C-13 NMR-STUDIES OF CO ADSORBED ON SUPPORTED PLATINUM AND PALLADIUM CATALYSTS USING MAGIC ANGLE SAMPLE SPINNING

The 13C solid-state NMR of 13CO chemisorbed on supported Pt and Pd catalysts has been investigated by static and magic angle spinning (MAS) NMR methods. The observation of a significant Knight shift of the 13C resonance by other workers is confirmed for both Pt and Pd catalysts. The sources of line broadening for the static pattern have been reinvestigated and the ultimate resolution by MAS methods has been tested. The bulk of the 13CO line width under MAS is shown to be the consequence of a site-to-site distribution in shift-like interactions from MAS and TOSS experiments. On Pd the CO is motionally narrowed by rapid diffusion on the surface at room temperature, giving a relatively narrow line unaffected by MAS. Several possibilities are discussed for the source of the shift distribution, including the effects of stable paramagnetic sites or a distriution in isotropic Knight shifts. These measurements and previous bulk susceptibility measurements are analyzed with respect to the resolution in other catalyst systems. Experimental support for a two-stage diffusion model in which the rate-limiting step is edge-to-face conversion is also discussed. © 1990 American Chemical Society.