Positive effect of Fe3+ ions on Bi2WO6, Bi2MoO6 and BiVO4 photocatalysis for phenol oxidation under visible light†
Abstract
It is known that Bi2WO6 (BiW), Bi2MoO6 (BiMo), and BiVO4 (BiV) under visible light are able to initiate organic oxidation by O2. Herein we report a profound effect of Fe3+ ions on phenol oxidation in aqueous suspensions at an initial pH value of 3. Three solids were made via a hydrothermal method followed by calcination at 350 °C for 3 h. In the absence of Fe3+, phenol degradation followed pseudo-first-order kinetics. The normalized rate constant with respect to the solid surface area was BiV > BiMo > BiW, a trend similar to those of solid crystallite size and spectral absorption. In the presence of Fe3+, phenol degradation was fast initially and then became slow. Meanwhile, H2O2 formation was enhanced by Fe3+, while Fe2+ was produced with a yield of up to 100%. As the Fe3+ concentration increased, the initial rate of phenol degradation (R0) changed. The maximum R0 values of BiV, BiMo, and BiW were 4, 17, and 86 times larger than those measured without Fe3+, respectively. But the normalized R0 to the initial amount of Fe3+ adsorption was in the order BiMo > BiW > BiV, a trend matching that of the Fe3+ adsorption constant. In the slow stage, interestingly, the first-order kinetics of phenol degradation resumed, with a rate constant much larger than that obtained without Fe3+. A study of electrochemistry showed that Fe2+ was oxidizable by O2 to regenerate Fe3+. A possible mechanism is proposed involving Fe3+ reduction on different sites of bismuth photocatalysts.