An investigation of chlorofluorocarbon decomposition using thermal- and photo-catalysis techniques.

摘要

Several scientific findings over the last two decades have revealed striking correlation between stratospheric ozone depletion and chlorofluorocarbon (CFC) emissions. The present study focuses on safe disposal of two most widely used CFCs which also possess maximum ozone depletion potential, namely CFC11 (CCl{dollar}sb3{dollar}F) and CFC12 (CCl{dollar}sb2{dollar}F{dollar}sb2{dollar}).; Catalytic oxidation is one of the most economical and effective methods for destruction of halogenated hydrocarbons. The present study employed thermal as well as photocatalytic techniques for the complete oxidative destruction of CFCs. The catalysts compared in this study included H-Y, Co-Y, Ce-Y and Cr-Y zeolites, titanium dioxide and also titania as a photocatalyst.; Catalysts were characterized in a fixed bed reactor to determine their initial activity, initial selectivity and also stability. Thermal catalytic experiments were carried out at temperatures of 150-400{dollar}spcirc{dollar}C and at space velocities of 5000 or 10500 h{dollar}sp{lcub}-1{rcub}{dollar}. A CFC feed concentration of 1500-2000 ppm was studied under three different feed conditions (with additional oxygen or water or both present). The photocatalytic experiments were carried out at (or near) ambient temperature and at space velocities of 20-1200 h{dollar}sp{lcub}-1{rcub}{dollar}. As photodecomposition has been reported to yield considerably greater destruction of olefinic compounds, photodecomposition of trifluoroethylene (CHF=CF{dollar}sb2{dollar}) was also studied for comparison with single-carbon CFCs. A UV light source, emitting radiation in the range of 300-400 nm was used for photoactivating the titania catalyst. A CFC feed concentration of {dollar}sim{dollar}300 ppm was utilized and water was always added to the feed stream.; Deactivation experiments for up to 4 days during thermal catalysis and 1 day during photocatalysis were carried out to determine the stability of the catalysts.; Reactor results indicated that both Y zeolites and TiO{dollar}sb2{dollar} were effective for complete oxidation of CFC11 and CFC12. Close to 100% conversion was achieved at 300{dollar}spcirc{dollar}C for CFC11 and at 400{dollar}spcirc{dollar}C for CFC12 feed. Among the Y zeolites, Cr-Y exhibited better resistance against deactivation which could be further improved by the addition of water vapor in the feed. However, the TiO{dollar}sb2{dollar} catalyst showed maximum stability (under wet feed conditions); a constant 85-90% conversion of CFC12 at 300{dollar}spcirc{dollar}C could be maintained for 4 days. The conversion of CFC11 was only 3-7% during photodecomposition on TiO{dollar}sb2{dollar} as compared to {dollar}sim{dollar}69% for trifluoroethylene. The reaction rate for the photocatalytic destruction of CFC11 was very small as compared to the rate for thermal catalytic destruction.; In situ IR techniques were employed to study the reaction intermediates on the surface of the titania catalyst during thermal as well as photocatalytic decomposition of CFCs. Based on the results of the reactor as well as in situ IR experiments, primary interaction between adsorbed CFC feed and surface hydroxyls during CFC decomposition on TiO{dollar}sb2{dollar} catalyst is suggested. (Abstract shortened by UMI.)