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

First-principles Investigation of the Stability of Surface Gold Oxides on Au(111). Hongqing Shi and Catherine Stampfl School of Physics, The University of Sydney, Sydney, Australia. “Structure-gap, materials-gap, water-gap”. “Pressure-gap, temperature-gap”.

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

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  1. First-principles Investigation of the Stability of Surface Gold Oxides on Au(111) Hongqing Shi and Catherine Stampfl School of Physics, The University of Sydney, Sydney, Australia

  2. “Structure-gap, materials-gap, water-gap” “Pressure-gap, temperature-gap” • Nanometric-size gold particles act as catalysts at or below room temperature [1] • UHV results often thought to be transferable to “real” high-pressure, high temperature catalysis • Dynamic environment + labile surface morphology at corresponding partial pressure and temperature need to be included. Efficient Gold-based catalysts: e.g. ; • 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). [1] M.Haruta, Catal. Today, 36, 153 (1997).

  3. Aim and Theoretical Approach Density-functional Theory (DFT) 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 • Energy cutoff of 36.75 Ry (500 eV) • Equivalent k-point sampling, 21 k-points in (1x1) IBZ • Full atomic relaxation of top three Au layers and O atoms with 5 layers slab, vacuum region of 15 Å [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. On-surface and sub-surface oxygen adsorption Ofcc/Otetra-I tetra II octa vacancy structure tetra I

  5. (4x4)Au3O2+OF+OH (4x4)Au3O2 (4x4)Au3O2+OH Surface oxide structures: (4x4) (4x4)Au3O2-Au3O

  6. Electronic structure of surface oxide phases Ofcc/Otetra-I On-surface fcc 0.06 ML 0.11 ML (4x4)Au3O2-Au3O 0.25 ML 1.0 ML (4x4)Au3O2-Au3O

  7. By defining , 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 <360 K, thin oxide-like structures are stable For atmospheric pressure, T>360 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

  9. 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 360 K • Could play an important role in low temperature catalytic reactions Outlook • Investigate chemical reactions (e.g. CO oxidation) on the most stable surface oxide Acknowledgements • We gratefully acknowledge support from: • the Australian Research Council (ARC) • the National Supercomputing Facility (APAC) • the Australian Centre for Advanced Computing and Communications (ac3)

  10. Convergence tests Table I. Convergence tests for fcc bulk gold of our first principles DFT method. The first line is our present calculation. The parameters a0, B and Ecoh are the lattice constant, bulk modulus and cohesive energy, respectively. The DZS represents the basis function of double- for the s orbital. a. Reference 33. b. Reference 34. c. Referecne 35. The calculation used VASP. d. Reference 36. 33. J. M. Soler, M. R. Beltrán, K. Michaelian, I. L. Garzón, P. Ordejón, D. Sánchez-Portal, and E. Artacho, Phys. Rev. B 61, 5771 (2000). 34. B. D. Yu and M. Scheffler, Phys. Rev. B 46, R15 569 (1997). 35. L. L. Wang and H. P. Cheng, Phys. Rev. B 69, 165417 (2004). 36. C. Kittel, Introduction to Solid State Physics (Wiley, New York, 1996).

  11. Convergence tests Table II. Structural and energetic data for clean Au(111) surface. The parameters , Esurf, d12 and d23 are the work function, surface energy, the first and second interlayer distance relaxation for the clean Au(111) surface, respectively. a. Reference 39. b. Reference 40. c. Reference 41. The value comes from our calculation with their equation and parameter. 35. L. L. Wang and H. P. Cheng, Phys. Rev. B 69, 165417 (2004). 38. Y. Yourdshahyan, H. K. Zhang, and A. M. Rappe, Phys. Rev. B 63, 081405 (2001). 39. G. V. Hansson and S. A. Flodstrom, Phys. Rev. B 18, 1572 (1978). 40. M. A. Van Hove and S. Y. Tong, Surface Crystallography by LEED: Theory, Computation, and Structural Results (Springer-Verlag, Berlin, 1979). 41. S. G. J. Mochrie, D. M. Zehner, B. M. Ocko, and D. Gibbs, Phys. Rev. Lett. 24, 2925 (1990).

  12. Convergence tests

  13. Appendix • A sufficiently high energy cut-off is crucial for accurate surface binding/adsorption energy calculations particularly for low coverage. • High quality quantitative calculation is necessary. In VASP, set tag PREC=High. ref. VASP manual at http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html

  14. 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

  15. 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.

  16. 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 ,

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