Mechanism of zeolite A nanocrystal growth from colloids at room temperature.
ABSTRACT The formation and growth of crystal nuclei of zeolite A from clear solutions at room temperature were studied with low-dose, high-resolution transmission electron microscopy in field emission mode and with in situ dynamic light scattering. Single zeolite A crystals nucleated in amorphous gel particles of 40 to 80 nanometers within 3 days at room temperature. The resulting nanoscale single crystals (10 to 30 nanometers) were embedded in the amorphous gel particles. The gel particles were consumed during further crystal growth at room temperature, forming a colloidal suspension of zeolite A nanocrystals of 40 to 80 nanometers. On heating this suspension at 80 degrees C, solution-mediated transport resulted in additional substantial crystal growth.
- SourceAvailable from: Cui Xue-minJournal of Membrane Science 11/2013; 447:66-72. · 4.09 Impact Factor
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ABSTRACT: The influences of the crystallization temperature, time, and alkalinity of the reaction system and aluminum sources (aluminum chloride and aluminum sulfate) on the synthesis of NaA zeolite have been systematically investigated using the dry-gel conversion method (DGC). The phase and morphology of the synthesis products were characterized using X-ray diffraction and SEM, respectively. This study showed that the alkalinity and temperature affected the NaA zeolite crystallization process and the size distribution of the crystalline end products. Furthermore, as the reaction time increased, the metastable phase transformed into a more stable phase. It was worth mentioning that the final NaA zeolite product contained a small quantity of NaX zeolite when aluminum chloride was used as the aluminum source while using aluminum sulfate as the aluminum source, the final product was mostly sodalite, indicating that the anions in the different aluminum sources have a significant effect on the crystallization process, most likely due to their electrostatic and steric interactions with the zeolite framework. To obtain NaA zeolite crystals with uniform sizes and high crystallinity, the optimal synthesis parameters for the two aluminum sources were determined.Powder Technology. 11/2013; 243:184–193.
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ABSTRACT: Phenomena preceding precipitation of gold(i) sulfide in the reaction of aqueous tetrachloroauric acid with sodium sulfide have been studied applying several time-resolved in situ techniques. As can be deduced from UV-vis absorption spectroscopy, soluble species ("pre-nucleation clusters") emerge within several seconds and slightly change with time; the spectra, along with the previous data for the immobilized products, are indicative of disordered matter with Au(i)-S bonding and the gap of about 1.5 eV. In situ tapping-mode atomic force microscopy (AFM) directly imaged "soft" droplet-like species from 20-50 nm to about 200 nm in the lateral size and the continuous film condensed on the highly oriented pyrolytic graphite (HOPG) support. Small-angle X-ray scattering (SAXS) revealed quasi-particles ("liquid clusters") growing to about 12 nm for 30 min, and slowly afterwards. Dynamic light scattering (DLS) was preferentially due to the larger species, although the liquid clusters within 20 nm comprise more than 80% of the total volume of the fluid products. The entities, which exist in solution for many hours before sudden gold sulfide sedimentation, are considered as "dense liquid" intermediates involved in a non-classical nucleation pathway.Physical Chemistry Chemical Physics 01/2014; · 3.83 Impact Factor