engleski

Study of the Microstructure of Amorphous Silicate Gel Before and After Its Hydrothermal Treatment

The design of zeolites with pre-defined features requires the understanding of their nucleation and growth mechanisms. Because of the wide applications of zeolites as catalysts, molecular sieves, additives etc, there is a growing need to purpose-tailor and control the desired performance of zeolites. Although there are well established techniques such as nuclear magnetic resonance (NMR), infrared spectroscopy (IR), Raman spectroscopy, low-angle scattering, light scattering, X-ray diffraction, high resolution transmission electron microscopy (HR-TEM), etc, - to measure the bulk structure of zeolites, the external surface structure is studied insufficiently, except some studies performed by HR-TEM. In this study atomic force microscopy (AFM) was used to investigate the very early stages of nucleation and crystallization on the surface of gel precursor. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM), as additional characterization tools are used to complete the information provided by the other techniques. Positron annihilation lifetime spectroscopy was also employed to study the change of the embedded free volume during crystallization. For this purpose the solid phase separated from freshly prepared sodium silicate hydrogel and through its hydrothermal treatment at 443 K was investigated. It was obtained that the freshly prepared gel is mainly composed of sphere-shaped primary particles but also of partially or even fully crystalline entities. AFM analysis of the solids separated from the hydrogel at various stages of its hydrothermal treatment (heating at 443 K) indicates that the particles of the partially and/or fully crystalline phase serve as nuclei for the further crystallization of zeolite.

zeolite; amorphous gel; hydrothermal treatment; nucleation; crystallization

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