SERS for Probing Highly Reactive Pd Sites with Elongated Pd–Pd Bonds on the Au Surface

Bond length is an important structural parameter that can affect the activity of catalysts. In core–shell gold–palladium (Au@Pd) nanocatalysts, sites with elongated Pd–Pd bonds arising from lattice mismatches between Au and Pd exhibit distinct catalytic activity. Despite their impact on catalysis, bond-length alternations have not been directly observed or characterized. Herein, we used the surface-enhanced Raman scattering (SERS) spectra of chemisorbed 2,6-dimethylphenyl isocyanide (2,6-DMPI) structural reporter to spectroscopically probe elongated Pd–Pd bonds on Au@Pd. In a catalyzed Suzuki–Miyaura coupling reaction, we found that strained sites with elongated Pd bonds are >100 times more active than unstrained sites. SERS spectra and theoretical simulations reveal that changes in bond lengths influence the electronic state of Pd atoms more profoundly than substituting coordination atoms or reducing coordination numbers. Further, we discovered that Pd growth begins as a continuous atomic chain rather than a single atom as thought previously.