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.