Hongqing Shi and Catherine Stampfl School of Physics, The University of Sydney, Sydney, Australia First-principles Investigation of Oxidation and Catalysis over 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 [1] Quantum size effects Charging of the gold particles by interaction with defects in the oxide Availability of low coordinated sites, and strain Combined effects of the gold particles and the oxide support M. Valden et al. Sci. 281, 1647 (1998). “Pressure-gap, temperature-gap” Efficient Gold-based catalysts: e.g. ; [1] M. Haruta, Catal. Today, 36, 153 (1997). “Structure-gap, materials-gap, water-gap”

Oxidation over Au(111) 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).

On-surface and sub-surface oxygen adsorption tetra II tetra I octa O fcc /O tetra-I vacancy structure

Surface oxide structures: (4x4) (4x4)Au 3 O 2 (4x4)Au 3 O 2 +O F +O H (4x4)Au 3 O 2 +O H (4x4)Au 3 O 2 -Au 3 O

Electronic structure of surface oxide phases 0.06 ML 0.11 ML 0.25 ML 1.0 ML O fcc /O tetra-I On-surface fcc (4x4)Au 3 O 2 -Au 3 O

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 Propose thin Au-oxide-like structures could play a role in the low temperature catalytic reactions Ab initio surface phase diagram

Nudged Elastic Band (NEB) Method [4] To investigate reaction barrier of on gold surface Density-functional Theory (DFT) The pseudopotential and plane-wave method VASP Projector augmented-wave method (PAW) Generalized gradient approximation (GGA) for the exchange-correlation functional Full atomic relaxation of top two Au layers and O atoms with 3 layers slab, vacuum region of 15 Å 5 k-points in (4  4) IBZ Energy cutoff of Ry (400 eV) [4] 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

CO adsorption on thin Au-oxide-like surface (a)(b) CO adsorption energy (eV) C-O bond-length ( Å) 1.14 C-Au bond-length ( Å)

CO 2 on Au-oxide-like surface The C-O bond-length at CO 2 is 1.18 Å (a)C sits 3.05 Å and 5.48 Å higher than upmost Au plane and the intact plane of Au(111). (b) C sits 4.02 Å and 6.70 Å higher than upmost Au plane and the intact plane of Au(111).

Reaction Path (a) O sub path (b) O up path

The Minimum Energy Path (MEP) (a) O sub path Reaction energy barrier: 0.82 eV

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).

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 surface oxide- like structures are stable up to 420 K The reaction energy barrier for CO with Osub is predicted to be 0.82 eV.

Acknowledgement We gratefully acknowledge support from: the Australian Research Council (ARC) the National Supercomputing Facility (APAC) the Australian Centre for Advanced Computing and Communications (ac3)

Convergence test: Oxygen molecule

Convergence test: Oxygen adsorption

Convergence test: 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,