(190h) Exploring Surface Structures of Pt3-Mn Alloys: A Comprehensive Analysis through a Cluster Expansion Methodology

Pt-based alloy catalysts are well known for propane dehydrogenation (PDH) chemistry.¹ However, rapid deactivation due to coking and sub-optimal selectivity of propylene continue to pose significant challenges. Recent studies of Pt₃Mn catalysts have shown its promising selectivity and prolonged stability for PDH², but current understanding of Pt₃Mn surface structures and atomic arrangements under the reaction conditions is still limited.

While bulk-terminated Pt₃Mn surfaces exhibit a uniform distribution of Mn atoms, preferential Pt or Mn atom distribution can occur at the edge and/or the corner sites. In addition, local ordering such as segregation, islanding, and site isolation may also occur and are known to influence PDH reactivity.² To provide molecular insights into these structural transformations, we utilize a combination of density functional theory (DFT) calculations, cluster expansion (CE) fitting, and Monte Carlo simulations to elucidate the surface structures of Pt₃Mn catalysts in realistic reaction environments. The combination of DFT and CE-based fitting of interatomic potentials, in turn, permits accurate and efficient analysis of the large configurational space of Pt and Mn atom arrangements.

DFT-analysis reveals energetically favorable anti-ferromagnetic (AFM) spin distributions among the Mn atoms in Pt₃Mn surface slabs. A framework is therefore developed to assign AFM spins to Mn atoms, and resulting DFT-calculated energies for (111) and (211) facets are separately used to fit the CE potentials, achieving accuracy in the order of 1 meV/atom. Subsequent Monte Carlo simulations, using the CE potential, predict low-energy atomic ensembles at different reaction conditions. Finally, PDH activity and selectivity descriptors, coupled with a microkinetic model, are used to determine site-averaged PDH reactivity on the Pt₃Mn alloy surfaces. These results provide insights into the impact of metastable surface alloy structures on propane conversion and propylene selectivity at high temperatures.

References:

1. Sattler et al., Chem. Rev. 2014

2 Wu, et al., JACS 2018