CO-oxidation model with superlattice ordering of adsorbed oxygen. I. Steady-state bifurcations
- Ames Laboratory and Department of Mathematics, Iowa State University, Ames, Iowa 50011 (United States)
- Ames Laboratory, Iowa State University, Ames, Iowa 50011 (United States)
We analyze a model for CO oxidation on surfaces which incorporates both rapid diffusion of adsorbed CO, and superlattice ordering of adsorbed immobile oxygen on a square lattice of adsorption sites. The superlattice ordering derives from an {open_quotes}eight-site adsorption rule,{close_quotes} wherein diatomic oxygen adsorbs dissociatively on diagonally adjacent empty sites, provided that none of the six additional neighboring sites are occupied by oxygen. A {open_quotes}hybrid{close_quotes} formalism is applied to implement the model. Highly mobile adsorbed CO is assumed randomly distributed on sites not occupied by oxygen (which is justified if one neglects CO{endash}CO and CO{endash}O adspecies interactions), and is thus treated within a mean-field framework. In contrast, the distribution of immobile adsorbed oxygen is treated within a lattice{endash}gas framework. Exact master equations are presented for the model, together with some {ital exact} relationships for the coverages and reaction rate. A precise description of steady-state bifurcation behavior is provided utilizing both conventional and {open_quotes}constant-coverage ensemble{close_quotes} Monte Carlo simulations. This behavior is compared with predictions of a suitable analytic pair approximation derived from the master equations. The model exhibits the expected bistability, i.e., coexistence of highly reactive and relatively inactive states, which disappears at a cusp bifurcation. In addition, we show that the oxygen superlattice ordering produces a symmetry-breaking transition, and associated coarsening phenomena, not present in conventional Ziff{endash}Gulari{endash}Barshad-type reaction models. {copyright} {ital 1999 American Institute of Physics.}
- OSTI ID:
- 686520
- Journal Information:
- Journal of Chemical Physics, Vol. 111, Issue 14; Other Information: PBD: Oct 1999
- Country of Publication:
- United States
- Language:
- English
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