Ceria Nanoshapes Research Articles

X-ray photoelectron and Raman spectroscopy of nanostructured ceria in soot oxidation under operando conditions

Ceria nanoshapes exhibiting different amounts of {100}, {110} and {111} facets (cubes, rods, octahedra and polyhedra) were prepared, characterized, tested in the carbon soot oxidation reaction, and studied by operando near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando Raman spectroscopy up to 550 °C. The specific soot oxidation reaction rate (mgC·m−2·min−1) clearly indicated that the {110} and {100} crystallographic planes of ceria were more active than the {111} ones for the oxidation of carbon soot. As deduced from the Ce 3d and O 1s signals recorded using different photon energies, all ceria nanoshapes experienced progressive reduction upon increasing the temperature under Ar, which was accompanied by the formation of oxygen vacancies. Raman studies revealed that, at this stage, isolated graphene layers in carbon soot reacted with ceria lattice oxygen atoms following a Mars-Van Krevelen mechanism. After exposure to O2 at 550 °C, a broad signal in the O 1s region at 531.2–532.5 eV was ascribed to surface active oxygen species, such as peroxide (O22−) and superoxide (O2−) species, generated from the interaction of molecular O2 with oxygen vacancies, which proved to be highly reactive to oxidize the graphitic structures of carbon soot.

Water Promotion (or Inhibition) of Condensation Reactions Depends on Exposed Cerium Oxide Catalyst Facets

Nanoparticles with well-defined facets enable quantitative correlations between surface features and catalytic activity. Here, we explore the role of geometric and acid–base properties in the mechanism of aldol condensation catalyzed by ceria nanoshapes. In the crucial C–C coupling step, the two carbonyl adsorbates are bound to two adjacent cations. On the CeO2(110) plane, all atoms lie on the same layer, favoring the interaction between adsorbates and facilitating the bimolecular C–C coupling. Thus, the initial unimolecular deprotonation is rate-limiting. In contrast, the (100) and (111) planes have an open structure with O on the top layer and Ce on the layer below. The former O layer interferes between adsorbates linked to Ce sites, making the C–C coupling rate-limiting. Water helps in overcoming this spatial hindrance by remote bond polarization via H-bonds. Therefore, water promotion is only observed on those planes for which the bimolecular step is rate-limiting.

Catalytic performance and an insight into the mechanism of CeO2 nanocrystals with different exposed facets in catalytic ozonation of p-nitrophenol

Ceria based catalysts have been widely applied in ozonation of organic pollutants in wastewater treatment. Ceria with different exposed facets have not been used in catalytic ozonation. In this study, ceria nanorods (R-CeO2), nanocubes (C-CeO2) and nanooctahedra (O-CeO2) with different exposed facets (110) + (100), (100) and (111)) respectively were used in catalytic ozonation of p-nitrophenol. Two calcination temperatures 300 and 500 °C were selected for comparative catalytic activity. Ceria nanoparticles (NP-CeO2) were also prepared for comparative catalytic activity. Approximately 86%, 71%, 68%, 64%, 60% and 40% TOC was removed by ozonation catalyzed by R300-CeO2, C300-CeO2, CeO2-NP, O300-CeO2, R500-CeO2 and ozonation alone respectively. Nanorod calcined at 500 °C (R500-CeO2) showed lower catalytic activity as compared to R300-CeO2. O3 was also rapidly decomposed by R300-CeO2 as compared to other catalysts and single ozonation. Inorganic hydroxyl radical scavengers and several characterization techniques were applied for investigating the underlying mechanism of catalytic ozonation. Hydroxyl radicals, surface peroxide and surface atomic oxygen as reactive oxygen species were generated for enhancing the catalytic activity. Furthermore, irrespective of surface area, the difference in catalytic activity of ceria nanoshapes were assigned to the differences in the abundance of surface basic sites, defects densities/oxygen vacancies (OVs) and coordination number of surface atoms. This study provides an insight to use other metals with variety of exposed facets for catalytic ozonation.

Structural Determination of Catalytically Active Subnanometer Iron Oxide Clusters

Supported subnanometer clusters exhibit superiority in catalytic performance compared to common nanoparticles, due to their higher fraction of exposed surfaces and larger number of active species at the metal–support interface, responding to the size effect and the support effect in heterogeneous catalysis. Here, we report the synthesis of subnanometer iron oxide clusters anchored to the surfaces of two types of ceria nanoshapes (nanorods and nanopolyhedra), as well as the structure–activity relation investigation for Fischer–Tropsch synthesis. On the basis of the comprehensive structural characterizations including aberration-corrected scanning transmission electron microscopy (STEM) and X-ray absorption fine structure (XAFS), we demonstrated that the subnanometer clusters of iron oxide are stable and catalytically active for the Fischer–Tropsch synthesis reaction. Furthermore, it is identified that finely dispersed iron oxide clusters (Fe–Ox–Fey) consisted of partially reduced Feδ+ (δ = 2.6–2.9) species...

Surface-Dependence of Defect Chemistry of Nanostructured Ceria

The defect chemistry of reduced ceria nanoshapes was investigated using in situ Raman and FTIR spectroscopy. Octahedral- and rod-shaped particles behave similarly upon exposure to CO in terms of formation of anion Frenkel pair defects and oxygen vacancies. This similarity is attributed to the preferential exposure of (111) surfaces in both type of particles. Cube-shaped particles terminated with (100)-oriented surfaces exhibit very different defect behavior in CO, revealing formation of oxygen vacancy defects at the expense of existing anion Frenkel pairs. Octahedra and rods, prereduced in H2, can be further reduced with CO. In contrast, prereduced cubes can reactively adsorb CO, forming surface-bicarbonate, only via converting Frenkel-pair defects to oxygen vacancies, without any further net reduction of the ceria.

Spectroscopic Investigation of Surface-Dependent Acid–Base Property of Ceria Nanoshapes

In addition to their well-known redox character, the acid–base property is another interesting aspect of ceria-based catalysts. Herein, the effect of surface structure on the acid–base property of ceria was studied in detail by utilizing ceria nanocrystals with different morphologies (cubes, octahedra, and rods) that exhibit crystallographically well-defined surface facets. The nature, type, strength, and amount of acid and base sites on these ceria nanoshapes were investigated via in situ IR spectroscopy combined with various probe molecules. Pyridine adsorption shows the presence of Lewis acid sites (Ce cations) on the ceria nanoshapes. These Lewis acid sites are relatively weak and similar in strength among the three nanoshapes according to the probing by both pyridine and acetonitrile. Two types of basic sites, hydroxyl groups and surface lattice oxygen are present on the ceria nanoshapes, as probed by CO2 adsorption. CO2 and chloroform adsorption indicate that the strength and amount of the Lewis base sites are shape dependent: rods > cubes > octahedra. The weak and strong surface dependence of the acid and base sites, respectively, are a result of interplay between the surface structure dependent coordination unsaturation status of the Ce cations and O anions and the amount of defect sites on the three ceria nanoshapes. Furthermore, it was found that the nature of the acid–base sites of ceria can be impacted by impurities, such as Na and P residues that result from their use as structure-directing reagent in the hydrothermal synthesis of the ceria nanocrystals. This observation calls for precaution in interpreting the catalytic behavior of nanoshaped ceria where trace impurities may be present.

Pulse Studies to Decipher the Role of Surface Morphology in CuO/CeO2 Nanocatalysts for the Water Gas Shift Reaction

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over CuO/CeO2 catalysts with two different ceria particle morphologies, in the form of nanospheres (ns) and nanocubes (nc). To understand the strong dependence of the WGS reaction activity on the ceria nanoshapes, pulses of CO (without and with water vapor) were employed during in situ X-ray diffraction and X-ray absorption near edge structure measurements done to characterize the catalysts. The results showed that CuO/CeO2 (ns) exhibited a substantially better activity than CuO/CeO2 (nc). The higher activity was associated with the unique properties of CuO/CeO2 (ns), such as the easier reduction of highly dispersed CuO to metallic Cu, the stability of metallic Cu and a larger concentration of Ce3+ in CeO2 (ns).

ChemInform Abstract: Shape‐Controlled Ceria‐Based Nanostructures for Catalysis Applications

AbstractReview: 104 refs.

Shape‐Controlled Ceria‐based Nanostructures for Catalysis Applications

Among oxide catalysts, ceria is a technologically important material because of its wide applications as a promoter in three-way catalysts for the elimination of toxic exhaust gases, low-temperature water-gas-shift reaction, oxygen sensors, oxygen permeation membrane systems, and fuel cells. The catalytic activities of cerium oxide are highly dependent on interfacial structures and nanocrystal morphologies. This Minireview highlights the recent progress in the research of ceria nanoshapes as both catalysts and catalyst supports, including the synthesis, structure characterization, catalytic properties, surface chemistry, as well as reaction mechanisms. Insights from in situ spectroscopy study and theoretical modeling of nanostructured ceria-based materials have shed light on the origin of the ceria shape effect. It is suggested that the surface structure of ceria controls the catalytic activity and selectivity through structure-dependent surface-site geometry, surface vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria. The morphology-dependent catalysis in ceria has offered a new strategy to finely tune the catalytic activity and selectivity through shape control without altering the catalyst composition. A brief summary and an outlook on this research field will be presented at the end.

Ceria Nanocatalysts: Shape Dependent Reactivity and Formation of OH

AbstractCeria nanoshapes (octahedra, wires, and cubes) were investigated for CO adsorption and subsequent reaction with water. Surprisingly, the reactivity of specific OH groups was explicitly determined by the ceria nanoshape. The nanoshapes showed different levels of carbonates and formates after exposure to CO, the amount of carbonates increasing from octahedral≪wires<cubes. Subsequent reaction with water at 200 °C was also found to be shape dependent, resulting in different amounts of recovered OH groups and removed carbonates and formates on the different ceria nanoshapes.

A Raman Spectroscopic Study of the Speciation of Vanadia Supported on Ceria Nanocrystals with Defined Surface Planes

AbstractVanadia (VOx) supported on ceria (CeO2) nanocrystals with defined surface planes, which includes rods, cubes and octahedra, was synthesized and used to explore the effect of support surface structure on the speciation of surface vanadia. The vanadia structures on these ceria “nanoshapes” were identified by in situ visible and UV Raman spectroscopy as a function of loading and calcination temperature, and they include monomeric, dimeric, trimeric, polymeric vanadia, and eventually crystalline V2O5 and CeVO4 as vanadia loading increases. As expected, the faceted ceria nanocrystals provide a rather homogeneous platform for anchoring the vanadia. At low vanadia surface density, only monomeric vanadia exists on the ceria nanoshapes, in contrast to vanadia supported on polycrystalline CeO2 in which multiple vanadia species coexist. Formation of CeVO4 from the reaction between surface vanadia and ceria upon high temperature calcination was compared for the three ceria nanoshapes with similar surface vanadia density (≈1/4 monolayer). It was found that both the surface structure and the amount of defect sites on the ceria nanoshapes play major roles in the production of CeVO4. The easier formation of CeVO4 on ceria rods, compared with cubes or octahedra, is attributed to the rods’ lowest surface oxygen vacancy formation energy and largest amount of defect sites.

On the structure dependence of CO oxidation over CeO2 nanocrystals with well-defined surface planes

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({110}+{100}), cubes ({100}), and octahedra ({111}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water–gas shift reaction, and CO disproportionation reaction, reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods>cubes>octahedra. The mobility of lattice oxygen also shows similar dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO2, while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.