Catalytic Efficiency of Ceria–Zirconia and Ceria–Hafnia Nanocomposite Oxides for Soot Oxidation

The catalytic oxidation of soot particulates has been investigated over CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO₂, Ce_0.75Zr_0.25O₂ and Ce_0.8Hf_0.2O₂ phases in case of CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce³⁺←O^2? and Ce⁴⁺←O^2? respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr⁴⁺ and Hf⁴⁺ into the CeO₂ lattice. The CeO₂–HfO₂ combination catalyst exhibited better activity than the CeO₂–ZrO₂. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.     

XAFS and XRD Analysis of Ceria–Zirconia Oxygen Storage Promoters for Automotive Catalysts

Three types of CeO₂–ZrO₂ (Ce:Zr = 1:1 molar ratio) compounds with different oxygen storage/release capacity (OSC) were characterized by means of the Ce K-edge and Zr K-edge XAFS. In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO₂–ZrO₂ solid solution, the OSC performance increased. Additionally, from the XRD analysis, the homogeneous CeO₂–ZrO₂ solid solution has an ordered cation arrangement, and exhibits the highest OSC. The crystal structure of this CeO₂–ZrO₂ solid solution is usually termed as “κ-CeZrO₄ phase”. However, the OSC performance of κ-CeZrO₄ degrades upon a high-temperature treatment under an oxidative atmosphere. The fresh κ-CeZrO₄ was aged at 973, 1,273 and 1,473 K under an oxidative atmosphere, respectively. The OSC performance deteriorated as: the fresh sample ≈973 > 1,273 > 1,473 K-aged samples. We also found that, if the temperature was beyond 1,273 K, the Ce/Zr ordered arrangement would collapse and the local structure around Ce and Zr ions would also changed remarkably. These results indicated that OSC was strongly dependent on its atomic structure.     

Synthesis of Zirconia Powder in Zirconia Aqueous Sol

Spherical zirconia particles formed by coagulation of colloidal particles in zirconia aqueous sol followed by calcination.The zirconia sol was prepared from zirconium oxychloride aqueous solution by adjusting its pH value.The coag-ulation resulted from addition of preopanol in the sol the coagulation mechanism was discussed based on collicd theory.     

Further Insights into the Key Features of Ceria–Zirconia Mixed Oxides Governing the Catalysed Soot Combustion Under NO x /O2

Eighteen Ce_0.76Zr_0.24O₂ samples have been prepared by different procedures and characterised by N₂ adsorption at –196 °C, XRD, Raman spectroscopy, H₂-TPR and XPS. The catalytic activity for soot combustion depends both on the BET surface area and on the Ce/Zr atomic surface ratio, which strongly depends on the preparation procedure.     

Development of Corundum—Mullite—Zirconia Clinkers

The properties of corundum-mullite-zirco-nia clinkers made by reaction-sintering have been investigated.It is shown that corundum-mullite-zirconia clinkers exhibit lower sintering tempera-ture as well as better thermal shock resistance and corrosion resistance,The carbon-bonded alu-mina-mullite-zirconia material possesses satis-factory properties and may have prospects for application in severe process environments such as continuous casting.     

Thermomechanical Properties of Corundum—Mullite—Zirconia Materials

The mechanical properties at elevated tempera-tures of corundum-mullite materials and the effects of zirconia and titania additions have been investi-gated.The results are correlated with changes in microstructure,Based on the results of these investi-gations ,reaction-sintered corundum-mullite-zirco-nia materials with improved hot strength and ther-mal shock resistance have been developed for refractor-ries usage.     

Ceria and Gold/Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons: An In Situ DRIFTS Study

Gold catalysts supported on nano-crystalline ceria prepared by deposition precipitation have been characterised and tested for the total oxidation of naphthalene. Two different precipitation methods were used to prepare the nano-crystalline ceria supports and it was observed that although both supports were active materials for naphthalene oxidation, ceria synthesized by homogeneous precipitation with urea was markedly more active than CeO₂ precipitated by carbonate. The addition of gold to both active CeO₂ catalysts resulted in different effects for the total oxidation of naphthalene. Gold addition promotes the naphthalene conversion to CO₂ when ceria is prepared by precipitation with carbonates, whilst the light off temperature is shifted towards higher temperatures when gold is added to ceria synthesized by the urea method. This behaviour has been related to a change in the support characteristics and a removal of the carbonate surface species, when gold is deposited onto the ceria support.     

Influence of Various Zirconia Additives on High Temperature Properties of Magnesia Spinel Brick

Magnesia spinel refractory is a promising material because of its good properties at high temperature,good exfoliation-and coorosion-resistances,Three kinds of additives containing ZrO2 were added o magnesia spinel refacto-ries to get better exfoliation resistance,corrosion resistance and adhensiveness,Three kinds of ZrO2-containing additives added to optimize the properties of the material are particularly discussed in this paper.     

Oxygen Storage Behavior of Ceria–Zirconia-Based Catalysts in the Presence of SO2

This paper investigates the effect of the presence of SO₂ in the dynamics of oxygen storage on ceria and ceria–zirconia. The introduction of SO₂ under reaction conditions at T<873 K negatively affects CO conversion under oscillating conditions on all the supports studied, owing to the formation of sulfate species. Deactivation is observed on all supports and activity is recovered only after desorption of SO₂, which occurs at 950<T<1000 K, depending on catalyst composition (Ce/Zr ratio) and treatment atmosphere. The amount of sulfur adsorbed is higher over solid solutions, reaching a maximum with Ce _x Zr_1–x O₂ (0.5<x<0.68). However this does not adversely affect activity compared to ceria. In the presence of Rh and Pd, reactivation is favored under reaction conditions. More generally, it appears that the removal of sulfates is facilitated in reductive atmospheres (both hydrogen and CO) over mixed oxides. No differences are observed following regeneration under oxidizing conditions.     

Effects of Deagglomeration and Zirconia Wear Debris on Sintering Behavior of α-Alumina Powder

This work investigates sintering behaviors of α-alumina powder that were treated by attrition milling with zirconia media. While zirconia wear debris delays initial sintering shrinkage and increases the temperature to reach the maximum shrinkage rate, the agglomerates also influence the final sintering. The final density of the sintered bulk is limited because of the existence of pores among agglomerates. Zirconia wear debris effectively inhibits the grain growth of alumina during the sintering. Varying the degree of deagglomeration and the amounts of zirconia wear debris together can control the microstructure during the final stage of the sintering.     

Zirconia toughening of mullite–zirconia–zircon composites obtained by direct sintering

Although multi-phase ceramic materials were always used, nowadays composite materials have an important industrial and technological role, because they enlarge the design capability of the manufacturer in properties and behaviors.Some mullite–zirconia–zircon composites were recently processed and characterized which presented satisfactory properties for structural applications under severe chemical and thermomechanical conditions. The objective of the present work is to study the influence of the starting composition in the mechanical and fracture properties of mullite–zirconia–zircon composites, with different microstructures, obtained by direct sintering of binary mixtures of electrofused mullite–zirconia (MZ) and micronized zircon. The materials were consolidated by slip casting of concentrated aqueous suspensions in plaster molds from a wide range of powder compositions (between 15–85 wt% and 85–15 wt% of the two raw materials used).Flexural strength (σ_f), dynamic elastic modulus (E), toughness (K_IC) and fracture surface energy (γ_NBT) were evaluated. The results were explained by microstructure and the XRD-Rietveld analysis.At low proportion, the zircon was thermally dissociated. The ZrO₂ was a product of this reaction and also influenced the mechanical and fracture properties of these materials through several combined mechanisms, principally as a result of the development of microcracks due to the volume change of the zirconia grains caused by the martensitic transformation during the cooling of these composites from sintering temperature.Composites prepared with higher MZ in the starting powders showed a higher fracture toughness and initiation energy. Microstructure consisting of mullite as a continuous predominant phase in which zircon and zirconia grains were distributed showed better mechanical and fracture properties.     

The Effect of Bauxite Substitution on High Temperature Strength Properties of Zirconia—corundum Mullite Material

Investigations on the effect of bauxite substi-tution for industrial alumina on high temperature strength and thermal shock resistance of reaction-sintered zirconia-corundum-mullite material(ZrO2 15%) have indicated that bauxite substitution would lead to increase in modulus of rupture at 1000-1400℃ as well as improvement in thermal shock resistance,Hot strength value reaches a maximum at 15% buxite addition.The mecha-nism of mechanical behavior at elevated tempera-tures is discussed in association with changes in mi-crostructural characteristics.     

Spectroscopic Study on the Nature of Active Entities in Copper–Ceria CO-PROX Catalysts

A series of oxidised copper-ceria catalysts with varying copper loadings and prepared by two different methods (impregnation of ceria and coprecipitation of the two components within reverse microemulsions) have been examined with the aim of determining the nature of active entities for the two main (CO and H₂) competing oxidation reactions that take place during preferential oxidation of CO in H₂-rich streams over this type of catalysts. The analysis is mainly based on operando spectroscopic exploration by DRIFTS and XANES as well as in situ analysis by Raman. The results allow establishing a model of the catalytic behaviour of this type of catalysts which can provide keys to control their CO-PROX catalytic properties.     

PAC Characterization of Nontransformable Tetragonal t′ Phase in Arc-Melted Zirconia–2.8 mol% Yttria Ceramics

Tetragonal compact bodies obtained by quenching from the melt a ZrO₂–2.8 mol% Y₂O₃ commercial powder have been investigated between room temperature and 1150°C, using mainly perturbed angular correlations spectroscopy. The resulting nontransformable t′ phase observed by optical micrography is characterized at nanoscopic level by a hyperfine interaction describing defective and disordered Zr⁴⁺ neighborhoods very different from those of the regular tetragonal phase. A small amount of remaining Zr⁴⁺ sites corresponds to a scarcely distorted tetragonal structure. As the compacts are heated, two processes activate: the movement of vacancies and the gradual and irreversible conversion of t′ defective sites into less defective ones, probably resulting from oxygen absorption.     

Ceria–zirconia mixed oxides: Synthetic methods and applications

The primary objective of this review was to illustrate the significance of ceria–zirconia (CZ) mixed oxides as catalysts and catalyst supports as employed for a wide variety of catalytic applications both in the liquid and gaseous phases. In particular, we were interested in bringing together the recent literature pertaining to these mixed oxides with catalysis perspective. The most prominent application of CZ mixed oxides is in three-way catalysis (TWC) as oxygen storage and release material for several years by replacing cerium dioxide as it shows better efficiency and a high thermal stability. Doping with zirconium oxide, as it is alone a non-reducible oxide, makes the CZ mixed oxide a highly reactive, thermally stable, and more reducible with elevated oxygen storage capacity (OSC) that are important for TWC applications. Apart from the TWC use, the CZ mixed oxides have a huge number of applications, as a direct component or a support, ranging from water–gas shift reaction, reforming of hydrocarbons, dehydration of alcohols, CO₂ utilization, catalytic combustion of pollutants, fine chemicals production, photocatalysis, and so on. All these applications are mainly dependent on three parameters of the mixed oxides, namely, OSC or redox nature, acid–base properties, and crystalline phases. Besides, most of the applications are influenced by the physical properties such as specific surface area, pore volume, pore diameter, crystallite size, and so on. In this review, many details pertaining to the synthesis of these mixed oxides by various conventional and non-conventional methods, their characterization by several techniques, and their application for various reactions of energy and environmental significance, as reported in the literature, are assessed.     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.