Synergistic effects of oxygen vacancies and the chelation of tetracycline with metallic ions for enhanced degradation of tetracycline over photocatalysts La2−xKxNiMnO6

Abstract

Oxygen vacancies (OVs) can trap photogenerated electrons (e⁻) to promote the separation of photogenerated e⁻ and holes (h⁺), and can serve as reactive sites in a variety of catalytic reactions, which benefits the photocatalytic activity of materials. In this study, doping with potassium was employed to introduce OVs into the La₂NiMnO₆ crystal lattice. A series of potassium doped La₂NiMnO₆ catalysts (La_2−xK_xNiMnO₆: x = 0, 0.025, 0.05, 0.075, and 0.10, denoted LKNMO: LNMO, LKNMO0025, LKNMO005, LKNMO0075, and LKNMO010, respectively) were synthesized by a sol–gel method. According to the XRD refinement results by the Rietveld method, the as-prepared catalysts crystallized into a rhombohedral structure (space group: Rc). The existence of OVs was validated by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). The photocatalytic performances of the as-prepared catalysts were evaluated by degrading tetracycline (TC) under Xe lamp irradiation. LKNMO0075 exhibited the highest photocatalytic performance in degrading TC (100%) after 150 min. Additionally, the degradation apparent rate constant for the degradation of TC by LKNMO0075 was 2.32 times that of pure LNMO. Based on photoelectrochemical measurements, the separation of photogenerated e⁻ and h⁺ was prolonged by the OV, thereby improving the transfer efficiency of photogenerated e⁻ and h⁺. With the synergistic effects of the OV and the chelation of TC with metallic ions (La³⁺, Ni³⁺ and Mn²⁺) on the surfaces of LKNMO photocatalysts, the photocatalytic degradation of TC was effectively enhanced. The trapping experiments demonstrated that the superoxide radical (˙O₂⁻) and h⁺ were the prominent active species in the photocatalytic reaction system, where the effect of ˙O₂⁻ was more significant. This work will be useful for the design of other double perovskite photocatalytic systems for the treatment of wastewater containing TC.