Quantum size effect in sodium: NMR and ESR in small Na particles in zeolite y

Abstract

The Na exchanged zeolite Y has been reacted with sodium or cesium metal vapor. For low concentration of the metal vapor, hyperfine ESR signal was observed, characteristic of ionic cluster species Na 43+ in the sodalite cages of the zeolite. Exposure to higher concentration leads to a single ESR narrow line which is attributed to Na small metallic particles inside the zeolite cavities. The metallic signal linewidth shows no temperature dependence in the range 4 to 300 K, which indicates a quenching of the bulk metal relaxation mechanisms. The signal intensity obeys Curie law in the same temperature range, in agreement with theoretical models for small metal particles. 23 Na NMR spectrum of (Na-Y) highly loaded with Na shows a very broad unshifted line with respect to sodium diamagnetic compound. This line is attributed to Na even metallic particles with a large size distribution. In the case of (Na-Y) highly loaded with Cs, the room temperature 23Na NMR spectrum shows two highly shifted narrow lines and a broad unshifted line. The latter should have the same origin as in the concentrated (Na-Y) with Na. The shifted lines are assigned to the Knight shifts in (Na/Cs) alloy particles which are formed by an exchange between the cesium and the Na cations of the zeolite framework. Such observation is being reported for the first time in the alkali-metals loaded zeolites.