Active Site Ensembles
An active site ensemble is the group of neighboring surface atoms that together bind an adsorbate or stabilize a transition state. Its size (nuclearity, the number of active atoms in an isolated cluster) and shape can control activity and selectivity, an idea usually called the ensemble effect; alloys and ordered intermetallics let the ensemble be tuned by diluting an active metal in a less reactive host.
Enumerating nuclearity and shape on intermetallic surfaces (2024)
Sharma, Nguyen, Kitchin, Ulissi and Janik built a Surface Nuclearity Calculator that represents the surface active atoms of a slab as a graph (Voronoi neighbors with periodic images), reads nuclearity as the size of each connected component, classes shapes by graph isomorphism, and flags stripe-like (1D-infinite) or mesh-like (2D-infinite) ensembles by comparing a unit cell with a 2 x 2 supercell. Applied to 29,002 low-index surfaces of 1,323 near-hull AFLOW intermetallics of Pd, Pt, Rh, Ru, Ir or Ag with Zn, Al, Ga, In or Cd, it found finite nuclearities (0 to 16) on about 80% of surfaces, with monomers making up 60% of the finite cases, and 21 distinct shapes up to nuclearity 5. Nuclearity ranges widened with active mole fraction and depended on Bravais lattice, so composition and lattice type can prune a search for a target site geometry (Sharma et al. 2024). Summary: sharma2024.