Adsorption of polar and nonpolar compounds onto complex nanooxides with silica, alumina, and titania

Adsorption of low-molecular adsorbates (nonpolar hexane, nitrogen, weakly polar acetonitrile, and polar diethylamine, triethylamine, and water) onto individual (silica, alumina, titania), binary (silica/alumina (SA), silica/titania (ST)), and ternary (alumina/silica/titania, AST) fumed oxides was studied to analyse the effects of morphology and surface composition of the materials. Certain aspects of the interfacial phenomena dependent on the structural characteristics of oxides were analysed using calorimetry, 1H NMR, and Raman spectroscopies, XRD, and ab initio quantum-chemical calculations. The specific surface area SBET,X-to-SBET,N2 ratio (X is an organic adsorbate) changes from 0.68 for hexane adsorbed onto amorphous SA8 (degassed at 200 °C) to 1.85 for acetonitrile adsorbed onto crystalline alumina (degassed at 900 °C). These changes are relatively large because of variations in orientation, lateral interactions, and adsorption compressing of molecules adsorbed onto oxide surfaces. Larger SBET,X/SBET,N2 values are observed for mixed oxides with higher crystallinity of titania or/and alumina phases in larger primary nanoparticles with greater surface roughness and hydrophilicity. Polar adsorbates can change the structure of aggregates of oxide nanoparticles that can, in turn, affect the results of adsorption measurements.