Reactivity of atomically dispersed Pt2+ species towards H-2: model Pt-CeO2 fuel cell catalyst

The reactivity of atomically dispersed Pt2+ species on the surface of nanostructured CeO2 films and the mechanism of H-2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt2+ sites are found to be inactive towards H-2 dissociation due to high activation energy required for H-H bond scission. Trace amounts of metallic Pt are necessary to initiate H-2 dissociation on Pt-CeO2 films. H-2 dissociation triggers the reduction of Ce4+ cations which, in turn, is coupled with the reduction of Pt2+ species. The mechanism of Pt2+ reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt-CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt2+ reduction.