Ternary alloys have important applications as catalysts in the chemical process industry. Furthermore, ternary alloys can exhibit a typical multi-component segregation behaviour that does not exist in binary alloys: co-segregation, site competition and blocking are typical phenomena that may occur in ternary alloys. In this contribution, segregation is studied for the ternary alloy system Pt-Pd-Rh, with a previously derived MEAM potential for the pure elements Pt, Pd and Rh and for the binary Pt-Rh, Pt-Pd and Pd-Rh alloys. Combined with Monte Carlo (MC) simulations, these MEAM potentials nicely reproduce the experimentally observed segregation behaviour in the binary sub-systems.
First, the heat of mixing at 1400 K is determined by MC/MEAM simulations for the whole of the ternary diagram. Subsequently, the ternary bulk phase diagram is constructed from MC/MEAM simulations. To our knowledge, this is the first attempt to construct a phase diagram for this system.
Next, MC/MEAM simulations are performed in order to study the segregation behaviour to the (111) surface as a function of bulk composition. For all compositions, at 1200 K, a strong Pd segregation is observed. Furthermore, a region of Pd-Pt co-segregation can be identified close to the binary Pt-Rh axis of the phase diagram.
Finally, segregation is studied as a function of temperature for three particular compositions. For Pt83Pd15Rh2, good agreement with the experimental data is obtained. Secondly, Pt20Pd40Rh40 behaves in a way that is analogous to the binary Pd-Rh system: at low temperatures a cherry-like configuration develops with the Pd-rich phase of lower surface energy in the sub-surface layers and it is from this phase that segregation occurs. The third alloy Pt40Pd10Rh50 shows co-segregation of Pt and Pd at higher temperatures. At low temperatures, only Pd segregates to the surface, as this is the most exothermic process. At higher temperatures (T > 1000 K), the Pd segregation diminishes and the less exothermic segregation of Pt becomes possible. Segregation energies of Pt and Pd are derived from our MC/MEAM simulations and compared to the segregation energies obtained from the Quasi-Chemical Approximation.
|