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

Mesoporous chromia aerogels with a surface area of 484-735 m² g^-1, a pore volume of 0.4-0.9 cm³ 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 N₂-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 m² 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 α-CrOOH, in which half of the O 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 α-CrOOH crystals built on [Cr(OH)₃O₃] 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 α-Cr₂O₃ and a surface area <200 m² g^-1.