Surface and structural characterisation of coprecipitated Ce x Zr1−x O2 (0 ≤ x ≤ 1) mixed oxides

Abstract

CexZr1−xO2-mixed oxides with three different Ce/Zr ratios (Ce0.8Zr0.2O2, Ce0.5Zr0.5O2 and Ce0.2Zr0.8O2) along with pure cerium and zirconium oxides were prepared by coprecipitation of the metal hydroxides in alkali media and subsequent calcinations at 500 °C, using two different cerium precursors (Ce(NO3)3·6H2O or (NH4)2Ce(NO3)6). These samples were characterised by N2 adsorption at −196 °C, XRD, Raman spectroscopy, XPS and H2-temperature programmed reduction. Besides, the two mixed oxides with higher cerium content were calcined at higher temperature (1000 °C) with the additional purpose of studying their thermal stability and phase homogeneity. XRD and Raman spectroscopy confirm a significant improvement in the insertion of zirconium cations into the ceria lattice when the samples Ce0.8Zr0.2O2 and Ce0.5Zr0.5O2 are synthesised with (NH4)2Ce(NO3)6 instead of Ce(NO3)3·6H2O. This is attributed to a more homogeneous coprecipitation of cerium and zirconium hydroxides, leading to mixed oxides with better bulk oxygen mobility and smaller lattice parameter. Moreover, the mixed oxides prepared with the (NH4)2Ce(NO3)6 precursor and calcined at 1000 °C exhibit a single phase whereas phase segregation occurs in the counterpart mixed oxides prepared with the Ce(NO3)3·6H2O precursor. XPS analysis reveal correlations among O/(Ce + Zr) surface atomic ratio and total cerium content for both cerium precursors. Among the samples calcined at 1000 °C, Ce0.8Zr0.2O2 synthesised with Ce(NO3)3·6H2O is the only one that preserves the low-temperature surface reduction peak, and also shows a BET surface area slightly higher than those of the rest of samples calcined at high temperature.