Properties of CeO₂ and Ce₁_-<i>_x</i>Zr<i>_x</i>O₂ Nanoparticles:  X-ray Absorption Near-Edge Spectroscopy, Density Functional, and Time-Resolved X-ray Diffraction Studies

Abstract

In this article the structural and electronic properties of CeO 2 and Ce 1 - x Zr x O 2 nanoparticles are investigated using time-resolved X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), and density functional calculations. CeO 2 and Ce 1 - x Zr x O 2 ( x ≤ 0.5) particles in sizes between 4 and 7 nm were synthesized using a novel microemulsion method. The atoms in these nanoparticles adopted a cubic or pseudocubic crystal structure. The lattice constant decreased with increasing Zr content, varying from 5.4019 Å in CeO 2 to 5.3066 Å in Ce 0.5 Zr 0.5 O 2 . Within the cubic structure, the Zr atoms exhibited structural perturbations that led to different types of Zr−O distances and nonequivalent O atoms in the Ce 1 - x Zr x O 2 compounds. Upon the addition of Zr to CeO 2, the Zr positive charge in Ce 1 - x Zr x O 2 is smaller than in pure ZrO 2 whereas the Ce positive charge is larger than in pure CeO 2 . Combination of these geometrical and electronic effects produced Zr L III -edge and O K-edge XANES spectra with a distinctive line-shape not seen in pure ZrO 2 or CeO 2 . The doping with Zr increases the thermal stability of the ceria nanoparticles and their chemical reactivity toward hydrogen. At temperatures between 300 and 900 °C, the Ce 1 - x Zr x O 2 nanoparticles reacted with H 2 and water evolved into gas phase. XANES showed the generation of Ce 3+ cations (without reduction of Zr 4+ ) but an absence of diffraction lines different from fluorite-type ones was noted. There was an expansion in the unit cell of the reduced particles probably as a consequence of a partial Ce 4+ → Ce 3+ transformation and the sorption of hydrogen into the bulk of the material. The Ce 1 - x Zr x O 2 nanoparticles interact with H 2 and reduce at lower temperatures than bulk Ce 1 - x Zr x O 2 systems. This important difference could originate in an enhancement in chemical reactivity characteristic of nanostructured materials.