Stoichiometric Ceria Research Articles

N2 as an Efficient IR Probe Molecule for the Investigation of Ceria-Containing Materials.

Ceria and ceria-based catalysts are very important in redox and acid-base catalysis. Nanoceria have also been found to be important in biomedical applications. To design efficient materials, it is necessary to thoroughly understand the surface chemistry of ceria, and one of the techniques that provides such information about the surface is the vibrational spectroscopy of probe molecules. Although the most commonly used probe is CO, it has some disadvantages when applied to ceria and ceria-based catalysts. CO can easily reduce the material, forming carbonate-like species, and can be disproportionate, thus modifying the surface. Here, we offer a pioneering study of the adsorption of 15N2 at 100 K, demonstrating that dinitrogen can be more advantageous than CO when studying ceria-based materials. As an inert gas, N2 is not able to oxidize or reduce cerium cations and does not form any surface anionic species able to modify the surface. It is infrared and transparent, and thus there is no need to subtract the gas phase spectrum, something that often increases the noise level. Being a weaker base than CO, N2 has a negligible induction effect. By using stoichiometric nano-shaped ceria samples, we concluded that 15N2 can distinguish between surface Ce4+ sites on different, low index planes; with cations on the {110} facets and on some of the edges, Ce4+-15N2 species with IR bands at 2258-2257 cm-1 are formed. Bridging species, where one of the N atoms from the molecule interacts with two Ce4+ cations, are formed on the {100} facets (2253-2252 cm-1), while the interaction with the {111} facets is very weak and does not lead to the formation of measurable amounts of complexes. All species are formed by electrostatic interaction and disappear during evacuation at 100 K. In addition, N2 provides more accurate information than CO on the acidity of the different OH groups because it does not change the binding mode of the hydroxyls.

Synthesis and optimization of ceria variants via sol-gel combustion method for environmental remediation

The study delves into the comprehensive exploration of ceria nanoparticle synthesis, explicitly focusing on distinct variants: fuel-rich, fuel-deficient, and stoichiometric proportions. These ceria variants are meticulously investigated using the sol-gel combustion method to uncover their unique characteristics. The exceptional performance of stoichiometric ceria is particularly notable, as it exhibits a well-defined crystal structure and balanced elemental composition. Conversely, fuel-deficient and fuel-rich ceria variants display altered lattice structures due to variations in their oxygen-to-cerium ratios. The study extends to the incorporation of these variants of ceria nanoparticles into PVDF polymeric membranes by flat sheet membrane synthesis for water application. The different ceria variant’s surface charges and electronic vacancies play a significant role in water purification and emulsion separation. Capitalizing on their individual properties, ceria-modified membranes are systematically evaluated, for membrane performance in terms of flux, recovery ratio and reusability. Stoichiometric ceria-incorporated membranes emerge as superior performers with 1.7x oil-water separation efficiency and reusability compared to pristine membranes. Therefore, ceria nanoparticle incorporated membrane can be used for sustainable environmental remediation of pollutants from the water.

Water on ceria{111}: Comparison between 23 experimental vibrational studies in the literature and new modeling.

Theoretical and experimental vibrational signatures of H2O and OH- (dissociated water) adsorbed on stoichiometric ceria{111} surfaces are compared. The experimental ones were collected from low-coverage experiments in the literature, and the theoretical anharmonic frequencies were generated using density functional theory calculations employing the optPBE-vdW functional for coverages from 0.5 to a few monolayers. It is found that (i) the experiments and our calculations overall agree well, lending credibility to both; (ii) the calculations manage to resolve the large class of H-bonded motifs into frequency classes that can guide experimental assignments; (iii) it is possible to find a geometrical H-bond definition that also captures the OH vibrational frequency downshifts well: R(H⋯O) ≤2.5 Å and the O-H⋯O angle θ ≥ 100°; and (iv) the frequency vs electric field relations for water and hydroxides (i.e., dissociated water) follow different and well-separated curves.

Structural dynamics of Schottky and Frenkel defects in CeO2: a density-functional theory study

Cerium dioxide CeO2 (ceria) is an important material in catalysis and energy applications. The intrinsic Frenkel and Schottky defects can impact a wide range of material properties including the oxygen storage capacity, the redox cycle, and the ionic and thermal transport. Here, we study the impact of Frenkel and Schottky defects on the structural dynamics and thermal properties of ceria using density functional theory. The phonon contributions to the free energy are found to reduce the defect formation free energies at elevated temperature. The phonon dispersions of defective CeO2 show significant broadening of the main branches compared to stoichiometric ceria. Phonon modes associated with the defects are identifiable in the infrared spectra through characteristic shoulders on the main features of the stoichiometric fluorite structure. Finally, the presence of Frenkel and Schottky defects are also found to reduce the thermal conductivity by up to 88% compared to stoichiometric CeO2.

Green synthesis of Tungsten-doped CeO2 catalyst for selective catalytic reduction of NOx with NH3 using starch bio-template

Herein, a novel tungsten-doped CeO2 catalyst was fabricated via the spread self-combustion (SSC) method using sweet potato starch as bio-template. The doping of tungsten refines the pore diameter of CeO2 and restrains the growth of cubic CeO2 crystal formed in it. However, the annealing at 700 °C promotes the growth of cubic CeO2 crystal and the formation of Ce4W9O33 crystal instead of the amorphous tungsten species compared with the catalyst calcined at 550 °C. It is also found that the doping of tungsten increases the concentration of the adsorbed oxygen on the surface of CeO2-550 and enhances the reducibility of the surface oxygen of stoichiometric ceria and the acid sites. The as-prepared Ce4W2Oz-550 catalyst exhibits higher catalytic performance than Ce4W2Oz-CP-550 synthesized via the urea hydrothermal co-precipitation method due to the better dispersion of cerium species and the larger molar ratio of Oα/(Oα + Oβ). However, the enhancement of annealing temperature from 550 °C to 700 °C decreases the acidic and the redox properties of Ce4W2Oz, and thereby depresses the promotional effect of tungsten-doped on the NH3-SCR activity of CeO2-550. The suitable starch dosage (liquid/solid ratio), ion concentration, alkali titration and annealing temperature are 1 g (10 mL/g), 1 mol/L, ammonia water (pH ≈ 7.0) and 400–550 °C for the as-prepared Ce4W2Oz catalyst, respectively.

Ni Nanoparticles on CeO2(111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO2-x(111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO2-x(111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO1.95(111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape showed an increase in Ce3+/Ce4+ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO2 catalysts for which metal-oxide interactions play a very important role in the activation of O-H and C-H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.

Atomic structures and local electronic properties of K- and Rh-modified ceria/Pt(111) inverse model catalysts.

Ceria has been widely applied as a support in heterogeneous catalysis due to its unique capability to store and release oxygen. As a typical inverse model catalyst, a ceria/Pt(111) system has attracted much attention due to its strong metal-oxide interaction. The structural and electronic properties of the ceria/Pt(111) system can be effectively modified by the introduction of alien K and Rh atoms. Here, the K- and Rh-modified ceria/Pt(111) inverse model catalysts have been investigated with high resolution scanning tunneling microscopy and apparent local work function measurement. The experimental results indicate that the K atoms prefer to occupy the top sites of the stoichiometric ceria, while the Rh atoms are prone to stay at the electron-rich ceria island edges. The K and Rh atoms act as an electron donor and acceptor on ceria/Pt(111), respectively. Such a study on the modification of the ceria-based catalysts should help understand strong metal-oxide interaction in heterogeneous catalysis at the atomic level.

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.

Density Functional Theory Study of the Mechanism of Formaldehyde Oxidation on Mn-Doped Ceria

Formaldehyde is an indoor pollutant, whose removal under mild conditions is of growing importance. Mn-doped CeO2 is a promising catalyst for the oxidation of formaldehyde to water and carbon dioxide. We have theoretically investigated the origin of the high activity of Mn-doped ceria as compared with ceria. DFT+U calculations were used to identify adsorption modes and compare different reaction mechanisms. The reaction mechanism involves HCHO adsorption, two C–H bond cleavage steps involving reactive O atoms (either structural O atoms of the support or adsorbed O2), H2O formation, and H2O and CO2 desorption. On the stoichiometric surface, a Mars–Van Krevelen mechanism occurs, which involves ceria surface O atoms. The lower coordination number of these O atoms in the stoichiometric Mn-doped ceria results in decreased barriers for C–H bond cleavage. In the presence of defects which will be ubiquitous in the Mn-doped surface, a Langmuir–Hinshelwood mechanism becomes feasible, as O2 can strongly adsorb on the...

Mechanistic Insights and Electrochemical Performance of Ceria Based Materials for Solid Oxide Electrolysis of CO2

Electrocatalytic reduction of CO2 to CO in a high temperature solid oxide electrolysis cell (SOEC) was performed. Gadolinium doped ceria (GDC), was used as an electrocatalyst for CO2 reduction in a SOEC having yttria stabilized zirconia (YSZ) as the electrolyte and lanthanum strontium manganite (LSM) as the oxygen electrode. Physical characterization of CuO/GDC cathode was performed by X-Ray diffraction (XRD), Figure 1(A), and scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) was utilized to determine the ohmic and polarization losses associated with electrolyte and electrode components of SOEC. Open circuit impedance spectra of GDC cathode in SOEC were obtained at 1023 K with varying ratio of CO2/CO gas in the inlet flow, as shown in Figure 1(B). As the CO2 concentration in the fuel side environment of SOEC was decreased from 100% to 10%, the measured ohmic losses were observed to decrease from 47 to 35 Ωcm2. This might occurred because of the reducing environment in which Ce4+ is partially reduced to Ce3+, resulting in mixed ion-electron conduction leading to higher ionic transport at the electrode-electrolyte interface. Furthermore, open circuit potential (OCP) was observed to be increased from 0.11 V to 0.90 V as the CO2 percentage was decreased from 100% to 90%, possibly due to the same reason. In order to understand the reaction mechanism of CO2 reduction on ceria, density function theory (DFT) calculations were performed to study the formation of CO and methanol from CO2 reduction on CeO2(110) surface. CO2 molecule sitting in the vicinity of oxygen vacancy site on the surface, is activated to form bent carbonate CO3 δ- species which dissociates into CO via the incorporation of the oxygen atom into the vacancy. The calculated activation barrier and reaction energy for this redox reaction is 258.9 kJ/mole and 238.6 kJ/mole respectively. The effect of lateral interactions were studied by performing calculations for the same reaction step on two oxygen vacancy (di-vacancy) on 2x2 supercell unit. The activation barrier and reaction energy on a di-vacancy were significantly reduced to 134.3 and 127.3 kJ/mole respectively. DFT calculations showed that the hydrogen atom co-adsorbed on the surface could further assist the CO2 dissociation reaction. In the presence of a hydrogen atom the dissociation reaction occurs in two exothermic steps: CO2+H→COOH, COOH→CO+OH. The adsorbed CO2 or CO could hydrogenate to methanol via formate (HCOO) or carboxyl (COOH) mediated mechanisms. The formate intermediate, produced by the hydrogenation of CO2 (CO2+H→ HCOO), was observed to be more stable with a binding energy of -222.9 kJ/mole on the stoichiometric ceria surface as compared to the carboxyl intermediate Ebinding = -36.0 kJ/mole[1]. Therefore, HCOO is likely to act as a spectator and may not participate in further hydrogenation reaction. Alternatively, carboxyl mediated reaction route involves exothermic reaction steps except only to the dissociation of COOH to CO and OH which was calculated to be endothermic with a reaction energy of 5.0 and 24.4 kJ/mole on stoichiometric and reduced ceria surface respectively. The intrinsic activation barriers of all steps involved in the carboxyl mechanism were calculated and are shown in Figure 1(C). COOH dissociation step was calculated to be of maximum activation barrier (126 kJ/mole) and is likely to be rate determining. The activation energy of this step was calculated to be lowered by ~50kJ/mole on reduced ceria surface. Ceria based materials have been suggested to possess electrocatalytic activity for CO2 reduction which can be further improved by aliovalent metal dopants. Surface of ceria doped with aliovalent dopants such as gadolinium (Gd), praseodymium (Pr) and samarium (Sm), have been suggested to of mixed ionic and electronic conductive nature. Classical molecular dynamic simulations were utilized to determine the oxide ion diffusivity (D) in the Gd doped ceria (GDC) at different temperatures (Figure 1(D) and Table 1). The calculated diffusivity was of 1.15x10-7 cm2s-1 at 1073 K. The activation energy of oxide ion diffusion in GDC was estimated to be of 41.0 kJ/mole. Experimental and theoretical results thus suggest the feasibility of CO2 reduction reaction on ceria surface. Combined with high catalytic activity and fast oxide ion transport, ceria based materials could be a potential candidate for electrocatalytic reduction of CO2. Reference 1. Kumari, N., Sinha, N., Haider, M. A. & Basu, S. CO2 reduction to Methanol on CeO2 (110) Surface: a density functional theory study. Electrochimica Acta 177 (2015) 21–29 Figure 1

Density Functional Theory Study of CO2 Adsorption and Reduction on Stoichiometric and Doped Ceria

Periodic density functional theory (DFT) calculations were performed to estimate the adsorption energies of the reactant, product and intermediates for CO2 reduction to methanol on stoichiometric and doped CeO2 (110) surface. A planar structure of CO2 was considered to adsorb on stoichiometric ceria surface with a binding energy of -23.4 kJ/mol. The planar structure is likely to participate in CO2 hydrogenation reactions via the formation of formate (HCOO) or carboxyl (COOH) intermediate. Between the two intermediates, the carboxyl mediated route shows all the way exothermic steps and is likely to produce methanol. Ceria based materials have been suggested to possess high catalytic and electrocatalytic activity for CO2 reduction which can be further improved by aliovalent metal dopants. Surface of ceria was doped with aliovalent dopants such as Gd, Pr and Sm. The calculated adsorption energies of CO2 on doped ceria were comparable to stoichiometric ceria. Electronic structure calculations plotted in the form of projected density of states showed reduced band gaps for Gd doped ceria as compared to bulk ceria, suggesting towards higher electronic conductivities of the doped ceria materials.

Energetics of Cu Adsorption and Adhesion onto Reduced CeO2(111) Surfaces by Calorimetry

The morphology and interfacial energetics of vapor-deposited Cu on slightly reduced CeO2(111) surfaces at 300 K have been studied using single crystal adsorption calorimetry (SCAC), He+ low-energy ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Copper grows as three-dimensional nanoparticles with a density of ∼1013 particles/cm2 on CeO2–x(111) (x = 0.05, 0.1, and 0.2). The initial heat of adsorption of Cu decreased with the extent of reduction, showing that stoichiometric ceria adsorbs Cu more strongly than oxygen vacancies. On CeO1.95(111), the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Cu clusters on stoichiometric steps, followed by the Cu particles spreading onto less favorable sites (step vacancies and terraces). Above ∼0.1 ML (>0.8 nm in diameter), the Cu adsorption energies showed no variation with extent of ceria reduction: the heat of adsorption increased slowly with coverage (particle ...

Mechanistic Insights of CO2 Reduction to Methanol: DFT Study

Theoretical calculations were performed, using density functional theory (DFT), to estimate the energetics of the elementary steps involved in the conversion of CO2 to methanol on the CeO2 (110) surface. Due to high oxygen storage capacity and mixed ionic electronic conductive nature, ceria based materials have been suggested to enhance the electrocatalytic activities in solid oxide electrolysis cell. However, no study so far has highlighted the role of extended ceria surface alone in carrying out the CO2 reduction reaction. The energetics of suggested mechanisms, via formate (HCOO) and carboxyl (COOH) intermediates, for CO2 hydrogenation to methanol were studied on stoichiometric and reduced ceria surface. Calculations were performed to determine the most favorable adsorption orientation and corresponding binding energy of reaction intermediates. The adsorbed formate intermediate species on the stoichiometric ceria surface, were observed to be stable with higher binding energy (-223 kJ/mole) as compared to the carboxyl (Ebinding = -37 kJ/mole), and is likely to be a spectator. In order to produce methanol via formate mediated routes, HCOO requires to be subsequently hydrogenated to H2COOH which ultimately needs to dissociate into H2CO and OH. The dissociative elementary step of this route is significantly endothermic (DErxn=64 kJ/mol). On the other hand, the mechanistic routes involving carboxyl (COOH) shows all the way exothermic steps on the CeO2 (110) surface, except only to the dissociation of COOH into CO and OH, to produce methanol. The dissociation step is thermoneutral (DErxn~5 kJ/mol) on stoichiometric ceria and slightly endothermic (DErxn=24 kJ/mol) on reduced ceria. Activation barrier for the dissociation step (COOH → CO+OH) is estimated to be 127 kJ/mole, which is significantly higher as compared to the other expected rate limiting, CO2 hydrogenation (CO2+H→COOH) step (Ea=37 kJ/mole). We will extend this work to analyze the activation barriers on reduced ceria surface which will ultimately give an inference of the effect of oxygen vacancy on the reduction of CO2 to hydrocarbons.

ReaxFF Force-Field for Ceria Bulk, Surfaces, and Nanoparticles

We have developed a reactive force-field of the ReaxFF type for stoichiometric ceria (CeO2) and partially reduced ceria (CeO2–x). We describe the parametrization procedure and provide results validating the parameters in terms of their ability to accurately describe the oxygen chemistry of the bulk, extended surfaces, surface steps, and nanoparticles of the material. By comparison with our reference electronic structure method (PBE+U), we find that the stoichiometric bulk and surface systems are well reproduced in terms of bulk modulus, lattice parameters, and surface energies. For the surfaces, step energies on the (111) surface are also well described. Upon reduction, the force-field is able to capture the bulk and surface vacancy formation energies (Evac), and in particular, it reproduces the Evac variation with depth from the (110) and (111) surfaces. The force-field is also able to capture the energy hierarchy of differently shaped stoichiometric nanoparticles (tetrahedra, octahedra, and cubes), and of partially reduced octahedra. For these reasons, we believe that this force-field provides a significant addition to the method repertoire available for simulating redox properties at ceria surfaces.

CO2 Reduction to Methanol on CeO2 (110) Surface: a Density Functional Theory Study

Numerical basis-set based density functional theory (DFT) calculations were performed to investigate the routes for CO2 reduction to methanol on extended CeO2(110) surface. Thermochemistry of elementary steps for the suggested routes of CO2 reduction to CH3OH was evaluated. Calculations were performed to determine the most favorable adsorption orientation and corresponding binding energy of reaction intermediates. Mechanistic routes considering the formation of the carboxyl (COOH) or the formate (HCOO) intermediate species were considered. Formate was observed to be stable with a binding energy of -222.9kJ/mole on the surface of the stoichiometric ceria as compared to the carboxyl (Ebinding=-36.0kJ/mole). Formate species on subsequent hydrogenation produce H2COOH. In order to produce methanol, H2COOH is required to dissociate into H2CO and OH which is a significantly endothermic (ΔErxn=64.0kJ/mol) step. On the contrary, the overall route involving a carboxyl intermediate is exothermic, on both stoichiometric and reduced ceria surface, except for the dissociation of carboxyl (COOH) into CO and OH (COOH->CO+OH) which is endothermic by 5.0kJ/mole and 24.4kJ/mole on stoichiometric and reduced ceria respectively. On the stoichiometric ceria, the activation barrier (Ea=126.9kJ/mole) for COOH dissociation was estimated to be significantly higher as compared to the expected rate limiting step of CO2 hydrogenation (Ea=36.5kJ/mole). A possible lateral route for the reduction of CO2 to methane via the fomation of adsorbed CO was considered. CO dissociation on the ceria surface was estimated to be highly endothermic (ΔErxn=772.9kJ/mole) and is unlikely to produce methane.

Surface structure of catalytically-active ceria nanoparticles

Catalytic mechanisms, and therefore activity, depend on the structure of catalyst surfaces. In turn, surfaces may reconstruct and/or exhibit local configurations that vary from bulk composition and structure. CeO2 (ceria) is a redox catalyst of interest in numerous automotive, energy and, increasingly, biomedical applications. Previous studies aimed at understanding catalytic mechanisms on ceria have limited consideration to systems with bulk-like stoichiometric or sub-stoichiometric surfaces. Here we summarize previous computational studies on ceria surfaces, nanoclusters, and nanoparticles, and highlight challenges in constructing physically-representative ceria nanoparticle (CNP) structures. Setting aside assumptions of bulk-like stoichiometric or sub-stoichiometric ceria surface terminations, we report results of DFT+U calculations and show that sufficiently small CNPs are not bulk-terminated, but rather are stabilized by the formation of Oxq groups (−2⩽q⩽0, x⩽3) at corners, edges, and {100} facets. These surface structures, not the annihilation and regeneration of O-vacancies, may directly control reduction/oxidation catalysis at CNPs below a critical size. As anion groups other than Oxq groups could be incorporated in stable CNP surfaces, this suggests the possibility of tailoring small CNP structures and mechanisms for particular catalytic reactions.

Reactive oxygen species in stoichiometric ceria: Bulk and low‐index surfaces

Abstractmagnified imageWe have calculated the stabilities of some reactive oxygen species (ROS) in stoichiometric bulk ceria and at the low‐index (111) and (110)‐surfaces, both in vacuum and in the presence of additional O2 molecules. We find that the formation of intrinsic ROS, here oxygen superoxides (O2–) and peroxides (O22–), is always endothermic at vacuum conditions and that the superoxide formation always leads to a higher formation energy than the peroxide formation. In the presence of additional O2 molecules, intrinsic peroxide formation becomes exothermic at the (110)‐surface in conjunction with the formation of extrinsic superoxide ions from adsorbed O2 molecules. This coexistence of intrinsic and extrinsic ROS species is anticipated to be stable at low temperatures, and can be important for understanding the ROS chemistry for nanoceria used in low‐temperature applications.magnified imageOxygen‐assisted reduction of the ceria(110) surface.(© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ceria in an oxygen environment: Surface phase equilibria and its descriptors

Ceria offers great promise in catalysis, due to its facile switchable oxidation state manifested from its oxygen buffering capability. This work presents the first equilibrium surface phase diagram of ceria exposed to various oxygen providing reservoirs (e.g., a pure O2 reservoir as well as NO/NO2, H2/H2O, or CO/CO2 redox environments) using first principles thermodynamics. For a pure O2 environment, the stoichiometric ceria (111) surface is favored at ambient conditions. Any appreciable surface reduction is observed only at ultra-low oxygen partial pressures (<10−30atm) and room temperature or at temperatures >2000K and ambient pressures. On the other hand, a redox environment promotes surface reduction at temperatures as low as 300K. Semi-local, Hubbard modified semi-local, and hybrid electronic exchange-correlation functionals are used to capture the energetics and phase transitions. We observe no difference between the three theories in the energetics governing ceria surface reduction in the dilute limit. The predicted phase transitions by all levels of theory are in agreement with each other and in excellent agreement with literature experimental data. Creation of oxygen vacancies in the sub-surface is energetically favored over surface vacancies, and is pivotal in determining the key features of the surface phase diagram. Consequently, we identify the oxygen vacancy formation energy as a descriptor for the surface reactivity of ceria in various oxygen environments. This single governing factor could be used in future studies to probe the surface reactivity of ceria and also to design improved ceria-based materials for redox reactions.

Copper-ceria interaction: A combined photoemission and DFT study

Stoichiometric and partially reduced ceria films were deposited on preoxidized Ru(0001) crystal by Ce evaporation in oxygen atmosphere of different pressures at 700 K. Copper-ceria interaction was investigated by deposition of metalic copper on both types of substrate. The samples were characterized by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) of core states and resonant photoelectron spectroscopy (RPES) of the valence bands. Copper adsorption on stoichiometric ceria caused reduction of CeO2, while on the oxygen-defficient ceria it partially reoxidized the substrate. This is in agreement with DFT+U calculations of copper adsorption on stoichiometric and defective ceria surfaces.

Role of step sites on water dissociation on stoichiometric ceria surfaces

The adsorption and dissociation of water on CeO2(111), CeO2(221), CeO2(331), and CeO2(110) has been studied by means of periodic density functional theory using slab models. The presence of step sites moderately affects the adsorption energy of the water molecule but in some cases as in CeO2(331) is able to change the sign of the energy reaction from endo- to exothermic which has important consequences for the catalytic activity of this surface. Finally, no stable molecular state has been found for water on CeO2(110) where the reaction products lead to a very stable hydroxylated surface which will rapidly become inactive.