Nanostructured electrode surfaces studied by electrochemical NMR

Abstract

Electrochemical nuclear magnetic resonance (EC-NMR) is a powerful local probe, which combines solid state NMR with electrochemistry. It is a unique technique that permits a unified, electronic-level study of the metal and adsorbate side of the electrochemical interface. Experiments can be performed either under direct potentiostatic control and in situ potential adjustment, or with samples prepared in a separate electrochemical cell and transferred to an NMR cell, where the potential is both known and constant. A phenomenological two-band model was applied to the NMR parameters to yield quantitative information about the Fermi level local density of states ( E f-LDOS) that are relevant to the type of chemisorption bond involved in the systems under investigations. A layer-model analysis was found to be effective in interpreting the 195Pt-NMR spectra of carbon-supported Pt nanoparticles. The surface peak of the 195Pt-NMR spectrum was found to be very sensitive to the chemical nature of the adsorbate present. The 195Pt Knight shifts show a direct correlation with the electronegativity of the adsorbate, and the 13C Knight shift of the CO adsorbate shows a correlation with the clean metal surface E f-LDOS. The electrode potential dependence of 13C-NMR spectra of CO adsorbed on Pt and Pd black show evidence of the alterations to the electrochemical interface by the application of the electric field. EC-NMR of Pt electrode surfaces modified by spontaneous deposition of ruthenium has provided new insights into the enhancement in CO-tolerance of these catalysts for methanol oxidation.