Structure and surface properties of ZrO2, CeO2, and Zr0.5Ce0.5O2 prepared by a microemulsion method according to X-ray diffraction, TPR, and EPR data

Abstract

It was established by X-ray diffraction, TPR, and EPR that microemulsion (m.e.) synthesis yields the binary oxides ZrO2(m.e.) and CeO2(m.e.) and the mixed oxide Zr0.5Ce0.5O2(m.e.) in the form of a tetragonal, cubic, and pseudocubic phase, respectively, having crystallite sizes of 5–6 nm. The bond energy of surface oxygen in the (m.e.) samples is lower than in their analogues prepared by pyrolysis. Hydrogen oxidation on the oxides under study occurs at higher temperatures than CO oxidation. ZrO2(m.e.) and CeO2(m.e.) are active in O 2 − formation during NO + O2 adsorption, while CeO2 is active during CO + O2 adsorption, too. However, its amount here is one-half to one-third its amount in the pyrolysis-prepared samples, signifying a reduced number of active sites, which are Zr4+ and Ce4+ coordinatively unsaturated cations and Me4+-O2− pairs. O 2 − radical anions are stabilized in the coordination sphere of Zr4+ coordinatively unsaturated cations via ionic bonding, and in the sphere of Ce4+ cations, via covalent bonding. Ionic bonds are stronger than ionic-covalent bonds and do not depend on the ZrO2 phase composition. Zr0.5Ce0.5O2 is inactive in these reactions because of the strong interaction of Zr and Ce cations. It is suggested that Ce(4 + β)+ coordinatively unsaturated cations exist on its surface, and their acid strength is lower than that of Zr4+ and Ce4+ cations in ZrO2 and CeO2, according to the order ZrO2 > CeO2 ≥ Zr0.5Ce0.5O2. Neither TPR nor adsorption of probe molecules revealed Zr cations on the surface of the mixed oxide.