X-ray photoelectron spectroscopy study of Pd and Pt ions in type Y-zeolite. Electron transfer between metal aggregates and the support as evidenced by X-ray photoelectron spectroscopy and electron spin resonance

Abstract

Spectroscopic study by ESCA and e.s.r. techniques was performed for Pt and Pd ions in type Y-zeolites. Pt and Pd ions were found to be ionically bonded to lattice oxygen ions. Atomically dispersed Pt^O and Pd^O were shown to give significant X.P.S. positive chemical shifts: +1.3 and +1.4 eV for Pt 4f and Pd 3d lines, respectively. These shifts were assigned to smaller electron relaxation energy.

Metal aggregates of small diameter (ϕ⩽ 20 Å) and narrow size distribution were prepared in zeolite or on silica. The X.P.S. chemical shift was quite significant (∼+0.7 eV) for Pt on zeolite support with respect to those (< +0.3 eV) for Pd on zeolite or Pt or Pd on silica support and did not depend on the particle size. Electron donor and electron acceptor properties of the materials were studied by following the e.s.r. formation of charge transfer complexes (radical ion) as a function of Pt or Pd loading. Pt aggregates were shown to form charge transfer complexes with Lewis acid sites and to increase electron donor properties of the zeolite no matter what the particle size. This increase was shown to arise from electron donation from the metal to the zeolite lattice. However, Pd aggregates were shown by e.s.r. not to give rise to significant electron transfer.