Resolving a structural issue in cerium-nickel-based oxide: a single compound or a two-phase system?

CeNiO3 has been reported in the literature in the last few years as a novel LnNiO3 compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol-gel route. A direct comparison between the equimolar physical mixture (n(CeO2) : n(NiO) = 1 : 1) and compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) detector, and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO2 + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce2Ni2O7) while the other two solutions suggest that RB1 is a two-phase system of either CeO2 + NiO or Ce1-xNixO2 and NiO.

Different morphologies on Cu-Ce/TiO2 catalysts for the selective catalytic reduction of NOx with NH3 and DRIFTS study on sol-gel nanoparticles

The copper-cerium binary oxide catalysts supported by titanium dioxide with nanosphere core-shell structures, nanotube (TNT) core-shell structures, impregnation (imp) nanoparticles and sol-gel nanoparticles were prepared for NH3-SCR of NOx under medium-low temperature conditions. The effect of different morphologies on the Cu-Ce/TiO2 catalysts was comprehensively studied through physicochemical characterization. The results showed that the sol-gel nanoparticles exhibited 100% NOx reduction efficiency in the temperature range of 180-400 °C. Compared with the other catalysts, the sol-gel nanoparticle catalyst had the highest dispersion and lowest crystallinity, indicating that morphology played an important role in the NH3-SCR of the catalyst. The in situ DRIFTS study on the sol-gel nanoparticle catalyst shows that cerium could promote Cu2+ to produce abundant Lewis acid sites, which would significantly increase the adsorption reaction of ammonia on the catalyst surface, thereby promoting the occurrence of the Eley-Rideal (E-R) mechanism. With the Ce-Ti interaction on the atomic scale, the Ce-O-Ti structure enhanced the redox properties at a medium temperature. In addition, cerium oxide enhances the strong interaction between the catalyst matrix and CuO particles. Therefore, the reducibility of the CuO species was enhanced.

Environmentally Benign pSOFC for Emissions-Free Energy: Assessment of Nickel Network Resistance in Anodic Ni/BCY15 Nanocatalyst

Yttrium-doped barium cerate (BCY15) was used as ceramic matrix to obtain Ni/BCY15 anode cermet for application in proton-conducting solid oxide fuel cells (pSOFC). Ni/BCY15 cermets were prepared in two different types of medium, namely deionized water (W) and anhydrous ethylene glycol (EG) using wet chemical synthesis by hydrazine. An in-depth analysis of anodic nickel catalyst was made aiming to elucidate the effect of anode tablets' preparation by high temperature treatment on the resistance of metallic Ni in Ni/BCY15-W and Ni/BCY15-EG anode catalysts. On purpose reoxidation upon high-temperature treatment (1100 °C for 1 h) in air ambience was accomplished. Detailed characterization of reoxidized Ni/BCY15-W-1100 and Ni/BCY15-EG-1100 anode catalysts by means of surface and bulk analysis was performed. XPS, HRTEM, TPR, and impedance spectroscopy measurements experimentally confirmed the presence of residual metallic Ni in the anode catalyst prepared in ethylene glycol medium. These findings were evidence of strong metal Ni network resistance to oxidation in anodic Ni/BCY15-EG. Enhanced resistance of the metal Ni phase contributed to a new microstructure of the Ni/BCY15-EG-1100 anode cermet getting more stable to changes that cause degradation during operation.

Synergistic impacts of sonolysis aided photocatalytic degradation of water pollutant over perovskite-type CeNiO3 nanospheres

The current study reports on the preparation of perovskite-type CeNiO3 nanostructures as a sonophotocatalyst via a facile hydrothermal approach followed by annealing at 800 °C.

Co3O4/CeO2 multi-shelled nanospheres derived from self-templated synthesis for efficient catalytic CO oxidation

Co3O4/CeO2 multi-shelled nanospheres with various Ce/Co ratios for low temperature CO oxidation were fabricated by using a bimetallic coordination polymer as a self-sacrificial template. The obtained Co0.9Ce0.1O2-δ catalyst exhibited superior CO oxidation performance, which reached 100% conversion at 80 °C. After five-cycle tests, the conversion could also be maintained at approximately 100% with almost no activity loss. Furthermore, the effects of the CeO2 component on catalytic performance have been investigated by various advanced physicochemical characterization methods. This indicated that the addition of cerium can significantly increase the surface area, so as to enrich the oxygen vacancies and provide a higher degree of accessible active sites. This work proposed an alternative approach to construct multi-shelled nanosphere composites.

How the Morphology of NiOx-Decorated CeO2 Nanostructures Affects Catalytic Properties in CO2 Methanation

Effects of morphology and exposed crystal planes of NiO_x-decorated CeO₂ (NiCeO₂) nanostructured catalysts on activity during CO₂ methanation were examined, using nanorod (NR), nanocube (NC), and nanooctahedron (NO) structures. The NiCeO₂ nanorods (NiCeO₂-NR) showed superior activity to NiCeO₂-NC and NiCeO₂-NO along with excellent selectivity for CH₄. This material also demonstrated exceptional durability, with no significant loss of catalytic activity or structural change after use. Comprehensive physicochemical characterization as well as density functional theory calculations determined that the high performance of the NiCeO₂-NR was closely related to the large quantity of surface oxygen vacancies and the high degree of reversibility associated with the Ce⁴⁺ ↔ Ce³⁺ redox cycle of the support. These effects originate from the enhanced reactivity of oxygen atoms on the (110) surfaces of the oxide compared with the (100) and (111) surfaces. This information is expected to assist in the rational design of practical catalysts for the activation of CO₂ molecules and other important transformations.

Characterization and catalytic activity of soft-templated NiO-CeO2 mixed oxides for CO and CO2 co-methanation

Nanosized NiO, CeO2 and NiO-CeO2 mixed oxides with different Ni/Ce molar ratios were prepared by the soft template method. All the samples were characterized by different techniques as to their chemical composition, structure, morphology and texture. On the catalysts submitted to the same reduction pretreatment adopted for the activity tests the surface basic properties and specific metal surface area were also determined. NiO and CeO2 nanocrystals of about 4 nm in size were obtained, regardless of the Ni/Ce molar ratio. The Raman and X-ray photoelectron spectroscopy results proved the formation of defective sites at the NiO-CeO2 interface, where Ni species are in strong interaction with the support. The microcalorimetric and Fourier transform infrared analyses of the reduced samples highlighted that, unlike metallic nickel, CeO2 is able to effectively adsorb CO2, forming carbonates and hydrogen carbonates. After reduction in H2 at 400 °C for 1 h, the catalytic performance was studied in the CO and CO2 co-methanation reaction. Catalytic tests were performed at atmospheric pressure and 300 °C, using CO/CO2/H2 molar compositions of 1/1/7 or 1/1/5, and space velocities equal to 72000 or 450000 cm3·h−1·gcat−1. Whereas CO was almost completely hydrogenated in any investigated experimental conditions, CO2 conversion was strongly affected by both the CO/CO2/H2 ratio and the space velocity. The faster and definitely preferred CO hydrogenation was explained in the light of the different mechanisms of CO and CO2 methanation. On a selected sample, the influence of the reaction temperature and of a higher number of space velocity values, as well as the stability, were also studied. Provided that the Ni content is optimized, the NiCe system investigated was very promising, being highly active for the COx co-methanation reaction in a wide range of operating conditions and stable (up to 50 h) also when submitted to thermal stress.

Structural, Textural, and Catalytic Properties of Ni-CexZr1−xO2 Catalysts for Methane Dry Reforming Prepared by Continuous Synthesis in Supercritical Isopropanol

A series of 5%Ni-CexZr1−xO2 (x = 0.3, 0.5, 0.7) catalysts has been prepared via one-pot solvothermal continuous synthesis in supercritical isopropanol and incipient wetness impregnation of CexZr1−xO2 obtained by the same route. The textural, structural, red-ox, and catalytic properties in methane dry reforming (MDR) of Ni-modified Ce-Zr oxides synthesized by two routes have been compared. It was shown by XRD, TEM, and Raman spectroscopy that the method of Ni introduction does not affect the phase composition of the catalysts, but determines the dispersion of NiO. Despite a high dispersion of NiO and near-uniform distribution of Ni within Ce-Zr particles observed for the one-pot catalysts, they have shown a lower activity and stability in MDR as compared with impregnated ones. This is a result of a low Ni concentration in the surface layer due to segregation of Ce and decoration of nickel nanoparticles with support species.

Promising Utilization of CO2 for Syngas Production over Mg2+- and Ce2+-Promoted Ni/γ-Al2O3 Assisted by Nonthermal Plasma

Dry reforming of methane is conducted in a catalyst packed-bed dielectric barrier discharge (DBD) reactor aiming to improve the reaction efficiency. The MgO- and CeO₂-promoted Ni/γ-Al₂O₃ catalyst is tested to carry out the reaction. An interesting observation is that Ni/MgO_Al₂O₃ integration provides ∼35 and 13% conversion of CH₄ and CO₂, respectively. The highest syngas ratio of 0.94 is obtained with Ni/MgO_Al₂O₃, whereas the ratio is only 0.57 with Ni/CeO₂_Al₂O₃ and 0.64 with bare DBD. In addition, Ni/CeO₂_Al₂O₃ offers the highest selectivity (68%) of CO due to the oxygen buffer property of CeO₂. Finally, the optimal acid/base property is highly desirable for the dry reforming reaction.

Effects of Ni loading on the physicochemical properties of NiOx/CeO2 catalysts and catalytic activity for NO reduction by CO

The NO reduction by CO reaction was investigated using NiO_x/CeO₂ catalysts with different Ni loadings. Surface NiO_x controls the catalytic activity which was related to the molecular structure and reducibility of the catalysts.

0D/2D CeO2 quantum dot/NiO nanoplate supported an ultralow-content Pt catalyst for the efficient oxidation of formaldehyde at room temperature

The 0D/2D CeO₂ quantum dot/NiO nanoplate supporting ultralow content of the Pt catalyst shows enhanced catalytic decomposition of formaldehyde at room temperature.

Effect of Nickel Oxide Doping to Ceria-Supported Gold Catalyst for CO Oxidation and Water-Gas Shift Reactions

Ceria-supported gold catalyst has drew much research interest owing to its high reactivity on CO oxidation and water-gas shift (WGS) reactions. However, till now, there were relatively limited studies on the effect of secondary metal/metal oxide component into gold-ceria system to enhance its catalytic performance. In this work, we synthetized the ceria supported gold-nickel samples via a deposition-precipitation method with the base of NaHCO3 to adjust final pH value of 8~9. We found that the addition of nickel oxide drove off the gold species from the stock solution during synthesis, and thus resulted in a dramatical decrease on doped Au concentration. No crystallized phases of gold and nickel were observed on the surface of ceria nanorods in both X-ray diffraction (XRD) and transmission electron microscopy (TEM). The valence of nickel was maintained as Ni2+ for all the measured samples by X-ray photoelectron spectroscopy (XPS), while gold was oxidized with the increased nickel amount after analysis of X-ray absorption near edge spectroscopy (XANES). The corresponding catalytic tests showed that with the introduction of nickel oxide, the activity of gold-ceria catalyst was promoted for the WGS reaction, but inhibited for the CO oxidation reaction.

Catalytic Performance of Gold Supported on Mn, Fe and Ni Doped Ceria in the Preferential Oxidation of CO in H2-Rich Stream

Ceria supported metal catalysts often exhibit high activity in the preferential oxidation (PROX) of CO in H2-rich stream and doping the ceria support with other metals proves to be rather effective in further enhancing their catalytic performance. Therefore, in this work, a series of ceria materials doped with Mn, Fe and Ni (CeM, where M = Mn, Fe and Ni; M/Ce = 1/8) were synthesized by a modified hydrothermal method; with the doped ceria materials (CeM) as the support, various supported gold catalysts (Au/CeM) were prepared by the colloidal deposition method. The influence of metal dopant on the performance of these ceria materials supported with gold catalysts in CO PROX was then investigated in detail with the help of various characterization measures such as N2 sorption, XRD, TEM, Raman spectroscopy, H2-TPR, XPS and XAS. The results indicate that the incorporation of Mn, Fe and Ni metal ions into ceria can remarkably increase the amount of oxygen vacancies in the doped ceria support, which is beneficial for enhancing the reducibility of ceria, the metal-support interaction and the dispersion of gold species. Although the gold catalysts supported on various doped ceria are similar in the size and state of Au nanoparticles, the CO conversions for CO PROX over Au/CeMn, Au/CeFe and Au/CeNi catalysts are 65.6%, 93.0% and 48.2%, respectively, much higher than the value of 33.6% over the undoped Au/CeO2 catalyst at ambient temperature. For CO PROX over the Au/CeNi catalyst, the conversion of CO remains near 100% at 60–130 °C, with a PROX selectivity to CO2 of higher than 50%. The excellent performance of Au/CeNi catalyst can be ascribed to its large amount of oxygen vacancies and high reducibility on account of Ni incorporation. The insight shown in this work helps to clarify the doping effect of other metals on the physicochemical properties of ceria, which is then beneficial to building a structure-performance relation for ceria supported gold catalyst as well as developing a better catalyst for removing trace CO in the hydrogen stream and producing high purity hydrogen.

How Transparent Oxides Gain Some Color: Discovery of a CeNiO3 Reduced Bandgap Phase As an Absorber for Photovoltaics

In this work, we describe the formation of a reduced bandgap CeNiO₃ phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO₃ phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of Ce _xNi_{1- x}O _y is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce-Ni-O materials. The library displays a reduced bandgap throughout, calculated to be 1.48-1.77 eV, compared to the starting materials, CeO₂ and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO₃ phase is realized. The CeNiO₃ reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured by surface photovoltage spectroscopy, which shows that the source of the photovoltaic activity is the reduced bandgap CeNiO₃ phase, making it a viable material for solar energy.

The effect of CuO modification for a TiO2 nanotube confined CeO2 catalyst on the catalytic combustion of butane

A modified confined catalyst with CeO2 on the interior and CuO on the exterior surface of TiO2 nanotubes (Ce-in-TNT-Cu-out) was prepared and investigated for the combustion of butane catalytically. Compared with the Ce-in-TNT and TNT-Cu-out, the Ce-in-TNT-Cu-out presents a higher activity for butane oxidation, with a conversion of 10% at 200°C and a conversion of 90%) at 300°C. XPS analysis indicates that more Ce(IV) and Cu(I) components exist in the Ce-in-TNT-Cu-out catalyst. It is proposed that electron transfer ability between encapsulated CeO2 and loaded CuO is significantly enhanced by the confinement effect of the TiO2 nanotubes, facilitating the formation and migration of active oxygen species in the catalyst. This result shows that modulating the electronic property of the active component can further improve the catalytic combustion performance of the confined catalysts.

Synthesis and characterization of nickel-doped ceria nanoparticles with improved surface reducibility

Nickel-doped ceria nanoparticles (Ni_0.1Ce_0.9O_2−x NPs) were fabricated from Schiff-base complexes and characterized by various microscopic and spectroscopic methods. Clear evidence is provided for incorporation of nickel ions in the ceria lattice in the form of Ni³⁺ species which is considered as the hole trapped state of Ni²⁺. The Ni_0.1Ce_0.9O_2−x NPs exhibit enhanced reducibility in H₂ as compared to conventional ceria-supported Ni particles, while in O₂ the dopant nickel cations are oxidized at higher valence than the supported ones.

Nickel-doped ceria nanoparticles (Ni_0.1Ce_0.9O_2−x NPs) were fabricated from Schiff-base complexes and characterized by various microscopic and spectroscopic methods.

Self-assembly of near-unity helical Ce1−xMxO2 (x = 0.1, M = Ni and Bi) solid solutions with tunable optical activity

Ce_1−xM_xO₂ solid solutions show tunable optical activity throughout the whole UV–visible range ascribed to electronic exciton coupling between left-handed helical structures.

Low temperature CO oxidation catalysed by flower-like Ni–Co–O: how physicochemical properties influence catalytic performance

In this work, mesoporous Ni–Co composite oxides were synthesized by a facile liquid-precipitation method without the addition of surfactant, and their ability to catalyse a low temperature CO oxidation reaction was investigated. To explore the effect of the synergetic interaction between Ni and Co on the physicochemical properties and catalytic performance of these catalysts, the as-prepared samples were characterized using XRF, XRD, LRS, N₂-physisorption (BET), SEM, TEM, XPS, H₂-TPR, O₂-TPD and in situ DRIFTS characterization techniques. The results are as follows: (1) the doping of cobalt can reduces the size of NiO, thus massive amorphous NiO have formed and highly dispersed on the catalyst surface, resulting in the formation of abundant surface Ni²⁺ ions; (2) Ni²⁺ ions partially substitute Co³⁺ ions to form a Ni–Co spinel solid solution, generating an abundance of surface oxygen vacancies, which are vital for CO oxidation; (3) the Ni_0.8Co_0.2 catalyst exhibits the highest catalytic activity and a satisfactory stability for CO oxidation, whereas a larger cobalt content results in a decrease in activity, suggesting that the amorphous NiO phase is the dominant active phase instead of Co₃O₄ for CO oxidation; (4) the introduction of Co can alter the morphology of catalyst from plate-like to flower-like and then to dense granules. This morphological variation is related to the textural properties and catalytic performance of the catalysts. Lastly, a possible mechanism for CO oxidation reaction is tentatively proposed.

The flower-like catalyst possesses highly dispersed amorphous NiO and a high concentration of surface oxygen vacancies which are the central points for CO oxidation.

Methane dry reforming over Ni catalysts supported on Ce–Zr oxides prepared by a route involving supercritical fluids

Ce0.5Zr0.5O2mixed oxides were prepared in a flow reactor in supercritical isopropanol with acetylacetone as a complexing agent. Variation of the nature of the Zr salt and the temperature of synthesis affected the phase composition, morphology and specific surface area of oxides. X-ray diffraction and Raman spectroscopy studies revealed formation of metastable t” and t’ phases. Oxides are comprised of agglomerates with sizes depending on the synthesis parameters. Loading NiO decreases the specific surface area without affecting X-ray particle sizes of supports. Such sintering was the most pronounced for a support with the highest specific surface area, which resulted in the lowest surface content of Ni as estimated by X-ray photoelectron spectroscopy and in the formation of flattened NiO particles partially embedded into the support. The catalytic activity and stability of these samples in the dry reforming of methane were determined by the surface concentration of Ni and the morphology of its particle controlled by the metal-support interaction, which also depends on the type of catalyst pretreatment. Samples based on ceria-zirconia oxides prepared under these conditions provide a higher specific catalytic activity as compared with the traditional Pechini route, which makes them promising for the practical application.

Reaction Sensitivity of Ceria Morphology Effect on Ni/CeO2 Catalysis in Propane Oxidation Reactions

CeO₂ nanocubes (c-CeO₂), nanoparticles (p-CeO₂), and nanorods calcined at 500 °C (r-CeO₂-500) and 700 °C (r-CeO₂-700) were used as supports to synthesize a series of Ni/CeO₂ catalysts for the propane combustion and oxidative dehydrogenation of propane (ODHP) reactions. The Ni-CeO₂ interaction greatly promotes the reducibility of CeO₂, but CeO₂ morphology-dependent Ni-CeO₂ interaction was observed to form different speciation of Ni species (Ni²⁺ dissolved in CeO₂, highly dispersive NiO, NiO aggregate) and oxygen species (strongly activated oxygen species, medially activated oxygen species, weakly activated oxygen species) in various Ni/CeO₂ catalysts. Ni-CeO₂ interaction is stronger in Ni/c-CeO₂ catalysts than in other Ni/CeO₂ catalysts. Different morphology-dependences of Ni/CeO₂ catalysts in propane combustion and ODHP reactions were observed. The Ni/r-CeO₂-500 catalyst with the largest strongly activated oxygen species is most catalytic active in the propane combustion reaction while the Ni/c-CeO₂ catalyst with the largest amount of weakly activated oxygen species exhibits the best catalytic performance in the ODHP reaction. Thus, the CeO₂ morphology engineering strategy is effective in finely tuning the metal-CeO₂ interaction and the reactivity of oxygen species to meet the requirements of different types of catalytic oxidation reactions.

Baize-like CeO2 and NiO/CeO2 nanorod catalysts prepared by dealloying for CO oxidation

Baize-like monolithic CeO₂ and NiO/CeO₂ nanorod catalysts were prepared by combined dealloying and calcination and the catalytic activities were evaluated using CO catalytic oxidation. The CeO₂ catalysts were composed of nanorods and exhibited a three-dimensional supporting structure with pores. After introduction of NiO, dispersed NiO nanosheets and nanoparticles were supported on the surface of CeO₂ nanorods and they were not well-crystallined due to CeO₂ inhibiting the NiO crystallization. The Raman and x-ray photoelectron spectroscopy analyses revealed that the introduction of NiO species into CeO₂ generated more coordinate unsaturated Ni atoms, oxygen vacancies, defects and active sites for CO catalytic reactions. The reaction activation energy of NiO/CeO₂ nanorod catalyst prepared from the Al₈₃Ce₁₀Ni₇ precursor alloy was just 31.2 kJ mol^-1 and the CO conversion can reach up to 97% at 240 °C, which was superior to that of pure CeO₂ and nanoporous NiO. The enhanced catalytic activity of baize-like NiO/CeO₂ nanorods can be attributed to the strong synergistic effects between finely dispersed NiO species and surface oxygen vacancies in CeO₂ nanorods.

Design, controlled synthesis, and properties of 2D CeO2/NiO heterostructure assemblies

Synthetic protocols to generate well-integrated frameworks of known composition are important for the rational design of advanced materials.

Modified carbon felt made using CexA1−xO2 composites as a cathode in electro-Fenton system to degrade ciprofloxacin

Carbon felt (CF) was modified by Ce_xA_1−xO₂ (A = Zr, Cu and Ni) and the role of these Ce_xA_1−xO₂/CF (A = Zr, Cu and Ni) cathode materials in the oxidative degradation of antibiotic ciprofloxacin (CIP) was investigated in the electro-Fenton system.

Low-temperature CO oxidation over CeO2 and CeO2@Co3O4 core–shell microspheres

The excellent CO catalytic activity and stability of CeO₂@Co₃O₄ composite were ascribed to the synergistic interactions between Co₃O₄ and CeO₂.