Tuning the Integration Rate of Ce(Ln)O₂ Nanoclusters into Nanoparticulated ZrO₂ Supports: When the Cation Size Matters

Three nanostructured catalysts with low total rare earth elements (REEs) content (i.e., 15 mol.%) were prepared by depositing CeO₂ or Ln³⁺-doped CeO₂ (Ln³⁺ = Y³⁺ or La³⁺; Ln/Ce = 0.15) on the surface of ZrO₂ nanoparticles, as nanometre-thick, fluorite-type clusters. These samples were subjected to successive reduction treatments at increasing temperatures, from 500 to 900 °C. A characterisation study by XPS was performed to clarify the diffusion process of cerium into the bulk of ZrO₂ crystallites upon reduction to yield Ce_xZr_1-xO_2-δ surface phases, and the influence of the incorporation of non-reducible trivalent REE cations, with sizes smaller (Y³⁺) and larger (La³⁺) than Ce⁴⁺ and Ce³⁺. For all nanocatalysts, a reduction treatment at a minimum temperature of 900 °C was required to accomplish a significant cerium diffusion. Notwithstanding, the size of the dopant noticeably affected the extent of this diffusion process. As compared to the undoped ZrO₂-CeO₂ sample, Y³⁺ incorporation slightly hindered the cerium diffusion, while the opposite effect was found for the La³⁺-doped nanocatalyst. Furthermore, such differences in cerium diffusion led to changes in the surface and nanostructural features of the oxides, which were tentatively correlated with the redox response of the thermally aged samples.