Dr David Nicholson's Page : Dr Julian Gale's Page : Theory and Simulation Group
Animated gif of a chabazite pore Tom Grey
Postgraduate Student,
Department of Chemistry,
Imperial College,
London.
SW7 2AY
United Kingdom
email: tjg1@ch.ic.ac.uk
The animated image above (rendered using the excellent povray freeware raytracer) shows a computer generated flight down the pore of the zeolite chabazite. Zeolites are inorganic materials containing long pores or channels of molecular proportions. When small molecules enter these pores, they experience attractive interactions with the walls of the pores. This means that the densities of gases in the pores tends to be higher than in the atmosphere surrounding the zeolite. This phenomenon is called 'adsorption' or, sometimes simply 'sorption'.
My PhD project, supervised by Dr David Nicholson and Dr Julian Gale, is to use computer simulation to study the adsorption of nitrogen into the pores of chabazite. I will be using Grand Canonical Monte-Carlo simulation (GCMC). This is a technique for sampling millions of possible molecular configurations of a chemical system. It is a very powerful method of studying the equilibrium properties of systems.
To perform a computer simulation of an adsorption system you need a mathematical model for that system. This consists chiefly of a model for the solid (the adsorbent), a model for the gas adsorbed (the adsorbate) and a model for the energy interaction between them. For zeolites which contain only silicon and oxygen, the first of these is the positions of the ions as found from X-Ray crystallography (together with parameters for the ions). The image below shows twenty-seven unit cells of chabazite:
Image of twenty-seven unit cells of chabazite
If you have the free Chime plugin from MDL you can view an interactive model of chabazite by clicking on the image. Most real samples of chabazite are not this simple. Some of the silicon ions are substituted by aluminium ions and this gives rise to a negative charge on the zeolite framework. This negative charge is countered by either hydrogen or small cations. These ions are often termed 'extra-framework cations' as they reside in the pore spaces rather than as part of the framework. I have spent much of my first year using GULP to find plausible locations for the calcium ions in chabazite and have submitted a paper detailing our findings.
With the start of my second year the project focus has shifted to the third of the requirements for simulation, a model for the energy interaction between the adsorbate and the adsorbent. We are going to try to extend the PN1 potential model [See: Advances in Colloid and Interface Science; 179, 76, (1998)] to model zeolites containing extra-framework cations. We have been doing ab initio quantum mechanics on nitrogen interacting with calcium cations and DFT quantum mechanics on periodic systems. I have also been adding code to the group's FORTRAN77 program to allow it to model zeolites.
As the different pieces of work come together we hope to explore the relationship between structure and adsorption properties and to develop workable approaches to simulating zeolites containing aluminium and extra-framework cations.
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When not working on my project I am very interested in 3D computer graphics and also in java programming. I have written a well received java applet which can display texture-mapped 3D models from the games quake and quake2 (by id software) in their native formats. I'm also working on a molecular dynamics applet.
To prove I can still leave my computer with only a minimal mental effort, I like to swim several evenings a week, although my swimming abilities are occasionally put to shame by small toddlers and routinely by everyone else.
Dr David Nicholson's Page : Dr Julian Gale's Page : Theory and Simulation Group

The sorry state of this page was last adjusted by T.J.Grey. 2 December 1999