EPR and NMR studies of amorphous aluminium borates

Abstract

Amorphous aluminium borates, Al_2(1–x)B_2xO₃ 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 (T_t⩽ 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.