Robust interaction of ZnO and TiO2 nanoparticles with layered graphitic carbon nitride for enhanced photocatalytic oxidative desulfurization of fuel oil: mechanism, performance and stability

Abstract

Sulfur compounds in fuel such as thiophene, benzothiophene and dibenzothiophene are the primary source of SO_x emissions, leading to environmental pollution and acid rain. In this study, we synthesized a layered oxygen-doped graphitic carbon nitride (OCN) structure and integrated ZnO and TiO₂ nanoparticles onto the OCN surface through a microwave-assisted sol–gel method. The X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) results confirmed a robust interaction between the ZnO and TiO₂ nanoparticles and the oxygen-doped g-C₃N₄ (OCN) surface, as indicated by the formation of C–N–Ti and C–O–Ti bonds. This interaction notably improved the optoelectronic properties of the ZnO–TiO₂/OCN composite, yielding increased visible light absorption, reduced charge recombination rate, and enhanced separation and transfer of photogenerated electron–hole pairs. The oxygen doping into the CN network could alter the band structure and expand the absorption range of visible light. The ZnO–TiO₂/OCN photocatalyst demonstrated remarkable desulfurization capabilities, converting 99.19% of dibenzothiophene (DBT) to dibenzothiophene sulfone (DBT-O₂) at 25 °C, and eliminating 92.13% of DBT from real-world fuel oil samples. We conducted in-depth analysis of the factors impacting the redox process of DBT, including the ZnO ratio, initial DBT concentration, catalyst dosage, stability, and O/S molar ratio. Radical trapping experiments established that ˙O₂⁻, ˙OH and h⁺ radicals significantly influence the reaction rate. The obtained results indicated that the ZnO–TiO₂/OCN photocatalyst represents a promising tool for future fuel oil desulfurization applications.