Determination by X-ray photoelectron spectroscopy of the electronic state of Pd clusters in Y zeolite

Abstract

X-Ray photoelectron spectroscopy (XPS) data of Pd clusters in NaY, MgY and HY zeolites show that the chemical shift of the Pd 3d_5/2 line is dependent on the concentration of the protons and their position in the zeolite. The positive charge or ‘electron deficiency’ of the Pd particles is significant even for Pd/NaY which contains only the protons created by H₂ reduction of Pd²⁺ ions. In these samples the Pd 3d_5/2 binding energy exceeds that of neutral Pd by ca. 0.4 eV. The positive charge of the Pd clusters is much higher in HY and MgY, reflecting the higher proton concentration in the supercages; the binding energy is shifted by 0.8 eV. Additional relevant information on the metal cluster environment is obtained from analysis of the Auger parameter of Pd. It decreases with increasing number of protons interacting with the Pd particles, and it increases upon substituting protons by polarizable ions, e.g. Na⁺ or NH⁺₄. These data are consistent with the model which suggests that metal–proton adducts, [Pd_n–H_m]^m+ with m 1, are formed in proton-containing zeolites.