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ChE 553 Lecture 3 . Binding Of Molecules To Surfaces: 1. Objective. General Overview Of Binding Of Molecules To Metal Surfaces What are the key forces What are trends What are adsorbed layers like. Key Terms. Adsorbate Adsorbent.

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Che 553 lecture 3

ChE 553 Lecture 3

Binding Of Molecules To Surfaces: 1


Objective
Objective

  • General Overview Of Binding Of Molecules To Metal Surfaces

    • What are the key forces

    • What are trends

    • What are adsorbed layers like


Key terms
Key Terms

Adsorbate

Adsorbent

http://chsfpc5.chem.ncsu.edu/~franzen/CH795N/dft_modules/surface_module/ni_111_co_binding.htm


Overview
Overview

Molecules bind to surfaces via

Physical Forces (Physisorption)

Dipole-Dipole interactions

Correlation

Chemical Forces (Chemisorption)

Densities similar to liquids

(1 gm/cm3)=1015 molecules/cm2


Today chemisorption on metals
Today: Chemisorption On Metals

  • Metals have many free electrons

  • Adsorbates bind to the free electrons

    • Adsorbate bonding changes

  • Bond is delocalized

  • Electrons easy to move


Overview of chemisorption
Overview Of Chemisorption:

Chemical bonds form between surface and adsorbate.

Figure 3.4 A comparison of the structure of various molecules in the gas phase and on a solid surface. (Geometric data from Lin et al. [1987] and Farkis [1935].)




Often good correlation between electron density electronegativity and properties
Often Good Correlation Between Electron Density, Electronegativity and Properties

Gas Phase ethylene

Bond Order Of Adsorbed Ethylene

All sigma bonds


General view of binding across the periodic table
General View Of Binding Across The Periodic Table Electronegativity and Properties

Exception

Figure 3.5 Classification of metals and semiconductors according to the chemical reactivity of their surfaces. (After Trapnell and Hayward [1971].)

Surfaces that Have similar electron densities, electronegativities behave similarly


Qualitative effects
Qualitative Effects Electronegativity and Properties

Strong bonds but insufficient electron density, no d’s

0 - no uptake

1 - uptake at 100 K but not 300 K

2 - Activated adsorption

3 – Rapid uptake at room temperature


Qualitative effects1
Qualitative Effects Electronegativity and Properties


Qualitative effects2
Qualitative Effects Electronegativity and Properties


Also varies with surface structure
Also Varies With Surface Structure Electronegativity and Properties

Nitrogen on Tungsten

Figure 3.6 The rate of adsorption of nitrogen on tungsten as a function of the position of the plane within the stereographic triangle. (Data of Ehrlich and Hudda [1963], Delchar and Ehrlich [1965], and Adams and Germer [1971].)


Molecular adsorption vs dissociate desorption
Molecular Adsorption vs Dissociate Desorption Electronegativity and Properties

Figure 3.7 Part of the periodic table showing which metals dissociate various gases at 10-6 torr and 100 or 300 K, and which do not. (This is an updated version of a figure presented by Brodén et al. [1976].)


More complex behavior
More Complex Behavior Electronegativity and Properties

Figure 3.10 The mechanism of ethylene decomposition on Pt(111). (Proposed by Kesmodel et al. [1979] and confirmed by Ibach and Lehwald [1979].)


Also surface structure sensitive
Also Surface Structure Sensitive Electronegativity and Properties

Figure 3.11 The mechanism of ethylene decomposition on (1x1)Pt(100). Proposed by Hatzikos and Masel [1987] and confirmed by Sheppard [1988].)


Next geometry of adsorbed layer
Next: Geometry Of Adsorbed Layer: Electronegativity and Properties

Key idea: adsorbates often form ordered structures when they adsorb - take order of substrate

Figure 3.12 Langmuir’s model of the adsorption of gases on surfaces. The black dots represent possible adsorption sites, while the white ovals represent adsorbed molecules.


Examples of surface structure co on pt 111
Examples Of Surface Structure Co on Pt(111) Electronegativity and Properties

Figure 3.14 The binding sites for CO adsorption on Pt(111). (Proposed by Crossley and King [1980].)


Also get incommensurate adsorption
Also Get Incommensurate Adsorption Electronegativity and Properties

(2x2) Domain

Domain wall

Domain wall

Figure 3.15 The domain wall structure of CO on Pt(100). (Proposed by Persson et al. [1990].)


Wood s notation still applies
Wood’s Notation Still Applies Electronegativity and Properties

Pt(110)(1x2)


Example a square lattice
Example A Square Lattice Electronegativity and Properties


Square lattice continued
Square Lattice Continued Electronegativity and Properties


Primitive vs centered lattices
Primitive vs Centered Lattices Electronegativity and Properties


More examples square lattice
More Examples: Square Lattice Electronegativity and Properties


Examples from hw set
Examples From HW Set Electronegativity and Properties


Summary
Summary Electronegativity and Properties

Two kinds of adsorption

  • Chemisorption & Physisorption

  • Physisorption – small changes in molecules

  • Chemisorption – large change in molecules often for complex overlayer structures

  • Wood’s notation


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