CHEM465/865, 2004-3, Lecture 11-13, 4th Oct., 2004

Specific Adsorption

Objective: understanding interfacial structure at metal|solution interface

Considered several models – assumptions:

Ø Ideal metal surface, no explicit electronic structure taken into

account, uniformly distributed surface charge density, Mσσσσ ,

controlled by electrode potential E

Ø Ions in solution: characterized by magnitude of charges and possibly

their radii (Stern, Grahame models), solvation shells, partial or

complete desolvation, besides that: ignore their chemical identities

⇒ Non-specifically adsorbed

⇒ So far, only long-range electrostatic effects as origin for charge

accumulation/depletion in space charge region

Experimental observation: as the electrode potential becomes more

positive – favouring accumulation of negative charges in its vicinity! –

chemical identity of ions becomes important!

Example: Electrocapillary curves of surface tension vs potential for Hg in

contact with solutions of indicated electrolytes at 18°C [from D.C. Grahame,

Chem. Rev. 41, 441, 1947. ]

At negative potentials, zE E<<<< : surface tension on the metal is independent

of composition of the electrolyte – results are in line with prediction of

Gouy-Chapman and Stern models � no specific adsorption.

At positive potentials, zE E>>>> : behaviour depends specifically on

composition, major effect due to anion excess � specific adsorption of

anions on mercury, anions are tightly bound due to strong interactions

Potential profiles in interfacial zone in presence of specific adsorption for

Hg in contact with NaCl (Cl-, Br-, I- specifically adsorb on Hg, F- does not)

[from D.C. Grahame, Chem. Rev. 41, 441, 1947.]

Specific adsorption of anions

at positive potentials induces

an excess of cations in the

diffuse layer!

What happens upon

increasing the electrolyte

concentration?

Ø More adsorption � shift

to more negative

potentials at inner

Helmholtz plane!

Ø PZC shifts to more

negative values

Adsorption on metal electrodes

Concentration of species at interface larger than accounted for by

electrostatic interactions

� specific adsorption

most important quantity: binding or adsorption energy

Ø Chemical interactions between adsorbate and electrode

� chemisorption binding energies > 0.5 eV

Ø Weaker physical interactions

� physisorption binding energies < 0.5 eV

Adsorption involves partial desolvation

Cations (smaller radius) � firmer solvation sheath than anions

� less likely to be adsorbed

Amount of adsorbed species: coverage θθθθ – fraction of surface sites

(adsorption sites) covered with adsorbate

� number of adsorbed species

number of surface atoms of the substrateθθθθ ====

Nowadays: most electrochemical studies are carried out with well-defined

single-crystal solid surfaces of metals or semiconductors

chemisorption: distinct positions possible – depending on crystallographic

structure of the surface

Experimental probes of adsorption phenomena:

Ø Electrochemical methods, i.e. macroscopic probes (electrocapillarity,

cyclic voltammetry, transient measurements – chronoamperometry,

e.g. CO monolayer oxidation)

Ø Spectroscopic and microscopic methods (surface enhanced Raman

spectroscopy SERS, IR spectroscopy, scanning tunneling

microscopy)

Study specific adsorption of particular ionic species: add excess (high

concentration) of inert, non-adsorbing electrolyte → supporting electrolyte

Why supporting electrolyte? No interference of adsorption phenomena with

double layer charging effects (problem sets).

Adsorption isotherms

How does the coverage of a species A on an electrode surface vary with

concentration cA of this species in the bulk solution (all other variables are

fixed, in particular the temperature)?

Adsorption is a stochastic process between free surface sites on electrode

and species A in solution.

What are the rates/probabilities of elementary reaction events, i.e.

adsorption and desorption?

Need a theory of the kinetics of individual processes – not limited to

thermodynamic equilibrium states!

Use absolute rate theory (a.k.a. transition state theory or activated complex

theory): adsorption and desorption are activated processes – potential

energy barrier has to be crossed, borrow required potential energy from

kinetic energy of environmental degrees of freedom

Rate of adsorption proportional to

Ø Probability of (((( ))))1 θθθθ−−−− finding

free surface site

Ø Probability of having species

A near surface, cA

Ø Probability of overcoming

activation barrier

GA

G†

activated

complex

species in

solution

adsorbate

∆∆∆∆GadGad

(((( )))) Aad ad A

†

1 expG G

v K cRT

θθθθ −−−−= − −= − −= − −= − −

where †G is the molar Gibbs free energy of the activated complex

and AG is the molar Gibbs free energy of A in solution

Similar: rate of desorption

addes des

†

expG G

v KRT

θθθθ −−−−= −= −= −= −

where adG is the molar Gibbs free energy of the adsorbate

Kad, Kdes are constants (statistical mechanics, quantum theory). They

determine the time scale of both processes.

At (dynamic) equilibrium: ad des

dd

0v vt

θθθθ = − == − == − == − =

ad adad des A

des

exp1

K Gv v c

K RT

θθθθθθθθ

∆∆∆∆==== ⇒⇒⇒⇒ = −= −= −= − −−−−

where adG∆∆∆∆ is the molar Gibbs free energy of adsorption.

Several cases:

Ø adG∆∆∆∆ is independent of θθθθ , i.e. no surface heterogeneities, no

effective interactions between adsorbate molecules

� Langmuir isotherm

Ø effective interactions (mean field) � phenomenological

ad ad0

G G γθγθγθγθ∆ = ∆ +∆ = ∆ +∆ = ∆ +∆ = ∆ + � Frumkin isotherm

(((( ))))0

ad adA

des

where exp exp ,1

K Gc g

Kg

RR TT

θθθθ γγγγθθθθθθθθ

∆∆∆∆= − −= − −= − −= − − −−−− ====

g is the Frumkin interaction factor:

repulsion: 0g >>>>

attraction: 0g <<<< adsorption more facile, cooperative

Frumkin isotherms for various values of g

Dependence on potential:

The molar Gibbs energy of adsorption depends on potential, different

dependence for anions, cations and neutral species

Consider adsorption and discharge according to

zadzA e A

+ −+ −+ −+ −++++ ����

Langmuir isotherm with potential dependence of molar Gibbs free energy

of adsorption

(((( ))))0ad ad z

0G G F ϕ ϕϕ ϕϕ ϕϕ ϕ∆ = ∆ + −∆ = ∆ + −∆ = ∆ + −∆ = ∆ + −

Resulting isotherm:

(((( ))))A

z0

exp1

Fc K

RT

ϕ ϕϕ ϕϕ ϕϕ ϕθθθθθθθθ

−−−−= −= −= −= − −−−−

Simple adsorption isotherm, which should be viewed as an ideal reference

case.

Study potential dependence of adsorption reaction: potential sweep

Ø Start in region with negligible θθθθ;

Ø vary potential slowly with constant sweep rate s

dd

vt

ϕϕϕϕ====

small enough: equilibrium, no double layer charging current,

large enough: sizable current); practice: ~ few mV s-1

Ø measure resulting current.

Resulting current (with above isotherm):

(((( ))))s

d zd0 0

1F

I Q Q vt RT

θθθθ θ θθ θθ θθ θ = = − −= = − −= = − −= = − −

symmetry!

Q0 is the total charge corresponding to the adsorption of one monolayer.

Maximum current: 1/ 2θθθθ ====

Coverage at a given potential:

(((( )))) (((( ))))s

d10 0

1Q I

Q Q v

ϕϕϕϕ

ϕϕϕϕ

ϕϕϕϕθ ϕ ϕθ ϕ ϕθ ϕ ϕθ ϕ ϕ= == == == = ∫∫∫∫

The structure of single crystal surfaces

Most solids are not crystalline on their surface (restructuring, amorphous,

oxidized).

Is it academic to study crystalline surfaces? – No!

Ø Well-defined structure reproducibility

Ø Periodicity facilitates theoretical description, diffraction methods

Ø Semiconductor industry

Many metals important in electrochemistry (Au, Ag, Cu, Pt, Pd, Ir)

fcc structure (face centered cubic)

conventional unit cell, lattice constant a

fcc lattice:

Specify surface structure (cuts through certain points of a unit cell):

� bulk crystal structure + orientation of cutting plane

A particular surface plane is

defined through the components

of normal vector to that plane:

Miller indices

How are they determined ?

Ø Find intersection of

cutting plane with crystal

axes, e.g. (for the simple

cubic lattice on the right)

the components are 3,1,2

Ø Take inverse of these values, e.g. 1/3, 1/1, 1/2

Ø Use smallest possible multiplicator, e.g. 6

� Miller indices (263)

Important surface planes of fcc lattice

3a

1a

2a

(a: lattice constant)

atop

site

threefold

hollow site

bridge

site

fourfold

hollow site

Different crystal surfaces: particular sites for adsorption.

Densities of surface sites:

Pt: lattice constant a = 3.9 Å

Pt(100): density -2 cm15

2

21.3 10

a= ⋅= ⋅= ⋅= ⋅

Pt(110): -2 cm15

2

20.93 10

a= ⋅= ⋅= ⋅= ⋅

Pt(111): -2 cm15

2

41.5 10

3a= ⋅= ⋅= ⋅= ⋅