Catalytic surface reactions: pattern formation

Nonlinear kinetics of reactions on surfaces demonstrate rich variety of phenomena such as pattern formation, global oscillations, and even chaotic behavior. Explaining nonlinear kinetics of surface processes requires detection of a sequence of reactions that leads to the time-dependent reaction rates (e.g. an oscillation). For many systems such a sequence is known. For example, in the generally-accepted mechanism for CO oxidation on Pt(100) and Pt(110) the crucial factor is surface reconstruction. This can be lifted by adsorption of CO. The existence of an oscillatory cycle in itself does not determine the kinetics. For most reaction conditions, and possibly even always, it occurs everywhere on the surface but out-of-phase. The result is either a normal equilibrium or steady state, or some form of pattern formation. To obtain a global oscillation, the phases of the local oscillations need to be synchronized.
 
Monte Carlo simulations form a bottom-up approach in which first a microscopic model of a system is used. That model is simulated and the results are compared with experimental data. Such an approach has the advantage that it is very clear what the relation is between the microscopic processes and the macroscopic kinetics. The disadvantage is that this approach is computationally very demanding, and it is therefore hard to obtain an overview of all the properties of a model. In Fig.1 the results of Monte Carlo (MC) simulations for CO oxidation on Pt(110) are presented [1-3]. The model still can give global oscillations, but only for restricted reaction conditions, in agreement with experiment. The surface reconstructs and forms Turing-like structures with a definite width. The diffusion length needs only to exceed this width, to trigger global oscillations. If the diffusion is slower, one gets pattern formation in the adlayer. These patterns also have a characteristic length scale, which is however differ from that of the Turing-like structures which are present at the same time. Thus, we suggested a new oscillatory synchronization mechanism without gas-phase coupling, coupling via stochastic resonance, or lateral interactions, which yields global oscillations or pattern formation as observed experimentally.
 
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Papers:
  1. O. Kortlüke, V.N. Kuzovkov, and W. von Niessen, Oscillation phenomena leading to chaos in a stochastic surface reaction model. - Phys. Rev. Letters, 1998, 81, p. 2164-2167.
  2. V.N. Kuzovkov, O. Kortlüke, and W. von Niessen, Nucleation and island growth kinetics on reconstructing surfaces. - Phys. Rev. Letters, 1999, 83, p. 1636-1639.
  3. O. Kortlüke, V.N. Kuzovkov, and W. von Niessen, Global synchronisation via homogeneous nucleation in oscillating surface reaction. - Phys. Rev. Letters, 1999, 83, p. 3089-3092.