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Abstract

CeO(2) nanotubes have been grown electrochemically using a porous alumina membrane as a template. The resulting material has been characterized by means of scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy, high-angle annular dark-field scanning transmission electron microscopy tomography, high-resolution electron microscopy (HREM), and electron energy loss spectroscopy. According to SEM, the outer diameter of the nanotubes corresponds to the pore size (200 nm) of the alumina membrane, and their length ranges between 30 and 40 microm. HREM images have revealed that the width of the nanotube walls is about 6 nm. The catalytic activity of these novel materials for the CO oxidation reaction is compared to that of a polycrystalline powder CeO(2) sample prepared by a conventional route. The activity of the CeO(2) nanotubes is shown to be in the order of 400 times higher per gram of oxide at 200 degrees C (77.2 x 10(-2) cm(3) CO(2) (STP)/(gxs) for the nanotube-shaped CeO(2) and 0.16 x 10(-2) cm(3) CO(2) (STP)/(gxs) for the powder CeO(2)).

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... For this reason several synthesis methods that yield nanostructured CeO 2 (nanotubes, nanorods, nanowires, nanopolyhedra, etc.) have been developed. These nanostructured CeO 2 oxides demonstrate enhanced catalytic behavior compared with bulk CeO 2 obtained by conventional synthesis methods [20][21][22]. However, only a few papers focus on the use of both mixing with a second metallic element and nanostructuration to improve the performance of CeO 2 -based materials [23,24]. ...
... A template-based electrodeposition method was employed to synthesize the nanotubes following a procedure similar to that previously reported in [22,30]. Whatman Anodisc commercial anodic alumina membranes with a thickness of 60 m and with pore diameters of approximately 200 nm, were employed as a template. ...
... The solution was added to the cell 1 h before starting the electrodeposition to assure that the pores were completely filled. Since the chemistry of praseodymium compounds is very similar to that of their equivalent of cerium, we can expect that the nanostructures formed using the proposed synthesis route, which in fact promotes the co-precipitation of the cations via electro-reduction of their nitrates, will be made up of Ce(OH) 3 and Pr(OH) 3 as already reported in [22,31,32]. Finally, the nanotube/membrane systems were oxidized at 250 • C in air for 1 h. ...
Article
Ceria-praseodimia nanotubes (Ce0.8Pr0.2O2−δ–NT) have been synthesized for the first time employing a template-based electrodeposition method inside the pores of anodic aluminum oxide (AAO) membranes. Various electron microscopy techniques such as Field Emission Gun Scanning Electron Microscopy (FEG-SEM), Scanning Transmission Electron Microscopy working in High Angle Annular Dark-Field mode (STEM-HAADF), High Resolution Transmision Electron Microscopy (HRTEM), Energy-dispersive X-ray Spectroscopy (X-EDS), Electron Energy Loss Spectroscopy (EELS) and Energy Filtered Transmision Electron Microscopy (EFTEM) have been used to characterise the morphology, structure and chemical composition of the nanotubes. The results indicate that nanotubes are formed by nanocrystals of cerium and praseodymium mixed oxide. Furthermore, the system Ce0.8Pr0.2O2−δ–NT/AAO shows a better performance in the CO oxidation reaction than both powdered Ce0.8Pr0.2O2−δ prepared by conventional methods and CeO2–NT/AAO. These results have been discussed and related to the synergistic effect of doping and nanostructuration of CeO2.
... Although the unique crystal plane has provided a good platform for understanding the role of active sites in catalysis, the relatively low ratio of surface active sites to the total atom numbers makes the quantification of a modelperformance relationship a great challenge (Fig. 1a). Fortunately, recent studies have revealed that nanoscale crystallites such as CeO 2 nanostructures are beneficial to the CO oxidation reaction thanks to their downsizing effect [24][25][26][27][28][29] . For example, Botana et al. 25 have found that CeO 2 nanotubes exhibited 400-fold higher CO oxidation compared to polycrystalline CeO 2 powder owing to their superior redox properties. ...
... Fortunately, recent studies have revealed that nanoscale crystallites such as CeO 2 nanostructures are beneficial to the CO oxidation reaction thanks to their downsizing effect [24][25][26][27][28][29] . For example, Botana et al. 25 have found that CeO 2 nanotubes exhibited 400-fold higher CO oxidation compared to polycrystalline CeO 2 powder owing to their superior redox properties. Also, Tsang et al. have demonstrated that the quantum size effect could enable the nanosize CeO 2 (below 5 nm) to possess dramatically increased surface superoxide species 30 , which undoubtedly contributed to improve the catalytic oxidation activity [30][31][32] . ...
... In other words, the active centres consisting of pit-surrounding Ce sites were beneficial to O 2 activation, CO adsorption and diffusion. In addition, the extremely high fraction of surface Ce atoms as well as the quantum size effect in the ultrathin CeO 2 sheets could also contribute to lower their overall activation energy and definitely accelerate the speed of CO catalytic reaction compared with the bulk counterpart [24][25][26][27][28][29][30][31][32] , which could be verified by the CO conversion curves and the corresponding activation energies of the intact ultrathin CeO 2 sheets and bulk counterpart in Fig. 4d,e. As such, the different activation energies between the ultrathin CeO 2 sheets with numerous surface pits and the bulk counterpart were not only due to the presence of surface pits but also due to the quantum effects at the nano-size. ...
Article
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Finding ideal material models for studying the role of catalytic active sites remains a great challenge. Here we propose pits confined in an atomically thin sheet as a platform to evaluate carbon monoxide catalytic oxidation at various sites. The artificial three-atomic-layer thin cerium(IV) oxide sheet with approximately 20% pits occupancy possesses abundant pit-surrounding cerium sites having average coordination numbers of 4.6 as revealed by X-ray absorption spectroscopy. Density-functional calculations disclose that the four- and five-fold coordinated pit-surrounding cerium sites assume their respective role in carbon monoxide adsorption and oxygen activation, which lowers the activation barrier and avoids catalytic poisoning. Moreover, the presence of coordination-unsaturated cerium sites increases the carrier density and facilitates carbon monoxide diffusion along the two-dimensional conducting channels of surface pits. The atomically thin sheet with surface-confined pits exhibits lower apparent activation energy than the bulk material (61.7 versus 122.9 kJ mol(-1)), leading to reduced conversion temperature and enhanced carbon monoxide catalytic ability.
... Among various shaped CeO2 structures, CeO2 nanotubes (NT-CeO2) are generally prepared by electrochemical deposition in anodie aluminum oxide (AAO) film or template methods (Ce(OH)3, Ce(OH)CO3, etc.) [10][11][12][13][14][15]. Practically, the polycrystalline NT-CeO2 were produced [10][11][12][13]16]. ...
... Among various shaped CeO2 structures, CeO2 nanotubes (NT-CeO2) are generally prepared by electrochemical deposition in anodie aluminum oxide (AAO) film or template methods (Ce(OH)3, Ce(OH)CO3, etc.) [10][11][12][13][14][15]. Practically, the polycrystalline NT-CeO2 were produced [10][11][12][13]16]. In contrast, synthesis of single crystalline NT-CeO2 is still challenging due to the lack of layered features of CeO2. ...
Article
Here, we report a facile synthetic methodology to prepare uniform single crystalline CeO2 nanotubes through a hydrothermal transformation of CeO2 nanorods in aqueous Ce(NO3)3 solution. A chemically driven etching-dissolution-deposition mechanism is proposed, involving the surface Ce3+ hydrolysis and dissolution at tips of nanorods and subsequent redeposition and crystallization on the outer sides of nanorods. Compared to CeO2 nanorods, CeO2 nanotubes exhibited the richer structural defects, higher reducibility and larger surface area, leading to a higher haloperoxidase-like activity.
... Therefore, various templating methods have been established for preparing CeO 2 nanorods and nanowires. Porous anodic alumina membranes (AAM) are commonly used in such synthesis as hard templates by virtue of their modulated pore diameters, ideally rod-shaped pores and exceptionally narrow size distribution [18][19][20]. CeO 2 nanowires were successfully synthesized with an improved sol-gel method within the hexagonally ordered nanochannels of the AAM. Ce 3+ cations and corresponding anions reacted directly inside the nanochannels to form intermediates with 1D nanostructures. ...
... Ce 3+ cations and corresponding anions reacted directly inside the nanochannels to form intermediates with 1D nanostructures. After posttreatment, the intermediates were transformed into the arrays of CeO 2 nanowires within the pores of AAM template [20]. CeO 2 nanorods can also be synthesized with solvothermal/hydrothermal methods [21][22][23], which generally feature controlled composition and morphology, low aggregation and high crystallinity, because of the diffusion-controlled growth in solvent media in a closed system. ...
Article
Rare earths (RE) refer to the lanthanide elements La–Lu together with Sc and Y. Conventionally, they have found applications in phosphors, magnets, catalysts, fuel cell electrodes/electrolyte. Here in this chapter, we discuss the synthesis, assembly and applications of rare earth based anisotropic nanomaterials. Regarding synthesis, the anisotropic growth behaviors of these nanocrystals are predominantly governed by their own unique crystal structures. Yet for wet-chemistry synthetic methods where a number of parameters could be finely tuned, the addition of particular coordination agents, templating agents or mineralizers has proven to be an effective way to direct the growth of nanocrystals into some anisotropic structures. Regarding applications, anisotropic nanomaterials, compared to their isotropic counterparts, often exhibit distinct properties. For example, the luminescence of anisotropic nanomaterials can display polarization and site-specific features. As for rare earth nanomaterials as magnetic resonance imaging (MRI) contrast agents, the high surface area of anisotropic nanostructures can give rise to superior performances. And for catalysis applications, anisotropic nanomaterials expose rich, highly active facets, which is of great importance for facet-selective catalytic reactions. In the chapter, we will start with introduction of the crystal structures of rare earth compounds, then briefly summarize the synthesis and assembly of rare earth anisotropic nanomaterials, and discuss their properties and applications in three realms, namely, luminescence, magnetism and catalysis.
... For example, Zhou et.al 13 claimed that CeO 2 nanorods with well-defined reactive planes {001} and {110} show 3-fold improvement in CO oxidation rates over CeO 2 nanoparticles. Leandro and co-workers 14 reported that the activity of the CeO 2 nanocubes exposed with planes {100} is shown to be in the order of 400 times higher per gram of oxide than powder CeO 2 for the CO oxidation reaction. ...
... Among them, the hydrothermal synthesis is considered as the most popular and effective method but special organic reagents, surfactants, or templates are generally needed. In addition, it is also a batch reaction process with long reaction time that typically lasts up to 12-24 h 10,14,25,26 . Therefore, development of facile, controllable, and effective methods for creating ceria architectures with defined exposed surfaces remains an important challenge. ...
Article
In this paper, we present a new method for preparing γ-alumina nanoparticles with a bimodal pore size distribution by using an efficiently mixing membrane dispersion microreactor. NH4HCO3 and Al2(SO4)3∙18H2O were reacted under vigorous mixing to give an ammonium aluminum carbonate hydroxide (AACH) precursor. γ-Alumina was obtained by calcination of AACH at 550 °C for 6 h. The effects of NH4HCO3 concentration, pH during aging, and reaction temperature were investigated. The mechanism of bimodal pore formation was clarified. The results showed that large pores (10–100 nm) were mainly formed in the reactor and during aging, and small pores (0–10 nm) were mainly formed during calcination. When the concentration of NH4HCO3 was 1.5 mol/L, the aging pH was 9.2, and the reaction temperature was 80 °C, γ-alumina with a specific surface area of 504.7 m²/g and pore volume of 1.76 mL/g was obtained. The average size of the large pores was about 30 nm, and the average size of the small pores was about 4 nm.
... Researchers have described numerous techniques of manufacturing CeO 2 micro/nanostructures and their distinct morphologies over the years [132][133][134][135]. Furthermore, many dimensionalities (such as 0D, 1D, 2D, and 3D) and morphologies are noted in studies. ...
Article
Emergence of supercapacitor's utilization is been clearly observed in recent years due to the advancement in energy conversion technologies as well as in energy storage systems. The supercapacitors are seemed to be attractive mainly due to its high-energy density, stable cycle life and longer lifetime. Additionally, sustainable nanotechnology played a crucial role in pushing the ongoing progress of supercapacitor technology. Among various popular materials, cerium oxide (CeO2) is one of the ceramic materials that have depicted positive electrochemical activity. CeO2 and its nanostructured composite materials are broadly investigated as a promising active material for supercapacitor applications. The enhancement in electrochemical performance due to addition of different elements to CeO2 was also been explained. The morphologies of prepared binary or ternary composite using CeO2 were also briefly discussed in this article.
... Using this synthesis method, we have prepared ceria− praseodymia solid solution nanoparticles which maintain a nanocube morphology with amount of Pr incorporated into the ceria nanocube structure up to 15 mol %.EELS can be also used to probe the oxidation state of cerium. The fine structure details of the EELS spectra of Ce 3+ and Ce 4+ differ in terms of the exact energy position of the M 4 and M 5 white lines as well as in their relative intensity (Figure S3).42 Moreover, both the M 4 and M 5 peaks present a low-intensity shoulder on their high-energy side in the EELS spectrum of Ce 4+ , which are not observed in the case of Ce 3+ . ...
Article
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In this study, ceria nanocubes (NC) modified with increasing concentrations of praseodymium (5, 10, 15 and 20mol.%) have been successfully synthesized by a hydrothermal method. The as-synthesized Pr-modified ceria nanocubes exhibit an enhanced oxidase-like activity on the organic dye TMB within a wide range of concentrations and durations. The oxidase activity increases with increasing Pr amounts in Pr-modified ceria nanocubes within the investigated concentration range. Meanwhile, these Pr-modified ceria nanocubes also show higher reducibility than pure ceria nanocubes. The kinetics of their oxidase mimetic activity is fitted with Michaelis-Menten equation. A mechanism has been proposed on how the Pr incorporation could affect the energy level of the bands in ceria, and hence facilitate the TMB oxidation reaction. The presence of Pr3+ species on the surface also contributes to the increasing activity of the Pr-modified ceria nanocubes present higher oxidase activity than pure ceria nanocubes.
... Deposition of some long­chain organic materi­ als on the solid surface leads to such coatings (Swalen et al. 1987), while roughened surfaces can be obtained by introducing inorganic nanosized particles, such as SiO 2 (Xu et al. 2010), ZnO (Wu et al. 2005), and TiO 2 (Sun et al. 2011). Recently, CeO 2 was paid much attention due to its multifunction as a catalytic material and UV blocker (González­Rovira et al. 2009;Godinho et al. 2010;Liu et al. 2011;Tsoncheva et al. 2013). CeO 2 has a relatively small band gap (3.1 eV) compared to TiO 2 (3.27 eV) and ZnO (3.37 eV) (Yang et al. 2009;Periyat et al. 2011;Shirke et al. 2011;Sun et al. 2012a,c), which means that the coating is con­ ducive via CeO 2 valence electrons. ...
Article
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Wood in outdoor exposure undergoes photo-chemical degradation mainly caused by the combination of UV radiation and moisture, thus appropriate coatings are desirable. In this paper, the wood surface was first treated with CeO 2 sol and then treated with a layer of octa-decyltrichlorosilane (OTS) and the created coating was characterized by field emission scanning electron micro-scopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDXA), X-ray diffraction (XRD), and FTIR spectroscopy (FTIR). The surface became superhydrophobic as the contact angle (CA) for water of the modified wood was 152 ° . Accelerated aging tests revealed that the modified wood has an excellent weath-ering resistance.
... According to Tana et al. (2009), the CeO 2 nanowires exhibit higher CO oxidation activity than NRs and NPs. Furthermore, Rovira et al. (2009) reported that the catalytic CO oxidation using CeO 2 nanotubes was 400 times higher than that of CeO 2 powders, and they concluded that the high catalytic activity was mainly due to the structure of nanotube. In the present study, the enhanced oxidation performance of ZnO NWs and NRs can be related to the larger surface to volume ratios than that of the NPs, namely, they can provide larger reactive boundaries available for the decomposition of butane than the NPs do. ...
Article
In order to explore the effects of the shape of ZnO nanomaterials on the plasma-catalytic decomposition of butane and the distribution of byproducts, three types of ZnO nanomaterials (nanoparticles (NPs), nanorods (NRs) and nanowires (NWs)) were prepared and coated on multi-channel porous alumina ceramic membrane. The structures and morphologies of the nanomaterials were confirmed by X-ray diffraction method and scanning electron microscopy. The observed catalytic activity of ZnO in the oxidative decomposition of butane was strongly shape-dependent. It was found that the ZnO NWs exhibited higher catalytic activity than the other nanomaterials and could completely oxidize butane into carbon oxides (COx). When using the bare or ZnO NPs-coated ceramic membrane, several unwanted partial oxidation and decomposition products like acetaldehyde, acetylene, methane and propane were identified during the decomposition of butane. When the ZnO NWs- or ZnO NRs-coated membrane was used, however, the formation of such unwanted byproducts except methane was completely avoided, and full conversion into COx was achieved. Better carbon balance and COx selectivity were obtained with the ZnO NWs and NRs than with the NPs.
... Selective surface plane exposure can be successfully achieved by controlling the morphology of the material at the nanoscale. Consequently, a variety of methods have been developed to synthesise nanosized ceria including the use of templates, special organic reagents, hydrothermal treatment, electrochemical methods, etc [16,17] to form a wide range of morphologies from particles [10], polyhedrons [18], rods [19], tubes [20,21], spheres [6], cubes [22], etc. ...
Article
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This paper reveals the key importance of surface oxygen defects in the oxidation catalytic activity of nanostructured ceria. A series of nanostructured rods and cubes with different physico-chemical properties have been synthesised, characterised and tested in the total toluene oxidation. The variation of the temperature and base concentration during the hydrothermal syntheses of nanostructured ceria leads not only to different ceria morphologies with high shape purity, but also to structures with tuneable surface areas and defect concentrations. Ceria nanorods present a higher surface area and a higher concentration of bulk and surface defects than nanocubes associated with their exposed crystal planes, leading to high oxidation activities. However, for a given morphology, the catalytic activity for toluene oxidation is directly related to the concentration of surface oxygen defects and not the overall concentration of oxygen vacancies as previously believed.
... Furthermore, the intensity of the Ce M5 peak is higher than that of the Ce M4 peak in the Co−Mn/CeO 2 catalyst compared with that of the Mn/CeO 2 and Co/CeO 2 catalysts. These observations indicate the improved reducible properties of Ce in the bimetallic Co−Mn/CeO 2 catalyst.34 ...
Article
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Understanding the role of nanointerface structures in supported bimetallic nanoparticles is vital for the rational design of novel high-performance catalysts. This work reports the synthesis, characterization, and the catalytic application of Co-Mn-oxide nanoparticles supported on CeO2 nanocubes with the specific aim of investigating the effect of nanointerfaces in tuning structure-activity properties. High resolution transmission electron microscopy (HRTEM) analysis reveals the formation of different types of Co-Mn nanoalloys with a range of 6 ± 0.5 to 14 ± 0.5 nm on the surface of CeO2 nanocubes which are in the range of 15 ± 1.5 to 25 ± 1.5 nm. High concentration of Ce3+ species are found in Co-Mn/CeO2 (23.34%) compared to that of Mn/CeO2 (21.41%), Co/CeO2 (15.63%), and CeO2 (11.06%) as evidenced by X-ray photoelectron spectroscopy (XPS) analysis. Nanoscale electron energy loss spectroscopy analysis, in combination with XPS studies show the transformation of Co2+ to Co3+ and simultaneously Mn4+/3+ to Mn2+. The Co-Mn/CeO2 catalyst exhibits the best performance in solvent-free oxidation of benzylamine (89.7% benzylamine conversion) compared with Co/CeO2 (29.2% benzylamine conversion) and Mn/CeO2 (82.6% benzylamine conversion) catalysts for 3 h at 120 oC using air as the oxidant. Irrespective of the catalysts employed, a high selectivity towards the dibenzylimine product (97-98%) was found compared to the benzonitrile product (2-3%). The interplay of redox chemistry of Mn and Co at the nanointerface sites between Co-Mn nanoparticles and CeO2 nanocubes as well as abundant structural defects in cerium oxide play a key role in the efficiency of the Co-Mn/CeO2 catalyst for the aerobic oxidation of benzylamine.
... Various methods have been developed for the synthesis of nanotubes, such as high temperature evaporation6, colloidal growth7, hydrothermal synthesis89, anodic oxidation101112, and template- and surfactant-assisted growth tech-niques131415161718192021222324. Recently, there is increasing interest in the template-assisted method because it has been used to synthesize various nanotubes composed of many types of materials, such as silica2526, metal oxides272829, polymers3031, and biological macromolecules32. An important characteristic of template synthesis method is the ability to control the dimensions of nanotubes obtained33. ...
Article
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Here we explored a novel ZnO nanorod array template-assisted electrodeposition route to synthesize large-scale single-walled polypyrrole (PPy) nanotube arrays (NTAs) and multi-walled MnO(2)/PPy/MnO(2) NTAs. The structures of nanotubes, such as external and inner diameters, wall thicknesses, and lengths, can be well controlled by adjusting the diameters and lengths of ZnO nanorods and deposition time. The synthesized hybrid MnO(2)/PPy/MnO(2) triple-walled nanotube arrays (TNTAs) as electrodes showed high supercapacitive perporties, excellent long-term cycling stability, and high energy and power densities. The PPy layers in MnO(2)/PPy/MnO(2) TNTAs provide reliable electrical connections to MnO(2) shells and uniquely serve as highly conductive cores to support the redox reactions in the active two-double MnO(2) shells with highly electrolytic accessible surface area. The fabricated multi-walled NTAs allow high efficient utilization of electrode materials with facilitated transports of ions and electrons. The outstanding performance makes MnO(2)/PPy/MnO(2) TNTAs promising candidates for supercapacitor electrodes.
... A lot of effort has been given to prepare oneedimensional structures like nanorods [15,16], nanowires [14], nanotubes [20,21]. Haiyan He et al. prepared different nanostructures CeO 2 like nanorods, nanowires, nonocubes via hydrothermal synthesis, and reported better catalytic activity in CO oxidation for 1-D nanostructure than nanocubes [22]. ...
Article
A two-step hydrothermal synthesis of nanostructured CeO2 assemblies is reported. The main benefit of the presented approach is the usage of simple reagents and standard laboratory equipment which allows obtaining novel CeO2 morphology. The CeO2 nanostructured materials are characterised with respect to their surface (N2-BET, XPS), bulk (XRD, Raman) and microscopic characteristics (HRTEM, EELS). The effect of calcination temperature of the hydrothermally obtained precursor on the crystallinity of the final products is reported. Since ceria is very often used in catalytic applications as an active phase or as a support, the catalytic total oxidation of methane was used as a model reactivity test. The physicochemical characterisation and reactivity results are compared with CeO2 materials obtained via simple precipitation method. The materials obtained with the presented facile hydrothermal method are characterised by a unique ordered nanorod morphology and present catalytic activity in total methane oxidation improved with respect to classical synthesis protocols. The surface of the nanorod-type assemblies was found to be enriched with Ce³⁺, especially when calcined at a lower temperatures. Due to the availability of the active centres within the obtained nanocrystalline rod-type assemblies and exposition of a reduced surface, they can be interesting in catalytic applications as a support with the active phase confined in their structure.
... The CeO 2 nanowires that exposed more active planes exhibited the highest activity. González-Rovira et al. [23] prepared CeO 2 nanomaterials with a tubular structure by an electrochemical method. The outer diameter of the nanotubes was about 200 nm with lengths between 30 and 40 µm. ...
Article
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Ceria in nanoscale with different morphologies, rod, tube and cube, were prepared through a hydrothermal process. The structure, morphology and textural properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and isothermal N2 adsorption-desorption. Ceria with different morphologies were evaluated as catalysts for CO oxidation. CeO2 nanorods showed superior activity to the others. When space velocity was 12,000 mL·gcat−1·h−1, the reaction temperature for 90% CO conversion (T90) was 228 °C. The main reason for the high activity was the existence of large amounts of easily reducible oxygen species, with a reduction temperature of 217 °C on the surface of CeO2 nanorods. Another cause was their relatively large surface area.
... They proved that adjusting the formation of the oxygen vacancy in CeO 2 is necessary to make it more efficient for application in catalysis. González-Rovira et al. [91] used a single-step process to prepare CeO 2 nanotubes with improved catalytic activity. Fig. 8f shows the light-off curve of CeO 2 nanotubes and powder CeO 2 . ...
Article
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CeO2, as a significant functional material, Has a widespread application in many fields due to its excellent properties. In this review, we sum up a serious of methods to prepare and differentiate nanostructured cerium oxide with various morphologies based on dimensionality and its main applications in industry. We mainly summary the different strategies to synthetic the CeO2 with 0-D, 1-D, 2-D and 3-D and the key parameters which may affect the nanostructures. We hope that this review helps researchers master can look up CeO2 related knowledge more quickly upon the synthetic methods and comparing various morphologies or fabricating ways to explore more convenient and economical procedures when they embark on new research on synthesis cerium oxide.
... Gonz alez-Rovira et al. 24 synthesized the CeO 2 nanotubes using a porous alumina membrane as template, and investigated their catalytic performance for CO oxidation. The CeO 2 nanotubes showed better catalytic activity than the conventional CeO 2 powders, and the light-off temperature (189 C) over the former was much lower than that (300 C) over the latter. ...
Article
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Volatile organic compounds (VOCs), methane, carbon monoxide, soot, automotive exhaust, and nitrogen oxides are harmful to the atmosphere and human health. It is urgent to strictly control their emissions. Heterogeneous catalysis is an effective pathway for the removal of these pollutants, and the critical issue is the development of novel and high-performance catalysts. In this review, we briefly summarize the preparation methods, physicochemical properties, catalytic activities, and related reaction mechanisms for the above pollutants removal of the rare earth oxides, mixed rare earth oxide, rare earth oxide-supported noble metal, and mixed rare earth oxide-supported noble metal catalysts that have been investigated by our group and other researchers. It was found that catalytic performance was associated with the factors, such as specific surface area, pore structure, particle size and dispersion, adsorbed oxygen species concentration, reducibility, reactant activation ability or interaction between metal nanoparticles and support. Furthermore, we also envision the development trend of such a topic in future work.
... CeO 2 纳米管表面存在着 大量的氧缺陷位和晶粒界面 [85] , 可作为氧化反应的 活性中心. 比如, 直径为 200 nm、长为 30~40 μm 的 CeO 2 纳米管催化 CO 氧化的反应速率(200 ℃)是纳 米粒子的 400 倍 [86] . 二维各向异性的 CeO 2 纳米材料, 尤其是超薄的纳米片, 也具有明显的催化形貌效应. ...
... 17 To date, the previous works have mostly involved the use of zerodimensional (0D) nanocrystals such as cubes, 16 spheres, 18 and octahedral, 19 or one-dimensional (1D) nanostructures, such as nanorods, 16 nanowires, 18 and nanotubes. 20 Since the discovery of graphene, 21 much more attention has also been drawn to twodimensional (2D) nanocatalysts. For instance, Choi et al. synthesized zeolite nanosheets with only 2 nm thick, which showed superior catalytic performance for the catalytic conversion of large organic molecules due to the unique properties derived from their exceptionally small thicknesses and possible quantum size effects. ...
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Copper-ceria sheets catalysts with different loadings of copper (2 wt.%, 5 wt.% and 10 wt.%) supported on ceria nanosheets were synthesized via a deposition-precipitation (DP) method. The prepared catalysts were systematically characterized with various structural and textural detections including X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), and temperature-programmed reduction by hydrogen (H2-TPR), and tested for the CO oxidation reaction. Notably, the sample containing 5 wt.% of Cu exhibited the best catalytic performance as a result of the highest number of active CuO species on the catalyst surface. Further increase of copper content strongly affects the dispersion of copper and thus leads to the formation of less active bulk CuO phase, which was verified by XRD and H2-TPR analysis. Moreover, on the basis of in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) results, the surface Cu⁺ species, which are derived from the reduction of Cu²⁺, are likely to play a crucial role in the catalyzing CO oxidation. Consequently, the superior catalytic performance of the copper-ceria sheets is mainly attributed to the highly dispersed CuOx cluster rather than Cu-[Ox]-Ce structure, while the bulk CuO phase is adverse to the catalytic activity of CO oxidation.
... Random orientation of small-sized nanocrystals to each other results in the formation of nanotubes as confirmed from high-resolution electron microscopy (HREM) image. 36 By varying the solvent composition, shape of nanostructured CeO 2 can be tuned through electrochemical deposition method. Different CeO 2 nanorod arrays (NRAs) can be obtained by facile electrochemical deposition process just by adjusting ammonium chloride concentration. ...
Article
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To encounter the increasing risk of melanoma as well as sunburns and premature ageing, tremendous efforts are being put forwarded by the researchers to improve the existing sunscreen materials used in cosmetics. Skin irritation and photoinstability issues of the organic filters motivate researchers to develop suitable alternatives based on inorganic oxides such as titanium dioxide (TiO 2) and zinc oxide (ZnO). Though these inorganic oxide-based UV filters are commonly used in sunscreens, their high oxidation catalytic activity, photocatalytic ability, toxicity, and unaesthetic formulations further encourage researchers to recognize other inorganic alternatives. In this direction, cerium oxide (CeO 2)-based nanocomposites including CeO 2 are considered to be attractive. However, catalytic oxidation is the prime limitation of such composites toward their applicability in commercial sunscreens. A thorough review of the attempts made by the researchers to understand such bottleneck of CeO 2-based nanocomposites is thus essential. In this review article, the UV shielding ability of CeO 2-based nanocomposites and CeO 2 is primarily focused. In particular, the approaches taken so far to maximize the usefulness in terms of UV shielding, photostability, oxidation stability, and toxicity issues of CeO 2-based nanocom-posites as potential inorganic UV filters in commercial sunscreens over most common inorganic counterparts, i.e., TiO 2 and ZnO are highlighted and emphasized.
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Understanding the interface-induced effects of heteronanostructured catalysts remains a significant challenge due to their structural complexity, but it is crucial for developing novel applied catalytic materials. This work reports a systematic characterization and catalytic evaluation of MnOx nanoparticle-dispersed CeO2 nanocubes for two important industrial applications, namely, diesel soot oxidation and continuous-flow benzylamine oxidation. The X-ray diffraction and Raman studies reveal an unusual lattice expansion in CeO2 after the addition of MnOx. This interesting observation is due to conversion of smaller sized Ce4+ (0.097 nm) to larger sized Ce3+ (0.114 nm) in cerium oxide led by the strong interaction between MnOx and CeO2 at their interface. Another striking observation noticed from transmission electron microscopy, high angle annular dark-field scanning transmission electron microscopy, and electron energy loss spectroscopy studies is that the MnOx species are well-dispersed along the edges of the CeO2 nanocubes. This remarkable decoration leads to an enhanced reducible nature of the cerium oxide at the MnOx/CeO2 interface. It was found that MnOx/CeO2 heteronanostructures efficiently catalyze soot oxidation at lower temperatures (50% soot conversion, T50 ∼660 K) compared with that of bare CeO2 nanocubes (T50 ∼723 K). Importantly, the MnOx/CeO2 heteronanostructures exhibit a noticeable steady performance in the oxidation of benzylamine with a high selectivity of the dibenzylimine product (∼94–98%) compared with that of CeO2 nanocubes (∼69–91%). The existence of a strong synergistic effect at the interface sites between the CeO2 and MnOx components is a key factor for outstanding catalytic efficiency of the MnOx/CeO2 heteronanostructures.
Article
A set of catalysts comprised of gold on different CeO2 supports has been prepared by a nanocasting route and characterized by several physicochemical techniques. These catalysts have been tested for CO oxidation and show outstanding catalytic activity. Higher calcination temperatures of the hard template, producing a poorly ordered silica template, have led to a higher amount of oxygen vacancies on the surface of CeO2. The presence of surface oxygen defects in the support combined with the deposition of Au nanoparticles (ca. 3 nm) homogeneously dispersed on the CeO2 support may explain the excellent behaviour for low temperature CO oxidation. Surprisingly, it has been observed that the degree of inverse replication of the template is not relevant in the catalytic performance, as in all cases neither the characteristics of the ceria surface nor the dispersion and oxidation state of gold are greatly modified by the formation of inter-particle bridges.
Article
CeO2 nanorods were synthesized by a hydrothermal method and used as the support for preparing a series of Ni/CeO2 nanorod catalysts. The surface area of the catalysts decreased when the Ni percent over the CeO2 nanorods was increased. SEM results showed that the CeO2 is formed by nanorods approximately 1 μm in length. TEM and HREM revealed that the width of the nanorods is about 8 nm and it grew along the [1 1 1[combining macron]] axis. The catalytic activity of the catalysts was improved as the Ni was loaded onto CeO2 nanorods. The exposed planes of the CeO2 nanorod structure along the zone axis [0 1 1] for Ni impregnation were (1[combining macron] 1[combining macron] 1), (1 1 1[combining macron]), (1 1[combining macron] 1), (1[combining macron] 1 1[combining macron]), (2 0 0) and (2[combining macron] 0 0) and they were more reactive for methanol conversion than (2[combining macron] 2[combining macron] 0), (2[combining macron] 0 2[combining macron]), (0 2 2[combining macron]), (0 2[combining macron] 2), (2 0 2) and (2 2 0) planes from the [1 1 1[combining macron]] axis (growth direction of the nanorod). This finding is mainly ascribed to the synergistic effect of the CeO2 nanorods and the Ni.
Article
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For the first time we synthesized mesoporous CeO2 nanosheets with single-crystal-like, ultrathin and uniform-sized structures through the thermal decomposition of specially prepared intermediates of Ce(OH)CO3 nanosheets. The resulting single-crystal-like CeO2 porous nanosheets are only 2.4 nm thick, as measured by AFM. BET analysis indicated that the pores of the nanosheets were centered at 3.7 nm. A novel structure transformation mechanism from 1D to 0D, then to a 2D assembly was clearly revealed. Moreover, such ultrathin CeO2 porous nanosheets exhibited much higher photocatalytic activity for dye degradation than that of polycrystalline CeO2 nanoparticles.
Article
Development of nanomaterials-based enzymatic mimics has gained considerable attention in recent years, because of their low cost, high stability and efficiently catalytic ability. Here, CeO2 was successfully incorporated into the coral-like NiO nanostructures assembled by nanoflakes with high surface area, forming the coral-like CeO2/NiO nanocomposites. The morphology and composition of CeO2/NiO nanocomposites were characterized by XRD, SEM, element mapping and XPS. The results of characterization showed that cerium was highly dispersed in the coral-like NiO nanostructures. The peroxidase-like activity of CeO2/NiO nanocomposites was investigated, and they exhibited enhanced peroxidase-like activity in comparison to that of pure NiO or CeO2. The catalytic activity was dependent on the cerium content, and the optimal content was 2.5%. The enhanced catalytic activity of CeO2/NiO nanocomposites arised from their high ability of electron transfer because of cerium incorporation. The catalytic performance of CeO2/NiO nanocomposites was evaluated by steady-state kinetic, which showed that the CeO2/NiO nanocomposites exhibited higher affinity for the substrates and similar catalytic efficiency compared with natural peroxidase. Based on the efficient peroxidase-like activity, CeO2/NiO was used for H2O2 determination. The constructed colorimetric H2O2 sensor had fast response for only 5 min, a wide linear range from 0.05 to 40 mM and a low detection limit with 0.88 μM. The CeO2/NiO nanocomposites were expected to have potential applications in clinical diagnosis and biotechnology as enzymatic mimics.
Article
The collisional reactions of composition-selected cerium oxide cluster cations, CenOm(+) (n = 2-6; m ≤ 2n), with CO and NO have been investigated under single collision conditions using a tandem mass spectrometer. At near-thermal energy, oxidation of CO and NO is observed only for the stoichiometric clusters, CenO2n(+) (n = 3-5), and the cross sections for the NO oxidation are found to be larger than those for the CO oxidation. In addition, the collision-energy dependence of the reaction cross sections reveals that the CO oxidation has a small activation barrier, whereas the NO oxidation is a barrierless process. These experimental findings are supported by density functional theory calculations.
Article
A morphological phase diagram is determined to relate the effect of base concentration and temperature during the hydrothermal synthesis with the final ceria nanostructured morphology. Representative samples of nanoparticles, nanorods and nanocubes have been characterized by XRD, N2 adsorption, TEM, XPS and Raman and catalytically tested for the total oxidation of naphthalene as a model polycyclic aromatic hydrocarbon. Ceria nanoparticles present the highest surface area and smallest crystalline size, leading to the most active of these structures. However if the catalytic activity is normalized by unit of surface area, the observed reactivity trend (nanorods < nanocubes < nanoparticles) is directly related to the concentration of surface oxygen vacancies as a result of the exposure of the (1 1 0) and (1 0 0) preferential planes.
Article
Mesoporous CeO2 nanobelts have been synthesized by a facile hydrothermal route via controlling cationic type and concentration of alkali without any surfactant or template. The key synthesis of CeO2 nanobelts is forming the beltlike precursors in the presence of enough NaOH (Na/Ce molar ratio ⩾16.3 at 120 °C) during hydrothermal process. The enough OH− ions induce the two steps of Cannizzaro disproportionation reaction to result in conversion of formate partly into carbonate, and Na+ ions with small ionic radius allow coexistence of carbonate and formate in structure and accordingly are favorable to anisotropic growth of beltlike precursors. The increased hydrothermal temperature can promote the formation of carbonates and the minimal required Na/Ce molar ratio is decreased from 16.3 at 120 °C to 10.8 at 140 °C. When Na+ is substituted by the same concentration of K+ or NH4+, the obtained precursor products are finally nanowires or irregular morphology. XRD measurement proves that the sodium ions do not enter the crystalline frame of CeO2 nanobelts and can be easily removed by simple washing with deionized water. After washing, CeO2 nanobelts with enlarged mesoporous pores show superior catalytic performance for CO oxidation compared with CeO2 nanoparticles prepared with traditional methods.
Article
The design and fabrication of solid nanomaterials are the key issues in heterogeneous catalysis to achieve desired performance. Traditionally, the main theme is to reduce the size of the catalyst particles as small as possible for maximizing the number of active sites. In recent years, the rapid advancement in materials science has enabled us to fabricate catalyst particles with tunable morphology. Consequently, both size modulation and morphology control of the catalyst particles can be achieved independently or synergistically to optimize their catalytic properties. In particular, morphology control of solid catalyst particles at the nanometer level can selectively expose the reactive crystal facets, and thus drastically promote their catalytic performance. In this review, we summarize our recent work on the morphology impact of Co3O4, CeO2 and Fe2O3 nanomaterials in catalytic reactions, together with related literature on morphology-dependent nanocatalysis of metal oxides, to demonstrate the importance of tuning the shape of oxide-nanocatalysts for prompting their activity, selectivity and stability, which is a rapidly growing topic in heterogeneous catalysis. The fundamental understanding of the active sites in morphology-tunable oxides that are enclosed by reactive crystal facets is expected to direct the development of highly efficient nanocatalysts.
Article
The design and fabrication of catalytic materials is a key issue in heterogeneous catalysis to achieve desired performance. Traditionally, the main theme is to reduce the size of the catalyst particles as small as possible for increasing the number of active sites. In recent years, the rapid advancement in materials science has enabled us to fabricate catalyst particles with tunable shape at nanometer level. Through morphology control of nanoparticles by exposing highly reactive crystal planes, their catalytic properties can be drastically enhanced. Therefore, both size modulation and shape control of catalyst nanoparticles can be achieved independently or synergistically to optimize their catalytic behavior. We highlight, in this review, the recent progress in shape control of CeO2 materials that are widely used as crucial components or structural and electronic promoters in heterogeneous catalysts. We first summarize the major synthetic strategies and characteristics of shape-controlled CeO2 nanomaterials. We then survey morphology-dependent nanocatalysis of CeO2 and Au-CeO2 catalysts. We understand now that the enhanced catalytic property of the Au-CeO2 system is closely related to the unique interaction between the gold nanoparticles and the ceria support; such an interaction originates from the particular shape of ceria, especially the exposed facets. Finally, we present our understanding of the morphology-dependent nanocatalysis and provide our perspectives on their future potential and development. The fundamental understanding of the nature of the intrinsic active sites of the shape-tunable ceria nanostructures, enclosed by reactive crystal planes/facets with unique properties, is expected to provide highly efficient nanocatalysts for practical applications.
Article
High-quality, noncrystalline alloy Co–B, Co–Mo–B, and Co–W–B nanotubes were prepared by chemical co-reduction in cobalt chloride and ammonium heptamolybdate (or ammonium metatungstate) containing a liquid crystal system. The nanotubes were characterized by powder X-ray diffraction, inductively coupled plasma spectrometry, N2 adsorption, transmission electron microscopy, and X-ray photoelectron spectroscopy. The catalytic performances of the as-prepared materials were carefully investigated for the selective hydrogenation of cinnamaldehyde in the liquid phase under moderate pressure. Higher catalytic activity and selectivity to cinnamylalcohol were exhibited by the nanotubular catalysts in the selective hydrogenation of cinnamaldehyde as compared with those of corresponding nanoparticles. The excellent performance of the nanotubes was correlated to their high specific surface area and tubular morphology. Mo and W effectively promoted the overall rate and yield of unsaturated alcohol. Aside from the higher catalytic activity, the structural stabilities of the nanotubular catalysts were also improved by doping in consecutive catalytic runs.
Article
The metal/semiconductor hetero-nanostructures perform widely applications in the light-driven physical and chemical processes, especially the nanomaterials based on the anisotropic metal nanocrystals with tunable plasmonic property have attracted intensive interest. Herein we report an efficient hydrothermal method for the synthesis of well-defined ceria (CeO2) coated gold nanorods (AuNRs) by employing the original hexadecyltrimethylammonium bromide (CTAB) as the ligands and soft-template. Importantly, the Au/CeO2 core/shell NRs have well maintained the tunable longitudinal plasmon resonance of AuNR in the near-infrared (NIR) region. We demonstrate that this hetero-nanostructure can accelerate the ceria dependent Fenton-like reaction through the plasmon-induced hot-electron injection under NIR light illumination. The generation of hot-electron is further revealed by detecting the NIR-light-driven photocurrent of a photoelectrochemical (PEC) cell base on the Au/CeO2 core/shell NRs modified electrode.
Article
This work investigates the structure-activity properties of CuOx-decorated CeO2 nanocubes with a meticulous scrutiny on the role of the CuOx/CeO2 nano-interface in the catalytic oxidation of diesel soot, a critical environmental problem all over the world. For this, a systematic characterization of the materials has been undertaken using TEM, TEM-EDS, HAADF-STEM, STEM-EELS, XRD, Raman, N2 adsorption-desorption, and XPS techniques. The TEM images show the formation of nanosized CeO2 cubes (~25 nm) and CuOx nanoparticles (~8.5 nm). The TEM-EDS elemental mapping images reveal the uniform decoration of CuOx nanoparticles on CeO2 nanocubes. The XPS and Raman studies show that the decoration of CuOx on CeO2 nanocubes leads to improved structural defects, such as higher concentrations of Ce3+ ions and abundant oxygen vacancies. It was found that CuOx-decorated CeO2 nanocubes efficiently catalyze soot oxidation at much lower temperature (T50 = 646 K, temperature at which 50% soot conversion is achieved) compared with that of pristine CeO2 nanocubes (T50 = 725 K) under tight contact conditions. Similarly, a huge 91 K difference in the T50 values of CuOx/CeO2 (T50 = 744 K) and pristine CeO2 (T50 = 835 K) was found in the loose-contact soot oxidation studies. The superior catalytic performance of CuOx-decorated CeO2 nanocubes is mainly attributed to the improved redox efficiency of CeO2 at the nano-interface sites of CuOx-CeO2 as evidenced by Ce M5,4 EELS analysis, supported by XRD, Raman, and XPS studies, a clear proof for the role of nano-interfaces in the performance of hetero-structured nanocatalysts.
Thesis
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Cerium oxide (ceria, CeO2-x where x is 0 to 0.5) has been one of the most widely used heterogeneous catalysts particularly in three way catalytic converters. Most of the catalytic traits can be attributed to two properties of ceria: first, the high mobility and storage capacity of oxygen within the lattice; second, the ease with which cerium changes between Ce3+ and Ce4+ states. These properties, combined with the abundance of cerium on earth, make ceria a low-cost highly effective alternative to noble metal catalysts. Recent research has been focused on the nanoscale properties of ceria. The effect on the catalytic activity of cerium oxide caused by varying the density of oxygen vacancy defects (OVD) has not been previously studied experimentally. This is due to the perceived inability to engineer stable defects not attributed to the presence of dopant atoms. It was found that the number of stable OVDs on cerium oxide nanoparticles and nanotubes can be increased with annealing at elevated temperatures under low pressure. The oxidative catalytic activity of these nanostructured catalysts was evaluated. Samples with higher densities of OVD were found to have much lower light-off temperatures when compared to that of their bulk counterpart. The chemical equilibrium reactions on the catalysts surface under low pressure were hypothesized to explain the unusual increase in the OVD density of the reported cerium oxide nanostructured catalysts. Cerium oxide is well known to exfoliate from the surface of cerium metal in the same way that rust exfoliates from iron or steel. A two-step process to fabricate nanoporous ceria membranes via anodization and subsequent calcinations is reported. These membranes have the potential to be used in solid oxide fuel cells and solid-state oxygen sensors. Cerium metal foil was first anodized into adherent porous cerium hydroxide film, followed by calcination for conversion into ceria membranes. These membranes are composed of ribbon-like structures that form the backbone of the porous framework. A proposed anodization model for the growth of the nanoribbons is discussed.
Article
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Ceria is an important rare earth metal oxide for numerous catalytic applications. In the present work, a combined study of structural characterization and catalytic activity of nanocrystalline Ce-Gd-O mixed oxides was thoroughly investigated towards CO oxidation. The Ce-Gd mixed oxides with different Gd-doping amounts (10 and 20 mol% Gd with respect to Ce) were prepared by means of an economical and simple coprecipitation method with aqueous NH3 solution as the precipitant. The resulting samples were treated at different calcination temperatures to evaluate their structural homogeneity and thermal stability. An extensive physicochemical characterization was done by means of XRD, Raman, TEM, BET analysis, H-2-TPR, XPS, and UV-Vis DRS techniques. XRD studies revealed the formation of nanocrystalline single phase Ce-Gd solid solutions. Raman studies further disclosed the formation of Ce-Gd solid solutions associated with deformed F-2g band and additional bands pertaining to oxygen vacancy defects. The Gd-doping remarkably reduced the CeO2 crystallite size, which is in nanoscale range as evidenced by TEM images. The BET surface area and oxygen vacancy defects of CeO2 were significantly enhanced after Gd3+ incorporation. The ability of CeO2 to store and release of oxygen (oxygen storage capacity, OSC) is markedly improved 3 and 4 times for Ce0.9Gd0.1O2-delta and Ce0.8Gd0.2O2-delta samples, respectively. Among the investigated catalysts, the Ce0.8Gd0.2O2-delta sample calcined at 773 K showed better catalytic activity due to smaller crystallite size, higher BET surface area, enhanced reducible nature, and superior OSC. It is found that the catalytic performance of the Ce-Gd sample strongly depends on the Gd-loading and calcination temperature.
Article
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Interfacial oxidation–reduction reaction is herein developed to prepare hollow binary oxide nanostructures. Ce–Mn nanotubes are fabricated by treating Ce(OH)CO3 templates with KMnO4 aqueous solution, where MnO4− is reduced to manganese oxide and the Ce3+ in Ce(OH)CO3 is simultaneously oxidized to form cerium oxide, followed by selective wash with HNO3. The resulting Ce–Mn binary oxide nanotubes exhibit high catalytic activity towards CO oxidation and show significant adsorption capacity of Congo red. Moreover, guided by the same interfacial-reaction principle, binary oxide hollow nanostructures with different shapes and compositions are synthesized. Specifically, hollow Ce–Mn binary oxide cubes, and Co-Mn and Ce-Fe binary oxide hollow nanostructures are achieved by changing the shape of the Ce(OH)CO3 templates from rods to cubes, by changing the tempates from Ce(OH)CO3 nanorods to Co(CO3)0.35Cl0.20(OH)1.10 nanowires, and by replacing the oxidant of KMnO4 with another strong one, K2FeO4, respectively. This work is expected to open a new, simple avenue for the general synthesis of hollow binary oxide nanostructures.
Article
Carbon monoxide binds to hemoglobin much more tightly than does molecular oxygen, so CO is highly toxic to humans and animals. Oxidation of CO into CO2 is a major solution to removal of carbon monoxide in air purification. CO is also a simple molecule and CO oxidation serves as a prototypical reaction for heterogeneous processes. Atomic clusters composed of limited numbers of atoms are experimentally and theoretically tractable models for heterogeneous catalysis. During the last decade, a number of publications have been devoted to understanding the CO oxidation at a molecular level based on the investigations of gas phase atomic clusters under controlled and well reproducible conditions. In this review, we will go through the reported CO oxidations by gas phase atomic clusters. Advantages, new insights, and problems to be solved involved with the cluster approach for the title reaction will be summarized.
Article
In this work, we developed a simple, cost-effective and controllable electrochemical method to synthesize free-standing CeO2 hierarchical nanorods and nanowires. Due to their hierarchical one-dimensional nanostructures and increased surface areas, both the CeO2 hierarchical nanorods and nanowires exhibit substantially higher photocatalytic performance than the commercial CeO2 nanoparticles in the degradation of methyl orange.
Technical Report
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Since CeO2-ZrO2 solid solutions have been demonstrated to have better catalytic properties than pure CeO2, they may also have properties that make them attractive for application as hydrocarbon reforming or hydrocarbon activation catalysts in Solid Oxide Fuel Cells (SOFCs). Moreover, the redox behavior, thermal stability and catalytic performance of zirconia-ceria mixed oxides are strongly related to their crystal structure. In the present work, novel ZrO2-CeO2 mixed oxide nanotubes with 50, 70 and 90 mol% CeO2 were synthesized following a very simple, high yield procedure and their structural properties were characterized mainly by synchrotron radiation X-ray diffraction (SR-XRD) and by high resolution transmission electron microscopy (HRTEM).
Article
In this study, we develop a facile hydrothermal route to build a nanostructure of porous CeO2 nanobundles with hierarchical architectures by the design and growth of anisotropic CeO2 precursors in a carbonate-assisted formaldehyde hydrothermal system without templates and surfactants. The synthetic key is to control the formation of their anisotropic CeO2 precursors with formate and carbonate promoted by carbonate and ammonium ions during the employed hydrothermal treatment. The as-prepared CeO2 nanobundles have a hierarchical porous structure assembled by numerous nanorods with small diameter and show much higher catalytic activities for CO oxidation compared with CeO2 nanorods, nanowires and nanoparticles prepared by formaldehyde-assisted hydrothermal treatment and traditional precipitation methods. The outstanding catalytic performance for the nanobundles is attributed to their excellent physicochemical properties, such as larger lattice cell parameters, much larger surface areas and the best redox behaviour of surface oxygen on CeO2 surface.
Article
h i g h l i g h t s Porous broom-like CeO 2 was fabricated for the first time by a template-free approach. Based on reaction time and temperature, morphology evolution mechanism has been proposed. Compared with other morphology, reduction property of porous broom-like CeO 2 was enhanced. g r a p h i c a l a b s t r a c t a r t i c l e i n f o a b s t r a c t In this study, CeO 2 with a broom-like porous hierarchical structure was successfully prepared by a simple template-free hydrothermal method. In the whole hydrothermal process, reaction time and temperature play important roles in morphology control. The morphology of the as-prepared samples was character-ized by field emission scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. Structure information was obtained by using an X-ray diffractometer and Raman analysis. Analyses of elements and chemical valence analysis were carried on X-ray photoelectron spectroscopy. Absorption edge and band gap energy were investigated by UV–vis diffuse reflectance. In addition, the optimum experimental conditions for target products were determined. Compared with various morphology samples' BET data, relative intensity of Raman peaks, catalytic evaluation, we can draw a conclusion that CeO 2 with a broom-like porous hierarchical structure has a higher concentration of oxygen vacancies that results in enhancement of catalytic activity. An Ostwald ripening process with orientative self-assembly was proposed for formation of the broom-like porous hierarchical structure. Ó 2014 Elsevier B.V. All rights reserved.
Article
Density functional theory calculations with on-site Coulomb interaction correction (DFT + U) have been performed to study the structures and catalytic activities of 2 x 1 reconstructed surface of CeO2(110). The reconstructed surface gives better thermal stability compared with the bulk truncated one and exhibits unique surface activity. We comprehensively calculated the O vacancy formation and diffusion on the reconstructed surface and found that the vacancy formation energy corresponding to the removal of one subsurface four-fold coordinated O is 1.71 eV only, which is smaller than that of the top-surface O vacancy or the sub-surface O vacancy at the bulk truncated surface. Accordingly, the O vacancy diffusion at 2 x 1 reconstructed surface is also much more feasible than that at the unreconstructed CeO2(110) with the highest diffusion barrier of only 0.84 eV. By calculating the detailed pathways of CO reaction with lattice O, we found that CO2 can directly occur without forming a bent negatively charged CO2- intermediate on the reconstructed surface, which may reduce the chance for the carbonate formation. It has also been clearly shown that strong localization characteristics of Ce 4f orbital indeed favor electron transfer from reaction intermediates to the CeO2 support.
Article
Porous CeO2 nanostructures were synthesized by a simple template method using cotton fibers as biomaterial template. The phase structure and morphology of the product were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM). The UV-blocking property was evaluated by UV-Vis spectrophotometry. Experimental results showed that the as-synthesized CeO2 was in a cubic fluorite structure and presented fiber-like structure. The fiber-like CeO2 nanostructures completely recorded the original morphology of cotton fibers, which consisted of large amounts of coalescent CeO2 nanoparticles with average particle size in the range of 6-10 nm. The UV test result showed that the UV transmittance of the product was closer to zero in the wavelength below 350 nm, indicating that the product exhibited excellent UV-blocking property.
Article
This study presents a novel and highly efficient CeO2-based type of additives to improve the performance of PEM fuel cells. Cerium(IV) oxide (CeO2) nanorods and CeO2 nanorod decorated nitrogen-doped reduced graphene oxide (NrGO) (CeO2/NrGO) were synthesized and utilized to enhance the ORR performance of PEMFCs. With a low energy transition between Ce⁺³ to Ce⁺⁴ and its high catalytic activity, CeO2 could promote O2 reduction. The structural properties of CeO2 nanorods and the CeO2/NrGO composite were investigated using XRD, RAMAN, SEM, TEM, TGA, BET, and XPS. The synthesized CeO2 nanorods and CeO2/NrGO composite were mixed with a low platinum loading (0.09 mgPt.cm⁻¹), and ex-situ electrochemical characterizations were performed (CV and LSV) to evaluate their catalytic activities. Fuel cell performance tests and in-situ impedance analyses were also conducted to confirm the electrochemical results. Compared to commercial Pt/C-based MEA (269 mW.cm⁻²), the addition of both CeO2 nanorods (382 mW.cm⁻²) and CeO2/NrGO (389 mW.cm⁻²) to the catalyst layer showed a significant enhancement in fuel cell performance, due to the unique oxygen buffer ability of the synthesized additives.
Article
In this report, a novel nanocomposite of highly dispersed CeO2 on TiO2 nanotube was designed and proposed as a peroxidase-like mimic. The best peroxidase-like activity was obtained for CeO2/nanotube-TiO2 when the molar ratio of Ce/Ti was 0.1/1, which was much higher than that for CeO2/nanowire-TiO2, CeO2/nanorod-TiO2, CeO2/nanoparticle-TiO2 with similar molar ratio of Ce/Ti. Moreover, in comparison with other nanomaterials based peroxidase mimics, CeO2/nanotube-TiO2 nanocomposites exhibited higher affinity to H2O2 and TMB. Kinetic analysis indicated that the catalytic behavior was in accordance with typical Michaelis-Menten kinetics. Ce3+ sites were confirmed as the catalytic active sites for the catalytic reaction. The first interaction of surface CeO2 with H2O2 chemically changed the surface state of CeO2 by transforming Ce3+ sites into surface peroxide species causing adsorbed TMB oxidation. Compared with CeO2/nanowire-TiO2, CeO2/nanorod-TiO2, CeO2/nanoparticle-TiO2, the combination of TiO2 nanotube with CeO2 presented the highest concentration of Ce3+ thus leading to the best peroxidase-like activity. Based on the high activity of CeO2/nanotube-TiO2, the reaction provides a simple method for colorimetric detection of H2O2 and glucose with the detection limits of 3.2 μM and 6.1 μM, respectively.
Article
Cerium oxide nanorods having a variety of aspect ratios can be produced by providing a first mixture that includes a cerium precursor material, and using microwave to heat the first mixture to a first temperature for a period of time to produce first plurality of cerium oxide nanorods having a first range of aspect ratios. A second mixture that includes a cerium precursor material heated using microwave to a second temperature for a period of time to produce second plurality of cerium oxide nanorods having a second range of aspect ratios. The first plurality of cerium oxide nanorods and the second plurality of cerium oxide nanorods are mixed to produce third plurality of cerium oxide nanorods having the third range of aspect ratios that is broader than the first range or the second range .
Article
Novel Ag-CeO2 nanotubes were synthesized by the precipitation method based on preformed Ag NW added as active phase precursors. This nanotubular material was used as catalysts to evaluate the methanol reforming reaction for H-2 production. Nanotubular structures with an external diameter from 120 to 280 nm and internal diameter from 40 to 80 nm were identified by electron microscopy techniques. These nanostructures were mainly composed of CeO2 nanoparticles (similar to 11 nm). The X-ray powder diffraction patterns of the Ag-CeO2 nanotubes showed diffraction peaks characteristic of the cubic structure of the CeO2 and metallic Ag. No diffraction peaks corresponding to those of the Ce-Ag alloys or other impurities were found in these samples. The catalytic activity was improved as the nanotubes concentration was increased in the catalysts. However, the H-2 production was diminished, so, an increase in the Ag particle size promotes the methanol combustion, decreasing hydrogen production and increasing the water formation as well as the CO2 production. It seems that the Ag is the phase mainly responsible of hydrogen production although the large particles reduce the CO formation.
Chapter
The concept of morphology-dependent nanocatalysis has mainly been explored for precious metal nanoparticles, mostly platinum, palladium, rhodium, and silver. Metal nanoparticles are typically used for liquid-phase reactions that are conducted at relatively lower temperatures, favoring to maintain their sizes and shapes. Several prominent examples of morphology-dependent nanocatalysis of metal oxides have recently been reported. The morphology of the metal oxides, through a preferential exposure of the reactive crystal planes, affects the catalytic performance more significantly than the conventionally considered parameters such as particle size and surface area. This chapter summarizes the recent progress on morphology control of cobalt oxides, ferric oxides, and cerium dioxide nanomaterials and demonstrated that tuning the shape of these metal oxides modulated the surface atomic configurations and consequently promoted their catalytic properties.
Article
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A simple protocol has been reported here to successfully perform a controllable conversion between Ce(OH)4 nanorods [Ce(OH)4-NR] and Ce(OH)4 nanoflowers [Ce(OH)4-NF] based on a prolonged mechanical force-driven stirring process. Results show that the Ce(OH)4 nanostructures undergo a morphology transformation from the initial nanorods to irregular nanoflowers, then to nanoflowers emanating from one center only by varying the stirring time before solvothermal. The detailed study confirmed that the mechanical force significantly improved the mass transport of the solution and drove the seeds of Ce(OH)4-NR [seeds-NR] to generate the seeds of Ce(OH)4-NF [seeds-NF]. The final CeO2 products (CeO2 nanorods [CeO2-NR] and CeO2 nanoflowers [CeO2-NF]) inherited original morphology were obtained by annealing Ce(OH)4-NR and Ce(OH)4-NF, respectively. To further optimize the performance of the final products, Au/CeO2-NR and Au/CeO2-NF were synthesized by a simple oxidation-reduction process, which led to increase surface areas and promising potential in CO oxidation.
Article
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Nanocrystalline CeO 2 has been synthesized at room temperature using water-in-oil (w/o) microemulsion technique. The structure and properties of the nanocrystalline CeO 2 were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and gas adsorption desorption measurement. XRD results showed the synthesized CeO 2 has a face centered cubic structure with crystallite size of about 5.2 nm. TEM observation also indicated the presence of nanometer sized particles of CeO 2. Coarser particles were also observed due to agglomeration. Gas adsorption desorption isotherms showed the behavior of fine particles with mesoporous structure.
Article
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An electrochemical synthetic route is developed to produce cerium-cobalt mixed oxide nanotubes with tunable composition. The process consists of a template-assisted electrogeneration of base from a cerium-cobalt nitrate solution of different compositions. From scanning electron microscopy studies it is seen that Ce-Co oxide nanotubes of high aspect ratio and ordered structures are formed. Energy-dispersive X-ray analysis shows a linear relationship between the solution and the oxide composition [Ce/ (Ce+Co) atomic ratio].
Article
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A series of CeMn composites with different compositions was prepared by co-precipitation. All samples were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). According to their size and chemical composition, two different kinds of particles were observed: (a) small crystallites of CeMn solid solution (<7 nm), and (b) large particles of manganese oxides with different reduction degrees (>50 nm). These results were used to explain differences between the surface and bulk compositions of the samples as well as the specific surface area. Furthermore, from XPS studies, the presence of various Mn species with different oxidation states could be stated. This fact might clarify the exceptional catalytic behaviour of the samples for the catalytic wet oxidation of toxic organic compounds. Copyright © 2004 John Wiley & Sons, Ltd.
Article
The influence of the cMez+ in acidic solutions of Cu, Ni, Co, and Sn salts on the amount of deposited metal, mMe, by ac electrolysis in the pores of various alumina templates grown in commonly used aluminium anodizing solutions of sulfuric, oxalic, phosphoric and chromo-boric acids was investigated. Behaviour of the porous alumina under ac voltage control manifests itself through mMe–cMez+ parabola dependencies. The peaks of mMe–cMez+ plots was found to be dependent on the alumina pore diameter, dp, and composition of the metal salt solution used. An increase in dp and decrease in a solution pH shifts mMe versus cMez+ dependencies and the optimal cMez+ toward smaller solution concentrations.From the results of the voltammetric and chronopotentiometric studies a surprisingly large value of a few seconds of the transition time for hydrogen ions discharge, occurring before the metal ion reduction at the bottom of alumina pores, was obtained. This result makes it reasonable to suggest that the amount of Me(OH)x, formed in the pore base region due to the hydrogen evolution from the acidic solutions with bulk concentration 0.1–0.2M, is the most favourable to the deposition of metal arrays at the highest rate if 13.5≤dp≤37.5nm.
Article
High quality arrays of Ln(OH)3 (Ln = La, Nd) nanowires have been successfully fabricated for the first time by an electrochemical process using anodic alumina membrane templates. A physico-chemical characterisation of electrodeposited hydroxides has been carried out by different techniques (XRD, SEM and EDX). The results show that the synthesized nanostructures are crystalline, dense, continuous, well aligned, and with high aspect ratio, suggesting further development of possible applications in the lanthanide family species.
Article
The present paper deals with a systematic experimental study and theoretical evaluation of the shape and weight for the characteristic M45 rare earth edge and K oxygen edge in the family of rare earth oxides. An innovative method is used for processing the recorded spectra; it consists of modelling the experimental data as a sum of predicted contributions. The results obtained for the k factors, cross-sections and oscillator strengths are generally in good agreement with the atomic calculations in the Hartree-Slater model which considers only transitions towards final states in the continuum. The position and relative weight of the white-lines on the M45 edges corresponding to transitions towards final bound states of f symmetry are also measured and compared either with measurements from other techniques e.g. X-ray absorption Spectroscopy (XAS) or with calculations of the multiplet terms.
Article
By using a combination of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques, we have followed in detail the structural and chemical changes occurring on the very surface layers of powder CexPr1-xO2 mixed oxides, used as supports, during the preparation of rhodium-supported catalyst by wet impregnation techniques. Results demonstrate the occurrence of Pr3+ leaching out of the Ce-Pr solid solution during the impregnation step, which promotes a severe spatial redistribution of lanthanide cations at the surface of the mixed oxide. A highly heterogeneous surface composition is reached after the metal deposition. Mobilization of the support also affects the features of the final surface structure of the metal-supported nanoparticles, which become decorated by patches of support material. All these facts seem crucial to properly understand the redox and catalytic behavior of this kind of materials.
Article
The rapid advances in nanotechnology and the ever decreasing size of features in the microelectronics industry brings with it the need for advanced characterisation with high spatial resolution in two and three dimensions. Stereo microscopy allows some insight into the three-dimensional nature of an object but for true quantitative analysis, one has to turn to tomography as a way to reconstruct a three-dimensional object from a series of two-dimensional projections (images). X-ray tomography allow structures to be imaged at relatively large length scales, atom probe tomography at the atomic level. Electron tomography offers an intermediate resolution (of about 1nm) with a field of view of hundreds of nm making it ideal for the characterisation of many nanoscale devices. Whilst electron tomography has been used in the biological sciences for more than 30 years, it is only now being applied to the physical sciences. In this paper, we review the status of electron tomography, describe the basis behind the technique and some of the practicalities of recording and analysing data for tomographic reconstruction, particularly in regard to solving three-dimensional problems that are encountered in materials science at the nanometre level. We present examples of how STEM dark-field imaging and energy-filtered TEM can be used successfully to examine nearly all types of specimens likely to be encountered by the physical scientist.
Article
A series of catalytic wet oxidation (CWO) reactions, at temperatures of 430-500 K and in a batch bench-top pressure vessel were carried out utilizing a strong wastewater composed of landfill leachate and heavily organic halogen polluted industrial wastewater. A CeO2-SiO2 mixed oxide catalyst with large surface area to assure optimal oxidation performance was prepared. The catalytic process was examined during batch reactions controlling Chemical Oxygen Demand (COD) and Adsorbable Organic Halogen (AOX) parameters, resulting AOX abatement to achieve better effect. Color and pH were also controlled during batch tests. A simple first order-two stage reaction behavior was supposed and verified with the considered parameters. Finally an OUR test was carried out to evaluate biodegradability changes of wastewater as a result of the catalytic reaction.
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