Highly Oxidized Palladium Nanoparticles Comprising Pd4+ Species: Spectroscopic and Structural Aspects, Thermal Stability, and Reactivity

Oxidized palladium nanoparticles, PdOx (x approximate to 1.3), measuring approximately 3 nm in size were prepared by RF-discharge under an oxygen atmosphere. The Pd3d X-ray photoelectron spectra (XPS) of oxidized palladium nanoparticles show two main peaks with binding energy E-b(Pd3d(5/2)) at similar to 336.5 and 338.6 eV, which were assigned to Pd2+ and Pd4+ species, respectively. Attempts to synthesize pure Pd4+ nanoparticles by plasma treatment without the presence of Pd2+ were unsuccessful. High-resolution transmission electron microscopy (HRTEM) data show the defect structure of the palladium nanoparticles. The particles' thermal stability is relatively high, being stable up to similar to 425-450 K. The oxidized palladium species (Pd4+) were found to be highly reactive toward CO at room temperature. These results demonstrate the necessity of further investigation of highly oxidized palladium species as possible active centers in CO oxidation reactions.