Improved Photocatalytic Activity of WO3 through Clustered Fe2O3 for Organic Degradation in the Presence of H2O2

Photocatalytic degradation of organic substrates over WO3 in an aerated aqueous suspension is very slow due to the difficulty of O-2 reduction by the conduction band electron on WO3. In this work, we report on H2O2 as an electron scavenger significantly accelerating the photodegradation of phenol and azo-dye X3B in water under UV or visible light. More importantly, an iron-containing WO3 (FeW) synthesized through thermal decomposition of a ferrotungstenic acid displayed a much higher activity than pure WO3 (HW) prepared in parallel. As the sintering temperature increased, both FeW and HW showed an exponential increase in activity. The maximum rate constant of phenol degradation obtained with FeW at 400 degrees C was about 2 times larger than that with HW at 600 degrees C. Sample characterization with electron paramagnetic resonance (EPR) spectroscopy and other techniques revealed that ferric species (0.3 wt % Fe2O3) were mainly present as clusters on the oxide surface at 120 degrees C and then they diffused toward the lattice sites of WO3 at high temperature, which was detrimental to the photocatalytic reaction. 5,5-Dimethyl-1-pyrroline N-oxide spin-trapping EPR showed that the production of hydroxyl radicals was greatly enhanced upon the addition of H2O2, the trend of which among different catalysts was the same as that of the rate of phenol degradation. The catalysts after excitation at 350 nm displayed a blue emission centered at 469 nm, the intensity of which varied with the catalyst activity nearly as expected. A possible mechanism for the improved photoactivity of WO3 is proposed involving the electron transfer from WO3 to Fe2O3 and the reaction of the reduced oxide with H2O2 to generate hydroxyl radicals.