STOCHASTIC-MODEL FOR COMPLEX SURFACE-REACTION SYSTEMS WITH APPLICATION TO NH3 FORMATION

A stochastic model is introduced that is appropriate to describe surfa ce-reaction systems. These reaction systems are well suited for the de scription via master equations using their Markovian behavior. In this representation an infinite chain of master equations for the distribu tion functions of the state of the surface, of pairs of surface sites, etc., arises. This hierarchy is truncated by a superposition approxim ation. The resulting lattice equations are solved in a small region wh ich contains all of the structure-sensitive aspects and can be connect ed to continuous functions which represent the behavior of the system for large distances from a reference point. In the present paper, we f ocus our interest on the development of the formalism and its use when applied to the formation of NH3. The results obtained (phase-transiti on points and densities of particles on the surface) are in agreement with Monte Carlo and cellular-automata simulations. The stochastic mod el can easily be extended to other reaction systems and is therefore a n elegant alternative to the description via Monte Carlo and cellular- automata simulations.