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Indium oxide (In 2 O 3 ) has emerged as a highly active catalyst for methanol synthesis by CO 2 hydrogenation. In this work we elucidate the reduction behavior and oxygen dynamics of cubic In 2 O 3 nanoparticles by in situ Raman and UV−vis spectra in combination with density functional theory (DFT) calculations. We demonstrate that application of UV and visible Raman spectroscopy enables, first, a complete description of the In 2 O 3 vibrational structure fully consistent with theory and, second, the first theoretical identification of the nature of defect-related bands in reduced In 2 O 3 . Combining these findings with quasi in situ XPS and in situ UV−vis measurements allows the temperature-dependent structural dynamics of In 2 O 3 to be unraveled. While the surface of a particle is not in equilibrium with its bulk at room temperature, oxygen exchange between the bulk and the surface occurs at elevated temperatures, leading to an oxidation of the surface and an increase in oxygen defects in the bulk. Our results demonstrate the potential of combining different in situ spectroscopic methods with DFT to elucidate the complex redox behavior of In 2 O 3 nanoparticles.

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...

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The mechanism of the low-temperature water–gas shift (LT-WGS) reaction over Au/CeO2 catalysts with different ceria terminations, i.e., (111), (110), and (100) facets, was investigated. Using combined operando Raman and UV-Vis...

The oxygen storage-release dynamics in ceria nanorods is elucidated by using a combination of in situ multi-wavelength Raman spectroscopy and density functional theory calculations. Ceria nanorods exposing CeO2(100) and CeO2(110) facets are shown to be characterized by highly facet-dependent properties regarding molecular oxygen activation and decomposition as well as lattice oxygen dynamics. Temperature-dependent Raman results show that oxygen is stored in the form of peroxide species on the (100) facets, which are then released as gaseous oxygen, whereas lattice oxygen is involved with the (110) facets. On the latter, peroxide species first decompose into oxygen atoms that heal vacant lattice oxygen sites before being released as gaseous oxygen. The exposure of different facets makes ceria nanorods an interesting material for catalytic applications, because they allow the use of a mixture of oxygen storage-release functions, as well as their synergistic interactions, in a single system.

In this combined Raman/UV-Vis and DFT study, structure-activity relations for CO oxidation over ceria nanorods (with/without gold) with CeO2(110) and CeO2(100) termination are elucidated using ceria nanocubes with CeO2(100) termination as reference.

In this work we elucidate the main steps of the CO oxidation mechanism over Au/CeO2(111), clarifying the course of CO adsorption at a broad variety of surface sites as well as of transmutations of one CO species into another. By combining transient spectroscopy with DFT calculations we provide new evidence that the active centers for CO conversion are single gold atoms. To gain insight into the reaction mechanism, we employ Modulation Excitation (ME) DRIFT spectroscopy in combination with the mathematical tool of Phase Sensitive Detection to identify the active species and perform DFT calculations to facilitate the assignments of the observed bands. The transient nature of the ME-DRIFTS method allows us to sort the observed species temporally, providing further mechanistic insight. Our study highlights the potential of combined transient spectroscopy and theoretical calculations (DFT) to clarify the role of adsorbates observed and to elucidate the reaction mechanism of CO oxidation over supported gold and other noble-metal catalysts.