Surface investigation on CexZr1-xO2 compounds

Abstract

The surface of Ce_xZr_1-xO₂ compounds has been studied by Fourier transform infrared (FTIR) spectroscopy using adsorbed probe species. Methoxy species, from methanol dissociative adsorption, compensate the coordinative unsaturation of surface cations. From the ν(OC) vibration of methoxy species it was shown that mixed compounds are solid solutions with a surface composition equal to the nominal bulk composition. This result was confirmed by X-ray photoelectron spectroscopy (XPS). However, compared to pure compounds, an increase of exposed Ce⁴⁺ cationic sites related to capping oxygen lability was observed for mixed compounds. From the adsorption of CO and pyridine probes it was shown that the Lewis acidities of mixed compounds were intermediate between those of pure ceria and zirconia, which are weak, being slightly higher for zirconia. Using CO₂ as a probe for basicity investigation of the surface, it was shown that the various carbonate species observed for mixed compounds were already observed for pure ceria or zirconia with no striking variation of the overall carbonatation. However, from a proposed interpretation of the relative importance of various species, such as monodentate or polydentate carbonates, it was inferred that surface oxygen species are more easily relaxed in ceria-rich mixed compounds than in ceria itself, and that the surface framework of cerium cations is stabilised by zirconium, the effect being the opposite in the case of the surface framework of zirconium ions.