Obtenção e caracterização de (hidr)oxidos de ferro

AUTOR(ES)
DATA DE PUBLICAÇÃO

1991

RESUMO

Magnetite was produced by heating iron (III) hydroxoacetate (lHA) under nitrogen. By varying the conditions of lHA preparations (acetate concentration in solids, starting reagents and association with other metaIs) and its heating temperature, materiaIs of different properties were prepared. Iron (III) hydroxoacetate gels were obtained through hydrogenolysis of iron nitrate or chloride with ammonia, followed by acetate and/or acetic acid sorption. The association with other metaIs was done by impregnation and coprecipitation methods. ln the first case, iron nitrate was hydrolised with ammonia in the presence of silica or alumina. ln the second case, iron and chromium nitrates, or iron and aluminum nitrates, were hydrolised simultaneously to produce mixed hydroxoacetates. lHA samples impregnated with chromium were prepared by chromium sorption in iron (III) hydroxide, followed by acetate sorption. AlI gels were dried in an oven at 120°C. The solids thus obtained were heated under nitrogen in the range of 150-420°C and underwent chemical and physical transformations. Magnetite formation was monitored by characterization of the solids, produced in several temperatures using the methods: Fe(lI), Fe(total) and carbon determination, infrared spectroscopy, surface area determination, thermal analysis (DSC and TGA), particle size distribution, X - ray diffraction, electron diffraction and transmission and scanning electron microscopy. The experimental results given in this work demonstrated that lHA produces magnetite via Fe(III) reduction by acetate and/or acetic acid; simultaneously it decomposes itself to give acetone and CO2. 0,04 mole of acetate / mole of iron was calculated as the minimum amount of acetate required to convert alI the lHA into magnetite. By heating lHA samples containing different acetate concentrations, magnetite was produced at temperatures around 400°C; below 360°C solids with acetate/iron = 0,04 and 0,25 give a mixture of hematite and magnetite , probably due to solid domains exempt of the reducing agent. In the other cases, only magnetite was formed. The use of iron chloride instead of iron nitrate does not modify the magnetite chemical and morphological characteristics. Heating of lHA formed in the presence of alumina yields magnetite non-uniformly deposited on the support. These particles are smaller and less crystalline than those prepared in the absence of alumina. On the other hand mixed aluminum and iron hydroxoacetate forms a non crystalline solid with high surface area (363 mg). Silica inhibits magnetite formation; silicates with ferric and ferrous ions were produced instead. By heatings IHA samples containing chromium, substoichiometric magnetite with high surface area was produced. Impregnation and coprecipitation methods give products with different cristallinity degrees, surface areas and particle sizes. Due to its morphological properties the solids produced in this work can probably be used as catalysts. The methods used in this work are good options for their preparation, instead of using the traditional fusion methods.

ASSUNTO(S)

oxido de ferro quimica do estado solido

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