In Situ Hard X-ray Photoelectron Study of O2 and H2O Adsorption on Pt Nanoparticles

To improve the efficiency of Pt-based cathode catalysts in polymer electrolyte fuel cells, understanding of the oxygen reduction process at surfaces and interfaces in the molecular level is essential. In this study, H2O and O2 adsorption and dissociation as the first step of the reduction process were investigated by in situ hard X-ray photoelectron spectroscopy (HAXPES). Pt 5d valence band and Pt 3d, Pt 4f core HAXPES spectra of Pt nanoparticles upon H2O and O2 adsorption revealed that H2O adsorption has a negligible effect on the electronic structure of Pt, while O2 adsorption has a significant effect, reflecting the weak and strong chemisorption of H2O and O2 on the Pt nanoparticle, respectively. Combined with ab initio theoretical calculations, it is concluded that Pt 5d states responsible for Pt–O2 bonding reside within 2 eV from the Fermi level.