Cerium catalysis

Catalysis Cerium Permanent magnets Neodymium and Samarium... 

THE ROLE OF CERIUM-BASED OXIDES USED AS OXYGEN STORAGE MATERIALS IN DeNO CATALYSIS... 

Dienes such as 90 can be accessed by a multi-component reaction under ruthenium catalysis involving an allene 88 and an enone (methyl vinyl ketone in this case), with cerium(m) chloride as an additive in DMF (Scheme 26).95,96 With an allene concentration of 0.25 M, yields are moderate to good. Different ruthenium catalysts and additives were tested in order to optimize this reaction. CpRu(COD)Cl 89 and CpRu(MeCN)3PF6 appeared to be more versatile ones. The mono-, di-, tri-, and tetrasubstituted allenes have been investigated with methyl vinyl... 

Catalog of Teratogenic Agents, 25 209 Catalysis 5 200-254. See also Acid-base catalysis Catalyst entries Catalytic entries Heterogeneous catalysis Homogeneous catalysis Photocatalysis of aromatic reactions, 16 844 cerium applications, 5 685-688... 

Rare earths have also been included as desirable SOx catalyst components in early patents but the catalytic behavior of cerium, in particular, had not been clarified. This paper has presented evidence that cerium catalyzes the oxidative adsorption of SO2 on high alumina cracking catalyst, alumina, and magnesia. We also have shown the catalytic character of platinum. The details of the catalysis especially by cerium, however, remain unexplained. 

The reasoning which led the author to make this first shot in the dark regarding the usefulness of combinations of solid compounds as ammonia catalysts was as follows If we assume that a labile iron nitride is an interminate in the catalytic ammonia synthesis, every addition to the iron which favors the formation of the iron nitride ought to be of advantage. In other words, the hypothesis was used that surface catalysis acts via the formation of intermediate compounds between the catalyst and one or more of the reactants. An experimental support for this theory was the fact that a stepwise synthesis via the formation and successive hydrogen reduction of nitrides had been carried out with calcium nitrides (Haber), and cerium nitrides (Lipski). Later, the author found molybdenum nitride as being the best intermediate for such a stepwise synthesis. 

Since 1962 rare earths have been used to stabilize zeolite cracking catalysts for the petroleum industry (1, 2. Until recently this application to catalysis has been the only commercially significant one. Currently, however, a number of new applications of potential commercial significance are appearing. One of the most important of these is the use of cerium in catalysts for automobile exhaust emission control. We will emphasize this application in our review without neglecting other applications. 

Metal oxides have often been used as catalysts for the autoxidation of hydrocarbons.1 In many cases the metal probably dissolves in the reaction medium and catalysis involves homogeneous metal complexes. However, according to a recent report56 cerium oxide catalyzes the liquid phase oxidation of cyclohexanone in acetic acid (5-15 bar and 98-118°C) without dissolving in the reaction medium. 

The analogous system which employs cerium(III) acetate and hydrogen peroxide offers three principle advantages over the CAB/H202 process. Firstly, only low levels of cerium(III) acetate are required for effective catalysis, typically about five times less on a molar basis compared to the CAB/H202... 

Pale yellow cerium dioxide (ceria, ceric oxide) has the fluorite structure and is used in catalysis" ", solid oxide fuel cells (SOFC)", thin film optical waveguides" , reversible oxygen storage materials for automobile catalysts" and for doping copper oxide superconductors". The diverse cerium enolate precursors and deposition methods used in the formation of cerium oxide thin films are summarized in Table 6, whereby the most common precursor for ceria is Ce(thd)4. 

Figures 3 and 4 summarise the catalysis experiments results for the mixed copper-cerium oxide and pristine ceria samples for the NO denox reaction. It can be seen from Figure 3 that the selectivity towards N2 was very high, and for one of the samples higher than for pristine ceria. However, as it can be seen fi om Figure 4 the...

Catalysis of the decomposition by several metal ions has been reported . The order in peroxomonosulphate was found to be one, except for reaction with cobalt(II) and cerium(IV) (both second-order in the sulphate) and manga-nese(II) (half-order). Formation of a complex with cerium(IV) and peroxomonosulphate has been both asserted and denied . Catalysis by copper(II) is not significant unless manganese is also added, at least in the presence of some hydrogen peroxide ... 

The applications of ceria based materials are related to a potential redox chemistry involving Cerium(III) and Cerium(lV), high affinity of the element for oxygen and sulfur, and absorption / excitation energy bands associated with its electronic structure. Important areas for application of cerium based materials are catalysis and chemicals, glass and ceramics, phosphors and metallurgy. 

The use of mechanical milling is also a suitable method for powder preparation. The feature of this procedure is to obtain powders of small crystallite size of a few nanometers with a high concentration of lattice defects. There are many mechanochemical studies on the synthesis of alloys, solid solutions, nanophasc materials, and intermetallic compounds. Of course, the mechanical alloying also has been applied to prepare mixed oxides containing cerium oxide to enhance catalysis, such as CeOj-TbO, CeOj-HfOj, Ce02 Zr02-Mn0, and CeOt ... 

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

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