Adsorption – some concepts

Adsorbent,Substrate

Absorption- uptake into the bulk

Sorption

AdsorbateAdsorptive

Adsorption

Adsorption

• Adsorption reduces the free energy caused by unbalanced attractive forces at the interface.

• ∆Hads < 0 for all adsorption onto solids in gas phase.

• Physical forces bind the adsorbed material to the surface, sometimes this turns into a chemical bond.

• Small variations in the interaction forces may have great impact on the fate of adsorbed molecules (e.g. dissociation, partial desorption…)

• Adsorption phenomena are the base of colloidal processes and catalysis, and have found practical use in separation and purification processes, drying of gases, pumping of vacuum systems, etc. etc.

Physical and chemical adsorption

Physical adsorptionPhysisorption

• Non-specific bindning to the surface, no directed bond.

• Often weak bonds (dispersive, ∆Hads

~ 20 kJ/mol), but can correspond to the strength of a covalent bond if Coulombinteraction is involved.

• Always present upon adsorption, often precedes chemisorption, long range interaction.

• Multilayers and condensates in pores or capillaries are always physisorbed.

• Small effect on the adsorbed moelcule.

Chemical adsorptionChemisorption

• Chemical bond, directed (angle-dependent), ∆Hads ~ 200-800 kJ/mol.

• Governed by short-ranged forces; 1-2 Å.

• Often slow and/or irreversible.

• Only monolayers can be chemisorbed.

• Strong influence on the structure of the adsorbed molecule. Implies electron exchange between adsorbate and substrate.

Example:

Adsorption of

a diatomic

molecule

Potential fortwo separateatomsapproachingthe surface.

Potential for a diatomic molecule approaching the surface.

Physisorption typically resultsin a lowering of the activationenergy for chemisorption!

The fate of the molecule isdetermined by the shape ofthe two potential curves, andin particular where they crosseach other.

An AB-molecule adsorbs unaffected;only physical adsorption. Dissociation (and chemisorption) is thermallly avtivated, with activation energy Ea.

Ea

After physical adsorption there is no barrier preventing chemisorption, but the molecule will not dissociate, so the whole molecule is chemically bonded to the surface.

Molecular

physisorption

Molecular

chemisorption

Dissociative

chemisorption

At large distance from the surface, the molecule is stable, but after physisoprtion there is no barrier against dissociation, and the atoms A and B are each individually chemically bonded to the surface.

Temperature

dependence of the

adsorption

Physical adsorption

No physical adsorption,but slow chemisorption.

Chemical adsorption

Scattering ”Trapping” ”Sticking”

Ek Ek’

Elastic Ek = Ek’

Inelastic Ek’ < Ek

Ek Ek’ Ek Ek’

EkEk’ << Ek’ = 0

Collisions with solid surfaces

“Trapping”:• Energy is transfered to the surface via excitation of lattice vibrations (phonons).

The capacity of the surface to absorb energy determines the trapping.• Trapping decreases with increasing temperature, since the surface must then

absorb more kinetic energy per trapping event.

S(0) or S0 S(Θ)

”Sticking”

Number of molecules sticking to the surface

Number of molecules hitting the surfaceDefinition: S =

• S normally depends on the coverage; S(Θ). S(0) is the initial “sticking”.

• Is determined by the surface’s capacity to absorb energy, i.e. the trapping,and its capacity to form a surface chemical bond. “Trapping” might thus be considered a necessary precursor for sticking.

• “Sticking” increases or remains constant as the temperature is increased.

• By comparing the temperature dependence of “trapping” and “sticking” therate determining process can be determined.

T = To +βt

β [K/s]

Add heat

Bonds arebroken

Collect desorbedmolecules or fragmentsin a mass spectrometer

m/e

”TPD trace”Sample

Number/s

T

Tp1

Tp2

Desorption enthalpy ∆Hdes

correlates with Tp

Vakuum

Temperature programmed desorption

(TPD)

Physisorbed multilayer monolayer ”on top” monolayer ”bridging”

Number/s

T

Peak area ∝coverage

Tp ∝ ∆Hdes

Information content in TPD

Molecules or atoms in different binding sites have different binding energies,and desorb at different temperatures.

Example: N on W(001)

Deposited monolayer equivalents (ML)

Saturation

4-fold hollow

Step

From atoms adsorbed todefects (steps) on the crystal.

N N

N N

N

Multilayers90 ºC

Monolayers,”end-on” ”side-on”170 ºC

N N

N N

N

N N

N N

N

N N

N N

N

Multilayer

Monolayer

Desorption of Adenine from Gold

Östblom et al., J. Phys. Chem. B 2005, 109, 15150-15160

Initially disordered adsorption to random

sites

Equilibrium structure

Mobility at surfaces

Thermally activateddiffusion

( )

0( , )ACTE

RTD T D e

Θ−Θ =

Diffusioncoefficient [cm/s2]

Diffusion constant [cm/s2]

Activation energy for diffusion [kJ/mol]

The diffusion equation:

Random walk-motion and diffusion

z = no. of nearest neighbours2 in 1D; 4 on a square lattice

6 on a hexagonal lattice

l = hop lengthν0 = hop frequency(ν0t = number of hops!)D = diffusion coefficient

Experimental determination of D0 and EACT

Plot lnD versus 1/T :

lx

”Random walk”: Adatoms hop between surface sites uponthermal excitation.

( ) /0( , ) ACTE RT

D T D e− ΘΘ =

2 20x l tν=

2 20

x lD

zt z

ν= =

2

0

1ln ln ACT

x ED

t R T= −

z

y = m - k x

Transport mechanisms

Xiao, Phys. Rev. B, 70, 033402 (2004) Se även Kellogg, Phys. Rev. Lett., 64, 3143 (1990)

Hopping

Tunneling

Ex: Hydrogen on Cu(100)T > 60 K: Arrhenius law, n ~ 1013 /s, EACT = 0.2 eVT < 60 K: Tunnelling, T-independent diffusionLauhon PRL 85, 4566 (2000)

Exchange

Vacancy diffusion

Vacancies on Ge(111)-c(2x8)Brihuega, PRB 70, 165410 (2004)

(110)

Increasingdiffusion rate

Increasingroughness

Features of lattice planes

(111)(100)

The activation energy depends onthe crystallographic direction!(orientational anisotropy)

EACT (100) > EACT (110) > EACT (111)

Steps on the surface actsas potential minima! Self-diffusion on Rhodium. On (110), (311) and (331), the

diffusion is one-dimensional along [110] ; on (111) and (100), two-dimensional. (Ayrault, J. Chem. Phys. 60, 281 (1974))

(Diff

usio

n ra

te)

Adsorption enthalpy and

activation energy

Potential energy

∆EACT

∆Hads

∆EACT

∆Hads

∆Hads

∆EACT

∆EACT

r

1 2 3

1→2

2←3

1←2 2→3

Adsorption enthalpy ∆Hads versus activation energy for diffusion ∆EACT

∆Hads ≠∆EACT

Effects of lateral

surface interactions

as

a0

a0

as

Commensurate overlayer: a0= nas

n = integer

Non-commensurate overlayer: a0 ≠ nas

Interactions between adsorbates:Electrostaticvan der Waals forcesElectron exchange

Impact ondiffusion:

T = 0

T > 0

Domains on an ideal surfaceupon attraction:

Models for film growth

Ideal layer-by-layer growth

Stranski-Krastinov

Volmer-Weber islandformation

Simultaneous multilayerformation

(Can be controlled via wettability, or surface free energy!)

Field ionisation microscopy (FIM)

Charged particles (He+ in FIM, e- in FEM) are accelerated from a charged metal tip towards afluorescent screen,where they providea magnified image(x106) of the atomdistribution on thetip. This is used todetermine crystalstructure.

Diffusion of a Rhenium atom onW(211) at 327 K (60 s intervalbetween pictures).

G. Ehrlich, CRC Crit. Rev. Solid.State and Matls. Sci. 10, 391 (1982).

Adsorption of oxygen onto Ni(111)

Atop

Bridge

hcp

fcc

Unit cell

Comparison between computation (density functional theory, DFT) and LEED.

Adsorption sites and theunit cell on Ni(111)

Reconstruction of the Ni surface,with adsorbed oxygen.

Yamagishi, Surface Science 543,12–18 (2003)

(cont.) oxygen on Ni(111)

O2 adsorbs dissociatively on Ni(111) and forms anoverlayer structure p(√3 × √3)R30°, with adsorption in “fcc hollow sites”

Yamagishi, Surface Science 543,12–18 (2003)

1 N2(g) + * N2*

2 N2* + * 2N*

3 N* + H* NH* + *

4 NH* + H* NH2* + *

5 NH2* + H* NH3* + *

6 NH3* NH3(g) + *

7 H2(g) + 2* 2H*

Fe ~400 °C

A2

X2

A2

X2 X

AAX

AX

Catalyst at high temperatures

Reactants Product

N2 (g) + 3H2 (g) 2NH3 (g)

Synthesis of ammonia

Fe

Adsorption-splitting-diffusion-reaction-desorption

Heterogeneous catalysis – revisited!Synthesis of ammonia on Fe – crystal

plane effects

Reaction rates on different crystal planes

Strongin, J. Catal., 103, 129 (1982)

TPD-diagram afterammonia synthesis;β1 and β2 peaksoriginate from C7 sites.

7-coordinated sites

Synthesis of ammonia on Fe – surface

modification

The activity is increasedby surface reconstructionand pre-treatment withwater.

For e.g. Fe(110)this results in a rateincrease of about400 times!

Synthesis of ammonia on Fe – additives

TPD-diagram for desorption of ammonia from clean Fe(111) and Fe(111) with adsorbed potassium: Potassium lowers the adsorption energy of ammonia! Variation in the ”sticking” coefficient for N2 at

various K additions to Fe(100) at 430 K.

Surface concentration of atomic nitrogen vs. N2 exposure for some crystal planes.

To be

continued...