= Solution
A bulk fcc atom has twelve nearest neighbours, whereas surface atoms are undercoordinated: ideal (111) terrace atoms have nine and (100) terrace atoms eight. Steps, kinks, edges and corners can have still lower coordination and different bond-angle environments. These sites change electronic structure, <adsorption> energies and activation barriers, making geometrically different sites catalytically inequivalent.
In a Pt-Pd particle, a site's neighbours may be Pt, Pd or a mixture; <surface segregation> means surface composition need not equal overall composition. Alloying also changes strain and electronic interactions. Surface relaxation, reconstruction, adsorbed <ligands>, oxide or contamination can further alter accessibility and chemical state. A high nominal Pt fraction therefore does not establish either accessible Pt sites or a particular catalytic activity.
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