1. PC4259 Chapter 4 Adsorption on Solid Surfaces & Catalysis
When atom or molecule is trapped by an attractive interaction on a solid surface, it becomes an adsorbate with adsorption energy Eads
Physisorption: Eads  100 meV, attracted by van der Waals force, little change in electronic configurations
Chemisorption: Eads  0.5 eV, chemical bond is formed between adsorbate and substrate, significant changes in electronic configurations

2. - - r Van der Waals (London) Interaction p1 p2 + +
Neutral atoms can induce (fluctuating) dipole moments in each other p1 p2 + - + - r
( = polarizability)
p1 =0 but p12 ≠ 0
Interaction between mutual induced dipoles:
Full potential energy:
Repulsion between atoms at small distance ~
Lennard-Jones potential

3. Physisorption Potential
Modeled as the interaction of an induced adsorbate dipole with its image dipole
Physisorption potentials of He atoms on some metals calculated with jellium model

4. Chemisorption: electronic structures of adsorbate & surface go through significant reconfiguration, form chemical bond (metallic, covalent or ionic)
DFT calculation results of charge densities of some chemisorbed atoms on a jellium substrate
E-donation from Li
E-capture by Cl
In chemisorption, Eads ~ 1 eV/atom = 96.5 kJ/mol = 23.1 kcal/mol

5. Dissociative chemisorption: a molecule dissociates, and the breaking species form chemical bonds with surface (e.g., O2  O + O on Fe)
Dissociation energy of molecule AB:
Ediss
Ediss = 4.5 eV, 5.2 eV and 9.8 eV for H2, O2 and N2
Dissociative adsorption energy:
For O2 on Fe, since O + Fe bond strength is ~ 4.2 eV, the dissociative Eads is ~ 3.2 eV

6. Transition Between Physisorption & Chemisorption statesZ’
Molecular physisorption & dissociative chemisorption potential curves intersect at transition point z’
Activation energy for chemisorption Eact
Precursor state for chemisorption
Barrier from precursor to chemisorption state:
a = Eact + d

7. Evolution of molecular bond in chemisorption
bridge site a = 0.5 eV
Transition point
on-top site a = 0.7 eV
H2 on Pd(100), bridge site on-top site
H2 on Cu(100)

8. Desorption from Surface
Desorption: Adsorbed species gain sufficient energy to leave the surface
Thermal desorption: desorption process activated by thermal energy (e.g., by raising temperature)
Stimulated desorption: desorption activated by energy transfer from photons, electrons, ions,…
Reaction before desorption: adsorbed atoms form molecules, then the molecules leave surface

9. Activation Energy for Desorption
Physisorbed & non-dissociative chemisorbed species:
Edes = Eads
Desorption of recombined dissociative chemisorbed species:
Edes = Eads + Eact

10. Arrangement of Adsorbates on Surface
Depends on coverage , adsorbate-substrate & adsorbate-adsorbate interactions, and T
 , in unit of ML (monolayer), can be measured using XPS, AES or EELS
Low  & high T,  2-D gas phase
High  & low T,  2-D order phase
High  & high T,  2-D liquid phase
Phase diagram & transition

11. Types of Adsorbate-adsorbate Interactions
Van der Waals attraction between mutually induced dipoles, important only for physisorbed inert gas at low T
Dipole force between permanent dipoles of adsorbed molecules (e.g. H2O, CO, NH3), or due to charge transfer in bond formation, often repulsive due to parallel dipoles
Orbital overlap between adsorbates at neighboring sites, often repulsive due to Pauli exclusion
Substrate-mediated interactions: Adsorbate disturbs electronic or mechanic structures (e.g. charge transfer or elastic distortion) at nearby sites, make them more favorable or unfavorable for others to occupy, corresponding effective attraction or repulsion
Mainly consider nearest neighbor (nn) and next (or 2nd) nearest neighbor (nnn) interactions

12. If nn-interaction repulsive but nnn-interaction is attractive 
H2 on graphite at low T
Quite Common

13. Adsorption sites on hexagonal surfaces of metals
CO take on-top sites on Rh(111), but bridge sites on Ni(111)

14. Si(111)
-Ga
Each Ga atom bonds with three Si atoms on surface, so all Si dangling bonds are saturated, while the dangling bond on top of a Ga atom is completely empty, satisfying electron counting rule

15. Si(111)
-Pb
STM image
More than one adsorbate may be accommodated in each supercell
Need both STM (or LEED) and XPS (or AES) data

16. Si(111)
-Sb
trimer

17. Superstructures formed by both adsorbed & substrate atoms
Simple two-layer case
fl + fu = 1 fu fl
Si(111)
-Ag

18. Dynamic Adsorption & Desorption Measurements
To find out binding energy, activation barrier for adsorption, etc.
A flux F can come from a gas-phase ambient of pressure p:
A flux can also be generated by a gas doser, a molecule beam or an evaporator in vacuum
At constant F or p for a period t, Ft or pt is the total exposure
Unit of Ft: monolayer (ML)
pt is often in unit of Langmuir (L), 1 L = 10-6 torr-s

19. Adsorption Kinetics
Under a flux F, surface coverage  increases at a rate:
Probability of sticking or sticking coefficient:
 = condensation coefficient, reflecting effects of orientation (steric factor), energy dissipation of adsorbed particles
f() = coverage factor, represents the probability of finding available adsorption sites. Sticking may be hindered by adsorbates already on surface
exp(-Eact/kT) = Boltzmann factor, comes in if there is a barrier for adsorption

20. Langmuir adsorption model: each adsorption site only accommodate 1 particle,   1 ML
Non-dissociative adsorption (n = 1) 
Dissociative adsorption of diatomic molecule (n = 2)
Dissociative adsorption of n-atom molecules
n = order of adsorption
(non-activated)

21. In physisorption or atomic chemisorption with Edes >> kT, initial sticking coefficient s0  1 & independent of T
In dissociative chemisorption with a physisorption precursor state of binding energy d and a barrier to chemisorption a, s0 depends on T
Molecule precursors of coverage p
Rate to desorb:
Rate to chemisorption
Initial sticking coefficient:

22. Eact = a - d from Arrhenius plot: ln(1/s0 -1) vs 1/T
Initial sticking coefficient in dissociative chemisorption
Eact = a - d from Arrhenius plot: ln(1/s0 -1) vs 1/T

23. Coverage factor in nth-order activated chemisorption
If precursor physisorption can occur at all sites, conversion to chemisorption requires n empty sites, introducing ka(1 - )n factor
Overall coverage factor:
(K = ka/kd)

24. Sb4 chemisorption on Si surfaces (n = 4)
T-dependence of K
Case of decreasing K at higher T, indicating εa > εd,

25. Mass Spectrometer for desorption measurement
Sample
TemperatureControl
Isothermal desorption: T fixed
Programmed desorption: T varies with time

26. (Polanyi-Wigner equation)
Desorption rate:
If adsorbates occupy identical sites, for nth-order desorption (e.g. n adsorbed atoms recombine first and then desorb as a molecule)
(Polanyi-Wigner equation)
n = 0: desorption of 2-D dilute gas in equilibrium with a 2-D solid, e.g. adatoms on a multilayer film
In isothermal desorption (T fixed):

27. Edes from Arrhenius plot
Isothermal desorption of 2-D gas of Ag in equilibrium with 3 different 2-D solid phases
Edes from Arrhenius plot

28. 1st-order (n = 1) Isothermal Desorption
: attempt frequency
~ 1013 s-1
(0 = 1 ML, Eads = 3 eV)
2nd-order (n = 2, e.g. O + O  O2) kinetics for associative diatomic molecular desorption:
(in Homework 8)

29. Temperature Programmed Desorption (TPD)
Analyze bonding and reaction properties of adsorbed species
Sample
Programmed heating
Mass spectrometer
Linear T ramping: T(t) = T0 + t
When T is low, desorption rate is low due to Boltzmann factor
At a very large t (or T), surface is run out of adsorbates, desorption rate is also low.
At Tm, desorption flux reaches a peak

30. 0th-order TPD First-order TPD
TPD n = 0
Peak is reached right be before all adsorbates have desorbed
First-order TPD
TPD n = 1
Peak at:
In 1st-order TPD, Tm is independent of 0

31. Edes from 1st-order TPD 2nd-order TPD ~ 1013 s-1 m
Tm decreases as 0 increases
Spectra are more symmetric

32. TPD spectra show a combination of a few kinetic models
Inhomogeneous substrate
Multilayer desorption
0th-order followed by 1st-order

33. Adsorption Isotherm
The coverage  on a surface in equilibrium with a gas ambient of pressure p satisfies , or:
with
In first-order Langmuir adsorption system
&
Langmuir adsorption isotherm

34. HREELS: for adsorbate bond configurations of atoms and molecules
Also can be detected with optical scattering method
Bond orientation from polarization dependence
Large shift

35. Electron Stimulated Desorption (ESD)
Through excitation of electronic system of adsorbates
Desorption of ionic or neutral species

36. Electron Stimulated Desorption Ion Angular Distribution (ESDIAD)
Flying away direction
At low 
0.5<<1 e H H O
0.2 <  <1
H+ ESDIAD from Ru(0001)

37. Adsorption Induced Work Function Variation
Dipole moment p = qd : intrinsic & induced
In-plane dipole has no effect

38. Cs-Induced Work Function Variation
Cs: large ion size, e-donor
Dipole-dipole interaction introduces a depolarization factor:
 = polarizability

39. Cs adsorption on Semiconductor
With:
On p-type GaAs
Bands bend downward
Evac  EC
negative electron affinity
high-flux photo-cathode

40. Adsorption Induced Change in LDOS near EF
Ni(111)-O
Depletion of LDOS at EF
6 L
100 L
1000 L
Surfactants: adsorbates to purposely modify surface property

41. Find a reaction path with lower barriers
Kinetic Barrier in Chemical Reaction
CO oxidation:
CO + ½O2  CO2
Energy gain: Hr = 283 kJ/mol
O2 dissociation barrier: ~ 5 eV
Haber-Bosch synthesis of NH3
½N2 + 3/2H2  NH3
Energy gain: Hr = 46 kJ/mol
Find a reaction path with lower barriers
N2 dissociation barrier: ~ 9.8 eV!

42. O2, H2 and N2 may easily dissociate when adsorbed on some surfaces
Basis of Heterogeneous Catalysis: Chemical reaction via adsorption-dissociation-reaction-desorption path often only encounters moderate barriers
Catalyst: accelerates certain chemical reaction, but is not consumed in reaction

43. Gerhard Ertl: 2007 Nobel Prize in Chemistry
for his pioneering studies of chemical processes on solid surfaces. He developed quantitative description of how H organizes on surfaces of catalytic metals such as Pt, Pd, and Ni. He also produced key insights into mechanism of Haber-Bosch process of NH3 synthesis
Haber-Bosch synthesis of NH3 on Fe
N2 dissociation not a major obstacle, but H addition to N is up-hill

44. CO oxidation on Pt(111): main barrier now is only 105 kJ/mol, while in gas phase O2 dissociation requires ~ 490 kJ/mol
Catalyst to convert CO to CO2, NO to N2 and HC to H2O in a car exhaust contains Pt, Pd, Rh and Ir

45. LDOS(EF), d-band center & Reactivity
LDOS at EF and surface reactivity are closely correlated E
Noble metal EF
Transition metal EF
d-band
sp-band
Downward shift of d-band center & increase of N2 dissociation barrier on Ru(0001) induced by adsorption of N, O or H,
DOS at EF in noble or transition metals

46. Raise nitrogen sticking probability by 102
K as electronic promoter in NH3 synthesis
Enhance LDOS at EF
Lower physisorption potential curve of N2
Raise nitrogen sticking probability by 102

47. Poisoning of catalyst
Poisoning often occurs due to coverage of S or graphitic C
On clean Pd(100), H2 dissociation is barrier-less
On p(22)S/Pd(100), H2 dissociation barrier = 0.1 eV
On c(22)S/Pd(100), H2 dissociation barrier = 2 eV, blocked
S adsorption shifts Pd d-band downward, surface becomes more repulsive for H2 adsorption & dissociation

48. General suitability of material as catalyst: should be just moderately reactive
Methanation of CO
CO + 3H2  CH4 + H2O
Fischer-Tropsch reaction facilitated by Fe-Co catalysts doped with K & Cu
Volcano curve