Xianzhi Fu

The gas-phase photocatalytic mineralization of benzene on porous titania-based catalysts

Photocatalytic degradation of benzene in oxygen-containing gaseous feed streams was investigated using titania and platinized (0.1 wt.-%) titania photocatalysts. The titania catalyst was synthesized using sol-gel techniques. Results of this study indicate that, when using this particular photocatalyst, benzene was oxidized to carbon dioxide and water without forming any detectable organic reaction products in the reactor effluent, although only some of the benzene reacted. Both the overall conversion of benzene and its mineralization were improved by platinizing the titania. When the titania catalyst was platinized, both photocatalytic and thermocatalytic reactions were promoted. Rates of photocatalytic reactions were significantly enhanced at reaction temperatures between 70 and 90°C, while at temperatures above 90°C the rates of thermocatalytic oxidation reactions were noticeably increased. It proved possible to obtain the continuous and essentially complete mineralization of benzene by using the platinized titania catalyst and optimizing such parameters as the reaction temperature, space time, and the concentrations of oxygen and water vapor in the feed stream.

Effects of reaction temperature and water vapor content on the heterogeneous photocatalytic oxidation of ethylene

The photocatalytic degradation of ethylene in airstreams has been studied over the temperature range 30–110 °C using a packed bed reactor containing sol-gel-derived TiO2 or platinized TiO2 particulates. Results of this study indicate that the reactivity of ethylene is greatly enhanced at increased temperatures and that the fraction of ethylene that reacts is stoichiometrically oxidized to CO2 under all operating conditions. The effect of raising the temperature has been ascribed to decreasing adsorption of water for both types of catalysts, as well as an increase in conventional heterogeneous catalytic reactions occurring on the Pt/TiO2 catalyst. In addition, platinizing the TiO2 photocatalyst and increasing the content of water vapor in the gaseous feed streams both decrease the rate of photocatalytic oxidation of ethylene. The activation energies for the photocatalytic and heterogenous catalytic oxidation of ethylene were determined to be 13.9–16.0 kJ mol−1 and 82.8 kJ mol−1 respectively.

Chlorinated byproducts from the photoassisted catalytic oxidation of trichloroethylene and tetrachloroethylene in the gas phase using porous TiO2 pellets

The photocatalytic degradation of trichloroethylene and tetrachloroethylene in the gas phase, when porous TiO2 pellets were used, produced such undesirable chlorinated byproducts as chloroform (CHCl3) and carbon tetrachloride (CCl4). The formation of byproducts was reduced by increasing oxygen mole fractions in the feed gas stream and/or using the TiO2 pellets fired at a lower temperature. Theoretical calculations indicate that the Cl-radical-initiated reaction produces these byproducts. On the basis of the experimental and the theoretical results, a method to prevent the formation of the chlorinated byproducts is discussed.

Applications in photocatalytic purification of air

Publisher Summary Semiconductor-mediated photocatalytic oxidation has received wide interest as a promising technique for remediating environmental pollution. The semiconductor mediated gas-solid heterogeneous photocatalytic oxidation technique provides a promising approach for the purification of air and, in conjunction with air stripping, purification of soils and water that are contaminated with volatile organic compounds (VOCs). Compared to liquid–solid photocatalytic reactions, the gas-phase photocatalytic degradation of organic compounds has much higher reaction rates and photoefficiencies. Most efforts to apply the photocatalytic oxidation process for the destruction of environmental contaminants have focused on purification of water. The effect of water vapor on the rates of oxidation in these systems appears to be system specific. However, if a given VOC requires water for its complete oxidation, water vapor must be present in the feed stream or else the reaction rate will eventually decrease to near zero. In general, increasing the reaction temperature increases the rate of oxidation, although some exceptions have been noted.

Factors controlling activity of zirconia-titania for photocatalytic oxidation of ethylene.

Small photocatalytic devices were developed to remove ethylene from closed plant growth units flown in space. The devices utilized sol-gel-derived catalyst pellets of zirconia-titania. This study was undertaken to understand the significance of different factors on the photocatalytic activity of the catalyst. Increasing reaction temperatures and decreasing humidity of the air significantly increased oxidation of ethylene. The quantity of ethylene oxidized per unit time increased linearly with increasing flow rates, and increasing concentrations of ethylene. Zirconia-titania pellet size and heel depth had little effect on oxidation of ethylene. Platinizing the zirconia-titania significantly increased ethylene oxidation. The catalyst was found to absorb large quantities of water when the humidity of the air stream was elevated and this greatly decreased catalytic activity.

Field trials of a TiO2 pellet‐based photocatalytic reactor for off‐gas treatment at a soil vapor extraction well

Abstract A field trial of a pilot‐scale TiO2 photocatalytic reactor for treatment of off‐gases from a soil vapor extraction (SVE) well at a chlorinated solvent spill site at the Savannah River Site near Aiken, SC, is described. Trichloroethylene (TCE), perchloroethylene (PCE), 1,1‐dichloroethylene (1,1 ‐DCE), and 1,1,1 ‐trichloroethane (1,1,1 ‐TCA) were treated at flow rates up to 6 1/min and space times of 5.1 x 107 to 1.2 x 109 g/mol. The TiO2 used was in the form of porous pellets with a surface area of 150 m2/g. Operation of the reactor with the undiluted waste stream (5000 ppmv) at 80, 100, and 110°C yielded many undesirable byproducts, such as phosgene, chloroform, carbon tetrachloride, and penta‐ and hexachloroethane, even though the conversion of PCE and TCE approached 100%. After diluting the waste stream with ambient air to below 1000 ppmv and maintaining space times around 5 x 108 g/mol, a conversion >99.5% was achieved with the production of only small amounts (<10 ppmv) of hexachloroethane. T...