Hongqing Shi and Catherine Stampfl

School of Physics, The University of Sydney, Sydney, Australia

Investigation of the Role of Surface Oxides in Catalysis by Gold

• UHV results often thought to be transferable to “real” high temperature, high-presure catalysis

• Dynamic environment + labile surface morphology at corresponding partial temperature and presure need to be included.

• Nanometric-size gold particles act as catalysts at or below room temperature

M. Valden et al. Sci. 281, 1647 (1998).

“Pressure-gap, temperature-gap”

Efficient Gold-based catalysts for oxidation reactions: e.g.

;

M. Haruta, Catal. Today, 36, 153 (1997).

“Structure-gap, materials-gap, water-gap”

22 CO1/2O CO

Introduction

322 SO1/2OSO

Calculation method First step: to investigate chemisorption of oxygen on Au(111) and the stability of surface

oxides, taking into account the effect of pressure and temperature

Density-Functional Theory (DFT)• The pseudopotential and plane-wave method

VASP [1,2]• Projector augmented-wave method (PAW)• Generalized gradient approximation (GGA) for the

exchange-correlation functional • Full atomic relaxation of top three Au layers and O

atoms with 5 layers slab, vacuum region of 15 Å• Equivalent k-point sampling, 21 k-points in (1x1) IBZ• Energy cutoff of 36.75 Ry (500 eV)

[1] G. Kresse et al., PRB 47, 558 (1993); 49, 14251 (1994);54, 11169 (1996); 59, 1758 (1999).[2] G. Kresse and J. Furthmüller, Comput. Mater. Sci. 6, 15 (1996).[3] P. E. Blöchl, PRB 50, 17953 (1994).

Oxygen adsorption and thin surface-oxides

tetra IItetra I octa

Ofcc/Otetra-I vacancy structure

(4x4)-oxide

Au(111)2x2-O fcc Au(111)2x2-O hcp

lower O

upper O

12 thin surface-oxides

[i]

a b c d

e f g h

j k l

lower O

upper O

Schnadt et al. Phys. Rev. Lett. 96, 146101 (2006); Michaelides et al. J. Vac. Sci. Technol. A 23, 1487(2005). (4x4)-O/Ag(111)

Surface oxide structures: (4x4)

(4x4)-oxide(4x4)-oxide

sdsp

5d

lower

lower O

upper O

upper

Ab initio atomistic thermodynamics

Two chemical reservoirs are used:

1. Chemical potential of oxygen, μO from ideal gas, O2

2. Chemical potential of metal, μM from bulk metal, M

⇅

⇅BULK

SURFACE

O2 GAS

⇅

⇅

⇅

⇅BULK

SURFACE

O2 GAS

C. Stampfl, Catal. Today, 105 (2005) 17; W.X. Li, C. Stampfl and M. Scheffler, Phys. Rev. Lett. 90 (2003) 256102; K. Reuter and M. Scheffler, Phys. Rev. B, 65 (2002) 035406

CONFTOT

CONFVIBROTTRANSTOT

),(

),(

FETpG

pVFFFFETpG

OOMMMM/OO A1

)( μμμ NNGGG ΔΔ

By defining ,2O2

1OO EμμΔ

OOMMbOOO 22

1

A1

)( μμμ ΔΔΔΔ NNENG

By defining ,2O2

1OO EμμΔ

OOMMbOOO 22

1

A1

)( μμμ ΔΔΔΔ NNENG

• For atmospheric pressure and temperature <420 K, thin oxide-like structures are stable• For atmospheric pressure, T>420 K, no stable species

Could thin Au-oxide-like structures play a role in the low temperature catalytic reactions?

Ab initio surface phase diagram

(4x4)-oxide

Reactivity of surface oxide for CO oxidation

Nudged Elastic Band (NEB) method [1]

• Full atomic relaxation of top two Au layers and O atoms with 3 layers slab, vacuum region of 15 Å

• Energy cutoff of 29.40 Ry (400 eV)

[1] H. Jónsson, G. Mills, and K. W. Jacobsen, in ‘Classical Quantum Dynamics in Condensed Phase Simulations’, edited by B. J. Berne, G. Ciccotti, and D. F. Coker (World Scientific, Singpore, 1998), p. 385

Two oxidation reaction paths:

1. CO reacts with upper oxygen to form CO2

2. CO reacts with lower oxygen to form CO2

22 COO2

1CO lower O

upper O

Initial and final states

CO on (4x4)-oxide

CO2 on (4x4)-oxide (CO reacts with lower O)

CO adsorption energy (eV) 0.37

C-O bond-length (Å) 1.14

C-Au bond-length (Å) 2.05

• The C-O bond-length at CO2 is 1.18 Å

• C sits 3.05 Å and 5.48 Å higher than uppermost Au plane and the intact plane of Au(111), respectively

The Minimum Energy Path (MEP)

Reaction energy barrier:0.82 eVTS state: C-O 1.18 Å, C-Olower 1.51 Å

CO+Olower CO2 pathway

Conclusion

• Acquired the ab initio (p,T) phase diagram for O/Au(111) system

• On/Sub-surface oxygen overlayer structures unstable

• At atmospheric pressure, thin (4x4) surface oxide-like structures are stable up to 420 K

• The CO oxidation reaction with lower O is more favourable than upper O.

• Activation energy barrier relatively high, further studies into this system

Acknowledgement

We gratefully acknowledge support from:

• the Australian Research Council (ARC)

• the Australian National Supercomputing Facility (APAC)

• the Australian Centre for Advanced Computing and Communications (ac3)

Convergence tests: Oxygen molecule

Convergence tests: Oxygen adsorption

Convergence tests: Oxygen adsorption

Convergence tests: CO molecule

Convergence tests: CO molecule

Convergence tests: CO molecule

Convergence tests

VASP

1.23

-3.14

1558

Ab Initio Atomistic Thermodynamics

MOTIVATION: To bridge the “pressure” gap, ie. to include finite temperature and pressure effects.

OBJECTIVE: To use data from electronic structure theory (eg. DFT-calculated energies) to obtain appropriate thermodynamic potential functions, like the Gibbs free energy G.

ASSUMPTION: Applies “only” to systems in thermodynamic equilibrium.

C. Stampfl, Catal. Today, 105 (2005) 17; W.X. Li, C. Stampfl and M. Scheffler, Phys. Rev. Lett. 90 (2003) 256102; K. Reuter and M. Scheffler, Phys. Rev. B, 65 (2002) 035406

Computation of Gibbs free energy

G(p,T) = ETOT + FTRANS + FROT + FVIB + FCONF + pV

For condensed matter systems,

ETOT Internal energy DFT-calculated value

FTRANS Translational free energy M∝ -1 → 0

FROT Rotational free energy M∝ -1 → 0

FVIB Vibrational free energy phonon DOS

FCONF Configurational free energy “menace” of the game

pV V = V(p,T) from equation of state (minimal variation) → 0 for p < 100 atm

To simplify calculations,

We set FTRANS = FROT = zero and FVIB will be calculated by finite-differences and approximated by the Einstein model.

Hence the Gibbs free energy of a condensed matter system, G(p,T) ≈ ETOT + FCONF at low temperatures.

⇅

⇅BULK

SURFACE

O2 GAS

Surface in contact with oxygen gas phase

Two chemical reservoirs are used:

1. Chemical potential of oxygen, μO from ideal gas, O2

2. Chemical potential of metal, μM from bulk metal, M

Neglecting FVIB and FCONF for the moment,

By defining ,

MMOOMM/OA1

),( μμ NNGGTpG ΔΔ

MMOOMM/OA1

),( μμ NNEETpG ΔΔ

OOMMbOOO 22

1

A1

)( μμμ ΔΔΔΔ NNENG

2O21

OO EμμΔ

The Transition State (TS)

TS at Osub path:

• C-O 1.18 Å

• C-Osub 1.51 Å

• The angle of O-C-Osub is 123

• Osub lifted vertically from its original site by 0.2 Å

• C sits 0.71 Å above the uppermost Au atom plane.

• C sits 3.20 Å above the intact plane of Au(111).