Photocatalytic transformations of chlorinated methanes in the presence of electron and hole scavengers

Abstract

The photocatalytic degradation in the aqueous phase of CCl₄, CHCl₃ and CH₂Cl₂ over irradiated TiO₂ has been investigated in the presence of electron and hole scavengers, including atmospheric oxygen. Chloromethanes degrade through combined reductive and oxidative processes, the importance of which varies on going from CCl₄, to CHCl₃ and CH₂Cl₂.

Under aerated conditions at pH 5, almost complete conversion into CO₂ and HCl has been observed. Although chloride ions were evolved stoichiometrically, at pH 11 persistent formation of formaldehyde and formic acid was observed. Stable intermediates, either chlorinated or dechlorinated (formic acid, formaldehyde and methanol) have been quantified. An initial reductive dominance was observed for CCl₄ even in the presence of oxygen. CH₂Cl₂ degradation was largely oxidative, even from the early stages of degradation, whereas the roles of the oxidative and reductive pathways were comparable for CHCl₃.

The effect of hole and electron scavengers highlights the importance of reductive and oxidative pathways. Reductants suchas alcohols (methanol, propan-2-ol, tert-butanol) enhance CCl₄ degradation remarkably, have a limited effect on CHCl₃ andstrongly decrease the degradation rate of CH₂Cl₂. Electron scavengers like periodate and peroxydisulfate decrease the degradation rate of CCl₄ and have limited effects on CHCl₃. Halides (chloride and bromide) show a peculiar behaviour. Acting as hole scavengers, they produce active species (Cl^Nsbd and Br^Nsbd) that participate in reactions with transient intermediates. During degradation of CHCl₃ in the presence of chloride, formation of CCl₄ was reported. Interestingly, for CCl₄ the degradation rate decreases and in the presence of bromide CBrCl₃ is detected at trace level.