LOCAL METAL TO NONMETAL TRANSITION ON OXYGEN-COVERED PLATINUM PARTICLES FROM PT-195 NUCLEAR-MAGNETIC-RESONANCE

The nuclear magnetic resonance (NMR) parameters (shift, relaxation tim e) in a metal are determined by the Fermi-level local density of state s (LDOS), and the nuclear spin-lattice relaxation rate is proportional to the temperature (Korringa relation). In the Pt-195 NMR spectrum of platinum catalysts, signals from surface atoms and from atoms deeper in the particles can be distinguished; furthermore the NMR parameters are sensitive to the chemisorption of gases (oxygen in the present cas e). Using the Korringa relation as the criterion, we find that at mode rate oxygen coverage (O/surface Pt ratios of 0.16 and 0.40) part of th e surface atoms are non-metallic (have zero LDOS), while at zero or at saturation coverage (O/surface Pt = 0.75) all atoms show metallic beh aviour. In addition, some possible relations between NMR-derived varia tions of the LDOS and chemisorption properties are indicated.