Effects of alkaline earth metals on the surface, structure, and reactivity of α-alumina

A series of strontium- and barium-doped alumina samples were prepared by hydrolysis, in neutral medium, starting from commercial Al 2 O 3 , SrCO 3 , and BaCO 3 materials. The precursors thus obtained were calcined under air at 700 °C; then, the bulk and surface properties of the resulting mixed oxides were characterized by nitrogen physisorption, X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H 2 -TPR), thermogravimetry (TGA), and differential thermal analysis (DTA). Contrary to SrCO 3 , an addition of BaCO 3 to α-Al 2 O 3 increases slightly the specific surface area. XRD patterns essentially reveal the characteristic reflections assigned to α-Al 2 O 3 . In agreement with TGA and XRD analysis, strontium and barium carbonates remain after calcination at 700 °C, their decomposition starting above 800 °C. Let us note that this decomposition occurs more readily on AlSr-100 than on AlBa-100 with no apparent relationship with the evolution observed on the specific surface areas. H 2 -TPR experiments underline a significant bulk reduction of barium and strontium carbonates taking place significantly above 900 °C with similar trend noticed during TGA regarding their thermal decomposition. However, the most relevant observation is related to a sharp enhancement of the reducibility of AlSr-y with the appearance two reduction ranges highlighting the existence of different types of interactions with strontium and the alumina substrate.