Model of the Structure of Cerium Oxide Crystalline Surfaces

Journal of Physical Chemistry C, 2020

Surface morphology is known to affect catalytic activity, as some surfaces show greater activity than others. One of the key challenges is to identify strategies to enhance the expression of such surfaces and also to prevent their disappearance over time. Here, we apply density functional theory to the catalytic material CeO2 to predict the effect of adsorbed CO2 on the morphology of the material as a function of temperature and pressure. We predict that CO2 adsorbs as surface carbonates and that the magnitude of the adsorption energy is surface dependent, following the order {100} > {110} > {111}. We show that this difference leads to selective thermodynamic enhancement of {100} surfaces as a function of CO2 partial pressure and temperature. Finally, we show how the calculated surface free energies as a function of external conditions can be deployed to predict changes in the equilibrium particle morphology. These include the prediction that ceria nanoparticles prepared in the presence of supercritical CO2 will favour enhanced cube-like morphologies.

Journal of Solid State Chemistry, 2000

X-ray absorption spectroscopy has been used to study the structural and electronic properties of cerium atoms in nanocrystalline cerium oxide. These nanocrystalline cerium oxides were prepared by precipitation followed by the aging process. To increase the particle size, the as-prepared cerium oxides were calcined at various temperatures. The nanocrystalline phase was retained, even when the samples were calcined at 6003C. The analyses of X-ray absorption near edge structures (XANES) for cerium oxides show that the increase in the relative intensity of the transition to 2p4 f 1 5d*L 5nal state and its transition energy shifts toward higher energy are due to the increase in covalence between cerium and oxide ligands with increasing particle size. The extended X-ray absorption 5ne structure (EXAFS) results for cerium oxide show that the third Ce+O shell is degraded and the coordination number around cerium is decreased, resulting in a decrease in the bond distances of R Ce+O and R Ce+ Ce for particle size ( (15 nm. However, when the particle size is increased, the third Ce+O shell started growing and the coordination number around cerium attained normal coordination. The present results provide evidence for the formation of nanocrystalline cerium oxide prepared by precipitation followed by the aging process.

Surface Science Spectra, 2000

X-ray photoelectron spectroscopy measurements of cerium dioxide thin films have been performed. The samples were prepared by chemical vapor deposition ͑CVD͒ using Ce͑dpm͒ 4 ͑Hdpm ϭ 2,2,6,6tetramethyl-3,5-heptanedione͒ as precursor on alumina and glass substrates. In this work, the spectra of the principal core levels for a CeO 2 film on glass are presented. The Ce 3d photopeak has the typical structure of Ce͑IV͒ compounds. By deconvolution of the O 1s signal, the presence of -OH groups and adsorbed water are evidenced. Sputtering treatments confirm that carbon contamination is limited to the outermost layers, while resulting hydrated species are bonded tenaciously to the oxide network.

J. Mater. Chem. A, 2014

Cerium dioxide (CeO 2 , ceria) nanoparticles possess size-dependent chemical properties, which may be very different from those of the bulk material. Agglomeration of such particles in nanoarchitectures may further significantly affect their properties. We computationally model the self-assembly of Ce n O 2n particles (n ¼ 38, 40, 80)zero-dimensional (0D) structuresin one-and two-dimensional (1D and 2D) nanoarchitectures by employing density-functional methods. The electronic properties of 1D Ce 80 O 160 and 2D Ce 40 O 80 resemble those of larger 0D crystallites, Ce 140 O 280 , rather than those of their building blocks. These 0D, 1D and 2D nanostructures are employed to study the size dependence of the formation energy of an oxygen vacancy, E f (O vac ), a central property in ceria chemistry. We rationalize within a common electronic structure framework the variations of the E f (O vac ) values, which are computed for the Ce n O 2n nanostructures with different sizes and dimensionalities. We identify: (i) the bandwidth of the unoccupied density of states projected onto the Ce 4f levels as an important factor, which controls E f (O vac ); and (ii) the corner Ce atoms as the structural motif essential for a noticeable reduction of E f (O vac ). These results help to understand the size dependent behaviour of E f (O vac ) in nanostructured ceria. † Electronic supplementary information (ESI) available: Figure for Ce 40 O 80 and

Physica A: Statistical Mechanics and its Applications, 2017

Cerium oxide is an interesting mixed valence compound of great technological importance. We model the growth of thin films of this substance by describing the statistical nucleation of atomic units containing Ce 3+ and Ce 4+. The theoretical results are compared with available experimental values of the magnetic susceptibility of the material, which is related to the proportion of magnetic atoms in the solid. The model is able to predict the composition of the final solid under different preparation conditions, namely the oxygen content of the precursor and the temperature of the substrate.

The Journal of Physical Chemistry Letters, 2013

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO 2 films. The method yields bulk-truncated layers of cubic c-Ce 2 O 3. Compared to CeO 2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce 2 O 3 (111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO 2 (111). In addition, c-Ce 2 O 3 layers represent an experimental realization of a normally unstable polymorph of Ce 2 O 3. During interfacial reaction, c-Ce 2 O 3 nucleates on the interface between CeO 2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal−oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.

Journal of Nanoparticle Research, 2013

A correlation between the particle size and the lattice parameter has been established in cerium oxide nanoparticles. The variation in the lattice parameter is attributed to the lattice strain induced by the introduction of Ce 3? due to the formation of oxygen vacancies. Lattice strain was observed to decrease with an increase in the particle size. The Ce 4? to Ce 3? ratio in CeO 2 nanoparticles increases with increasing the calcination temperature in air atmosphere. Such anomalous behavior is due to the physical effect of nanoparticle sizes on increasing the oxidation state of Ce ions in CeO 2. Keywords Cerium oxide Á XPS Á XRD Ceria (CeO 2) is an inorganic compound broadly used in sensors, electrochromic, and anticorrosive coatings, and also in diverse catalysts and in the capacity of an abrasive material. Upon transition into a nanocrystalline state, ceria significantly changes its physicochemical properties, at that, the character of these changes is unusual enough (Ivanov et al. 2009). One of the major intrinsic properties of nanocrystalline ceria is a clearly marked dependence of a unit cell parameter on particle size. It is generally accepted that such an effect originates from partial reduction of Ce 4? into Ce 3? and corresponding formation of oxygen vacancies in ceria crystal lattice (Tsunekawa et al. 1999; Wu et al. 2004). Strongly pronounced non-stoichiometry gives rise to one of the most intriguing properties of ceria, namely its biological activity. It has been shown that CeO 2-x powders and sols (aqueous and nonaqueous) are effective scavengers of free radicals and other reactive oxygen species (ROS) and possess autoregenerative properties (Chen et al. 2006). Such a combination of properties as well as nontoxic nature and excellent biocompatibility makes nanocrystalline ceria a unique material that can protect cells from oxidative stress. In particular, CeO 2-x particles can increase cell longevity and survival potential of various micro-and macroorganisms (Ivanov et al. 2008; Colon et al. 2009). It was also shown that CeO 2-x is promising in view of treatment of various human diseases including cancer (Chen et al. 2006; Colon et al. 2009). In view of the fact that nanoceria biological activity is size-dependent (Chen et al. 2006), extensive study of physical and chemical properties of CeO 2-x is required paying special attention to its oxygen non-stoichiometry. Several reports were made dealing with dependence of oxygen non-stoichiometry and unit cell parameter on the particle size in CeO 2-x (Tsunekawa et al. 1999; Wu et al. 2004; Hailstone et al. 2009).

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2011

X-ray Emission Spectroscopy (XES) and X-ray Absorption Spectroscopy (XAS), have been used to investigate the photon emission and absorption associated with the Ce3d 5/2 and Ce3d 3/2 core-levels in CeOxide. A comparison of the two processes and their spectra will be made.

Physical Chemistry Chemical Physics, 2015

CeO2 films were prepared evaporating Ce on Cu(111) in O2 gas; a nearly perfect wetting could be achieved at pO2 = 5 × 10−7 mbar.

ChemPhysChem, 2011

Cerium dioxide nanoparticles are highly effective for oxygen buffering in redox reactions. [1] This functionality has potential for use in automotive three-way catalytic converters (TWCC) in which they can reduce pollutants by promoting catalytic oxidation of CO and hydrocarbons with simultaneous reduction of NO x . [1b,c] This ability to promote the catalysis of redox reactions is due to the dual valence of cerium, which is stable in both the 3 + and 4 + oxidation states, and the mobility of oxygen anions even at relatively low temperatures (573 K) within the CeO 2 (fluorite) lattice. The high electronic and ionic conductivity of this material has also contributed to the successful use of CeO 2 nanoparticles as catalytic anodes in solid oxide fuel cells (SOFC). [1d,e] However, the detailed mechanism for oxygen buffering in these materials is relatively poorly understood. In part, this is due to the complex nature of partially reduced nanocrystalline CeO 2 , which is exaggerated in nanoparticles where surfaces play a crucial role in catalytic activity and selectivity and for which theoretical prediction of surface configurations using density functional theory is not straightforward. [4] Atomic force microscopy has provided experimental verification of theoretically predicted structures for bulk single crystals, but it has not previously been possible to determine the surface chemistry of active oxide nanoparticles experimentally. [4b] Herein, we show that spherical aberration-corrected transmission electron microscopy (TEM) can be used to determine the surface termination of oxide nanoparticles. We have used a combination of direct aberration corrected TEM, and computational exit wavefunction restoration, [7] to examine the structure of specific surfaces of catalytic CeO 2 nanoparticles during a reversible beam induced redox reaction at ambient temperature. a shows the phase of the specimen exit surface wavefunction of a representative ceria nanoparticle viewed close to a < 110 > direction, restored from a focal series of aberration-corrected images. The nanoparticle surfaces pres-ent in the restored exit wave phase parallel to the electron beam were characterised by extracting average line profiles and matching these to equivalent profiles extracted from simulated data ). We emphasise that this comparison with simulated data is simplified compared to that using single images since reconstruction of the exit wavefunction computationally compensates the residual aberrations of the microscope leaving the projected structure and specimen thickness as the only unknown parameters.