Probing by NMR the effect of surface charges on the chemisorption bond

The C-13 NMR line position of CO chemisorbed on Pt and Pd electrodes was me asured as a function of the potential of the electrode with respect to a Ag /AgCl reference, in the ranges of -0.3 to 0.2 V for Pt and -0.7 to -0.3 V f or Pd. The lines shifted at rates of -71 +/- 20 ppm/V for Pt and -136 +/- 5 1 ppm/V for Pd. This effect is much too large to be ascribed to shielding e ffects due to local electric fields. It is assigned to a change in the C-13 Knight shift, as demonstrated from the associated change in the spin-latti ce relaxation rate. This change in Knight shift with potential is indicativ e of charge transfer processes between the electrode and the adsorbed CO.