Structure and morphology evolution of silica-modified pseudoboehmite aerogels during heat treatment

• Silica-doped boehmites were prepared by sol–gel method with supercritical drying.
• Ultrathin two-dimensional crystallites of pseudoboehmite were obtained.
• Changes in structure and morphology upon calcination were studied.
• Simulation of XRD patterns was performed with use of the Debye Scattering Equation.
• Thermal stability of alumina depended on morphology inherited from pseudoboehmite.

Silica-modified pseudoboehmite aerogels (0, 10, 20 at% of Si) were prepared by sol–gel method followed by supercritical drying. The phase transformations, changes in structure and morphology upon calcination were thoroughly investigated by advanced X-Ray diffraction (XRD) techniques and high-resolution transmission electron microscopy (HRTEM). Obtained pseudoboehmite samples had specific nanostructure: ultrathin two-dimensional (2D) crystallites were loosely packed. The silica dopant drastically enhanced the crystallite anisotropy. Thus, the aerogel with Al:Si atomic ratio of 9:1 consisted of the pseudoboehmite nanosheets with thickness of one unit cell (average dimensions of 14.0×1.2×14.5 nm). The specific nanostructure caused remarkable features of experimental XRD patterns, including anisotropic peak broadening and appearance of forbidden reflection. Direct simulation of XRD patterns with using the Debye Scattering Equation allowed the size and morphology of pseudoboehmite crystallites to be determined. The silica addition strongly delayed formation of γ-alumina and further phase transformations upon calcinaton. Thermal stability of alumina was suggested to be affected by the particle morphology inherited from the pseudoboehmite precursor.

Pseudoboehmite samples had specific nanostructure: ultrathin two-dimensional (2D) crystallites were loosely packed.Figure optionsDownload as PowerPoint slide