Magnetic resonance studies on porous alumina doped with iron and chromium

Abstract

A combined magic-angle spinning (MAS) NMR and electron paramagnetic resonance (EPR) study has been performed on Fe- and Cr-doped alumina as a function of the heat-treatment temperature. Between 300 and 600 °C large quantities of tetra- and penta-coordinated Al were produced 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 γ- to α-Al₂O₃ and were mainly found within the alumina granules. On the other hand, Cr ions in high oxidation states (Cr⁵⁺, Cr⁶⁺) hampered the transition from γ- to α-Al₂O₃ and it has been proposed that they are preferentially situated on the surfaces of the granules.