CeO ₂ Nanoparticle Doping as a Probe of Active Site Speciation in the Catalytic Hydrolysis of Organophosphates

Abstract

Organophosphate hydrolysis is important for degrading environmentally harmful compounds and recovering phosphate ions in biological molecules. CeO 2 nanoparticles have been well-studied for dephosphorylation via hydrolysis owing to the accessible and tunable distribution of Ce 3+ and Ce 4+ ions. However, there remains uncertainty in the literature regarding which surface defect properties direct catalytic activity, such as the Ce 3+ /Ce 4+ distribution, oxygen vacancies, faceting, and dopants, and to what degree they contribute to efficient hydrolysis. Trivalent (M 3+ ) dopants serve as a tool for manipulating defects, including the concentration of Ce 3+ and oxygen vacancies, thereby influencing the hydrolytic activity of CeO 2 . Herein, trivalent metal ions (M = Y 3+, Cr 3+, In 3+, and Gd 3+ ) were employed to modulate the active sites on the CeO 2 nanoparticle surface, and their effects on organophosphate hydrolysis were investigated. M-doped CeO 2 nanoparticles were synthesized via hydrothermal methods, followed by annealing to remove ligands and prime the nanocrystal surface for catalysis. Catalytic performance was evaluated using dimethyl- p -nitrophenyl phosphate (DMNP) as a model organophosphate substrate, with degradation monitored over time using ultraviolet–visible (UV–vis) absorption spectroscopy. Powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy revealed successful doping of CeO 2 in all cases, albeit with distinctive characteristics demonstrating how M 3+ dopants affect catalysis. We show that CeO 2 exhibits high sensitivity to dopants that generate Ce 3+ ions, oxygen vacancy defects, and lattice strain. Consequently, achieving high catalytic efficiency within CeO 2 requires a balanced active site ensemble, wherein defects are maintained at optimal concentrations and distributions on the nanocrystal surface.

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