adsorption –some concepts adsorption - linköping university · • adsorption phenomena are the...
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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...