Surface and Bulk Redox Behaviors of CeO₂ and Strong Influence of Impurities

The Ce⁴⁺/Ce³⁺ redox cycle and accompanying formation/elimination of oxygen defects lie at the core of surface chemistry and catalytic performance of CeO₂. Herein, via careful ESR characterization of commercial CeO₂ with trace impurities, impurity-free pure CeO₂ prepared by calcination of Ce(NO₃)₃ in air, and Ce(NO₃)₃ with ESR at 4 and 140 K, in combination with CO reduction followed by H₂ adsorption, we unambiguously assign the ESR signal at g ≈ 1.96 measured at 140 K for commercial CeO₂, but not for pure CeO₂, to d¹ configuration of 3d¹ impurity transitional cations like Ti³⁺ and the ESR signals at g ≈ 3.4 and 2.9 measured at 4 K to 4f¹ configuration of Ce³⁺ species on the surface and in the bulk associated with surface and bulk oxygen defects in CeO₂, respectively. The F⁺ centers and associated Ce³⁺ species are more stable on the CeO₂ surface while the oxygen vacancies (V_O) and associated Ce³⁺ species are more stable in the bulk. H₂ homolytic dissociation to the hydride species at the Ce³⁺ species and F⁺ centers occurs at 303 K on the CeO₂ surface and at 573 K in the CeO₂ bulk, leading to the oxidation of the Ce³⁺ species to the Ce⁴⁺ species and of the F⁺ centers to oxygen vacancies. The bulk redox behaviors of pure CeO₂ and commercial CeO₂ are similar, but the surface redox behaviors of commercial CeO₂ are dominated by the impurities and very different from those of pure CeO₂. These results deepen our fundamental understanding of surface and bulk redox behaviors of CeO₂ and highlight the strong influence of impurities.