Facet-dependent dual active sites of CeO₂ govern distinct adsorption mechanisms for fluorinated organics and inorganic fluoride

Understanding how crystal facets dictate the adsorption mechanisms of multifunctional oxides is crucial for designing selective materials for fluorine-containing contaminants. Here, CeO₂ nanocrystals with dominant (111) facets in octahedra (Oct), (110) facets in rods (Rod), and (100) facets in cubes (Cube) were synthesized to elucidate facet-dependent interactions with fluorinated organics and inorganic fluoride. Oct exhibited the highest capacity and kinetics for perfluorooctanoic acid (PFOA) adsorption, while Rod showed superior performance toward F^–. Spectroscopic analyses and site-quenching tests revealed distinct active sites: surface hydroxyl groups and oxygen vacancies governed F^– uptake via ion exchange and vacancy filling, whereas unsaturated Ce(IV) Lewis acid sites drove PFOA adsorption through C–F bond polarization and lattice oxygen substitution. Pyridine-IR and phosphate inhibition experiments confirmed the key role of facet-dependent Lewis acidity, following the order Oct > Rod > Cube. DFT calculations further demonstrated that the (110) facet favors hydroxyl and F^– binding, while the (111) facet exhibits the strongest Lewis acidity and charge transfer with fluorinated organics. This study establishes the structure–facet–function correlation in CeO₂, unveiling dual active-site mechanisms that differentiate inorganic and organic fluorine adsorption and offering mechanistic insights for rational design of advanced fluorine-selective materials.