Low temperature coadsorption of hydrogen and carbon monoxide on Ni(100) I. TPD, Δφ, and UPS studies

Abstract

Ultraviolet photoemission (UPS), work function change (Δ) and temperature programmed desorption (TPD) data are reported for carbon monoxide and hydrogen coadsorbed on Ni(100). Predosed hydrogen and postdosed carbon monoxide are intimately mixed and strongly coupled on Ni(100) at 100 K, but there is no evidence for direct C-H or O-H bonding. TPD shows sharp desorption peaks of both CO and H 2 around 210 K ( Σ states). UPS shows that a single type of weakly adsorbed CO, Σ -CO, exists on an H-presaturated Ni(100) surface at 100 K. This species is irreversibly converted to β -CO after heating to 260 K to desorb some CO and H 2 . The chemical bonding of the adsorbed CO changes significantly with the conversion from Σ to β -CO. Work function measurements show that the surface dipole associated with Σ -CO is nearly zero. The results suggest that Σ -CO has an electronic structure readily derivable from chemisorbed CO perturbed by a through-Ni interaction with hydrogen bonded to the same Ni atom or atoms.