Pyrophosphate effect on the photocatalytic degradation of phenol over bare and Pt-deposited Bi2WO6

Abstract

Pyrophosphate anions (PPA) are widely present in wastewater, but their effect on semiconductor photocatalyis for organic degradation has not been established yet. In this study, we report a synergistic effect between Pt and PPA, that greatly improves photocatalytic activity of Bi₂WO₆ (BiW) for phenol degradation in aqueous solutions at pHs 5–11. BiW was synthesized by a hydrothermal method, followed by photochemical deposition of 0.5% Pt (Pt/BiW). Upon addition of PPA, rates of phenol degradation on BiW and Pt/BiW decreased and increased, respectively. The former is due to decreased O₂ adsorption, as inferred from photocatalytic and electrochemical reductions of O₂. (Photo)electrochemical measurements revealed that Pt and PPA act as electron and hole mediators, respectively. It is proposed that adsorbed PPA on the catalyst is first oxidized to a pyrophosphate anion radical, followed by phenol oxidation to regenerate PPA. However, as PPA concentration increased beyond 0.50 mM, rate of phenol degradation on Pt/BiW started to decrease, due to self-recombination of pyrophosphate anion radicals into a less reactive product.