Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

Macroporous ceramics with pore sizes from 400 nm to 4 mm and porosity within the range 20%–97% have been produced for a number of well-established and emerging applications, such as molten metal filtration, catalysis, refractory insulation, and hot gas filtration. These applications take advantage of the unique properties achieved through the incorporation of macropores into solid ceramics. In this article, we review the main processing routes that can be used for the fabrication of macroporous ceramics with tailored microstructure and chemical composition. Emphasis is given to versatile and simple approaches that allow one to control the microstructural features that ultimately determine the properties of the macroporous material. Replica, sacrificial template, and direct foaming techniques are described and compared in terms of microstructures and mechanical properties that can be achieved. Finally, directions to future investigations on the processing of macroporous ceramics are proposed.

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Processing Routes to Macroporous Ceramics: A Review

It is well known that urbanization and industrialization have resulted in the rapidly increasing emissions of volatile organic compounds (VOCs), which are a major contributor to the formation of secondary pollutants (e.g., tropospheric ozone, PAN (peroxyacetyl nitrate), and secondary organic aerosols) and photochemical smog. The emission of these pollutants has led to a large decline in air quality in numerous regions around the world, which has ultimately led to concerns regarding their impact on human health and general well-being. Catalytic oxidation is regarded as one of the most promising strategies for VOC removal from industrial waste streams. This Review systematically documents the progresses and developments made in the understanding and design of heterogeneous catalysts for VOC oxidation over the past two decades. It addresses in detail how catalytic performance is often drastically affected by the pollutant sources and reaction conditions. It also highlights the primary routes for catalyst deactivation and discusses protocols for their subsequent reactivation. Kinetic models and proposed oxidation mechanisms for representative VOCs are also provided. Typical catalytic reactors and oxidizers for industrial VOC destruction are further discussed. We believe that this Review will provide a great foundation and reference point for future design and development in this field.

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Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources.

Synthesis methods for nanosized hydroxyapatite with diverse structures.

Magnetic nanoscaled Fe3O4/CeO2 composite was prepared by the impregnation method and characterized as a heterogeneous Fenton-like catalyst for 4-chlorophenol (4-CP) degradation. The catalytic activity was evaluated in view of the effects of various processes, pH value, catalyst addition, hydrogen peroxide (H2O2) concentration, and temperature, and the pseudo-first-order kinetic constant of 0.11 min–1 was obtained for 4-CP degradation at 30 °C and pH 3.0 with 30 mM H2O2, 2.0 g L–1 Fe3O4/CeO2, and 0.78 mM 4-CP. The high utilization efficiency of H2O2, calculated as 79.2%, showed a promising application of the catalyst in the oxidative degradation of organic pollutants. The reusability of Fe3O4/CeO2 composite was also investigated after six successive runs. On the basis of the results of metal leaching, the effects of radical scavengers, intermediates determination, and X-ray photoelectron spectroscopic (XPS) analysis, the dissolution of Fe3O4 facilitated by CeO2 played a significant role, and 4-CP was decom...

Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient Fenton-Like Heterogeneous Catalyst for Degradation of 4-Chlorophenol

SiC with a woodlike microstructure was produced by vacuum-infiltrating charcoal with tetraethyl orthosilicate (TEOS). The TEOS in the cell structure was hydrolyzed in an ammoniacal solution to form SiO 2 gel. The SiO 2 -replicated charcoal was fired at 1400°C in Ar to form α-SiC. After residual carbon was burned off at 700°C in air, porous α-SiC with the original wood structure, analogous with fossilized wood, was produced.

Biomimetic Process for Producing SiC “Wood”

The morphologic and microstructural development of natural fish bone through heat treatment is examined in view of fish-waste management and as a possible route for hydroxyapatite ceramics. Fish bone heated at temperatures ≤1300°C maintains a porous structure, with a sintered wall and a major crystalline phase of hydroxyapatite. Fish-originated ceramics are potentially useful as bioactive media as well as for environmentally compatible materials.

Microstructural Development of Natural Hydroxyapatite Originated from Fish‐Bone Waste through Heat Treatment

Role of cerium-zirconium mixed oxides as catalysts for car pollution : A short review

Innovative three-way catalysts containing ceria-zirconia solid solutions (CZ) with a ZrO 2 content ranging from 5 to 75 mol% were developed Such catalysts have very high oxygen storage/release capacity (OSC) The OSC of'catalysts containing first-generation CZ was found to increase linearly with the amount of zirconia dissolved in the solid solution in the 0-20 mol%( ZrO 2 range The thermal stability of the catalyst containing first-generation CZ was higher than that containing ceria Second-generation CZ with greater OSC than first-generation CZ was developed by a homogeneous coprecipitation process, followed by heat-treatment below 1000 °C, enabling dissolution of zirconia into ceria up to 75 mol% zirconia as estimated by XRD Third-generation CZ, CZ-doped alumina (ACZ), was developed by homogeneous coprecipitation based on the "diffusion barrier concept" The OSC of ACZ was 23 times larger than that of pure ceria The NOx emission over the catalyst containing ACZ was reduced by about a factor of five compared with that containing pure ceria Toyota Motor Corp has put three-way catalysts containing first-, second-, and third-generation CZ into practical use globally

Development of Innovative Three-Way Catalysts Containing Ceria–Zirconia Solid Solutions with High Oxygen Storage/Release Capacity

Transition alumina, such as 7-A1203, 0-A1203 and 6-A1203, has been widely used as a catalyst support. The surface area of the support significantly decreases at high temperature operations, for example in purifying automotive exhaust gas [1]. In this case, it is important to maintain the support with high surface area free of sintering or phase transition, otherwise precious metals (platinum, rhodium etc.) supported on alumina will consequently sinter. Cerium and other rare earth elements have been used as a promoter to improve the catalytic activities for purifying carbon monoxide, nitrogen oxides and hydrocarbons emitted from automotive engines [2-4]. It has been suggested that the addition of cerium plays serveral roles, including the catalysis of the water-gas shift reaction, the oxygen storage in the lattice for oxidation catalysis under rich air-fuel conditions, and the inhibition of the growth of noble metal particles. The present letter describes the influence of the addition of cerium to 7-A1203 catalyst supports on the phase transition of 7-A1203 to ~-A1203 and the sintering of transition alumina below the transition temperature. A series of cerium-added alumina was prepared by the impregnation technique. Powdered 7-A1203 with surface area 170 m 2 g ~ was used as a support, and an aqueous cerium nitrate was used as an impregnation solution. The samples were then dried at 110 ° C for 8 h and calcined at 600°C for 3 h. The starting alumina contained 0.05 wt % impurities, mainly iron and trace silicon, calcium and sodium. The cerium contents of the samples were 0.5, 1, 2, 3, 5 and 10mol% by the CeO2 to the total moles of oxides (CeO2 + A1203). The samples were further calcined in air at a temperature between 1 000 ° C and 1 200 ° C for 5 h in order to examine the thermal stability of the alimina support. The surface area of the samples was derived by application of the BET equation, from nitrogen adsorption data obtained at 77K by a standard volumetric procedure. The phase transition temperature was measured with a DTA apparatus at heating rates of 2.5, 5, 10 and 20Kmin ~ using 0¢-A1203 powder as a reference material. Kissinger's plots [5] were applied to the evaluation of the activation energy of the phase transition from active alumina to c¢-A12 03. Phase analysis was done by the powder X-ray diffraction (XRD) technique. The amount of ~-A1203 in the samples was determined by the XRD intensity of the (1 00) diffraction line of 0~-Al203. The standard data were obtained from the XRD data of the mixtures of u-A1203 powder and 0-A1203 powder which were formed by calcining 7-A1203 at 1 300°C and l 000°C for 10 h. The samples containing 0.01 mol % gadolinium were prepared by the same procedure as above, but using an aqueous solution of gadolinium nitrate. Fig. 1 shows the Surface area of alumina supports containing cerium with different molar concentrations, calcined in air at 1 000 ° C, l 100°C and 1 200°C for 5 h. The thermal stability of alumina was improved by the addition of only 0.5 to 2 mol % cerium, and a gradual decline in surface area was found with increasing cerium content. XRD analysis of the samples calcined in air at 1 200°C indicated that 0.5 mol % Ce-A1203 consisted of 0-, ~and ~-A1203, and that pure A1203 completely transformed to ~-A1203 (Fig. 2). CeO2 phase was found in the samples with more than 2 mol % cerium. These results clearly show that the addition of cerium improves the thermal stability of alumina at 1 200°C by preventing the transformation to ~-A1203. However, the overloading of cerium decreases the surface area of the alumina because of the growth of CeO2 particles. The XRD data of the samples calcined at 1 100°C indicated the formation of 0and ~-A12 03. The decrease in surface area was inhibited by the cerium loading at 1 100 ° C. Kissinger's plots of the data obtained by DTA for both pure AI203 and 0.5 tool % Ce-AI203 and given in Fig. 3 as the relation of ln[(dT/dt)Tn72] with l/Tm. (dT/dt) is the heating rate, and Tm represents the maximum rate temperature of the phase transition. The activation energy of the phase transition was calculated at 581kJmol ~ for pure AI203 and 582 kJ tool ~ for 0.5 mol % Ce-A1203. Although these values are rather high compared with those in the literature [6, 7], it can be found that the alumina in this study has the constant activation energy of the phase transition regardless of the cerium addition. Scanning electron microscopy (SEM) observation of the samples calcined at ! 200 ° C revealed the formation of ~-A1203 particles, 0.1 to 0.3 #m in diameter. The particle size was independent of the content of cerium in the samples. These results can be explained by assuming that the cerium addition decreased the ~-A1203 transformation by influencing the nucleation process of ~-A1203 but not its growth process. Fig. 4 compares the 1 250 ° C isothermal transformation data for pure A1203 with those for 0.5mol % Ce-A1203. The data were analysed using the following empirical kinetic equation

Masakuni Ozawa

Papers

Biomimetic Process for Producing SiC “Wood”

Microstructural Development of Natural Hydroxyapatite Originated from Fish‐Bone Waste through Heat Treatment

Role of cerium-zirconium mixed oxides as catalysts for car pollution : A short review

Development of Innovative Three-Way Catalysts Containing Ceria–Zirconia Solid Solutions with High Oxygen Storage/Release Capacity

Effect of cerium addition on the thermal stability of gamma alumina support