Electron-hole interactions in choline-phosphotungstic acid boosting molecular oxygen activation for fuel desulfurization

Abstract

Many studies have been conducted regarding the separation behavior of carriers (electrons and holes) because of involving the generation of superoxide radicals (O2−), hydroxyl radicals (HO), and hydrogen peroxide (H2O2) in the photocatalytic process of heteropolyacids. Instead, relatively little attention has been focused on the potential Coulomb interactions between photogenerated electrons (e−) and holes (h+). Herein, choline-phosphotungstic acid (Ch3-HPW) was synthesized via one-step acid-base neutralization reaction method, and characterized. The electronic excited state analysis of Ch3-HPW showed that the formation of singlet oxygen (1O2) was related to the electron-hole interactions in the photocatalytic process of ground state molecular oxygen (3O2) activation. Subsequently, a facile fuel photocatalytic oxidative desulfurization and extraction system was established on the basis of Ch3-HPW, air, and acetonitrile (MeCN), to better understand the 3O2 activation in specific applications. The main photocatalytic reaction conditions affecting the desulfurization process, including the amount of Ch3-HPW, the volume ratio of MeCN to model oil, the initial S-concentration, air/N2 bubbling, sulfur compounds, and fuel composition, were systematically investigated under UV radiation. The sulfur removal for model oil and straight-run gasoline in the system were 99.6% and 89.9%, respectively. The results of radical scavenger experiments, electron spin-resonance (ESR) spectroscopy, and density functional theory (DFT) calculations further demonstrated that 1O2, H2O2, and h+ played important roles in the oxidation of sulfur-containing compounds. A new method was developed for the desulfurization of liquid fuels using green and inexpensive O2 in this work to promote the development of photocatalytic process of exciton-involved HPA-based photocatalysts.