(478a) First Principles Investigation of Ceria Surface Phase Diagram in Oxygen-Rich and Lean Environments

            Ceria is a promising
material in the field of heterogeneous catalysis, given its oxygen buffering
capability and the ability to modify its oxygen stoichiometry depending on the operating
environment.¹ Besides providing structural support for metal catalysts,
it can also serve as an active component in the chemical reactions. Several
Water-Gas Shift (WGS) reaction studies have shown increased activity for metal
catalysts supported on ceria as compared to other supports or the metal itself.^2,3,4
Given the importance of the redox mechanism on ceria, a fundamental understanding
of how ceria behaves under a wide range of operating conditions is needed. The
oxygen chemistry of ceria, including oxygen vacancies, has been studied
extensively^5,6,7; however, comprehensive first principles
investigations to derive the ceria surface phase diagram have not been
conducted. This work focuses on understanding the fundamentals governing the
active role of ceria in reaction mechanisms involving oxygen, using first
principles thermodynamics (FPT) as well as conventional semi-local and hybrid
exchange-correlation density functionals. On the most stable (111) plane of
ceria,⁸ more than 25 oxygen configurations (sub-surface oxygen
vacancies, surface oxygen vacancies, and oxygen ad-atom coverages) are studied,
which cover the entire range from an oxidized surface to a surface/sub-surface
with no oxygen. FPT relations are used to determine the most stable configurations
in a wide temperature-pressure regime, as shown in the ceria surface phase
diagram. Several features predicted by our surface phase diagram are in
excellent agreement with prior experiments.^9,10

Surface
phase diagram of ceria in oxygen-rich and lean environments. ?vac' stands for
vacancies. Experimental data [10] (symbols) indicate the region, where ceria is
expected to undergo vacancy formation leading to a non-stoichiometric surface.

References

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