Creation of Well-Defined Pd Surface Sites on Single Crystal Pd33Ag67: From Ensembles to Single Atoms

The composition and distribution of Pd atoms on the surface were controlled by thermal treatments following sputtering of the surface of PdAg single crystal of composition Pd33Ag67 at 100 K. The Pd distribution at the (111) surface was varied from small ensembles to single atoms by thermal annealing between 400 and 820 K. On Pd ensembles, D2 dissociates readily, populating the surface with two types of atomic D/H. D2 (H2) desorbs around 295 K (beta-state) from pure Pd sites, while it desorbs from bimetallic or pure silver sites desorbs around 185- 230 K (gamma-state). In contrast, single Pd sites did not dissociate D2 or H2 under the conditions used herein. In the annealing process rapid segregation of Pd from the topmost surface into the near subsurface occurs with annealing within minutes, and a subsequent, much slower diffusion into the bulk occurs on the time scale of hours. Moreover, subsurface Pd can be employed for fine adjustment of the chemical bonding of adsorbates to these surfaces. This study establishes a method for tuning the compositional and configurational surface properties of well-defined substitutional bimetallic bulk alloys.