الفهرس | Only 14 pages are availabe for public view |
Abstract Nanostructure ferrites have become an important area of research in magnetism, catalysis technological applications and in biosciences. Nanoparticle ferrites are among the most important magnetic materials that can be widely used in many fields, such as high-density information storage, ferrofluids, magnetic devices , flexible recording media, sensors, color imaging, bimolecular separation and so on . Furthermore, spinel ferrites are well known catalysts for various processes like oxidative dehydrogenation of hydrocarbons, decomposition of alcohols, and selective oxidation of carbon monoxide. These properties are strongly dependent on their shape, size, crystallinity and the distribution of the cations among the tetrahedral and octahedral sites of the spinel structure. It is a well-known fact that the properties of ferrite materials are strongly influenced by the material’s composition and microstructure, which are sensitive to the preparation method used in their synthesis . In addition, the sintering conditions employed and the impurity levels present in or added to these materials also change their properties. The usual conventional ceramic method of preparing ferrites, gives inhomogeneous final microstructure with low surface areas. The recently developed low temperature co-precipitation method yields homogeneous and fine ferrite powders with high surface areas and catalytic activity. Why copper manganese ferrite? Mn containing ferrites have various applications such as transformers, electromagnetic interference, asymmetric digital subscriber line etc. These properties of ferrites mainly depend upon chemical composition, method of preparation, sintering time and temperature. By introducing small amount of foreign ion can change the electrical and magnetic properties of the ferrite. Copper ferrites have attracted the attention of investigators, for its uniqueness condition. It shows remarkable effect in structural resistivity. Several studies have been reported on the addition of Mn4+ ions in copper ferrite and other ferrites. In the present work, a detailed investigation of the manganese doped copper ferrite with composition and temperature dependence of surface and catalytic activity properties and thermoelectric power were carried out. The prevailing investigation comprises a trial of correlating the catalytic activity of the above mentioned spinels that have the general form CuxMn1-x Fe2O4, where (x=0.0, 0.2, 0.4, 0.6, 0.8, 1) with its structure and texture towards the decomposition of ethanol. 1) The parent materials were prepared by the interaction between aluminum oxide and the metal nitrates (in presence of urea as a precipitating agent). Interaction process was affected by impregnation and the calcination is affected in air atmosphere at a range of temperatures 600-1200°C for 5 hours. 2) In this work different coordinated physicochemical techniques have been used in order to characterize the different catalysts and identify the nature and different types of acid sites. These physicochemical techniques include i) phase changes and structural investigation of the catalysts were achieved by X-ray diffractometer. Thus, it was possible to define phase changes accompanying annealing in air as well as to assert spinel formation being ferrite, and ii) absorption and/or adsorption of volatile amine such as pyridine have been used to identify qualitatively the nature of acid sites on solid catalysts. 3) Nitrogen sorption isotherms were measured on the parent mixtures and their calcination products. Applying the BET equation the corresponding specific surface areas were calculated and the values obtained were discussed in terms of influencing of x values for the different mixtures. Furthermore, analysis of sorption isotherms was performed. Va-t plots were constructed, and it was possible to investigate the porosity of the various adsorbents. Pore size distribution analysis from cumulative calculations was also done according to de Boer’s method. The necessary calculations were run through a computer programme designed for this purpose. 4) The catalytic activity of the different catalyst series calcined at 600 up to 1200°C, was tested for the catalytic decomposition of ethanol, using a standard flow system, in which the reactor was heated by fixed bed technique. The catalytic experiments, in the temperature range 250-450°C, showed that diethyl ether, ethylene and acetaldehyde are the products of both dehydration and dehydrogenation reactions, respectively. Eventually, the surface structures were correlated with the catalytic activity, where the catalyst selectivity towards the dehydration and dehydrogenation pathways was also taken in consideration. The activity patterns for the different spinels were understood in terms of the total acidity of the catalysts. 5) The appropriate reaction mechanism for both the surface dehydration and dehydrogenation reactions was postulated. |