ChE 553 Lecture 4

1
ChE 553 Lecture 4

• Models For Physisorption And Chemisorption I

2
Objective For Today

• Quantify the results from lect 3
• Forces that determine bonding
• Large trends
• Physical forces
• Electronegativity
• Hardness/density of states

3
Forces Between a Molecule and Metal Surface

• Dipole-Induced dipoles
• Correlation instantaneous dipole-dipole
  interactions
• Electron reorganization /bonding

http//chsfpc5.chem.ncsu.edu/franzen/CH795N/dft_m
odules/surface_module/ni_111_co_binding.htm
4
Literature Discusses Two Types Of Adsorption

• Physisorption
• Dipoles and correlations dominate
• Chemisorption
• Electron reorganizations dominate

5
Usually Not A Clear Distinction

6
Physisorption Chemisorption Usually Treated
Differently In The Literature

• Physisorption
• Add up physical interactions assuming that there
  are no electronic rearrangements
• Chemisorption
• Considering electron reorganization

7
Modeling Physisorption

• Usual model add up the physical forces

8
Working Out The Algebra

• Assume Leonard-Jones potential
• Pages of algebra

Expected Theoretically for induced dipole/induced
dipole
9
A Comparison Of Heats Of Adsorption Calculated To
Measure
10
A Comparison Of Heats Of Adsorption Calculated To
Measure
Experiment
Calc
Starting to see reorganization of electrons
11
New Topic Modeling Chemisorption

• Several different models
• Local chemical bonds
• Bonds to free electrons
• Ionic forces
•  
• Local chemical bonds works on some semiconductors
•  
• Bonds to free electrons dominate on metals
•  
• Ionic forces dominate on oxides and other
  insulators

12
Modeling Bonds To Free Electrons

• Three models
• Algebraic models
• Jellium models
• Full QM
• Clusters
• Slabs

13
Algebraic Model (Pauling Electro Negativity Model)

• Expand energy as a function of the number of
  electrons around each atom, molecule, surface as
  a Taylor series
• Assume electrons exchanged when molecules
  interact but Taylor coefficients constant
• Minimize energy as electrons transferred

14
Derivation

• Taylor series
• ?A Electronegativity
• ?A hardness
• number of electrons

15
Derivation For The Interaction Of A and B
16
Derivation For The Interaction Of A and B
17
Result
Interaction
?H0
18
Numerical Comparison

TABLE 3.4 A Comparison of Eleys 1950
Calculations of Heats of Adsorption to Measured
Values
Key Conclusion Electronegativity and Hardness
Key
Works for metals on metals, hydrogen on metals,
sigma bonded species Only works modestly for
pi-bonding.
19
For Ionic Systems The Equation Becomes

• EQU 3.48

20
Key Implication Of Theory Hard-Hard And
Soft-Soft Interactions

• Hard acids interact strongly with other hard
  acids and very strongly with hard bases.
• Soft acids interact strongly with other soft
  acids and very strongly with soft bases.
• Hard/soft interactions weak.

21
Definitions

• Hard acid An acceptor with no low-lying
  unoccupied orbitals so that it has a small
  affinity for electrons and remains positively
  charged during a reaction. Such species will
  have a small ?ßAB and a very negative Ebmo
  (hardness). Examples include solvated ions of
  Al3, Mg2, H, and surfaces such as alumina or
  silica.
•  
• Hard base A donor with no high-lying donor
  orbiatls, so that it has little capacity to
  donate electrons and a small value of ?ßAB.
  Examples include F-, OH- , H2O, amines and
  surfaces such as MgO or TiO2.

22
Definitions Continued

• Soft acid A species that easily accepts charge.
  Generally, the species will have a high affinity
  for electrons, and a high polarizability (i.e.,
  large ?ßAB) so that it can easily form covalent
  bonds. Examples include Hg2, Ag, and Pt, and
  most small metal clusters.
• Soft base A species that easily gives up
  charge. Generally, the species will have a high
  affinity for electrons, and a high polarizability
  (i.e., large ?ßAB) so that it can easily form
  covalent bonds. Examples include I-, RS-, and
  H-, and most metal surfaces.

23
Rules

• Hard acids bind strongly to hard bases
• Soft acids bind strongly to soft bases
• Hard-soft interactions weak
• Example Binding of H2O and H2S on platinum and
  alumina
• Limitations of method still not properly
  considered molecules with discrete bonds.

24
Corrections For Molecular Adsorbates (Fukui
Functions)

• Key idea the electronegativity is not constant
  around a molecule so it is easier to add
  electrons in some places than others.

Figure 3.26 The LUMO (a) and HOMO (b) for CO.
25
Jellium Model
Figure 3.33 The electron density outside of a
charge compensated jellium surface for rs 2 and
5, after Halloway and Nørskov, 1991. (a)
Actual electron density, (b) scaled electron
density.
26
Newns Anderson Jellium Model
Figure 3.34 A schematic of the density of states
calculated via Equation 3.62 for the interaction
of an adsorbate with a surface with (a) a narrow
band and (b) a wide band.
27
Key Prediction Of Newns Anderson Model

• Bonds are dynamic - there is continuous exchange
  of electrons between bond and surface - one
  electron pairs up with an adsorbate then leaves,
  then another electron forms a bond.
•  
• Implications
• Very mobile, rather reactive surface species
• Energy levels broaden due to the uncertainty
  principle

28
Data Verifies Rapid Exchange
Figure 3.35 A comparison of the UPS spectrum of
N2O adsorbed on a W(110) surface to the UPS
spectrum of N2O in the gas phase. (Data of Masel
et al. 1978.)
29
Quantification Of Model Effective Medium Model

• Add up effects of electrons and d-electrons to
  get predictions
•  
• Assume only sigma bonds
•  
• Key implication- bonding goes as electron
  density.

30
Table Of Electron Density
Source Calculated by Morruzzi et al. 1978
and as fit to data by DeBoer 1988. The values
from DeBoer should be multiplied by 0.9 to make
them compatible with Morruzzis
values. Morruzzis value.
31
Comparison To Data
Figure 3.40 A correlation between the bonding
mode of ethylene on various closed packed metal
surfaces at 100 K and the interstitial electron
density of the bulk metal. (After Yagasaki and
Masel 1994.)
32
Comparison To Ethylene Data
Figure 3.47 A correlation between the
vibrational frequency of the C-C stretch in C2D4
adsorbed on a series of closed packed metal
surfaces at 100 K and the interstitial electron
density of the metal.
Figure 3.41 A correlation between the
carbon-carbon bond order on adsorbed ethylene on
various closed packed metal surfaces at 100 K and
the interstitial electron density of the bulk
metal. (After Yagasaki and Masel 1994.)
33
Comparison To Ethylene Data

Figure 3.47 A correlation between the
vibrational frequency of the C-C stretch in C2D4
adsorbed on a series of closed packed metal
surfaces at 100 K and the interstitial electron
density of the metal.
34
CO Data
Figure 3.46 A correlation between the low
coverage limit of the vibrational frequency of CO
adsorbed on a series of closed packed metal
surfaces and the interstitial electron density of
the metal.
Fails because not properly considering
delta-bonds (model only considers sigma bonds)
35
Summary

• Physisorption
• Physical forces dominate
• Add up the forces
• Chemisorption Metals
• Metalic bonds dominate
• Radicals attached to Jellium
• Rapid exchange of electrons
• Hard-hard, soft-soft interactions strong
• Hard-soft interactions weak