Photocatalyzed Destruction of Chlorinated Hydrocarbons

Abstract

Semiconductor photocatalysis with a primary focus on TiO₂ as a durable photocatalyst has been applied as a method for water and air purification. In this thesis, the basic electronic and chemical processes underlying the quantum efficiencies of the TiO₂/UV process are investigated. Time-resolved microwave conductivity experiments provide the recombination lifetimes and interfacial charge transfer rate constants of eight different TiO₂ catalysts. Their quantum efficiencies towards the photooxidation of chlorinated hydrocarbons vary from 0.04 to 0.44%. A direct correlation between (1) the quantum efficiencies and (2) the recombination lifetimes and the interfacial charge transfer rate constants is observed. The charge-carrier recombination rate in size-quantized particles ( 1-4 nm) is increased due to the mixing of states that relaxes the selections rules of an indirect bandgap semiconductor. The effects of protonation (i.e., pH 7-12) of amphoteric ZnO surface states on cross-sections for electron capture at the surface are studied by time-resolved radio frequency conductivity. Electrostatic repulsion due to a negatively-charged ZnO-surface leads to decreasing surf ace recombination rates with increasing pH. Vanadium doped into TiO₂ affects the quantum efficiency. Depending on the preparation method, vanadium plays three distinct roles. First, vanadium is present as surficial > VO₂⁺ and promotes charge-carrier recombination through electron-trapping followed by hole elimination. Second, V(IV) impurities in surficial V₂O₅ islands result in enhanced charge-carrier recombination through hole-trapping followed by electron elimination. Third, V(IV) is substitutional in the TiO₂ lattice in the form of a solid solution, Vₓ Ti₁₋ₓO₂. The V(IV) centers trap both electrons and holes and thus yield enhanced charge-carrier recombination. The addition of inorganic oxidants such as HSO₅⁻, ClO₃⁻, IO₄⁻, and BrO₃⁻ increases the quantum efficiency. BrO₃⁻ acts by scavenging conduction-band electrons and reducing charge-carrier recombination. When ClO₃⁻ is present, however, competitive adsorption for the TiO₂ surface occurs among 4-CP, ClO₃⁻, and O₂, and the heterogeneous photodegradation of 4-chlorophenol follows three parallel pathways. ClO₃⁻ favors a reaction pathway involving the thermal oxidation of the reactive intermediates.