Theory of the NO+CO surface-reaction model.

Abstract

We derive a pair approximation (PA) for the NO+CO model with instantaneous reactions. For both the triangular and square lattices, the PA, derived here using a simpler approach, yields a phase diagram with an active state for CO-fractions y in the interval y(1)<y<y(2), with a continuous (discontinuous) phase transition to a poisoned state at y(1) (y(2)). This is in qualitative agreement with simulation for the triangular lattice, where our theory gives a rather accurate prediction for y(2). To obtain the correct phase diagram for the square lattice, i.e., no active stationary state, we reformulate the PA using sublattices. The (formerly) active regime is then replaced by a poisoned state with broken symmetry (unequal sublattice coverages), as observed recently by Kortlüke et al. [Chem. Phys. Lett. 275, 85 (1997)]. In contrast with their approach, in which the active state persists, although reduced in extent, we report here the qualitatively correct theory of the NO+CO model on the square lattice. Surface diffusion of nitrogen can lead to an active state in this case. In one dimension, the PA predicts that diffusion is required for the existence of an active state.