In situ ligand-modulated activation of inert Ce(III/IV) into ozonation catalyst for efficient water treatment

As a classic strategy to maximize catalytic activity, modulation of the electronic structure of central metal using organic ligands encounters great challenge in radical reactions exemplified by advanced oxidation processes (AOPs) due to operando destruction of employed ligands. Herein, we provide a paradigm achieving in situ ligand-modulated activation of the originally inert Ce(III/IV) for catalytic ozonation as a representative AOP widely applied in full-scale water treatment. Among the small-molecule carboxylates typically produced from pollutant degradation during ozonation, we find oxalate (OA) is a potent ligand to activate Ce(III/IV), inducing 11.5- and 5.8-fold elevation in rate constants of O₃ decomposition and atrazine degradation, respectively. The Ce(III)-OA complex is proved the catalytic active species to boost pollutant degradation, while the catalytic ozonation unusually involves both •OH-dependent and •OH-independent pathways with comparable contributions. Both experiment and density functional theory calculation results show the pronounced electron donating effect of OA as evidenced by the substantial decreases in the charge residing on Ce, the ionization potential, and the Ce(III/IV) electrode potential, affords the activation of the Ce center for efficient ozonation. A comprehensive kinetic model involving 67 reactions is established to verify and elaborate the catalytic mechanism. Moreover, with in situ OA production, trace Ce³⁺ enables autocatalytic mineralization and codegradation of typical contaminants, which are not observed in case of Fe²⁺ or Cu²⁺. In addition, Ce³⁺ outperforms numerous state-of-the-art ozonation catalysts in terms of mass activity. This study sheds light on sustainable activation of the metal center harnessing operando ligands produced from the catalyzed reaction.