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.