Journal of the Chemical Society - Faraday Transactions p. 2663 - 2670 (1994)
Update date:2022-08-17
Topics:
Simon, Simion
Pol, Andre van der
Reijerse, Edward J.
Kentgens, Arno P. M.
Moorsel, Geert J. van
Boer, Engbert de
Amorphous aluminium borates, Al2(1-x)B2xO3 with 0 a function of composition and heat-treatment temperature (Tt a narrow temperature 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 (>/= 1E7 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. 27Al 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 deg 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 interacts strongly with the pore surface when it contains a large fraction of four- and five-coordinate Al ions.
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