Investigation of Photocatalytic Activity of Zn (II) and Mn (II) Polyoxometalates Supported on ZnO for Degradation of Organic Dyes

This paper describes the synthesis of two Keggin-type substituted polyoxometalates [(n-C4H9)4N)]5PMo2W9O39(Zn2+OH2).3H2O and [(n-C4H9)4N)]5PMo2W9O39(Mn2+OH2).3H2O supported on ZnO. The heterogeneous catalysts were characterized by IR, UV-Vis, UV-Vis DRS, XRD, FESEM, EDX, TG, BET, and ICP techniques. These prepared catalysts exhibited high photocatalytic activity for the degradation of methylene blue (cationic), rhodamine B (cationic), bromothymol blue (indicator), and methyl orange (anionic, azo) dyes in the presence of hydrogen peroxide under sunlight irradiation. The kinetics of the oxidation degradation of the four dyes were controlled to be first-order reactions according to the Langmuir-Hinshelwood kinetic model for ZnO, two homogeneous polyoxometalates, and two heterogeneous photocatalysts. The photocatalytic degradation activity of the heterogeneous species was superior to that of the homogeneous ones. Moreover, these systems can be recovered and reused for six catalytic cycles without a significant decrease in the dye degradation efficiency.