杂多酸存在下X3B染料光降解和Cr(Ⅵ)光还原的协同反应机理

Reaction Mechanism on the Synergistic Photodegradation of Dye X3B and Photoreduction of Dichromate in the Presence of Polyoxometalate

以H3PW12O40(PW)和H4SiW12O40(SiW)杂多酸(POM)为催化剂,波长大于320nm的高压氙灯为光源,研究了混合水溶液中活性艳红染料X3B的光致降解和重铬酸根(Cr(VI))的光致还原.结果表明,POM-X3B-Cr(VI)三元体系的反应效率高于POM-X3B、POM-Cr(VI)和X3B-Cr(VI)二元体系的反应效率,PW的光活性高于SiW,且X3B光降解和Cr(VI)光还原之间存在明显的协同作用.通过考察各组分起始浓度以及N2、O2、H2O2和乙醇的影响,实验发现,激发态POM*与H2O反应产生POM-和·OH是反应的决速步骤.X3B光降解和Cr(VI)光还原分别主要通过·OH和POM-进行,而X3B和Cr(VI)之间光化学反应的贡献较小.在二元和三元体系中POM浓度对反应速率表现出不同的影响,表明激发态POM*与H2O之间的反应具有可逆性.

Photodegradation of a textile dye X3B and photoreduction of dichromate （Cr（Ⅵ）） in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates （POM）, H3PW12O40 （PW） and FLSiW12O40 （SiW）. The reactions in POM-X3B-Cr（Ⅵ） system were faster than those in POM-X3B, POM-Cr（Ⅵ） and X3B-Cr（Ⅵ） systems. For all reactions, PW was more photoactive than SiW. The reaction rates were proportional to the initial concentration of each component. The effects of N2, O2, and air were small but regular, indicating Cr（Ⅵ） photoreduction by a reduced POM. Quenching experiments with H2O2 and ethanol revealed that X3B photodegradation mainly occurred through hydroxyl radical （·OH）. It was proposed that the production of · OH and a reduced POM^- by the reaction between H2O and excited POM^ ＊ was the rate determining step, with which all evidence could be well interpreted. Different effects of POM concentration in two- or three-component system on the reaction rates suggested that the reaction between H2O and excited POM^＊ was reversible.