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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Binding Of Molecules To Surfaces: 1
Molecules bind to surfaces via
Physical Forces (Physisorption)
Chemical Forces (Chemisorption)
Densities similar to liquids
(1 gm/cm3)=1015 molecules/cm2
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.  and Farkis .)
Gas Phase ethylene
Bond Order Of Adsorbed Ethylene
All sigma bonds
Figure 3.5 Classification of metals and semiconductors according to the chemical reactivity of their surfaces. (After Trapnell and Hayward .)
Surfaces that Have similar electron densities, electronegativities behave similarly
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
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 , Delchar and Ehrlich , and Adams and Germer .)
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. .)
Figure 3.10 The mechanism of ethylene decomposition on Pt(111). (Proposed by Kesmodel et al.  and confirmed by Ibach and Lehwald .)
Figure 3.11 The mechanism of ethylene decomposition on (1x1)Pt(100). Proposed by Hatzikos and Masel  and confirmed by Sheppard .)
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.
Figure 3.14 The binding sites for CO adsorption on Pt(111). (Proposed by Crossley and King .)
Figure 3.15 The domain wall structure of CO on Pt(100). (Proposed by Persson et al. .)
Two kinds of adsorption