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Atmospheric chemistry of (CF)CF-C#N - A replacement compound for the most potent industrial greenhouse gas, SF
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Mads Peter Sulbaek Andersen, Mildrid Kyte, Simone Thirstrup Andersen, Claus Jørgen Nielsen, and Ole John Nielsen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03758 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on January 4, 2017
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Environmental Science & Technology
Atmospheric chemistry of (CF3)2CF-C≡N - A replacement compound for the most potent industrial greenhouse gas, SF6
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Mads P. Sulbaek Andersen1,2 *, Mildrid Kyte2, Simone Thirstrup Andersen2, Claus J. Nielsen3, and Ole John Nielsen2
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Department of Chemistry and Biochemistry, California State University, Northridge, California 91330, USA 2
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Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark
Department of Chemistry, University of Oslo, P.O.Box 1033 Blindern, 0315 Oslo, Norway
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* Corresponding author (MPSA): Phone: +1 818 677 2492, Fax: +1 818 677 4068, E-mail:
[email protected].
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Abstract
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FTIR/smog chamber experiments and ab initio quantum calculations were performed to
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investigate the atmospheric chemistry of (CF3)2CFCN, a proposed replacement compound for the
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industrially important sulfur hexafluoride, SF6. The present study determined k(Cl +
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(CF3)2CFCN)= (2.33 ± 0.87) × 10–17, k(OH + (CF3)2CFCN) = (1.45 ± 0.25) × 10-15 and k(O3 +
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(CF3)2CFCN) ≤ 6×10-24 cm3 molecule–1 s–1, respectively, in 700 Torr of N2 or air diluent at 296 ±
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2 K. The main atmospheric sink for (CF3)2CFCN was determined to be reaction with OH
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radicals. Quantum chemistry calculations, supported by experimental evidence, shows that the
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(CF3)2CFCN + OH reaction proceeds via OH addition to –C(≡N), followed by O2 addition to –
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C(OH)=Ṅ, internal H-shift and OH regeneration. The sole atmospheric degradation products of
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(CF3)2CFCN appears to be NO, COF2 and CF3C(O)F. The atmospheric lifetime of (CF3)2CFCN
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is approximately 22 years. The integrated cross section (650-1500 cm-1) for (CF3)2CFCN is (2.22
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± 0.11) × 10-16 cm2 molecule-1 cm-1 which results in a radiative efficiency of 0.217 W m-2 ppb-1.
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The 100-year Global Warming Potential (GWP) for (CF3)2CFCN was calculated as 1490, a
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factor of 15 less than that of SF6.
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1. Introduction
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Sulphur hexafluoride, SF6, is a compound with important industrial applications such as a
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dielectric insulator in high-voltage transformers, electric cables or buses, and circuit breakers or
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switchgear. The usage of SF6 has been increasing since 1985 and current emissions are
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approaching 10kt/a [1,2]. SF6 has a lifetime of 3200 years in the atmosphere and a Global
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Warming Potential (GWP100) of 23,500 [1], which makes SF6 the most potent greenhouse gas.
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Currently, the atmospheric mole fraction of SF6 is 7.28 ppq corresponding to a radiative forcing
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of 0.0041 w/m2 [1]. Finding a suitable replacement technology for this compound would be
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highly desirable. Heptafluoro-isobutyronitrile, (CF3)2CFCN, is a non-toxic compound and a
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potential dielectric insulator replacement of SF6 [3,4]. Detailed knowledge of the atmospheric
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chemistry of (CF3)2CFCN is warranted to access its potential environmental impact, before any
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large scale production and industrial use of the compound. The present study investigates the
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atmospheric chemistry of (CF3)2CFCN. Both smog chamber experiments and ab initio
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calculations were conducted to determine the kinetics of reactions with OH radicals, with
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chlorine atoms and with O3, the atmospheric oxidation mechanism, the atmospheric lifetime and
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the global warming potential(s) of (CF3)2CFCN.
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2. Experimental
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2.1 Photo-reactor experiments
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The experimental part of the present work was conducted in the recently updated
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CCAR (Copenhagen Center for Atmospheric Research) photo-reactor. At the core of this
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setup is a 101 liter quartz reactor interfaced with a Bruker IFS 66v/s FTIR spectrometer. See
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Nilsson et al. [5] for details. All experiments in the present work were performed at 296 ± 1 K
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in 700 Torr of air diluent. Using an analytical path length of 50.01-53.42 m, IR spectra were
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obtained from averaging of 32 interferograms with a spectral resolution of 0.25 cm-1.
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Quantitative analysis of reactant and reference compounds was performed using absorption
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features over the following wavenumber ranges: CH4: 2860-3200; CF3CF2H: 869, 3001 cm-1;
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CF3CH3: 1407, 1440 cm-1; (CF3)2CFCN: 2272 cm-1; O3: 2720-2785 cm-1; COF2: 1943 cm-1
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and CF3C(O)F: 1897 cm-1. 2 ACS Paragon Plus Environment
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Environmental Science & Technology
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Ozone was produced from pure O2 using a commercially available ozone-discharge
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generator from O3-Technology, and pre-concentrated using a silica gel trap submerged in a
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dry ice/isopropanol cooling bath (-77 oC), significantly reducing the amount of O2 introduced
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into the chamber. (CF3)2CFCN was supplied by 3M with a purity of >99% and degassed in
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several freeze-pump-thaw cycles before use. All other reagents used in in the present work
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were purchased from commercial sources and certified with purities of >99%.
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Chlorine atoms were produced by photolysis (Osram Eversun L100/79 UVA lamps, emission peak at 368 nm) of Cl2 according to reaction (1):
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Cl2 + hv → 2 Cl
(1)
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Hydroxyl radicals (OH) were generated effectively by photolysis of O3 using UVB lamps
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(Waldmann F85/100 UV6, Wavelength region 280-360 nm) in the presence of H2:
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O3 + hν → O(1D) + O2
(2)
1
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H2 + O( D) → OH + H
(3)
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O3 + H → OH + O2
(4)
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H + O2 + M → HO2 + M
(5)
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HO2 + O3 → OH + 2O2
(6)
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HO2 + HO2 → H2O2 + O2
(7)
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H2O2 + hν → 2 OH
(8)
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OH + H2 → H + H2O
(9)
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OH + O3 → HO2 + O2
(10)
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OH + H2O2 → HO2 + H2O
(11)
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OH + HO2 → H2O + O2
(12)
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In 700 Torr of total pressure, quenching of O(1D) is significant, but the resulting O(3P) will
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readily recombine with O2 to reform O3, which renders the reaction of O(3P) with any of the
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organics of no importance at the levels present in the chamber. Reaction 3 and 4 produce OH
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radicals in the ground state and also in excited vibrational states with ν ≤ 4 and ν ≤ 9 for reaction
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3 and 4, respectively [6,7,8]. The excited OH species can be assumed to undergo complete
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deactivation in collisions with N2 and O2 before reacting with any of the organics: firstly,
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vibrational relaxation of OH by reaction with N2 and O2 occurs with rate coefficients of 10−15 3 ACS Paragon Plus Environment
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and 10−13 cm3 molecule−1 s−1, respectively,[9] and secondly, the mixing ratios of O2 and N2 are
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significantly larger (4−5 orders of magnitude) than any of the organic reactants.
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One must also consider that the rate coefficients for O(1D) reaction with reference compounds
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methane and H2 are of comparable magnitude, 1.75 × 10−10 and 1.2 × 10−10 cm3 molecule−1 s−1 at
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298 K, [10] respectively (the reference rate coefficient for O(1D) reaction with CF3CF2H is 1.03
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× 10-11 cm3 molecule−1 s−1 [10]; no rate coefficient exist for the reaction of O(1D) with
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(CF3)2CFCN). However, the mixing ratios of H2 are typically 2-3 orders of magnitude larger than
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those of the organics. Thus, and reaction of O(1D) with the organics these can be safely ignored
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(see section 3.2).
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Quantitative analysis of the reactants and products concentrations was achieved using
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in situ FTIR spectroscopy and by process of spectral stripping in which a previously
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quantified reference spectra was subtracted from the spectrum of interest. The reference
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spectra employed here were calibrated by expanding known volumes of reference compounds
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into the photo-reactor.
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Kinetic measurements of chlorine atoms or OH radical reactions were conducted using
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the well-established relative rate method. The loss of (CF3)2CFCN was measured relative to
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one or more reference compounds and plotted using the expression:
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[( ) ]
ln [(
) ]
( )
=
[ ! ]
ln [
! ]
(I)
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where [(CF3)2CFCN]t0, [(CF3)2CFCN]t, [Reference]t0, and [Reference]t are the concentrations
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of the reactant and the reference at times t0 and t, and k(CF3)2CFCN and kreference are the rate
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coefficients for the reactant and the reference. Plots of ln[(CF3)2CFCN]t0/[(CF3)2CFCN]t)
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versus ln([Reference]t0/[Reference]t) should be linear, pass through the origin, and have a
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slope of k(CF3)2CFCN/kreference.
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Kinetic measurements for the O3 reaction were conducted using an absolute rate
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method, where the pseudo first order loss of (CF3)2CFCN was determined in the presence of
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excess O3.
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Complications due to photolysis and heterogeneous reactions which can lead to
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unwanted loss of reactants, reference compounds, and products, need to be considered.
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Control experiments, in which mixtures of (CF3)2CFCN and reference compounds were 4 ACS Paragon Plus Environment
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Environmental Science & Technology
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subjected to 30 mins of UV radiation in the absence of oxidants (Cl atoms, O3 or OH radicals)
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were performed. Mixtures obtained after UV irradiations were also allowed to remain in the
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chamber in the dark for 30 minutes. Neither set of control experiments showed any significant
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loss of reactants or products (
