Site-specific chemical states of adsorbed CO on Pt(997): A high resolution XPS study

We investigated the adsorbed states of CO on Pt(997) using high-resolution X-ray photoelectron spectroscopy (HR-XPS) with synchrotron radiation. Four kinds of adsorbed surfaces were prepared: (1) the saturation at 200 K, (2) the saturation at 310 K, (3) “on-top at terrace” and “on-top at step” CO species, and (4) only step-CO species. HR-XPS measurements of these surfaces were carried out at 90 K or 310 K. In the case of the saturation coverage at 310 K, three sharp peaks were clearly observed at binding energies of 285.91, 286.24, and 286.64 eV in the C 1s region. These peaks were assigned to the “bridge at terrace”, “on-top at step” CO and “on-top at terrace”, respectively. In addition, the Pt 4f7/2 spectra were measured under the same conditions. In the case of the Pt(997) clean surface, three components, i.e., step, terrace and bulk Pt atoms, were identified at 70.6, 70.8, and 71.1 eV in the Pt 4f7/2 spectrum, respectively. For the CO saturated Pt(997) surface at 200 K, three shifted surface components were newly observed at 71.5, 72.1 and 71.8 eV. They are related to the “bridge at terrace,” “on-top at terrace,” and “on-top at step” CO species, respectively. We discuss the charge transfer between CO and surface Pt atoms based on the chemical shifts of both C 1s of CO and Pt 4f7/2. The charge transfer occurs not only from the step Pt atom but also from surrounding terrace Pt atoms in the case of the “on-top at step” CO species.