EPR and NMR studies of amorphous aluminium borates

Amorphous aluminium borates, Al2(1–x)B2xO3 with O ⩽x⩽ 0.5, prepared from mixtures of aluminium nitrate, boric acid and glycerol, have been studied by EPR and 27Al MASNMR as a function of composition and heat-treatment temperature (Tt⩽ 860 °C). EPR studies showed the presence of physisorbed NO2, NO and O2 molecules, produced by decomposition reactions during the thermal treatment. The O2 molecules in the gaseous state were observed in a narrow temperture interval around 60 K and in the condensed phase at low temperature (<20 K). The D value for condensed O2 amounts to 109 GHz, significantly lower than the value for 'free' O2, which is 119 GHz. Above 20 K the NO2 molecules in all samples rotate rapidly ( 107 Hz) about an axis parallel to the interatomic oxygen–oxygen direction; this mobility decreases with increasing heat-treatment temperature. Some EPR lines were tentatively ascribed to pairs or clusters of the abovementioned paramagnetic molecules. 27 Al MASNMR studies showed the presence of six-, five- and four-coordinate Al atoms, their relative concentrations being strongly dependent on the thermal history and composition of the samples. The fractions of tetra- and penta-coordinated Al atoms were maximum at heat-treatment temperatures between 300 and 600 °C and decreased considerably after the samples were exposed to air. Therefore the low coordinated Al atoms are predominantly located at the surface.The decreased mobility of NO2 molecules, at high treatment temperatures, indicates that NO2 intereacts strongly with the pore surface when it contains a large fraction of four- and five-coordinate Al ions.