| Abstract |
Tropospheric end-oxidation-products of 1,1,1,2-tetrafluoroethane (CF3CH2F, HFC-134a) were qualitatively and quantitatively determined and the degradation mechanism was thoroughly investigated, in this work. HFC-134a belongs to the second generation of chlorofluorocarbon alternatives (CFCs) and it is, even todays, one of the most widely used compounds in industry and everyday life. Its exceptional physicochemical properties, the negligible ozone depletion potential (ODP) and the substantially lower global warming potential (GWP) compared to CFCs have led to its widespread use in a variety of applications, with the most well-known one its use in Mobile Air-Conditioning (MAC) units. However, HFC-134a atmospheric implications evaluation and the effect on Air-Quality, Climate and water resources requires both qualitative and quantitative determination of end-oxidation-products, once released in the Atmosphere, so as to globally estimate and predict its impact during its atmospheric lifetime (Full Life - Cycle Assessment, FLCA).
Within this context, the kinetics for the reaction of Cl atoms with HFC-134a were studied in the temperature range 243 - 363 K and 700 Torr total pressure (synthetic air, 80% N2, 20% O2), in the gas phase. Next, quantum yields of the major end-oxidation-products were determined as a function of temperature and pressure, between 243 - 363 K and 100 - 700 Torr, respectively. Experiments were carried out using the Thermostated PhotoChemical Reactor technique (TPCR) coupled with Fourier Transformed InfraRed Spectroscopy (FT-IRS), allowing the in-line monitoring of all the stable reactants and products. Polluted environment case was also simulated by conducting product-yield measurements in the presence of nitrogen oxides (NOx). The major products that were identified using Infrared spectroscopy were perfluorinated acetaldehyde (CF3C(O)F), fluoro- (HC(O)F) and perfluorinated or difluoro- (FC(O)F) formaldehyde. Further, Beer-Lambert law was employed to quantitatively measure reactants consumption and products formation and to look into the oxidation mechanism of HFC-134a. The focus of the present thesis was mainly to quantitatively determine the CF3C(O)F product-yield, since it is one of the mostly well-known trifluoroacetic acid (CF3C(O)OH, TFA) precursors, of which the environmental impact is currently debatable. The pressure dependent product-yield measurements shown there is no significant variation between 300 and 700 Torr, demonstrating the minor role of the third body interactions in this region. However, product-yields were sensitive at different molecular oxygen (O2) concentrations. Therefore, O2 role was thoroughly studied, as a function of O2 number density and temperature, at infinite pressure limit and the results from this study demonstrated that oxidation proceeds via two parallel mechanistic channels that lead to a. CF3C(O)F and b. HC(O)F and FC(O)F. CF3C(O)F product yield showed an increase at higher O2 concentrations and decreased temperatures. On the contrary, HC(O)F and FC(O)F production channel obeys a positive temperature and inversed O2 partial pressure dependence, respectively. Finally, experiments in the presence of NOx, showed that in polluted areas the product yields for HC(O)F and FC(O)F increase further, while the corresponding ones for CF3C(O)F formation were decreased.
|
Search
