Toward an Atomic-Level Understanding of Ceria-Based Catalysts: When Experiment and Theory Go Hand in Hand

Abstract

Conspectus Because ceria (CeO2) is a key ingredient in the formulation of many catalysts, its catalytic roles have received a great amount of attention from experiment and theory. Its primary function is to enhance the oxidation activity of catalysts, which is largely governed by the low activation barrier for creating lattice O vacancies. Such an important characteristic of ceria has been exploited in CO oxidation, methane partial oxidation, volatile organic compound oxidation, and the water–gas shift (WGS) reaction and in the context of automotive applications. A great challenge of such heterogeneously catalyzed processes remains the unambiguous identification of active sites. In oxidation reactions, closing the catalytic cycle requires ceria reoxidation by gas-phase oxygen, which includes oxygen adsorption and activation. While the general mechanistic framework of such processes is accepted, only very recently has an atomic-level understanding of oxygen activation on ceria powders been achieved by combined experimental and theoretical studies using in situ multiwavelength Raman spectroscopy and DFT. Recent studies have revealed that the adsorption and activation of gas-phase oxygen on ceria is strongly facet-dependent and involves different superoxide/peroxide species, which can now be unambiguously assigned to ceria surface sites using the combined Raman and DFT approach. Our results demonstrate that, as a result of oxygen dissociation, vacant ceria lattice sites are healed, highlighting the close relationship of surface processes with lattice oxygen dynamics, which is also of technical relevance in the context of oxygen storage-release applications. A recent DFT interpretation of Raman spectra of polycrystalline ceria enables us to take account of all (sub)­surface and bulk vibrational features observed in the experimental spectra and has revealed new findings of great relevance for a mechanistic understanding of ceria-based catalysts. These include the identification of surface oxygen (Ce–O) modes and the quantification of subsurface oxygen defects. Combining these theoretical insights with operando Raman experiments now allows the (sub)­surface oxygen dynamics of ceria and noble metal/ceria catalysts to be monitored under the reaction conditions. Applying these findings to Au/ceria catalysts provides univocal evidence for ceria support participation in heterogeneous catalysis. For room-temperature CO oxidation, operando Raman monitoring the (sub)­surface defect dynamics clearly demonstrates the dependence of catalytic activity on the ceria reduction state. Extending the combined experimental/DFT approach to operando IR spectroscopy allows the elucidation of the nature of the active gold as (pseudo)­single Au+ sites and enables us to develop a detailed mechanistic picture of the catalytic cycle. Temperature-dependent studies highlight the importance of facet-dependent defect formation energies and adsorbate stabilities (e.g., carbonates). While the latter aspects are also evidenced to play a role in the WGS...

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

“…In these conditions, the second O could remain loosely bound to the just refilled O vacancy and be available to react with new incoming reactant species, such as CO or H 2 . Recently, Ziemba et al [60] have reported a study of the interaction of O 2 with reduced ceria surfaces, demonstrating a close correlation between the outcomes from state-of-the-art spectroscopy techniques and theory. Herein, the participation of these reactive oxygen species in the CO-PROX mechanism has been carefully investigated (vide infra).…”

Insights on catalytic mechanism of CeO2 as multiple nanozymes

“…Besides, the defective CeO 2−x catalysts show the features of high oxygen mobility, high surface reducibility, large oxygen storage capacity, etc. Thus, many attractive catalytic behaviors of CeO 2−x are discovered, where it is recognized as the catalytic active components, functional supports, or additives in heterogeneous catalysis [70][71][72][73][74][75][76][77][78][79][80][81][82][83].…”

“…The electronic and geometrical configurations of those interfacial species significantly affect the catalytic performance of CeO 2 -based catalysts. Considering those species generated from the structural defects of oxygen vacancy, the controllability of the ratios of Ce 3+ /Ce 4+ as well as the concentrations of oxygen vacancies becomes critical, in which the surface Ce 3+ fraction has been widely adopted as the descriptor for their catalytic performance [76][77][78][79]. Generally, the abundance of the structural defects of oxygen vacancy shows an evidently size-dependent correlation, in which CeO 2−x with smaller size always introduces more oxygen vacancies [84].…”

“…Raman spectroscopy can characterize the density of oxygen vacancy in CeO 2 . Combination of XPS and Raman practically delivers the information of the structural defect of CeO 2 nanostructures [2,44,51,74,77]. Nuclear magnetic resonance (NMR) spectroscopy is a sensitive technique to qualitatively and quantitatively provide information (e.g., type, concentration, and distribution) of the interfacial acidic and basic sites [36,95].…”

Proper usage of Scherrer's and Guinier's formulas in X-ray analysis of size distribution in systems of monocrystalline CeO2 nanoparticles

“…For nanomaterials functionality in general, size and size distribution can be key parameters, as well as morphology and other structural variables [1][2][3][4]. In the case of ceria (CeO 2 ) nanoparticles (NPs), morphology is known to play an essential role in determining their redox and catalytic performances in many applications [5][6][7]. Although controlling of morphologies have been one of the main issue in the synthesis of ceria NPs [8][9][10][11][12][13][14], the effective surface area of the NP systems can also impact their performance.…”