Accelerating electron transfer in heterogeneous catalytic ozonation via ligand effect: A dual-pathway synergistic mechanism

Abstract

Heterogeneous catalytic ozonation (HCO) has emerged as a promising route for eliminating refractory organic contaminants from wastewater, relying on catalyst-assisted ozone activation to generate reactive oxygen species (ROS). However, the low electron transfer efficiency between active sites of the catalyst and ozone molecules hinders the efficient and sustainable ROS generation, thereby impacting the overall HCO performance. Herein, we propose a ligand-based strategy for expediting electron transfer and valence circulation of catalyst for HCO. We found that oxalic acid (OA), a representative terminal product generated during the ozonation of various aromatic organics, induced a 3.3-fold increase in the rate constant of catalytic ozonation of ibuprofen (IBU) by CeO₂, surpassing other low-molecular-weight organic acids. Experiments and characterizations proved that OA not only accelerated the Ce³⁺/Ce⁴⁺ valence cycle for activating ozone into hydroxyl radical (·OH), but also could be transformed into carbon-centered radicals (C₂O₄^•− and CO₂^•−) that participated in the degradation of pollutants, achieving a dual-pathway synergy to promote the catalytic performance of HCO. Moreover, we unveiled that the self-accelerating phenomenon of OA prevailed in the HCO of multi-structured pollutants. Overall, these findings highlight the potential of organic ligands to regulate electron transfer processes and redox dynamics in catalytic ozonation, offering new insight into the design of more efficient water treatment systems.

Graphical abstract

Keywords

Ligand effect / Catalytic ozonation / Carbon-centered radicals / Wastewater treatment

Highlight

	● Oxalic acid (OA) could expedite electron transfer and valence cycle of Ce3+/Ce4+.
	● OA was converted into carbon-centered radicals for pollutant degradation.
	● The CeO2/OA/O3 system could remove refractory pollutants rapidly.
	● Prevailing self-accelerating behavior of OA in HCO was revealed.

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