AL-27 NUCLEAR-MAGNETIC-RESONANCE STUDY OF ALUMINAS AND THEIR SURFACES

The nature of the surface of alpha-, gamma-, and transition-aluminas has been explored via variable-temperature solid-state Al-27 NMR spectroscopy. Results from a previous study of aluminas using Al-27 NMR at room temperature showed a loss in signal with increasing surface area. The conclusion based upon these results was that the aluminum atoms in the first two molecular layers of alumina did not contribute to the Al-27 NMR signal. Further, the observed signal loss was due to the surface aluminum atoms experiencing a large electric field gradient. Results from the study presented here indicate that a loss in Al-27 signal with increasing surface area is real but that it is due instead to dynamic events occurring at the surface. Low-temperature studies served to slow these dynamics, thereby increasing the observable signal by amount greater than expected from the Boltzmann factor alone. We present these data herein and propose mechanisms by which these surface dynamics may occur.