Catalytic hydrogenation reactions are important in many industrial applications. While Pt is catalytically active towards hydrogenation, it is very costly, and can suffer from poisoning by CO, and coke. Previously, Ag based catalysts have been modified by alloying Pt or Pd for applications in highly selective heterogeneous catalysis, promising for catalyst design since Ag is cheaper than Pt and more resilient to poisoning. By analogy to our single-atom alloy approach in other systems such as PtCu and PdCu, alloying Pt into Ag has the potential to greatly enhance Pt’s catalytic selectivity while reducing the cost of precious metal required to catalyze industrially relevant reactions and reduce poisoning, due to weaker binding on Ag. The atomic-scale surface structure of dilute PtAg alloys has not been reported to date. Using scanning tunneling microscopy (STM) and STM-based spectroscopies, we characterized the surface structure and local geometry of Pt deposited on Ag(111) as a function of temperature. At low temperatures, intermixing of PtAg is driven by a negative mixing enthalpy, resulting in different metastable states such as isolated Pt atoms in, and islands on, Ag terraces, as well as Pt rich brims located along Ag step edges. Increasing the alloying temperature results in an increased concentration of Pt atoms along Ag steps edges as well as direct exchange of Pt atoms into Ag terraces. At higher temperatures, there is sufficient thermal energy for Pt atoms to fully disperse in the Ag(111) surface layer as isolated atoms, forming single-atom alloys. This characterization of PtAg surface alloys will enable us to correlate reaction activity and selectivity to the atomic-scale structure of the alloy and potentially tune catalytic selectivity and resilience to poisoning.

Atomic-scale characterization and reactivity of PtAg surface alloys

2019 Fall National Meeting

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