Continuous-Flow Electron Spin Resonance Measurements of Hydroxyl Radicals Produced during Photocatalytic Water Oxidation

Hydroxyl radicals (^•OH) generated on semiconductor oxide photocatalysts are expected to facilitate the decomposition and selective oxidation of organic compounds. However, the efficiency and behavior of photocatalytic ^•OH production have not been fully understood. In this study, we developed a flow system in which an electrolyte containing the spin-trapping agent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), was passed through a photoelectrochemical cell, and the radical species generated by the photoanodic water oxidation were continuously analyzed by electron spin resonance (ESR) spectroscopy. This flow-based ESR measurement enabled us to simultaneously quantify the current density and radical production rate under photocatalytic reaction, and to measure the Faradaic efficiency (FE) for the formation of the spin adduct (^•DMPO-OH) with near-real-time response. We utilized a tungsten oxide (WO₃) electrode as a typical photocatalyst to investigate ^•OH formation during water oxidation. When the incident photon-to-photocurrent conversion efficiency (IPCE) was 7.1 to 12.7% at an electrode potential of 1.20 V vs RHE, the FE of ^•OH formation was found to be low, ranging from 0.63 to 0.92%. The ^•OH FE increased with decreasing applied electrode potential or light intensity, suggesting that the surface density of photogenerated holes may influence the ^•OH formation.