First, the electronic structure change under dry and humidified oxygen conditions at room temperature was measured by in situ HAXPES equipped with an ambient cell. The valence band (mainly Pt 5d) and Pt 4f spectra were successfully obtained under up to 1 mbar for the first time. Both valence band and Pt 4f spectra show that O2/H2O co-adsorption hindered oxygen adsorption. Based on our first principles calculation of valence band density-of-states (DOS) we have found that DOS just below Fermi level strongly decreases upon oxygen adsorption, while valence band spectra remain almost unchanged upon H2O adsorption. These results suggest that H2O molecules may occupy the oxygen adsorption sites on Pt surface more easily than oxygen, resulting in hindering the successive oxygen adsorption.
However, under the more realistic condition at atmospheric pressure (1 bar) quite different results that the formation of higher oxidation states of Pt in Pt Pt L3-edge absorption spectra was enhanced by water adsorption were obtained by high resolution (FWHM of about 2.5 eV) in situ XAFS, taking advantage of life-time free measurements1). It is difficult to observe these changes by conventional XAFS spectra with poor energy resolution (FWHM of about 5.2 eV). At the atmospheric pressure more frequent attack by oxygen molecules onto water-adsorbed Pt surface may replace water molecules by oxygen, resulting in the formation of hydrated hydroxyl intermediates and higher oxidation states which hinder oxygen reduction reaction. The molecular dynamics simulation2) also confirms the oxygen replacement on water-adsorbed Pt surfaces. This enhanced oxygen adsorption is more clearly observed for Pt nano-particles, probably because Pt nano-particles with stronger Pt-O bonding than Pt-Co nano-particles may further stabilize Pt-O bonding by additional adsorbed water. On the other hand, oxygen molecules on Pt-Co nano-particles may be difficult to form hydrated hydroxyl intermediates with adsorbed water, leading to less water effect on oxygen adsorption on Pt-Co.
These results would be much helpful to understand why Pt-Co nano particles show higher ORR activity than Pt nano particles.
References:
1) K. Hamalainen et al., Phys. Rev. Lett. 67, 2850 (1991).
2) R. Li, et al., New J. Chem., 38, 683 (2014).