Structure, Bonding and Chemistry of Water and Hydroxyl on Transition Metal Surfaces

University dissertation from Stockholm : Fysikum

Abstract: The structure, bonding and chemistry of water and hydroxyl on metal surfaces are presented. Synchrotron based x-ray photoelectron- and x-ray absorption spectroscopy along with density functional theory calculations mainly form the basis of the results. Conditions span the temperature range 35 - 520 K and pressures from ultra-high vacuum (~10 fAtm) to near ambient pressures (~1 mAtm). The results provide, e.g, new insights on the importance of hydrogen bonding for surface chemical kinetics.Water adsorbs intact on the Pt(111), Ru(001) and Cu(110) surfaces at low temperatures forming 2-dimensional wetting layers where bonding to the metal (M) mainly occurs via H2O-M and M-HOH bonds. Observed isotope differences in structure and kinetics for H2O and D2O adsorption on Ru(001) are due to qualitatively different surface chemistries. D2O desorbs intact but H2O dissociates in kinetic competition with desorption similar to the D2O/Cu(110) system. The intact water layers are very sensitive to x-ray and electron induced damage.The mixed H2O:OH phase on Ru(001) consists of stripe-like structures 4 to 6 Ru lattice parameters wide where OH decorates the edges of the stripes. On Pt(111), two different long-range ordered mixed H2O:OH structures are found to be inter-related by geometric distortions originating from the asymmetric H-bond donor-acceptor properties of OH towards H2O.Water adsorption on Cu(110) was studied at near ambient conditions and compared to Cu(111). Whereas Cu(111) remains clean, Cu(110) holds significant amounts of water in a mixed H2O:OH layer. The difference is explained by the differing activation barriers for water dissociation, leading to the presence of OH groups on Cu(110) which lowers the desorption kinetics of water by orders of magnitude due to the formation of strong H2O-OH bonds. By lowering the activation barrier for water dissociation on Cu(111) by pre-adsorbing atomic O, generating adsorbed OH, similar results to those on Cu(110) are obtained.

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