Octahedral CeO2 Research Articles

A kinetic study of catalytic hydrothermal reactions of acetaldehyde with cubic CeO2 nanoparticles

The hydrothermal reactions of acetaldehyde using cubic CeO2 nanoparticles were performed on the assumption that repolymerisation of monolignin by aldehydes can be suppressed by catalytic oxidation in the presence of CeO2 during depolymerisation of lignin. The catalytic potential of cubic CeO2 nanoparticles was evaluated by comparing them with octahedral CeO2 via their structural, morphological, catalytic characterisation and in terms of catalytic reactivity. In the cubic CeO2 system, the conversion of acetaldehyde and the selectivity of acetic acid (oxidation of acetaldehyde) were higher than those in octahedral CeO2, while the selectivity of crotonaldehyde produced by aldol condensation was lower than that in octahedral CeO2. The overall reaction mechanism was also developed based on a kinetic study on the primary products of acetaldehyde using the first-rank Delplot method. According to its catalytic oxidation rate constant, cubic CeO2 has 3–5 times higher oxidative catalytic potential than octahedral CeO2, suggesting that the repolymerisation of monolignin by aldehydes can be effectively suppressed by cubic CeO2 nanocatalysts.

Highly Selective Production of Acrylic Acid from Glycerol via Two Steps Using Au/CeO2 Catalysts

Using biomass resources for chemical production can provide more sustainable development in the chemical industry. In this work, acrylic acid was selectively produced from glycerol via two steps: Glycerol was efficiently converted to allyl alcohol by formic acid-mediated deoxydehydration (DODH), and then, the obtained allyl alcohol was oxidized without purification into acrylic acid in a basic aqueous solution. The Au/CeO2 catalysts were used for the selective oxidation, and it worked well even in the presence of residual formic acid and impurities. A high yield of 87% was obtained for the production of acrylic acid from glycerol: 94.5% from glycerol to allyl alcohol and 92% from allyl alcohol to acrylic acid. The different shapes of CeO2 such as rods, octahedra, and cubes were used as supports to deposit the Au active phase. Au deposited on octahedral CeO2 presented the highest yield toward acrylic acid, and it was the most stable for the repeated oxidations. The effects of reaction time, temperature, O2...

The synergy between atomically dispersed Pd and cerium oxide for enhanced catalytic properties

We report a photochemical synthesis of Pd/CeO2 catalysts with atomically dispersed Pd. Compared to atomically dispersed Pd/CeO2 with a cubic CeO2 support (Pd/CeO2-CP), atomically dispersed Pd/CeO2 with a truncated octahedral CeO2 support (Pd/CeO2-TOP) exhibited higher activity and selectivity, owing to the synergy between Pd atoms and the (111) surface of CeO2. When compared to Pd/CeO2 with Pd clusters and nanoparticles via chemical reduction, Pd/CeO2-TOP showed excellent activity with an enhancement factor of 324 in CO oxidation, as well as an activity enhancement by a factor of 344 in selective oxidation of benzyl alcohol.

Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation

Octahedral CeO2 microparticles possessing a stair-like microstructure comprising self-assembled nano-sized rectangular blocks were synthesized by a hydrothermal synthetic route. The samples were characterized by X-ray diffraction, scanning/transmission electron microscopy, and nitrogen adsorption/desorption techniques. The structural functions of the as-synthesized CeO2 as support for the Pd catalyst were investigated by the catalytic oxidation of low-concentration methane. The results showed that the unique stair-like structure of CeO2 improved the stability of the Pd catalyst for continuous conversion of low-concentration methane, which suggests that the as-synthesized CeO2 has potential application in the removal of low-concentration methane from ventilation air and other oxidations.

Surface Structure Dependence of SO2 Interaction with Ceria Nanocrystals with Well-Defined Surface Facets

The effects of the surface structure of ceria (CeO2) on the nature, strength, and amount of species resulting from SO2 adsorption were studied using in situ IR and Raman spectroscopies coupled with mass spectrometry, along with first-principles calculations based on density functional theory (DFT). CeO2 nanocrystals with different morphologies, namely, rods (representing a defective structure), cubes (100 facet), and octahedra (111 facet), were used to represent different CeO2 surface structures. IR and Raman spectroscopic studies showed that the structure and binding strength of adsorbed species from SO2 depend on the shape of the CeO2 nanocrystals. SO2 adsorbs mainly as surface sulfites and sulfates at room temperature on CeO2 rods, cubes, and octahedra that were either oxidatively or reductively pretreated. The formation of sulfites is more evident on CeO2 octahedra, whereas surface sulfates are more prominent on CeO2 rods and cubes. This is explained by the increasing reducibility of the surface oxygen in the order octahedra < cubes < rods. Bulk sulfites are also formed during SO2 adsorption on reduced CeO2 rods. The formation of surface sulfites and sulfates on CeO2 cubes is in good agreement with our DFT results of SO2 interactions with the CeO2(100) surface. CeO2 rods desorb SO2 at higher temperatures than cubes and octahedra nanocrystals, but bulk sulfates are formed on CeO2 rods and cubes after high-temperature desorption whereas only some surface sulfates/sulfites are left on octahedra. This difference is rationalized by the fact that CeO2 rods have the highest surface basicity and largest amount of defects among the three nanocrystals, so they bind and react with SO2 strongly and are the most degraded after SO2 adsorption cycles. The fundamental understanding obtained in this work on the effects of the surface structure and defects on the interaction of SO2 with CeO2 provides insights for the design of more sulfur-resistant CeO2-based catalysts.

Near-Infrared Light Manipulated Chemoselective Reductions Enabled by an Upconversional Supersandwich Nanostructure.

Core-satellite is one of the most powerful superstructures since it leads to enhanced or completely new properties through compatible combination of each component. Here we create a novel ceria-based core-shell-satellite supersandwich structure with near-infrared (NIR) light manipulated catalytic activity by integrating the upconversion luminescent and catalytic functionality of CeO2 nanoparticles. Specifically, lanthanide-doped octahedral CeO2 nanoparticles (o-CeO2) are coated with silica layer (o-CeO2@SiO2) to enhance their luminescence intensity. The pH-dependent catalytic active cubic CeO2 nanoparticles (c-CeO2) are then assembled on the surface of o-CeO2@SiO2 to form the supersandwich structure (o-CeO2@SiO2@c-CeO2) following a classic chemical reaction. The upconversion quantum yield of o-CeO2 in this nanostructure can be nearly doubled. Furthermore, under NIR light irradiation, the o-CeO2@SiO2@c-CeO2 supersandwich structure based composite catalyst displays superior catalytic activity in selective reduction of aromatic nitro compounds to corresponding azo compounds, and the composite photocatalyst can be easily recycled for several times without significant loss of catalytic activity. This strategy may serve as a universal method for the construction of multifunctional nanostructures and shed light on the green chemistry for chemical synthesis.

Dual-functional CeO2:Eu3+ nanocrystals for performance-enhanced dye-sensitized solar cells.

Single-crystalline, octahedral CeO2:Eu3+ nanocrystals, successfully prepared using a simple hydrothermal method, were investigated to determine their photovoltaic properties in an effort to enhance the light-harvesting efficiency of dye-sensitized solar cells (DSSCs). The size of the CeO2:Eu3+ nanocrystals (300-400 nm), as well as their mirrorlike facets, significantly improved the diffuse reflectance of visible light. Excitation of the CeO2:Eu3+ nanocrystal with 330 nm ultraviolet light was re-emitted via downconversion photoluminescence (PL) from 570 to 672 nm, corresponding to the 5D0→7FJ transition in the Eu3+ ions. Downconversion PL was dominant at 590 nm and had a maximum intensity for 1 mol % Eu3+. The CeO2:Eu3+ nanocrystal-based DSSCs exhibited a power conversion efficiency of 8.36%, an increase of 14%, compared with conventional TiO2 nanoparticle-based DSSCs, because of the strong light-scattering and downconversion PL of the CeO2:Eu3+ nanocrystals.

Morphology Effect of CeO2 Support in the Preparation, Metal–Support Interaction, and Catalytic Performance of Pt/CeO2 Catalysts

AbstractPt/CeO2 catalysts with various Pt loadings were prepared by a conventional incipient wetness impregnation method that employed CeO2 cubes (c‐CeO2), rods (r‐CeO2), and octahedra (o‐CeO2) as the support and Pt(NH3)4(NO3)2 as the metal precursor. Their structures and catalytic activities in CO oxidation in excess O2 and the preferential oxidation of CO in a H2‐rich gas (CO‐PROX) were studied, and strong morphology effects were observed. The impregnated Pt precursor interacts more strongly with CeO2 rods and cubes than with CeO2 octahedra, and the reduction/decomposition of the Pt precursor impregnated on CeO2 octahedra is easier than that on CeO2 rods and cubes. With the same Pt loading, the Pt/o‐CeO2 catalyst contains the largest fraction of metallic Pt, whereas the Pt/c‐CeO2 catalyst contains the largest fraction of Pt2+ species. The reducibility of pure CeO2 and CeO2 in the Pt/CeO2 catalysts follows the order r‐CeO2&gt;c‐CeO2&gt;o‐CeO2, and the reducibility of CeO2 depends on the Pt loading for the Pt/c‐CeO2 catalysts but not much for the Pt/r‐CeO2 and Pt/o‐CeO2 catalysts. The catalytic performance of Pt/CeO2 catalysts in both CO oxidation and the CO‐PROX reaction follows the order Pt/r‐CeO2&gt;Pt/c‐CeO2&gt; Pt/o‐CeO2. The Pt0‐CeO2 ensemble is more active than the Pt2+‐CeO2 ensemble in the catalysis of CO oxidation in excess O2. H2‐assisted CO oxidation catalyzed by the Pt/CeO2 catalysts was observed in the CO‐PROX reaction, and the Pt2+ species and CeO2 with a large concentration of oxygen vacancies constitute the active structure of the Pt/CeO2 catalyst for the CO‐PROX reaction. The effect of the morphology of the CeO2 support in the preparation, metal–support interaction, and catalytic performance of Pt/CeO2 catalysts can be correlated the exposed crystal planes and surface composition/structure of the CeO2 support with different morphologies. These results not only demonstrate that the structure and catalytic performance of oxide‐supported catalysts can be tuned by controlling the morphology of the oxide support but also deepens the fundamental understanding of CO oxidation reactions catalyzed by Pt/CeO2 catalysts.

Surface structure dependence of selective oxidation of ethanol on faceted CeO2 nanocrystals

Shaped CeO2 nanoparticles have been used to explore the effect of surface structure upon the surface chemistry and catalytic selectivity for the ethanol selective oxidation reaction. CeO2 octahedra, cubes, and rods were synthesized using previously published methods. Adsorption and desorption behavior on these nanoshapes was determined by a combination of temperature-programmed desorption (TPD) and in situ DRIFTS. Activity and selectivity were measured in steady-state reaction and in temperature-programmed surface reaction (TPSR). Shape-dependent differences are observed in surface adsorbates, their transformation temperatures, and the selectivity for dehydration, dehydrogenation, and decomposition. Ethoxide and acetate are the primary surface species present under both TPD and TPSR conditions for all shapes. Different rates of α- and β-CH bond scission on the different shapes are responsible for different product selectivity. Structure-dependent, reductive vacancy formation and availability of reactant O2 combine to control surface H which in turn plays a role in controlling product selectivity.

Catalytic Cracking Reaction of Heavy Oil in the Presence of Cerium Oxide Nanoparticles in Supercritical Water

Catalytic cracking of Canadian oil sand bitumen in supercritical water was performed to clarify the effect of CeO2 nanoparticles. The cracking was performed at 723 K to promote a redox reaction between the water, bitumen, and catalyst for the production of hydrogen and oxygen. As the catalyst, CeO2 with two different morphologies was employed because the redox reaction of CeO2 with water and organics is expected and its activity can be controlled by its structure. In this study, two roles of water were considered as well. Water is attractive as a high potential medium with low dielectric constant and density at near the critical point (374 °C, 22.1 MPa) that allows formation of highly crystalline smaller metal oxides particles. However, the chemical effects of water are investigated with heavy oil catalytic cracking. Transmission electron microscopy images indicated that CeO2 nanoparticles with cubic and octahedral shape were synthesized using a plug-flow reactor under hydrothermal conditions. The particles sizes were 8 and 50 nm for cubic and octahedral CeO2, respectively. At 773 K, it was found that the oxygen storage capacity (OSC) of the cerium oxide nanoparticles with cubic {100} facets was nearly 3.4 times higher than that of the cerium oxide nanoparticles with octahedral {111} facets. Heavy oil fractions of bitumen were cracked in a batch-type reactor at 723 K in order to produce as much light oil as possible, and the effect of the catalyst loading and reaction conditions on the conversion rate and coke formation were investigated. As a result, it was demonstrated that it is possible to obtain a high conversion rate by increasing the exposed surface area and reducing the particle size of the catalyst. The highest conversion was obtained in the presence of 20 mg loading of cubic CeO2 nanoparticles (8 nm) with reaction time of 1 h.

Synthesis of CePO&lt;SUB&gt;4&lt;/SUB&gt; Nano-Wires with Improved Photoluminescent Properties by Co-Crystallizing with Nano-Sized CeO&lt;SUB&gt;2&lt;/SUB&gt;

By adjusting experimental parameters, such as reaction time and ratios, CeO2/CePO4 composites have been prepared through a single-step hydrothermal process, in which CeO2 octahedra and CePO4 nanowires mingled with each other to form many interfaces. The formation of the particular structure can be explained by heterogeneous nucleation theory: the existed CePO4 nanowires at the early stage of hydrothermal process provide many thermodynamically favorable sites, on which the nucleation energy of octahedral CeO2 is the lowest. The photoluminescent (PL) properties of CeO2/CePO4 composites showed that the strongest PL emission can be achieved with initial Ce/P of 4 and hydrothermal time of 24 h. And final Ce/P of the samples at this point was the maximum value by the EDS analysis. So it was proposed that the interaction between more CeO2 and the more coatings of CeO2 to CePO4 are responsible for the best PL properties.

Synthesis of monodisperse CeO2 octahedra assembled by nano-sheets with exposed {001} facets and catalytic property

Octahedron-like CeO2 particles assembled by nanosheets with exposed (001) facets have been successfully synthesized. It has been demonstrated that polyvinyl pyrrolidone (PVP) plays an important role in the formation of octahedral CeO2 assembled by the nanosheets. The octahedral CeO2 assembled by the nanosheets exhibit high CO oxidation activity due to their special structures.

Exploitation of Surface‐Sensitive Electrons in Scanning Electron Microscopy Reveals the Formation Mechanism of New Cubic and Truncated Octahedral CeO2 Nanoparticles

AbstractDevelopment of new analytical tools for nanostructures directly contributes to the study of catalysts. By using scanning electron microscopy (SEM) with a newly designed signal enhancer, we study cubic and truncated octahedral cerium oxide (CeO2) nanoparticles, which are composed of smaller primary octahedral CeO2 and are formed through bond formation with hexanedioic acid. The signal enhancer is designed to efficiently collect secondary electrons of kinetic energy less than 10 eV; thus, it greatly improves the S/N ratio. On the basis of the observed SEM images and electron backscattered diffraction patterns of the cross section of the nanoparticles, we discuss the formation mechanism of the nanoparticles and speculate that the primary CeO2 nanocrystals share their edges in the cubic nanoparticles and truncated octahedral nanoparticles. These results will contribute to the preparation of nanostructured metal oxide surfaces with controlled morphologies that could enhance catalytic activity.

Controllable Electrochemical Synthesis and Photocatalytic Activity of CeO2 Octahedra and Nanotubes

Single-crystalline CeO2 octahedra were successfully grown on Cu substrates via a facile electrochemical approach, and free-standing CeO2 nanotubes with high crystallinity were also obtained by adjusting the reaction temperature. A ferromagnetic behavior at 5 K arising from their intrinsic defects is observed for the CeO2 octahedra and nanotubes. Furthermore, the CeO2 octahedra and nanotubes exhibit a good photocatalytic performance in the degradation of an organic pollutant in waste water.

Three-Dimensional Structure of CeO2 Nanocrystals

Visualization of three-dimensional (3D) structures of materials at the nanometer scale can shed important information on the performance of their applications and provide insight into the growth mechanism of shape-controlled nanomaterials. In this paper, the 3D structures and growth pathway of CeO2 nanocrystals prepared by the hydrothermal method were investigated by electron tomography and high-resolution transmission electron microscopy (HRTEM). Two typical shapes (regular truncated octahedrons and irregular compressed truncated octahedron with {111} and {001} as the major facets) were identified in samples synthesized under different conditions. The homogeneous growth environment in solution with polyvinylpyrrolidone (PVP) molecules led to the formation of regular octahedral CeO2 nanocrystals with small {001} facet truncations. When the PVP surfactant was removed, the aggregation of regular truncated octahedral CeO2 particles through a lattice matched interface generated irregular compressed truncated ...

Electron Tomography of SPM Probes, Nanoparticles and Precipitates

ABSTRACTNanoscale tomographic reconstructions from objects with diameters of 100nm or smaller can only be achieved non-destructively with transmission electron tomography. The application of this technique to W tips, which are common probes for scanning tunneling microscopy and nanoindentation, is demonstrated with emphasis on visualizing oxide layers and functionally attached nanoparticles. For the reconstruction of facetted free-standing catalyst nanoparticles, such as CeO2 octahedra, we propose a combination of energy-filtered (EF) and bright field (BF) TEM tomography to achieve high fidelity of the projection relationship via EFTEM, due to its incoherent imaging mode, and high resolution definition of the particle circumference from the edge-enhancement effects of the BF tomogram. Finally, electron tomography applications to CeO2 nanoprecipitates embedded in a multicomponent oxide glass matrix are shown, which comprises the first tomographic 3D reconstruction of a nanoscale dendrite.