Che 553 lecture 3
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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

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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].)


Trends over the periodic table

Trends Over The Periodic Table


Properties over periodic table

Properties Over periodic Table


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

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

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


Qualitative effects2

Qualitative Effects


Also varies with surface structure

Also Varies With Surface Structure

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

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

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

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:

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)

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

(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

Pt(110)(1x2)


Example a square lattice

Example A Square Lattice


Square lattice continued

Square Lattice Continued


Primitive vs centered lattices

Primitive vs Centered Lattices


More examples square lattice

More Examples: Square Lattice


Examples from hw set

Examples From HW Set


Summary

Summary

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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