Materiais nanoestruturados magnéticos à base de ferro recobertos por carbono: síntese, caracterização e aplicações

Abstract

In this work it was investigated the preparation and characterization of materials containing magnetic nanostructures based on iron coated with carbon layer and/or filaments and carbon nanotubes by a simple and low cost CVD process. Temperature Programmed Reaction (TPRe) was used to monitor the process. The carbon sources studied were methane (CH4) and ethanol (CH3CH2OH). The materials investigated as precursors of magnetic phases were synthetic hematite (-Fe2O3), ferrites of Co, Cu and Ni (CoFe2O4, CuFe2O4 and NiFe2O4), the supported mixed oxide Ca2Fe2O5/CaO and the iron rich industrial waste red mud. It was also prepared a carbon matrix containing magnetic iron nanoparticles by the pyrolysis of Fe3+ dispersed in sucrose. Preliminarytests in environmental and catalytic applications as magnetic adsorbents of dye and catalyst support were performed for most of the materials produced. Initially, methane and ethanol were tested in reactions with synthetic hematite (-Fe2O3), the most common iron oxide. The reaction between methane and hematite between 600-700oC essentially formed magnetite (Fe3O4) and at higher temperatures magnetite and wustite (Fe1-xO) in different proportions. The results suggested the formation of 4 % carbon after reaction with methane and the presence of particlessmaller than 200 nm in some of the materials produced. Better results were found for the ethanol reaction with hematite with higher carbon contents mainly as filaments (up to 44 % of carbon for reactions above 700oC). After TPRe with ethanol at 600oC, all hematite converts to magnetite, wustite and metallic iron (Fe0). At higher temperatures,the composition is basically formed by iron carbide phases (Fe3C) and metallic iron in different proportions. The carbon activation with CO2 showed an increase of 65 % in the surface area. It was also observed the presence of particles smaller than 200 nm in some of the material produced. The reaction of ethanol was used to modify the waste red mud. The results showed that the TPRe of ethanol with the waste provides the formation of metallic iron cores covered by carbon deposits in the form of nanofilaments and suggested the presence of particles smaller than 50 nm. The carbon content estimated for the materials was between 7 and 35 %, approximately. In the next step it was studied the use of the ferrites CoFe2O4, CuFe2O4 and NiFe2O4. The results showed the formation of reduced phases such as -Fe for CuFe2O4, Ni-Fe and Fe-Co for the ferrites NiFe2O4 and CoFe2O4, respectively, and the presence of phases Fe3C and -Fe(C). The carbon contents ranged between 37 and 45 %for different ferrites prepared. It was observed in general large amounts of carbon filaments and abundant presence of magnetic particles smaller than 100 nm. The next Fe precursor investigated in reactions with ethanol was the supported mixed oxide formed by Ca2Fe2O5/CaO. At this stage the objective was to study the influence of using the support in the formation of magnetic particles. After reaction with ethanol it was found the formation of the phases Fe0 Fe3C, CaCO3, Ca(OH)2 andCaFe2O4 besides carbon in the form of irregular nanofilamentos containing a series of iron particles with dimensions of ca. 42 nm. One advantage of this material is that the magnetic nanofilaments can be separated from the support by simple dissolution with acid treatment.Finally, it was investigated a different process to produce magnetic nanoparticles based on the pyrolysis of an iron salt containing sucrose. The analyses showed that the thermal treatment at 400oC produced mainly magnetite, besides carbon, whereas treatments at higher temperatures such as 600 and 800oC produces the formation ofphases such as metallic iron and iron carbide. Particles smaller than 50 nm encapsulated by carbon were identified. Different carbon contents were found, according to the sample with values between 61 and 95 %.