Na-Influenced Bulk and Surface Properties of the So-Called Iota(ι)-Alumina: Spectroscopy and Microscopy Studies

The test alumina (the so-called ι-Al₂O₃) was thermally recovered at 1,100°C from chitosan-AlO_x hybrid films and found to contain Na and Ca impurity ions inherited from the parent chitosan. Two different modifications of pure alumina, namely, γ- and α-Al₂O₃, were adopted as control samples. The test and control aluminas were examined for 1) the bulk elemental constitution by atomic absorption spectroscopy (AAS), 2) the surface chemical composition by X-ray photoelectron spectroscopy (XPS), 3) the bulk phase composition by X-ray powder diffractometry (XRD), ex-situ Fourier-transform infrared spectroscopy (IR), and Laser Raman (LRa) spectroscopy, 4) the surface area, topography, and morphology by N₂ sorptiometry, and atomic force (AFM) and scanning electron microscopy (SEM), 5) the surface adsorptive interactions with pyridine and 2-propanol gas-phase molecules by in-situ IR spectroscopy of the adsorbed species, and 6) the surface catalytic interactions with 2-propanol gas-phase molecules by in-situ IR spectroscopy of the gas phase. Results obtained could clearly show that the test alumina (ι-Al₂O₃) is only hypothetically pure alumina since in reality its bulk structure is majored by mullite-type Na-aluminate (Na_0.67Al₆O_9.33/NaAlO₂) and minored by Na-β-alumina (Na_1.71Al₁₁O₁₇) and β-alumina (NaAl₁₁O₁₇). Consistently, observed Na-influenced modifications of the surface chemistry, topology, and morphology, as well as adsorptive and catalytic interactions with pyridine and 2-propanol gas-phase molecules, showed significant deviations from those exhibited by the control pure aluminas (γ- and α-Al₂O₃).