Influence of the Synthesis Method on the Properties of Ceria-Doped Alumina

Abstract

Ceria-doped alumina sol-gel materials were obtained by two synthesis methods at low temperature; using method A, 2-propanol-diluted cerium precursor was slowly added at the time of the aluminum sol formation in acidic environment; using method B, the cerium salt was mixed with the aluminum alkoxide before sol formation in a basic environment. The supports were characterized by N₂ physisorption, thermogravimetric and thermal differential analyses (TGA and DTA), X-ray diffraction (XRD), ²⁷Al Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR), 2-propanol reactions, and ammonia temperature-programmed desorption (NH₃-TPD). The samples obtained by Method B present similar values in properties such as specific areas, pore volumes, pore size distribution, and acidity compared to those of pure alumina; the alumina structure was not modified, but segregated crystallites of CeO₂ were found in samples calcined at 1000 ^∘C, as observed by XRD. The ceria-containing materials synthesized by method A show a thermal behavior similar to that of alumina, with no appreciable segregation of CeO₂ detected by XRD and modifications in the amounts of tetra, penta, and octa-hedral aluminum coordination as determined by NMR. 2-propanol reactions showed a good correlation with acid density determined by NH₃-TPD. As the percentage of ceria in the material increases, surface area, pore volume, and acidity decrease. These changes can be correlated with an increase of pentacoordinated aluminum content. The results indicate that CeO₂ is well dispersed in the alumina framework when method A is used, but synthesis method B does not have the same effect on the CeO₂incorporation.