Chem120A+Notes+-+Surface+Chemical+Bond

Chem120A+Notes+-+Surface+Chemical+Bond - The Surface...

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The Surface Chemical Bond
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Bonding Trends across the Periodic Table The heat of adsorption generally increases from right to left in the periodic table.
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• The bonding energy depends on the number of electrons initially in the orbital ε 1 . •The bonding energy also depends on the relative position between different energy levels. Bonding and Anti-bonding Orbital σ * Consider two atomic orbitals coupled to form a bonding and an anti-bonding molecular orbital. Assume that, before the formation of the bond, there are n electrons in the atomic orbital 1 , and one electron in the atomic orbital 2 , then the bonding energy Energy Bonding 1 , n Anti-bonding 2 *1 2 *2 * 1 () [ 2 ( 1 ) ] ( ) () ( ) ( ) B En n n n σσ εε =+ + =− + (2) (1) 0, that is, The increase of weakens the chemical bond. BB EE n −= > Interaction between two electronic states
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The Effective-Medium Theory Consider a surface d band coupled to an molecular orbital ε forms a bonding and an anti-bonding surface state. • Transition metals have less valence d electrons, so the Fermi level is low, and the surface d band is partially filled before the formation of surface bond. After the surface bonding, the anti-bonding band is empty, so the surface bond is strong. • Noble metals have more valence d electrons, so the Fermi level is high and the surface d band is fully filled before surface bonding. After the surface bonding, the anti-bonding band is almost fully filled, so the surface bond is weaker than that of transition metal. Bonding Anti-bonding Energy d band Density of states Fermi level for noble metals Fermi level for transition metals d the energy level at the center of the band the energy level of the molecular orbital d d
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1 σ Energy d band F E 2 3 4 1 π 2 5 5 % 5* % 2* % 2 % Metal Surface bonding Free CO Bond Formation between CO and Metal Surfaces
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Comparison with Experimental results
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Cluster-Like Bonding of Molecular Adsorbates l C-C (Å) l C-M (Å) r M (Å) r C (Å) α ( o ) Ethylene chemisorption at ~300K on the (111) crystal faces of various transition metals • Ethylene is rearranged to from ethylidyne on surfaces • The C-C bond distance on the surface is slightly less than the free C-C bond length of 1.54 Å, as in the cluster compounds. • The C-C bond order reduction from two to nearly one indicates charge transfer from molecules into the metal surface.
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Alkylidyne Group in Surface and Organometallic Chemistry
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Structure and Vibrational Spectrum Benzene 7 o
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Complex System with Multiple Bonding Sites for a Single Molecule Pyridine
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Adsorption Induced Reconstruction
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Adsorption induced Restructuring of the Ni(100) surface Ideal fcc (100)-c(2x2) Ni (100)-c(2x2)-C with substrate restructing Ni C 3D view of Ni (100)-c(2x2)-C Top view
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Adsorption Induced restructuring of the Fe(110) Surface Ideal fcc (110)-(2×2) Fe (110)-(2×2)-S with substrate restructuring Top view 3D view of Fe (110)-(2×2)-S
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Sulfur-Chemisorption-Induced missing-row (2x2) Restructuring on Iiridium (110) Surface Ideal fcc (110) Ir (110)-(2×2)-2S 3D view of Ir (110)-(2×2)-2S Top view S Ir
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Hydrogen: 1.7 atm. 730 Å× 700 Å Oxygen: 1 atm.
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This note was uploaded on 09/28/2009 for the course CHEM 120A taught by Professor Whaley during the Spring '07 term at Berkeley.

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Chem120A+Notes+-+Surface+Chemical+Bond - The Surface...

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