(600ai) Phase Transition of Ceria Using First Principles Calculations

            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.^1,2 The 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.

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

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B. ?Structural, redox and catalytic chemistry of ceria based materials.? Bulletin
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