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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
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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
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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
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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:
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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).
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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
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(((( )))) 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.
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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
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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.
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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= == == == = ∫∫∫∫
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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:
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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
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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= ⋅= ⋅= ⋅= ⋅