Atomic and electronic structure of unreduced and reduced CeO2 surfaces: A first-principles study

Theoretical study of structure sensitivity on ceria-supported single platinum atoms and its influence on carbon monoxide adsorption

Density functional theory (DFT) calculations explore the stability of a single platinum atom on various flat, stepped, and defective ceria surfaces, in the context of single-atom catalysts (SACs) for the water-gas shift (WGS) reaction. The adsorption properties and diffusion kinetics of the metal strongly depend on the support termination with large stability on metastable and stepped CeO2(100) and (210) surfaces where the diffusion of the platinum atom is hindered. At the opposite, the more stable CeO2(111) and (110) terminations weakly bind the platinum atom and can promote the growth of metallic clusters thanks to fast diffusion kinetics. The adsorption of carbon monoxide on the single platinum atom supported on the various ceria terminations is also sensitive to the surface structure. Carbon monoxide weakly binds to the single platinum atom supported on reduced CeO2(111) and (211) terminations. The desorption of the CO2 formed during the WGS reaction is thus facilitated on the latter terminations. A vibrational analysis underlines the significant changes in the calculated scaled anharmonic CO stretching frequency on these catalysts.

Two decades of ceria nanoparticle research: structure, properties and emerging applications

Cerium oxide nanoparticles (CeNPs) are versatile materials with unique and unusual properties that vary depending on their surface chemistry, size, shape, coating, oxidation states, crystallinity, dopant, and structural and surface defects. This review encompasses advances made over the past twenty years in the development of CeNPs and ceria-based nanostructures, the structural determinants affecting their activity, and translation of these distinct features into applications. The two oxidation states of nanosized CeNPs (Ce3+/Ce4+) coexisting at the nanoscale level facilitate the formation of oxygen vacancies and defect states, which confer extremely high reactivity and oxygen buffering capacity and the ability to act as catalysts for oxidation and reduction reactions. However, the method of synthesis, surface functionalization, surface coating and defects are important factors in determining their properties. This review highlights key properties of CeNPs, their synthesis, interactions, and reaction pathways and provides examples of emerging applications. Due to their unique properties, CeNPs have become quintessential candidates for catalysis, chemical mechanical planarization (CMP), sensing, biomedical applications, and environmental remediation, with tremendous potential to create novel products and translational innovations in a wide range of industries. This review highlights the timely relevance and the transformative potential of these materials in addressing societal challenges and driving technological advancements across these fields.

Local environmental engineering for highly stable single-atom Pt1/CeO2catalysts: first-principles insights

Single-atom Pt1/CeO2catalysts may cope with the high cost and durability issues of fuel cell electrocatalysts. In the present study, the stability and underlying interaction mechanisms of the Pt1/CeO2system are systematically investigated using first-principles calculations. The Pt adsorption energy on CeO2surfaces can be divided into chemical interaction and surface deformation parts. The interaction energy, mainly associated with the local chemical environment, i.e. the number of Pt-O bonds, plays a major role in Pt1/CeO2stability. When forming a Pt-4O configuration, the catalytic system has the highest stability and Pt is oxidized to Pt2+. An electronic metal-support interaction mechanism is proposed for understanding Pt1/CeO2stability. In addition, our calculations show that the Pt1/CeO2(100) system is dynamically stable, and the external O environment can promote the further oxidation of Pt to Ptn+(2 ≤n< 4). The present study provides useful guidance for the experimental development of highly stable and efficient electrocatalysts for fuel cell applications.

Unravelling the role of dopants in the electrocatalytic activity of ceria towards CO2 reduction in solid oxide electrolysis cells

CO₂ reduction in Solid Oxide Electrolysis Cells (SOECs) is a key-technology for the transition to a sustainable energy infrastructure and chemical industry. Ceria (CeO₂) holds great promise in developing highly efficient, cost-effective and durable fuel electrodes, due to its promising electrocatalytic properties, and proven ability to suppress carbon deposition and to tolerate high concentrations of impurities. In the present work, we investigate the intrinsic electrocatalytic activity of ceria towards CO₂ reduction by means of electrochemical impedance spectroscopy (EIS) on model systems with well-defined geometry, composition and surface area. Aiming at the optimization of the intrinsic catalytic properties of the material, we systematically study the effect of different dopants (Zr, Gd, Pr and Bi) on the reaction rate under varying operating conditions (temperature, gas composition and applied polarization) relevant for SOECs. The electrochemical measurements reveal the dominant role of the surface defect chemistry of the material in the reaction rate, with doping having only a mild effect on the rate and activation energy of the reaction. By analyzing the pO₂ and overpotential dependence of the reaction rate with a general micro-kinetic model, we are able to identify the second electron transfer as the rate limiting step of the process, highlighting the dominant role of surface polarons in the energy landscape. These insights on the correlation between the surface defects and the electrocatalytic activity of ceria open new directions for the development of highly performing ceria-based technological electrodes.

Effect of Ce/Zr Composition on Structure and Properties of Ce1-xZrxO2 Oxides and Related Ni/Ce1-xZrxO2 Catalysts for CO2 Methanation

Ce_1-xZr_xO₂ oxides (x = 0.1, 0.25, 0.5) prepared via the Pechini route were investigated using XRD analysis, N₂ physisorption, TEM, and TPR in combination with density functional theory calculations. The Ni/Ce_1-xZr_xO₂ catalysts were characterized via XRD analysis, SEM-EDX, TEM-EDX, and CO chemisorption and tested in carbon dioxide methanation. The obtained Ce_1-xZr_xO₂ materials were single-phase solid solutions. The increase in Zr content intensified crystal structure strains and favored the reducibility of the Ce_1-xZr_xO₂ oxides but strongly affected their microstructure. The catalytic activity of the Ni/Ce_1-xZr_xO₂ catalysts was found to depend on the composition of the Ce_1-xZr_xO₂ supports. The detected negative effect of Zr content on the catalytic activity was attributed to the decrease in the dispersion of the Ni⁰ nanoparticles and the length of metal-support contacts due to the worsening microstructure of Ce_1-xZr_xO₂ oxides. The improvement of the redox properties of the Ce_1-xZr_xO₂ oxide supports through cation modification can be negated by changes in their microstructure and textural characteristics.

Interfacial Unit-Dependent Catalytic Activity for CO Oxidation over Cerium Oxysulfate Cluster Assemblies

Atomically precise cerium oxo clusters offer a platform to investigate structure-property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe70 torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce70 rings linked by monomeric cation units, for CO oxidation. CuCe70 emerged as the best performing MCe70 catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe70 as compared to CeCe70. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe70 exhibited a faster formation of Ce3+ and contained CO bridging sites absent in CeCe70. Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe70 as compared to CeCe70. The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. This study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe70 family to thus tune their catalytic activity.

Surface restructuring of a perovskite-type air electrode for reversible protonic ceramic electrochemical cells

Reversible protonic ceramic electrochemical cells (R-PCECs) are ideally suited for efficient energy storage and conversion; however, one of the limiting factors to high performance is the poor stability and insufficient electrocatalytic activity for oxygen reduction and evolution of the air electrode exposed to the high concentration of steam. Here we report our findings in enhancing the electrochemical activity and durability of a perovskite-type air electrode, Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3-δ (BCFN), via a water-promoted surface restructuring process. Under properly-controlled operating conditions, the BCFN electrode is naturally restructured to an Nb-rich BCFN electrode covered with Nb-deficient BCFN nanoparticles. When used as the air electrode for a fuel-electrode-supported R-PCEC, good performances are demonstrated at 650 °C, achieving a peak power density of 1.70 W cm⁻² in the fuel cell mode and a current density of 2.8 A cm⁻² at 1.3 V in the electrolysis mode while maintaining reasonable Faradaic efficiencies and promising durability.

One limiting factor to the high-performing reversible protonic ceramic electrochemical cells is the poor stability and electrocatalytic activity of air electrodes. Here the authors report a water-promoted surface restructuring process to enhance the performance of Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3-δ air electrode.

Oxygen vacancy distributions and electron localization in a CeO2(100) nanocube

Oxygen vacancy distributions in a 5 nm CeO2 nanocube were determined using the Reverse Monte Carlo method. The oxygen vacancies tend to be located on the surface of the CeO2 nanocube, with far fewer in subsurface and internal regions.

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

To study the dependence of the relative stability of surface (V^A) and subsurface (V^B) oxygen vacancies with the crystal facet of CeO₂, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce³⁺polarons, are facet- and concentration-dependent. At the higher vacancy concentration, theV^Ais more stable than theV^Bat the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both theV^Aand theV^Bhave similar stability. The stability of theV^Avacancies, compared to that of theV^B, is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce³⁺-Ce³⁺interactions.

Electronic excitation spectra of cerium oxides: from ab initio dielectric response functions to Monte Carlo electron transport simulations

Nanomaterials made of cerium oxides CeO₂ and Ce₂O₃ have a broad range of applications, from catalysts in automotive, industrial or energy operations to promising materials to enhance hadrontherapy effectiveness in oncological treatments. To elucidate the physico-chemical mechanisms involved in these processes, it is of paramount importance to know the electronic excitation spectra of these oxides, which are obtained here through high-accuracy linear-response time-dependent density functional theory calculations. In particular, the macroscopic dielectric response functions  of both bulk CeO₂ and Ce₂O₃ are derived, which compare remarkably well with the available experimental data. These results stress the importance of appropriately accounting for local field effects to model the dielectric function of metal oxides. Furthermore, we reckon the energy loss functions Im(-1/) of the materials, including the accurate evaluation of the momentum transfer dispersion from first-principles calculations. In this respect, by using Mermin-type parametrization we are able to model the contribution of different electronic excitations to the dielectric loss function. Finally, from the knowledge of the electron inelastic mean free path, together with the elastic mean free path provided by the relativistic Mott theory, we carry out statistical Monte Carlo (MC) electron transport simulations to reproduce the major features of the reported experimental reflection electron energy loss (REEL) spectra of cerium oxides. The good agreement with REEL experimental data strongly supports our approach based on MC modelling, whose main inputs were obtained using ab initio calculated electronic excitation spectra in a broad range of momentum and energy transfers.

A computational investigation of the adsorption of small copper clusters on the CeO2(110) surface

We report a detailed density functional theory (DFT) study of the geometrical and electronic properties, and the growth mechanism of a Cu_n (n = 1-4) cluster on a stoichiometric, and especially on a defective CeO₂(110) surface with one surface oxygen vacancy, without using pre-assumed gas-phase Cu_n cluster shapes. This gives new and valuable theoretical insight into experimental work regarding debatable active sites of promising CuO_x/CeO₂-nanorod catalysts in many reactions. We demonstrate that CeO₂(110) is highly reducible upon Cu_n adsorption, with electron transfer from Cu_n clusters, and that a Cu_n cluster grows along the long bridge sites until Cu₃, so that each Cu atom can interact strongly with surface oxygen ions at these sites, forming stable structures on both stoichiometric and defective CeO₂(110) surface. Cu-Cu interactions are, however, limited, since Cu atoms are distant from each other, inhibiting the formation of Cu-Cu bonds. This monolayer then begins to grow into a bilayer as seen in the Cu₃ to Cu₄ transition, with long-bridge site Cu as anchoring sites. Our calculations on Cu₄ adsorption reveal a Cu bilayer rich in Cu⁺ species at the Cu-O interface.

General synthesis of 2D rare-earth oxide single crystals with tailorable facets

Two-dimensional (2D) rare-earth oxides (REOs) are a large family of materials with various intriguing applications and precise facet control is essential for investigating new properties in the 2D limit. However, a bottleneck remains with regard to obtaining their 2D single crystals with specific facets because of the intrinsic non-layered structure and disparate thermodynamic stability of different facets. Herein, for the first time, we achieve the synthesis of a wide variety of high-quality 2D REO single crystals with tailorable facets via designing a hard-soft-acid-base couple for controlling the 2D nucleation of the predetermined facets and adjusting the growth mode and direction of crystals. Also, the facet-related magnetic properties of 2D REO single crystals were revealed. Our approach provides a foundation for further exploring other facet-dependent properties and various applications of 2D REO, as well as inspiration for the precise growth of other non-layered 2D materials.

CO2 adsorption on the pristine and reduced CeO2 (111) surface: Geometries and vibrational spectra by first principles simulations

First principles simulations of carbon dioxide adsorbed on the ceria (CeO₂) (111) surface are discussed in terms of structural features, stability, charge transfer, and vibrational modes. For this purpose, different density functional theory methods, such as Perdew-Burke-Ernzerhof (PBE) PBE and Hubbard correction, hybrid functionals, and different basis sets have been applied and compared. Both the stoichiometric and the reduced (111) surfaces are considered, where the electronic structure of the latter is obtained by introducing oxygen vacancies on the topmost or the subsurface oxygen layer. Both the potential energy surfaces of the reduced ceria surface and the adsorbate-surface complex are characterized by numerous local minima, of which the relative stability depends strongly on the electronic structure method of choice. Bent CO₂ configurations in close vicinity to the surface oxygen vacancy that partially re-oxidize the reduced ceria surface have been identified as the most probable stable minima. However, the oxygen vacancy concentration on the surface turns out to have a direct impact on the relative stability of possible adsorption configurations. Finally, the vibrational analyses of selected adsorbed species on both the stoichiometric and reduced surfaces show promising agreement with previous theoretical and experimental results.

Adsorption of Arsenic Ions Transforms Surface Reactivity of Engineered Cerium Oxide Nanoparticles

Cerium oxide (CeO₂) nanoparticles (NPs) are massively used as abrasives in the chemical and mechanical polishing (CMP), an essential process to manufacture semiconductor wafers. The CMP process for arsenide-based semiconductor materials produces wastewater with co-occurring arsenic (As) ions and CeO₂ NPs. We found that CeO₂ NPs adsorbed both arsenite (As(III)) and arsenate (As(V)) ions and the adsorption isotherms suggested different adsorption energies and capacities of the two species. Applying the ferric reducing ability for nanoparticle assay, we revealed that the adsorbed As(III) and As(V) each reduced CeO₂ NP surface reactivity but followed different mechanisms. The adsorbed As(III) ions below a critical coverage (110 mmol/kg) increased occupation of Ce 4f orbitals and thus reduced electron mobility of the original CeO₂ NPs. The adsorbed As(V) ions withdrew electrons from Ce 4f orbitals and likely became oxidizing agents that greatly inhibited the original surface reducing ability. Electron paramagnetic resonance analysis further revealed that adsorbed As(III) and As(V) ions decreased the propensity of CeO₂ NPs to produce reactive oxygen species. This work highlights the importance of examining NPs in their post-use phases in which surface reactivity and hazard potential can be greatly altered by chemical exposure history and NP surface transformations.

Colossal oxygen vacancy formation at a fluorite-bixbyite interface

Oxygen vacancies in complex oxides are indispensable for information and energy technologies. There are several means to create oxygen vacancies in bulk materials. However, the use of ionic interfaces to create oxygen vacancies has not been fully explored. Herein, we report an oxide nanobrush architecture designed to create high-density interfacial oxygen vacancies. An atomically well-defined (111) heterointerface between the fluorite CeO₂ and the bixbyite Y₂O₃ is found to induce a charge modulation between Y³⁺ and Ce⁴⁺ ions enabled by the chemical valence mismatch between the two elements. Local structure and chemical analyses, along with theoretical calculations, suggest that more than 10% of oxygen atoms are spontaneously removed without deteriorating the lattice structure. Our fluorite–bixbyite nanobrush provides an excellent platform for the rational design of interfacial oxide architectures to precisely create, control, and transport oxygen vacancies critical for developing ionotronic and memristive devices for advanced energy and neuromorphic computing technologies.

Oxygen vacancies can impart interesting properties in complex oxides, but specific architectures designed to create high-density oxygen vacancies are largely unknown. Here the authors report a fluorite-bixbyite nanobrush platform to tune interfacial oxygen and show that an atomically well-defined heterointerface can induce charge modulation.

Approaches for Selective Oxidation of Methane to Methanol

Methane activation chemistry, despite being widely reported in literature, remains to date a subject of debate. The challenges in this reaction are not limited to methane activation but extend to stabilization of the intermediate species. The low C-H dissociation energy of intermediates vs. reactants leads to CO2 formation. For selective oxidation, nature presents methane monooxygenase as a benchmark. This enzyme selectively consumes methane by breaking it down into methanol. To assemble an active site similar to monooxygenase, the literature reports Cu-ZSM-5, Fe-ZSM-5, and Cu-MOR, using zeolites and systems like CeO2/Cu2O/Cu. However, the trade-off between methane activation and methanol selectivity remains a challenge. Density functional theory (DFT) calculations and spectroscopic studies indicate catalyst reducibility, oxygen mobility, and water as co-feed as primary factors that can assist in enabling higher selectivity. The use of chemical looping can further improve selectivity. However, in all systems, improvements in productivity per cycle are required in order to meet the economical/industrial standards.

Divergent influence of {1 1 1} vs. {1 0 0} crystal planes and Yb3+ dopant on CO oxidation paths in mixed nano-sized oxide Au/Ce1−xYbxO2−x/2 (x = 0 or 0.1) systems

In this paper, the fundamental information on interactions in systems concerning nanocrystalline gold disperses on the shaped (octahedron-like or cube-like) Ce_1−xYb_xO_2−x/2 (x = 0 or 0.1) support has been discussed.

Oxidation of Reduced Ceria by Incorporation of Hydrogen

The interaction of hydrogen with reduced ceria (CeO2-x ) powders and CeO2-x (111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce3+ species to hydrogen, which results in the formation of Ce4+ , and thus in oxidation of ceria.

Atomistic and experimental study on thermal conductivity of bulk and porous cerium dioxide

Cerium dioxide (CeO₂) is a surrogate material for traditional nuclear fuels and an essential material for a wide variety of industrial applications both in its bulk and nanometer length scale. Despite this fact, the underlying physics of thermal conductivity (k_L), a crucial design parameter in industrial applications, has not received enough attention. In this article, a systematic investigation of the phonon transport properties was performed using ab initio calculations unified with the Boltzmann transport equation. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space, mode Grüneisen parameter and mean free path (MFP) distributions were also conducted. To further augment theoretical predictions of the k_L, measurements were made on specimens prepared by spark plasma sintering using the laser flash technique. Since the sample porosity plays a vital role in the value of measured k_L, the effect of porosity on k_L by molecular dynamics (MD) simulations were investigated. Finally, we also determined the nanostructuring effect on the thermal properties of CeO₂. Since CeO₂ films find application in various industries, the dependence of thickness on the in-plane and cross-plane k_L for an infinite CeO₂ thin film was also reported.

Oxygen diffusion and vacancy migration thermally-activated govern high-temperature magnetism in ceria

Several experimental works currently demonstrate that metallic nano-oxides and carbon nanomaterials expected to be diamagnets, in fact, behave as ferromagnets at room temperature. More than scientifically intriguing, this unconventional and unexpected ferromagnetism pave the way for innovation products and novel nanotechnological applications, gathering the magnetism to interesting functionalities of these nanomaterials. Here, we investigate the non-conventional ferromagnetism observed at high temperatures in nanocrystalline cerium dioxide (CeO₂or nanoceria) thin films that are optically transparent to visible light. Nanoceria exhibits several concrete applications in catalytic processes, photovoltaic cells, solid-state fuel cells, among others, which are mostly due to natural presence of oxygen vacancies and easy migration of the oxygen through the structure. The ferromagnetism in non-stoichiometric nanocrystaline ceria can be consistently described by ab initio electronic structure calculations, which support that oxygen vacancies cause the formation of magnetic moments and can provide a robust interconnectivity within magnetic polarons theoretical framework. Additionally, we present a conceptual model to account the oxygen transport to the non-conventional ferromagnetism at temperatures well above room temperature. The approach is complementary to the thermally-activated effective transfers of charge and spin around oxygen vacancy centers.

Magnetism and plasmonic performance of mesoscopic hollow ceria spheres decorated with silver nanoparticles

We investigate the role of interfaces and surfaces in the magnetic and surface enhanced Raman spectroscopy (SERS) properties of CeO2 hollow spheres decorated with Ag nanoparticles (H-CeO2@Ag). The composites, H-CeO2@Ag, were synthesized using a newly developed two-step process. The CeO2 hollow sphere diameter ranges from 100 nm to 2 μm and the grafted Ag nanoparticle (NP) size varies from 5 to 50 nm with a controllable coverage ratio. Spectroscopic and microscopic characterization confirms the formation of an interface between the Ag and ceria and shows different charge rearrangements occurring at both the interface and the surface. Room temperature ferro-magnetism was observed in all composites, and is associated mostly with ceria surface defects. A strong SERS effect was reported with a detection limit down to 10-14 M for the rhodamine 6G analyte. Scanning transmission electron microscopy and electron energy loss spectroscopy investigation reveals that hot-spots are associated with the silver NP surfaces and also with the Ag/CeO2 interface. This interfacial hot spot occurs for metallic particles above 30 nm and is strongly red shifted with respect to the Ag surface plasmon. The strong SERS activity is then attributed to the presence of several types of hot-spots and the geometrical features (buoyant hollow sphere and size dispersion) of the composite.

Insights into the CO2 deoxygenation to CO over oxygen vacancies of CeO2

The reversibly regenerated oxygen vacancies of CeO₂ can catalyze CO₂ deoxygenation and the reaction is initially surface reaction limited.

Shape-selective synthesis of nanoceria for degradation of paraoxon as a chemical warfare simulant

Repeated attacks using organophosphorus compounds, in military conflicts or terrorist acts, necessitate developing inexpensive and readily available decontamination systems. Nanosized cerium oxide is a suitable candidate when presents {111} facets.

Ab initio insights into the structural, energetic, electronic, and stability properties of mixed CenZr15−nO30 nanoclusters

In this work, we report a DFT investigation combined with Spearman correlation analysis of the energetic, structural and electronic properties of mixed Ce_nZr_15−nO₃₀ nanoclusters as a function of the composition (n = 0, 1,…,14, 15).

Breaking H2 with CeO2: Effect of Surface Termination

The ability of ceria to break H₂ in the absence of noble metals has prompted a number of studies because of its potential applications in many technological fields. Most of the theoretical works reported in the literature are focused on the most stable (111) termination. However, recently, the possibility of stabilizing ceria particles with selected terminations has opened new avenues to explore. In the present paper, we investigate the role of termination in H₂ dissociation on stoichiometric ceria. We model (111)-, (110)-, and (100)-terminated slabs together with the stepped (221) and (331) surfaces. Our results support a dissociation mechanism proceeding via the formation of a hydride/hydroxyl CeH/OH intermediate. Both the stability of such an intermediate and the activation energy depend critically on the termination, the (100)-terminated surfaces being the most reactive: the activation energy is 0.16 eV, and the CeH/OH intermediate is stable by -0.64 eV for the (100) slab, whereas the (111) slab presents 0.75 and 0.74 eV, respectively. We provide structural, energetic, electronic, and spectroscopic data, as well as chemical descriptors correlating structure, energy, and reactivity, to guide in the theoretical and experimental characterization of the Ce-H surface intermediate.

Understanding the ionic conductivity maximum in doped ceria: trapping and blocking

Materials with high oxygen ion conductivity and low electronic conductivity are required for electrolytes in solid oxide fuel cells (SOFC) and high-temperature electrolysis (SOEC). A potential candidate for the electrolytes, which separate oxidation and reduction processes, is rare-earth doped ceria. The prediction of the ionic conductivity of the electrolytes and a better understanding of the underlying atomistic mechanisms provide an important contribution to the future of sustainable and efficient energy conversion and storage. The central aim of this paper is the detailed investigation of the relationship between defect interactions at the microscopic level and the macroscopic oxygen ion conductivity in the bulk of doped ceria. By combining ab initio density functional theory (DFT) with Kinetic Monte Carlo (KMC) simulations, the oxygen ion conductivity is predicted as a function of the doping concentration. Migration barriers are analyzed for energy contributions, which are caused by the interactions of dopants and vacancies with the migrating oxygen vacancy. We clearly distinguish between energy contributions that are either uniform for forward and backward jumps or favor one migration direction over the reverse direction. If the presence of a dopant changes the migration energy identically for forward and backward jumps, the resulting energy contribution is referred to as blocking. If the change in migration energy due to doping is different for forward and backward jumps of a specific ionic configuration, the resulting energy contributions are referred to as trapping. The influence of both effects on the ionic conductivity is analyzed: blocking determines the dopant fraction where the ionic conductivity exhibits the maximum. Trapping limits the maximum ionic conductivity value. In this way, a deeper understanding of the underlying mechanisms determining the influence of dopants on the ionic conductivity is obtained and the ionic conductivity is predicted more accurately. The detailed results and insights obtained here for doped ceria can be generalized and applied to other ion conductors that are important for SOFCs and SOECs as well as solid state batteries.

Density Functional Theory plus Hubbard U Study of the Segregation of Pt to the CeO2- x Grain Boundary

Grain boundaries (GBs) can be used as traps for solute atoms and defects, and the interaction between segregants and GBs is crucial for understanding the properties of nanocrystalline materials. In this study, we have systematically investigated the Pt segregation and Pt-oxygen vacancies interaction at the ∑3 (111) GB in ceria (CeO₂). The Pt atom has a stronger tendency to segregate to the ∑3 (111) GB than to the (111) and (110) free surfaces, but the tendency is weaker than to (112) and (100). Lattice distortion plays a dominant role in Pt segregation. At the Pt-segregated-GB (Pt@GB), oxygen vacancies prefer to form spontaneously near Pt in the GB region. However, at the pristine GB, oxygen vacancies can only form under O-poor conditions. Thus, Pt segregation to the GB promotes the formation of oxygen vacancies, and their strong interactions enhance the interfacial cohesion. We propose that GBs fabricated close to the surfaces of nanocrystalline ceria can trap Pt from inside the grains or other types of surface, resulting in the suppression of the accumulation of Pt on the surface under redox reactions, especially under O-poor conditions.

A new insight into the theoretical design of highly dispersed and stable ceria supported metal nanoparticles

How to design and develop ceria supported metal nanoparticles (M/CeO₂) catalysts with high performance and sintering resistance is a great challenge in heterogeneous catalysis and surface science. In the present work, we propose two ways to improve the anti-sintering capability of M/CeO₂ catalysts. One is to introduce Ti atom on CeO₂ (1 1 1) to form monatomically dispersed Ti, TiO_x or TiO₂-like species on ceria. Density functional theory calculations show that the much stronger interactions between Au and Ti modified CeO₂ (1 1 1) occur compared with that on CeO₂ (1 1 1). According to the electronic analysis, the strong interactions are attributed to the electron transfer from the Ti modified ceria substrate to Au. The other is to dope Ti into CeO₂ (1 1 1) to form Ti_xCe_1-xO₂. This also leads to the interaction enhancement between Au and Ti_xCe_1-xO₂ (1 1 1). Electronic analysis indicates that the charge protuberance of surface O atoms near Ti atom results in the strong interactions between metal and ceria. This work provides new ideas for preparing M/CeO₂ catalysts with high dispersity and stability, and sheds light into the theoretical design of catalysts.

Well-defined palladium–ceria interfacial electronic effects trigger CO oxidation

The electron transfer from Pd cubes to CeO₂ rods via the interfaces triggered low-temperature CO oxidation.

A Simple Descriptor to Rapidly Screen CO Oxidation Activity on Rare-Earth Metal-Doped CeO2: From Experiment to First-Principles

Ceria (CeO₂) is an attractive catalyst because of its unique properties, such as facile redoxability and high stability. Thus, many researchers have examined a wide range of catalytic reactions on ceria nanoparticles (NPs). Among those contributions are the reports of the dopant-dependent catalytic activity of ceria. On the other hand, there have been few mechanistic studies of the effects of a range of dopants on the chemical reactivity of ceria NPs. In this study, we examined the catalytic activities of pure and Pr, Nd, and Sm-doped CeO₂ (PDC, NDC, and SDC, respectively) NPs on carbon monoxide (CO) oxidation. Density functional theory (DFT) calculations were also performed to elucidate the reaction mechanism on rare-earth (RE)-doped CeO₂(111). The experimental results showed that the catalytic activities of CO oxidation were in the order of CeO₂ > PDC > NDC > SDC. This is consistent with the DFT results, where the reaction is explained by the Mars-van Krevelen mechanism. On the basis of the theoretical interpretation of the experimental results, the ionic radius of the RE dopant can be used as a simple descriptor to predict the energy barrier at the rate-determining step, thereby predicting the entire reaction activity. Using the descriptor, a wide range of RE dopants on CeO₂(111) were screened for CO oxidation. These results provide useful insights to unravel the CO oxidation activity on various oxide catalysts.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Theoretical insights into ω-alkynylfuran cycloisomerisation catalyzed by Au/CeO2(111): the role of the CeO2(111) support

ω-Alkynylfuran cycloisomerisation on CeO₂(111)-supported Au clusters with different sizes was explored to unveil the role of the CeO₂(111) support, including charge transfer effects and interactions.

Study on the CO Oxidation over Ceria-Based Nanocatalysts

A series of ceria nanocatalysts have been prepared to study the structure dependency of the CO oxidation reaction. The ceria samples with well-defined nanostructures (nanocubes/Ce-NC and nanorods/Ce-NR) have been prepared using the hydrothermal method. Mesoporous ceria (Ce-MES) and ceria synthesized with solution combustion technique (Ce-SCS) have also been prepared for comparison. The lowest CO oxidation temperature has been reached by using ceria nanocubes (Ce-NC). This high activity draws immense contributions from the highly reactive (100) and (110) surfaces of the truncated nanocubes. The Ce-MES and Ce-SCS samples, despite their high surface areas, are unable to outdo the activity of Ce-NC and Ce-NR due to the abundant presence of (111) crystalline planes. This finding confirms the structure sensitivity of CO oxidation reaction catalyzed with ceria.

Morphology-dependent interplay of reduction behaviors, oxygen vacancies and hydroxyl reactivity of CeO2 nanocrystals

Reduction behaviors, oxygen vacancies and hydroxyl groups play decisive roles in the surface chemistry and catalysis of oxides. Employing isothermal H2 reduction we simultaneously reduced CeO2 nanocrystals with different morphologies, created oxygen vacancies and produced hydroxyl groups. The morphology of CeO2 nanocrystals was observed to strongly affect the reduction process and the resultant oxygen vacancy structure. The resultant oxygen vacancies are mainly located on the surfaces of CeO2 cubes and rods but in the subsurface/bulk of CeO2 octahedra. The reactivity of isolated bridging hydroxyl groups on CeO2 nanocrystals was found to depend on the local oxygen vacancy concentration, in which they reacted to produce water at low local oxygen vacancy concentrations but to produce both water and hydrogen with increasing local oxygen vacancy concentration. These results reveal a morphology-dependent interplay among the reduction behaviors, oxygen vacancies and hydroxyl reactivity of CeO2 nanocrystals, which deepens the fundamental understanding of the surface chemistry and catalysis of CeO2.

Oxide Nanocrystal Model Catalysts

Model catalysts with uniform and well-defined surface structures have been extensively employed to explore structure-property relationships of powder catalysts. Traditional oxide model catalysts are based on oxide single crystals and single crystal thin films, and the surface chemistry and catalysis are studied under ultrahigh-vacuum conditions. However, the acquired fundamental understandings often suffer from the "materials gap" and "pressure gap" when they are extended to the real world of powder catalysts working at atmospheric or higher pressures. Recent advances in colloidal synthesis have realized controlled synthesis of catalytic oxide nanocrystals with uniform and well-defined morphologies. These oxide nanocrystals consist of a novel type of oxide model catalyst whose surface chemistry and catalysis can be studied under the same conditions as working oxide catalysts. In this Account, the emerging concept of oxide nanocrystal model catalysts is demonstrated using our investigations of surface chemistry and catalysis of uniform and well-defined cuprous oxide nanocrystals and ceria nanocrystals. Cu2O cubes enclosed with the {100} crystal planes, Cu2O octahedra enclosed with the {111} crystal planes, and Cu2O rhombic dodecahedra enclosed with the {110} crystal planes exhibit distinct morphology-dependent surface reactivities and catalytic properties that can be well correlated with the surface compositions and structures of exposed crystal planes. Among these types of Cu2O nanocrystals, the octahedra are most reactive and catalytically active due to the presence of coordination-unsaturated (1-fold-coordinated) Cu on the exposed {111} crystal planes. The crystal-plane-controlled surface restructuring and catalytic activity of Cu2O nanocrystals were observed in CO oxidation with excess oxygen. In the propylene oxidation reaction with O2, 1-fold-coordinated Cu on Cu2O(111), 3-fold-coordinated O on Cu2O(110), and 2-fold-coordinated O on Cu2O(100) were identified as the active sites, respectively, to produce acrolein, propylene oxide, and CO2. Ceria rods enclosed with the {110} and {100} crystal planes, ceria cubes enclosed with the {100} crystal planes, and ceria octahedra enclosed with the {111} crystal planes exhibit distinct morphology-dependent oxygen vacancy concentrations and structures that can be well correlated with the surface compositions and structures of exposed crystal planes. Consequently, the metal-ceria interactions, structures, and catalytic performances of ceria-supported catalysts depend on the CeO2 morphology. Our results comprehensively reveal the morphology-dependent surface chemistry and catalysis of oxide nanocrystals that not only greatly deepen the fundamental understanding of oxide catalysis but also demonstrate a morphology-engineering strategy to optimize the catalytic performance of oxide catalysts. These results adequately exemplify the concept of oxide nanocrystal model catalysts for the fundamental investigations of oxide catalysis without the "materials gap" and "pressure gap". With the structure-catalytic property relationships learned from oxide nanocrystal model catalyst studies and the advancement of controlled-synthesis methods, it is promising to realize the structural design and controlled synthesis of novel efficient oxide catalysts in the future.

Skin Bond Electron Relaxation Dynamics of Germanium Manipulated by Interactions with H2 , O2 , H2 O, H2 O2 , HF, and Au

Although germanium performs amazingly well at sites surrounding hetero-coordinated impurities and under-coordinated defects or skins with unusual properties, having important impact on electronic and optical devices, understanding the behavior of the local bonds and electrons at such sites remains a great challenge. Here we show that a combination of density functional theory calculations, zone-resolved X-ray photoelectron spectroscopy, and bond order length strength correlation mechanism has enabled us to clarify the physical origin of the Ge 3d core-level shift for the under-coordinated (111) and (100) skin with and without hetero-coordinated H2 , O2 , H2 O, H2 O2 , HF impurities. The Ge 3d level shifts from 27.579 (for an isolated atom) by 1.381 to 28.960 eV upon bulk formation. Atomic under-coordination shifts the binding energy further to 29.823 eV for the (001) and to 29.713 eV for the (111) monolayer skin. Addition of O2 , HF, H2 O, H2 O2 and Au impurities results in quantum entrapment by different amounts, but H adsorption leads to polarization.