Cerium oxide is an important material for catalytic and fuel cell applications. We present an ab initio density functional theory (DFT) study of the vibrational properties of ceria focusing on the interpretation of Raman spectra of polycrystalline powder samples, with vibrational bands in the frequency region between 250 and 1200 cm −1 . The model systems include the oxidized CeO 2 as well as the reduced CeO 2−x and Ce 2 O 3 bulk materials together with the CeO 2 (111) and oxygen defective CeO 2−x (111) surfaces. The experimentally observed band at 250 cm −1 is assigned to a surface mode of the clean CeO 2 (111) surface, in agreement with our Raman spectra of ceria (CeO 2 ) powders with varying crystal size (Filtschew, A.; Hofmann, K.; Hess, C., J. Phys. Chem. C 2016, 120, 6694). The reduced model systems display signature vibrational bands in the 480−600 cm −1 region associated with the presence of oxygen defects and reduced Ce 3+ ions. In the high-frequency region between 800 and 900 cm −1 , characteristic peroxide (O 2 2−) stretching vibrations at the oxidized and defective ceria surfaces are obtained, and a systematic study with respect to the peroxide coverage provides the basis for a correlation between the position of the peroxide stretching mode and its adsorption geometry and concentration. The present theoretical analysis allows for a consistent description of the experimental Raman spectra of polycrystalline ceria. The outlined approach serves as a reference for the description of vibrational properties of other metal oxides.

Supported gold catalysts are highly active for a variety of reactions including low-temperature CO oxidation. It has been shown that reducible support materials, e.g., ceria or titania, may significantly alter the catalytic performance. In this contribution, we provide the first direct evidence for ceria (CeO2) support dynamics and its relevance in Au/CeO2 catalysts during room temperature CO oxidation. In particular, combined operando Raman and UV–vis spectroscopy are employed to monitor the surface and subsurface defect dynamics of ceria quantitatively and in real time. The results clearly show a dependence of catalytic activity on the reduction state of the ceria support. In fact, the prereduced CeO2 catalyst support increases the activity during CO oxidation initially by 100%. The reduction is not limited to the CeO2 surface but also affects the CeO2 subsurface due to oxygen mobility and charge transfer in CeO2–x . Our results highlight the enormous importance of the support properties for a mechanistic understanding of oxidation reactions over metal/ceria catalyst materials.

Ceria is widely used in heterogeneous catalysis owing to its redox properties. Engineering the shape of ceria particles offers a powerful tool to develop materials with enhanced catalytic properties. In this study, we provide evidence for the shape-dependent dioxygen adsorption and activation of ceria nanoparticles with (111) and (100) facets by in situ Raman spectroscopy and relate these properties to unique adsorption sites employing density functional theory. Temperature- and gas-phase-dependent experiments demonstrate facilitated formation of peroxide, superoxide, and weakly bound dioxygen species on the (100) facets as rationalized by calculated vibrational frequencies of O2 2–, O2 –, and O2 species on CeO2–x (100) surfaces. Our results show that localization of the excess charge, driving the Ce4+ → Ce3+ reduction, significantly affects the stretching vibrations. Our approach provides a powerful basis for future developments of ceria-based catalysts by bridging the materials gap between idealized and real catalytic systems.

Supported-metal (Au, Pt) ceria-based catalysts are considered as promising candidates for the water−gas shift (WGS) reaction at low temperatures. Two main mechanisms have been proposed in the literature, the redox and associative mechanisms. A key step in both mechanisms has been considered to be the cleavage of O−H bonds. In this mechanistic study, the role of surface and bulk oxygen species involved in the WGS reaction over ceria-supported gold catalysts (Au/CeO 2 ) was elucidated directly using operando Raman spectroscopy combined with isotope labeling and supported by DFT+U calculations. Exposure of Au/CeO 2 to pure H 2 18 16 O), and (iii) large amounts of 18 O in the subsurface of the ceria support due to oxygen transfer from the surface to the ceria subsurface, highlighting the oxygen dynamics of the ceria support. While the results of this study are fully consistent with a redox mechanism involving a reaction pathway for replenishment of surface oxygen ions O 2− from terminal hydroxyl groups (O−H) accessible also in the absence of CO in the gas phase, other reaction mechanisms cannot be ruled out.

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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...

The large-scale production and ecotoxicity of urea make its removal from wastewater a health and environmental challenge. Whereas the industrial removal of urea relies on hydrolysis at elevated temperatures and high pressure, nature solves the urea disposal problem with the enzyme urease under ambient conditions. We show that CeO2-x nanorods (NRs) act as the first and efficient green urease mimic that catalyzes the hydrolysis of urea under ambient conditions with an activity (kcat = 9.58 × 101 s-1) about one order of magnitude lower than that of the native jack bean urease. The surface properties of CeO2-x NRs were probed by varying the Ce4+/Ce3+ ratio through La doping. Although La substitution increased the number of surface defects, the reduced number of Ce4+ sites with higher Lewis acidity led to a slight decrease of their catalytic activity. CeO2-x NRs are stable against pH changes and even to the presence of transition metal ions like Cu2+, one of the strongest urease inhibitors. The low costs and environmental compatibility make CeO2-x NRs a green urease substitute that may be applied in polymer membranes for water processing or filters for the waste water reclamation. The biomimicry approach allows the application of CeO2-x NRs as functional enzyme mimics where the use of native or recombinant enzyme is hampered because of its costs or operational stability.

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