Hongqing Shi and Catherine Stampfl School of Physics, The University of Sydney, Sydney, Australia First-principles Investigation of the Stability of Surface.

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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)

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. 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”

Aim and Theoretical Approach 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 Energy cutoff of 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, (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 By defining, 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 <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

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)

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 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, R (1997). 35. L. L. Wang and H. P. Cheng, Phys. Rev. B 69, (2004). 36. C. Kittel, Introduction to Solid State Physics (Wiley, New York, 1996).

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, (2004). 38. Y. Yourdshahyan, H. K. Zhang, and A. M. Rappe, Phys. Rev. B 63, (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).

Convergence tests VASP

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

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,