EPR investigation of paramagnetic species on palladium-promoted yttria and lanthana

Abstract

Electron paramagnetic resonance studies have been carried out on Pd/Y₂O₃ and Pd/La₂O₃ systems with low Pd loading. The samples were prepared by coprecipitation and by decomposition of amorphous citrate precursors. Oxidation of yttria-containing samples at 870 K leads to the formation of Pd³⁺, which converts into Pd⁺ on evacuation at 870 K, both species being stabilized in the bulk. Mild reduction of doped yttria and lanthana in hydrogen gives another type of Pd⁺ ions localized within the surface and subsurface layers. Formation of O^–₂ radical anions coordinated with Y³⁺ or La³⁺ cations occurs readily at 77 K upon oxygen admission. The decay and formation of adsorbed radical anions is sensitive to the electron-donating properties of the sample (i.e. the surface concentration of Pd⁺).