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Engineering the Surface Architecture of Highly Dilute Alloys: An ab Initio Monte Carlo Approach

Highly dilute alloys of platinum group metals (PGMs) - (Pt, Rh, Ir, Pd, and Ni) with coinage metals (Cu, Au and Ag) serve as highly selective and coke-resistant catalysts in a number of applications. The catalytic behaviour of these materials is governed by the size and shape of the surface “ensembles” of PGM atoms. Therefore, establishing a means of control over the topological architecture of highly dilute alloy surfaces is crucial to optimising their catalytic performance. In the present work, we use on-lattice Monte Carlo (MC) simulations that are parameterised by density functional theory (DFT) derived energetics, in order to investigate the surface aggregation of PGM atoms under vacuum conditions and in the presence of CO. We study several highly dilute alloy surfaces at various PGM loadings, including Pd/Au(111), Pd/Ag(111), Pt/Cu(111), Rh/Cu(111), Ir/Ag(111) and Ni/Cu(111). Under vacuum conditions, we observe a thermodynamic preference for dispersion of PGM as single atoms in the surface of the coinage metal host, on all examined alloy surfaces except Ir/Ag(111), where Ir atom aggregation and island formation is preferred. By evaluating the alloy surface structure in the presence of CO, we determine that the size and shape of PGM ensembles can be manipulated by tuning the partial pressure of CO (PCO) on the Pd/Au(111), Pd/Ag(111), Ir/Ag(111) and Ni/Cu(111) surfaces. In contrast, we determine that Pt/Cu(111) and Rh/Cu(111) highly dilute alloys are unresponsive to changes in PCO with Rh and Pt dispersing as isolated single atoms within the host matrix, irrespective of gaseous composition. Our findings suggest that it may be possible to fine-tune the surface architecture of highly dilute binary alloys for optimised catalytic performance.