Mechanism of Photooxidation of Trichloroethylene on TiO2: Detection of Intermediates by Infrared Spectroscopy

Charge Trapping and Photoadsorption of O2 on Dehydroxylated TiO2 Nanocrystals—An Electron Paramagnetic Resonance Study

The interaction of photogenerated charges with molecular oxygen was investigated on TiO2 nanocrystals by means of paramagnetic resonance (EPR) spectroscopy. Compared to photoactivation experiments in vacuum at P < 10(-6) mbar and T = 140 K, the presence of O2 enhances the concentration of persistently trapped electron and hole centres--by a factor of ten--due to the formation of adsorbed O2- species. The photoadsorption of oxygen was also tracked quantitatively by pressure measurements, and the number of trapped charges, hole centres and O2- was found to correspond to ten electron-hole pairs per TiO2 nanocrystal. Conversely, in experiments at P < 10(-6) mbar with one trapped electron-hole pair per particle, charge separation is not persistent and completely reversible with respect to temperature. Heating to 298 K causes the total annihilation of photogenerated and trapped charges.

Electrochemical Mass Spectroscopic and Surface Photovoltage Studies of Catalytic Water Photooxidation by Undoped and Carbon-Doped Titania

Carbon-doped TiO2, demonstrated as an efficient photocatalyst in visible light photooxidation of organic compounds, was prepared with different doping concentrations and investigated via differential electrochemical mass spectroscopy (DEMS) and capacitive surface photovoltage (SPV) measurements in the form of thin layer electrodes. In all cases the total photocurrent as well as the surface photovoltage of the doped materials decreased markedly in relation to the undoped one. No detectable oxygen evolved from the doped electrodes in acidic solution under UV-light excitation. Since an oxidation of formic acid is still apparent, it is concluded that this oxidation occurs via isolated, catalytically poorly active trap states within the forbidden energy region. The existence of these states is confirmed by capacitive SPV measurements.

Gas phase trichloroethylene (TCE) photooxidation and byproduct formation: photolysis vs. titania/silica based photocatalysis

Photooxidation of trichloroethylene (TCE) was examined in comparative study using photolysis and photocatalysis. Degussa P25 titania coated on reactor wall and deposited on silica based microporous support were used as photocatalyst. The destruction of TCE and formation of potential byproducts were investigated under steady state conditions using annular photoreactors. Experimental work involved passing polluted air containing TCE through the UV photoreactor at varying concentrations and residence times. Ultraviolet illumination was provided by low pressure mercury lamps with outputs at either 254 nm, 365 nm, or 185/254 nm. Silica supported photocatalyst yielded maximum removal capacity of up to about 6 kg TCE per m3 per hour, nearly twice that provided by the coated titania. Direct photolysis with ozone generating UV also provided very high TCE conversion of up to 6kg TCE per m3 per hour. However, major quantities of phosgene and dichloroacetyle chloride (DCAC) were produced as byproducts. TCE removal using silica based photocatalyst did not result in any detectable DCAC. Only phosgene along with trace amounts of chloroform and carbon tetrachloride were identified as oxidation byproducts with silica based photocatalyst.

Thin film trichloroethylene electrochemical sensor

Pt-Ti and Pb-Pt-Ti thin films were deposited on alumina substrates by sputtering in Ar gas. In this study, an electrodeposited Pb-modified Pt-Ti thin film working electrode was prepared. Optimal sensing conditions were found to be -2.10 V (versus Ag/Ag+ with 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile (AN) solution) sensing potential, 250 rpm agitation rate. At room temperature, the response time was 15 s (90% response time). The correlation of sensing response current, id, and trichloroethylene (TCE) concentration, CL, is id = 2.86CL in the range from 100 to 700 ppm TCE. Additionally, the rate constant of (TCE) cathodic reduction was found to be 2.434 x 10(-3) cm(-1) s(-1).

Primary intermediates of oxygen photoevolution reaction on TiO2 (Rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements

Primary intermediates of oxygen photoevolution (water photooxidation) reaction at the TiO2 (rutile)/aqueous solution interface were investigated by in situ multiple internal reflection infrared (MIRIR) absorption and photoluminescence (PL) measurements. UV irradiation of TiO2 in the presence of 10 mM Fe3+ in the solution caused the appearance of a new peak at 838 cm(-1) and a shoulder at 812 cm(-1). Detailed investigations of the effects of solution pH, the presence of methanol as a hole scavenger, and isotope exchange in water (H2(16)O-->H2(18)O) on the spectra have shown that the 838- and 812-cm(-1) bands can be assigned to the O-O stretching mode of surface TiOOH and TiOOTi, respectively, produced as primary intermediates of the oxygen photoevolution reaction. The results give strong support to our previously proposed mechanism that the oxygen photoevolution is initiated by a nucleophilic attack of a H2O molecule on a photogenerated hole at a surface lattice O site, not by oxidation of surface OH group by the hole. The conclusion is supported by PL measurements. A plausible reaction scheme is proposed for the oxygen photoevolution on TiO2 (rutile) in aqueous solutions of pH less than about 12.

Photocatalytic oxidation of gaseous 2-chloroethyl ethyl sulfide over TiO2

Photocatalytic oxidation of gaseous 2-chloroethyl ethyl sulfide (2-CEES, ClCH2CH2SCH2CH3) over TiO2 illuminated with UV light and maintained at 25 or 80 degrees C in air has been investigated. 2-CEES was found to suffer progressive oxidation to yield ethylene (CH2CH2), chloroethylene (ClCHCH2), ethanol (CH3CH2OH), acetaldehyde (CH3C(O)H), chloroacetaldehyde (ClCH2C(O)H), diethyl disulfide (CH3CH2S2CH2CH3), 2-chloroethyl ethyl disulfide (ClCH2CH2S2CH2CH3), and bis(2-chloroethyl) disulfide (ClCH2CH2S2CH2CH2Cl) as the main primary intermediates, and water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), surface sulfate ions (SO4(2-)), and hydrogen chloride (HCl) as the final products. Trace concentrations of gaseous 2-chloroethanol (ClCH2CH2OH), ethanesulfonyl chloride (CH3CH2SO2Cl), ethyl thioacetate (CH3CH2SC(O)CH3), and considerable amounts of acetic acid (CH3C(O)OH), crotonaldehyde (CH3CHCHC(O)H), methyl acetate (CH3C(O)OCH3), and methyl formate (CH3OC(O)H) were also detected in the gas phase during the photooxidation conducted at 80 degrees C. Increase in temperature from 25 to 80 degrees C accelerates formation of gaseous ethanol, acetaldehyde, chloroacetaldehyde, diethyl disulfide, 2-chloroethyl ethyl disulfide, and bis(2-chloroethyl) disulfide but suppresses ethylene and chloroethylene production at initial stages of the process. Some aspects of the possible reaction mechanism leading to this wide array of intermediates and final products are discussed.