Probing by NMR the Effect of Surface Charges on the Chemisorption Bond

Abstract

The ${}^{13}\mathrm{C}$ NMR line position of $\mathrm{CO}$ chemisorbed on $\mathrm{Pt}$ and $\mathrm{Pd}$ electrodes was measured as a function of the potential of the electrode with respect to a $\mathrm{Ag}/\mathrm{AgCl}$ reference, in the ranges of $\ensuremath{-}0.3$ to $0.2\mathrm{V}$ for $\mathrm{Pt}$ and $\ensuremath{-}0.7$ to $\ensuremath{-}0.3\mathrm{V}$ for $\mathrm{Pd}$. The lines shifted at rates of $\ensuremath{-}71\ifmmode\pm\else\textpm\fi{}20\mathrm{ppm}/\mathrm{V}$ for $\mathrm{Pt}$ and $\ensuremath{-}136\ifmmode\pm\else\textpm\fi{}51\mathrm{ppm}/\mathrm{V}$ for $\mathrm{Pd}$. This effect is much too large to be ascribed to shielding effects due to local electric fields. It is assigned to a change in the ${}^{13}\mathrm{C}$ Knight shift, as demonstrated from the associated change in the spin-lattice relaxation rate. This change in Knight shift with potential is indicative of charge transfer processes between the electrode and the adsorbed $\mathrm{CO}$.