CuPt alloys are well-known catalysts for CO oxidation and for hydrocarbon reactions. A detailed knowledge of the interactions that drive the surface composition of CuPt-alloys helps to understand these catalytic processes. In view of the conflicting experimental findings for the (111) surface composition of Cu3Pt, we have developed a method that adequately describes the segregation to various surfaces of CuPt alloys.
With First Principles (FP) methods alone, it remains difficult to predict properties of large model systems in order to investigate partially ordered and disordered systems. Approximate modelling techniques are able to extrapolate the principal results of FP methods to fill this gap. At present, Cluster Expansion Methods (CEM) are the most versatile to include an arbitrary number of ground-states in their parameterisation. However, structural defects or variations in local atomic volume are not easily described by standard CEM calculations. In contrast, the Modified Embedded Atom Method (MEAM) explicitly describes the distance dependence of interatomic interactions. We succeeded in formulating a novel method that makes optimal use of CEM and MEAM to accurately describe surface segregation in CuPt-alloys. This is the first method in which one single set of interaction parameters is able to calculate the energies of CuPt surfaces and correctly reproduces a L11-ground state for the equiatomic alloy and a L12 ground state for the Cu3Pt and the CuPt3 alloys.
The critical order-disorder transition temperatures, the surface energies of ordered alloys and the temperature dependent surface segregation to the low-index surfaces of Cu3Pt, CuPt and CuPt3 alloys are successfully described. The surface energy results are in excellent agreement with DFT-PW91 and DFT-LDA test sets. The calculations confirm the Cu-enrichment at various CuPt surfaces, and find evidence of Pt-enrichment at the (111) surface of the L11 CuPt alloy. This remarkable new observation was not found in previous theoretical studies of surface segregation in CuPt-alloys but is confirmed in this work by a double set of FP calculations.
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