Structural and textural modifications of ZrO₂ induced by La₂O₃ addition, thermal treatment and reducing process

Lanthana-doped zirconium oxide may strongly influence the solid state reaction. A series of samples denoted Zr_xLa_1–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, 900, 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; XRD results show that Zr_xLa_1–x are formed basically by three mixed oxides: La₂Zr₂O₇, La_0.5Zr_0.5O_1.75, La_0.46Zr_0.54O_1.77, 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.