Hongqing Shi and Catherine Stampfl

1
Investigation of the Role of Surface Oxides in
Catalysis by Gold

• Hongqing Shi and Catherine Stampfl
• School of Physics, The University of Sydney,
  Sydney, Australia

2
Introduction
Efficient Gold-based catalysts for oxidation
reactions e.g.

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

M. Haruta, Catal. Today, 36, 153 (1997).
M. Valden et al. Sci. 281, 1647 (1998).
Structure-gap, materials-gap, water-gap
Pressure-gap, temperature-gap

• 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.

3
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).
4
Oxygen adsorption and thin surface-oxides
Au(111)2x2-O fcc
octa
Au(111)2x2-O hcp
tetra II
tetra I
(4x4)-oxide
Ofcc/Otetra-I
vacancy structure
lower O
upper O
5
12 thin surface-oxides
a
b
c
d
g
h
f
e
lower O
upper O
i
j
k
l
Schnadt et al. Phys. Rev. Lett. 96, 146101
(2006) Michaelides et al. J. Vac. Sci. Technol.
A 23, 1487(2005). (4x4)-O/Ag(111)
6
Surface oxide structures (4x4)
(4x4)-oxide
(4x4)-oxide
lower O
upper
upper O
lower
s
d
s
p
5d
7
Ab initio atomistic thermodynamics

• Two chemical reservoirs are used
• Chemical potential of oxygen, µO from ideal gas,
  O2
• Chemical potential of metal, µM from bulk metal, M

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
8
Ab initio surface phase diagram
(4x4)-oxide

• For atmospheric pressure and temperature lt420 K,
• thin oxide-like structures are stable
• For atmospheric pressure, Tgt420 K, no stable
  species
• Could thin Au-oxide-like structures play a
  role in the low temperature catalytic reactions?

9
Reactivity of surface oxide for CO oxidation

• Nudged Elastic Band (NEB) method 1

lower O

• Two oxidation reaction paths
• CO reacts with upper oxygen to form CO2
• CO reacts with lower oxygen to form CO2

upper O

• 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
10
Initial and final states

• CO on (4x4)-oxide

CO adsorption energy (eV) 0.37
C-O bond-length (Å) 1.14
C-Au bond-length (Å) 2.05
CO2 on (4x4)-oxide (CO reacts with lower O)

• 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

11
The Minimum Energy Path (MEP)
COOlower ?CO2 pathway
Reaction energy barrier 0.82 eV TS state C-O
1.18 Å, C-Olower 1.51 Å
12
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

13
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)

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Convergence tests Oxygen molecule
16
Convergence tests Oxygen adsorption
17
Convergence tests Oxygen adsorption
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Convergence tests CO molecule
19
Convergence tests CO molecule
20
Convergence tests CO molecule
21
Convergence tests
VASP
1.23
-3.14
1558
22
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
23
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 lt 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.
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Surface in contact with oxygen gas phase

• Two chemical reservoirs are used
• Chemical potential of oxygen, µO from ideal gas,
  O2
• Chemical potential of metal, µM from bulk metal, M

Neglecting FVIB and FCONF for the moment,
By defining ,
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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).