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Lattice Defects and Oxygen Storage Capacity of Nanocrystalline Ceria and Ceria-Zirconia
Abstract
The atomic structures of nanocrystalline powders of ceria, CeO2, and ceria-zirconia solid solution, (Ce,Zr)O2, were studied by the pulsed neutron diffraction technique. Ceria is used as an oxygen storage component in automotive exhaust emission control systems, but the degradation of its oxygen storage capacity (OSC) after extended use at high temperatures has been a problem. Our results for the first time establish a direct correlation between the concentration of vacancy-interstitial oxygen defects and OSC. The surface area, on the other hand, exhibits much less correlation with OSC. The results also show that zirconia, which is known to retard the degradation when incorporated into ceria, reduces ceria and preserves oxygen defects. It is suggested that oxygen defects are the source of OSC in ceria-based catalyst supports, and the preservation of oxygen defects is critical for the stability of OSC against thermal aging.
... Substitution of Ce 4+ for metal cations of lower valence (e.g., La 3+ or Pr 3+ ) causes the formation of oxygen vacancies, which could act as sites for CO 2 activation [43]. Doping of CeO 2 with isovalent Zr 4+ but of different size compared to Ce 4+ causes distortion of its lattice structure and displacement of oxygen atoms from their normal lattice positions [17,41], Ce-O bonds are elongated and weakened [44], resulting in a higher fraction of oxygen vacant sites and oxygen mobility. ...
... The main CO 2 -TPO peak shown in Fig. 10 was attributed to a nano-filament form of carbon [47,48]. Also, the larger extent of coking of 18NiCo with larger metal particles (43 nm) compared to 3NiCo (7.8 nm) is in good agreement with the literature [15,16,35,44,52]. ...
... The pore volume of KPtCZ was improved, as shown in Table 1. The elemental analysis using energy dispersive X-ray diffraction (EDX) analysis of the RuCZ, PdCZ, PtCZ and KPtCZ on Figure 4 confirmed the presence of dopants of Ru, Pd, Pt and K. ionic substitution of Pt and Pd was due to the atomically dispersed Pt, Pd and Ru in the support, which can oxidize by utilizing bulk oxygen or oxygen vacancies in the support [39,40]. As the active metals were substituted in the support material, slight deviations in the interplanar spacing values were possible compared to noble metal reference data. ...
... The resistance towards sintering and effective dispersion lead to high catalytic activity. The ionic substitution of Pt and Pd was due to the atomically dispersed Pt, Pd and Ru in the support, which can oxidize by utilizing bulk oxygen or oxygen vacancies in the support [39,40]. As the active metals were substituted in the support material, slight deviations in the interplanar spacing values were possible compared to noble metal reference data. ...
Water-gas shift (WGS) is an industrial process to tackle CO abatement and H 2 upgradation. The syngas (CO and H 2 mixture) obtained from steam or dry reformers often has unreacted (from dry reforming) or undesired (from steam reforming) CO 2 , which is subsequently sent to downstream WGS reactor for H 2 upgradation. Thus, industrial processes must deal with CO 2 and H 2 in the reformate feed. Achieving high CO 2 or H 2 selectivities become challenging due to possible CO and CO 2 methanation reactions, which further increases the separation costs to produce pure H 2. In this study, M/Co 3 O 4-ZrO 2 (M = Ru, Pd and Pt) catalysts were prepared using sonochemical synthesis. The synthesized catalysts were tested for WGS activity under three feed conditions, namely, Feed A (CO and steam), Feed B (CO, H 2 and steam) and Feed C (CO, H 2 , CO 2 and steam). All the catalysts gave zero methane selectivity under Feed A conditions, whereas the methane selectivity was significant under Feed B and C conditions. Among all catalysts, PtCZ was found to be the best performing catalyst in terms of CO conversion and CO 2 selectivity. However, it still suffered with low but significant methane selectivity. This best performing catalyst was further modified with an alkali component, potassium to suppress undesirable methane selectivity. All the catalysts were well characterized with BET, SEM, TEM to confirm the structural properties and effective doping of the noble metals. Additionally, the apparent activation energies were obtained to showcase the best catalyst.
... In the case of catalysts supported on inert substances such as SiO 2 , the mechanism follows a monofunctional pathway, in which the metal acts as the sole catalyst to activate both reactants [18]. Once carbon is formed through the dehydrogenation of methane, subsequent CO 2 activation and carbon-carbon reactions are constrained, which results in catalyst deactivation [19][20][21][22][23][24][25]. With an acidic (Al 2 O 3 ) or basic (La 2 O 3 , CeO 2 , and MgO) support, a bifunctional process occurs [26,27]. ...
... Due to its redox behavior, capacity to behave as an acid-base bifunctional catalyst, reducibility, and excellent thermal stability, ZrO 2 is particularly promising as a catalyst and support in the catalysis disciplines. Zirconia is well recognized for slowing the rate of cerium deterioration at high temperatures [25]. Zirconia doping in ceria is thought to have a positive effect by restricting heat sintering. ...
The dry reforming of methane is a highly popular procedure since it can transform two of the most abundant greenhouse gases, methane and carbon dioxide, into useful syngases that can be further processed into valuable chemicals. To successfully achieve this conversion for the effective production of syngas, an optimal catalyst with advantageous physicochemical features must be developed. In this study, a variety of Ni-based catalysts supported by zirconia alumina (5Ni-10Zr + Al) were prepared by using the impregnation approach. Different loadings of Fe promoter were used, and the performances of the resulting catalysts in terms of activity and stability were investigated. The catalyst used in this study had an active metal component made of 5% Ni and x% Fe supported on 10ZrO2 + Al2O3, where x = (1, 2, 3, and 4). The physicochemical characteristics of both freshly calcined and used catalysts were studied using a range of characterization techniques, such as: N2 adsorption–desorption isotherms, XRD, H2-TPR, Raman spectroscopy, TGA, and TEM. An investigation of the effects of the Fe promoter on the catalytic activity of the catalyst (5Ni + xFe-10Zr + Al) was conducted. Amongst the studied catalysts, the 5Ni + 3Fe-10Zr + Al catalyst showed the best catalytic activity with CH4 and CO2 conversions of 87% and 90%, respectively, and had an H2/CO ratio of 0.98.
... Substitution of Ce 4+ for metal cations of lower valence (e.g., La 3+ or Pr 3+ ) causes the formation of oxygen vacancies, which could act as sites for CO 2 activation [43]. Doping of CeO 2 with isovalent Zr 4+ but of different size compared to Ce 4+ causes distortion of its lattice structure and displacement of oxygen atoms from their normal lattice positions [17,41], Ce-O bonds are elongated and weakened [44], resulting in a higher fraction of oxygen vacant sites and oxygen mobility. ...
... The main CO 2 -TPO peak shown in Fig. 10 was attributed to a nano-filament form of carbon [47,48]. Also, the larger extent of coking of 18NiCo with larger metal particles (43 nm) compared to 3NiCo (7.8 nm) is in good agreement with the literature [15,16,35,44,52]. ...
The effects of metal loading and particle size of NiCo alloy deposited on Ce0.75Zr0.25O2-δ carrier on the carbon reaction paths of dry reforming of methane at 750 °C with respect to the observed long-term activity profiles was elucidated. Small NiCo nanoparticles (7.8 nm) showed progressive deactivation as opposed to the larger ones (43 nm) which exhibited self-regeneration. Towards this goal, appropriate transient kinetic isotopic experiments (use of 18O2 and 13CO2) and other catalyst characterization studies (transient isothermal reduction, XPS, HAADF-STEM-EDX) were conducted. This work paves the way for the design of highly active and coke-resistant reducible metal oxides-supported NiCo.
... A study by Mamontov and Egami used neutron diffraction, electron paramagnetic resonance, and temperature programmed reduction to study oxygen vacancies and interstitial oxygen defects in CeO 2 and Ce 0.7 Zr 0.3 O 2 and concluded that the presence of zirconium showed a stabilizing influence on these defects via the reduction of Ce 4+ to Ce 3+ , giving increased resistance to thermal sintering. 34 This work claimed that interstitial oxygen defects are the source of cerium-zirconium oxides oxygen storage capacity properties and that further stabilization of these defects would further improve the thermal resistance of these supports. Figure 11(b) shows a histogram of the cation-cation distances. ...
... Crystallite size differences could be partially responsible for the difference in redox properties between the two samples, as smaller crystallites are often accredited with more surface area and, thus, active cerium atoms; however, various studies have found that crystallite size itself is not the defining factor but rather the removal of oxygen defects that also accompanies sintering. 34 The difference in TPR results may also be ascribed to the nano-domain phase segregation present in the reduced-oxidized sample since homogeneous ceria-zirconia solid solutions are expected to have a better performance, and the presence of interstitial oxygen ions, even at a low level, could conceivably alter the oxygen transport properties of the materials. ...
A reverse Monte Carlo analysis of neutron and x-ray total scattering data from two ceria-zirconia samples of composition Ce 0.75 Zr 0.25 O 2 is performed to analyze the distribution of cations and to examine the possibility of oxide-ion disorder. For the first material, heated in air under moderate conditions (800 °C), the structure is a single-phase solid-solution with the statistical distribution of cations, but a local tetragonal symmetry is found, consistent with the different coordination preferences of Ce and Zr. For the second material, heated under H 2 at 1050 °C followed by reoxidation at 400 °C, the structure shows a considerable disorder, with evidence for oxygen interstitials (Frenkel-ion defects) and a non-statistical distribution of cations with significantly higher concentrations of like–like cation nearest neighbors, highlighting the existence of cation-rich nano-domains. The results highlight the dynamic nature of this solid-solution, with structural evolution upon thermal treatment, which is of relevance to understanding its stability under redox catalytic conditions in practical applications.
... This latter phyllosilicate, due to the coating with cerium oxide, showed another band at 561 cm − 1 related to the presence of oxygen vacancies [41]. At these species, formed due to the presence of Ce 3+ ions, are assigned also the bands at 226 and 632 cm − 1 also showed in the Ni/Ce-ps sample [42]. Furthermore, the Ni/Ce-ps catalyst exhibited a band at 968 cm − 1 probably due to the second-order longitudinal optical mode of ceria [43]. ...
One of the strategies to mitigate the greenhouse effect is converting the CO 2 into high-added value products. In this work, a photothermo-catalytic approach, using peculiar Ni-phyllosilicates samples, was applied for the CO 2 conversion into solar fuels. In particular, the Ni-phyllosilicates were modified with the introduction of the Ce ions in their structure and successively they were covered with the CeO 2 semiconductor. The structural, morphological , textural, optical and reducibility properties as well as the interaction with CO 2 were investigated. The Ni/ Ce-phyllosilicate covered with CeO 2 achieved a CO 2 conversion of 87% after 5 hours of photothermo-catalytic test using simulated solar irradiation at 120 • C, producing 15.8 μmol/g cat •h of CO and 5.6 μmol/g cat •h of CH 4. The same sample was tested in an integrated approach where the CO 2 was evolved by the catalytic oxidation of toluene and then was converted into CO and CH 4 , obtaining a CO 2 conversion of 50% and 8.8 μmol CO /g cat •h and 3.3 μmol CH4 /g cat •h. The presence of Ni in the phyllosilicates structure guaranteed a good catalytic stability whereas the deposition of CeO 2 allowed to exploit the improved thermal (redox) properties of cerium oxide and favoured the CO 2 adsorption on its basic sites and oxygen vacancies. Furthermore, the high surface area of the as-synthetized Ni-phyllosilicates permitted to efficiently expose the CeO 2 surface-active sites to the solar radiation. The here investigated catalysts showed versatile properties ideal for hybrid catalytic approaches as the photothermo-catalysis, allowing to propose new solutions for the CO 2 valorization.
... A high rate of simultaneous conversion of all these pollutants can only be achieved within a narrow operating window near the stoichiometric air-to-fuel ratio (~14.6) [19]. Although the catalytic oxidation of CO is theoretically a simple reaction but it has high importance to meet the rigorous environmental regulations. ...
Background: Ceria (CeO2) is the focus of constant and diverse research interest due to its wide industrial applications. In particular, ceria-based mixed oxides with nanostructures exhibit better catalytic properties owing to high specific surface area, improved sintering properties, and high oxygen storage-release characteristics in comparison to the individual bulk materials. This work is aimed at analyzing the significance of doping in the ceria lattice to enhance CO and soot oxidation activity over a series of CeO2-La2O3, CeO2-Sm2O3, CeO2-Eu2O3, and CeO2-Gd2O3 mixed oxides. Methods: The investigated CeO2 and Ce0.8M0.2O2-δ (M= La, Sm, Eu, and Gd) mixed oxides were prepared by a simple and environmental friendly co-precipitation method, characterized in terms of composition, crystalline structure, particle size, hydrogen consumption, and oxidation state by the state-of-art techniques, namely, XRD, RS, TEM, TPR, UV-vis DRS, BET SA and OSC, and evaluated for CO and soot oxidation reactions. Results: Systematic characterization of the synthesized mixed oxides by various techniques provided interesting information. XRD results confirmed that doped materials exist as single-phase fluorite struc-tured oxides with significant changes in the lattice parameter. Raman spectroscopy results established the presence of oxygen vacancies in various proportions. The incorporation of trivalent cations into the ceria lattice greatly enhanced the OSC of the materials. H2-TPR results confirmed that doped samples are more reducible than pure ceria. Catalytic studies revealed an enhanced activity for both the reactions in contrast to pure ceria. Conclusion: Among various catalysts investigated, the Ce0.8Sm0.2O2-δ combination exhibited better catalytic activity owing to a considerably high OSC and facile reduction at significantly lower temperatures. The catalytic activity results confirmed that CO and soot oxidation activity could be improved by doping the ceria with appropriate trivalent cations.
... Thus, employing anti-biofouling coatings can significantly lower operating costs, particularly for naval vessels, resulting in cost savings [4,5]. Various methods to control surface fouling include using biocides, toxic coatings, etc. Antifouling paints based on organic tin Numerous studies have reported the enhanced thermal stability, corrosion resistance behaviour and OSC of CZ NPs [29][30][31]. The antibacterial efficiency of ceria-zirconia nanoparticles has also been evaluated [32,33]. ...
Biofouling, the accumulation of microorganisms, plants, algae on wet surfaces is one of the major issues adversely affecting the overall hydrodynamic performance of the marine vessels. Ceria (CeO 2) nanoparticles (NPs) are effectively used as anti-biofouling agent to prevent the deterioration of steel structures, due to their excellent redox capacity. Various approaches are being investigated to enhance the antifouling activity of ceria NPs. Here, we report the development of novel polydopamine (PDA) functionalised ceria-zirconia nanoparticles filled water-borne epoxy nanocomposite coating to prevent the microbial-induced corrosion of mild steel. Ceria NPs were functionalised with PDA to enhance the dispersibility and improve their ability to resist biofouling in water-borne epoxy resin coatings against microbial species. As the anti-biofouling activity of ceria depends on their oxygen storage capacity, zirconium was incorporated to create a defective crystal structure with more active oxygen storage and release sites. Ceria and ceria-zirconia NPs were synthesised by precipitation method and functionalised with PDA. The functionalisation of ceria-zirconia (CZ) NPs was confirmed by Fourier Transform Infrared (FTIR) spectroscopy, X-ray Diffraction (XRD), Confocal Raman spectroscopy and X-ray Photon spectroscopy (XPS). Thermogravimetric analysis (TGA) shows that incorporation of zirconium increases the amount of oxygen vacancies in the crystal lattice there by enhancing ceria's redox potential. Anti-biofouling and anti-biocorrosion properties of the coatings were explored against different microbial strains. The antibac-terial tests show that colony-forming units (CFU) of PDA functionalized ceria-zirconia epoxy (EPCZ) nano-composites were suppressed by 93 % and 87 % in the case of Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa) bacteria, respectively. Biofilm inhibition studies show that EPCZ nanocomposite coatings have higher biofilm inhibition efficiency against gram-negative bacteria (96.37 %) than gram-positive bacteria (62.67 %) due to the differences in their morphology. The practical applicability of the nanocomposite coatings was studied using cultured seawater consortia, and the results reveal that PCZ nanoparticles provide superior biofilm inhibition. Electrochemical impedance spectroscopy (EIS) reveals that the EPCZ coatings exhibit significant enhancement in charge transfer resistance and coating resistance. The synergistic effect of PDA functionalisation and zirconium incorporation in ceria leads to the exceptional biocorrosion resistance in corrosive bacterial environments.
... Some researchers have put forward the idea that Zr +4 tends to favor a 7-fold coordination, as opposed to an 8-fold coordination. This preference for 7-fold coordination could potentially contribute to the creation of more oxygen vacancies by reducing binary metal cations [84][85][86]. The same approach can be applied to the reduction of bismuth Bi to Bi +3 in cyclic voltammetry. ...
Phyto-mediated nanocomposite materials possess significant potential in morphological control as well as in electrocatalytic activity, which is the main reason for steered interest in the design of new materials for their applications in water splitting. In this work, nanocomposites of Bi2O3-ZrO2 have been prepared by using Amaranthus viridis L. (AVL)–based aqueous extract as a reducing agent. The synthesized nanomaterial’s composition and charge storage properties have been examined in detail. A crystalline structure for the synthesized Bi2O3-ZrO2-based nanocomposite has been elucidated by XRD analysis, while analysis through FTIR, UV, and SEM along with EDX also confirmed the successful biogenic synthesis of the desired nanocomposite. The synthesized biocomposite material is further applied on the nickel foam to be utilized in supercapacitor applications as a working electrode. Results manifest an exceptional specific capacitance value of 283.9 F/g at 2 mV/s by the AVL-Bi2O3-ZrO2 electrode. The columbic efficiency of the electrode was determined to be 99% even after 5000 GCD cycles. The synthesized Bi2O3-ZrO2 nanocomposite is considered to be an efficient electrocatalyst with a comparatively lower band gap of 2.7 eV. This lower bandgap further manifests its potential role in environmental remediation processes through photocatalysis. Additionally, the Bi2O3-ZrO2 composite has also been tested for cyclic stability through chronoamperometry showing reasonable stability. The Ragone plot has been used to determine the power density and energy density as 12 WhKg−1 and 3 KWKg−1 manifesting the higher capacitive behavior of nanocomposite. The synthesized bio-based nanocomposite has a good potential as an electrocatalyst for water splitting having highest productivity of the electrode up to 16.5 h as displayed by hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To the best of our knowledge, this study represents the first account on the green synthesis of Bi2O3-ZrO2 nanocomposites, wherein Amaranthus viridis L. (AVL) serves as the eco-friendly source, in combination with Bi2O3, which has reduced toxicity, and ZrO2, renowned for its exceptional stability. Our investigation further highlights the importance of synthesis of materials through phytoextract-assisted route for utilization in overall water splitting and energy storage applications.
... Two types of PNAs have recently garnered significant interest in both academia and industry: noble metal (Pt, Pd, and Ag)-supported metal oxides (Al 2 O 3 , TiO 2 , CeO 2 , or their solid solutions) and Pd-supported zeolites (Pd/SSZ-13, Pd/Beta, and Pd/ZSM-5) adsorbents [5][6][7][8][9][10][11]. Among these, CeO 2 is a candidate for PNA support because of its excellent redox properties and abundant oxygen vacancies, which facilitate the adsorption and storage of NO x [12][13][14][15][16][17]. Additionally, noble metals supported on CeO 2 will further improve the NO x adsorption capacity, and the desorption temperature is about 250-460 • C [18]. ...
At present, passive NOx adsorbers (PNAs) represent one of the most effective technologies for addressing NOx emissions from diesel engines during cold-start periods. Conventional PNAs, which primarily consist of noble metals (such as Pt, Pd, and Ag) loaded on metal oxides or zeolites, share the common drawback of high production costs. Consequently, developing low-cost PNAs with outstanding NOx storage performance remains a significant challenge. In this study, a series of CuxBa5Ce adsorbents were synthesized using the impregnation method, and a monolithic adsorbent was employed to evaluate NOx storage and release performance. Techniques such as XRD, UV-Vis DRs, H2-TPR, XPS, and in situ DRIFTs confirmed the crucial roles of Cu and Ba in NOx storage and release. Specifically, the incorporation of Cu into CeO2 enhanced NOx storage performance. Moreover, in the Cu3Ba5Ce adsorbent, the addition of Ba not only introduced new storage sites and altered the stability of NOx adsorption species but also helped prevent the aggregation of CuO, thereby prolonging the complete NOx storage duration and satisfying desorption temperature requirements. The Cu3Ba5Ce adsorbent exhibited the most favorable NOx storage performance, including a complete NOx storage time of 135 s and a NOx storage efficiency exceeding 50% at 80 °C over a 10 min period. While PNAs loaded with noble metals, such as Pd/CeO2 and Pt/CeO2, exhibited NOx storage efficiencies below 50% after adsorbing for 5 min at 80 °C. Therefore, this research offered a crucial strategy for developing non-noble-metal-loaded, Ce-based PNAs.
... Ceria-based catalysts (CeO 2 ), due to their oxygen storage capacity (OSC), are used in automotive catalysts to widen the operating window, i.e., the air-fuel ratio range [1]. Notably, ceria demonstrates significant activity for the selective catalytic reduction (SCR) of NO x [2], particularly when doped with a different valence metallic cation, such as iron [3][4][5][6]. ...
We demonstrate the effect of two preparation methods of gold catalysts (
... Ni 2+ has a smaller ionic radius (0.72 nm) than Ce 3+ (1.11 nm) and Ce 4+ (1.01 nm). As a result, doping can result in an O vacancy when Ni 2+ replaces Ce 3+ or Ce 4+ (Li et al. (2009);Mamontov et al. (2000)). On the other hand, when a tetravalent Ce is switched out for a diatomic Ni, an O vacancy is produced to achieve electrical neutrality. ...
Doping transition metal ions into cerium oxide (CeO2) results in interesting modifications to the material, including an increase in surface area, a high isoelectric point, biocompatibility, greater ionic conductivity, and catalytic activity. Herein, various concentrations (1-5%, 10% and 20%) of nickel (Ni) doped CeO2 nanoparticle have been made by a facile chemical process. Using a variety of cutting-edge analytical techniques, the structural, optical, and photocatalytic properties of undoped and varied concentrations (1-5%, 10%, and 20%) of Ni doped CeO2 nanoparticles have been investigated. Pure cubic fluorite structure with average crystallite sizes in the region of 12-15 nm was determined by X-ray diffraction (XRD) investigation. High resolution electron microscopy (HR-TEM), which revealed highly homogeneous hexagonal shape of the particles with average size of 15 nm, was also used to determine microstructural information. According to the optical absorption, the band gaps of Ni doped and undoped CeO2 nanoparticles were found to be 2.96 eV and 1.95 eV, respectively. When exposed to sunlight, the narrow band gap Ni doped CeO2 nanoparticles worked as an active visible light catalyst to remove the dyes Rose Bengal (RB) and Direct Yellow (DY). The best photodegradation efficiencies for RB and DY dyes were found about 93% and 97%, respectively, using the 5% Ni-doped CeO2 catalyst. The apparent rate constant values of 0.039 for RB and 0.040 min-1 were attained for DY. As well, the treated, untreated dye solution and control solutions were utilized to assess the toxicity of commercially accessible Vigna Radiata seeds. In this study exhibits percentages of length and germination increased by 30-35% when compared to dye pollutant solution. The Ni doped CeO2 can provide a substantial alternative for current industrial waste management because of its quick photocatalytic activity and remarkable seed germination results.
... This prompted the investigation of defect structures at the atomic scale, focusing on nanoscale properties. One of the first applications of total scattering on undoped ceria demonstrated the presence of interstitial oxygen ions triggered by suitable annealing processes [35]. A more recent work still based on neutron PDF investigated defects on NPs of different shapes, suggesting the formation of surface oxygen defects consistent with reduced Ce3O5+δ [36]. ...
We present a combined real and reciprocal space structural and microstructural characterization of CeO2 nanoparticles (NPs) exhibiting different crystallite sizes; ~3 nm CeO2 NPs were produced by an inverse micellae wet synthetic path and then annealed at different temperatures. X-ray total scattering data were analyzed by combining real-space-based Pair Distribution Function analysis and the reciprocal-space-based Debye Scattering Equation method with atomistic models. Subtle atomic-scale relaxations occur at the nanocrystal surface. The structural analysis was corroborated by ab initio DFT and force field calculations; micro-Raman and electron spin resonance added important insights to the NPs' defective structure. The combination of the above techniques suggests a core-shell like structure of ultrasmall NPs. These exhibit an expanded outer shell having a defective fluorite structure, while the inner shell is similar to the bulk structure. The presence of partially reduced 2 − species testifies to the high surface activity of the NPs. On increasing the annealing temperature, the particle dimensions increase, limiting disorder as a consequence of the progressive surface-to-volume ratio reduction.
... The CeO 2 surface can be modified by increasing the number of surface active sites by varying specific surface areas, pore sites, the ratio of Ce 3+ /Ce 4+ , and the number of oxygen vacancies and acid-base sites [59]. There are many processes that can be used for modifying a CeO 2 surface, such as annealing under vacuum [60], hydrogen treatment [61], thermal activation [62], thermal annealing treatment [35,63], electron beam irradiation [64], and pressure control [65]. However, these processes are difficult to control and require expensive equipment and tools. ...
This study developed a facile and effective approach to engineer the surface properties of cerium oxide (CeO2) nanospindle catalysts for the direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol. CeO2 nanospindles were first prepared by a simple precipitation method followed by wet chemical redox etching with sodium borohydride (NaBH4) under high intensity ultrasonication (ultrasonic horn, 20 kHz, 150 W/cm²). The ultrasonically assisted surface modification of the CeO2 nanospindles in NaBH4 led to particle collisions and surface reduction that resulted in an increase in the number of surface-active sites of exposed Ce³⁺ and oxygen vacancies. The surface modified CeO2 nanospindles showed an improvement of catalytic activity for DMC formation, yielding 17.90 mmol.gcat⁻¹ with 100% DMC selectivity. This study offers a simple and effective method to modify a CeO2 surface, and it can further be applied for other chemical activities.
... CeO 2 may repeatedly offer a procedure for adsorbing, activating, and cleaving CO 2 molecules, then creating the adsorbed CO species for further hydrogenation into CH 4 during the CO 2 methanation process due to its redox characteristic. It is the quantity of oxygen lattice that is integrated into the CeO 2 structure that is referred to as OSC [164,165]. The morphology, form, defects, and OSC in the nanostructure of CeO 2 nanoparticles were all influenced by the preparation method [166,167]. ...
It is generally accepted that carbon dioxide (CO2) is one of the most important greenhouse gases and a primary factor in the acceleration of climate change. Methanation, which involves converting CO2 to methane in the presence of renewable hydrogen, is a plausible technique for achieving net-zero atmospheric CO2 concentrations. Nevertheless, the development of effective catalytic systems continues to be a significant barrier tor CO2 methanation. CeO2-based catalysts for CO2 methanation could benefit from tuning their oxygen vacancies (OVs) to increase catalytic performance. This review examines and discusses in depth a number of different characterization methodologies for measuring OVs. Additionally, this review focuses specifically on the role that OVs play in various CeO2-based catalysts as well as the numerous tuning strategies that may be used to increase the number of OVs in these catalysts. This study could give scientists new ideas for how to improve catalytic CO2 methanation.
The study is presented on the influence of the composition of a ceria-zirconia support on the structure and the activity in water gas shift reaction of platinum catalysts (Pt/Ce0.75Zr0.25O2 и Pt/Ce0.4Zr0.5Y0.05La0.05O2). The structure diagnostics of the samples were performed using high-resolution transmission electron microscopy, powder X-ray diffraction, CO chemisorption and X-ray atomic pair distribution function method. It was shown that the catalysts contain highly dispersed platinum particles not exceeding 2 nm in size. Platinum particles supported on Ce0.75Zr0.25O2 are smaller due to the higher specific surface area of the support. The catalysts Pt/Ce0.75Zr0.25O2 and Pt/Ce0.4Zr0.5Y0.05La0.05O2 proved to have similar efficiency while having the same platinum content. It was assumed that the catalysts supported on Ce0.4Zr0.5Y0.05La0.05O2 demonstrate a slightly higher turnover frequency per platinum surface atom, but it is likely compensated by the difference in the supported metal particle size.
Designing highly active and robust catalysts for the oxygen evolution reaction is key to improving the overall efficiency of the water splitting reaction. It has been previously demonstrated that evaporation induced self‐assembly (EISA) can be used to synthesize highly porous and high surface area cerate‐based fluorite nanocatalysts, and that substitution of Ce with 50% rare earth (RE) cations significantly improves electrocatalyst activity. Herein, the defect structure of the best performing nanocatalyst in the series are further explored, Nd 2 Ce 2 O 7 , with a combination of neutron diffraction and neutron pair distribution function analysis. It is found that Nd 3 + cation substitution for Ce in the CeO 2 fluorite lattice introduces higher levels of oxygen Frenkel defects and induces a partially reduced RE 1.5 Ce 1.5 O 5 + x phase with oxygen vacancy ordering. Significantly, it is demonstrated that the concentration of oxygen Frenkel defects and improved electrocatalytic activity can be further enhanced by increasing the compositional complexity (number of RE cations involved) in the substitution. The resulting novel compositionally‐complex fluorite– (La 0.2 Pr 0.2 Nd 0.2 Tb 0.2 Dy 0.2 ) 2 Ce 2 O 7 is shown to display a low OER overpotential of 210 mV at a current density of 10 mAcm ⁻² in 1M KOH, and excellent cycling stability. It is suggested that increasing the compositional complexity of fluorite nanocatalysts expands the ability to tailor catalyst design.
CeO2 nanorod based catalysts for the base‐free synthesis of azoxy‐aromatics via transfer hydrogenation of nitroarenes with ethanol as hydrogen donor have been synthesized and investigated. The oxygen vacancies (Ov) and base sites are critical for their excellent catalytic properties. The Ov, i.e., undercoordinated Ce cations, serve as the sites to activate ethanol and nitroarenes by lowering the energy barrier to transfer hydrogen from α‐Csp3‐H in ethanol to the nitro group coupling it to the redox reactions between Ce3+ and Ce4+. At the same time, the base sites catalyze the condensation step to selectively produce azoxy‐aromatics. The catalytic route opens a much improved way to use non‐noble metal oxides without additives for the selective functional group reduction and coupling reactions.
CeO2 nanorod based catalysts for the base‐free synthesis of azoxy‐aromatics via transfer hydrogenation of nitroarenes with ethanol as hydrogen donor have been synthesized and investigated. The oxygen vacancies (Ov) and base sites are critical for their excellent catalytic properties. The Ov, i.e., undercoordinated Ce cations, serve as the sites to activate ethanol and nitroarenes by lowering the energy barrier to transfer hydrogen from α‐Csp3‐H in ethanol to the nitro group coupling it to the redox reactions between Ce3+ and Ce4+. At the same time, the base sites catalyze the condensation step to selectively produce azoxy‐aromatics. The catalytic route opens a much improved way to use non‐noble metal oxides without additives for the selective functional group reduction and coupling reactions.
NiO x /CeO 2 catalysts were synthesized under various pretreatment conditions. Different pretreatment conditions significantly influenced the activity of the NO reduction by CO reaction.
In recent years, heterogeneous catalysts have led to environment friendly transformations with better yields and reusability. Pd, one of the initial metals employed in heterogeneous organic synthesis, suffered from limitations like its high cost. This justifies the need for development of catalysts with abundant, low‐cost metals has been receiving a lot of attention in the scientific community. In this work, bimetallic oxide catalyst, CeO 2 ‐ZrO 2 is synthesized by a sol‐gel route. The structure and morphology of the catalyst is investigated using XRD, SEM, EDAX, TGA, BET, and TPD. It is utilized for obtaining quinoxaline derivatives at room temperature. 2,3‐Diphenylquinoxaline is obtained via a simple condensation reaction between 1,2‐Diaminobenzene and 1,2‐diketones, catalyzed by CeO 2(50) ‐ZrO 2(50) with 87% yield in 15 min. Quinoxalines are known for their biological and therapeutic activities; hence, they are essential molecules. The biological activity of the synthesized quinoxaline derivatives has been evaluated against bacterial and fungal strains.
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The oxygen storage capacity of ceria-based catalytic materials is influenced by their size, morphology, and surface structure, which can be tuned using surfactant-mediated synthesis. In particular, the cuboidal morphology exposes the most reactive surfaces; however, when the capping agent is removed, the nanocubes can agglomerate and limit the available reactive surface. Here, we study ceria nanocubes, lanthanum-doped ceria nanocubes, and ceria nanocubes embedded inside a highly porous silica aerogel by high-energy resolution fluorescence detection—X-ray absorption near edge spectroscopy at the Ce L3 edge. In situ measurements showed an increased reversibility of redox cycles in ceria nanocubes when embedded in the aerogel, demonstrating enhanced reactivity due to the retention of reactive surfaces. These aerogel nanocomposites show greater improvement in the redox capacity and increased thermal stability of this catalytic material compared to the surfactant-capped nanocubes. Ex situ measurements were also performed to study the effect of lanthanum doping on the cerium oxidation state in the nanocubes, indicating a higher proportion of Ce⁴⁺ compared to that of the undoped ceria nanocubes.
Amorphous materials are metastable solids with only short-range order at the atomic scale, which results from local intermolecular chemical bonding. The lack of long-range order typical of crystals endows amorphous nanomaterials with unconventional and intriguing structural features, such as isotropic atomic environments, abundant surface dangling bonds, highly unsaturated coordination, etc. Because of these features and the ensuing modulation in electronic properties, amorphous nanomaterials display potential for practical applications in different areas. Motivated by these elements, here we provide an overview of the unique structural features, the general synthetic methods, and the potential for applications covered by contemporary research in amorphous nanomaterials. Furthermore, we discussed the possible theoretical mechanism for amorphous nanomaterials, examining how the unique structural properties and electronic configurations contribute to their exceptional performance. In particular, the structural benefits of amorphous nanomaterials as well as their enhanced electrocatalytic, optical, and mechanical properties, thereby clarifying the structure-function relationships, are highlighted. Finally, a perspective on the preparation and utilization of amorphous nanomaterials to establish mature systems with a superior hierarchy for various applications is introduced, and an outlook for future challenges and opportunities at the frontiers of this rapidly advancing field is proposed.
Ceria and ceria-zirconia nanomaterials of different origin were studied in order to elucidate the role of their structural and texture characteristics in controlling the performance towards capturing CO 2 from effluent...
In this study, a novel three-dimensional (3D)-OMm-Co3O4/SiO2-0.5AP (OMm = ordered macro–meso porous, AP = aluminum phosphate) monolithic catalyst was for the first time constructed successfully with the hierarchical Co-phyllosilicate ultrathin nanosheets growth on the surface of 3D printed ordered macropore–mesoporous SiO2 support. On the one hand, we discovered that the construction of ordered macropore–mesoporous structures is beneficial to the diffusion and adsorption of reactants, intermediates, and products. On the other hand, the formation of hierarchical Co-phyllosilicate ultrathin nanosheets could provide more active Co&+ species, abundant acid sites, and active oxygen. The above factors are in favor of improving the catalytic performance of benzene oxidation, and then a 3D-OMm-Co3O4/SiO2-0.5AP catalyst exhibited the superior catalytic activity. To explore the effect of catalysts structure and morphology, various Co-based catalysts were also constructed. Simultaneously, the 3D-OMm-Co3O4/SiO2-0.5AP catalyst has excellent catalytic performance, water resistance, and thermal stability in the catalytic combustion of benzene due to the strong interactions between Co&+ species and SiO2 in the phyllosilicate. Therefore, this study proposes a new catalyst synthesis method through 3D printing, and presents considerable prospects for the removal of VOCs from industrial applications.
Oxynitrides of cerium are expected to have many useful properties but have not been synthesized so far. We identified possible modifications of a not-yet-synthesized Ce3O3N compound, combining global search (GS) and data mining (DM) methods. Employing empirical potentials, structure candidates were obtained via global optimization on the energy landscape of Ce3O3N for different pressure values. Furthermore, additional feasible structure candidates were found using data mining of the ICSD database. The most promising structure candidates obtained were locally optimized at the ab initio level, and their E(V) curves were computed. The structure lowest in total energy, Ce3O3N-DM1, was found via local optimization starting from a data mining candidate and should be thermodynamically metastable up to high pressures.
In this article, the ongoing development progress of the polarized neutron technique developments at the China Spallation Neutron Source (CSNS) is presented. The general goal of establishing polarized neutron capability in parallel to CSNS beamline construction is also explained. The key instrument development projects of developing the Spin-Exchange Optical Pumping (SEOP) ³ He systems, polarized neutron devices, and their application on neutron beamlines are demonstrated. Performance parameters of the developed techniques are also discussed.
The Fenton/Fenton-like reaction is an advanced oxidation process, which is widely recognized for its efficient removal of recalcitrant organic contaminants. In Fenton/Fenton-like oxidation systems, multivalent metals are important catalysts, such as Fe, Cu, Co, Mn, Ce, Ag, Cr, Ru, W, Mo, V, Ti, etc., which can activate H2O2 to produce reactive oxygen species (ROS) by low-valent metal (Mⁿ⁺) with reduction properties and high-valent metal (M(n+z)+) with oxidation properties due to the dual roles of H2O2 as reductant and oxidant. Moreover, Fenton/Fenton-like oxidation is a radical reaction process, in which the redox reaction between multivalent metals and radicals may occur. Therefore, the conversion and recycle of valence state of multivalent metals are crucial in the production of ROS and the degradation of organic contaminants. In this review, the recent advances in the conversion and recycle valence states of multivalent metals, including Fe³⁺/Fe²⁺, Cu²⁺/Cu⁺, Co³⁺/Co²⁺, Mn⁴⁺/Mn²⁺, Ce⁴⁺/Ce³⁺, Ru³⁺/Ru²⁺, Cr⁶⁺/Cr³⁺, Ti⁴⁺/Ti³⁺, Mo⁶⁺/Mo⁴⁺ and the like, were systematically reviewed. Firstly, the conversion and influencing factors of valence states of multivalent metals in Fenton processes were introduced; Secondly, the strategies for regenerating low-valent metals were summarized; Thirdly, the formation, determination methods and roles of high-valent metals in contaminant degradation were analyzed and discussed; Finally, the concluding remarks and future perspectives were proposed. This review could provide valuable information for the development of new catalysts of multivalent metals, deepen the understanding of the mechanisms of Fenton/Fenton-like processes and provide a reference for expanding the application of the multivalent metals catalytic Fenton/Fenton-like process in water and wastewater treatment.
The charge and discharge working mechanisms in lithium sulfur batteries contain multi-step complex reactions involving two-electron transfer and multiple phase transformations. The dissolution and diffusion of lithium polysulfides cause a huge loss of active material and fast capacity decay, preventing the practical use of lithium sulfur batteries. Herein, CeO2 nanorods supported bimetallic nickel cobalt oxide (NiCo2Ox) was investigated as a cathode host material for lithium sulfur batteries, which can provide adsorption-catalysis dual synergy to restrain the shuttle of polysulfides and stimulate rapid redox reaction for the conversion of polysulfides. The polar CeO2 nanorods with abundant surface defects exhibit chemisorption towards lithium polysulfides and the excellent electrocatalytic activity of NiCo2Ox nanoclusters can rev up the chain transformation of lithium polysulfides. The electrochemical results show that the battery with NiCo2Ox/CeO2 nanorods can demonstrate high discharge capacity, stable cycling, low voltage polarization and high sulfur utilization. The battery with NiCo2Ox/CeO2 nanorods unveils a high specific capacity of 1236 mAh g-1 with a very low capacity fading of 0.09% per cycle after 100 cycles at a 0.2C current rate. Moreover, the excellent performance with high sulfur loading (>5 mg cm-2) verifies a huge promise for future commercial applications.
In this work, a new continuum model, informed by DFT simulations, is developed to predict the chemical expansion observed in non-stoichiometric oxides and is applied to study the expansivity of oxygen vacancies in CeO 2 . The chemical expansion is a summation of two competing processes: the formation of oxygen vacancy and the change in cation radius. We introduce an elastic dipole tensor to determine the elastic energy per defect introduced to the system around the oxygen vacancy. We show that this tensor, which can be accurately predicted from first-principle DFT calculations, can be used to predict the chemical expansion of ceria as well as other fluorite structure oxides. Compared to previous work which obtains expansivity based on empirical potentials, our work provides an efficient way of computing it directly by DFT calculations. Furthermore, we discuss how the elastic dipole tensor can predict the O 2 partial pressure vs O/Ce ratios in strained systems and show that CeO 2 can be reduced more easily in the presence of tensile strains.
Shale gas exploration generates methane, mostly used as fuel, and natural gas liquids (NGLs), mainly used as chemical feedstocks. These NGLs are typically extracted using expensive turbo-expansion and cryogenic distillation unit operations and an alternative modular approach using a solid-state electrochemical reactor configuration is proposed. This electrochemical approach offers advantages over other process intensification techniques due to its modularity and potential delivery of an electric by-product. In this study, a solid oxide cell setup using rare-earth and alkali-earth based perovskites – LaFeO3 and SrFeO3 – was demonstrated for thermally efficient dehydrogenation of NGLs to generate olefins for downstream chemical production. This demonstration showed that ethane conversion via oxidative dehydrogenation (up to 0.1 mmol/min/cm2 of ethylene) was significant in comparison to thermal cracking (up to 0.01 mmol/min/cm2 of ethylene) under the conditions tested and illustrates the viability of this approach to process intensification. Furthermore, modifications to the current cell configuration are needed before the electric by-product can be generated.
This study developed a facile and effective approach to engineer the surface properties of cerium oxide (CeO2) nanospindle catalysts for the direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol. CeO2 nanospindles were first prepared by a simple precipitation method followed by wet chemical redox etching with sodium borohydride (NaBH4) under high intensity ultrasonication (ultrasonic horn, 20 kHz, 150 W/cm2). The ultrasonically assisted surface modification of the CeO2 nanospindles in NaBH4 led to particle collisions and surface reduction that resulted in an increase in the number of surface-active sites of exposed Ce3+ and oxygen vacancies. The surface modified CeO2 nanospindles showed an improvement of catalytic activity for DMC formation, yielding 17.90 mmol·gcat−1 with 100 % DMC selectivity. This study offers a simple and effective method to modify a CeO2 surface, and it can further be applied for other chemical activities.
Ceria-based materials have been highly desired in photocatalytic reactions due to their redox properties and strong oxygen storage and transfer ability. Herein, we report the structures of one CeCe70 oxysulfate cluster and four MCe70 clusters (M = Cu, Ni, Co, and Fe) with the same Ce70 core. As noted, single-crystal X-ray diffraction confirmed the structures of CeCe70 and the MCe70 series, while Raman spectroscopy indicated an increase in oxygen defects upon the introduction of Cu and Fe ions. The clusters catalyzed the oxidation of 4-methoxybenzyl alcohol under ultraviolet light. CuCe70 and FeCe70 exhibited enhanced reactivity compared to CeCe70 and improved aldehyde selectivity compared to control experiments. In comparison with their homogeneous congeners, the CeCe70/MCe70 clusters altered the location of radical generation from the bulk solution to the clusters' surfaces. Mechanistic studies highlight the role of oxygen defects and specific transition metal introduction for efficient photocatalysis. The mechanistic pathway in this study provides insight into how to select or design a highly selective catalyst for photocatalysis.
The Ce0.5Zr0.5O2 solid solution with a homogeneous structure is, respectively, modulated by a hard template strategy (P-Ce0.5Zr0.5O2), the ligand decomposition method (T-Ce0.5Zr0.5O2), and the co-precipitation method (O-Ce0.5Zr0.5O2) and employed in the CO2 conversion reaction. The different catalytic performances of Ce0.5Zr0.5O2 obtained are attributed to the homogeneity degree. The T-Ce0.5Zr0.5O2 catalyst with a complete homogeneous structure presents both higher CO2 conversion and CO selectivity in the range 250-450 °C than the P-Ce0.5Zr0.5O2 catalyst with a surface Ce-rich structure and the O-Ce0.5Zr0.5O2 catalyst with a surface Zr-rich structure. Interestingly, a larger number of oxygen vacancies are observed over the T-Ce0.5Zr0.5O2 catalyst, which leads to strong CO2 and H2 adsorption ability as well as fast regeneration of hydroxyl groups. In addition, the product formation mechanisms are discussed, as analyzed by high-pressure in situ diffused reflectance infrared Fourier transform spectroscopy, where CO is thought to be produced directly by the decomposition of monodentate formate, and CH3OH generation is observed to be inclined to proceed through the following reaction route: bicarbonates/carbonates → bidentate formate → methoxy → methanol.
Alkylation of benzene using CO2 and H2 to toluene and xylene (TX) can not only give value-added chemical products but also alleviate the greenhouse effect. The key to achieving this goal is to develop a highly efficient catalyst. Herein, a class of bifunctional catalysts containing series of ZnxCeyZrzO metal oxide and HZSM-5 was prepared and studied for the catalytic properties. Higher CO2 conversion and alkylation utilization were achieved using the ZnxCeyO coupling with HZSM-5 compared to its counterpart ZnO and CeO2, revealing synergy between the ZnO and CeO2. More strikingly, the ZnxCeyZrzO, synthesized by insertion of Zirconia (Zr) atoms into CeO2, can not only improve the concentration of oxygen vacancy but also facilitate CO2 adsorption. After coupling with HZSM-5, the alkylation utilization of CO2 was significantly improved and the unwanted CO selectivity was obviously reduced by hindering the RWGS reaction. The optimal bifunctional catalyst Zn3Ce1Zr3O/Z5 showed alkylation utilization of 24%, unwanted CO selectivity of only 64% and the TX selectivity as high as 82%. In situ DRIFTS results confirmed that the methanol generated on the ZnxCeyZrzO metal oxide via the formate-methoxy intermediates mechanism was rapidly alkylated on HZSM-5 with benzene to produce TX.
In this work, we introduce a Rhodium-Ceria-Zirconia (Rh/CZ) internal reforming catalyst layer to a high-performance metal-supported solid oxide fuel cell (MS-SOFC). The catalyst is applied by infiltrating Rh, CeO2, and ZrO2 precursors into the stainless steel (SS 430) support. The cell is tested by directly feeding an ethanol solution (45 vol%) into the anode at 600 °C. Our experimental results show that the button cell with the infiltrated 5 wt% Rh/CZ demonstrates an improved performance over the button cell without the catalyst layer by enhancing the internal reforming activity of ethanol toward the production of synthesis gas. The maximum current density improved from 0.3 A cm⁻² to 0.4 A cm⁻² while the long-term stability was also greatly improved. Post mortem cell analysis reveals that the infiltrated catalyst layer can prevent severe coke deposition from the cell's anode functional layer. The proposed integrated reforming catalyst and MS-SOFC system is a promising pathway to enable bioethanol fed-SOFC technology for future electric vehicles.
Computer simulation techniques have been used to model cubic CeO2-ZrO2 solid solutions in the whole composition range. Aspects related with the oxygen storage capacity of these materials are emphasized. The energetics of the Ce4+/Ce3+ bulk reduction reaction as well as the activation energy for oxygen migration in the lattice are investigated and compared with the corresponding quantities in pure CeO2. It is found that even small additions of ZrO2 decrease the bulk reduction energy of Ce4+ to values comparable to those reported for surface reduction in pure CeO2. Activation energy calculations indicate an almost monotonic increase of oxygen mobility with increasing zirconia content.
We have observed that the insertion of trivalent cations into the lattice of Ce0.6Zr0.4O2 solid solution improves the oxygen storage capacity at low temperatures by decreasing the temperature of the reduction in the bulk of the solid solution compared to the undoped sample. The effect depends on the type of dopant and its concentration. Reduction of the solid solution at moderate temperatures is strongly improved upon aging in H-2/Ar independently of the extensive sintering of the sample. (C) 1997 Academic Press.
Reduction/oxidation behaviour of high surface area CeO2and Ce0.5Zr0.5O2mixed oxide is compared. It is shown that introduction of ZrO2into the CeO2framework with formation of a solid solution strongly modifies the reduction behaviour in comparison to that seen with CeO2alone. Remarkably, upon repetitive reduction/oxidation processes, the temperature of the reduction of the solid solution decreases from 900 to 700 K. Conversely, in the aged CeO2sample almost no reduction occurs below 900 K. Textural characterisation shows that the redox process favours a large sintering of both the samples and, unexpectedly, induces formation of a new mesoporosity centered at about 20 nm. The expansion/contraction of the lattice parameter upon, respectively, reduction/oxidation, detected by X-ray diffraction, is suggested to be responsible for the textural modification. The suppression of the redox capacities of CeO2at moderate temperature is associated with the decrease of the surface area. The sintering process induces a structural modification of Ce0.5Zr0.5O2which promotes the reduction in the bulk, resulting in a high efficiency of the Ce4+→Ce3+redox cycle at moderate temperatures.
A definition of structural defects in amorphous solids in terms of the distribution of the internal stresses on the atomic level and of the symmetry of the environment of individual atoms is introduced. This definition does not require an ideal reference structure. The concept of the internal stresses on the atomic scale has been previously applied to describe the core structure of crystalline dislocations. In this paper it has been applied to the model amorphous structure generated by a computer simulation. It was found that there is a significant variation in the magnitude and direction of internal stresses, and that there are regions of 10 to 20 atoms over which the stresses remain either high or low. A method of calculating the symmetry coefficients at atomic sites has been proposed, and applied to the same system. It has been shown that there are significant correlations between the internal stresses and the local symmetry. The low-stress, high-symmetry regions resemble microcrystalline clusters, while the high-stress, low-symmetry regions show a stress-distribution pattern and symmetry similar to the crystalline defects. The structural defects in amorphous structures are defined as the latter regions, and the significance of such a definition in elucidating the properties of amorphous solids is discussed.
The local structure of the M–O bond in CeO2–ZrO2mixed oxides is investigated with the aim of finding a correlation between structural parameters and oxygen exchange properties. It is found that insertion of ZrO2into the CeO2lattice strongly perturbs the symmetry of the M–O bond. As the content of CeO2in CeO2–ZrO2solid solution is increased from 20 to 80 mol%, tetragonal, (t,t′,t″) and cubic phases are formed. The local symmetry of the Zr–O bond is strongly perturbed by the increase in CeO2content while no significant modification of the first Ce–O coordination sphere is found, except for lengthening of the Ce–O bond consistent with lattice parameter increase. The perturbation of the Zr–O coordination sphere, which leads to highly disordered oxygen in the lattice, is indicated as responsible for the high oxygen mobility in CeO2–ZrO2mixed oxides.
The ionic disorder in single crystals of the fluorite-type solid solutions Ba1-xLaxF2+x (with x=0.209 and x=0.492) has been studied in the temperature range from room temperature to 800 degrees C by diffuse neutron scattering, ionic conductivity, and specific heat measurements. From the diffuse neutron scattering it was found that the disorder was dominated by 222 clusters, which at low temperatures (T<400 degrees C) were ordered along the (100) direction in aggregates of up to four 222 clusters. The correlation between the different 222 clusters in the aggregates is slowly lost when entering into the fast ion phase. The lifetime of the 222 clusters could not, even at the highest temperatures, be determined by neutron scattering ( tau >>10-10s), in agreement with NMB results which suggest a jump frequency below 75 MHz. The temperatures at which the steepest slopes are found in the loss of correlations and in the conductivity coincide at approximately 650 degrees C. At this temperature no clear anomaly is observed in the specific heat. Based on these findings the authors propose a conduction mechanisms where F- ions are moving through the lattice by means of rearrangements of the 222 clusters.
Rh/CeO2-ZrO2 mixed oxides show improved redox properties compared to Rh/CeO2 due to a low-temperature reduction in the bulk. In the present work we have investigated the Zr-O and Ce- O local structure in Rh/Ce0.5Zr0.5O2 by means of EXAFS and Raman spectroscopy. By introducing Zr4+ into the CeO2 lattice, the oxygen sublattice is strongly modified compared to the ideal fluorite structure of CeO2: the Ce4+ ions retain their coordination while the number of oxygens around the Zr4+ decreases from 8 to 6 due to a displacement. of two oxygens to a nonbonding distance. It is suggested that this distortion of the oxygen sublattice generates some mobile oxygens in Rh/Ce0.5Zr0.5O2 which are responsible for the improved redox properties. (C) 1997 Academic Press
The temperature programmed reduction, oxygen uptake, and surface area stability of high-surface-area Rh-, Pt-, and Pd- loaded Ce0.5Zr0.5O2 solid solution have been investigated. It is observed that the presence of the noble metal strongly favors the reduction of the support compared with the metal- free Ce0.5Zr0.5O2. A further improvement is observed on sintering induced by repetitive reduction/oxidation processes. Indeed, the temperature of support reduction decreases from 650-950 to about 450 K for the Rh- and Pt-loaded samples and to 520 K in the presence of Pd. In contrast, the low-temperature reduction of Rh/CeO2 (surface area 194 m(2) g(-1)) is strongly encumbered after such treatments. The roles of noble metal and added ZrO2 in promoting low-temperature reduction are discussed. (C) 1999 Academic Press
Cerium (IV) oxide prepared at high temperatures in air shows ESR absorption signals. The oxide prepared at low temperatures shows very diffuse signals. Heating the oxide in vacuum at 1000° or in hydrogen at 298° results in a reduction of the intensity of the signal. A theory was developed to show that the ESR signal is obtained from the quasi free electron in non‐stoichiometric ceria.
The complex oxides in the CeO2ZrO2 system were examined for the improvement of oxygen storage capacity in automotive catalysts. The formation of CeZr oxide solid solution improved the thermal stability and activity of CeO2. The CeZr addition enhanced the removal activity for CO, NOx and hydrocarbons under dynamic air-fuel ratio condition. The automotive catalyst was designed and developed through research on the oxides in the CeO2ZrO2 system.
Adsorption of O2, using either normal oxygen or 17O-enriched mixtures, on CeO2 outgassed at different temperatures has been studied by EPR and FTIR. Different signals, assigned to O2– species bonded to surface cerium ions, were observed depending on the vacuum treatment temperature Tv, a parameter that determines the type of defects generated at the CeO2 surface. Depending on their linewidths and lowest g values, the O2– signals observed in EPR spectra can be grouped into two types, related to species absorbed at isolated and aggregated oxygen vacancies, respectively. The EPR parameters indicate that the bonds of these species to the surface have different degrees of covalency, which might also influence their IR absorption coefficient. While the two oxygen atoms in the O2– species formed on isolated surface oxygen vacancies are EPR equivalent, they are non-equivalent in those formed on aggregated vacancies produced at the higher temperatures. The different EPR signal parameters and the variations in the intensities of the latter, observed with different Tv and thermal treatments of the samples after adsorption, indicate that EPR of adsorbed O2– and related species can be used as a probe to study the generation and properties of defects on CeO2 surfaces.
The mathematical functions necessary for Rietveld refinement of time-of-flight neutron powder diffraction patterns from spallation sources are developed and a computer program for least-squares analysis is described. The results of Rietveld refinements of nickel and a low-carbon steel are described and discussed. The method fully exploits the high resolution (Δd/d ~ 0.3 ~ 0.5%) available with powder diffractometers currently in operation on these sources and examples are given of precise determination of atom coordinates, thermal parameters, lattice parameters and the detection of small strains.
EPR and FTIR have been used to obtain information on the oxygen adsorption sites generated on CeO2/γ-Al2O3 samples by outgassing treatments at different temperatures. Several types of O2– species, showing EPR signals with different line shapes, have been detected depending on the type of surface sites where the radical is adsorbed. The main difference in these EPR signals lies in the gx principal value of the g tensor, which takes values between 2.026 and 2.008; this is ascribed to variations in the covalent character of the Ce4+–O2– bond, which is affected by ceria–alumina interactions and by the mild reduction treatments. The correlation between the kind of cerium oxide entities which may exist on the alumina surface for several samples, with different cerium contents and varying preparation methods, and the characteristics of the EPR signals allows us to assign the O2– adsorption sites to cerium cations located at specific places on those cerium oxide entities. It is shown that the adsorption strengths towards O2 and the ease of reoxidation of these sites are substantially lower than those of the sites existing at pure CeO2 surfaces. Some of the consequences of these differences for the performance of car-exhaust catalytic converters are discussed.
The nature of the oxygen species adsorbed on slightly reduced CeO2/SiO2 supported catalysts has been investigated using oxygen enriched with 17O2. The observed hyperfine splitting was found to be associated with the central component of the g tensor; it indicated that the corresponding orbital contained the unpaired electron and therefore the central component was defined as gxx. With this assumption, the e.s.r. spectra are consistent with the adsorption of oxygen as O–2 at 77 K with gyy= 2.0109, gxx= 2.0158 and gzz= 2.028 with a hyperfine splitting (Axx) of 75 G about gxx. The O–2 is adsorbed with the internuclear axis along the surface of the catalyst at a site corresponding to a cerium ion. There is some indication that interaction with the metal ion orbitals is perturbing the energy levels of the adsorbed O–2 ion.
We have studied the structure of ultra-thin CeO2 overlayer on single crystals of Y stabilized cubic zirconia (ZrO2) by energy dispersive surface X-ray scattering. The overlayers were formed by depositing cerium metal and annealing in oxygen atmosphere. Three different crystallographic surfaces, (001), (011) and (111), were examined. We observed formation of CeO2 overlayer with thickness of 10–40Å and lateral coherence length of 20–70Å depending on the sample. In addition, we performed in situ temperature study of a sample with (001) surface. A zirconia crystal with cerium metal deposited on the surface was placed on a substrate heater and was annealed in air during in situ surface X-ray diffraction. We observed the formation of a ceria epitaxial film at 420°C followed by a preferential grain growth at higher temperatures and finally the diffusion of Ce into the bulk above 700°C.
Temperature-programmed reduction in a H2/Ar mixture of Rh-loaded CeO2-ZrO2 solid solutions with a ZrO2 content varying between 10 and 90% mol and of monoclinic, tetragonal, and cubic structures is reported. It is shown that incorporation of ZrO2 into a solid solution with CeO2 strongly promotes bulk reduction of the Rh-loaded solid solutions in comparison to a Rh/CeO2 sample. The promotion of the bulk reduction results in high oxygen storage capacity (OSC) as measured by oxygen uptake. A structural dependence of both reduction and oxidation processes is observed which is attributed to a higher oxygen mobility in the cubic structure compared to the tetragonal and monoclinic ones.
The nature of weakly bound oxygen on ceria films was studied using temperature-programmed desorption with labeled 18O2. For α- Al2O3(0001)-supported ceria, a desorption feature between 800 and 1300 K is shown to result from partial reduction of ceria. However, this oxygen accounts for only a small fraction of the total oxygen in the ceria film and isotopic labeling studies suggest that this oxygen does not exchange freely with the remaining oxygen in the film. In contrast, results for zirconia-supported ceria demonstrate that much more oxygen desorbs in the low temperature regime below 1300 K, and that there is significant isotope exchange throughout the ceria film and with the zirconia substrate. Finally, exposure of reduced, zirconia-supported ceria to water at 670 K resulted in reoxidation of the ceria film. Oxygen from ceria was then shown to react with CO adsorbed on supported Rh particles, completing the catalytic cycle for the water-gas-shift reaction.
Temperature-programmed desorption (TPD) has been used to compare the O2 desorption characteristics of a thin film of ceria prepared by vapor deposition on an α-Al2O3(0001) substrate with that of a CeO2(111) surface. On CeO2(111), there is no substantial desorption of O2 below 1300 K, while a low-temperature state, between 800 and 1250 K, was observed from the ceria film. Reexposure of the ceria film to O2 above 600 K repopulated this desorption feature, showing that adsorption−desorption cycles did not alter the morphology of the film. By adding Rh to the ceria film and then measuring the reaction of adsorbed CO on the Rh, it was demonstrated that this low-temperature feature is responsible for the oxidation of CO during TPD. The implications of these results for understanding the oxygen storage function of ceria-supported catalysts are discussed.
The adsorption and reaction properties of Rh particles supported on a CeO2(100) surface and on polycrystalline CeO2 films annealed at low (LT, 970 K) and high (HT, 1720 K) temperatures were studied using temperature-programmed desorption (TPD) and steady-state, CO oxidation kinetics. SEM and XRD of the polycrystalline samples showed significant growth of the ceria crystallite size and change in the surface morphology upon high-temperature annealing. In TPD, a substantial fraction of the CO adsorbed on Rh supported on the LT polycrystalline film was found to desorb as CO2, with the oxygen coming from the CeO2 support. In contrast, oxidation of CO to CO2 was not observed in significant amounts during TPD from Rh on the CeO2(100) sample or the HT film. In agreement with the adsorption measurements, steady-state CO oxidation exhibited a second, ceria-mediated process for Rh on the LT film which was not observed on the HT film. These results indicate that the structure of ceria plays an important role in the reactivity of Rh/ceria catalysts.
For Pt.I see abstr. A12758 of 1972. The diffuse scattering of long wavelength (7-8AA) neutrons by fluorite solid solutions of YF3 in CaF2 in the composition range Ca(Y)F2.1 to Ca(Y)F2.32 has been studied. The local distribution of defects is revealed by the variation of scattering with sin theta / lambda . Earlier inferences (part I) from the Bragg neutron diffraction measurements on the clustering of the two types of fluoride interstitial in this system are supported by the present results.
Some materials with the fluorite structures show a pronounced specific heat anomaly well below their melting temperature. This anomaly is a consequence of lattice disorder and is associated with the onset of fast-ion conduction. This paper presents the results of a series of experiments in which the coherent diffuse quasielastic neutron scattering from single crystals of three such fluorite compounds PbF2, SrCl2 and CaF2, was investigated. The diffuse scattering intensity, and its energy width, increases with temperature into the fast-ion phase, and when integrated over energy transfer the intensity exhibits a characteristic variation with scattering vector, falling on an anisotropic shell in reciprocal space and peaking in certain directions. The diffuse intensity indicates that dynamic correlations exist between the defective anions in the fast-ion-phase. A model of short-lived clusters comprising anion Frenkel interstitials, anion vacancies and relaxed anions has been developed which satisfactorily accounts for the distribution of intensity.
Bragg neutron diffraction studies have been carried out on fluorite solid solutions of YF3 in CaF2. The samples investigated were a single crystal containing 6 mol% YF3 (studied at room temperature (RT)) and polycrystalline samples of compositions (Ca/Y)F2.10 (500 degrees C), (Ca/YF)F2.15 (RT and 900 degrees C), (Ca/Y)F2.25 (RT) and (Ca/Y)F2.32 (RT). The results in all cases indicate two types of interstitial site located along the (110) and (111) directions from the cubic, 1/2, 1/2, 1/2 positions (F, and F" respectively). Vacancies in the normal fluorine sites (Fn) are also detected. Variations in the relative numbers of F', F" and Fn atoms with excess fluoride content are observed. The clustering of defects has been considered and possible models which would account for the observed occupation numbers have been proposed. A revised neutron scattering length for calcium of 0.477+or-0.004*10-12 cm is reported.
Pulsed neutron diffraction measurements on nano-scale powder of ceria, CeO2, uncovered new structural features which appear to be intimately related to the function of this material as an oxygen storage medium in automotive three-way catalytic converters. The results of the pair-distribution function (PDF) analysis and the Rietveld refinement of the neutron diffraction data indicate the presence of interstitial oxygen defects in the octahedral sites of the fluorite structure. The defects were found to disappear following high-temperature treatment. It is suggested that these weakly bound interstitial oxygen defects provide oxygen mobility that facilitates the oxygen storage capacity of ceria in the catalytic converters.
The redox behavior and the structural properties of Ce0.6Zr0.4O2 mixed oxides of high surface area and dense ceramic-type materials are compared. It is shown that due to the insertion of ZrO2 into the CeO2 lattice the oxygen sublattice becomes significantly distorted: Zr–O coordination of the type 5+2 and 4+2 is found in the former and latter samples, respectively. The nature of these distortions is correlated with the origin of the sample and it appears also to be related to the particle size of the Ce0.6Zr0.4O2 mixed oxide. Noticeably, the reduction behavior of these samples as investigated by the temperature-programmed reduction method appears correlated with their structural properties.
Steady-state, CO-oxidation kinetics at 515 K have been measured on model, Pd catalysts, prepared by vapor deposition of Pd onto either zirconia, praseodymia, ceria, or a ceria-zirconia mixed oxide. A second rate process (RE2), associated with both the metal and the oxide support and observed previously on ceria-supported catalysts in excess CO [7], was also found for Pd supported on ceria-zirconia, but neither zirconia nor praseodymia had any effect on CO oxidation under the conditions of our study. For ceria and ceria-zirconia, deactivation, through the loss of RE2, caused by high-temperature calcination, was examined, with the Pd added after calcination so that the metal particle size was not a factor in deactivation. For ceria, there was a strong dependence on calcination temperature, with almost complete loss of RE2 above 1170 K. XRD showed that the loss was accompanied by a large increase in the crystallite size. Results for ceria-zirconia showed that the loss in this case was more gradual, with CO oxidation activity due to RE2 maintained to much higher calcination temperatures. Taking the importance of RE2 as a measure of the ability of the catalyst to use oxygen from the oxide, the implications of these results with respect to oxide structure and the effect of aging on oxygen-storage properties of reducible oxides are discussed.