Oxidation mechanism of iodate to metaperiodate on a tungsten trioxide photoanode

The mechanisms involved in the photoelectrochemical oxidation of iodate (IO3−) to metaperiodate (IO4−) and H2O to O2 on WO3 were investigated using density functional theory. The oxygen evolution reaction (OER) included four proton-coupled electron transfer (PCET) steps via H2O2 intermediates. After the first PCET of the rate-limiting step, instead of H2O adsorption, IO3− was adsorbed and the second PCET step generated IO4− species (PI1 mechanism). Although another mechanism, PI2, with the two PCET steps followed by the IO3− adsorption was found, IO4− was energetically generated via PI1 rather than via PI2. Based on the free energy changes in the PCET steps, alkaline cations such as Li+, Na+, and K+ on WO3 suppressed the OER process. In addition, Na+ promoted the IO3− adsorption and subsequent PCET step of PI1. The computational results verified that using NaIO3 aqueous solution on WO3 effectively inhibits OER and facilitates IO4− generation, resulting in a higher (lower) selectivity of IO4− (O2) than using LiIO3 and KIO3 aqueous solutions.