Phase separation and surface segregation in ceria-zirconia solid solutions

TL;DR

This study combines computational methods to show that ceria-zirconia solid solutions tend to phase-separate at high temperatures, with surface segregation favoring cerium enrichment, impacting catalytic material stability.

Contribution

It provides the first detailed thermodynamic and surface segregation analysis of ceria-zirconia solid solutions using DFT and statistical mechanics.

Findings

Ceria-zirconia solutions are metastable and tend to phase-separate.

Maximum Zr solubility in CeO2 at 1373 K is about 2%.

Surface enrichment of Ce occurs near the (111) surface at high temperatures.

Abstract

Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria - zirconia solid solutions are thermodynamically metastable with respect to phase separation into Ce-rich and Zr-rich oxides. We estimate that the maximum equilibrium concentration of Zr in CeO2 at 1373 K is ~2%, and therefore equilibrated samples with higher Zr content are expected to exhibit heterogeneity at the atomic scale. We also demonstrate that in the vicinity of the (111) surface, cation redistribution at high temperatures will occur with significant Ce enrichment of the surface, which we attribute to the more covalent character of Zr-O bonds compared to Ce-O bonds. Although the kinetic barriers for cation diffusion normally prevent the decomposition/segregation of ceria-zirconia solid solutions in typical catalytic applications, the separation behaviour described here can be expected to occur in modern three-way catalytic converters, where very high temperatures are reached.