The nature of chemisorbates formed from ammonia on gold and palladium electrodes as discerned from surface-enhanced Raman spectroscopy

The chemisorbates formed from ammonia-containing alkaline electrolyte on gold and palladium electrodes have been identified using surface-enhanced Raman spectroscopy (SERS). On gold, a potential-dependent band at ca. 365¯385 cm-1 is observed, consistent with the metal¯nitrogen stretch for molecular adsorbed ammonia on the basis of the frequency redshift observed upon deuteration. A similar feature is also observed on palladium, at 440¯455 cm-1, again consistent with chemisorbed ammonia on the basis of the H/D shift. On palladium, but not on gold, however, transfer of the electrode to ammonia-free electrolyte yielded a vibrational band at 455¯465 cm-1. The near-zero H/D frequency shift obtained for this irreversibly adsorbed component indicates the formation of chemisorbed atomic nitrogen on palladium. This finding is discussed in terms of the mechanism for ammonia electro-oxidation.