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Chemisorption of oxygen, chlorine, hydrogen, hydroxide, and ethylene on silver clusters: A model for the olefin epoxidation reaction
A41 Surface Science 209 (1989) 229-242 North-Holland, Amsterdam
229
SHIFT IN XPS LEVELS IN IONIC ADSORBATE TO ELECTROSTATIC EFFECTS
LAYERS DUE
P e t e r A. S C H U L T Z
Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, USA and R i c h a r d P. M E S S M E R
Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, USA and General Electric Corporate Research and Development, Schenectady, N Y 12301, USA Received 9 March 1988; accepted for publication 26 July 1988 Using a point charge model, the electrostatic contribution to the core level electron binding energies in ionic adsorbate layers is examined. The system considered as an illustrative example is K and CO coadsorbed on transition metal surfaces. In an ionic model for coadsorption, the shifts in the K, C, and O core levels with respect to the singly adsorbed systems have two dominant contributions: a chemical shift due to charge transfer from K to CO and an offsetting shift due to a Madelung term deriving from Coulombic interaction of the core hole with the ionic surface layer. The Madelung potential is evaluated explicitly while the change in screening due to the addition or removal of a valence electron upon the formation of a core hole is obtained via an "equivalent core" approximation. Taken together, the two terms cause a shift to lower binding energy of all adsorbate core levels, offering a potential resolution to an apparent paradox for this coadsorbed system. For adsorbate surface densities typically observed for K + CO, it is found that the Madelung shift would be large, of order 7-8 eV, and must be considered to understand the photoemission results.
Surface Science 209 (1989) 243-289 North-Holland, Amsterdam
243
CHEMISORPTION OF OXYGEN, CHLORINE, HYDROGEN, HYDROXIDE, AND ETHYLENE ON SILVER CLUSTERS: A MODEL
FOR TIlE OLEFIN
E m i l y A. C A R T E R
EPOXIDATION
REACTION
* a n d W i l l i a m A. G O D D A R D
*
III
Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA Received 10 June 1988; accepted for publication 30 August 1988 We have examined various postulated pathways for the catalytic epoxidation of olefins by silver using ab initio quantum mechanical methods to study likely intermediates in this reaction. In particular, we predict preferred binding sites, geometries, vibrational frequencies, and binding energies for O, 02, C1, H, OH, and C2H 4 on a cluster model for Ag aggregates present on supported catalysts. These calculations suggest the presence of two nearly degenerate states of chemisorbed atomic oxygen. The calculated binding energy for these surface oxides are 78-79
A42 kcal/mol (experimental values are 77-78 kcal/mol). One surface oxide has the form of an oxyradical anion and is predicted to be selective for olefin epoxidation. The other surface oxide is a closed shell state expected to be less active and non selective for olefin epoxidation. These results lead to a detailed mechanistic model that explains: (i) why C2H 4 exhibits high selectivity for epoxide while higher olefins do not; (ii) the difference in activity per surface site between the (110) and (111) surfaces of Ag; and (iii) the role of both electropositive (e.g. Cs) and electronegative (e.g. C1) promoters in increasing selectivity. A number of experiments are proposed which would test key points of this new mechanism.
Surface Science 209 (1989) 291-313 North-Holland, Amsterdam REACTION
OF ATOMIC
J. A B R E F A H
291
HYDROGEN
WITH CRYSTALLINE
SILICON
and D.R. OLANDER
Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory and Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA Received 16 August 1988; accepted for publication 4 October 1988 The kinetics of the gas-solid reaction of crystalline silicon, Si(111), and atomic hydrogen (thermally generated in a tungsten oven) have been studied by the modulated molecular beam mass spectrometric technique. Volatile silicon tetrahydride, Sill4, and recombined molecular hydrogen were the only reaction products detected from ambient temperature to 1000 K. At room temperature the majority of the incident H atoms recombined on the Si surface and was re-emitted as H2; part of it diffused into the Si lattice; and a small fraction ( - 3%) reacted to produce Sill 4. The etch rate increased with beam intensity but decreased with increasing temperature. The proposed mechanism involves saturation of the surface dangling bonds by H atoms, formation of Sill and Sill 2 surface complexes, and reaction of the latter with a weakly-bound mobile overlayer of H atoms. Rate constants characterizing the elementary steps of the mechanism were determined by fitting the model to the data.
314
Surface Science 209 (1989) 314-334 North-Holland, Amsterdam
ADSORPTION J. K N A L L ,
AND JI)ESO~ON
S.A. B A R N E T T * ,
KINETICS
OF In ON Si(100)
J.-E. S U N D G R E N
Department of Physics, LinkSping University, S-58183 Linki~ping, Sweden and
229
SHIFT IN XPS LEVELS IN IONIC ADSORBATE TO ELECTROSTATIC EFFECTS
LAYERS DUE
P e t e r A. S C H U L T Z
Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, USA and R i c h a r d P. M E S S M E R
Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104, USA and General Electric Corporate Research and Development, Schenectady, N Y 12301, USA Received 9 March 1988; accepted for publication 26 July 1988 Using a point charge model, the electrostatic contribution to the core level electron binding energies in ionic adsorbate layers is examined. The system considered as an illustrative example is K and CO coadsorbed on transition metal surfaces. In an ionic model for coadsorption, the shifts in the K, C, and O core levels with respect to the singly adsorbed systems have two dominant contributions: a chemical shift due to charge transfer from K to CO and an offsetting shift due to a Madelung term deriving from Coulombic interaction of the core hole with the ionic surface layer. The Madelung potential is evaluated explicitly while the change in screening due to the addition or removal of a valence electron upon the formation of a core hole is obtained via an "equivalent core" approximation. Taken together, the two terms cause a shift to lower binding energy of all adsorbate core levels, offering a potential resolution to an apparent paradox for this coadsorbed system. For adsorbate surface densities typically observed for K + CO, it is found that the Madelung shift would be large, of order 7-8 eV, and must be considered to understand the photoemission results.
Surface Science 209 (1989) 243-289 North-Holland, Amsterdam
243
CHEMISORPTION OF OXYGEN, CHLORINE, HYDROGEN, HYDROXIDE, AND ETHYLENE ON SILVER CLUSTERS: A MODEL
FOR TIlE OLEFIN
E m i l y A. C A R T E R
EPOXIDATION
REACTION
* a n d W i l l i a m A. G O D D A R D
*
III
Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA Received 10 June 1988; accepted for publication 30 August 1988 We have examined various postulated pathways for the catalytic epoxidation of olefins by silver using ab initio quantum mechanical methods to study likely intermediates in this reaction. In particular, we predict preferred binding sites, geometries, vibrational frequencies, and binding energies for O, 02, C1, H, OH, and C2H 4 on a cluster model for Ag aggregates present on supported catalysts. These calculations suggest the presence of two nearly degenerate states of chemisorbed atomic oxygen. The calculated binding energy for these surface oxides are 78-79
A42 kcal/mol (experimental values are 77-78 kcal/mol). One surface oxide has the form of an oxyradical anion and is predicted to be selective for olefin epoxidation. The other surface oxide is a closed shell state expected to be less active and non selective for olefin epoxidation. These results lead to a detailed mechanistic model that explains: (i) why C2H 4 exhibits high selectivity for epoxide while higher olefins do not; (ii) the difference in activity per surface site between the (110) and (111) surfaces of Ag; and (iii) the role of both electropositive (e.g. Cs) and electronegative (e.g. C1) promoters in increasing selectivity. A number of experiments are proposed which would test key points of this new mechanism.
Surface Science 209 (1989) 291-313 North-Holland, Amsterdam REACTION
OF ATOMIC
J. A B R E F A H
291
HYDROGEN
WITH CRYSTALLINE
SILICON
and D.R. OLANDER
Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory and Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA Received 16 August 1988; accepted for publication 4 October 1988 The kinetics of the gas-solid reaction of crystalline silicon, Si(111), and atomic hydrogen (thermally generated in a tungsten oven) have been studied by the modulated molecular beam mass spectrometric technique. Volatile silicon tetrahydride, Sill4, and recombined molecular hydrogen were the only reaction products detected from ambient temperature to 1000 K. At room temperature the majority of the incident H atoms recombined on the Si surface and was re-emitted as H2; part of it diffused into the Si lattice; and a small fraction ( - 3%) reacted to produce Sill 4. The etch rate increased with beam intensity but decreased with increasing temperature. The proposed mechanism involves saturation of the surface dangling bonds by H atoms, formation of Sill and Sill 2 surface complexes, and reaction of the latter with a weakly-bound mobile overlayer of H atoms. Rate constants characterizing the elementary steps of the mechanism were determined by fitting the model to the data.
314
Surface Science 209 (1989) 314-334 North-Holland, Amsterdam
ADSORPTION J. K N A L L ,
AND JI)ESO~ON
S.A. B A R N E T T * ,
KINETICS
OF In ON Si(100)
J.-E. S U N D G R E N
Department of Physics, LinkSping University, S-58183 Linki~ping, Sweden and