Monte Carlo simulation of the adsorption of co on pt(111): thermodynamic considerations for the surface configuration of adsorbed species

We have developed a "two-site" lattice gas model which provides a theoretical framework for understanding adsorption equilibrium in systems for which the potential energy surface is inherently heterogeneous in terms of both adsorbate-adsorbate and adsorbate-potential energy surface interactions and also entropie factors associated with the existence of two types of sites. Utilizing this general framework, we have examined two model systems with general features of the CO-Pt(111) adsorption system. In the first of these models, we represent the CO-CO interaction with a repulsive dipole-dipole -- dipole-image potential and in the second, with a Lennard-Jones 6-12 potential. We delineate criteria for determining the ordered ground states of each of the systems in terms of the relative magnitudes of adsorbate-adsorbate and adsorbate-potential energy surface interactions and, utilizing Monte Carlo simulations, we calculate approximate phase diagrams for each. In addition to observing the expected order-disorder phase transitions, we observe a "bridge-to-top" transition associated with the transfer of molecules from bridge to energetically favored top sites as the temperature of the system is decreased. Both models produce phase diagrams which may be inferred from LEED studies of the CO-Pt(111) system at low coverages. In addition, the dipole-dipole -- dipole-image potential reproduces quantitatively the experimentally observed coverage dependence of both the heat of adsorption and the infrared frequency of linear CO, while the Lennard-Jones model fails to reproduce these trends. We have utilized our model to address several issues related to the equilibrium of the CO-Pt(111) system and we suggest avenues for future experimental study.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29459/1/0000541.pd

Cellular automaton version of the AB2 reaction model obeying proper stoichiometry

The cellular-automaton (CA) AB2 surface-reaction model of Chopard and Droz (1988) is modified so that the proper stoichiometry in both the B2 adsorption and the AB reaction is followed. Simulations show that the first-order kinetic phase transition of the Monte Carlo AB2 model is recovered. A mean-field analysis which predicts a first-order phase transition, is also presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48823/2/ja911539.pd

The interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

We have studied the interaction of polyaromatic hydrocarbons (PAHs) with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene and ovalene at sub-monolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of 2-D adsorbate islands up to the desorption temperature. Pre-exponential frequency factors are found to be in the range $10^{14}$-$10^{21} s^{-1}$ as obtained from both Falconer--Madix (isothermal desorption) analysis and Antoine's fit to vapour pressure data. The resulting binding energy per carbon atom of the PAH is $52\pm$5 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is $61\pm5$~meV/atom which is considerably larger than previously reported experimental values.Comment: 8 pages, 4 figures, 2 table

Self-sustained oscillations in a heterrogeneous catalytic reaction: a monte carlo simulation

We have utilized Monte Carlo methods to study the kinetics of a generic heterogeneous catalytic reaction, A+B-->AB. This reaction includes the elementary steps of adsorption and desorption of reactants A and B, surface reaction through the Langmuir--Hinshelwood mechanism, and desorption of product AB. It is shown that this model is capable of producing self-sustained oscillations in the rate of reaction. The oscillations are dependent on the rate of desorption and exhibit a time scale much greater than those of the adsorption and surface reaction steps in the model. We analyze the dynamic quality of the oscillations and discern that they stem from chaos. To our best knowledge, this is the first study in which chaos has been observed and characterized through a Monte Carlo simulation. With the results of this work, we have been able to analyze the fundamental components responsible for producing the chaos in our simulations. We discuss the implications of our results for actual catalytic systems with oscillatory behavior.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28245/1/0000698.pd