Correlation between the Stretching Frequency of Carbon Monoxide Adsorbed and the Fermi Level Local Density of States at Surfaces of Platinum Catalysts

Abstract

We have studied the stretching frequency (extrapolated to zero coverage) of adsorbed CO and the 195Pt NMR (before adsorption of CO) of Pt catalysts on zeolite-NaY (three dispersions), on zeolite-NaHY (two dispersions), and on zeolite-NaX (one dispersion); we have also measured the hydrogen and oxygen chemisorption capacity of these samples. For comparison, we studied three samples on an oxidic carrier (unpromoted and promoted by Li or K salts, all from the same initial batch) by infrared and by NMR. In the zeolite case, we find a direct correlation between the CO stretching frequency and the density of states at the Fermi energy on surface sites (surface Ef-LDOS) derived from the clean-surface NMR data, but the chemisorption results seem to be determined by steric as well as electronic effects, and show no simple relation to the two other quantities. The values for the oxidic samples appear to form a distinct set, with probably some promotor-specific effects. The zeolite data support an interpretation of ...