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

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 Density-Functional Theory (DFT) 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 • 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 Au(111)2x2-O hcp octa tetra I tetra II (4x4)-oxide Ofcc/Otetra-I vacancy structure lower O upper O

5. 12 thin surface-oxides d c a b g h f e lower O upper O [i] j l k 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. 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

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 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) CO+OlowerCO2pathway Reaction energy barrier: 0.82 eV TS state: C-O1.18 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)

15. Convergence tests: Oxygen molecule

16. Convergence tests: Oxygen adsorption

17. Convergence tests: Oxygen adsorption

18. Convergence tests: CO molecule

19. Convergence tests: CO molecule

20. Convergence tests: CO molecule

21. Convergence tests

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, ETOTInternal energy DFT-calculated value FTRANSTranslational free energy ∝ M-1 → 0 FROTRotational free energy ∝ M-1 → 0 FVIB Vibrational free energy phonon DOS FCONF 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 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.

24. O2 GAS ⇅ SURFACE ⇅ BULK 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 ,

25. 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).