Abstract: Phase Transition of Ceria Using First Principles Calculations (2012 Annual Meeting)

Venkatesh Botu, Ashish B. Mhadeshwar and Ramamurthy Ramprasad, Department of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, CT

Ceria can undergo phase transformation from its stable stoichiometric ratio at ambient conditions to a lower oxygen content material with increasing temperature and decreasing oxygen partial pressure, while preserving its fluorite structure (Fm-3m). However at a critical O/Ce ratio of 1.5, the hexagonal structure (P3/m1) of ceria becomes more prevalent.^1,2The transition from a stoichiometric to non-stoichiometric ceria creates oxygen vacancies along with cerium atoms with a reduced oxidation state.^3,4 Such characteristics make ceria a promising material for catalytic reactions.^1,5 One such case is the application of ceria in the Water-Gas Shift (WGS) reaction, where metals supported on ceria exhibit higher activities compared to no ceria support or a different support material.^6,7,8 Our work probes an atomic level understanding of the nature of ceria over a wide range of temperature and pressure conditions using Density Functional Theory (DFT) and First Principle Thermodynamics (FPT). The most stable ceria (111) surface^9,10 is modeled using a periodic 2x2 supercell image, as shown in Figure 1. More than 25 configurations of oxygen concentrations are considered to determine the stable phases in a wide temperature pressure region. This is the first ever comprehensive first principles investigation to predict the phase transition of ceria under various operating conditions.

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Left: Top view and Right: Side view of the ceria (111) plane. Blue: adsorbed oxygen; red: surface oxygen; black: sub-surface oxygen; green: surface cerium; and grey: cerium in the second layer.

References

1. Trovarelli, A. “Catalytic properties of ceria and CeO₂-containing materials” Catalysis Reviews 38 (1996): 439-520.

2. Zinkevich, M et al. “Thermodynamic modelling of the cerium–oxygen system.” Solid State Ionics 177 (2006): 989-1001.

3. Nolan, M. et al. “Oxygen vacancy formation and migration in ceria” Solid State Ionics 177 (2006): 3069-3074.

4. Torbrügge, S. et al. “Evidence of subsurface oxygen vacancy ordering on reduced CeO₂(111)” Physical Review Letters 99 (2007): 1-4.

5. Rao, G and Mishra, B. “Structural, redox and catalytic chemistry of ceria based materials.” Bulletin of the Catalysis Society of India 2 (2003): 122-134.

6. Wheeler, C. et al. “The water–gas-shift reaction at short contact times.” Journal of Catalysis 223 (2004): 191-199.

7. Hilaire, S. et al. “A comparative study of water-gas-shift reaction over ceria supported metallic catalysts” Applied Catalysis 215 (2001): 271-278.

8. Gorte, R. and Zhao S. “Studies of the water-gas-shift reaction with ceria-supported precious metals” Catalysis Today 104 (2005): 18-24.

9. Nolan, M. et al. “The electronic structure of oxygen vacancy defects at the low index surfaces of ceria” Surface Science 595 (2005): 223-232.

10. Désaunay, T. et al. “Modeling basic components of solid oxide fuel cells using density functional theory: Bulk and surface properties of CeO₂.” Surface Science 606 (2012): 305-311.

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