X-ray imaging of aggregation in silica and zeolitic precursors

The resolution available in the King's College London scanning transmission x-ray microscope (STXM) can be exploited to study aggregate structures over a length scale from 100 nm to 10 micrometers that overlaps with and complements that available from small-angle x-ray scattering (SAXS) data. It is then possible to use these combined sets of data to test between different growth models for the aggregates, using the fractal dimension of the structures as a way of distinguishing the different models. In this paper we show some of the first transmission x-ray images taken of silica gels and zeolite precursors, materials that are of great practical and economic importance for certain selective catalytic processes in the chemical industry, and yet for which there is still only limited understanding of the complicated processes involved in their preparation. These images reveal clearly the fractal aggregates that are formed by the specimens