Please use this identifier to cite or link to this item: http://www.repositorio.ufc.br/handle/riufc/13323
Title in Portuguese: Estudo de compostos lamelares contendo Fe usando Espectroscopia Mössbauer de 57Fe e técnicas complementares
Author: Gouveia, Daniel Xavier
Advisor(s): Mendes Filho, Josué
Co-advisor(s): Paiva, José Airton Cavalcante de
Keywords: Espectroscopia de Mossbauer
Issue Date: 2006
Citation: GOUVEIA, D. X. Estudo de compostos lamelares contendo Fe usando espectroscopia Müssbauer de 57Fe e tecnicas complementares. Fortaleza, 2006. 119 f. Tese (Doutorado em Física) - Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2006.
Abstract in Portuguese: As propriedades estruturais e de decomposição térmica das hidrotalcitas Mg-Fe e Co-Cu-Fe foram estudadas através de espectroscopia Mössbauer de 57Fe, análise termogravimétrica, difração de raios X, e espectroscopia de absorção no infravermelho (FTIR). No sistema Mg-Fe a destruição da estrutura lamelar ocorre em torno de 300 oC. O alargamento dos picos de difração de raios X observados nos difratogramas sugerem que os óxidos resultantes constituem uma solução sólida. Para as amostras tratadas em temperaturas maiores do que 500 oC a formação de fases do tipo MgO e MgFe2O4 é observada. A espectroscopia Mössbauer de 57Fe foi empregada para monitorar o ambiente químico do Fe na faixa 100-900 oC de temperatura. As medidas in situ de difração de raios X revelam que em 180 oC inicia-se uma contraçãoo interlamelar. Este fenômeno é atribuído ao processo de "grafting"no qual os ânions interlamelares ligam-se às camadas através de uma ligação covalente. A reconstrução estrutural da hidrotalcita Mg-Fe também foi investigada. A eficiência da reconstrução estrutural depende da temperatura de tratamento e da razão molar Mg/Fe. A estrutura das amostras reconstruídas são as mesmas da amostra inicial. As medidas in situ de espectroscopia Mössbauer de 57Fe foram realizadas na faixa 100-500 oC confirmaram uma desordem estrutural crescente nesta regi~ao de temperaturas. Os valores do desdobramento quadrupolar indicam que o máximo de desordem ocorre em 300 oC. Com relação ao sistema ternário Co-Cu-Fe observamos que devido ao efeito Jahn-Teller o sistema Cu-Fe somente é estabilizado na presença de Co2+. Para baixas concentrações Co2+ fases adicionais segregadas são observadas nos sólidos. Os padrões de difração de raios X indicam a presença de Cu(OH)2 e CuO. O processo de decomposição térmica foi investigado através de difração de raios X, espectroscopia Mössbauer de 57Fe in situ e de espectroscopia de absorção no infravermelho (FTIR). Aumentando a temperatura de tratamento térmico das amostras de 25 oC a 180 oC observamos um aumento da desordem estrutural. Este efeito tem sido atribuído a oxidação Co2+ para Co3+ uma vez que a decomposição foi realizada ao ar. Parte dos cátions Co3+ migram para a região interlamelar formando um composto metastável que ainda possui uma estrutura lamelar. O colapso da estrutura lamelar é observado a 300 oC. Com o aumento posterior da temperatura o sistema torna-se mais cristalino e a formação de Co3O4 é observada através do ensio de raios X. Nas hidrotalcitas com maior teor de Cu, alguns dos ânions carbonato são liberados somente acima de 550 oC sendo este fenômeno atribuído a formação de uma fase rica em carbonato. Os valores de área superficial específica apresentam um máximo na faixa de temperatura onde ocorre o colapso da estrutura lamelar.
Abstract: The structural and thermal decomposition properties of Mg-Fe and Co-Cu-Fe hy-drotalcites (HT) have been studied through thermogravimetric analysis, X ray powder diffraction (XRD), Fourier transform infrared spectroscopy and 57Fe MÄossbauer spectroscopy. In the Mg-Fe system, the destruction of the layered structure took place at about 300 oC. The broad peaks observed in the X ray diffractograms suggests that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500 oC the formation of the MgO and MgFe2O4 formation of the MgO and MgFe2O4 spinel phases is observed. 57Fe MÄossbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900 oC). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180 oC. This phenomenon is identifed as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the Mg-Fe HTs was also investigated and its e±ciency depends on the thermal annealing temperature and on the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure. The in situ 57Fe MÄossbauer experiments were performed in the 100-500 oC temperature range con¯rm an increasing structural disorder in this temperature range. The quadrupolar splitting indicates that the maximum disorder occurs at 300 oC. Regarding the Co-Cu-Fe ternary system we have observed that due to the strong Jahn-Teller e®ect the Cu-Fe layered system is stabilized only in the presence of Co2+. At low Co2+ contents, additional phases are segregated in the solids. X ray patterns di®raction The structural and thermal decomposition properties of Mg-Fe and Co-Cu-Fe hydrotalcites (HT) have been studied through thermogravimetric analysis, X ray powder diffraction (XRD), Fourier transform infrared spectroscopy and 57Fe Mossbauer spectroscopy. In the Mg-Fe system, the destruction of the layered structure took place at about 300 oC. The broad peaks observed in the X ray diffractograms suggests that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500 oC the formation of the MgO and MgFe2O4 formation of the MgO and MgFe2O4 spinel phases is observed. 57Fe Mossbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900 oC). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180 oC. This phenomenon is identified as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the Mg-Fe HTs was also investigated and its efficiency depends on the thermal annealing temperature and on the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure. The in situ 57Fe Mossbauer experiments were performed in the 100-500 oC temperature range confirm an increasing structural disorder in this temperature range. The quadrupolar splitting indicates that the maximum disorder occurs at 300 oC. Regarding the Co-Cu-Fe ternary system we have observed that due to the strong Jahn-Teller effect the Cu-Fe layered system is stabilized only in the presence of Co2+. At low Co2+ contents, additional phases are segregated in the solids. X ray patterns diffraction show the presence of Cu(OH)2 and CuO. The decomposition process was investigated by in situ X ray, in situ Mossbauer and FTIR experiments. By increasing the temperature from 25 oC up to 180 oC we observed that the structural disorder increases. This effect has been likely attributed to the Co 2+ to Co3+ oxidation since thermal decomposition was carried out under static air atmosphere. Part of the Co3+ cations could migrate to the interlayer region, thus forming a metastable compound that still has a layered structure. Collapse of the layered structure was observed at about 200 oC. By further increasing the temperature the system becomes more crystalline and the formation of Co3O4 is observed in the X ray patterns. In Cu-rich HT, some of the carbonate anions are released at temperatures higher than 550 oC and this phenomenon is attributed to the formation of a carbonate-rich phase. The specific surface area data present its highest values in the temperature range where the collapse of the layered structure takes place.
URI: http://www.repositorio.ufc.br/handle/riufc/13323
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