Title

Computer simulation for adsorption and desorption of molecules on solid surfaces

Date of Award

1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Cynthia S. Hirtzel

Keywords

reactive and non-reactive desorption, Macro State Markov Chain Model (MSMCM)

Subject Categories

Chemical Engineering

Abstract

This dissertation consists of two parts. In the first part, a Monte Carlo computer experiment for simulation of thermal desorption of gases from mixed adlayers is presented. This simulation is developed for mixed adlayers containing two species and for cases in which reactive and non-reactive desorption coexist. The effects of lateral interactions among adatoms, adsorbate islanding, surface diffusion, initial surface coverages and distribution of site energies are taken into account. It is observed that for a wide range of temperature, when the reaction energy of two species occupying adjacent sites is lower than the site energy of the original occupant, non-reactive desorption has very limited effect on the spectrum of the reactive desorption. Only in cases in which the activation energy is quite close to the site energy of the original occupant does the non-reactive desorption affect the reactive desorption significantly. This is a complex phenomena, of course, and initial coverages, lateral interactions, and energy distributions also affect the desorption spectrum in these cases.

In the second part, a new simulation model based on the grand canonical ensemble to study adsorption of molecules in zeolite molecular sieves is described. The existing Monte Carlo simulation models of adsorption proposed by many researchers to date use a sequential-state generating scheme which requires large amounts of computation time. In this study, a new Monte Carlo simulation model, called the Macro State Markov Chain Model (MSMCM), is developed. One primary advantage of this model is that it is suitable for implementation on massively-parallel machines, and hence the computational time required for the computer experiments can be greatly reduced. A massively-parallel algorithm for gas adsorption in zeolite 5A based on this model has been developed and implemented on the Connection Machine CM2. The results of this study are found to be in good agreement in general with the experimental data available in the literature, and the simulation time is dramatically reduced in comparison with conventional sequential schemes. The model can be used to estimate pure and multicomponent gas adsorption isotherms and is a powerful and flexible tool for studies of adsorption systems. This model and algorithm also appears to have great potential to investigate and improve the potential energy model which defines the structure and intermolecular forces of systems being investigated.

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