Article pubs.acs.org/est
Atmospheric Chemistry of (CF3)2CF−CN: A Replacement Compound for the Most Potent Industrial Greenhouse Gas, SF6 Mads P. Sulbaek Andersen,*,†,‡ Mildrid Kyte,‡ Simone Thirstrup Andersen,‡ Claus J. Nielsen,§ and Ole John Nielsen‡ †
Department of Chemistry and Biochemistry, California State University, Northridge, California 91330, United States 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 ‡
S Supporting Information *
ABSTRACT: FTIR/smog chamber experiments and ab initio quantum calculations were performed to investigate the atmospheric chemistry of (CF3)2CFCN, a proposed replacement compound for the industrially important sulfur hexafluoride, SF6. The present study determined k(Cl + (CF 3 ) 2 CFCN) = (2.33 ± 0.87) × 10 −17 , k(OH + (CF3)2CFCN) = (1.45 ± 0.25) × 10−15, and k(O3 + (CF3)2CFCN) ≤ 6 × 10−24 cm3 molecule−1 s−1, respectively, in 700 Torr of N2 or air diluent at 296 ± 2 K. The main atmospheric sink for (CF3)2CFCN was determined to be reaction with OH radicals. Quantum chemistry calculations, supported by experimental evidence, shows that the (CF3)2CFCN + OH reaction proceeds via OH addition to −C(N), followed by O2 addition to −C(OH)N·, internal H-shift, and OH regeneration. The sole atmospheric degradation products of (CF3)2CFCN appear to be NO, COF2, and CF3C(O)F. The atmospheric lifetime of (CF3)2CFCN is approximately 22 years. The integrated cross section (650−1500 cm−1) for (CF3)2CFCN is (2.22 ± 0.11) × 10−16 cm2 molecule−1 cm−1 which results in a radiative efficiency of 0.217 W m−2 ppb−1. The 100-year Global Warming Potential (GWP) for (CF3)2CFCN was calculated as 1490, a factor of 15 less than that of SF6.
1. INTRODUCTION
the atmospheric oxidation mechanism, the atmospheric lifetime, and the global warming potential(s) of (CF3)2CFCN.
Sulfur hexafluoride, SF6, is a compound with important industrial applications such as a dielectric insulator in highvoltage transformers, electric cables or buses, and circuit breakers or switchgear. The usage of SF6 has been increasing since 1985, and current emissions are approaching 10kt/a.1,2 SF6 has a lifetime of 3200 years in the atmosphere and a Global Warming Potential (GWP100) of 23 500,1 which makes SF6 the most potent greenhouse gas. Currently, the atmospheric mole fraction of SF6 is 7.28 ppq corresponding to a radiative forcing of 0.0041 w/m2.1 Finding a suitable replacement technology for this compound would be highly desirable. Heptafluoroisobutyronitrile, (CF3)2CFCN, is a nontoxic compound and a potential dielectric insulator replacement of SF6.3,4 Detailed knowledge of the atmospheric chemistry of (CF3)2CFCN is warranted to access its potential environmental impact, before any large scale production and industrial use of the compound. The present study investigates the atmospheric chemistry of (CF3)2CFCN. Both smog chamber experiments and ab initio calculations were conducted to determine the kinetics of reactions with OH radicals, with chlorine atoms, and with O3, © 2016 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Photoreactor Experiments. The experimental part of the present work was conducted in the recently updated CCAR (Copenhagen Center for Atmospheric Research) photoreactor. At the core of this setup is a 101 L quartz reactor interfaced with a Bruker IFS 66v/s FTIR spectrometer. See Nilsson et al.5 for details. All experiments in the present work were performed at 296 ± 1 K in 700 Torr of air diluent. Using an analytical path length of 50.01−53.42 m, IR spectra were obtained by averaging 32 interferograms with a spectral resolution of 0.25 cm−1. Quantitative analysis of reactant and reference compounds was performed using absorption features over the following wavenumber ranges: CH4: 2860−3200; CF3CF2H: 869, 3001 cm−1; CF3CH3: 1407, 1440 cm−1; (CF3)2CFCN: Received: Revised: Accepted: Published: 1321
July 27, 2016 December 5, 2016 December 12, 2016 December 12, 2016 DOI: 10.1021/acs.est.6b03758 Environ. Sci. Technol. 2017, 51, 1321−1329
Article
Environmental Science & Technology 2272 cm−1; O3: 2720−2785 cm−1; COF2: 1943 cm−1; CF3C(O)F: 1897 cm−1. Ozone was produced from pure O2 using a commercially available ozone-discharge generator from O3-Technology, and preconcentrated using a silica gel trap submerged in a dry ice/ isopropanol cooling bath (−77 °C), significantly reducing the amount of O2 introduced into the chamber. (CF3)2CFCN was supplied by 3 M with a purity of >99% and degassed in several freeze−pump−thaw cycles before use. All other reagents used in in the present work were purchased from commercial sources and certified with purities of >99%. Chlorine atoms were produced by photolysis (Osram Eversun L100/79 UVA lamps, emission peak at 368 nm) of Cl2 according to reaction 1: Cl 2 + hv → 2Cl
Quantitative analysis of the reactants and products concentrations was achieved using in situ FTIR spectroscopy and by the process of spectral stripping in which a previously quantified reference spectra was subtracted from the spectrum of interest. The reference spectra employed here were calibrated by expanding known volumes of reference compounds into the photoreactor. Kinetic measurements of chlorine atoms or OH radical reactions were conducted using the well-established relative rate method. The loss of (CF3)2CFCN was measured relative to one or more reference compounds and plotted using the expression: ⎛ [(CF3)2 CFCN]t ⎞ k(CF ) FCN ⎛ [Reference]t ⎞ 3 2 0 0 ⎟⎟ = ⎟⎟ ln⎜⎜ ln⎜⎜ kReference ⎝ [Reference]t ⎠ ⎝ [(CF3)2 CFCN]t ⎠
(1)
Hydroxyl radicals (OH) were generated effectively by photolysis of O3 using UVB lamps (Waldmann F85/100 UV6, wavelength region 280−360 nm) in the presence of H2: O3 + hν → O(1D) + O2
(2)
H 2 + O(1D) → OH + H
(3)
O3 + H → OH + O2
(4)
H + O2 + M → HO2 + M
(5)
HO2 + O3 → OH + 2O2
(6)
HO2 + HO2 → H 2O2 + O2
(7)
H 2O2 + hν → 2OH
(8)
OH + H 2 → H + H 2O
(9)
OH + O3 → HO2 + O2
(10)
OH + H 2O2 → HO2 + H 2O
(11)
OH + HO2 → H 2O + O2
(I)
where [(CF3)2CFCN]t0, [(CF3)2CFCN]t, [Reference]t0, and [Reference]t are the concentrations of the reactant and the reference at times t0 and t and k(CF3)2CFCN and kReference are the rate coefficients for the reactant and the reference. Plots of ln[(CF3)2CFCN]t0/[(CF3)2CFCN]t versus ln([Reference]t0/ [Reference]t) should be linear, pass through the origin, and have a slope of k(CF3)2CFCN/kreference. Kinetic measurements for the O3 reaction were conducted using an absolute rate method, where the pseudo first order loss of (CF3)2CFCN was determined in the presence of excess O3. Complications due to photolysis and heterogeneous reactions, which can lead to unwanted loss of reactants, reference compounds, and products, need to be considered. Control experiments, in which mixtures of (CF3)2CFCN and reference compounds were subjected to 30 min of UV radiation in the absence of oxidants (Cl atoms, O3, or OH radicals), were performed. Mixtures obtained after UV irradiations were also allowed to remain in the chamber in the dark for 30 min. Neither set of control experiments showed any significant loss of reactants or products (
