Journal of Molecular Catalysis A-Chemical, Vol.228, No.1-2, 11-19, 2005
Physical and chemical properties of Ce1-xZrxO2 nanoparticles and Ce1-xZrxO2(111) surfaces: synchrotron-based studies
In this article. we review a series of studies that use synchrotron-based techniques (high-resolution photoemission, time-resolved Xray diffraction (XRD), and X-ray absorption near-edge spectroscopy) to investigate the physical and chemical properties of Ce1-xZrxO2 nanoparticles and Ce1-xZrxO2(111) surfaces (x less than or equal to 0.5). CeO2 and Ce1-xZrxO2 particles in sizes between 4 and 7nm were synthesized using a novel microemulsion method. The results of XANES (O K-edge, Cc and Zr L-III-edges) indicate that the Ce1-xZrxO2 nanoparticles and Ce1-xZrxO2(111) surfaces have very similar electronic properties. For these systems, the lattice constant decreased with increasing Zr content. varying frorn 5.4 Angstrom in CeO2 to 5.3 Angstrom in Ce0.5Zr0.5O2, Within the fluorite structure, the Zr atoms exhibited structural perturbations that led to different types of Zr-O distances and non-equivalent O atoms in the Ce1-xZrxO2 compounds. The Ce1-xZrxO2 nanoparticles were more reactive towards H-2 and SO2 than the Ce1-xZrxO2(111) surfaces. The Ce1-2ZrxO2(111) surfaces did not reduce in hydrogen at 300 degreesC. At temperatures above 250 degreesC the Ce1-xZrxO2 nanoparticles reacted with H-2 and water evolved into gas phase. XANES showed the generation of Ce3+ cations without reduction of Zr4+. There was an expansion in the unit cell of the reduced nanoparticles probably as a consequence of a partial Ce4+ --> Ce3+ transformation and the sorption of hydrogen into the bulk of the material. S K-edge XANES spectra pointed to SO4 as the main product of the adsorption of SO2 on the Ce1-xZrxO2 nanoparticles and Ce1-xZrxO2(111) surfaces. Full dissociation of SO, was seen on the nanoparticles but not on the Ce1-xZrxO2(111) surfaces. The metal cations at corner and edge sites of the Ce1-xZrxO2 nanoparticles probably play a very important role in interactions with the H, and SO, molecules. (C) 2004 Elsevier B.V. All rights reserved.