CHEMISORPTION OF CARBON-MONOXIDE ON 3 METAL-SURFACES - NI(111), PD(111), AND PT(111) - A COMPARATIVE-STUDY

At one-third coverage of carbon monoxide, a (unroofed-radical-sign 3 X unroofed-radical-sign 3)R30-degrees low-energy electron diffraction (LEED) pattern has been observed on Ni(111), Pd(111), and Pt(111). Interestingly, the C-O stretching frequency indicates three completely different adsorption sites: a 2-fold bridge site for Ni(111), a 3-fold site for Pd(111), and an on-top site for Pt(111). As the coverage increases to one-half, a c(4 x 2) LEED pattern appears. For Ni(111) and Pd(111), the adsorbate seems to prefer the 2-fold site. On Pt(111), half of the chemisorbed CO sits on the on-top site while the other half is located at the 2-fold site. Further increase in coverage results in tilted CO molecules on Pt(111) and adsorption of CO at the on-top site of Ni(111) and Pd(111). To rationalize these observations, we carried out extended Huckel calculations within the tight-binding formalism. Our discussion is augmented by conceptual tools such as density of states and crystal orbital overlap population. We also examine the question of the adequacy of the Blyholder model in describing the adsorbate-surface interaction.