Direct mass spectrometric studies of the destruction of hazardous wastes. 2. Gas-phase photocatalytic oxidation of trichloroethylene over titanium oxide: products and mechanisms

Abstract

The gas-phase photocatalytic oxidation of trichloroethylene (TCE) over titanium dioxide was investigated as a potential method for destroying this common pollutant. The results from this study agree with earlier studies in that high levels of destruction of TCE were achieved. Accompanying these high rates of destruction were high quantum yields (approaching unity). However, directsampling mass spectrometry and gas-phase Fourier transform infrared (FTIR) spectroscopy revealed that there are significant quantities of byproducts produced [phosgene, dichloroacetyl chloride (DCAC), carbon monoxide, molecular chlorine]. The DCAC has been rationalized on the basis of a chemical reaction mechanism in which the TCE molecules are oxidized in a chain reaction involving C1 atoms. This mechanism appears to be validated by tests with other chlorinated ethylenes (perchloroethylene, dichloroethylenes). Phosgene may arise at least partially from the photocatalytic oxidation of DCAC, and molecular chlorine may result from the recombination of chlorine atoms. The results of this study are discussed relative to aqueous-phase photocatalytic oxidation of TCE where chlorinated intermediates have been observed.

Catalysis with Supported Size-selected Pt Clusters

“…Due to its chemical inertness, current TCE degradation methods are inefficient and energy intensive. Promising alternative approaches reported so far, include photocatalytic conversion of TCE (gasphase [48] and supported [49]) and the use of small noble metal particles to catalyze the conversion [50,51]. Yet, more efficient catalysts are desirable, however require a more fundamental knowledge of the activation of TCE towards degradation.…”

“…Using size-selected clusters as a part of a photocatalytic system, they might be active towards TCE degradation nonetheless. This might be a more successful approach, since the photocatalytic conversion of TCE has been achieved in both gasphase [48] and on supported clusters [49].…”

Pilot-Scale Demonstration of an Innovative Treatment for Vapor Emissions

“…The square root dependence was observed in a continuous flow reactor when TCE concentrations were less than 60 ppmv. 8 Higher concentrations yielded a linear dependence.…”

“…The blowers were located at the end of the air handling system so that the entire system was maintained under slight negative pressure. Because TCE oxidation can produce intermediates such as phosgene, 8,[15][16] keeping the system under vacuum prevented the release of any toxic vapors. The exhaust gases from the blower were returned for final treatment by a caustic contacting tower.…”

“…This is likely due to chain reactions that are propagated by chlorine radicals. [7][8][9] Because the photocatalytic reaction occurs with greater efficiency and because chlorinated ethylenes are commonly found at contaminated sites, 10 trichloroethylene (TCE) and perchloroethylene (PCE) are often chosen as representative pollutants for PCO tests. Approximately 90% of Superfund sites have TCE or PCE as pollutants.…”