Transcript Secondary Ion Mass Spectrometry

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
Van der Waals (London) Interaction
Neutral atoms can induce (fluctuating)
dipole moments in each other
p2  E ~
p1
p1
+
p2
-
+
-
r
r3
p1 =0 but p12 ≠ 0
( = polarizability)
Interaction between mutual induced
dipoles:
Eb ~
p1 p2
r
3
~
p12
r6
Full potential energy:
    r  
U ( r )  4  
 r
12
Lennard-Jones potential
6
Repulsion between atoms at
small distance ~ 1
r 12
Physisorption Potential
Modeled as the interaction of
an induced adsorbate dipole
with its image dipole
C ( )
V ( z)  
( z  z0 ) 3
Physisorption potentials of
He atoms on some metals
calculated with jellium model
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
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  E( A)  E( B)  E( AB)
Ediss = 4.5 eV, 5.2 eV and 9.8 eV
for H2, O2 and N2
Dissociative adsorption energy:
( AB)
Eads
 E( AB)  E(2S )  E0 ( A  S )  E0 (B  S )
For O2 on Fe, since O + Fe bond strength is ~ 4.2 eV, the
dissociative Eads is ~ 3.2 eV
Ediss
Transition Between Physisorption
& Chemisorption states
Z’
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
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)
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
If nn-interaction repulsive but nnn-interaction
is attractive  3  3
H2 on graphite at low T
3  3 Quite Common
Adsorption sites on hexagonal surfaces of metals
CO take on-top sites on Rh(111),
but bridge sites on Ni(111)
Si(111) 3  3 -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) 3  3 -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) 3  3 -Sb
trimer
Superstructures formed by both
adsorbed & substrate atoms
Simple two-layer case
fl + fu = 1
fl
 s 1  f u
fu
Si(111) 3  3 -Ag
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:
p
F
2m kT
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:
d
dt
Probability of sticking or
sticking coefficient:
 rads  sF
s  f ( ) exp(Eact / kT )
•
 = 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)
f ( )  1  

d
dt
 s0 F (1   )
  1  exp(s0 Ft)
Dissociative adsorption of
diatomic molecule (n = 2)
f ( )  (1   )2
Dissociative adsorption of n-atom
molecules
n
f
(

)

(
1


)
(non-activated)
n = order of adsorption
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: kd   p d exp( d / kT )
Rate to chemisorption ka   p a exp( a / kT )
Initial sticking coefficient: s0 
 
ka
    a 
 1  d exp  d

ka  kd   a
kT


1
Initial sticking coefficient in dissociative chemisorption
s0 
 
ka
    a 
 1  d exp  d

ka  kd   a
kT


1
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:
(1  K )(1   ) n
f ( ) 
1  K (1   ) n
(K = ka/kd)
Sb4 chemisorption on Si surfaces (n = 4)
T-dependence of K
ka  a
 a d 
K
 exp 

kd  d
kT 

Case of decreasing K at
higher T, indicating εa > εd,
Mass Spectrometer for desorption measurement
Mass
spectrometer
Sample
Temperature
Control
Isothermal desorption: T fixed
Programmed desorption: T varies with time
Desorption rate: rdes   * f * ( ) exp(Edes / kT )
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)
rdes  d / dt  kn n  kn0 n exp(Edes / kT )
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):  d / dt  k0
  0  k0t
Isothermal desorption of 2-D gas of Ag in equilibrium
with 3 different 2-D solid phases
kn  kn0 exp(Edes / kT )
Edes from Arrhenius plot
1st-order (n = 1) Isothermal Desorption
 d / dt  k1  k10 exp(Edes / kT )
  0 exp(k1t )
0
1
k
: 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:
 d / dt  k2 2  k20 2 exp(Edes / kT )
(in Homework 8)
Temperature Programmed Desorption (TPD)
Analyze bonding and reaction properties of adsorbed species
Mass
spectrometer
Linear T ramping: T(t) = T0 + t
rdes
•
Sample
Programmed
heating

Edes 
d
0 n

 kn exp

dt
k
(
T


t
)
0


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
0th-order TPD
rdes

Edes 
d
0

 kn exp

dt
k
(
T


t
)
0


TPD
n=0
Peak is reached right be before all
adsorbates have desorbed
First-order TPD
rdes

Edes 
d
0

 k1  exp

dt
k
(
T


t
)
0


Peak at:
kT
2
m

TPD
n=1
drdes
d 2
 2 0
dt
dt
E des
k10
 E des
exp 
 kT m



In 1st-order TPD, Tm is independent of 0
Edes from 1st-order TPD
 k10Tm
E 
Edes  kTm  ln
 ln des 

kTm 

 k10Tm

 kTm  ln
 3.64



2nd-order TPD
rdes

Edes 
d
0 2

 k 2  exp

dt
k
(
T


t
)
0


2k  m 
0
2
Edes
kTm
 Edes 

exp
 kTm 
Tm decreases as 0 increases
Spectra are more symmetric
TPD
n=1
k10 ~ 1013 s-1
m
TPD
n=2
TPD spectra show a combination of a few kinetic models
Inhomogeneous substrate
Multilayer desorption
0th-order followed by 1st-order
Adsorption Isotherm
The coverage  on a surface in equilibrium with a gas
ambient of pressure p satisfies rads  rdes, or:
1 f * ( )
p
K f ( )
with
K
 exp(Eads / kT )
 * 2m kT
In first-order Langmuir adsorption system
f ( )  1  
& f * ( )  
Kp
 ( p) 
1  Kp
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)
Flying away
direction
At low 
e
0.5<<1
H
H
O
0.2 <  <1
H+ ESDIAD from Ru(0001)
Adsorption Induced Work Function Variation
Dipole moment p = qd : intrinsic & induced
  
en dip p
0
In-plane dipole has no effect
Cs-Induced Work Function Variation
Cs: large ion size, e-donor
Dipole-dipole interaction
introduces a depolarization
factor:
f dep 
9n
3/ 2
dip
4 0
 = polarizability
Cs adsorption on Semiconductor
    eVs
With:  
  '
Vs  Vs  Vs '
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
Depletion of
LDOS at EF
0
6L
100 L
1000 L
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
O2, H2 and N2 may easily dissociate
when adsorbed on some surfaces
Basis of Heterogeneous
Catalysis: Chemical reaction via
adsorption-dissociation-reactiondesorption path often only
encounters moderate barriers
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
LDOS at EF and surface reactivity are closely correlated
E
Noble metal EF
Transition
metal EF
sp-band
d-band
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
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
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