Comparative Study of Homogeneous and Heterogeneous Photocatalytic Degradation of RhB under Visible Light Irradiation with Keggin-Type Manganese-Substituted Catalysts

This paper reported for the first time a comparative study of the photocatalytic activity of the Keggin-type Mn(H)-substituted heteropolyanion PW11O39Mn11(H2O)(5-)(PWIIMn) and its heterogeneous system PW11Mn/D301R through the degradation of rhodamine B (RhB) under visible light irradiation. A novel photocatalytic mechanism was suggested, and an interaction between PWIIMn and RhB was scrutinized by using visible absorption spectrum and fluorescence emission spectrum. Influences of some factors such as the RhB initial concentration, the PW11Mn concentration, the PW11Mn/D301R dosage, the solution pH, and different anions existing in solution on the photocatalytic degradation rate of RhB were also examined. The stability of PW11Mn/D301R was evaluated by the cycle photodegradation of RhB in the end. The experimental results showed that 100% of RhB degradation was reached at 100 min for the PW11Mn system and 40 min for the PW11Mn/D301R system when the solution containing 10 mu mol center dot L(-1)Rhb was exposed to visible light. The hydroxyl radicals were responsible for the destruction of dye. The photocatalysis mechanism was different from that of both semiconductor and Keggin parent catalysts. An electrostatic interaction and a coordination interaction between PW11Mn and RhB occurred simultaneously in an acidic aqueous solution. The coordination interaction slowed significantly the RhB degradation, but became weak obviously after PW11Mn was adsorbed onto the D301R resin. The influence of anions existing in solution on the RhB degradation followed the sequence of PO43- > SO42-> Cl-> NO3-. No matter if in neutral or in acidic aqueous solution, the photocatalyst PW11Mn/D301R was stable.