XPS evidence for the incongruent surface oxidation of minerals in the Pd-Te-Bi system

Selected members of the Pd-Te-Bi system (PdTe, PdTeBi and PdBi) have been synthesized from their constituent elements, their structure and phase integrity confirmed by x-ray diffraction and optical microscopy and their initial oxidation in air under ambient conditions investigated by means of XPS. Prior to air exposure, the Pd core-electron binding energies were 0.6-1.0 eV higher than the corresponding values for the pure metal, while the core-level shifts for the alloying elements were very small, In each case, oxidation was quite rapid, but the palladium did not appear to be directly involved in the initial reaction; no change in the Pd 3d spectrum other than in its intensity was observed, notwithstanding the formation of significant amounts of tellurium oxide and/or bismuth oxide, The electron spectra were consistent with an incongruent oxidation reaction leading to the formation of a layer of tellurium and/or bismuth oxide covering a palladium-enriched substrate.