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Abstract

Nonlinear dynamics in heterogeneous catalysis has been studied extensively over the past decades (see [[1]–[14]] for review articles). A large number of reactions, different types of catalysts and reactor designs have been used. In general, an understanding of pattern formation on catalytic surfaces requires not only detailed knowledge of the reaction and diffusion processes on the surface, but also needs to assess the influence of temperature changes, the flow of the relevant species (usually in the gas, sometimes in the liquid phase) and mixing effects in the fluid phase. The ’chemical part’ of the problem can be isolated by working at very low pressure (typically below 10⁻⁴ mbar) on single crystal surfaces (in order to minimize the effects of catalyst inhomogeneities). Naturally, the reaction rate is much smaller at low than at high pressure so that experiments can be conducted under strictly isothermal conditions and the heat balance need not be taken into account. Moreover, at 10⁻⁴ mbar there are typically 10⁵ site changes of an adsorbed particle for 1 adsorption event, i.e., diffusion is fast enough to guarantee a locally well-mixed adsorption layer. As a consequence, the pattern formation can be successfully modeled by reaction-diffusion equations. In this article we restrict ourselves to typical aspects of surface reactions which distinguish them from other reaction-diffusion systems (such as reactions in the liquid phase or in gels). These aspects include threshold kinetics (i.e., a reaction does not occur until a certain concentration exceeds a finite value), anisotropy (in particular dynamical changes of anisotropy) and global coupling (which occurs due to good mixing of the gas phase).