Preparation and characterization of aerogels derived from Al(OH)3 and CrO3

Aerogels containing both Al2O3 and Cr2O3 were prepared by the reduction, by alcohols, of a precursor salt solution derived from Al(OH)3 and CrO3, followed by supercritical drying in either CO2 or ethanol. TEM analyses showed a microstructure typical of aerogels, with a connected matrix of ∼10-nm diameter particles and an open pore network. Subsequent thermal processing converts the initial aerogels to a high surface-area material comprised of Al2O3 and Cr2C3. Addition of ∼6% SiO2, relative to Al results in an increased retention of surface area at high temperatures. Surface areas of the aerogels after supercritical drying ranged from 240 to 700 m2/g, while after treatment at 1000 °C values ranged from 110 to 170 m2/g. The composition which showed the greatest temperature stability was 2(0.94Al2O3 · 0.06SiO2)Cr2O3. After treatment at 1000 °C, all a samples contained a large number of crystallites of the Cr2O3 phase, eskolaite, with diameters ranging from 0.5 to 1.0 μm. An additional unidentified phase may also be present. The presence of these larger crystallites leads to a lower transmittance in the near-IR due to increased scattering.