Hydrodynamic Limits for the Monomer-Dimer Surface Reaction: Chemical Diffusion, Wave Propagation, and Equistability

For finite adspecies mobility, the lattice-gas monomer-dimer (A + B2) surface reaction model exhibits a discontinuous transition from a stable reactive steady state to a stable A-poisoned steady state, as the impingement rate PA for A increases above a critical value P*. The reactive (poisoned) state is metastable for PA just above (below) P*. Increasing the surface mobility of A enhances metastability, leading to bistability in the limit of high mobility. In the bistable region, the more stable state displaces the less stable one separated from it by a planar interface, with P* becoming the equistability point for the two states. This hydrodynamic regime can be described by reaction-diffusion equations (RDE’s). However, for finite reaction rates, mixed adlayers of A and B are formed, resulting in a coverage-dependent and tensorial nature to chemical diffusion (even in the absence of interactions beyond site blocking). For equal mobility of adsorbed A and B, and finite reaction rate, the prediction for P* from such RDE’s, incorporating the appropriate description of chemical diffusion, is shown to coincide with that from kinetic Monte Carlo simulations for the lattice-gas model in the regime of high mobility. Behavior for this special case is compared with that for various other prescriptions of mobility, for both finite and infinite reaction rates