Structural and Redox Properties of Ce1-xZrxO2-δ and Ce0.8Zr0.15RE0.05O2-δ (RE: La, Nd, Pr, Y) Solids Studied by High Temperature in Situ Raman Spectroscopy

Abstract

In situ Raman spectroscopy at temperatures up to 450 °C is used to probe the structural and redox properties of Ce1-xZrxO2-δ solids (x = 0-0.8) prepared by the citrate sol-gel and coprecipitation with urea methods. The anionic sublattice structure of the solids is dependent on the preparation route. The composition effects exhibited by the Raman spectra are adequate for characterizing the phases present and/or eventual phase segregations. For x = 0.5 the pseudocubic t″ phase occurs for the solid prepared by the citrate sol-gel method, while phase segregation (cubic, tetragonal) is evidenced for the corresponding material prepared by the coprecipitation with urea method. A larger extent of defects and interstitial O atoms is evidenced for the materials prepared by the citrate sol-gel method. The well-known "defect" ("D") band around 600 cm-1 for CeO2 as well as for Ce1-xZrxO2-δ consists of at least two components: "D1" above 600 cm-1 and "D2" below 600 cm-1. Doping of Ce0.8Zr0.2O2-δ with rare earth cations (La3+, Nd3+, Y3+, Pr3+) results in strengthening of the "D2" band that, however, is found to be insensitive under reducing conditions of flowing 5% H2/He at 450 °C. A novel approach based on sequential in situ Raman spectra under alternating oxidizing (20% O2/He) and reducing (5% H2/He) gas atmospheres showed that the "D1" band is selectively attenuated under reducing conditions at 450 °C and is therefore assigned to a metal-oxygen vibrational mode involving interstitial oxygen atoms that can be delivered under suitable conditions. A reversible temperature-dependent evolution of the anionic sublattice structures of Ce1-xZrxO2-δ solids is evidenced by in situ Raman spectroscopy. The results are corroborated by powder XRD and oxygen storage capacity measurements, and observed structure/function relationships are discussed. It is shown that at low temperatures (e.g., 450 °C) the function of oxygen release and refill is based on a mechanism involving oxygen atoms in interstitial sites rather than on defects induced by hetrovalent M4+→ RE3+ doping, the latter improving the pertinent function at high (e.g., >600 °C) temperatures. © 2017 American Chemical Society.