Mechanism Study on Photocatalytic Reduction of Cr（Ⅵ） Ion on the Surface of TiO₂

Semiconductor photocatalysis has been widely studied as an efficient means for treatment of environmental pollution. Toxic heavy metal ions can be reduced and detoxified by getting photo-induced electrons. Cr（Ⅳ） pollution has caused much attention because of its high toxicity. Photocatalytic reduction of Cr（Ⅳ） ion has been investigated in many papers. But the mechanism of this reduction process is still not very clear.The experimental results showed that Cr（Ⅵ） ion is reduced efficiently on P-25 TiO₂ semiconductor particles in acidic suspension under UV irradiation. Kinetic studies showed that photocatalytic reduction of Cr（Ⅵ） is in accordance with Langmuir-Hinshelwood law.
The mechanism of photocatalytic Cr（Ⅵ） reduction is studied in this paper with ESR and XPS methods. ESR results showed that Ti³⁺ is produced at the beginning of the reaction, but disappears along with the reaction time. It is pointal out that Ti³⁺ is involved in the reaction as an intermediate from which Cr（Ⅵ） gets electrons. Photo-induced conduction band electrons is captured by Ti⁴⁺ at the beginning of the reaction, and Ti³⁺ is formed. Then Cr（Ⅵ）gets electrons from Ti³⁺ and is reduced. So Cr（Ⅵ） ion is mainly reduced indirectly by getting elevtrons from Ti³⁺. At the same time, Cr（Ⅵ） reduCtion by directly capturing photo-induced conduction band electrons is possible. XPS results showed that Cr（Ⅵ） ions adsorbed at the surface of TiO₂ is totally redued to Cr（Ⅲ）. Cr（Ⅴ） and Cr（Ⅵ） may be produced as the reaction intermediates. The remarkable inhibition of phosphate ion on Cr（Ⅵ） reduction meantthat Cr（Ⅵ） adsorption on the surface of TiO₂ is the elementary step for Cr（Ⅵ） reduction.The decrease in Cr（Ⅵ） reduction in the presence Mn²⁺ ions may results from the reoxidation of the possible intermediate Cr（Ⅵ） spedes.