Adlayer structure and lattice size effects on catalytic rates predicted from KMC simulations: NO oxidation on Pt(111)

Repulsive and/or attractive interactions between surface adsorbates have an important effect on the structure of the adsorbate layer and consequently on the rate of heterogeneous catalytic reactions. Thus, developing reaction models that take into account adsorbate-adsorbate interactions is crucial for making accurate predictions of the catalytic rate and surface coverage during reaction. In the present work, we employ kinetic Monte Carlo simulation to model the catalytic NO oxidation on Pt (111), adopting a cluster expansion (CE) Hamiltonian approach for treating the aforementioned interactions. We investigate CEs of increasing complexity, ranging from pairwise 1st nearest neighbor to long-range and many-body terms. We show that energetic models incorporating solely short-range interactions result in ordered adlayer structures, which are disrupted by anti-phase boundaries and defective regions when the size of the periodic lattice is non-commensurate to the structure of the stable adlayer. We find that O₂ dissociates on sites located in these defective regions, which are predominantly responsible for the activity, and the predicted catalytic rate is strongly depended on the lattice size. Such effects are absent when employing non-periodic lattices, whereon the catalytic activity appears more intense on edges/corner sites. Finally, inclusion of long-range interactions in the model Hamiltonian induces relative disorder in the adsorbate layer, which is ascribed to the “softening” of the repulsive interactions between adspecies. Under these circumstances, the distribution of activation energies for O₂ dissociation is broader as compared to short-range interaction models and on this basis we explain the disparate catalytic rate predictions when using different CEs.