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.