adsorption at, desorption from, and chemical activity of ... · adsorption at, desorption from, and...
TRANSCRIPT
Adsorption at, Desorption from,and Chemical Activity of Metal
Surfaces
C. StampflFritz-Haber-Institut der Max-Planck-Gesellschaft,
Berlin-Dahlem, Germany
Northwestern University, Evanston Illinois USA
Outline
• Fundamental concepts: adparticles atsurfaces
• Adsorption of atoms and molecules at metalsurfaces: trends in dependence on site andsubstrate
• Surface chemical reactions: e.g. CO oxidation
• Combined ab initio and statistical mechanicalschemes to bridge micro and macroscopicscales: The way to go!
• macroscopic regimes
Surface energy barrier
0
-10
-20
-30
-40
0 2 4 6 8 10∋
F
Ene
rgy
(eV
)
Position perpendicular to surface (Å)
V V
Φ
esVxc ffe
cf. K. Reuter
O/Ru(0001)
Veff effective Kohn-Sham potential
Ves electrostatic potential
Vex exchange-correlation potential
Classical image effect
Veff(z)= -1 (e-)2
4πε0 4(z-z0)(From Scheffler and Stampfl In: Handbook of Surface Science, Vol. 2: Electronic Structure, 2000)
Adsorbate-substrate interaction
∆N2s
∆N1s
N0( )
a)
b)
c)
d)
1s 2s
2s derived∆DOS
1s derived∆DOS
energy levelsof free atom
DOS ofsubstratesurface(the choice ofεF refers to Pd)
bonding
bonding antibonding
d
sεd εF
antibonding
The example: H on Pd
2s
2s derivedbonding
2s derivedantibonding
1s derivedbonding
1s derivedantibonding
~
1s~
εd
(From Scheffler and Stampfl In: Handbook of Surface Science, Vol. 2: Electronic Structure, 2000)
Adsorbate-induced DOS versus distance
vacuum level adatom-induced DOS
width of O 2pderived resonance
εF
ΦRu(0001) = 5.4 eV
~ ~~ ~
vacuum level adatom-induced DOS
width of Na 3sderived resonance
εF
ΦAl(100) = 4.4 eV
Al Na Ru O
Na at Al(001) O at Ru(0001)
Distance dependence of the broadening and shift of the adsorbate energy level(Na 3s and O 2p)
For Na/Al partial charge transfer from adsorbate to substrateFor O/Ru partial charge transfer from substrate to adsorbate
(From Scheffler and Stampfl In: Handbook of Surface Science, Vol. 2: Electronic Structure, 2000)
DFT-LDA eigenvalues versus occupation
Ik=EN-1-EN=∫N dEN’/dN’ dN’ = - ∫0εk(fk) dfk≈ -εk (fk=0.5)
cf. M. Fuchs
The Na 3s-state as a function of the number of valence electrons
f3s=0 is the Na+ ion
f3s=1 is the neutral atom
f3s=2 is the Na- ion
For f3s=0.5 the eigenvaluegives the ionization energy(5.2 eV; expt. 5.14 eV)
(From Scheffler and Stampfl In: Handbook of Surface Science, Vol. 2: Electronic Structure, 2000)
For f3s=1.5 the eigenvaluegives the electron affinity (0.7 eV; expt. 0.55 eV)
� �
� ��� ���� � ���
����
����
����
���
����
�
number of valence electrons
� eV�
� �
1N-1
-1.500e-01
-1.000e-01
-5.000e-02
-2.000e-02
-1.000e-02
-5.000e-03
5.000e-03
1.000e-02
3.000e-02
5.000e-02
O/Ru(0001): electronegative adsorbate: chargetransfer to O
Na/Al(001): electropositive adsorbate: chargetransfer from Na
CO/Ru(0001):donation/back-donationBlyholder model
-1.5000e-02
-1.0000e-02
-5.0000e-03
-2.0000e-03
-1.0000e-03
-5.0000e-04
5.0000e-04
1.0000e-03
3.0000e-03
5.0000e-03
difference density: n∆(r) = n(r) - n0(r) - nad(r)
(MS & CS; Handbook (2000))(C. Stampfl et al. PRB (1998))
Bonding at surfaces: O, Na, CO
Binding energy at kink sites
1
2
345
6 7
The total energy of a system can be written as:
E=ΣI=1EIEI is the energy contribution due to atom I.
Eadkink= - (E1(6)+ E2(7)+ E3(8)+ E4(10)+ E5(12)+ E6(11)+ E7(9))
+ E1(0)+ E2(6)+ E3(7)+ E4(9)+ E5(11)+ E6(10)+ E7(8) = - E5(12)+ E1(0) The cohesive energy
Eadkink= - E after+ E before
N
EI can be expressed as a function ofcoordination of atom I fi ``bond-cutting model’’e.g. EI C
For adsorption of an atom from thegas phase at a kink site, the energy gain is:
(From Scheffler and StampflIn: Handbook of Surface Science,, 2000)
Why are processes at surfaceimportant?
• A surface is in contact with molecules from the gas or
liquid phase bonds may break and new ones formand chemical reactions take place
• Technological reasons:Heterogeneous catalysis, e.g., reactivity, role of promotersand poisonsCorrosion, e.g., protective coatingsElectronic and magnetic devices, e.g., growth, stability
• Fundamental reasons
For understanding need to first study simple processes thenmore complicated - compare for different but similar systems
Trends in adsorption energies fordifferent substrates
−4.0 −2.0 0.0 2.0d−band center, εd (eV)
−8.0
−6.0
−4.0
−2.0
0.0
Simple ModelDFT−GGAExp. (polycryst.)
Zr Nb Mo Tc Ru Rh Pd Ag−8.0
−6.0
−4.0
−2.0
0.0
O C
hem
isor
ptio
n po
tent
ial e
nerg
y re
l. to
1 − 2
O2
(eV
)
(Hammer and Norskov, Adv. Catal. 45 (2000) 71)
O adsorption on 4d transition metals
From left to right in thePeriodic Table, the weakerthe O-metal bond becomes.
This is due to lower lying(and more occupied) d-bandswhich causes antibondingO-metal states to become occupied.
Ag
Zr
Trends in adsorption energy fordifferently coordinated sites
−3.0 −2.5 −2.0Pt−εd (eV)
−2.0
−1.5
−1.0
CO
che
mis
orpt
ion
pote
ntia
l ene
rgy
(eV
)
−3.0 −2.5Cu−εd (eV)
0.0
0.5
1.0
H2 dissociation energy barrier (eV
)
Pt(100)−hex, low site
Pt(11 8 5), terrace
Pt(100)−hex, high site
Pt(111)Pt(100)
Pt(11 8 5), step
Pt(11 8 5), kink
Cu(111)
Cu(100)
(Hammer and Norskov, Adv. Catal. 45 (2000) 71)
CO on Pt
Correlation of CO adsorption energywith d-band center foradsorption in the topsite on Pt atoms withdifferent coordinations
Strongest bindingfor highest d-bandcenter
Coadsorption
0 2 4 6O neighbors
−1.1
−0.9
−0.7
−0.5
−0.3
−0.1
∆Ead
(eV
)
1O +1COh h
2O +1COh h
3O +1COhh
CO in the hcp site
0 2 4 6O neighbors
−0.11
−0.09
−0.07
−0.05
−0.03
−0.01
1O +1COh t2O +1COh t
CO in the top site
The CO adsorption energy decreases with increasingnumbers of O atoms that bond to the same Ru atom
Can remain practically unchangedor even exhibit a slight increase due to O-induced lateral weakeningof Ru-Ru bonds for non-O-bonded Ru atoms
CO+O on Ru(0001)
(Stampfl and Scheffler, to be published)
1O +1COh t 2O +1COh,f t 1O +2COh t,f 3O +1COh h
0.0 2.0 4.0 6.0ZC(A)
o
−1.5
−1.0
−0.5
0.0
0.5
1.0
1.5
Ene
rgy
(eV
) ∆Φ
∆Etot
0.0 2.0 4.0 6.0ZC(A)
o0.0 2.0 4.0 6.0
ZC(A)o
0.0 2.0 4.0 6.0ZC(A)
o
Coadsorption:barriers to adsorption
Pre-adsorbed O atoms can induce activation barriers to adsorptionof CO (in the hcp site) well above the surface
(Stampfl and Scheffler, to be published)
-0.5 0.0 0.5 1.01.0
1.5
2.0
2.5
4.8
3.6 1.2
1.2
0.0
2.4
1.2
1.2 -1.2
Z (A)Oo
ZC(A
)o
ZZO
C
Surface chemical reactions:CO oxidation - Eley-Rideal mechanism
Reaction coordinate
3.0
2.0
1.0
0.0
Pot
entia
l ene
rgy
(eV
)
CO(g)+ O (g)
O(a)+CO(g)
CO (g)
1.20
1.95
0.04
0.04
1-2 2
2
(Stampfl and Scheffler, PRL, 1997; Surf. Sci. (1999))
CO incidentfrom gas phase
CO2 leavessurface withlarge energygain
12 34
Reaction coordinate
3.0
2.0
1.0
0.0
Pot
entia
l ene
rgy
(eV
)
CO(g)+ O (g)
O(a)+CO(a)
O(a)+CO(g) CO (g)
21-2
2
0.66
1.64
0.85
0.30
CO oxidation -Langmuir-Hinshelwood mechanism
(Stampfl and Scheffler, PRL, 1997; Surf. Sci. (1999))
Atomic geometryalong minimumenergy pathway
CO and Oboth adsorbon the surfaceprior to reaction
Total electron density alongreaction pathway for CO oxidation
ORu
OC
(From Scheffler and Stampfl In: Handbook of Surface Science, Vol. 2: Electronic Structure, 2000)
Difference electron density alongreaction pathway
-7.50e-02
-6.00e-02
-4.50e-02
-3.00e-02
-1.80e-02
-3.00e-03
3.00e-03
3.30e-02
6.30e-02
9.30e-02
-3.30e-02
-2.70e-02
-2.10e-02
-1.50e-02
-9.00e-03
-3.00e-03
3.00e-03
9.00e-03
1.50e-02
-3.300e-02
-2.700e-02
-2.100e-02
-1.500e-02
-9.000e-03
-3.000e-03
3.000e-03
9.000e-03
1.500e-02
-3.30e-02
-2.70e-02
-2.10e-02
-1.50e-02
-9.00e-03
-3.00e-03
3.00e-03
9.00e-03
1.50e-02
2.0e-02
4.0e-02
6.0e-02
8.0e-02
1.0e-01
2.0e-01
3.0e-01
4.0e-01
2.0e-02
4.0e-02
6.0e-02
8.0e-02
1.0e-01
2.0e-01
3.0e-01
4.0e-01
2.0e-02
4.0e-02
6.0e-02
8.0e-02
1.0e-01
2.0e-01
3.0e-01
4.0e-01
2.0e-02
4.0e-02
6.0e-02
8.0e-02
1.0e-01
2.0e-01
3.0e-01
4.0e-01
(Stampfl and Scheffler, Surf. Sci. (1999))
Activation energy barriers:Trends CO oxidation
Pt Rh Ru−1
0
1
6
7
8
Ene
rgy
(eV
)
ECO,O
ads
Ebarrier
∆Ο
∆CO
Eint
(Z.-P. Liu and P. Hu, J. Chem Phys. 2001)
Energy barrier increasesfrom Pt to Rh to Ru
Correlates with totalchemisorption energy of O and CO
Significant contributionto barrier due to weakeningof O-metal bondBut also weakening of CO-metalbond important
Ru
O O RuO
RuO2
2 2 x
Processes relevant toheterogeneous catalysis
•Adsorption (often dissociatively) on a solid surface from the gas phase.•Diffusion on the surface, diffusion of adsorbates into the subsurface region and/or substrate atoms through a surface oxide layer.•Interactions between adparticles and chemical reaction.•Desorption of reaction products
cf. K. Reuter(C. Stampfl, M.V. Ganduglia-Pirovano, K. Reuter,
and M. Scheffler, Surf. Sci. 500 (2001), in press.)
Combined ab initio and statisticalmechanical approach
Increasingly the importance and role of the atmospheric environment, i.e. species and gas pressure, as well as the temperature, in trying to understand various processes (e.g catalytic reactions), are being appreciated
Processes at surfaces, and surfaces themselves, can be verydifferent under standard UHV conditions compared to under more ``realistic conditions’’ ``Pressure-gap’’
For detailed understanding of the complex behavior of atoms andmolecules at surfaces, require knowledge of both microscopic andmacroscopic processes predictive theoretical description that can link these differentscales, and take temperature and pressure into account
CombiningMicroscopic + Macroscopic Theories
Electronic StructureDFT
Potential Energy Surface
Stable and metastable structures
Interaction parametersVibrational frequencies
Lattice-gas Hamiltonian
Partition functionRate equations
Realistic description Kinetics and Thermodynamics
Lattice gas Hamiltonian
H = Es Σi ni + 1/2(V1n Σi,a ni ni+a + V2n Σi,b ni ni+b
+ V3n Σi,c ni ni+c + VtrioΣi,a,a’ ni ni+a ni+a’ + …)
Stampfl et al., PRL 83, 2993 (1999).
VbtV1n
V2nV3n
V1nh-f
V3nh-f V2n
h-f Vlt
Vtt
Pair and trio inter-atomic interactions
Schematic illustrationof the types of two- andthree-body interactionsincluded in the lattice-gas Hamiltonian
Namely, first, second,and third two-body interactionsfor adsorbates in fcc andhcp site, as well as in combined fcc-hcp sites;three types of three-body(or trio) interactions
Kinetically hindered regionStampfl et al.PRL 77(1996) PRB 53 (1996)
Adsorption energies of O on Ru(0001)
Site E θ=1/9 E θ=2/9 E θ=1/4 E θ=1/3 E θ=1/2 E θ=2/3 E θ=3/4 Eθ=1
hcp 2.503 2.417 2.577 2.370 2.307 2.150 2.091 1.895
fcc 2.152 2.107 2.145 2.105 2.025 2.015 1.942 1.865
Site E θ=2/9 E θ=1/2 E θ=5/4
hcp-fcc 2.294 2.209 1.492
Extracting Interaction Parameters
Eθ=2/9=V0+1/2(V1n+V3n)Eθ=1/4=V0+3V3n
Eθ=1/3=V0+3V2n
Eθ=1/2=V0+V1n+V2n+3V3n +Vlt Eθ=2/3=V0+3/2V1n+3V2n +3/2V3n+3Vbt
Eθ=3/4=V0+2V1n+2V2n+3V3n+2Vlt+2Vbt+2/3Vtt Eθ=1.0=V0+3(V1n+V2n+V3n)+3Vlt+6Vbt+2Vtt
Site V1n V2n V3n Vlt Vbt Vtt
hcp 0.265 0.044 -0.025 -0.039 -0.046 0.058
fcc 0.158 0.016 0.002 -0.052 -0.044 0.076
hcp-fcc 0.586 0.101 0.033
V3n
V3n
V1n
Extracting the interaction energies
Example:Obtaining an expressioninvolving the first andthird two-body interactionsat coverage 2/9.The adsorption energyis written as:
Eθ=2/9=V0+1/2(V1n+ V3n)
There is one first neighborinteraction in the 3x3 unitcell, V1n , and due to imagesin neighboring cells, thereare interactions at thirdneighbor distances, V3n
Rate equations: adsorption, desorption
dθ/ dt =Rad-Rdes Kinetic equation
Rate of adsorption:Rad=2Sdis(θ,T)Pmaλm/h
Atomic adsorbate in contact with a gas of homonuclear molecules
Rate of desorption: Rdes=2Sdis(θ,T)a kBTZvrq3
2/(hλm2q3
2(1-q)2) *exp{-(2V0-De
(g))/kBT}*exp{2µ(lat)/kBT}
Pm is the molecular pressure, a area of unit cell, λm=h/(2πmkBT)1/2 thethermal wavelength of molecule,Sdis is the dissociative sticking coefficient,
Zvr partition function: vibrations and rotations of molecule, De(g)dissociation
energy and µ(lat) chemical potential
Heating rate: 6 K/sStampfl et al.,PRL 83 (1999)
Temperature ProgrammedDesorption Spectra: O/Ru(0001)
Theory ExperimentO
2 -d
esor
ptio
n ra
te (
ML/
s)
temperature (K)
O2
-des
orpt
ion
rate
(ar
b.u.
)
0.0 0.2 0.4 0.6 0.8 1.0Coverage (θ)
2.0
3.0
4.0
5.0
6.0
Hea
t of A
dsor
ptio
n (e
V)
T=700 KT=1100 KT=1500 K
Ead=2EO-metal+EO2-gas
Peaks at low T predict stability of ordered structures at Θ=0.25,0.5,0.75,1.0(2x2)-O, (2x1)-O,(2x2)-3O,(1x1)-O
Effect of T greater forhigher O coverages withweaker adsorption energy
Heat of Adsorption
Summary
Hybrid approach, combining density-functionaltheory calculations with phenomenological methods, wherevarious thermodynamic properties of adsorbates at surfaceswere studied.
The methodology and concepts are general and can beapplied to various problems
Describes evolution of surface structures as a function of temperature, pressure, and coverage. Can treat disordered andordered, systems, as well as different species
Demonstrated through the example of oxygen at Ru(0001),reproducing experimental Temperature Programmed DesorptionSpectra and heat of adsorption.