Theory of the NO+CO surface reaction model

TL;DR

This paper develops a pair approximation model for the NO+CO surface reaction, accurately predicting phase diagrams and the effects of lattice structure and diffusion, providing the first qualitative theory for the square lattice case.

Contribution

The paper introduces a simplified pair approximation approach that accurately models the NO+CO reaction on different lattices, including the first qualitative theory for the square lattice.

Findings

PA predicts active and poisoned states depending on CO-fraction y.

Reformulated PA with sublattices captures broken symmetry in square lattice.

Surface diffusion enables active states on the square lattice.

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 state, we reformulate the PA using sublattices. The (formerly) active regime is then replaced by a poisoned state with broken symmetry (unequal sub- lattice coverages), as observed recently by Kortluke 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 first 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.