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The tilting of CO molecules chemisorbed on a Pt(110) surface: A molecular-orbital study
A252 Surface Science 185 (1987) 355 372 North-Holland, Amsterdam THE TILTING
355
OF CO MOLECULES
CHEMISORBED
ON A Pt(ll0) S U R F A C E : A M O L E C U L A R - O R B I T A L
STUDY
M . I . B A N *, M . A . V A N H O V E a n d G . A . S O M O R J A I
Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, CA 94720. USA Received 4 August 1986; accepted for publication 13 February 1987 The tilting of carbon monoxide molecules adsorbed on a Pt(110) surface has been modeled by using the extended Hiickel molecular-orbital (EHMO) method. Electronic properties, including total energies, were obtained for various adsorbate-substrate cluster systems in order to explore the qualitative features of this system. The tilting away from the surface normal is due to excessive close-packing of CO molecules along the atomic ridges of the Pt(ll0) surface. It is found energetically favorable for the tilting to alternate on either side of each ridge in a zig-zag fashion, as indicated by glide plane symmetry in observed LEED patterns. A polar tilt angle of - 1 6 ° is found, in reasonable agreement with experimental values. This yields CO-CO distances which are similar to the smallest values found in organometallic clusters and small compared with values in CO and CO 2 crystals. An azimuthal twist in the direction of the ridges is found to be slightly unfavorable energetically with EHMO, although it is thought to have been observed in some experiments and not in others. The optimal polar tilt angle slightly changes with the size of the metal cluster. To obtain a tilted equilibrium direction one needs at least three CO molecules on neighbouring Pt atoms, and six Pt atoms in two layers (directly beneath each other).
Surface Science 185 (1987) 373-393 North-Holland, Amsterdam THE METAL-METAL
OXIDE INTERFACE: A STUDY OF
THERMALLY-ACTIVATED USING
ELECTRON
J.G. CHEN
373
DIFFUSION
A T T H E N i / Ai 203 I N T E R F A C E
SPECTROSCOPIES
*, J.E. C R O W E L L
* * a n d J.T. Y A T E S , Jr.
Surface Science Center. Department of Chemisto,. University of Pittsburgh, Pittsburgh. PA 15260, USA Received 29 December 1986: accepted for publication 11 February 1987 The behavior of Ni atoms deposited on well-characterized AI203 films, prepared by oxidizing an atomically clean AI(Ill) surface, has been studied using high-resolution electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). At 200 K, the deposited Ni forms three-dimensional Ni clusters on the AI203/Al(lll ) substrate. Interaction of these Ni metal clusters with surface AI-O species is directly detected by observation of the preferential modification of surface phonon mode frequencies and intensities in contrast to the behavior of a bulk phonon mode of the A1203 substrate. Heating the Ni/AI203 deposit above 200 K gives rise to two thermally induced processes: Process I, smoothing (below - 400 K) of the three-dimensional Ni clusters and Process II, diffusion (400-700 K) of the Ni overlayer through channels in the AI20 ~ film. The metallic aluminum beneath the AI203 film is found to provide a major driving force for the inward diffusion of the Ni overlayer by Ni-AI alloy formation. This work represents the first application of EELS to the study of transport processes in a thin film system.
355
OF CO MOLECULES
CHEMISORBED
ON A Pt(ll0) S U R F A C E : A M O L E C U L A R - O R B I T A L
STUDY
M . I . B A N *, M . A . V A N H O V E a n d G . A . S O M O R J A I
Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, CA 94720. USA Received 4 August 1986; accepted for publication 13 February 1987 The tilting of carbon monoxide molecules adsorbed on a Pt(110) surface has been modeled by using the extended Hiickel molecular-orbital (EHMO) method. Electronic properties, including total energies, were obtained for various adsorbate-substrate cluster systems in order to explore the qualitative features of this system. The tilting away from the surface normal is due to excessive close-packing of CO molecules along the atomic ridges of the Pt(ll0) surface. It is found energetically favorable for the tilting to alternate on either side of each ridge in a zig-zag fashion, as indicated by glide plane symmetry in observed LEED patterns. A polar tilt angle of - 1 6 ° is found, in reasonable agreement with experimental values. This yields CO-CO distances which are similar to the smallest values found in organometallic clusters and small compared with values in CO and CO 2 crystals. An azimuthal twist in the direction of the ridges is found to be slightly unfavorable energetically with EHMO, although it is thought to have been observed in some experiments and not in others. The optimal polar tilt angle slightly changes with the size of the metal cluster. To obtain a tilted equilibrium direction one needs at least three CO molecules on neighbouring Pt atoms, and six Pt atoms in two layers (directly beneath each other).
Surface Science 185 (1987) 373-393 North-Holland, Amsterdam THE METAL-METAL
OXIDE INTERFACE: A STUDY OF
THERMALLY-ACTIVATED USING
ELECTRON
J.G. CHEN
373
DIFFUSION
A T T H E N i / Ai 203 I N T E R F A C E
SPECTROSCOPIES
*, J.E. C R O W E L L
* * a n d J.T. Y A T E S , Jr.
Surface Science Center. Department of Chemisto,. University of Pittsburgh, Pittsburgh. PA 15260, USA Received 29 December 1986: accepted for publication 11 February 1987 The behavior of Ni atoms deposited on well-characterized AI203 films, prepared by oxidizing an atomically clean AI(Ill) surface, has been studied using high-resolution electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). At 200 K, the deposited Ni forms three-dimensional Ni clusters on the AI203/Al(lll ) substrate. Interaction of these Ni metal clusters with surface AI-O species is directly detected by observation of the preferential modification of surface phonon mode frequencies and intensities in contrast to the behavior of a bulk phonon mode of the A1203 substrate. Heating the Ni/AI203 deposit above 200 K gives rise to two thermally induced processes: Process I, smoothing (below - 400 K) of the three-dimensional Ni clusters and Process II, diffusion (400-700 K) of the Ni overlayer through channels in the AI20 ~ film. The metallic aluminum beneath the AI203 film is found to provide a major driving force for the inward diffusion of the Ni overlayer by Ni-AI alloy formation. This work represents the first application of EELS to the study of transport processes in a thin film system.