Fluorinated And Fluorocarbon Coating Of TiO₂ By Pulsed Plasma Processing: Improved Photocatalytic Oxidation Of Gaseous M-Xylene And Mercury

Abstract

Titanium dioxide (TiO₂) is a preferred catalyst for photocatalytic oxidation of many air pollutants. In an effort to enhance its photocatalytic activity, TiO₂ was modified by pulsed plasma treatment. In this work, TiO₂ nanoparticles, coated on a glass plate, were treated with a plasma discharge of hexafluoropropylene oxide (HFPO) gas. By appropriate adjustment of discharge conditions, it was discovered that the TiO₂ particles can be either directly fluorinated (Ti-F) or coated with thin perfluorocarbon films (C-F). Specifically, under relatively high power input, the plasma deposition process favored direct surface fluorination. The extent of Ti-F formation increased with increasing power input. In contrast, at lower average power inputs, perfluorocarbon films are deposited on the surface of the TiO₂ particles. The plasma surface modified TiO₂ nanoparticles were subsequently employed as catalysts in the photocatalytic oxidation of m-xylene, as carried out inside a closed loop batch reactor. Both types of modified TiO₂ were significantly more catalytically active than that of the unmodified particles. For example, the rate constant of m-xylene degradation was increased from 0.012 min-1 with untreated TiO₂ to 0.074 min-1 with fluorinated TiO₂. The plasma treatment converts the TiO₂ particles from hydrophilic to hydrophobic. The more hydrophobic TiO₂ surfaces will more readily adsorb non-polar compounds, such as m-xylene, while simultaneously minimizing the competitive adsorption of water molecules. Additionally, it is believed that the presence of surface fluorine atoms can contribute to decreased electron-hole recombinations, thus further increasing TiO₂ photocatalytic activity. Similarly, the promotional effect was also found in elemental mercury, another non-polar compound, where the removal rate of elemental mercury obtained from fluorinated TiO₂ was 1.6 times greater than that of the untreated TiO₂. In contrast, fluorinated TiO₂ showed less reactivity than that of the untreated TiO₂ in degrading acetaldehyde, a polar compound. The study of the effect of relative humidity on m-xylene degradation revealed that the optimum relative humidity for m-xylene removal was 10% for the untreated TiO₂. The removal efficiency was observed to decrease progressively as the relative humidity was raised above the 10% level. When fluorinated TiO₂ was used as the catalyst, the impact of relative humidity range from 0 to 40% was minimal. The inhibition effect of water vapor was not observed until the relative humidity was raised to 60% and 80%. The photocatalytic reactivity of catalyst used in successive runs was lower than that obtained with fresh TiO₂. The catalytic deactivation observed apparently results from deposition of carbonaceous species deposited on the surface of the TiO₂ particles after repeated use. However, it was discovered that subjection of the used catalysts to an oxygen plasma for 10 minutes was successful in regenerating catalyst activity to 96% of its original activity.