Ceria (CeO2) owes its role in automotive three-way catalysts and oxidation catalysis to its oxygen storage capacity (OSC): reversible release and uptake of lattice oxygen through the Ce(4+)/Ce(3+) redox couple. For nearly stoichiometric, low-defect CeO2 nanoparticles (2–12 nm) prepared by a hydrothermal route from a tetravalent cerium precursor, Özkan et al. (2020) found that the surface-related OSC increases with decreasing particle size, as expected, but that the complete oxygen storage capacity (OSCc, CO-pulse reduction to saturation and O2-pulse re-oxidation at 430 °C) increases with particle size — from about 90 μmol(O) g⁻¹ at 5 nm to about 280 μmol(O) g⁻¹ at 12 nm — a counterintuitive trend without a satisfactory mechanistic explanation, in tension with the enhanced oxygen buffering reported and predicted for ceria particles below ~5 nm even though the source's OSCc series itself begins at ~5 nm, the particle size reached after the calcination the synthesis requires. The references cited below contain no study that has extended a low-defect CeO2 size series beyond 12 nm to determine whether a particle size of maximum total oxygen storage exists, or that has explained mechanistically why OSCc increases with size.

Background

Ceria (CeO2, crystallizing in the fluorite structure) is the archetypal oxygen storage material: under reducing conditions Ce(4+) is reduced to Ce(3+) with the formation of oxygen vacancies — lattice sites where an oxide ion is missing — releasing oxygen, and under oxidizing conditions the vacancies are refilled. This reversible redox buffering is the basis of ceria's function in automotive three-way catalysts, which must convert CO, hydrocarbons and NOx simultaneously and bridge oxygen-poor and oxygen-rich excursions of the engine, as well as in ceria catalysts for HCl oxidation (Deacon process) and chemical-looping oxygen carriers. The oxygen storage capacity (OSC) is measured by pulse experiments, alternating pulses of a reducing gas (typically CO or H2) and of O2 over the sample and quantifying the oxygen exchanged; OSC usually refers to the rapidly exchangeable, essentially surface oxygen, whereas the complete oxygen storage capacity (OSCc) is obtained by pulsing the reducing gas until the sample is saturated — no further oxygen extracted — and therefore probes oxygen from the particle interior as well. Full reduction to Ce2O3 corresponds to 0.25 oxygen atoms per CeO2, i.e. roughly 1.45 mmol(O) g⁻¹, far above typical measured values near 430 °C. For conventional micron-sized ceria, OSC at such temperatures scales with surface area, because reduction is confined to a near-surface shell.

Nanosizing is the classic lever for enhancing ceria's oxygen storage, and prior work established that very small ceria particles reduce more easily. Mamontov et al. (2000) correlated lattice defects with the OSC of nanocrystalline ceria. Density functional theory (DFT) calculations by Migani et al. (2010) showed that the oxygen-vacancy formation energy of ceria nanocrystallites decreases steeply as the particle diameter shrinks toward ~1 nm, predicting that smaller particles reduce more readily. Experimentally, Xu et al. (2010) measured by calibrated H2 temperature-programmed reduction (heating under flowing H2 while monitoring oxygen removal) that the total reducible oxygen of ceria nanocrystals rises dramatically once the diameter falls below about 5 nm, attributed to superoxide (O2−) species bound at the surface; Kullgren, Hermansson and Broqvist (2013) explained this 'supercharging' by DFT: small octahedral ceria nanoparticles are intrinsically understoichiometric and stabilize additional oxygen as superoxo ions, which enhances their low-temperature oxygen storage. Sun and Xue (2013) modeled the same size trend thermodynamically through the excess of surface Ce(4+) at edges and corners of small octahedra. Consistent with interior oxygen participating at few-nanometre sizes, mesostructured ceria built from roughly 6 nm crystallites shows elevated complete OSC (Möller et al. 2018; Lu et al. 2020).

Özkan et al. (2020) then isolated the size variable about as cleanly as available: they synthesized CeO2 nanoparticles of 2–12 nm by a hydrothermal route from a tetravalent cerium precursor, decoupling nucleation from growth, yielding particles with low microstrain and low bulk-defect concentration — nearly ideal CeO2.0 stoichiometry in the particle interior — unlike most earlier samples made from Ce(3+) precursors. At 430 °C, they found the expected trend that OSC increases with decreasing size, but, as their key result, that OSCc increases with particle size, from about 90 μmol(O) g⁻¹ for 5 nm particles (the size after the calcination the synthesis requires) to about 280 μmol(O) g⁻¹ at 12 nm — opposite to the small-particle enhancement documented in the earlier literature and counterintuitive in that OSCc, as a thermodynamic quantity of bulk oxygen exchange, should not depend on particle dimensions. The authors could not extend the series beyond 12 nm, offered only a tentative attribution (reduced oxygen-vacancy formation enthalpy and low lattice disorder of the largest particles) without a satisfactory explanation, noted that the broadening of the size distribution after heat treatment and the deviation from spherical morphology of the largest particles complicate interpretation, and measured only one condition (430 °C). The unresolved question is the particle-size–oxygen-storage relationship of low-defect ceria: whether a total-OSC optimum size exists beyond the measured range, and what mechanism produces the rising OSCc.

Problem Statement

For low-defect, near-stoichiometric CeO2 nanoparticles — the class characterized by low microstrain, low concentration of bulk defects and an almost ideal CeO2.0 stoichiometry in the particle interior, as realized in the source study by hydrothermal synthesis from a tetravalent cerium precursor (which yielded sizes of 2–12 nm): (1) determine how the oxygen storage capacity (OSC) and the complete oxygen storage capacity (OSCc) depend on particle size over a range extended beyond 12 nm — in particular, the functional form of the OSCc size dependence and its limit as the particle size increases, including whether OSCc passes through a maximum at some finite particle size; and (2) establish the mechanistic origin of the observed increase of OSCc with particle size — from about 90 μmol(O) g⁻¹ at 5 nm to about 280 μmol(O) g⁻¹ at 12 nm under CO-pulse reduction to saturation and O2-pulse re-oxidation at 430 °C — including its reconciliation with the predicted and observed facilitated reduction of very small ceria nanoparticles. These sub-questions jointly serve one scientific objective: a quantitative and mechanistically grounded particle-size–oxygen-storage relationship for low-defect ceria.

References

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