Citation
Kang, HwayChuan (1990) Model studies of adsorbate ordering, adsorption and reaction using MonteCarlo simulations. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd05152007125719
Abstract
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The application of MonteCarlo simulations to study thermally excited timedependent phenomena is examined. The diffusion coefficient and the exponent for domain growth for a square lattice gas experiencing equal and repulsive nearestneighbor and nextnearestneighbor interactions are calculated for three different dynamics: Kawasaki; Metropolis and energybarrier. All three dynamics satisfy detailed balance, but the diffusion coefficient is found to show a different temperature dependence for each. The growth exponents for Kawasaki and energybarrier dynamics are in close agreement, and larger than that for Metropolis dynamics. This difference arises because the domain sizes reached were not sufficiently large.
In further MonteCarlo simulations of the same lattice gas model using dynamics which allow precursormediated migration, the growth exponent of fourfold degenerate ordered (2x1) domains on a square lattice is found to be 1/2. If the growth law is written as [...], A is found to be proportional to [...], where D is the diffusion coefficient of the adsorbed particle. Kawasaki dynamics simulations at zero temperature are performed for the growth of [...] domains on a triangular lattice. The results show that the low temperature behavior is markedly dependent upon the details of the lateral interactions and the range of the particle hops. This latter result demonstrates the strong influence of a precursor state on growth kinetics.
MonteCarlo analysis of molecular beam reflectivity measurements of the probability of molecular adsorption of ethane on the Ir(110)(1x2) surface shows that a precursor state can also be rather important in adsorption. We show that the experimental data can be explained by adsorption occurring in two channels: direct and precursormediated. In this case the precursor is an ethane molecule trapped in a second layer on top of the first layer of molecularly adsorbed ethane. From the simulations we were also able to calculate the energy barriers for diffusion and desorption of an ethane molecule in the precursor state.
MonteCarlo simulations of a LangmuirHinshelwood reaction between two interacting species were also performed. The parametrization of the reaction rate coefficient that is implicit in an Arrhenius plot is examined. It is shown that the effective energy barrier and preexponential factor obtained from an Arrhenius plot show strong compensation when the overlayer configuration is strongly temperature dependent. This can explain the anomalously high reaction or desorption preexponential factors observed for some adsorbed systems.
It is also shown that a LangmuirHinshelwood reaction occurring between two species, even when they are noninteracting, can lead to configurational effects. Compact 'islands' consisting solely of either species A or species B are observed in simulations. An order parameter which allows an analogy between the reacting system and magnetic systems to be drawn is defined. The reactivity of the catalyst surface is inversely proportional to the 'island' size.
Item Type:  Thesis (Dissertation (Ph.D.)) 

Degree Grantor:  California Institute of Technology 
Division:  Chemistry and Chemical Engineering 
Major Option:  Chemical Engineering 
Thesis Availability:  Restricted to Caltech community only 
Research Advisor(s): 

Thesis Committee: 

Defense Date:  11 September 1989 
Record Number:  CaltechETD:etd05152007125719 
Persistent URL:  http://resolver.caltech.edu/CaltechETD:etd05152007125719 
Default Usage Policy:  No commercial reproduction, distribution, display or performance rights in this work are provided. 
ID Code:  1823 
Collection:  CaltechTHESIS 
Deposited By:  Imported from ETDdb 
Deposited On:  24 May 2007 
Last Modified:  26 Dec 2012 02:42 
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