The knowledge of the composition and surface structure of nanoalloy particles is crucial to explain their catalytic performance. In addition, the bonding of adsorbates may, in some cases, induce modifications in local atomic composition and surface structure, changing the activity and selectivity of the catalyst. These facts were observed for Au-Pd nanoparticles[1]-[2]. Indeed, although the gold surface enrichment is predicted to be thermodynamically favorable under vacuum conditions[3], a reversed segregation of Pd as a more active component to the surface is reported to occur in the presence of adsorbates[4]-[5].

In order to study how adsorption of CO molecules changes the surface composition of AuPd alloys, we develop a theoretical methodology which is able to take this effect into account[6]. An Ising model based on density functional theory calculations is derived to define interatomic potentials that describe metal–metal, metal–CO, and CO–CO interactions. Then, through the use of Monte Carlo simulations within the semi-grand canonical ensemble, the effect of adsorption-induced segregation for the AuPd(100) surface is well-reproduced for different temperatures and CO pressures. Segregation isotherms identify a Pd surface enrichment for low CO pressures, and CO surface saturation is reached at an intermediate coverage of θ = 0.5 ML. Furthermore, Pd chains induced by an ordering of adsorbed CO molecules appear at low temperature and intermediate CO pressures. These chains are the result of a competitive effect between CO–CO repulsions and metal–CO interactions.

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