IRREVERSIBLE SATURATION TRANSITIONS IN DIMER DIMER REACTION MODELS OF HETEROGENEOUS CATALYSIS

The dynamics and the critical behaviour of dimer-dimer surface reactions of the type 1/2A2 + B2 --> B2A are investigated by means of Monte Carlo simulations and finite-size analysis. Three models, which follow the Langmuir-Hinshelwood mechanism and involves the formation of intermediate AB-species are proposed and studied. Neglecting both diffusion and desorption of the reactants (model M1), a critical point (p1B2) is found at p1B2 = 2/3 (P(B2) is the partial pressure of B2-dimers in the gas phase), such that for p(B2) < 2/3 (p(B2) > 2/3) the surface becomes irreversibly saturated by a 'binary compound' of A and AB-species (B-species), respectively. The reaction proceeds only at p(B) = 2/3. Assuming B-diffusion (model M2) the critical point remains unchanged and the main features of M2 are basically the same as those of M1. The third model (M3) considers the recombination reaction of adsorbed B-species leading to a dimer desorption. M3 has a critical point at p1B2 congruent-to 0.7014, as it follows from a finite-size analysis. For p(B2) less-than-or-equal-to p1B2 the surface is saturated by A and AB-species, while for p(B2) > p1B2 a reaction window with B2A production is found. So, at p1B2 one has an irreversible transition from an off-equilibrium saturated state to a stationary regime with sustained reaction. The critical behaviour of the rate of production and the reactant's coverage is analysed and the corresponding critical exponents are evaluated.