Photochemistry of semiconductor particles. Part 4.—Effects of surface condition on the photodegradation of 2,4-dichlorophenol catalysed by TiO2 suspensions

Abstract

The photocatalysed degradation of 2,4-dichlorophenol (DCP) has been investigated in aqueous suspensions of TiO₂. A possible reaction scheme has been proposed for the degradation, in which chlorobenzoquinone (CBQ) was detected as a predominant intermediate present in the reaction solution. Kinetic details for the degradation steps have been analysed based on the experimental results. The essential role of oxygen was considered to be the capturing of the photogenerated electron to form the oxidizing species, such as H₂O₂, HO^˙₂ and OH˙. In anaerobic conditions, the photodegradation rate was quite low even with the adsorbed Cu²⁺ ion on the TiO₂ powder as an alternative electron scavenger. This is due to the rapid indirect recombination of the photogenerated electron and hole, which is mediated by the short-circuiting reaction of Cu²⁺. However, in aerobic conditions, oxygen takes up the photogenerated electron trapped at the adsorbed Cu²⁺ ion to prevent it from recombining with the photogenerated hole. As a result, the hole has sufficient opportunity to participate in the oxidizing reactions. The degradation rate was dependent to some extent on the surface charge of the TiO₂ particles. Positive charge always promotes the photodegradation, whereas negative change is detrimental. This was attributed to the effects of surface charge on the migration of electrons from the interior of the TiO₂ particles to the surface.