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