MAGNETIC-RESONANCE STUDIES ON POROUS ALUMINA DOPED WITH IRON AND CHROMIUM

A combined magic-angle spinning (MAS) NMR and electron paramagnetic re sonance (EPR) study has been performed on Fe- and Cr-doped alumina as a function of the heat-treatment temperature. Between 300 and 600 degr ees C large quantities of tetra- and penta-coordinated Al were produce d in porous xerogels. The pentacoordinated Al species have been found to be related to the catalytic activity. It has been found that the Fe ions accelerate the transition from gamma- to alpha-Al2O3 and were ma inly found within the alumina granules. On the other hand, Or ions in high oxidation states (Cr5+, Cr6+) hampered the transition from gamma- to alpha-Al2O3 and it has been proposed that they are preferentially situated on the surfaces of the granules.