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.
Use complementary composition, spatial and population measurements:
- Insufficient Pt: digest representative aliquots and measure calibrated ICP-OES or ICP-MS to obtain bulk Pt/Pd ratio and trace Fe. Blanks, recovery checks and standards control contamination and matrix effects. This is sensitive and quantitative but destructive and supplies no particle-by-particle spatial information.
- Poor Pt-Pd mixing: use STEM-EDS maps or line profiles of many individual particles. Characteristic X-ray energies identify the elements; background subtraction and calibrated sensitivity factors permit local composition estimates. HAADF-STEM supplies complementary atomic-number-sensitive contrast, with Pt generally brighter than Pd for comparable thickness. Core-shell segregation or distinct domains can then be tested. Projected overlap can imitate mixing, and low counts, thickness and beam damage limit nanoscale quantification; tomography or multiple views can resolve important ambiguities.
- Fe impurities: EDS/EELS can locate Fe-rich inclusions or surface-associated material, while the bulk digest establishes whether a trace signal is representative. Failure to detect Fe in a few TEM fields does not rule out dilute contamination below local detection limits. XPS can additionally assess near-surface composition and oxidation state, but is not a bulk assay.
- Size uniformity: obtain calibrated TEM images from randomly chosen independent fields, segment individual particles and report a number-weighted size histogram with sample count and shape descriptors. Avoid overlap and selection of only well-dispersed or attractive particles. A projected diameter is not a complete three-dimensional volume; scattering methods can corroborate ensemble size but weight particles differently.
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