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Adsorption at, Desorption from, and Chemical Activity of Metal Surfaces C. Stampfl Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin-Dahlem, Germany Northwestern University, Evanston Illinois USA

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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.