Ressonância magnética nuclear aplicada ao estudo de óxidos de alumínio nanoestruturados dispersos em materiais carbonosos porosos
| Ano de defesa: | 2013 |
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| Autor(a) principal: | |
| Orientador(a): | |
| Banca de defesa: | |
| Tipo de documento: | Dissertação |
| Tipo de acesso: | Acesso aberto |
| Idioma: | por |
| Instituição de defesa: |
Universidade Federal do Espírito Santo
BR Mestrado em Física Centro de Ciências Exatas UFES Programa de Pós-Graduação em Física |
| Programa de Pós-Graduação: |
Não Informado pela instituição
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| Departamento: |
Não Informado pela instituição
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| País: |
Não Informado pela instituição
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| Palavras-chave em Português: | |
| Link de acesso: | http://repositorio.ufes.br/handle/10/7446 |
Resumo: | This work is devoted to the study of porous carbon materials containing dispersed particles of oxides or hydroxides of aluminum using a combination of X-ray diffraction (XRD) and solid-state 27Al nuclear magnetic resonance (NMR) spectroscopy. Such materials are of great interest for technological applications, including the removal of fluoride ions contained in water, the adsorption of ammonia from gaseous mixtures and also their use as catalysts supports. The samples were obtained using two types of carbon precursors: a commercial activated carbon (denoted as AC) and a char obtained by carbonization of the endocarp of babassu coconut at 700 ºC (denoted as BC), with specific surface areas of 1300 and 340 m2/g, respectively. The alumina-carbon composites were prepared by aqueous impregnation of AC or BC with aluminum nitrate (samples named AC_Al or BC_Al, respectively), using excess ammonium hydroxide as the precipitating agent. Samples were also synthesized by impregnation of AC with control of the pH (pH = 11) of the reaction (sample named AC_Al_pH11). Additionally, another sample was synthesized under the same conditions but without any carbonaceous matrix (sample named Hid_Al_pH11). The as-prepared powders were heat-treated under an inert atmosphere, up to the temperature of 1500 ° C. Solid-state 27Al NMR was valuable for the characterization of the shortrange order occurring in the aluminum-containing phases present in the studied composites. The aluminum compounds formed in the as-synthesized AC_Al and AC_Al_pH11 samples were identified by XRD and solid-state 27Al MAS NMR as nanocrystalline aluminum oxyhydroxides or hydroxides. On the other hand, all aluminum-containing phases were X-ray amorphous in the as-synthesized BC_Al samples, with the presence of a distribution of AlO6 (octahedral Al site), AlO5 and AlO4 (tetrahedral Al site) units revealed xvii xviii by solid-state 27Al NMR. The quadrupole coupling parameters, isotropic chemical shifts and relative fractions of each component were determined by fitting the spectra to a set of second-order quadrupolar lineshapes broadened by a distribution of quadrupole couplings and chemical shifts associated with disorder in the 27Al environments. In the case of the AC_Al and AC_Al_ph11 samples, heat treatment at 400 ºC caused the thermal degradation of the hydroxides and oxyhydroxides and the development of X-ray amorphous or nanocrystalline transition aluminas. As for the BC_Al samples, no crystalline phase was detected up to 1000 ºC. For all samples, the relative fractions of AlO4, AlO5 and AlO6 sites changed as a function of the heat-treatment temperature, with a trend of increase of the AlO6 contribution at high temperatures associated with the formation of the thermally stable corundum (a-Al2O3) phase, which was fully developed at 1500 ºC. These results showed that the nature of the carbon material used to prepare the composites was of great relevance for the definition of the type and crystallinity of the alumina phases formed after heat treatments. Depending on the type of carbon matrix, the detailed synthesis conditions and the heat-treatment temperature, alumina-carbon composites containing either nanocrystalline or amorphous alumina phases were produced |