k1ref(298 K) = 5.70 ×10-11 cm3
molecule-1 s-1 and k2ref(298 K) = 4.00 ×10-11 cm3 molecule-1 s-1, respectively. The least square
analysis of the experimental data based on expression 1 lead to k3 determination as it is depicted in
figure 3. The rate coefficients determined using each of the reference were, k1(298 K) = (3.51
±0.04) ×10-11 cm3 molecule-1 s-1 and k1(298 K) = (3.41 ±0.08) ×10-11 cm3 molecule-1 s-1 using C2H6
and C3H8 as references, respectively. The quoted uncertainties are the 2σ (95 % level of confidence)
precision of the measurements.
Figure 3. Relative rate coefficients determination for the reaction of Cl + (CF3)2C=CH2 at T = 298
K and P = 760 Torr. Symbols and data analysis are given as insets, with the dashed lines to
corresponding to least-square analysis of the experimental data, based on expression 1.
Necessity/ intention of continuing work within the EUROCHAMP infrastructure
In continuation of the actual collaboration and due to the increased interest and impact of the
ongoing work, there is a mutual intention to perform additional measurements, in which the
temperature dependence of the absolute rate coefficient for the reaction of OH radicals with HFIB
will be determined using the PLP/LIF facility located in ICARE-CNRS. This will add extra value
on the project, since two independent and complementary techniques, along with the scheduled
experiments that are currently going on in University of Crete, will enable us to fully examine all
the different aspects of the project. More experiments are also planned to investigate the OHinitiated oxidation using CH3ONO as OH radicals precursor.
Outcomes (intended publication(s), PhD work, etc.)
Within the frame of this work the results will be published in peer-reviewed journals (at least one
publication) and some part of those will be included in M. Lendar’s (PhD candidate) PhD
dissertation.
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