Specific Adsorption
Transcript of Specific Adsorption
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= ⋅= ⋅= ⋅= ⋅