Predicting NO_x Catalysis by Quantifying Ce³⁺ from Surface and Lattice Oxygen

Abstract

Our work introduces a novel technique based on the magnetic response of Ce and molecular oxygen adsorbed on the surface of nanoceria and ceria-based catalysts that quantifies the number and type of defects and demonstrates that this information is the missing link that finally enables predictive design of NO catalysis in ceria-based systems. The new insights into ceria catalysis are enabled by quantifying the above for different ceria nanoparticle shapes (i.e., surface terminations) and O partial pressure. We used ceria nanorods, cubes, and spheres and evaluated them for catalytic reduction of NO by CO. We then demonstrated the quantitative prediction of the reactivity of nanomaterials via their magnetism in different atmospheric environments. We find that the observed enhancement of reactivity for ceria nanocubes and nanorods is not directly due to improved reactivity on those surface terminations but rather due to the increased ease of generating lattice defects in these materials. Finally, we demonstrate that the method is equally applicable to highly topical and industrially relevant ceria mixed oxides, using nanoscale alumina-supported ceria as a representative case-a most ill-defined catalyst. Because the total oxide surface is a mixture of active ceria and inactive support and ceria is not likely present as crystallographically well-defined phases, reactivity does not easily scale with surface area or a surface termination. The key parameter to design efficient NO reduction in ceria-based catalysts is knowing and controlling the surface localized excess Ce ion areal density.

Elucidating the Promotional Effect of Cerium in the Dry Reforming of Methane

“…As mentioned before, the Ce 3+ /Ce 4+ redox couple is mainly responsible for the enhanced catalytic activity. However, under‐stoichiometric CeO 2‐x is mostly present in small nanoparticles with oxygen deficiency concentrated in low‐coordinated cerium atoms on the surface [22–25] . This implies that, in a ceria supported system, the amount of Ce 3+ is proportional to the dispersion of the oxide.…”

“…However, understoichiometric CeO 2-x is mostly present in small nanoparticles with oxygen deficiency concentrated in low-coordinated cerium atoms on the surface. [22][23][24][25] This implies that, in a ceria supported system, the amount of Ce 3 + is proportional to the dispersion of the oxide. However, the reducibility of ceria in a redox environment will also influence its chemical state.…”

Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by Decoupling and Deconvolution of the Defect Structure of Redox-Active Nanoceria

“…Nanoceria (cerium oxide nanoparticles, CeO 2‑ x ) recently has attracted attention owing to its redox/photocatalytic, catalytic, and acid–base properties for energy, environmental, and biomedical applications. The potential performance of nanoceria is considered widely to be associated with the intrinsic defects Ce 3+ and charge-compensating oxygen vacancies (V Ö ) present at the surface or subsurface of the nanoparticles, where continuous reversible Ce 3+ ↔ Ce 4+ switching and associated changes in oxygen vacancy concentration ([V Ö ]) occur; changes in the pH in aqueous media are known to be one of the means of initiating this switching. − The insuperable challenge to researchers is that defect equilibria for semiconductors such as CeO 2‑ x typically are described for anhydrous and high-temperature conditions. Furthermore, there does not appear to be any analytical instrumentation capable of directly assessing defect equilibria under hydrated (water vapor) or aqueous (liquid water) conditions.…”

NiCoP–CeO2 composites for efficient electrochemical oxygen evolution

“… 32 Besides, the presence of Ce 3+ and adsorptive oxygen species indicates abundant oxygen vacancies in the sample, which plays a great role in enhancing the adsorption of H 2 O, increasing the electrical conductivity, reducing the activation energy of OER and consequently promoting the electrochemical catalysis performance. 33 …”