Enhanced photooxidative desulphurization of dibenzothiophene over fibrous silica tantalum: Influence of metal-disturbance electronic band structure

Fibrous silica tantalum (FSTa) and a series of metal oxides (silver oxide (AgO), copper oxide (CuO) and zinc oxide (ZnO)) supported on FSTa were prepared by hydrothermal and electrochemical method, respectively. X-ray diffraction, nitrogen adsorption-desorption analyses, Fourier-transform infrared, ultraviolet–visible diffuse reflectance spectroscopy, and photoluminescence were used to characterize the catalysts. The catalyst activity towards on photocatalytic oxidative desulfurization (PODS) of 100 mg L⁻¹ dibenzothiophene (DBT) was ranked in the following order: FSTa (3.03 × 10⁻³ mM min⁻¹) > Zn/FSTa (2.65 × 10⁻³ mM min⁻¹) > Cu/FSTa (2.33 × 10⁻³ mM min⁻¹) > Ag/FSTa (1.46 × 10⁻³ mM min⁻¹) under visible light irradiation for 150 min. This result demonstrated that the addition of metal oxides lowered the efficiency of PODS of DBT, most probably due to the unfit energy level of the photocatalyst towards redox potentials of superoxide anion radical (⋅O₂⁻) and hydroxyl radical (⋅OH). Nevertheless, among the metal oxides loaded FSTa, Zn/FSTa showed a higher desulfurization rate, which likely due to its higher valence band energy (E_VB = 3.12 eV) than the redox potential of the H₂O/⋅OH (+2.4 eV vs. NHE), which allowed the production of ·OH for oxidation of DBT into dibenzothiophene sulfone (DBTO₂). In parallel, the hole at the VB of ZnO can also directly oxidize DBT to DBTO_2, as confirmed by the scavenger experiment. A kinetics study using Langmuir–Hinshelwood model illustrated that the photodegradation over Zn/FSTa followed the pseudo-first-order, and adsorption was the rate-limiting step. These findings are believed to aid in the rational design of high-performance photocatalysts for various photocatalytic applications, especially the removal of sulphur-containing compounds from fuel oils.