Determination of O and OH adsorption sites and coverage in situ on Pt electrodes from Pt L(2,3) X-ray absorption spectroscopy.

The adsorption of atomic oxygen and hydroxide on a platinum electrode in 0.1 M HClO4 or H2SO4 electrolytes was studied in situ with Pt L(2,3) X-ray absorption spectroscopy (EXAFS and XANES). The Pt L3 edge absorption data, mu, were collected at room temperature in transmission mode on beamline X-11A at the National Synchrotron Light Source using a custom built cell. The Pt electrode was formed of highly dispersed 1.5-3 nm particles supported on carbon. A novel difference procedure (delta mu = mu(O[H]/Pt) - mu(Pt)) utilizing the L3 XANES spectra at different applied voltages was used to isolate the effects of O[H] (O or OH) adsorption in the XANES spectra. The Deltamu results are compared with results obtained from real-space full-multiple scattering calculations utilizing the FEFF8 code on model clusters. The experimental results, when compared with theoretical calculations, allow the adsorption site to be identified. At low coverages OH adsorbs primarily in 1-fold coordinated atop sites. As the coverage increases, O binds in the bridge/fcc sites, and at still higher coverages above 1.05 V RHE, O adsorbs into a higher coordinated n-fold or subsurface site, which is thought to be the result of Pt-O site exchange and oxide formation. These results are similar to those found in the gas phase. Direct specific adsorption of bisulfate anions in H2SO4 is spectroscopically seen in both the EXAFS and XANES data and is seen to impede oxygen adsorption consistent with previous reports.