An XPS study of metal-support interactions on [formula omitted] and [formula omitted

Abstract

The electronic properties of palladium supported on SiO 2 and La 2O 3 were investigated by X-ray photoelectron spectroscopy. Spectra were collected after each stage of catalyst preparation: deposition of the palladium chloride precursor, oxidation in air at 623 K, and reduction in H 2 at 573 K. The Pd 3d 5 2 binding energies recorded following precursor deposition and oxidation were the same on both catalysts. However, after reduction the Pd 3d 5 2 binding energies of the Pd La 2O 3 samples shifted below the corresponding values for metallic Pd, while the Pd 3d 5 2 binding energies of the Pd SiO 2 samples did not. Furthermore, the binding energies for the Pd La 2O 3 samples decreased with increasing metal loading, the largest Pd particles apparently exhibiting the greatest interaction. A maximum shift of −0.7 eV relative to the Pd foil value was observed for 8.8% Pd La 2O 3 . This binding energy shift is interpreted as arising from a change in the chemical state of Pd, i.e., that Pd supported on La 2O 3 is more electronegative than zero-valent Pd alone. A model is proposed which suggests that a thin covering of the La 2O 3 support lies on a portion of the Pd surface. This covering is partially reduced during H 2 treatment at 573 K and, because of the electropositive nature of La, the additional charge is distributed amongst the surface Pd atoms. It should be noted that the La 2O 3 support surface after H 2 reduction is a complex mixture of La(OH) 3, LaO(OH), La 2O 3, and La 2(CO 3) 3. The exact composition of the support depended strongly on the reduction temperature and the amount of Pd deposited.