Indications fromPt195NMR for a temperature-dependent metal-nonmetal transition of small platinum particles in zeolites

Abstract

The ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ nuclear-spin-lattice relaxation rate ${\mathit{T}}_1^{\mathrm{-}1}$ has been studied as a function of temperature in several Pt/zeolite catalysts with different metal particle-size distributions and chemisorption conditions. Both NaX and NaY zeolites have been used as carriers. At temperatures above 80 K all clean-surface samples show the Korringa relaxation mechanism with ${\mathit{T}}_1$T=const, characteristic of nuclei in the metallic state. At low temperatures, important deviations from this behavior are found: at 22 K only the nuclei in particles larger than 1.6–1.9 nm still have ${\mathit{T}}_1$T=const. Chemisorption of hydrogen lowers this size limit considerably; the effect of oxygen is less marked. Similar phenomena have not been found for oxide-supported catalysts: we propose a qualitative explanation in terms of broadening of the one-electron energy levels by breathing-mode vibrations of the metal particles.