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PC4259 Chapter 4
Adsorption on Solid Surfaces & Catalysis
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
When atom or molecule is trapped by
an attractive interaction on a solid
surface, it becomes an adsorbate
with adsorption energy Eads
Van der Waals (London) Interaction
+ - + -r
p1 p2
Interaction between mutual induced
dipoles:
Neutral atoms can induce
(fluctuating) dipole moments in
each other3
12 ~
rpEp
6
21
321 ~~
rp
rppEb
p1 =0 but p12 ≠ 0
Repulsion between atoms at
small distance ~ 121
r
Full potential energy:
6124)( rrrU
Lennard-Jones potential
( = polarizability)
Physisorption Potential
Modeled as the interaction of an induced adsorbate dipole with its image dipole
30 )(
)()(
zz
CzV
Physisorption potentials of
He atoms on some metals
calculated with jellium model
DFT calculation results of
charge densities of some
chemisorbed atoms on a
jellium substrate
Chemisorption: electronic structures of
adsorbate & surface go through significant reconfiguration, form chemical bond (metallic, covalent or ionic)
E-donation from Li
E-capture by Cl
In chemisorption, Eads ~ 1 eV/atom = 96.5 kJ/mol = 23.1 kcal/mol
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:
)()()( ABEBEAEEdiss
Ediss = 4.5 eV, 5.2 eV and 9.8 eV
for H2, O2 and N2
Ediss
Dissociative adsorption energy:
)()()2()( 00)( SBESAESEABEE BA
ads
For O2 on Fe, since O + Fe bond strength is ~ 4.2 eV, the
dissociative Eads is ~ 3.2 eV
Transition Between Physisorption &
Chemisorption states
Activation energy for
chemisorption Eact
Molecular physisorption & dissociative
chemisorption potential curves
intersect at transition point z’
Z’
Precursor state for chemisorption
Barrier from precursor to chemisorption state:
a = Eact + d
Evolution of molecular bond in chemisorption
Transition point
H2 on Pd(100), bridge site on-top site H2 on Cu(100)
on-top site a = 0.7 eV
bridge site a = 0.5 eV
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
Activation Energy for Desorption
Physisorbed & non-dissociative
chemisorbed species:
Edes = Eads
Desorption of recombined
dissociative chemisorbed species:
Edes = Eads + Eact
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
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
H2 on graphite at low T 33 Quite Common
If nn-interaction repulsive but nnn-interaction
is attractive 33
Adsorption sites on hexagonal surfaces of metals
CO take on-top sites on Rh(111), but bridge sites on Ni(111)
Si(111) 33 -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
Si(111) 33 -Pb
STM image
More than one adsorbate may be
accommodated in each supercell
Need both STM (or LEED) and XPS (or AES) data
Si(111) 33 -Sb
trimer
Superstructures formed by both adsorbed & substrate
atoms
fufl
Si(111) 33 -Ag
fl + fu = 1
us f1
Simple two-layer case
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:
mkT
pF
2
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
Adsorption Kinetics
Under a flux F, surface coverage increases at a rate:
sFrdtd
ads
Probability of sticking or
sticking coefficient: )/exp()( kTEfs act
• = 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
Langmuir adsorption model: each adsorption site only accommodate 1 particle, 1 ML
Non-dissociative adsorption (n = 1)
1)(f )1(0 Fsdtd
)exp(1 0Fts
Dissociative adsorption of diatomic molecule (n = 2)
2)1()( f
Dissociative adsorption of n-atom molecules nf )1()( n = order of adsorption(non-activated)
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: )/exp( kTk ddpd
Rate to chemisorption )/exp( kTk aapa
1
0 exp1
kTkk
ks ad
a
d
da
a
Initial sticking coefficient:
1
0 exp1
kTkk
ks ad
a
d
da
a
Initial sticking coefficient in dissociative chemisorption
Eact = a - d from Arrhenius plot: ln(1/s0 -1) vs 1/T
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: n
n
K
Kf
)1(1
)1)(1()(
(K = ka/kd)
Sb4 chemisorption on Si surfaces (n = 4)
kTk
kK da
d
a
d
a
exp
T-dependence of K
Case of decreasing K at
higher T, indicating εa > εd,
Mass Spectrometer for desorption measurement
Sample
TemperatureControl
Mass spectromete
r
Isothermal desorption: T fixed
Programmed desorption: T varies with time
Desorption rate: )/exp()(** kTEfr desdes
If adsorbates occupy identical sites, for nth-order
desorption (e.g. n adsorbed atoms recombine first
and then desorb as a molecule)
)/exp(/ 0 kTEkkdtdr desn
nn
ndes (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): 0/ kdtd
tk00
Isothermal desorption of 2-D gas of Ag in equilibrium
with 3 different 2-D solid phases
)/exp(0 kTEkk desnn
Edes from Arrhenius plot
1st-order (n = 1) Isothermal Desorption
)/exp(/ 011 kTEkkdtd des
)exp( 10 tk
2nd-order (n = 2, e.g. O + O O2) kinetics for
associative diatomic molecular desorption:
(0 = 1 ML, Eads = 3 eV)
01k : attempt frequency
~ 1013 s-1
)/exp(/ 202
22 kTEkkdtd des
(in Homework 8)
Temperature Programmed Desorption (TPD)
Linear T ramping: T(t) = T0 + t
Analyze bonding and reaction properties of adsorbed species
Sample
Programmed heating
Mass spectrometer
• 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
)(
exp0
0
tTk
Ek
dt
dr desn
ndes
)(
exp0
0
tTk
Ek
dt
dr des
ndes
0th-order TPD
First-order TPD
)(
exp0
01 tTk
Ek
dt
dr des
des
Peak at:
Peak is reached right be before all adsorbates have desorbed
02
2
dt
d
dt
drdes
m
desdesm kT
E
k
EkT exp
01
2
TPD n = 1
TPD n = 0
In 1st-order TPD, Tm is independent of 0
2nd-order TPD
Edes from 1st-order TPD
64.3ln
lnln
01
01
mm
m
desmmdes
TkkT
kT
ETkkTE
~ 1013 s-1 01k
)(
exp0
202 tTk
Ek
dt
dr des
des
m
des
m
desm kT
E
kT
Ek exp2 0
2
TPD n = 1
TPD n = 2
m
Tm decreases as 0 increases
Spectra are more symmetric
TPD spectra show a combination of a few kinetic models
Multilayer desorption0th-order followed by 1st-order
Inhomogeneous substrate
Adsorption Isotherm
The coverage on a surface in equilibrium with a gas
ambient of pressure p satisfies , or: desads rr
)(
)(*1
f
f
Kp with
mkT
kTEK ads
2*
)/exp(
In first-order Langmuir adsorption system
1)(f )(*f&
Kp
Kpp
1)(
Langmuir adsorption isotherm
HREELS: for adsorbate
bond configurations of
atoms and molecules
Also can be detected with
optical scattering method
Bond orientation from
polarization
dependence
Large shift
Electron Stimulated
Desorption (ESD)
Through excitation of
electronic system of adsorbates
Desorption of ionic or neutral species
Electron Stimulated Desorption Ion Angular Distribution (ESDIAD)
O
HH
e
Flying away direction
H+ ESDIAD from Ru(0001)
At low 0.5<<1
0.2 < <1
Adsorption Induced Work Function Variation
Dipole moment p = qd : intrinsic & induced
In-plane dipole has no effect
0pendip
Cs-Induced Work Function Variation
Cs: large ion size, e-donor
Dipole-dipole interaction
introduces a depolarization
factor:
0
2/3
4
9
dip
dep
nf
= polarizability
Cs adsorption on Semiconductor
sVe
'
'sss VVV
With:
On p-type GaAs
Bands bend downward
Evac EC
negative electron affinity
high-flux photo-cathode
Adsorption Induced Change in LDOS near EF
Ni(111)-O
0
6 L
100 L
1000 L
Depletion of LDOS at EF
Surfactants: adsorbates to purposely modify surface property
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
N2 dissociation barrier: ~ 9.8 eV! Find a reaction path with lower barriers
Basis of Heterogeneous
Catalysis: Chemical reaction via
adsorption-dissociation-reaction-
desorption path often only
encounters moderate barriers
O2, H2 and N2 may easily dissociate
when adsorbed on some surfaces
Catalyst: accelerates
certain chemical
reaction, but is not
consumed in reaction
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
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
LDOS(EF), d-band center & Reactivity
E
Noble metal EF
Transition metal EF
d-bandsp-band
LDOS at EF and surface reactivity are closely correlated
DOS at EF in noble
or transition metals
Downward shift of d-band center & increase
of N2 dissociation barrier on Ru(0001)
induced by adsorption of N, O or H,
K as electronic promoter in NH3 synthesis
Enhance LDOS at EF
Lower physisorption potential curve of
N2
Raise nitrogen sticking probability
by 102
Poisoning of catalyst
On p(22)S/Pd(100), H2 dissociation barrier = 0.1 eV
On clean Pd(100), H2 dissociation is barrier-less
On c(22)S/Pd(100), H2 dissociation barrier = 2 eV, blocked
S adsorption shifts Pd d-band downward, surface becomes
more repulsive for H2 adsorption & dissociation
Poisoning often occurs due to coverage of S or graphitic C
CO + 3H2 CH4 + H2O
Fischer-Tropsch reaction
facilitated by Fe-Co
catalysts doped with K &
Cu Volcano curve
General suitability
of material as
catalyst: should be
just moderately
reactive
Methanation of CO