Modelling adsorbate coverage on complex alloy surfaces

In order to extend catalysis theory to complex alloys and multiple adsorbates, we have to face the fact that the number of possible surface site-adsorbate combinations gets too large to calculate. We, instead, define rules for adsorbate-adsorbate interactions; specifically, blocking rules in terms of disallowed local adsorbate-adsorbate configurations. We then conduct simple simulations to investigate how different rules entail certain outcomes. For the PdAg intermetallic and PdAg solid solutions, we find that the presence of Ag atoms hinders O* species from covering the whole (111) surface, which is the case for unary Pd(111), and instead allows for adsorbed *OH species. We predict that the adsorbed *OH species improves the oxygen reduction reaction activity because they have adsorption energies at the top of the activity volcano. Experiments can use our results to distinguish between the different possible PdAg(111) alloy surface manifestations, and to better understand adsorbate coverage on complex alloys. Lastly, we use our approach on Ag₁₄Ir₁₆Pd₃₀Pt₁₄Ru₂₆ high-entropy alloys, but find that the choice of adsorbate-adsorbate interaction rules affects the oxygen reduction in less distinguishable ways compared to the binary PdAg alloys.