REACTANT SEGREGATION IN A LANGMUIR-HINSHELWOOD SURFACE-REACTION

We have performed Monte Carlo simulations of a Langmuir-Hinshelwood reaction between two species A and B adsorbed on a square lattice, with the goal of determining how spatial correlations between the species vary with reaction rate. Adsorption of each species occurs when a gas-phase molecule, either A or B, impinges upon a vacant lattice site. The probability that a molecule impinges upon and adsorbs successfully into a vacant lattice site per unit time is pa/2 for both species. Desorption is not allowed and the surface reaction is allowed to occur only between nearest-neighbor AB pairs. For each nearest-neighbor AB pair, the probability of reaction per unit time is p r. A novel feature of this investigation is that we explicitly simulate the diffusion of the particles on the lattice. The particles are allowed to migrate by hopping to vacant nearest-neighbor sites, where the probability of a hop per unit time is pm. In all these simulations we have set pm to be unity, and varied pr from 0.01 to unity. We have also set pa = pr/5 for all the simulations in order to maintain moderately low fractional surface coverages. "Islanding" of each type of particle occurs even for the lowest value of pr used, although the entire surface is never poisoned. For range of values of pr used, the "islands" grow to a finite steady-state size. We also found that the islands that are formed are consistent with a dimension of two. A nearest-neighbor correlation function φ is defined to describe the process of islanding, and the dependence of φ upon pm/pr is studied. By studying this simple model we show that quite large inhomogeneities can be reasonably expected to occur in catalytic systems even when reaction probabilities are small compared to diffusion rates, and that these inhomogeneities affect total reaction rates. © 1990 American Institute of Physics.