Probing Gas-Phase Pyrolysis of C2-C4 Perfluoroalkanes with Photoionization Mass Spectrometry

The pyrolytic gas-phase decomposition of C2-C4 perfluoroalkanes (PFA), namely hexafluoroethane (C2F6), octafluoropropane (C3F8), and decafluorobutane (n-C4F10), was investigated using a silicon carbide chemical microreactor coupled with tunable vacuum ultraviolet photoionization mass spectrometry (PIMS). This work characterizes the pyrolysis products over a temperature range of 293-1373 K, complementing existing theory-based kinetic models on PFA pyrolysis. A notable outcome of this study is the first observation of the parent-ion signals for C2F6, C3F8, and n-C4F10 utilizing the PIMS technique. The observed products from C2F6 pyrolysis were difluorocarbene (CF2) and trifluoromethyl (CF3). In contrast, pyrolysis of both C3F8 and n-C4F10 yielded fluoromethylidyne (CF), CF2, CF3, tetrafluoroethene (C2F4), and pentafluoroethyl (C2F5). Photoionization spectra were measured for each species observed in the experiments. Appearance energies (AEs) were obtained from the spectra and compared with adiabatic ionization energies (IEs) from quantum chemical calculations. For the three PFA and most of the pyrolysis products, there are significant disagreements between the AE values from the current work and literature values, which are discussed. However, the measured AEs are mostly in excellent agreement with the calculated adiabatic IEs. The observed products indicate that the pyrolysis of the three PFA and their radical products with more than two carbon atoms proceeds through C-C bond cleavage, whereas CF3 and CF2 radicals participate in addition reactions or decompose by unimolecular C-F bond cleavage.