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Theoretical Study of surface stabilities of CeO2 - a DFT and DFT+U study

Theoretical Study of surface stabilities of CeO2 - a DFT and DFT+U study. Yong Jiang 1 , James B Adams 2 1,2 Sci. & Engr. of Materials Program 2 Dept. of Chemical and Materials Engineering Arizona State University Presented to 2004 APS 4 Corners Fall meeting, UNM, Alburquerque, NM.

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Theoretical Study of surface stabilities of CeO2 - a DFT and DFT+U study

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  1. Theoretical Study of surface stabilities of CeO2 - a DFT and DFT+U study Yong Jiang 1, James B Adams 2 1,2 Sci. & Engr. of Materials Program 2 Dept. of Chemical and Materials Engineering Arizona State University Presented to 2004 APS 4 Corners Fall meeting, UNM, Alburquerque, NM

  2. Automobile Emission Limits • Emission regulations of the fossil-fuel burning engines have been tightened since the Clean Air Act was published in 1970's in the US. Fig.1 NO emission limit published by USA EPA (from http://www.fhwa.dot.gov/environment/aqfactbk/factbk12.htm)

  3. Three-way Catalysts (TWC) • Traditional Pt/Rh and Pt/Pd/Rh TWC contains Pt/Pd: for CO and HC oxidation Rh: efficient at NO reduction • Low-cost Pd-only catalysts only work efficiently in narrow air(O)-to-fuel (A/F) window centered at ratio=14.6 • High oxygen storage/release capacity (OSC) material is needed to stabilize the air-fuel ratio about this value

  4. CeO2 as OSC Material • Advantages of using pure CeO2 as OSC - elevated oxygen transport capacity - facile redox ability: CeO2↔ CeO2-x (0<x<0.178) - compatibility with noble metals • Nano-size particles with high surface area (in order of ~100 m2/g) is demanded 1 • But ceria particles may start to aggregate at T<700K 2 • People want to know the stable surfaces at certain chemical environment. 1 Fornasiero, P. et al, J. Catal. 164, 173, (1996) 1,2 El Fallah, J., et al. J. Phys. Chem. 98, 5522, 1994 3 Fornasiero, et al. J. Catal. 1996, 164, 173

  5. Simulation Theory - DFT • DFT - Based on the Hohenberg-Kohn theorems - using Kohn-Sham total energy functional (Exc added): - solve N one-electron equations self-consistently - Exchange-correlation Approximation : LDA, GGA (PW91/PBE) - Ion-electron pseudopotentials: Ultrasoft and PAW

  6. PBE-PAW PW91-PAW Total Energy (eV) LDA-PAW Volume (Å3) Validation of exchange-correlation • Necessary to test on all available exchange -correlation functionals and pseudopotentials Fig 2. Validation by reproducing bulk properties of CeO2 [1] Hill, S.E.; Catlow, C.R.A. J. Phys. Chem. Solids. 1993, 54, 411 [2] Eyring, L.; Handbook on the Physics and Chemistry of Rare Earths; Amsterdam, 1979, Vol.3, Chap.27 [3] Nakajima, A.; Yoshihara,A.; Ishigame,M. Phys. Rev. B. 1994, 50, 13297 (Polarized Raman-scattering at 10k) [4] Gerward, L.; Olsen, J.S.; Powder Diffr. 1993, 8, 127

  7. Simulation Theory – DFT+U • DFT limitation: VKS continuous, and fixed for all energy bands, not exact for a strongly correlated system - transition or rare-earth metals • DFT+U: the Coulomb d–d or f-f interaction is treated as a separate term 1/2UNiNj as in HF, and the delocalized s, p electrons are described by orbital-independent one-electron potential (LDA or GGA).

  8. The optimal values: U=7, J=0.7 eV LDA+U parameters (U and J) Fig 3. LSDA+U predictions for lattice constant, bulk modulus, and band gaps of bulk CeO2. The dotted lines stand for experimental measurements.

  9. LSDA+U LSDA+U LSDA LSDA 2p 4f 4f 5d 5p 5d 2s DOS of CeO2 bulk Figure 2. Comparison of partial electron density of states (PDOS) of bulk CeO2 by LDA (U, J = 0) and LDA+U (U = 7 eV, J = 0.7 eV). The left plot is for cerium ions (magnitude 2), and the right one is for oxygen ions..

  10. Surface Energy vs. T and PO2 • Surface energy is a function of T and PO2. in which the last two terms are normally ignored. The temperature dependence of the first two terms can be neglected for T=[ 0,1200 K]. (Tmelt>=2600K) • Our goal is to determine the relative ordering of surface stabilities of all possible surfaces. (1) (2) (3)

  11. CeO2 Surfaces of interest (a) 110 (I) (b) 111 (II) (c) 100 (III) (d) 210 (III) (e) 211 (I) (f) 310 (I) (g) 100_O (h) 100_Ce (i) 210_O (j) 210_Ce (l) 111_O (m) 111_Ce Fig 3.Views of the surface structures under study with all stoichiometric surfaces in the 1st row and all non-stoichiometric surfaces in the 2nd row (Red circles: O2-, white circles: Ce4+).

  12. Surface Energy Results a this work; b Z. Yang, et al, J. Chem. Phys. 120, 7741 (2004); cN. V. Skorodumova, et al,Phys. Rev. B 69, 075401 (2004); d T.X.T. Sayle, et al, Surf. Sci. 316, 329 (1994); e J. C. Conesa, Surf. Sci. 339, 337 (1995)

  13. Surface Stabilities at 300K • the stoichiometric (111) has the lowest free energy over almost all the range of pressure up to 1 atm • The cerium-rich (111)_Ce surface become the most stable one only at UHV. • The O-rich (210)_O may become the most stable one at PO2> 1 atm, but this range is beyond the limitation of our thermodynamic model

  14. Surface Stabilities at 1200K • the transition point moves to higher pressures gradually when temperature increases. • At T= 1200 K, the transition point positions at lnPO2 = ~ -80.

  15. Surface-Ce 4 Surface-O 1 Charge ELF 4f 2p 5d 4f 5p 2s 5d -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Surface electronic structure Figure 5. Charge, ELF, and Projected density of states for Ce and O ions of the stoichiometric (111) by LSDA+U (U = 7 eV, J = 0.7 eV).

  16. Surface-Ce 4 Surface-O 1 2p 4f 2s 4f 5d 5p 5d Surface electronic structure, cont’ Figure 6. Projected density of states of the O-rich (210) by LSDA+U (U = 7 eV, J = 0.7 eV).

  17. Summary • Ceria surface energies are investigated extensively by DFT as a function of temp and PO2 • Our results show that the stoichiometric (111) has the lowest surface energy over almost all the range of pressure • The transition to the cerium-rich (111)_Ce is found at UHV only.The transition point moves to higher pressures gradually when temperature increases. • DFT+U calculation with the optimal U=7 and J=0.7 eV leads to reasonable predictions for all considered physical properties of ceria Ongoing and future work oxygen vacancy in bulk and on surfaces: energy level in band-gap, vacancy formation energy, equilibrium concentration, and migration…

  18. Acknowledgements • Advisor: Dr. James Adams • All committee members - Dr. Mark Van Schilfgaarde - Dr. Peter Crozier - Dr. Renu Sharma • Dr. William Petuskey

  19. Yong Jiangtiger-paw @ asu.edu Ph.D in Computational Materials Science (4.0/4.0) , ASU, 2001-2005 M.S. in Solid State Device(3.8/4.0) , ASU, 2002-2004 To obtain a research position to broaden my expertise on computational materials science. Publications 1. Yong Jiang and James Adams, Oxygen Vacancy Formation and Migration in CeO2 bulk, to be submitted to Physcial Review B. 2. Yong Jiang and James Adams, Theoretical Study of CeO2 Surface Stabilities under the influence of temperature and oxygen partial pressure, submitted to Journal of Chemical Physics (2004) 3. Yong Jiang, James Adams, and Donghai Sun, Benzotriazole Adsorption on Cu2O (111) Surfaces: a First-Principles Study, Journal of Physical Chemistry B. 108 (2004) 12851 4. Yong Jiang and James Adams, First-Principles Study of Benzotriazole Adsorption onto Clean Cu(111),Surface Science, 529 (2003) 428 5. Yong Jiang, Dashu Peng, Dong Lu, and Luoxin Li, Analysis of Bimetal Sheet Bonding by Cold Rolling, Journal of Materials Processing Technology, 105 (2000) 32 6. Yong Jiang, Dashu Peng, and Langfei Liu, Stream Function Theory Calculations of Clad Sheet Bonding by Cold Rolling. Proceeding of the 6th National Conference on Plastic Deformation Mechanics of Materials, China, (1999)

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