Ceria Zirconia Solid Solutions Research Articles

SCR NOx mechanistic study with a mixture of hydrocarbons representative of the exhaust gas from coal combustion over Rh/Ce0.62Zr0.38O2 catalyst

The performance of rhodium supported on ceria–zirconia solid solution for selective catalytic reduction of NOx by unburned hydrocarbons in the presence of O2 was investigated. Three hydrocarbons (propane, propylene and toluene) were selected as model compounds of aliphatic and aromatic hydrocarbons existing in the coal combustion exhaust gas. The Rh/Ce0.62Zr0.38O2 catalyst was found to be active in coal deNOx reaction (57% at 555K for hydrocarbons mixture). X-ray diffraction, H2 temperature-programmed reduction, temperature-programmed desorption of NO, temperature-programmed surface reaction were used to determine and to explain the performance of our catalyst in deNOx process. In order to better understand the mechanism of this process, the nature of the species formed on the catalytic surface during catalytic reduction of NO by hydrocarbons was studied by infrared spectroscopy. It was found that the nitrates species reacts with the adsorbed hydrocarbons, during the deNOx process forming some carboxylates and some nitrites. The three functions are confirmed in deNOx process: (i) oxidation of NO to NO2 on Rh2O3 particles, (ii) mild oxidation of HC in the presence of NO2 to carboxylates on Rh2O3 and (iii) decomposition-reduction of NO to N2 with total oxidation of the oxygenated species on the Rh3+ species.

The influence of precipitation temperature on the properties of ceria–zirconia solid solution composites

The ceria–zirconia composites (CZ) with a Ce/Zr mass ratio of 1/1 were synthesized by a back-titration method, in which the influence of precipitation temperature on the properties of ceria–zirconia precipitates was investigated. The resulting precipitation and mixed oxides at different precipitation temperatures were then characterized by a range of techniques, including textural properties, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR) as well as oxygen storage capacity (OSC) measurement. The results revealed that ceria–zirconia composites were formed as solid solution and such structure is favored of thermostability and texture properties. In particular, the composite CZ-70 synthesized at 70°C exhibited prominent thermostability with a surface area of 32m2/g as well as a pore volume of 0.15cc/g after aging treatment at 1000°C for 5h. And this was found to be associated with the wider pore size distribution which maybe owed to the formation of large crystal at the primary stage of precipitation. Additionally, the composite CZ-70 showed excellent reduction property and OSC benefiting from stable texture and structure.

ChemInform Abstract: Catalytic Aspects of Ceria‐Zrconia Solid Solution: Part 1. An Update in the Synthesis, Properties and Chemical Reactions of Ceria Zirconia Solid Solution

AbstractReview: 348 refs.

Ceria–Zirconia Solid Solutions (Ce1–xZrxO2−δ, x ≤ 0.2) for Solar Thermochemical Water Splitting: A Thermodynamic Study

The redox behavior of ceria–zirconia solid solutions (or Zr-substituted ceria, ZSC) with a Zr content of up to 20 mol % is studied by thermogravimetry (TG) between 600 °C and 1490 °C under controlled atmospheres. Thermodynamic properties, specifically standard oxidation enthalpy, ΔHoxd⊖, and entropy, ΔSoxd⊖, are derived from TG data. The raw TG results show that the extent of reduction is significantly increased (compared with undoped ceria), even at a low Zr substitution level of 5 mol %. Concomitantly, the magnitude of the thermodynamic functions dramatically decreases as a function of Zr content, particularly at low values of oxygen non-stoichiometry, δ (<3 mol %). Thermochemical fuel production from Zr-substituted ceria generally increases with increasing Zr content under both two-temperature and isothermal cycling conditions. In the case of two-temperature cycling, the benefit is accompanied by a penalty in the (computed) steam-to-hydrogen conversion ratio, whereas it is accompanied by a gain in this...

ChemInform Abstract: Catalytic Aspects of Ceria Zirconia Solid Solution: Part‐II an Overview on Recent Developments in the Heterogeneous Catalytic Applications of Metal Loaded Ceria‐zirconia Solid Solution

AbstractReview: 234 refs.

Cyclic production of syngas and hydrogen through methane-reforming and water-splitting by using ceria–zirconia solid solutions in a solar volumetric receiver–reactor

For cyclic production of syngas and hydrogen by methane reforming (reduction) and water splitting (re-oxidation) under simulated solar-light irradiation, foam devices coated with ceria–zirconia (CexZr1−xO2, where x=0.3, 0.5 and 0.8) solid solutions and CeO2 were used. Reduction characteristics of the foam devices were investigated in the temperature range from 873 to 1173K. The solid solutions were found to be reduced more easily than pure CeO2 especially at low temperatures. The gas yields by the solid solutions increased with increasing Ce content. Depending on temperature and composition of the solid solutions, the optimum operating times for methane reforming, during which carbon deposition was negligible, were determined. Among the solid solutions, Ce0.8Zr0.2O2 showed the best stability and gas yields during repeated operations of the redox cycle at 1173K, and it was confirmed from the XRD analysis that sintering of Ce0.8Zr0.2O2 was the least. To find the effect of metallic foams on the gas productivity, a Ni foam coated with Ce0.8Zr0.2O2 was investigated, and it rendered significantly higher gas yields than the SiC foam device; however, its long-term stability was assessed to be inferior to the latter.

Defect and Transport Model of Ceria–Zirconia Solid Solutions: Ce0.8Zr0.2O2−δ—An Electrical Conductivity Study

The electrical conductivity (σ) of Ce0.8Zr0.2O2−δ was measured and modeled using a defect chemical-based model including contributions from singly and doubly ionized oxygen vacancies, acceptor impurities, and small polaron electrons. By analyzing the pO2 dependence of σ in terms of the defect model, a transition between an impurity dominated regime at high pO2 and lower temperature to one controlled by the simultaneous generation of electrons and doubly ionized oxygen vacancies at low pO2 and higher temperature is identified. At even lower pO2, or equivalently larger deviations from stoichiometry, evidence is presented for a further transition to singly ionized oxygen vacancies accompanying electron generation. Temperature induced conductivity relaxation measurements are successfully applied in deconvoluting electron generation and migration contributions to the activation energy. Key parameters are extracted including the enthalpy of reduction Hr of 2.87 ± 0.06 eV and the electron hopping or migration energy of 0.354 ± 0.005 eV. Both the activated electron mobility and the broad maximum in conductivity observed under the most reducing conditions support the small polaron model for electron transport in Ce0.8Zr0.2O2−δ. Consistent with earlier findings, Zr, though isovalent with Ce, markedly enhances the reducibility, and thereby the oxygen storage capability of ceria–zirconia solid solutions.

Selective Vapour Phase Oxidation of Benzyl Alcohol to Benzaldehyde over Mesoporous Ceria–Zirconia Solid Solution Synthesized Via a Facile Citrate Route

In this article, a facile solid state mixing of precursors with citric acid was employed for the synthesis of mesoporous ceria–zirconia solid solution (CZ), which further require a thermal treatment alone for the material development. The ceria and zirconia nanocrystals were also prepared by the same solid state mixing method. The as-synthesized samples were characterized by means of various structural and textural studies. The results depicted that the presence of citric acid is desired for the formation of a perfect CZ solid solution with microflake like morphology. It has an enhanced role in the determination of crystalline size, phase purity, morphology, surface area and pore parameters of the as-synthesized samples. The active phase over the CZ samples was found to be tetragonal. The selective vapour phase oxidation of benzyl alcohol (BzOH) to benzaldehyde was effectively carried in a fixed bed reactor in moderate air flow over the redox catalysts with a maximum BzOH conversion of 80 % with substantial durability and reusability.

A complex and de-complex strategy to ordered mesoporous Ce0.5Zr0.5O2 with comprehensive pilot scale performances

Synthesis of ordered mesoporous ceria-zirconia (CeO2–ZrO2, M-CZ) solid solution in water-based conditions have challenged scientists for a long time due to the extremely different chemical properties of cerium and zirconium cations. This study discuss the synthesis of highly ordered mesoporous Ce0.5Zr0.5O2 solid solution in aqueous solution through a new complex and de-complex method using di-cationic Gemini as the template. The method takes advantage of the coordinative capability of citric acid (CA) to allow the coexistence of inorganic resources in mild basic solutions and uses the strong oxidic capability of H2O2 to precipitate Ce and Zr cations homogeneously. Through this strategy, the combination between inorganic–organic groups in aqueous solution was avoided, whereas coordination between inorganic–organic species was realized. Mechanisms show that the formation of peroxycarboxylic groups (–COOOH) from the reaction between metal-CA molecular and hydroxyl radical (OH) was the key process. Meso-scale micelles were formed during hydrothermal treatment, while hexagonally arranged pores with narrow size distribution were obtained in the calcination procedure. The surface area of the calcined product was 228 m2 g−1 with a pore volume of 0.7 cm3 g−1, whereas oxygen storage capacity was 652 μmol g−1. The M-CZ product illustrated fairly good pilot-scale three-way catalytic (TWC) performances thanks to the thin pore walls in the mesostructures. A complete (100%) conversion of CO and NOy at 340 °C was detected in the engine pedestal tests. The complex and de-complex strategy may provide a new route for the preparation of ordered mesoporous composite oxides.

Propene oxidation over palladium catalysts supported on zirconium rich ceria–zirconia

Ceria–zirconia solid solution with the Ce/Zr ratio of 1/3 was prepared and used as support material for palladium catalysts. The palladium catalysts supported on the zirconium rich ceria–zirconia (Pd/CZ) were studied for propene oxidation reaction. In the activity tests, the influences of palladium loadings, pretreatment conditions, and reaction conditions were investigated. The catalytic activities of Pd/CZ with small amount of palladium component were strongly affected by the pretreatment conditions such as oxidative or reductive atmosphere. From IR measurements, it was suggested that the palladium species with high reducibility should be active for the complete oxidation of propene. The initial activities of Pd/CZ catalysts under the insufficient oxygen condition were equivalent to the activities under the stoichiometric condition due to the property of high oxygen storage capacity (OSC). It was revealed that CZ support helped the high dispersion of palladium oxide particles and its OSC property assisted the propene oxidation reaction.

Details Behind the Self‐Regeneration of Supported NiCo/Ce0.8Zr0.2O2 Bimetallic Catalyst in the CH4–CO2 Reforming Reaction

AbstractCeria zirconia solid solution with a high specific surface area and oxygen storage capacity was used to support NiCo bimetallic nanoparticles and was successfully employed as a catalyst in the methane–CO2 reforming reaction. During the reforming test with an equimolar CH4/CO2 ratio at 1023 K, an initial catalyst deactivation was observed, which was followed by a slow self‐reactivation. The catalyst reached its initial activity after approximately 400 h TOS with negligible carbon accumulation and H2 and CO yields of 71 and 85 %, respectively. Catalyst deactivation in the initial 25 h of reaction could be correlated to the oxidation of nanosized NiCo particles by water produced through the reversed water gas shift reaction pathway. With prolonged time on stream, sintering and growth of nanosized NiCo particles occurs, which makes them less susceptible to oxidation and slowly leads to their reduction and reactivation. In parallel, recrystallization of Ce0.8Zr0.2O2 support from cubic crystallites to a more irregular polyhedral shape occurs, which improves the oxygen storage capacity of the material and significantly contributes to the catalyst regeneration. The rate of catalyst regeneration mainly depends on the rate of Ce0.8Zr0.2O2 support recrystallization, which is driven by sintering, and is consequently much slower than the initial deactivation. Upon increasing the CH4/CO2 feed ratio to 1.5 and 2.33, water yields were significantly reduced and the previously observed catalyst deactivation could be strongly decreased or even completely avoided.

Optimization of Ce0.6Zr0.4−x Al1.3x O2 solid solution based on oxygen storage capacity

High oxygen storage capacity (OSC) materials are very much demanded for catalytic applications and ceria is one of the promising OSC materials widely used in three-way catalysts. The objective of the present work is to overcome the inherent limitations of ceria by doping Al3+ and Zr4+ into its lattice and optimize the concentration of dopant based on its OSC. A solid solution of Ce0.6Zr0.4−x Al1.3x O2 (CZA) (0.0 ≤ x ≤ 0.4) was synthesized by sol–gel technique and the concentration of Zr and Al was optimized based on the maximum OSC of CZA. In all the samples, the lattice defects on the structure were observed and confirmed by HRTEM and Raman spectroscopy. The crystal size of CZA samples was observed to be varied from 2.7 to 7.5 nm. The crystallite growth of CZA was observed to be less compared to that of ceria–zirconia solid solution after the ageing treatment. The OSC of the sample was calculated by the cyclic heating and cooling process in the temperature range of 30–800 °C using Thermogravimetric analysis and the same was observed to be varied from 0.108 to 0.230 μmol g−1 of CeO2. The coefficient of thermal expansion, which was qualitatively used to confirm the OSC, was measured using the thermomechanical analyser and it was observed to be high in Ce0.6Zr0.2Al0.26O2 compared to that of Ce0.6Zr0.4O2 and the same was found to be significant after 600 °C. It is concluded that Ce0.6Zr0.2Al0.26O2 has the highest OSC among all the samples, which could be explored for the potential catalytic applications.

Role of Ceria-Zirconia Solid Solution with High Oxygen Storage Capacity in Cermet Anodes of Solid Oxide Fuel Cells

A solid solution of ceria and zirconia is investigated as the ceramic component of the cermet anode for solid oxide fuel cells (SOFCs). Homogeneously dispersed NiO-Ce0.75Zr0.25O2-δ (CZO) composite powder is synthesized via a single-step combustion process based on the glycine-nitrate method. The electrolyte-supported cell composed of a Ni - CZO anode, yttria-stabilized zirconia (YSZ) electrolyte and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) - gadolinia-doped ceria (GDC) cathode is fabricated, and its electrochemical performance is compared with that of a reference cell employing the conventional Ni-YSZ anode. The Ni-CZO anode exhibits substantially higher performance than the Ni-YSZ anode, indicating that CZO actively participates in the anode's catalytic process, in contrast to YSZ. Impedance spectra of full-cells and half-cells are correlated to understand the electrode reaction mechanisms, and heavily overlapping impedance arcs are successfully deconvoluted using a differential relaxation time (DRT) technique. It is found that CZO promotes fuel oxidation through an oxygen spillover mechanism and enhances gas-solid interaction via H2 adsorption on the ceramic component of the cermet anode, resulting in a reduction of the electrode polarization resistance. The potential of the Ni-CZO anode for practical applications and a strategy for further development are discussed in detail.

Structural characterization and oxygen nonstoichiometry of ceria-zirconia (Ce1−xZrxO2−δ) solid solutions

The oxygen nonstoichiometry and crystalline structure of ceria-zirconia Ce1−xZrxO2−δ (CZO) (x=0.05, 0.1, 0.2, 0.5, and 0.8) solid solutions, commercially used as oxygen storage materials, have been examined using thermogravimetry, X-ray diffraction, and Raman spectroscopy. In this study detailed data describing oxygen vacancy concentrations, obtained at intermediate to high pO2, are interpreted with the aid of point defect equilibria relationships. For cubic CZO (x⩽0.2), the ease of reducibility dramatically increased with increasing Zr content, as reflected by an ultimate >40% decrease in reduction enthalpy, with a corresponding shift in onset of reduction to higher pO2. The impact of pre-existing oxygen vacancies on the larger reduction enthalpy found for Y doped CZO, as compared with this study, is discussed, as is evidence that Zr increases the electron migration energy in ceria by 50%. The reducibility of tetragonal CZO (x>0.2) was found to increase following redox cycling. This enhanced reducibility is believed to be related to ordering and is partially negated by a high temperature (1000°C) heat treatment.

Low temperature carbon monoxide catalytic oxidation at the Pd/Ce–Zr–Al–Ox catalyst

The homogeneous chemical composition ceria–zirconia–alumina (Ce–Zr–Al–Ox) nano-alloy were successfully synthesized by surfactant-assisted parallel flow co-precipitation method and applied as supports for low temperature CO oxidation. The experiment conditions were studied in detailed. At 0.92 wt% Pd loading, 30,000 ppm CO could be completely oxidized to CO2 at 30 °C at a WHSV of 4,380 ml g−1 h−1 over the Pd/Ce–Zr–Al–Ox (nCe:nZr = 3:1) catalyst. Pd/Ce–Zr–Al–Ox catalysts were systematical studied by mean of BET, XRD and TEM analysis. XRD characterization showed that zirconium element entered into cubic structure of ceria and leaded to structure distortion. Addition of aluminum increased specific surface area of ceria–zirconia solid solution substantially. The average pore diameter of Ce–Zr–Al–Ox support palladium catalysts were the key impact factor for CO oxidation. When the Pd/Ce–Zr–Al–Ox catalysts had highly dispersed palladium nanoparticles, large average pore diameter, suitable surface area and pore volume, the activity of CO oxidation was the best.

Hydrogen production from water-splitting reaction based on RE-doped ceria–zirconia solid-solutions

The effect of rare earth (RE = Tb, Pr and La) dopant on the catalytic performance of RE-doped ceria–zirconia (CZRE) solid-solutions for oxygen storage capacity and hydrogen production activity has been successfully investigated. The sustainability of the solid-solutions even after the reduction was confirmed by XRD. Raman analysis showed that the addition of RE element in CZ system significantly decreased the intensity of the characteristic fluorite peak (462–474 cm−1) indicating a highly deformed structure than CZ system which can enhance the oxygen mobility and redox property of these materials and the order of the intensity decrease was Pr > Tb > La. The XPS measurements revealed that the CZPr sample has a homogeneous distribution of Ce/Zr and also showed a high enrichment of Pr on the particle surface than the others. Among the CZRE solid-solution catalysts tested, CZPr catalyst showed the best catalytic performance for high OSC and hydrogen production from water-splitting reaction.

Optical properties of ceria–zirconia epitaxial films grown from chemical solutions

Epitaxial thin films of ceria–zirconia solid solutions were grown on yttria-stabilized zirconia (YSZ) substrates by chemical solution deposition. Characterization by X-ray diffraction, atomic force microscopy and variable angle spectroscopic ellipsometry show that high-quality, epitaxial, high density films, with a thickness in the range of 20–27 nm were obtained. Compositional dependence of the film optical constants (n and k) was determined from ellipsometry measurements. It was found that the optical properties for the mixed-oxide films smoothly vary between the values for the pure oxides and are best described by means of a Tauc–Lorentz (TL) model. Moreover, we found that the parameters of the TL model have a linear dependence on the cerium concentration. The obtained results can serve as a database for the thickness, composition and porosity characterization of CexZr1−xO2 thin films.

Vapor phase synthesis of cyclopentanone over nanostructured ceria–zirconia solid solution catalysts

The present study was undertaken to develop a novel and easy practical approach for synthesis of cyclopentanone, a versatile industrial ingredient. Accordingly, ceria–zirconia based nano-oxide catalysts, namely, CexZr1−xO2 and CexZr1−xO2/M (M=SiO2 and Al2O3) were prepared and evaluated for the title reaction. The physicochemical characterization has been achieved using different techniques, namely, XRD, BET surface area, XPS, Raman, OSC, and HREM. The catalytic results revealed that CexZr1−xO2 based nano-oxides are promising heterogeneous catalysts for synthesis of cyclopentanone. Amongst, the CexZr1−xO2/Al2O3 catalyst exhibited ∼100% conversion with 75% desired cyclopentanone product selectivity owing to its favorable physicochemical characteristics.

Effects of Cr on the NO oxidation over the ceria–zirconia solid solution

CrOx, Ce0.5Zr0.5O2 and 10Cr/Ce0.5Zr0.5O2 catalysts were prepared to investigate the separate effect of Cr and Ce0.5Zr0.5O2 for selective catalytic oxidation of NO. It was found that the co-actions of CrOx and Ce0.5Zr0.5O2 exhibited the higher catalytic activity than separate effect of CrOx or Ce0.5Zr0.5O2. The separate effects of Cr and Ce0.5Zr0.5O2 on the structure of the catalysts were systematically examined by XRD, TEM, BET, XPS, NO-TPD and O2-TPD. The XPS results showed that Cr species were enriched on the surface and the amount of Cr6+ increased after Cr was loaded on the Ce0.5Zr0.5O2. TPD results showed that NO was adsorbed on the sites of Cr, and O2 was adsorbed and activated on the ceria–zirconia solid solution. Finally, the adsorbed NO interacted with the adsorbed O2 to generate NO2 in the Langmuir–Hinshelwood mechanism.

Optimization of Ceria-Zirconia Solid Solution based on OSC Measurement by Cyclic Heating Process

Ceria is one of the rare earth metals having Oxygen Storage Capacity (OSC) characteristics, and it is widely used in Three- Way-Catalysts (TWC). Though Ceria has attractive properties, there are some inherent problems including limited OSC and thermal stability. In order to overcome the limitations of Ceria, an attempt is made to optimize the composition of CexZr1-xO2(0.4 ≤ x ≤ 0.8) (CZ) based on the OSC. The formation of homogenous solid solution, the lattice defect, cubic fluorite structureand OSC band of CZ were studied by suitable techniques. Thermogravimetric analysis was used to calculate the OSC of CZ solid solution with cyclic heating and cooling up to 800°C. It is concluded that OSC of CexZr1-xO2 was varied from 0.0435 to 0.1469μmol/gram of CeO2. Based on the highest OSC, Ce0.6Zr0.4O2 was confirmed to be a best solid solution among all the tested composition of CZ.