Limitations of Self-Regenerative Antioxidant Ability of Nanoceria Imposed by Oxygen Diffusion

Abstract

Nanoceria (CeO2–x ) is a unique antioxidant material with the ability to self-regenerate its properties after interaction with oxidants. In our paper, the dynamics of nanoceria–oxidant interaction accompanied by reversible Ce3+ ↔ Ce4+ transitions of cerium ions was studied for nanoceria water colloidal solutions using conventional spectroscopic techniques. We have shown that interaction of nanoceria with hydrogen peroxide leads to Ce3+ → Ce4+ oxidation, accompanied by quenching of Ce3+ luminescence of nanoceria, and the recovery of initial Ce3+ luminescence intensity occurs with a sufficient time delay (up to few days). The role of oxygen transport within ceria nanoparticles in the regeneration of antioxidant properties of nanoceria after interaction with an oxidant is discussed. Involvement of oxygen diffusion into recovery of nanoceria antioxidant properties hampers the redox activity of ceria nanoparticles making it size- and temperature-dependent.

Wavelength‐Selective Photoreduction of Colloidal CeO2–x Nanocrystals

“…However, the addition of HP leads to uniform decrease in the intensity of Ce 3+ luminescence band as whole indicating that not only surface but also bulk Ce 3+ ions are involved in the processes of nanoceria‐HP interaction. As was shown in previous study, the oxidation of surface Ce 3+ ions is followed by oxygen penetration inside nanoceria and the Ce 3+ → Ce 4+ oxidation of the Ce 3+ ions which are located in the nanoceria “core.” The oxidation phase accompanied by decrease in Ce 3+ luminescence intensity requires about 10 min, after that no decrease in Ce 3+ luminescence intensity was observed. The oxidation dynamics at different temperatures can be seen in the inset to Figure .…”

“…The resulting spectrum after full recovery of Ce 3+ luminescence intensity coincides with the one taken before oxidant addition (Figure ). The well‐pronounced temperature and size dependence (Figure S4, Supporting Information) of the Ce 4+ → Ce 3+ reduction phase allowed us to suggest the involvement of oxygen diffusion into the processes of Ce 3+ luminescence recovery . According to our concept described in previous study, the high oxygen deficiency of small ceria nanoparticles provides high oxygen mobility, which plays an important role during nanoceria interaction with oxidants of different nature (HP, potassium periodate, and sodium hypochlorite).…”

“…Hence, at such extremely high defects concentration in nanoceria, V O should form V O clusters, which, as was shown in the publication, can provide fast (few minutes) Ce 3+ → Ce 4+ oxidation controlled by single‐file oxygen diffusion. At the same time, the Ce 4+ → Ce 3+ recovery was extremely slow (up to few days) and controlled by standard oxygen diffusion . The Ce 4+ → Ce 3+ reduction phase of colloidal nanoceria could be accelerated by increasing the temperature and using smaller nanoparticles, but it still requires few hours to recover the initial Ce 3+ /Ce 4+ ratio .…”

“…At the same time, the stage of Ce 3+ → Ce 4+ oxidation only slightly changes with temperature increase and is independent of the laser irradiation. As was shown in a previous study, Ce 3+ → Ce 4+ oxidation is accompanied by fast oxygen diffusion into nanoceria due to high oxygen deficiency of nanoparticle (and, possibly involving single‐file diffusion mechanism), which determine its slight dependence on the temperature . According to the standard diffusion law: D = D 0 exp(− E a / kT ), where E a is the diffusion activation energy.…”

“…At the same time, the Ce 4+ → Ce 3+ recovery was extremely slow (up to few days) and controlled by standard oxygen diffusion . The Ce 4+ → Ce 3+ reduction phase of colloidal nanoceria could be accelerated by increasing the temperature and using smaller nanoparticles, but it still requires few hours to recover the initial Ce 3+ /Ce 4+ ratio . Given the promising prospects for the use of nanoceria as catalytic materials, it would be important to find ways and methods for the rapid reduction of nanoceria after oxidation to improve the performance of catalytic devices in a broad sense.…”