Texture and nanostructure of chromia aerogels prepared by urea-assisted homogeneous precipitation and low-temperature supercritical drying

Mesoporous chromia acrogels with a surface area of 484-735 m(2) g(-1), a pore volume of 0.4-0.9 cm(3) g(-1) and a pore diameter of 3-9 nm were prepared by urea-assisted homogeneous precipitation from an aqueous Cr(NO3)(3) solution, followed by continuous supercritical extraction with CO2 under different conditions (pressure and time) after replacement of the water with a hexane/2-butanol mixture. The texture and chemistry of the aerogels transformed by heating in air or an inert atmosphere and the structure of the nanoparticles were characterized by means of N-2-adsorption isotherms, AA, HRTEM, FTIR, a variety of thermoanalytical methods (TPD, DSC, TGA, TPO-TPK) and X-ray diffraction in combination with structure modeling. At the CO2 extraction stage, a pressure of about 400 bars was critical for production of aerogels with surface areas > 700 m(2) g(-1). The fresh chromia aerogels consisted of closely packed almost globular, 3- to 5-nm nanoparticles with a structure analogous to that of monoclinic alpha-CrOOH, in which half of the 0 atoms and OH groups were replaced with coordinately bonded water molecules. After dehydration at 550-600 K, the materials retained their texture, being converted to faceted 3- to 5-nm nanoparticles, consisting of two-dimensional fragments (clusters) of alpha-CrOOH crystals built on [Cr(OH)(3)O-3] octahedra without bonding along the Z-axis. The texture of dehydrated chromia, aerogels was stable at temperatures up to 650 K in air and up to 773 K in an inert atmosphere. At higher temperatures, the material underwent a glow transition, yielding microcrystalline 50-nm particles with the well-defined structure of alpha-Cr2O3 and a surface area < 200 m(2) g(-1). (C) 2002 Elsevier Science B.V. All rights reserved.