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A computational study of the structure and properties of aluminophosphates and carbon nitrides

Deifallah, M.K.; (2009) A computational study of the structure and properties of aluminophosphates and carbon nitrides. Doctoral thesis, UCL (University College London).

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Abstract

In this thesis we have studied computationally two types of inorganic materials which can be classified as aluminophosphates and carbon nitrides. Both semi-classical and QM techniques have been applied. Aluminophosphates (AlPOs) are inorganic microporous oxides that have found use in a variety of industrial processes including ion exchange, molecular sieving and size and shape selective catalysis. Initially we examined whether we could influence the defect conformation of silicon doped AlPOs by the choice of extraframework templating species; our calculations indicate that doubly charged extraframework species encourage a clustering of the defects within the AlPO framework whilst monovalent cations lead to formation of more dispersed dopant configurations in the material. Later we examine the chemistry of vanadium dopants in AlPO and conclude that isomorphous substitutions at either Al or P sites can be achieved with little structural distortions; however, incorporations as vanadyl ion, (VO)2+, results in either unstable energy minima conformations, or framework breakage. We have also investigated the use of doped AlPOs to trap hydrocarbons in an attempt to determine whether they could be used in conjunction with the catalytic converter to control the emission of harmful substances into the atmosphere. Our work indicates that AlPOs do show positive results for this, with the best performance displayed by silicon doped AlPOs. In a concluding part to the work on AlPOs, we have fully characterised a new layered AlPO polymorph and have also determined some of its characteristics when doped. Our work on carbon nitrides can be split into two parts: first we studied graphene-like single layered sheets, and second, we examined the high pressure behaviour of a 3-dimensional layered carbon nitride. In the first part of the work, which was encouraged by the recent synthesis of a single layer of pure carbon "graphene", we examine the interplay between the structural and electronic properties of single layer carbon nitride sheets. Our calculations indicate that carbon nitride "graphenes" are wide band-gap semiconductors whose semi-conducting properties can be altered by the presence of voids, adsorbates and compressional stress. A 3-dimensional layered carbon nitride was studied since predictions have been made that a dense phase of such a material of composition C3N4, may have "superhard" properties. By tracing the layered C,N solid’s reaction to compression, we aimed to investigate whether a dense phase could be yielded. Our calculations predict the emergence of new dense frameworks within the structure upon compression. Our calculations also indicate that structural phase boundaries are affected by the presence of HCl inside the C,N solid.

Type:Thesis (Doctoral)
Title:A computational study of the structure and properties of aluminophosphates and carbon nitrides
Language:English
Additional information:Authorisation for digitisation not received
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry

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