ChE 553 Lecture 3

1. ChE 553 Lecture 3 Binding Of Molecules To Surfaces: 1

2. Objective • General Overview Of Binding Of Molecules To Metal Surfaces • What are the key forces • What are trends • What are adsorbed layers like

3. Key Terms Adsorbate Adsorbent http://chsfpc5.chem.ncsu.edu/~franzen/CH795N/dft_modules/surface_module/ni_111_co_binding.htm

4. 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

5. 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

6. 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].)

7. Trends Over The Periodic Table

8. Properties Over periodic Table

9. Often Good Correlation Between Electron Density, Electronegativity and Properties Gas Phase ethylene Bond Order Of Adsorbed Ethylene All sigma bonds

10. 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

11. 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

12. Qualitative Effects

13. Qualitative Effects

14. 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].)

15. 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].)

16. 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].)

17. 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].)

18. 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.

19. 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].)

20. 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].)

21. Wood’s Notation Still Applies Pt(110)(1x2)

22. Example A Square Lattice

23. Square Lattice Continued

24. Primitive vs Centered Lattices

25. More Examples: Square Lattice

26. Examples From HW Set

27. 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