CORRELATION BETWEEN THE STRETCHING FREQUENCY OF CARBON-MONOXIDE ADSORBED AND THE FERMI-LEVEL LOCAL-DENSITY OF STATES AT SURFACES OF PLATINUM CATALYSTS

We have studied the stretching frequency (extrapolated to zero coverag e) of adsorbed CO and the Pt-195 NMR (before adsorption of CO) of Pt c atalysts on zeolite-NaY (three dispersions), on zeolite-NaHY (two disp ersions), and on zeolite-NaX (one dispersion); we have also measured t he hydrogen and oxygen chemisorption capacity of these samples. For co mparison, we studied three samples on an oxidic carrier (unpromoted an d promoted by ii or K salts, all from the same initial batch) by infra red and by NMR. In the zeolite case, we find a direct correlation betw een the CO stretching frequency and the density of states at the Fermi energy on surface sites (surface E-f-LDOS) derived from the clean-sur face NMR data, but the chemisorption results seem to be determined by steric as well as electronic effects, and show no simple relation to t he two other quantities. The values for the oxidic samples appear to f orm a distinct set, with probably some promotor-specific effects. The zeolite data support an interpretation of the concept of ''electron de ficiency'' not in terms of missing electronic charge, but in terms of a low density of states at the Fermi energy on surface sites. This is consistent with ideas derived from the frontier-orbital picture for me tal-adsorbate interaction. It remains unclear why the surface E-f-LDOS show a systematic variation with the acidity of the faujasite framewo rk.