Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2surfaces

CO adsorption on CeO2(111): A CCSD(T) benchmark study using an embedded-cluster model

A benchmark model that combines an embedded-cluster approach for ionic surfaces with wavefunction-based methods to predict the vibrational frequencies of molecules adsorbed on surfaces is presented. As a representative case, the adsorption of CO on the lowest index non-polar and most stable facet of CeO2, that is, (111) was studied. The CO harmonic vibrational frequencies were not scaled semiempirically but explicitly corrected for anharmonic effects, which amount to about 25 cm-1 with all tested methods. The second-order Møller-Plesset perturbation method (MP2) tends to underestimate the CO harmonic frequency by about 40-45 cm-1 in comparison with the results obtained with the coupled-cluster singles and doubles with perturbational treatment of triple excitation method [CCSD(T)] and independently from the basis set used. The best estimate for the CO vibrational frequency (low-coverage case) differs by 12 cm-1 with the experimental value obtained by infrared reflexion absorption spectroscopy of 1 monolayer CO adsorbed on the oxidized CeO2(111) surface. In addition, a conservative estimate of the adsorption energy of about -0.22 ± -0.07 eV obtained at the CCSD(T) level confirms the physisorption character of the adsorption of CO on the CeO2(111) surface.

Surface Structure and Chemistry of CeO2 Powder Catalysts Determined by Surface-Ligand Infrared Spectroscopy (SLIR)

ConspectusCerium is the most abundant rare earth element in the Earth's crust. Its most stable oxide, cerium dioxide (CeO2, ceria), is increasingly utilized in the field of catalysis. It can catalyze redox and acid-base reactions, and serve as a component of electrocatalysts and even photocatalysts. As one of the most commonly used in situ/operando characterization methods in catalysis, infrared (IR) spectroscopy is routinely employed to monitor reaction intermediates on the surface of solid catalysts, offering profound insight into reaction mechanisms. Additionally, IR vibrational frequencies of probe molecules adsorbed on solid catalysts provide detailed information about their structure and chemical states. Numerous studies in the literature have utilized carbon monoxide and methanol as IR probe molecules on ceria particles. However, assigning their vibrational frequencies is often highly controversial due to the great complexity of the actual surface of ceria particles, which include differently oriented crystal facets, reconstructions, defects, and other structural variations. In our laboratory, taking bulk ceria single crystals with distinct orientations as model systems, we employed a highly sensitive ultrahigh vacuum (UHV) infrared spectroscopy system (THEO) to study the adsorption of CO and methanol. It turns out that the theoretical calculations adopting hybrid functionals (HSE06) can bring the theoretical predictions into agreement with the experimental results for the CO frequencies on ceria single crystal surfaces. The obtained frequencies serve as reliable references to resolve the long-standing controversial assignments for the IR bands of CO and methanol adsorbed on ceria particles. Furthermore, these characteristic frequencies allow for the determination of orientations, oxidation states and restructuring of exposed crystal facets of ceria nanoparticles, which is applicable from UHV conditions to industrially relevant pressures of up to 1 bar, and from low temperatures (∼65 K) to high temperatures (∼1000 K). We also used molecular oxygen as a probe molecule to investigate its interaction with the ceria surface, crucial for understanding ceria's redox properties. Our findings reveal that the localization of oxygen vacancies and the mechanism of dioxygen activation are highly sensitive to surface orientations. We provided the first spectroscopic evidence showing that the oxygen vacancies on ceria (111) surfaces tend to localize in deep layers. In addition, we employed N2O as a probe molecule to elucidate the origin of the photocatalytic activity of ceria and showed that the photocatalytic activity is highly sensitive to the surface orientation (i.e., surface coordination structure). This Account shows that probe-molecule infrared spectroscopy serves as a powerful in situ/operando tool for studying the surface structure and chemistry of solid catalysts, and the knowledge gained through the "Surface Science" approach is essential as a crucial benchmark.

Lattice-Stabilized Chromium Atoms on Ceria for N2O Synthesis

The development of selective catalysts for direct conversion of ammonia into nitrous oxide, N2O, will circumvent the conventional five-step manufacturing process and enable its wider utilization in oxidation catalysis. Deviating from commonly accepted catalyst design principles for this reaction, reliant on manganese oxide, we herein report an efficient system comprised of isolated chromium atoms (1 wt %) stabilized in the ceria lattice by coprecipitation. The latter, in contrast to a simple impregnation approach, ensures firm metal anchoring and results in stable and selective N2O production over 100 h on stream up to 79% N2O selectivity at full NH3 conversion. Raman, electron paramagnetic resonance, and in situ UV-vis spectroscopies reveal that chromium incorporation enhances the density of oxygen vacancies and the rate of their generation and healing. Accordingly, temporal analysis of products, kinetic studies, and atomistic simulations show lattice oxygen of ceria to directly participate in the reaction, establishing the cocatalytic role of the carrier. Coupled with the dynamic restructuring of chromium sites to stabilize intermediates of N2O formation, these factors enable catalytic performance on par with or exceeding benchmark systems. These findings demonstrate how nanoscale engineering can elevate a previously overlooked metal into a highly competitive catalyst for selective ammonia oxidation to N2O, paving the way toward industrial implementation.

Oxidation and Reduction of Polycrystalline Cerium Oxide Thin Films in Hydrogen

This study investigates the oxidation state of ceria thin films' surface and subsurface under 100 mTorr hydrogen using ambient pressure X-ray photoelectron spectroscopy. We examine the influence of the initial oxidation state and sample temperature (25-450 °C) on the interaction with hydrogen. Our findings reveal that the oxidation state during hydrogen interaction involves a complex interplay between oxidizing hydride formation, reducing thermal reduction, and reducing formation of hydroxyls followed by water desorption. In all studied conditions, the subsurface exhibits a higher degree of oxidation compared to the surface, with a more subtle difference for the reduced sample. The reduced samples are significantly hydroxylated and covered with molecular water at 25 °C. We also investigate the impact of water vapor impurities in hydrogen. We find that although 1 × 10-6 Torr water vapor oxidizes ceria, it is probably not the primary driver behind the oxidation of reduced ceria in the presence of hydrogen.

Toward a Consistent Prediction of Defect Chemistry in CeO2

Polarizable shell-model potentials are widely used for atomic-scale modeling of charged defects in solids using the Mott-Littleton approach and hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) embedded-cluster techniques. However, at the pure MM level of theory, the calculated defect energetics may not satisfy the requirement of quantitative predictions and are limited to only certain charged states. Here, we proposed a novel interatomic potential development scheme that unifies the predictions of all relevant charged defects in CeO2 based on the Mott-Littleton approach and QM/MM electronic-structure calculations. The predicted formation energies of oxygen vacancies accompanied by different excess electron localization patterns at the MM level of theory reach the accuracy of density functional theory (DFT) calculations using hybrid functionals. The new potential also accurately reproduces a wide range of physical properties of CeO2, showing excellent agreement with experimental and other computational studies. These findings provide opportunities for accurate large-scale modeling of the partial reduction and nonstoichiometry in CeO2, as well as a prototype for developing robust interatomic potentials for other defective crystals.

Shape-Controlled Pathways in the Hydrogen Production from Ethanol Steam Reforming over Ceria Nanoparticles

The ethanol surface reaction over CeO₂ nanooctahedra (NO) and nanocubes (NC), which mainly expose (111) and (100) surfaces, respectively, was studied by means of infrared spectroscopy (TPSR-IR), mass spectrometry (TPSR-MS), and density functional theory (DFT) calculations. TPSR-MS results show that the production of H₂ is 2.4 times higher on CeO₂-NC than on CeO₂-NO, which is rationalized starting from the different types of adsorbed ethoxy species controlled by the shape of the ceria particles. Over the CeO₂(111) surface, monodentate type I and II ethoxy species with the alkyl chain perpendicular or parallel to the surface, respectively, were identified. Meanwhile, on the CeO₂(100) surface, bidentate and monodentate type III ethoxy species on the checkerboard O-terminated surface and on a pyramid of the reconstructed (100) surface, respectively, are found. The more labile surface ethoxy species on each ceria nanoshape, which are the monodentate type I or III ethoxy on CeO₂-NO and CeO₂-NC, respectively, react on the surface to give acetate species that decompose to CO₂ and CH₄, while H₂ is formed via the recombination of hydroxyl species. In addition, the more stable monodentate type II and bidentate ethoxy species on CeO₂-NO and CeO₂-NC, respectively, give an ethylenedioxy intermediate, the binding of which is facet-dependent. On the (111) facet, the less strongly bound ethylenedioxy desorbs as ethylene, whereas on the (100) facet, the more strongly bound intermediate also produces CO₂ and H₂ via formate species. Thus, on the (100) facet, an additional pathway toward H₂ formation is found. ESR activity measurements show an enhanced H₂ production on the nanocubes.

CO2 Activation over Nanoshaped CeO2 Decorated with Nickel for Low-Temperature Methane Dry Reforming

Dry reforming of methane (DRM) is a promising way to convert methane and carbon dioxide into H₂ and CO (syngas). CeO₂ nanorods, nanocubes, and nanospheres were decorated with 1-4 wt % Ni. The materials were structurally characterized using TEM and in situ XANES/EXAFS. The CO₂ activation was analyzed by DFT and temperature-programmed techniques combined with MS-DRIFTS. Synthesized CeO₂ morphologies expose {111} and {100} terminating facets, varying the strength of the CO₂ interaction and redox properties, which influence the CO₂ activation. Temperature-programmed CO₂ DRIFTS analysis revealed that under hydrogen-lean conditions mono- and bidentate carbonates are hydrogenated to formate intermediates, which decompose to H₂O and CO. In excess hydrogen, methane is the preferred reaction product. The CeO₂ cubes favor the formation of a polydentate carbonate species, which is an inert spectator during DRM at 500 °C. Polydentate covers a considerable fraction of ceria's surface, resulting in less-abundant surface sites for CO₂ dissociation.