Carbon-13 NMR spectra of carbon-13 monoxide adsorbed on silica-supported palladium particles: particle size dependence of the surface diffusion rate and the carbon-13 Knight shift

Abstract

The {sup 13}C NMR spectra of {sup 13}CO adsorbed on a 56% dispersion silica-supported Pd catalyst are found to be quite different from those reported for {sup 13}CO on a 19% dispersion Pd catalyst. The mobility of the {sup 13}CO is greatly reduced on these small Pd particles, which is interpreted as a consequence of the increase in the relative number of low-coordination, electron-deficient Pd sites such as on edges and corners and the concomitant increase in the tendency for CO to bind there in a linear fashion. This is supported by the observation that linear {sup 13}CO on this catalyst has very little Knight shift due to the decrease in the local electron density at the Fermi level for these sites, while bridging {sup 13}CO on terraces is still substantially Knight shifted. Comparison of the {sup 13}C NMR and FTIR spectra shows that the linear CO extinction coefficient is approximately half that for the bridging CO. These NMR results correlate well with previous work on the dispersion dependence of the magnetic susceptibility of silica-supported Pd particles and with the IR spectra for CO adsorbed on such catalysts.