ChE 553 Lecture 3

Binding Of Molecules To Surfaces: 1

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Objective

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

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Key Terms

Adsorbate

Adsorbent

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

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

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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

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Overview Of Chemisorption:

Chemical bonds form between surface and adsorbate.

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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

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Properties Over periodic Table

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12

3

4

5

67

89 10

Cu

Zn

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13Electron

egati

vity

Often Good Correlation Between Electron Density, Electronegativity and Properties

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Bond Order Of Adsorbed Ethylene

Gas Phase

ethylene

All sigma bonds

General View Of Binding Across The Periodic Table

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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

Exception

Qualitative Effects

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Adsorbing Gas

Metal

H2

O2

N2

CO

Group a 2 or 3 3 2 3 Group b 3 3 3 3 Group c 3 3 2 3 Group d 3 3 2 3 Group e 3 3 2 3

Cu 2 3 2 1 Ag 0 2 or 3 0 0 Au 0 0 0 3 Al 0 def-3 0 3 K, Na, Li 0 3 0 0

0 - no uptake1 - uptake at 100 K but not 300 K2 - Activated adsorption3 – Rapid uptake at room temperature

Strong bonds but insufficient electron

density, no d’s

Qualitative Effects

Adsorbing Gas

Metal

H2

O2

N2

CO

Si, Ge 0 or 2 3 0 0 InP 0 3 0 0 NiO 0 1 ZnO def-3 def-3 ? 1 MgO Al2O3 0 1 SiO2 def-2 def-3 NaCl 0 ? 1 0-100K LiF 1-40 K

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Qualitative Effects

Metal

C2H2

C2H4 CH4

C2H6

CH3OH

H2O Group a 3 ? ? 3 Group b 3 2 3 ? Group c 3 ? 3 1 Group d 3 2 3 1 Group e 3 2 3 1

Cu 1 or 3 0 1 1 Ag 1 0 1 1 Au 3 0 1 1 Al 2 ? 3 3 K, Na, Li 3 C2H2

0 C2H4 ? ? 3

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Also Varies With Surface Structure

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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].)

Nitrogen on Tungsten

Molecular Adsorption vs Dissociate Desorption

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2 2

DissociativeAdsorption

MolecularAdsorption

NoAdsorption

Molecular And

DissociativeAdsorption

ActivatedDissociativeAdsorption

LimitedData

PtIrOsRe

PdRhRu

NiCo

AuPt

AgPd

CuNi

Ir

Ni

AuPtIrOsRe

AgPdRhRu

CuNi

Sc Ti V Cr Mn Fe

Y Zr Nb Mo Tc

La Hf Ta W

CO300 K

Dissociated

Molecular

Sc Ti V Cr Mn Fe

Y Zr Nb Mo Tc

La Hf Ta W

N

Dissociated Molecular

Ti V Cr Mn Fe Co

Zr Nb Mo Tc Ru

Hf Ta W Re

NO300 K

Dissociated Molecular

Sc

Y

La

Rh

Os

Sc Ti V Cr Mn

Y Zr Nb Mo Tc

La Hf Ta W

NO100 K

Dissociated

Sc Ti V Cr Mn Fe

Y Zr Nb Mo Tc

La Hf Ta W

300 K

Dissociated Activated

Sc Ti V Cr Mn Fe Co Ni Cu

Y Zr Nb Mo Tc Ru Rh Pd Ag

La Hf Ta W Re Os Ir Pt

O300 K

Dissociated

O100 KAu

Key

AuPt

AgPd

Cu

IrOsRe

RhRu

Co

Fe Co

Molecular

Au

Ag

Cu

AuPtIrOsRe

Ag

CuNiCo

PdRhRu

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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

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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

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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:

Key idea: adsorbates often form ordered structures

when they adsorb - take order of substrate

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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)

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

Also Get Incommensurate Adsorption

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

Domain wallDomain wall

(2x2) Domain

Wood’s Notation Still Applies

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Pt(110)(1x1)

Pt(110)(1x2)

b 1

b 2

b 1

b 2

Pt(110)(1x2)

Example A Square Lattice

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Square Lattice Continued

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Primitive vs Centered Lattices

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More Examples: Square Lattice

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4.181010 R

Examples From HW Set

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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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