Coadsorption of potassium and CO on Pt(111)

Abstract

The adsorption of CO on a K-covered Pt(111) surface was studied at 160 and 300 K as a function of K coverage and compared to CO adsorption on clean Pt(111). Different adsorbed states of CO could be distinguished by O(1s) and C(1s) binding energies. CO on clean Pt(111) was characterized by O(1s) binding energies at 532.7 eV (on-top site) and 531.0 eV (bridge site) whereas K-promoted CO was measured at 531.3 eV binding energy. The peak height of this K-influenced CO increases linearly with K coverage. The peak at 532.7 eV decreased in intensity with increasing K coverage. The corresponding C(1s) spectra showed a broad maximum at 286.9 eV for CO on Pt(111) and a single maximum at 285.8 eV for CO on Pt + K. UPS spectra of CO adsorbed on Pt + K showed changes in the 4σ and 5σ + 1π molecular orbital peaks which are consistent with an increased 4σ−1π energy separation, indicative of an increased C-O bond length. The work function increased for CO on Pt + K while the magnitude of this increase was proportional to K coverage for θ K < 0.17. A dipole moment of CO for the Pt + K surface of 1.8 D was evaluated from the work function change with CO coverage. Core level binding energy shifts, UPS data and work function changes (dipole moment) for CO on Pt + K indicate a large negative charge on the adsorbed CO which is interpreted in terms of enhanced back donation of electrons into the 2π ∗ anti-bonding orbital of CO. This K-influenced CO species is likely to be adsorbed in an on-top site on the Pt(111) surface. Its adsorption energy is increased relative to that for CO on Pt(111). No dissociation of CO on K covered Pt is observed.