Ceria-based Mixed Oxides Research Articles

Facile fabrication of novel ceria-based nanocomposite (CYO-CSO) via co-precipitation: Electrochemical, photocatalytic and antibacterial performances

Ceria-based mixed metal oxide (rare-earth-based) nanocomposites have become well-known and play an important part in contemporary nanotechnology research due to their superior photocatalytic, biological, and electrochemical performance. A Ce0.8Y0.2 O2-δ–Ce0.8Sm0.2O2-δ (CYO-CSO) nanocomposite was fabricated utilizing a chemical precipitation technique for a variety of applications. For structural study, Fourier transfer infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD), and thermographic analysis (TGA) were used, as well as Scanning Electronic microscopy (SEM) and Energy dispersive X-ray analysis (EDX) for morphological and elemental analysis. There was no secondary phase seen during the creation of fluorite cubic crystal arrangement. XRD and FTIR were used to reveal the metal-oxygen connection. UV–Vis spectroscopy was used to calculate the optical bandgap of the produced nanocomposite. Electrochemical characteristics were discovered when conducting electrochemical impedance studies (EIS) at a frequency range of 40 Hz to 6 kHz and a voltage of 1.3 V. Methylene blue (MB) was utilized to estimate photocatalytic performance. The photocatalytic performance of CYO-CSO nanocomposite for MB dye was shown to be superior (93%). The antibacterial characteristic features of the synthesized CYO-CSO nanocomposite against foodborne pathogens are satisfactory.

Low temperature toluene and phenol abatement as tar model molecules over Ni-based catalysts: Influence of the support and the synthesis method

Ni addition over two ceria-based mixed oxides (Ce0.63Zr0.33Sm0.04O2− δ, and Ce2Y2O7) has been carried out by sol-gel (Ni-CZS and Ni-CY) and wet impregnation (Ni/CZS and Ni/CY) methods. The four catalysts were characterized by BET, XRD, TGA, XPS, H2-TPR and H2-chemisorption. Partial insertion of Ni2+ ions into both mixed oxide’s structure was found in the corresponding sol-gel catalysts. Their catalytic activity was evaluated in tar reforming, with toluene and phenol as model molecules, under isothermal condition at 400 and 550 °C for 6 h, using a typical ex-biomass syngas composition in order to mimic the real conditions of a gasification process. Among the four materials, Ni/CY was the best catalyst in terms of average tar conversion, whereas the best catalytic stability was obtained for Ni-CZS, owing to inserted Ni2+ species. This study demonstrates that Ni/CY can be a promising system for tar abatement at relatively low temperature through catalytic hot gas cleaning of a raw syngas generated by biomass gasification.

Hydrothermal Synthesis of ZnO–doped Ceria Nanorods: Effect of ZnO Content on the Redox Properties and the CO Oxidation Performance

The rational design of highly efficient, noble metal-free metal oxides is one of the main research priorities in the area of catalysis. To this end, the fine tuning of ceria-based mixed oxides by means of aliovalent metal doping has currently received particular attention due to the peculiar metal-ceria synergistic interactions. Herein, we report on the synthesis, characterization and catalytic evaluation of ZnO–doped ceria nanorods (NR). In particular, a series of bare CeO2 and ZnO oxides along with CeO2/ZnO mixed oxides of different Zn/Ce atomic ratios (0.2, 0.4, 0.6) were prepared by the hydrothermal method. All prepared samples were characterized by X-ray diffraction (XRD), N2 physisorption, temperature-programmed reduction (TPR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and transmission electron microscopy (TEM). The CO oxidation reaction was employed as a probe reaction to gain insight into structure-property relationships. The results clearly showed the superiority of mixed oxides as compared to bare ones, which could be ascribed to a synergistic ZnO–CeO2 interaction towards an improved reducibility and oxygen mobility. A close correlation between the catalytic activity and oxygen storage capacity (OSC) was disclosed. Comparison with relevant literature studies verifies the role of OSC as a key activity descriptor for reactions following a redox-type mechanism.

Effect of the Preparation Method on the Physicochemical Properties and the CO Oxidation Performance of Nanostructured CeO2/TiO2 Oxides

Ceria-based mixed oxides have been widely studied in catalysis due to their unique surface and redox properties, with implications in numerous energy- and environmental-related applications. In this regard, the rational design of ceria-based composites by means of advanced synthetic routes has gained particular attention. In the present work, ceria–titania composites were synthesized by four different methods (precipitation, hydrothermal in one and two steps, Stöber) and their effect on the physicochemical characteristics and the CO oxidation performance was investigated. A thorough characterization study, including N2 adsorption-desorption, X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM) and H2 temperature-programmed reduction (H2-TPR) was performed. Ceria–titania samples prepared by the Stöber method, exhibited the optimum CO oxidation performance, followed by samples prepared by the hydrothermal method in one step, whereas the precipitation method led to almost inactive oxides. CeO2/TiO2 samples synthesized by the Stöber method display a rod-like morphology of ceria nanoparticles with a uniform distribution of TiO2, leading to enhanced reducibility and oxygen storage capacity (OSC). A linear relationship was disclosed among the catalytic performance of the samples prepared by different methods and the abundance of reducible oxygen species.

H2 production by thermochemical water splitting with reticulated porous structures of ceria-based mixed oxide materials

In this work, we present for the first time the preparation and evaluation of Ceria-based mixed oxides reticulated porous ceramic (RPC) structures for H2 production by thermochemical water splitting. After appropriate screening of the powder materials, ceria-based materials modified with Co, Mn and Zr were discarded due to their low cyclability and/or hydrogen productivity, derived from segregation of active phases or sintering during the thermal reduction and reoxidation. Sponge replica method has been optimized to allow obtaining a Ce0.9Fe0.1Oy RPC sponge structure with an outstanding hydrogen production of 15 STPcm3/gmaterial·cycle at a maximum temperature of 1300 °C. This better performance, comparing to the powder, can be attributed to the open macroporosity of the reticulated porous structure which enhances both heat and mass transfer. The H2 production is maintained along several consecutive cycles without loss of activity, reinforcing the favorable prospects for large-scale hydrogen production.

In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O2-Based OSC Materials of Identical Composition

In situ Raman spectroscopy was used at temperatures in the 50–480 °C range under oxidizing (20% O2/He) and reducing (5% H2/He) flowing gas atmospheres to compare the spectra obtained for a series of industrial rare earth doped CexZr1−xO2−δ oxygen storage capacity (OSC) mixed metal oxide materials of identical at % composition, which were prepared by the same chemical synthesis route, in which one synthesis parameter of the aqueous chemistry was slightly varied. The Raman fingerprint of the anionic sublattice is very sensitive to O atom relocations within the bulk of the material matrix and to the pertinent defect topology in each case. A protocol of sequential Raman measurements and analysis was proposed to discern subtle differences between the oxygen vacancy and defect topologies of the examined materials. It can be concluded that for two materials under comparison for their structures, identical Raman spectra are obtained only if the procedures followed for their preparation are identical; a slight variation of one single parameter (e.g., in the aqueous chemistry stage) results in discernible differences in the Raman spectra. The proposed procedure can serve as a tool for proving or disproving infringement of IPR (Intellectual Property Rights) protected preparation methods of ceria-based mixed metal oxide materials.

A combined experimental and computational study of water-gas shift reaction over rod-shaped Ce0.75M0.25O2 (M = Ti, Zr, and Mn) supported Cu catalysts

Water-gas shift (WGS) reaction over a series of ceria-based mixed oxides supported Cu catalysts was investigated using a combined experimental and theoretical method. The mixed rod-shaped Ce0.75M0.25O2 (M = Ti4+, Zr4+, Mn4+) solid solutions, which majorly expose the (110) and (100) facets, are synthesized by hydrothermal method and used to prepare supported Cu catalysts. It is found that the Cu/Ce0.75Ti0.25O2 (Cu-CT) exhibits the highest CO conversion in the temperature range of 150–250 °C among all supported Cu catalysts. This is mainly attributed to (i) good dispersion of Cu; (ii) largest amount of moderate copper oxide; and (iii) strongest Cu-support interaction of Cu-CT. Compared to other mixed metals, periodic density functional theory calculations performed in this work further suggest that the introduction of Ti into CeO2 not only promotes oxygen vacancy formation and CO adsorption, but also facilitates the carboxyl (COOH) formation at the interface of the Cu cluster and the support, which leads to the enhanced catalytic activity of the Cu-CT toward WGS reaction.

Characterization of Ceria-Based Nano-Oxide Catalysts by Raman Spectroscopy

Ceria-based materials have drawn intense research focus due to variety of catalytic and energy related applications. Creation of oxygen vacancy defects imparts oxygen storage and release property (OSC) in the ceria, which facilitates the redox catalytic activity. Introduction of aliovalent dopant ions into the ceria nanocrystals enhances the OSC by generation of extrinsic defects. The current study describes synthesis and characterization of rare earth doped ceria-based mixed oxides. Characterization of the samples was carried out using XRD, BET surface area measurement, TEM, HRTEM, etc. The bulk defect features of the samples were studied employing visible Raman spectroscopy. F2g peak of the Raman spectra evidenced red shift and peak broadening, which could be attributed to change in lattice parameter, oxygen vacancy defects, and smaller crystallite size of the doped nanocrystals. Additional peak (D1) appeared due to the creation of oxygen vacancy defects. The ratio of intensity of D1 peak to F2g peak gave the defect concentration of the doped samples. O 2p and Ce 4f direct band gap energies of the samples were also evaluated. Decrease of band gap energy of the doped samples provided evidence of defect concentration enhancement. A correlation was found among the defect concentration and OSC of the prepared materials. Finally, CO oxidation reaction was performed with the doped materials and the activity was found to be in accordance with the enhancement of defect concentration and OSC.

Catalytic oxidation of soot on mesoporous ceria-based mixed oxides with cetyltrimethyl ammonium bromide (CTAB)-assisted synthesis

Mesoporous ceria and transition metal-doped ceria (M0.1Ce0.9O2 (M=Mn, Fe, Co, Cu)) catalysts were synthesized via CTAB-assisted method. The physicochemical properties of the prepared catalysts were characterized by XRD, DLS analysis, SEM, BET, Raman, H2-TPR and in situ DRIFT techniques. The catalytic activity tests for soot oxidation were performed under tight contact of soot/catalyst mixtures in the presence of O2 and NO+O2, respectively. The obtained results show that mesoporous ceria-based solid solutions can be formed with large surface areas and small crystallite size. Transition metals doping enhances the oxygen vacancies and improves redox properties of the solids, resulting in the increased NO oxidation capacity and NOx adsorption capacity. The soot oxidation activity in the presence of O2 is enhanced by doping transition metal, which may be related to their high surface area, increased oxygen vacancies and improved redox properties. The soot combustion is accelerated by the NO2-assisted mechanism under NO+O2 atmosphere, facilitating an intimate contact between the soot and the catalyst.

Aliovalent Metal Ion Doped Ceria Catalysts for CO and Soot Oxidation

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. Keywords: Ceria, solid solutions, CO oxidation, soot oxidation, redox property, H2-TPR.

Strain Field in Ultrasmall Gold Nanoparticles Supported on Cerium-Based Mixed Oxides. Key Influence of the Support Redox State.

Using a method that combines experimental and simulated Aberration-Corrected High Resolution Electron Microscopy images with digital image processing and structure modeling, strain distribution maps within gold nanoparticles relevant to real powder type catalysts, i.e., smaller than 3 nm, and supported on a ceria-based mixed oxide have been determined. The influence of the reduction state of the support and particle size has been examined. In this respect, it has been proven that reduction even at low temperatures induces a much larger compressive strain on the first {111} planes at the interface. This increase in compression fully explains, in accordance with previous DFT calculations, the loss of CO adsorption capacity of the interface area previously reported for Au supported on ceria-based oxides.

Catalytic dissolution of ceria–lanthanide mixed oxides provides environmentally friendly partitioning of lanthanides and platinum

The reductive dissolution of cerium–lanthanide mixed oxides was investigated under mild conditions in weakly acid media. The dissolution is catalyzed by platinum nanoparticles (Pt NPs) deposited onto oxide surfaces. The deposition of Pt NPs was performed by sonochemical reduction of Pt(IV). The dissolution efficiency was studied as a function of different parameters such as stirring, sonication at 20kHz, dissolution media and oxide composition. The quantitative dissolution of these oxides can be performed in 0.25M HNO3–1M HCOOH–0.2M [N2H5][NO3] and 0.125M H2SO4–1M HCOOH under stirring at 40°C in the presence of argon, or in 0.125M H2SO4 under sonication in the presence of Ar–CO gas mixture at 40°C. In nitric acid solutions mechanical stirring is more efficient than sonication due to the secondary denitration triggered by ultrasound. The incorporation of trivalent (Gd, Nd) or tetravalent (Tb) lanthanides into the ceria matrix resulted in faster dissolution. After dissolution, lanthanides and platinum can be separated by simple filtration. The catalytic dissolution of ceria-based mixed oxides offers a simple way of lanthanide and noble metal partitioning which could appear of interest for catalyst recycling.

Opportunities for ceria-based mixed oxides versus commercial platinum-based catalysts in the soot combustion reaction. Mechanistic implications

The aim of this paper is to study the activities of ceria–zirconia and copper/ceria–zirconia catalysts, comparing with a commercial platinum/alumina catalyst, for soot combustion reaction under different gas atmospheres and loose contact mode (simulating diesel exhaust conditions), in order to analyse the kinetics and to deduce mechanistic implications.Activity tests were performed under isothermal and TPR conditions. The NO oxidation to NO2 was studied as well. It was checked that mass transfer limitations were not influencing the rate measurements. Global activation energies for the catalysed and non-catalysed soot combustion were calculated and properly discussed.The results reveal that ceria-based catalysts greatly enhance their activities under NOx/O2 between 425°C and 450°C, due to the “active oxygen”-assisted soot combustion. Remarkably, copper/ceria–zirconia shows a slightly higher soot combustion rate than the Pt-based catalyst (under NOx/O2, at 450°C).

Surface/structure functionalization of copper-based catalysts by metal-support and/or metal–metal interactions

Cu-based catalysts have recently attracted great attention both in catalysis and electro-catalysis fields due to their excellent catalytic performance and low cost. Given that their performance is determined, to a great extent, by Cu sites local environment, considerable efforts have been devoted on the strategic modifications of the electronic and structural properties of Cu sites. In this regard, the feasibility of tuning the local structure of Cu entities by means of metal-support or metal–metal interactions is investigated. More specifically, the physicochemical properties of Cu entities are modified by employing: (i) different oxides (CeO2, La2O3, Sm2O3), or (ii) ceria-based mixed oxides (Ce1−xSmxOδ) as supporting carriers, and (iii) a second metal (Cobalt) adjacent to Cu (bimetallic Cu–Co/CeO2). A characterization study, involving BET, XRD, TPR, and XPS, reveal that significant modifications on structural, redox and electronic properties of Cu sites can be induced by adopting either different oxide carriers or bimetallic complexes. Fundamental insights into the tuning of Cu local environment by metal-support or metal–metal interactions are provided, paving the way for real-life industrial applications.

Pt/CexPr1−xO2 (x = 1 or 0.9) NOx storage–reduction (NSR) catalysts

Model Pt/Ce0.9Pr0.1O2 and Pt/CeO2 NOx storage–reduction catalysts were prepared via nitrate calcination, co-precipitation and carbon-templating routes. Raman spectroscopic data obtained on the catalysts indicated that the introduction of praseodymium into the ceria lattice increased the concentration of defect sites (vacancies), arising from the higher reducibility of the Pr4+ cation compared to Ce4+. For the Pr-promoted samples, H2-TPR profiles contained high temperature bulk reduction peaks which were less pronounced compared with their ceria analogs, indicating that the presence of praseodymium enhances oxygen mobility due to the creation of lattice defects. Under lean-rich cycling conditions, the cycle-averaged NOx conversion of the Pt/Ce0.9Pr0.1O2 samples was in each case substantially higher than that of the Pt/CeO2 analog, amounting to a difference of 10–15% in the absolute NOx conversion in some cases. According to DRIFTS data, a double role can be assigned to Pr doping; on the one hand, Pr accelerates the oxidation of adsorbed NOx species during the lean periods. On the other hand, Pr doping destabilizes the adsorbed NOx species during the rich periods, and the kinetics of nitrate decomposition are faster on Pt/Ce0.9Pr0.1O2, leading to improved catalyst regeneration. These results suggest that ceria-based mixed oxides incorporating Pr are promising materials for NOx storage–reduction catalysts intended for low temperature operation.

Reactivity of Doped Ceria-Based Mixed Oxides for Solar Thermochemical Hydrogen Generation via Two-Step Water-Splitting Cycles

Ceria-type materials were investigated as reactive chemical intermediates, in view of solar thermochemical hydrogen production via two-step water-splitting. Ceria/zirconia mixed oxides and ceria doped with yttrium, lanthanum, praseodymium, or gadolinium were studied using a thermobalance to evaluate their thermal reduction capacity in inert atmosphere and their subsequent reactivity with water steam to generate hydrogen. Ceria/zirconia materials present the highest reduction yields with a noticeable linear increase as a function of the zirconium content (in the range 0–54% Zr), while the gravimetric amount of O2 released during reduction tends to level off for Zr atomic contents above 25%. Temperature-programmed reduction experiments demonstrate that the zirconium insertion favors the bulk reduction. The addition of different dopants (among Y, La, Pr, and Gd) did not affect the global materials reducibility, although it should favor the material thermal stability during repeated cycles. The marked effect ...

Soot combustion performance and H2-TPR study on ceria-based mixed oxides

Ceria (CeO 2 ) catalysts doped with different metal ions were prepared by sol-gel or impregnation methods, and their catalytic activity for soot combustion was evaluated by thermogravimetry. Temperature-programmed reduction was used to investigate the influence of their redox properties on soot combustion performance. The results indicate that the oxidation temperature of soot can be significantly reduced using catalysts containing transition metals, which provide increased surface and lattice oxygen compared with undoped CeO 2 . The ability of each catalyst to deliver active oxygen at low temperature (200–400 °C) plays an important role in determining catalytic performance. However, incorporation of structure promoter, alkali metals, and alkaline earth metals increase the delivery of active oxygen only at moderate temperature (above 400 °C), which accelerate the combustion rate of soot. 采用溶胶-凝胶法或浸渍法制备了不同金属离子掺杂的铈基复合氧化物催化剂, 并采用热重法考察其催化碳烟燃烧的活性, 借助H 2 -TPR (程序升温还原)手段探讨了催化剂氧化还原性对碳烟燃烧性能的影响. 结果表明, 过渡金属的掺杂促使催化剂在低温下提供更多的表面氧和晶格氧, 显著降低了碳烟的氧化温度, 催化剂于200~400 °C释放的活性氧数量对于碳烟燃烧性能提高至关重要; 而结构性助剂金属、碱金属或碱土金属的掺入可提高中温活性氧数量, 虽然对碳烟起燃温度无明显改善, 但加快了碳烟的燃烧速率. The influence of doping ceria with different kinds of metal ions on its redox properties for soot combustion was investigated. The oxidation temperature of soot could be significantly reduced using ceria doped with transition metals because more active oxygen was generated at low temperature. In contrast, doping ceria with alkali metal and alkaline earth metals increased the combustion rate of soot.

High thermal stability of ceria-based mixed oxide catalysts supported on ZrO2 for toluene combustion

Cu–Mn–Ce mixed oxide catalysts (CMCs) supported on ZrO2 were prepared by an impregnation method. Their catalytic activity was evaluated by a model reaction, that of toluene combustion. 5% CMC/ZrO2 catalysts show a very high activity and thermal stability, and 90% of toluene conversion could be obtained at temperatures below 260 °C, even with the catalyst being annealed at 900 °C. A new phase of Zr0.88Ce0.12O2 in the interface was formed by the interaction between ZrO2 and CMC on the catalyst with low loading. Zr0.88Ce0.12O2 serves as the actual carrier of the active phase, and can significantly improve the thermal stability of the catalyst.

On the interaction of Mg with the (111) and (110) surfaces of ceria

The catalytic activity of cerium dioxide can be modified by deposition of alkaline earth oxide layers or nanoparticles or by substitutional doping of metal cations at the Ce site in ceria. In order to understand the effect of Mg oxide deposition and doping, a combination of experiment and first principles simulations is a powerful tool. In this paper, we examine the interaction of Mg with the ceria (111) surface using both angle resolved X-ray (ARXPS) and resonant (RPES) photoelectron spectroscopy measurements and density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). With DFT + U, we also examine the interaction of Mg with the ceria (110) surface. The experiments show that upon deposition of Mg, Ce ions are reduced to Ce(3+), while Mg is oxidised. When Mg is incorporated into ceria, no reduced Ce(3+) ions are found and oxygen vacancies are present. The DFT + U simulations show that each Mg that is introduced leads to formation of two reduced Ce(3+) ions. When Mg is incorporated at a Ce site in the (111) surface, one oxygen vacancy is formed for each Mg to compensate the different valencies, so that all Ce ions are oxidised. The behaviour of Mg upon interaction with the (110) surface is the same as with the (111) surface. The combined results provide a basis for deeper insights into the catalytic behaviour of ceria-based mixed oxide catalysts.

Textural, structural, and morphological characterizations and catalytic activity of nanosized CeO 2–MO x (M = Mg 2+, Al 3+, Si 4+) mixed oxides for CO oxidation

The present work focuses on the combination of ceria with another oxide of different ionic valences from period 3 (Mg 2+, Al 3+, and Si 4+) using coprecipitation method, followed by calcination at 450 and 750 °C, respectively. The textural, structural, morphological and redox properties of nanosized ceria–magnesia, ceria–alumina and ceria–silica mixed oxides have been investigated by means of N 2 physisorption, XRD, Raman, HRTEM, DRS, FT-IR, and H 2-TPR technologies. XRD results of these mixed oxides reveal that only nanocrystalline ceria ( ca. 3–6 nm for the 450 °C calcined samples) could be observed. The grain size of ceria increases with the increasing calcination temperature from 450 to 750 °C due to sintering effect. The highest specific surface area is obtained at CeO 2–Al 2O 3 mixed oxides when calcination temperature reaches 750 °C. Raman spectra display the cubic fluorite structure of ceria and the existence of oxygen vacancies, and displacement of oxygen ions from their normal lattice positions in the ceria-based mixed oxides. DRS measurements confirm that the smaller the grain size of the ceria, the higher indirect band gap energy. H 2-TPR results suggest that the reductions of surface and bulk oxygen of ceria were predominant at low and high calcination temperature, respectively. Finally, CO oxidation were performed over these ceria-based mixed oxides, and the combination of CeO 2–Al 2O 3 exhibited highest activity irrespective of calcination temperature, which may due to excellent textural/structural properties, good homogeneity, and redox abilities.