CO adsorption on Pd(111): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various CO-Pd(1 1 1) bonding configurations have been identified from the decomposition analysis of the adsorbate C 1s and the substrate Pd 3d(5/2) core-level photoemission spectra obtained after CO adsorption at 120 K. The c(4 x 2) structure at 0.5 monolayer (ML) and the (2 x 2)-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fee and hcp threefold hollow sites in the c(4 x 2) structure and of hollow and on-top sites in the (2 x 2) structure, as reported in the literature. For CO adsorption at 300 K, a different site occupation is evidenced by the presence of two components in the C 1s and Pd 3d(5/2) core-level and C-O stretching vibration lineshapes. At coverages up to 0.1 ML only fee threefold hollow sites in a (root3 x root3)R30 degrees structure are occupied. The population of these sites continues with increasing the CO coverage up to 0.32 ML, but in addition adsorption of CO in bridge sites takes place. The latter sites become predominant at the 300 K saturation coverage of 0.5 ML, and are considered to form domain boundaries in a poorly ordered (root3 x root3)R30 degrees phase with split low energy electron diffraction reflexes. Between 0.32 and 0.5 ML the domain boundary regions grow at the expense of the (root3 x root3)R30 degrees domains. (C) 2000 Elsevier Science B.V. All rights reserved.