Local reactivity of bimetallic overlayer andcluster systems

Ata Roudgar and Axel GroßChemistry Department, Simon Fraser University, Burnaby, Canada

Physik-Department T30, Technische Universitat Munchen, 85747 Garching, Germany

I. Introduction

Bimetallic surfaces are well-suited for the tailoring of the reactivity since they offer the possibilityto prepare specific surface compositions and structures. In bimetallic systems, strain as well aselectronic interaction effects modify the catalytic activity. We have tried to disentangle botheffects by performing density functional theory calculations using the VASP code (G. Kresse andJ. Furthmuller, Phys. Rev. B 54, 11169 (1996)). H and CO adsorption energies have beendetermined as a local probe of the reactivity and were analyzed in terms of the d band model

δEchem = V 2

|εd−εa|2 δεd (B. Hammer, O. H. Nielsen, J. K. Nørskov, Catal. Lett. 46, 31 (1997)).

II. Pd/Au(111) and Pd/Cu(111) overlayer: substrate andstrain effects

aCu = 3.64 A < The calculated Pd lattice constant aPd = 3.98 A < aAu = 4.18 A

H and CO adsorption energy of

Pd/Au(111)

Number of Pd overlayers on Au0 1 2 3 Pd@Au PdA

dsor

ptio

n en

ergy

Ead

s (eV

)

-2.4

-2.1

-0.6

-0.3

0.0

0.3

0.6 CO fcc hollowH fcc hollowH hcp hollowH bridgeH on-top

(a)(111)

d-band center on Pd/Au(111)

0 1 2 3 Pd@Au Pd

Number of Pd over layers

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

d-ba

nd p

ositi

on (e

V)

Surface (111)Subsurface (111)

Pd-Au interaction energy is 0.1 eV weaker than Pd-Pd therefore, both overlayer expansionand weak Pd-Au coupling lead to an effectively lower coordination of the Pd d band⇒ higherd band center and larger adsorption energies

H adsorption energies on Pd/Cu(111)

Cu Pd/Cu Pd@Cu Pd

Surface structure

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Ads

orpt

ion

ener

gy E

ads (e

V)

fcc sitehcp siteBridge siteon-top site

d-band center of Pd/Au(111)

Cu Pd/Cu Pd@Cu Pd-2.2-2.1-2.0-1.9-1.8-1.7-1.6-1.5-1.4

d-ba

nd c

ente

r (eV

)

Pd-Cu interaction energy is 0.245 eV stronger than Pd-Pd therefore, both overlayercontraction and stronger Pd-Cu coupling lead to an effectively higher coordination of the

Pd d band⇒ lower d band center and lower adsorption energies

III. Pdn cluster deposited on Au(111)

There are two opposit effects: The lower coordination of cluster atoms and reduction of Pd-Pddistances

Pd-Pd distances

2.76

2.76

2.65

2.76

2.76

2.77

(c)

(d)

(b)

(a)

NN distances: dAu = 2.95 A, dPd = 2.80 A

For Pd3 two effects almost canceled each other whilefor Pd7 one of them overcompensate the other

H and CO adsorption on Pd10 clusters

(c)

(b)

(a)

(d)

adsE =−0.449 (−0.646) eV

adsE =−0.468 (−0.559) eV

E =−0.349 (−0.189) eVads

adsE =−0.358 (−0.669) eV

H adsorption

(a)(c)

(b)(d)

E =−1.744 (−2.469) eVads

E =−2.172 (−2.429) eVads

E =−2.147 (−1.741) eVads

E =−2.003 (−2.035) eVads

CO adsorption

H and CO adsorption energies on Pd10/Au(111) (free Pd10) clusters

Adsorption energies on supported 3D clusterssignificantly reduced with respect to the overlayer system

H and CO adsorption energies

Pd Pd Overlayer

Ads

orpt

ion

ener

gy (

eV)

-2.4

-2.3

-2.2

-2.1

-0.7

-0.6

-0.5

H fccH hcpCO fccCO hcp

3 7

LDOS of Pd3 cluster

0

2

4

6

8

Pd with substrate Au

Pd without substrate

Pd With substrate Pd

0

2

4

6

0

2

4

6

LDO

S p

roje

cted

on

atom

ic o

rbita

ls

0

2

4

6

8

-4 -3 -2 -1 0 10

2

4

6

8

Pd (dxy)

Pd (dyz)

Pd (d3z^2-r^2)

Pd (dxz)

Pd (dx^2-y^2)

d band LDOS of the Pd3 cluster:d states with z component broad-ened⇒ metallic character

IV. Pd/Au(111) overlayer in the presence of the water

Water monomer and dimer on Pd/Au(111)

Emonoads = −0.316 eV, Edimer

ads = −0.427 eV/H2O

On-top site is the most favorable site forwater adsorption

EH−bond = (Edimerads − Emono

ads )× 2

⇒ EH−bond = −0.222 eV < EGasH−bond

Water adsorption energy as a function of water coverage

θ: 2/3 1 3/4 1/2 1/3 1/4Eads: ⇓ +3.135 -0.465 -0.419 -0.295 -0.308

H2O adsorption energies in eV/H2O

Possible structures for θ = 2/3

structure Eads(eV/H2O)H-down bilayer -0.536

H-down with shifted bilayer -0.512H-up bilayer -0.506

H-up with shifted bilayer -0.462half-dissociation bilayer -0.321

H2O adsorption energies in eV/H2O

The interaction between hexagonal waterring with metal is significantly small

Eads[M-H2O] = -0.12 eV/H2O

H-down bilayer for differentsubstrates

structure Eads(eV/H2O) L (A)Pd/Au -0.536 1.95

Pd/Pd@Au -0.520 1.95Pd -0.552 1.84

L=Hydrogen bond length (A)

Different water structures for θ = 2/3

(a) (b)

(d)(c)

Water structure: a) H-down bilayer, b) H-up bilayer, c)

half-dissociation bilayer, d) H-down with shifted bilayer

Lattice expansion⇒ H-bond length↑⇒ H-bond energy↓⇒ Eads↓

H and CO adsorption onPd/Au(111)

(a) (b)

(d)(c)

H and CO adsorption: a) H adsorption on fcc site, b)

H adsorption on hcp site (black circles are H

adsorbate), c) CO adsorption on-top site, d) CO

adsorption on fcc site

For CO adsorption, H-down andH-up with shifted bilayer wasused for fcc and hcp sites (the

most favorite)

H and CO adsorption energies

H adsorption energiesEads[H2O]↓ Eads[H2O]↑ Eads[Clean]

Fcc -0.661 -0.660 -0.690Hcp -0.596 -0.595 -0.655

On-top 0.155 — 0.075CO adsorption energies

Fcc -1.831 -1.894 -2.023Hcp -1.866 -1.923 -2.043

On-top -1.243 -1.317 -1.413All adsorption energies are in eV

CO adsorption energies for H-down and H-upbilayer are different⇒ relatively small

dipole-dipole interaction

H and CO adsorption energies changed by30-50 meV (5%) and 170-200 meV (10%)

respectively in the presence of water. i.e. theyare only slightly modified by the presence of

water.

V. Conclusions and publications

•We have presented DFT calculations for the adsorption of atomic H and molecular CO on thePd/Au overlayer and cluster in the presence and absence of the water.

• For Pd/Au(111) overlayer both lattice expansion as well as Pd-Au interaction lead to a higheradsorption energy.

• For Pd cluster deposited on Au(111) we found a lower adsorption energy compare toPd/Au(111) overlayer.

• For Pd/Au(111) overlayer in the presence of the water we found that the modification of Hand CO adsorption energies due to the relatively weak interaction of water are reduced by lessthan 50 meV and 200 meV respectively.

A. Roudgar and A. Gro, Local reactivity of metal overlayers: Density functional theory calculations of Pd on Au, Phys. Rev. B 67, 033409

(2003).

A. Roudgar and A. Groß, Local reactivity of thin Pd overlayers on Au single crystals, J. Electroanal. Chem. 548, 121 (2003).

A. Roudgar and A. Groß, Local reactivity of supported metal clusters: Pd on Au(111), Surf. Sci. 559, L180-L186 (2004).