PNNL

Abstract

In this article, we review a series of studies that use synchrotron-based techniques (high-resolution photoemission, time-resolved x-ray diffraction, and x-ray absorption near-edge spectroscopy) to investigate the physical and chemical properties of Ce1-xZrxO2 nanoparticles and Ce1-xZrxO2 (111) surfaces (x = 0.5). Ce O2 and Ce1-xZrxO2 particles in sizes between 3 and 7 nm were synthesized using a novel microemulsion method. The results of XANES (O K-edge, Ce and Zr LIII-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 from 5.40 Å in CeO2 to 5.27 Å 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 H2 and SO2 than the Ce1-xZrxO2 (111) surfaces. The Ce1-xZrxO2 (111) surfaces did not reduce in hydrogen at 300 °C. At temperatures above 250 °C, the Ce1-xZrxO2 nanoparticles reacted with H2 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 SO2 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 H2 and SO2 molecules.