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New interpretation of the XPS core hole spectra of CO on a Ni(100) surface
A594
and John L. Gland Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA Received 5 February 1991; accepted for publication 2 May 1991 Chemisorbed CO can be completely removed from the P t ( l l l ) surface in the temperature range 318 to 348 K for hydrogen pressures above 2 × 10- 2 Torr. Thermal desorption of CO in this temperature range in the absence of hydrogen removes only a fraction of the adsorbed CO. A series of in situ isothermal kinetic experiments are presented in this paper which show that CO displacement in the presence of 0.2 Torr of hydrogen is a first-order process in CO coverage with an activation energy of 10.q kcal/mol. We propose that the origin of this effect is that repulsive intereactions between coadsorbed atomic hydrogen and carbon monoxide induce high desorption rates of CO characteristic of high CO coverages, presumably due to lower values of the desorption activation energy. The importance of these results is to show that high coverages of coadsorbed hydrogen resulting from substantial overpressures of H 2 may substantially modify desorption activation energies, and thus the coverages and kinetic pathways available, even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which oCCUr at tiigh pressure.
Surface Science 258 (19911 82-90 North-Holland
Initial growth and structure of Pt on Ni(100) and oxidation of this layer structure S. Deckers 1, F. Bisschop, D. de Jager, S.H. Offerhaus, J. van Roijen, F.H.P.M. Habraken * and W.F. van der Weg Atomic and Interface Physics Department, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, Netherlands Received 10 April 1991; accepted for publication 14 June 1991 High-energy ion scattering in combination with shadowing and blocking, ellipsometry, Auger electron spectroscopy and low-energy electron diffraction have been used to study the growth and annealing behaviour of thin Pt layers on Ni(100) as well as the room-temperature oxidation of the resulting layer structures. The absolute Pt coverages were measured by means of Rutherford backscattering spectrometry. From Auger uptake curves, values for the electron attenuation lengths at different energies have been deduced. Room-temperature deposition of Pt on the Ni(100) surface proceeds through a disordered layer-by-layer growth. Above 250 °C crystallisation effects occur and above 350 ° C a surface alloy starts to develop. The room temperature oxidation of Ni(100) is strongly suppressed by Pt. For Pt coverages larger than 0.86 monolayer, no measurable oxidation occurs with the oxygen exposures (up to 80000 langmuir) applied. After surface alloy formation, oxidation is possible again to a limited extent. The results are compared with similar deposition and oxidation studies on Ni(l 11) and Fe(100).
Surface Science 258 (1991) 91-100 North-Holland
New interpretation of the XPS core hole spectra of CO on a Ni(100) surface M. Ohno Institut fiir Physikalisehe und Theoretische Chemie, Technische Universitat Braunschweig, D-3300 Braunschweig, Germany
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and John L. Gland Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA Received 5 February 1991; accepted for publication 2 May 1991 Chemisorbed CO can be completely removed from the P t ( l l l ) surface in the temperature range 318 to 348 K for hydrogen pressures above 2 × 10- 2 Torr. Thermal desorption of CO in this temperature range in the absence of hydrogen removes only a fraction of the adsorbed CO. A series of in situ isothermal kinetic experiments are presented in this paper which show that CO displacement in the presence of 0.2 Torr of hydrogen is a first-order process in CO coverage with an activation energy of 10.q kcal/mol. We propose that the origin of this effect is that repulsive intereactions between coadsorbed atomic hydrogen and carbon monoxide induce high desorption rates of CO characteristic of high CO coverages, presumably due to lower values of the desorption activation energy. The importance of these results is to show that high coverages of coadsorbed hydrogen resulting from substantial overpressures of H 2 may substantially modify desorption activation energies, and thus the coverages and kinetic pathways available, even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which oCCUr at tiigh pressure.
Surface Science 258 (19911 82-90 North-Holland
Initial growth and structure of Pt on Ni(100) and oxidation of this layer structure S. Deckers 1, F. Bisschop, D. de Jager, S.H. Offerhaus, J. van Roijen, F.H.P.M. Habraken * and W.F. van der Weg Atomic and Interface Physics Department, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, Netherlands Received 10 April 1991; accepted for publication 14 June 1991 High-energy ion scattering in combination with shadowing and blocking, ellipsometry, Auger electron spectroscopy and low-energy electron diffraction have been used to study the growth and annealing behaviour of thin Pt layers on Ni(100) as well as the room-temperature oxidation of the resulting layer structures. The absolute Pt coverages were measured by means of Rutherford backscattering spectrometry. From Auger uptake curves, values for the electron attenuation lengths at different energies have been deduced. Room-temperature deposition of Pt on the Ni(100) surface proceeds through a disordered layer-by-layer growth. Above 250 °C crystallisation effects occur and above 350 ° C a surface alloy starts to develop. The room temperature oxidation of Ni(100) is strongly suppressed by Pt. For Pt coverages larger than 0.86 monolayer, no measurable oxidation occurs with the oxygen exposures (up to 80000 langmuir) applied. After surface alloy formation, oxidation is possible again to a limited extent. The results are compared with similar deposition and oxidation studies on Ni(l 11) and Fe(100).
Surface Science 258 (1991) 91-100 North-Holland
New interpretation of the XPS core hole spectra of CO on a Ni(100) surface M. Ohno Institut fiir Physikalisehe und Theoretische Chemie, Technische Universitat Braunschweig, D-3300 Braunschweig, Germany
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