Structural and Textural Modifications of ZrO2 Induced By La2O3 Addition, Thermal Treatment and Reducing Process

Abstract

Lanthana-doped zirconium oxide may strongly influence the solid state reaction. A series of samples denoted ZrxLa1–x (where x = atomic percentage of the element) are prepared by hydrolysis in the neutral medium from ZrO2 and La2O3. These samples are calcined under air at 450 °C, 900 °C, and 1200 °C, then characterized by specific surface area (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermoreductions programmed under H2 (TPR–H2). The results show that after calcination at 450 °C, the lanthanum addition increases slightly the zirconia surface area and the XRD analysis does not reveal any interaction between two oxides. After calcination at 900 °C, sintering appears and oxides lose half of their surface area. Lanthanum is not inserted into the structure of ZrO2, while some interaction occurs between lanthanum and zirconium oxide. At 1200 °C, the sintering of the samples is very important; the specific surface is about 1 m2/g–1; XRD results show that ZrxLa1–x are formed basically by three mixed oxides: La2Zr2O7, La0.5Zr0.5O1.75, La0.46Zr0.54O1.77, which is confirmed by the SEM method. H2-TPR experiments confirm that changes in the reducibility can reflect some alterations of the nature of interactions between ZrO2 and La2O3. Preliminary experiments on zirconia do not reveal the occurrence of significant reduction processes. On the other hand, extensive reduction of La2O3 is much more accentuated for lanthana samples. At high temperatures, a significant lessening in the H2 consumption suggests that ZrO2 would likely interact with La2O3, which is confirmed in the results indicating the presence of the pyrochlore type of oxides mentioned above.