Theoretical adsorption studies are most often limited to one or more adsorbate species on a pure metal. However, metallic catalysts used in the chemical industry usually consist of binary metal alloys instead. The introduction of a second metal in the substrate significantly complicates the study of the adsorption process and chemical reaction details. Adsorption on binary metal surfaces is indeed often accompanied by ensemble and/or ligand effects. Furthermore, the detailed atomic configuration of the surface is not a priori known and the presence of the adsorbates may influence the composition and ordering of the atoms in the alloy surface.
In this contribution, first, a model is proposed for the different kinds of atomic interactions in the CO/Cu3Pt(111) system. For the metal-metal interactions, the MEAM with recently optimised surface-specific parameters is used. The ligand effects are accounted for by the subregular solution model. In this way, the metal-adsorbate bond strength may vary with the local composition of the metal atom. The adsorption energies are derived from DFT data, corrected with constant terms in order to reproduce the experimental results. The lateral interaction between the CO molecules are described by a soft dipole-dipole repulsion at large distances and a hard sphere repulsion at short distances.
Kinetic Monte Carlo (KMC) simulations are then set up in order to investigate the CO adsoption on Cu3Pt(111). First, the TPD spectrum of CO on both ordered and disordered, Pt-enriched Cu3Pt(111) is simulated. At low CO coverages, the KMC simulations reproduce the experimentally observed peak maximum at 330 K on ordered Cu3Pt(111), which is attributed to CO adsorbed on Pt top sites. Increasing the coverage leads to the additional occupation of mixed Cu-Pt bridge sites and desorption temperatures of 170 K and 220 K, also in good agreement with the experiments. Next, assuming a non-linear ligand effect, the experimentally observed peak shift from 330 K to 300 K on disordered Pt-enriched Cu3Pt(111) could be reproduced.
Finally, preliminary simulation results for the CO oxidation on Cu3Pt(111) as a function of temperature are presented. Due to higher temperatures, atomic diffusion in the metallic substrate is also accounted for.
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