The thermally induced transformation of pseudoboehmite gels-a comparison of the effects of corundum seeding and iron doping

The transformation of doped or seeded pseudoboehmite to corundum was studied by combining thermal analysis, X-ray diffraction, transmission electron microscopy, and electron paramagnetic resonance spectroscopy. The temperature of phase transformation to corundum was lowered by about 130 °C when Fe3+ or corundum seeds were added to the sols. Action of Fe3+ ions depends on the actual degree of thermally induced transformation of pseudoboehmite via transition aluminas to corundum and the ability of these alumina phases to incorporate Fe3+ ions. These ions tend to aggregate with increasing iron concentration of the alumina phases and can work as nucleation centers. Small (∼20 nm) corundum particles act as active nucleation sites whereas larger grains (200-400 nm) also present in the samples are less effective. For the first time trapping and stabilization of NO2 molecules in transition aluminas formed by a sol-gel route was shown.