Benzeneadsorption on binary Pt3M alloys and surface alloys: a DFT study

Abstract

Benzene adsorption on Pt₃M/Pt(111) surfaces and Pt₃M(111) bulk alloys (M = Fe, Co, Ni, Cu, Pd, Ag, Au) is analyzed using density functional theory calculations on 4-layered slabs in the framework of catalyst development for aromatics hydrogenation. Segregation in the top layers was allowed for, accounting for the actual stoichiometric composition of the top layers rather than using simplified ‘skin’ or ‘sandwich’ structures. On the surfaces that do not segregate (M = Pd, Ag, Au), the preferred benzene adsorption site is the hollow Pt₃-hcp⁰ site. On antisegregated “Pt-skin” surfaces (M = Fe, Co, Ni, Cu, Pd), which have a top layer composed entirely of Pt, benzene prefers bridge sites with a maximized number of solute atoms M in the subsurface layers. Benzene adsorption is weaker than on pure Pt(111), by 0.1–0.5 eV on the surface alloys and by 0.6–1.0 eV on bulk alloys, except for Pt₃Pd alloys, which behave similarly to pure Pt. On the fully segregated Pt₃Ag and Pt₃Au alloys, which have a Ag resp. Au monolayer on top, only physisorption occurs. Benzene adsorption does not change the segregation state of the catalyst. From various DOS-based catalyst descriptors, the occupied d-band center of the clean catalyst slab shows the best correlation with benzene adsorption energies, allowing the prediction of benzene adsorption energies on a range of other Pt-based bimetallic alloys.