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