Local metal to non-metal transition on oxygen-covered platinum particles from195Pt nuclear magnetic resonance

Abstract

The nuclear magnetic resonance (NMR) parameters (shift, relaxation time) in a metal are determined by the Fermi-level local density of states (LDOS), and the nuclear spin-lattice relaxation rate is proportional to the temperature (Korringa relation). In the ¹⁹⁵Pt 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 case). Using the Korringa relation as the criterion, we find that at moderate oxygen coverage (O/surface Pt ratios of 0.16 and 0.40) part of the surface atoms are non-metallic (have zero LDOS), while at zero or at saturation coverage (O/surface Pt=0.75) all atoms show metallic behaviour. In addition, some possible relations between NMR-derived variations of the LDOS and chemisorption properties are indicated.