X-ray diffraction, high-resolution transmission electron microscopy and Fourier-transform infrared study of Ca-doped Al2O3

Abstract

Three alumina preparations, doped with ca. 3% CaO and corresponding to the γ-, δ,θ- and α-Al2O3 phases, respectively, have been studied by XRD, TEM and FTIR spectroscopy. Owing to the presence of Ca, no bulk phase stabilization/destabilization effects were observed, but the particle morphology of the γ- and α-phases was modified (though to a different extent), and in the α-phase the segregation of bulk Ca aluminate(s) was noted.The IR spectra of the three materials, and the spectra of the adsorption of some probe molecules (pyridine, CO, CO2) indicated that, starting from the lowest calcination temperatures, Ca tends to accumulate at the surface of the alumina phase(s). In doing so, it modifies the surface hydrated layer, the basicity of the surface, and the strength and nature of the surface Lewis acidity.It was postulated that, long before the segregation of separate Ca aluminate phases, there is the gradual formation of a (two-dimensional) surface layer of Ca aluminate, whose formation occurs primarily, but not exclusively, at the expense of the surface cationic centres in which Al possesses tetrahedral coordination.