Heterogeneous photodegradation of aqueous hydroxy butanedioic acid by microporous polyoxometalates

A new type of heterogeneous photocatalytic materials, microporous polyoxotungstates such as H3PW12O40/SiO2, H4SiW12O40/SiO2 and Na4W10O32/SiO2, were synthesized by incorporating the polyoxotungstates into the silica matrix via a sol–gel technique, and some physico-chemical properties of as-synthesized composites were measured. The photocatalytic activity of the composites was tested via degradation of the model molecule, hydroxy-butanedioic acid (malic acid, (MA)). After 90 or 180 min irradiation in the presence of Na4W10O32/SiO2 or H3PW12O40/SiO2 (H4SiW12O40/SiO2), an aqueous MA (100 mg/l) was totally degraded into the intermediates such as oxalic acid, glyceric acid, tartaric acid, butenedioic acid, acetic acid, formic acid and so on. After subsequent 90 or 60 min irradiation under the same conditions, these intermediates were totally mineralized into CO2 and H2O. Disappearance of MA (in the range of 0–200 mg/l) followed Langmuir–Hinshelwood first-order kinetics, and the reaction rate constant and adsorption equilibrium constant were measured. Most of the intermediates, identified during the process of H4SiW12O40/SiO2-photocatalyzed MA degradation, are the same with those detected during the process of TiO2-photocatalyzed MA degradation. The mechanism studies showed that the desolubilized polyoxotungstates appears to photo-oxidative degradation of MA by both OH radical attack and excited state polyoxotungstate direct oxidation.