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” Introduction

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 Ry (500 eV) [1] G. Kresse et al., PRB 47, 558 (1993); 49, (1994); 54, (1996); 59, 1758 (1999). [2] G. Kresse and J. Furthmüller, Comput. Mater. Sci. 6, 15 (1996). [3] P. E. Blöchl, PRB 50, (1994).

Oxygen adsorption and thin surface-oxides tetra IItetra I octa O fcc /O tetra-I vacancy structure (4x4)-oxide Au(111)2x2-O fcc Au(111)2x2-O hcp lower O upper O

12 thin surface-oxides [i] ab c d e f g h j k l lower O upper O Schnadt et al. Phys. Rev. Lett. 96, (2006); Michaelides et al. J. Vac. Sci. Technol. A 23, 1487(2005). (4x4)-O/Ag(111)

Surface oxide structures: (4x4) (4x4)-oxide s d s p 5d5d lower lower O upper O upper

Ab initio atomistic thermodynamics Two chemical reservoirs are used: 1.Chemical potential of oxygen, μ O from ideal gas, O 2 2.Chemical potential of metal, μ M from bulk metal, M C. Stampfl, Catal. Today, 105 (2005) 17; W.X. Li, C. Stampfl and M. Scheffler, Phys. Rev. Lett. 90 (2003) ; K. Reuter and M. Scheffler, Phys. Rev. B, 65 (2002)

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 Ry (400 eV) [1] H. J ónsson, 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 CO 2 2.CO reacts with lower oxygen to form CO 2 lower O upper O

Initial and final states CO on (4x4)-oxide CO 2 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 CO 2 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 eV TS state: C-O 1.18 Å, C-O lower 1.51 Å CO+O lower  CO 2 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: CO molecule

Convergence tests VASP

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) ; K. Reuter and M. Scheffler, Phys. Rev. B, 65 (2002)

Computation of Gibbs free energy G(p,T) = E TOT + F TRANS + F ROT + F VIB + F CONF + pV For condensed matter systems, E TOT Internal energyDFT-calculated value F TRANS Translational free energy ∝ M -1 → 0 F ROT Rotational free energy ∝ M -1 → 0 F VIB Vibrational free energyphonon DOS F CONF Configurational free energy“menace” of the game pVV = V(p,T) from equation of state (minimal variation)→ 0 for p < 100 atm To simplify calculations, We set F TRANS = F ROT = zero and F VIB 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) ≈ E TOT + F CONF at low temperatures.

⇅ ⇅ BULK SURFACE O 2 GAS Surface in contact with oxygen gas phase Two chemical reservoirs are used: 1.Chemical potential of oxygen, μ O from ideal gas, O 2 2.Chemical potential of metal, μ M from bulk metal, M Neglecting F VIB and F CONF for the moment, By defining,

The Transition State (TS) TS at O sub path: C-O 1.18 Å C-O sub 1.51 Å The angle of O-C-O sub is 123  O sub 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).