Re-Evaluating CeO2 Expansion Upon Reduction: Noncounterpoised Forces, Not Ionic Radius Effects, Are the Cause

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Abstract

Ceria (CeO2) is widely used in reduction and oxidation processes such as catalysis, solid-oxide fuel cells and electrolyzers, and thermochemical redox processes. Counterintuitively, as ceria reduces and oxidizes, it expands and contracts, respectively. This has been attributed to the larger ionic radius of Ce3+ as compared with Ce4+. However, electronic structure calculations (DFT+U) detailed herein show that this is incorrect. While the presence of Ce3+ cations causes local expansion of their coordinating O anions, the expansion is compensated by the contraction of the O-Ce bonds in the second coordination shell. This results in only negligible changes in the Ce sublattice (Ce3+-Ce4+ distances of 3.90 Å rather than a 3.89 Å Ce4+-Ce4+ distance in oxidized ceria). The severing of Ce-O bonds upon the formation of an O vacancy results in noncounterpoised forces acting on the vacancy neighboring Ce cations, which relax toward the O anion opposite the vacancy, thereby expanding ceria (Ce4+ vac-Ce4+ vac distance of 4.14 Å). The relaxation of Ce4+ cations away from the vacancy rather than toward the vacancy, as is found in other materials, arises because ceria reduction results in the population f orbitals rather than d-O p antibonds. The corrected explanation for ceria expansion presented here will enable better design of ceria-based systems and modifications to ceria, such as doping, that will improve its performance.

Original languageEnglish (US)
Pages (from-to)8052-8059
Number of pages8
JournalJournal of Physical Chemistry C
Volume121
Issue number14
DOIs
StatePublished - Apr 13 2017
Externally publishedYes

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Cerium compounds
radii
expansion
causes
Vacancies
cations
Cations
Positive ions
anions
Anions
solid oxide fuel cells
Negative ions
sublattices
contraction
catalysis
electronic structure
orbitals
Solid oxide fuel cells (SOFC)
oxidation
Discrete Fourier transforms

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films

Cite this

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title = "Re-Evaluating CeO2 Expansion Upon Reduction: Noncounterpoised Forces, Not Ionic Radius Effects, Are the Cause",
abstract = "Ceria (CeO2) is widely used in reduction and oxidation processes such as catalysis, solid-oxide fuel cells and electrolyzers, and thermochemical redox processes. Counterintuitively, as ceria reduces and oxidizes, it expands and contracts, respectively. This has been attributed to the larger ionic radius of Ce3+ as compared with Ce4+. However, electronic structure calculations (DFT+U) detailed herein show that this is incorrect. While the presence of Ce3+ cations causes local expansion of their coordinating O anions, the expansion is compensated by the contraction of the O-Ce bonds in the second coordination shell. This results in only negligible changes in the Ce sublattice (Ce3+-Ce4+ distances of 3.90 {\AA} rather than a 3.89 {\AA} Ce4+-Ce4+ distance in oxidized ceria). The severing of Ce-O bonds upon the formation of an O vacancy results in noncounterpoised forces acting on the vacancy neighboring Ce cations, which relax toward the O anion opposite the vacancy, thereby expanding ceria (Ce4+ vac-Ce4+ vac distance of 4.14 {\AA}). The relaxation of Ce4+ cations away from the vacancy rather than toward the vacancy, as is found in other materials, arises because ceria reduction results in the population f orbitals rather than d-O p antibonds. The corrected explanation for ceria expansion presented here will enable better design of ceria-based systems and modifications to ceria, such as doping, that will improve its performance.",
author = "Christopher Muhich",
year = "2017",
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doi = "10.1021/acs.jpcc.6b12373",
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TY - JOUR

T1 - Re-Evaluating CeO2 Expansion Upon Reduction

T2 - Noncounterpoised Forces, Not Ionic Radius Effects, Are the Cause

AU - Muhich, Christopher

PY - 2017/4/13

Y1 - 2017/4/13

N2 - Ceria (CeO2) is widely used in reduction and oxidation processes such as catalysis, solid-oxide fuel cells and electrolyzers, and thermochemical redox processes. Counterintuitively, as ceria reduces and oxidizes, it expands and contracts, respectively. This has been attributed to the larger ionic radius of Ce3+ as compared with Ce4+. However, electronic structure calculations (DFT+U) detailed herein show that this is incorrect. While the presence of Ce3+ cations causes local expansion of their coordinating O anions, the expansion is compensated by the contraction of the O-Ce bonds in the second coordination shell. This results in only negligible changes in the Ce sublattice (Ce3+-Ce4+ distances of 3.90 Å rather than a 3.89 Å Ce4+-Ce4+ distance in oxidized ceria). The severing of Ce-O bonds upon the formation of an O vacancy results in noncounterpoised forces acting on the vacancy neighboring Ce cations, which relax toward the O anion opposite the vacancy, thereby expanding ceria (Ce4+ vac-Ce4+ vac distance of 4.14 Å). The relaxation of Ce4+ cations away from the vacancy rather than toward the vacancy, as is found in other materials, arises because ceria reduction results in the population f orbitals rather than d-O p antibonds. The corrected explanation for ceria expansion presented here will enable better design of ceria-based systems and modifications to ceria, such as doping, that will improve its performance.

AB - Ceria (CeO2) is widely used in reduction and oxidation processes such as catalysis, solid-oxide fuel cells and electrolyzers, and thermochemical redox processes. Counterintuitively, as ceria reduces and oxidizes, it expands and contracts, respectively. This has been attributed to the larger ionic radius of Ce3+ as compared with Ce4+. However, electronic structure calculations (DFT+U) detailed herein show that this is incorrect. While the presence of Ce3+ cations causes local expansion of their coordinating O anions, the expansion is compensated by the contraction of the O-Ce bonds in the second coordination shell. This results in only negligible changes in the Ce sublattice (Ce3+-Ce4+ distances of 3.90 Å rather than a 3.89 Å Ce4+-Ce4+ distance in oxidized ceria). The severing of Ce-O bonds upon the formation of an O vacancy results in noncounterpoised forces acting on the vacancy neighboring Ce cations, which relax toward the O anion opposite the vacancy, thereby expanding ceria (Ce4+ vac-Ce4+ vac distance of 4.14 Å). The relaxation of Ce4+ cations away from the vacancy rather than toward the vacancy, as is found in other materials, arises because ceria reduction results in the population f orbitals rather than d-O p antibonds. The corrected explanation for ceria expansion presented here will enable better design of ceria-based systems and modifications to ceria, such as doping, that will improve its performance.

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