Formation of Pd monomers and dimers on a single-crystal Pd ₃Fe(111) surface

Surface reconstruction of binary alloys is important in heterogeneous catalysis because it modifies both surface composition and structure and thus affects the catalytic activity and selectivity. We report here on segregation and surface morphology at a Pd₃Fe(111) single-crystal model catalyst investigated by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM). Annealing in vacuum causes Pd segregation, and STM reveals a complex surface structure with 0.17 monolayers of Pd monomer and dimer adatoms on top of the outermost alloy layer. This result is explained by density functional theory (DFT) calculations, which reveal that the contribution from vibrational free energy causes Pd atoms to detach from step edges at high temperature (>1200 K) and then become trapped at room temperature at Fe defect sites due to a large diffusion barrier. This adlayer structure differs from surface structures observed for other binary alloy systems and is likely to offer new opportunities for manipulating catalytic properties of bimetallic alloys.