Synchrotron radiation photoemission studies of CO chemisorption on Pt/Ta(110) and Ni/Ta(110)

Abstract

The effect of overlayer structure and electronic density near the Fermi level on CO chemisorption is studied for Pt or Ni deposited on Ta(110) using angle-resolved synchrotron radiation photoemission spectroscopy. Pt films form two-dimensional pseudomorphic islands at submonolayer coverage, but undergo a structural change to a close-packed Pt(111)-like structure at ∼0.7 monolayer coverage. Absence of CO induced photoemission features at monolayer coverage suggests that the CO–Pt bond has been weakened and this prevents the retention of CO on a close-packed Pt monolayer at room temperature. Close-packed Pt films three or more layers thick readily chemisorb CO. At liquid-nitrogen temperature, CO is retained on Pt/Ta(110), and photoemission features, such as the CO 4σ shake-up satellite seen for weakly bonded CO on noble metals, appear for the CO phase chemisorbed on a Pt monolayer. This photoemission feature is absent for CO chemisorbed on thicker Pt films. Photoemission examination of the CO–metal bonding geometry suggests that the CO molecules lie close to the surface normal for both Pt monolayers and the thicker Pt films. From the photoemission data, we conclude that the CO–metal bond strength is reduced for CO–Pt/Ta(110). Comparable studies of CO on Ni/Ta(110) were not possible because the three-dimensional clustering of Ni produced a more complicated surface which had a number of inequivalent CO bonding sites.