Stoichiometric Ceria Research Articles

Octahedrality versus tetrahedrality in stoichiometric ceria nanoparticles

We predict that tetrahedral Ce(n)O(2n) nanoparticles <2 nm in size become more stable than those experimentally observed at larger sizes with truncated octahedral morphologies, based on global optimisation and density functional calculations.

Mechanism of Sulfur Poisoning and Storage: Adsorption and Reaction of SO2 with Stoichiometric and Reduced Ceria Films on Cu(111)

We have performed a systematic synchrotron radiation photoelectron spectroscopy (SR-PES) study of the interaction of SO2 with stoichiometric CeO2(111) and reduced CeO2–x films supported on Cu(111). Resonant photoelectron spectroscopy (RPES) was utilized to monitor changes in the cerium oxidation states. Adsorption of SO2 at 150 and 300 K results mostly in formation of sulfite (SO32–) species on both reduced and stoichiometric ceria. Other species such as sulfate (SO42–), SO2 chemisorbed at cationic sites, anionic SO2–, atomic sulfur, surface sulfides, and bulk oxy-sulfides were detected. The origin of these species is discussed in relation to the changes in the oxidation state of ceria cations at the surface. The thermal evolution of the species formed upon adsorption at 150 and 300 K was studied systematically on both the stoichiometric and the reduced films. It was found that the sulfite partially desorbs to release SO2 and partially decomposes. In general, the reduced CeO2–x surface was found to be much more active toward cleavage of S–O bonds, as indicated by a higher amount of oxy-sulfide species produced. The decomposition pathways can be monitored via changes of the ceria oxidation state and are discussed in relation to the stoichiometry of the ceria surface.

The Electronic Structure and Chemical Properties of a Ni/CeO2Anode in a Solid Oxide Fuel Cell: A DFT +UStudy

Using a model for the Ni/ceria anode of a solid oxide fuel cell (SOFC), we have performed a DFT + U study of H2 and CH4 activation on the ceria surface and elucidated the mechanisms of charge transfer as a result of ceria reduction. Bader’s Atoms in Molecules (AIM) analysis scheme establishes that deposition of nickel on stoichiometric ceria to form the Ni/ceria anode results in the transfer of electron density from Ni to CeO2. The subsequent removal of an oxygen atom from the ceria surface of the Ni/ceria anode in the process of fuel oxidation results in a minor transfer of electron density from ceria to Ni. The modest amount of transferred electron charge following vacancy formation can be explained by the fact that the rather shallow vacancy-induced occupied band in ceria has a lower energy than the top of the Ni valence band. We also show that the removal of an oxygen atom not bonded to Ni is more favorable (by 28 kcal/mol) than extraction of an oxygen, originally bonded to both Ni and Ce at the Ni/Ce...

The Effect of Environment on the Reaction of Water on the Ceria(111) Surface: A DFT+U Study

The interaction of a water molecule with the (111) surfaces of stoichiometric and reduced ceria is investigated using first principle density functional theory with the inclusion of the on-site Coulomb interaction (DFT+U). It is found that on the stoichiometric ceria(111) surface, the water molecule is adsorbed spontaneously through single hydrogen bond configuration. In contrast, on the lightly reduced ceria(111), there exist both molecular adsorption (no-H-bond configuration) and dissociative adsorption (surface hydroxyl) modes. It is obvious that oxygen vacancies can enhance the interaction of water with the substrate. Phase diagrams for stoichiometric and reduced ceria(111) surfaces in equilibrium with water vapor in the complete range of experimentally accessible gas phase condition are calculated and discussed combining the DFT results and thermodynamics data using the ab initio atomistic thermodynamic method. We present a detailed analysis of the stability of the water−ceria system as a function of the ambient conditions, and focus on two important surface processes for water adsorption on the stoichiometric and on the lightly reduced surfaces, respectively.

Atomic and Electronic Structure of Cerium Oxide Stepped Model Surfaces

The atomic and electronic structure of ceria surfaces exhibiting step edges have been studied by means of periodic density functional (LDA+U and GGA+U) calculations. A variety of stoichiometric and nonstoichiometric models of increasing complexity have been designed. The electronic structure has been explored using the topological Bader analysis, the calculated magnetic moments and the ELF (electron localization function) maps. It is concluded that Ce3+ atoms may exist even in stoichiometric extended ceria samples and that the presence of oxygen vacancies in stepped surfaces also induces the presence of Ce3+ atoms although in both cases, the reduced atoms tend to occupy the sites with smallest possible coordination number.

Density functional theory study of water adsorption at reduced and stoichiometric ceria (111) surfaces

We study the structure and energetics of water molecules adsorbed at ceria (111) surfaces for 0.5 and 1.0 ML coverages using density functional theory. The results of this study provide a theoretical framework for interpreting recent experimental results on the redox properties of water at ceria (111) surfaces. In particular, we have computed the structure and energetics of various absorption geometries at the stoichiometric ceria (111) surface. We find that single hydrogen bonds between the water and the oxide surface are favored in all cases. At stoichiometric surfaces, the water adsorption energy depends rather weakly on coverage. We predict that the observed coverage dependence of the water adsorption energy at stoichiometric surfaces is likely the result of dipole-dipole interactions between adsorbed water molecules. When oxygen vacancies are introduced in various surface layers, water molecules are attracted more strongly to the surface. We find that it is very slightly energetically favorable for adsorbed water to oxidized the reduced (111) surface with the evolution of H(2). In the event that water does not oxidize the surface, we predict that the effective attractive water-vacancy interaction will result in a significant enhancement of the vacancy concentration at the surface in agreement with experimental observations. Finally, we present our results in the context of recent experimental and theoretical studies of vacancy clustering at the (111) ceria surface.

Structure, stability and morphology of stoichiometric ceria crystallites

Download options Please wait... Article information DOI https://doi.org/10.1039/A707052G Article type Paper Download Citation J. Chem. Soc., Faraday Trans., 1998,94, 427-434 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Structure, stability and morphology of stoichiometric ceria crystallites S. Vyas, R. W. Grimes, D. H. Gay and A. L. Rohl, J. Chem. Soc., Faraday Trans., 1998, 94, 427 DOI: 10.1039/A707052G To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Shyam Vyas Robin W. Grimes David H. Gay Andrew L. Rohl

Catalytic properties of nanostructured metal oxides synthesized by inert gas condensation

The correlation between defects in metal oxides and activity in heterogeneous catalytic oxidation was investigated. Processing of nanocrystalline metal oxides by inert gas condensation was employed to synthesize oxygen-deficient nonstoichiometric cerium oxide. Comparison with stoichiometric nanocrystalline ceria revealed the effect of nonstoichiometry on surface chemical reactivity. Stoichiometric nanocrystalline cerium dioxide was investigated for catalytic activity in two reactions; oxidation of CO by (i) SO 2 and (ii) O 2. Catalytic activity was found at significantly different temperatures of 560°C in the former and 360°C in the latter reaction. Complementary temperature programmed reduction (TPR) studies were performed and two distinct signals were found, which correlated with the light-off temperatures for each of the catalytic reactions. This demonstrated that the oxygen intermediates, participating in the redox mechanisms were different for both reactions. Nonstoichiometric nanocrystalline CeO 2−x exhibited catalytic activity in CO oxidation by O 2 and a corresponding TPR-signal at 200°C, which suggested the presence of an even more reactive oxygen species. Fourier-transform infrared spectroscopy (FT-IR) and electrical conductivity were used to study the interaction of chemisorbed CO and O 2 with the metal oxides. Surface defects and chemisorbed oxygen were discussed as possible origins of enhanced catalytic activity due to their effects on the nature of surface oxygen species. The combination of these studies yielded detailed insight into the different reaction mechanisms and the chemical interaction between solid surface and gaseous species that are critical towards the engineering of catalytic and gas sensor materials.

Catalytic redox activity and electrical conductivity of nanocrystalline non-stoichiometric cerium oxide

Nanocrystalline cerium was synthesized by magnetron sputtering and inert gas condensation. Controlled post-oxidation of the metallic nanoclusters resulted in formation of oxygen-deficient cerium oxide. Catalytic redox reactions such as SO 2 reduction by CO and CO oxidation in air were investigated. A comparison of activity with conventional stoichiometric ceria catalysts was used as a probe to distinguish different surface reactions. Electrical conductivity of nanocrystalline CeO 2− x was investigated by impedance spectroscopy and compared with literature data for single-crystal CeO 2. The results are discussed in terms of the microstructure and defect chemistry of nanocrystalline cerium oxide.