A thesis submitted for the degree of Doctor of Philosophy The Surface Structure and Growth of Natural Zeolites by Manisha Mistry University College London 2005 Davy Faraday Research Laboratory, The Royal Institution of Great Britain. Centre for Theoretical and Computational Chemistry, Department of Chemistry, University College London 1 UMI Number: U592160 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI U592160 Published by ProQuest LLC 2013. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 A b s t r a c t A systematic study has been carried out in order to investigate two aspects o f zeolite chemistry: to determine the external surface structure and to establish the mechanism of crystal growth. The natural zeolite edingtonite (EDI) was chosen as a model material and the surface structures were studied by finding the most stable termination for each face. For each of the four morphologically important faces the surface structure was found to minimise the number of broken bonds created upon cleaving the surface. In the absence of experimental data, the crystal morphologies were used as a proxy indicator of relative growth rate of different faces. Qualitative inspection indicates that only the equilibrium morphology is able to reproduce the habit of the natural crystal. However, quantitative analysis with the crystal habit observed in SEM images reveals a discrepancy in aspect ratios. By predicting the crystal morphology by using a newly proposed, less demanding method of calculating the number of broken bonds upon cleaving the surface per surface area (Nbb/S A ), one is able to accurately reproduce the crystal morphology with the correct aspect ratio. The utility of the NBb/S A method has been tested for a sample of zeolites, results show the Nbb/S A technique has correctly predicted the growth rate and crystal habit of merlinoite, analcime, natrolite, and zeolite A. The test on thomsonite was able to reproduce the correct order o f stability for each face, however the aspect ratios were not precisely predicted. By the use o f atomistic and ab-initio methods, this research has also investigated the growth mechanism of EDI. The total and condensation energies of a range o f possible solution fragments have been calculated and the most stable cluster along with the mechanism of formation has been suggested. The energetics of clusters along with the surface structural analysis has shown strong evidence that characteristic building units control the growth and aggregation o f EDI, which dictates the extended crystalstructure and the rate at which these building units condense onto the surface determines the crystal shape. 2 To Sandip an exceptional person who has inspired me to believe that goals can be achieved by reaching beyond material concepts 3 A cknowledgements I would firstly like to thank my supervisor Ben Slater for his encouragement, enthusiasm and guidance throughout my PhD. I would also like to acknowledge my secondary supervisors: Dewi Lewis for useful conversations, and C. Richard. A. Catlow for the opportunity to work at The Royal Institution o f Great Britain. I am also grateful to Matt Johnson for his help with SEM experiments. Personal thanks to my parents, Ramanbhai and Laxmiben Mistry, for their endless support, and giving me the opportunity to always achieve my goals. I am very grateful to my sister Mala Mistry, for her assistance with exhausting tasks. I would also like to thank my fiance Sandip Vekaria, who has always given me unparalleled strength and encouragement throughout the past four years. Finally I would like to thank my friends Iman Saadoune, Dinah Parker, Ilona Franklin, Naseem Ramsahye, Donna Arnold, Dave Plant, David Coombes, Martin Foster, Dan Wilson and Misbah Sarwar, who have made working at the RI an unforgettable experience; and other close friends Sarah Harper, Lin Yan Liu and Sarah Choudhury. 4 Ta b l e o f C o n t e n t s LIST OF FIGURES.................................................................................................................................................... 9 LIST OF TABLES.................................................................................................................................................... 13 CONVENTION.........................................................................................................................................................16 C h a p t e r O n e INTRODUCTION.................................................................................................................................................... 17 1.0 OBJECTIVES.................................................................................................................................................... 17 1.1 Zeolites................................................................................................................................................... 19 1.1.1 The geochemistry of natural zeolites ............................................................................................19 1.1.2 Classification of zeolites................................................................................................................. 20 1.1.3 Structure and composition..............................................................................................................20 1.2 Applications of zeolites .......................................................................................................................26 1.2.1 Ion exchange in zeolites .................................................................................................................. 26 1.2.2 Bulk catalysis................................................................................................................................... 28 1.2.3 Surface catalytic processes.............................................................................................................30 1.2.3.1 Pore mouth catalysis ............................................................................................................ 31 1.2.3.2 Key-lock catalysis .................................................................................................................32 1.2.3.3 Nest effect.............................................................................................................................. 33 REFERENCES......................................................................................................................................................... 34 C h a p t e r T w o CRYSTAL GROWTH: AN OVERVIEW OF THEORY AND EXPERIMENTAL TECHNIQUES 38 2.0 UNDERSTANDING THE CONCEPTS OF CRYSTAL GROWTH........................................................38 2.1 Crystal nucleation and growth ............................................................................................................ 39 2.1.1 Homogenous nucleation................................................................................................................. 39 2.1.2 Heterogeneous nucleation...............................................................................................................42 2.2 Models of a crystal surface................................................................................................................. 43 2.2.1 Surface point defects........................................................................................................................48 2.3 Atomistic nature of crystal growth .....................................................................................................50 2.4 Spiral growth model ............................................................................................................................. 52 2.5 Experimental surface analytical techniques .......................................................................................55 2.5.1 Scanning tunnel microscope...........................................................................................................55 2.5.2 Atomic force microscope................................................................................................................ 56 2.5.3 Transmission electron microscopy ................................................................................................58 2.5.4 Scanning Electron Microscopy.......................................................................................................59 2.5.5 Energy Dispersive X-ray Analysis ................................................................................................60 REFERENCES......................................................................................................................................................... 60 C h a p t e r T h r e e METHODOLOGY...................................................................................................................................................64 3.0 COMPUTATIONAL TECHNIQUES..........................................................................................................