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The crystal growth of the nanoporous zeolite L has been studied in this work using atomic force microscopy (AFM) to investigate late-phase crystallization processes occurring on its surface. The nanometer resolution of the AFM has identified the twodimensionallayer- by-Iayer growth mechanism of this zeolite on the {001} face, however the {1~O} face displays a previously unreported mode of attachment for this mechanism. The probability for growth in the long, c axis of the crystal is thousands of times more likely than growth in the orthogonal, a direction. Growth on this face was found to occur via the incorporation of cancrinite columns, with a measured step height of 1.2 nrn. The frustrated a-directional growth of the crystal can occur only by the bridging connection of two adjacent cancrinite columns, affording terraces 1.6 nm in height. The modification of the crystal growth mechanism of zeolite L was studied when its habit was modified with respect to the length and diameter of these hexagonal cylinder-shaped crystals. By varying synthetic parameters such as the composition and reaction time, controlled modifications in crystal habit could be observed. By studying the surfaces of a vast array of crystals, the alteration in growth mechanism and defect formation were identified. Extensive holes and cracks were observed on the surface of the {001} face that were formed as a consequence of low supersaturation conditions. The increased understanding of the growth mechanism of zeolite L was utilised to impart control over the resultant crystal habit by the addition of 21-crown-7. The function of this crown ether is likely to facilitate the lateral growth of cancrinite columns on the {1~O} face resulting in crystals that have decreased length and increased diameter. The first in-situ surface dissolution study of zeolite L was performed. The observation of a high degree of friction, detected exclusively on the dissolving parts of the crystal, enabled a detailed quantitative investigation to be carried out. This study provided evidence about the fine structural modification of the surface during dissolution on the {100} face. Additionally, the first methodology for calculating enthalpies for dissolution, activation energies, orders of reaction and dissolution rates from the friction data provided by AFM has been reported. A 1.6 nm terrace on the {10Q} face of zeolite L under basic conditions was found to dissolve with a total enthalpy of dissolution of 216 kJ mot", an activation energy of 25 kJ mer' and an order of reaction of 0.31. The dissolution, rate of the terrace was found to vary when the pH, applied load and temperature were varied. |
