Atmospheric Chemistry of c-C₅HF₇ and c-C₅F₈: Temperature-Dependent OH Reaction Rate Coefficients, Degradation Products, Infrared Spectra, and Global Warming Potentials

Abstract

c-C 5 HF 7 (1H-heptafluorocyclopentene) and c-C 5 F 8 (perfluorocyclopentene) are potent greenhouse gases presently used as replacement compounds in Si etching. A thorough understanding of their potential impact on climate and air quality necessitates studies of their atmospheric reactivity, radiative properties, and atmospheric degradation pathways. The predominant atmospheric removal process for these compounds is expected to be via reaction with the OH radical. In this study, rate coefficients, k, for the gas-phase reaction of the OH radical with c-C 5 HF 7 and c-C 5 F 8 were measured over a range of temperatures (242−370 K) and pressures (50−100 Torr, He) using a pulsed laser photolysis−laser-induced fluorescence technique. In addition, a complementary relative rate technique, employing multiple reference compounds, was used to study the reactions between 273 and 372 K at 100 Torr (He) total pressure. Reaction rate coefficients were found to be independent of pressure over this range of conditions with k 1 (296 K) = (4.59 ± 0.10) × 10 −14 cm 3 molecule −1 s −1 and k 1 (T) = (4.00 ± 0.40) × 10 −13 exp(−(631 ± 30)/T) cm 3 molecule −1 s −1 for c-C 5 HF 7 and k 2 (296 K) = (4.90 ± 0.14) × 10 −14 cm 3 molecule −1 s −1 and k 2 (T) = (3.59 ± 0.4) × 10 −13 exp(−(591 ± 25)/T) cm 3 molecule −1 s −1 for c-C 5 F 8 . Stable end-products were measured following the OH radical-initiated degradation of c-C 5 HF 7 and c-C 5 F 8 in the presence of O 2 . F(O)CCF 2 CF 2 CF 2 CH(O), CF 2 O, and CO 2 were observed as the major end-products in the oxidation of c-C 5 HF 7 with molar yields of 0.64, 1.27, and 0.53, respectively. For c-C 5 F 8 , F(O)CCF 2 CF 2 CF 2 CF(O), CF 2 O, and CO 2 were observed with molar yields of 0.66, 0.63, and 0.43, respectively. The total carbon mass balance in both systems was 1.0 ± 0.15. The high yield of a C5-dicarbonyl endproduct is consistent with a ring opening at the carbon−carbon double bond site for both c-C 5 HF 7 and c-C 5 F 8 . A comparison of the present kinetic and degradation product results with previously published studies is presented. A rate coefficient upper limit for the gas-phase reaction of O 3 with c-C 5 HF 7 and c-C 5 F 8 of 1 × 10 −21 cm 3 molecule −1 s −1 was measured as part of this work. Atmospheric lifetimes for c-C 5 HF 7 and c-C 5 F 8 are estimated to be 252 and 236 days, respectively. Infrared absorption spectra of c-C 5 HF 7 and c-C 5 F 8 were also measured and found to agree, to within 5%, with results from previous studies. The well-mixed and lifetime adjusted radiative efficiencies (RE, W m −2 ppb −1 ) and 100 year time horizon global warming potential (GWP) for c-C 5 HF 7 are 0.35, 0.24, and 46.7 and for c-C 5 F 8 are 0.38, 0.25, and 46.2, respectively.

Reaction of Cl Atom with c-C5F8 and c-C5HF7: Relative and Absolute Measurements of Rate Coefficients and Identification of Degradation Products

“…The products in dashed rectangles are carbonyl species that could account for the observed 1700–1950 cm –1 residual absorption after subtracting compounds with known reference spectra. The oxidation mechanism follows the peroxy and alkoxy chemistry as previously described in Gierczak et al and Liu et al for the OH reaction. , …”

“…The IR spectra of c-C 5 F 8 and c-C 5 HF 7 were taken from our previous study. 2 The infrared spectrometer was equipped with a multipass absorption cell (365 cm path length) with KBr windows mounted on the infrared radiation entry and exit ports. The detector was a liquid N 2 cooled HgCdTe/B semiconductor.…”

“…k 1 (296 K) and k 2 (296 K) are about 22 and 100 times faster than those of the corresponding OH radical reactions measured in our previous work. 2 Assuming a global average concentration of OH radical and Cl atom of 1 × 10 6 and 6 × 10 2 molecule cm −3 , 8 respectively, the Cl atom reaction is expected to contribute about 1% to the atmospheric removal of c-C 5 F 8 and 6% in the case of c-C 5 HF 7 . However, in regions where the Cl atom concentration can be as high as 1 × 10 4 molecule cm −3 , 8 the Cl reaction can contribute as much as 18% and 50% to the atmospheric removal of c-C 5 F 8 and c-C 5 HF 7 , respectively.…”

“…While the OH radical and Cl atom rate coefficients measurements are important for understanding the direct impact of these compounds on climate, the atmospheric oxidation mechanism and degradation product identification is also important in the evaluation of indirect environmental effects from the use of c -C 5 F 8 and c -C 5 HF 7 as replacement compounds. Our previous mechanistic studies for the OH + c -C 5 F 8 and c -C 5 HF 7 reactions revealed the formation of dicarbonyl species that were not directly observed in previous studies. , The Cl atom-initiated oxidation of c -C 5 F 8 and c -C 5 HF 7 has not been explored prior to this work and differences, or similarities, with the OH radical-initiated oxidation are not understood. Therefore, the characterization and quantification of intermediate and end products of the Cl-initiated oxidation is needed to complete our knowledge of the atmospheric chemistry and impact of c -C 5 F 8 and c -C 5 HF 7 .…”

“…c -C 5 F 8 and c -C 5 HF 7 , cyclic unsaturated compounds, are currently under consideration as etching agents for the semiconductor industry . The primary pathway for removal of these compounds from the atmosphere is expected to be reaction initiated by the OH radical addition to the carbon–carbon double bond as discussed in previous work. − Although these cyclic compounds are highly fluorinated, the OH radical reaction is sufficiently rapid that the compounds have relatively short atmospheric lifetimes and thus, low global warming potentials (GWPs). − The reaction of the Cl atom with hydrofluoroolefins, HFOs, typically have larger rate coefficients than that of the OH reaction , and could represent an additional atmospheric sink for these compounds, especially in locations where Cl atom concentration can be as high as ∼10 4 atom cm –3 …”