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Conversão de xilanas-a-furfural na presença de sólidos ácidos mesoporosos

Dias, Ana Sofia Vagueiro de Sousa
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Tese de Doutorado
POR
Relevância na Pesquisa
17.612117%
O objectivo deste trabalho era encontrar catalisadores ácidos mesoporosos para a conversão das xilanas em furfural, que pudessem substituir o ácido sulfúrico usado no processo industrial. Esta conversão ocorre, em meio ácido, por hidrólise da xilana em xilose que, por sua vez, dá origem ao furfural por desidratação e ciclização. O desempenho destes catalisadores foi estudado na reacção da xilose, o passo limitante, em dimetilsulfóxido ou num sistema bifásico (água/solvente orgânico). Nos Capítulos 2 a 4, descreve-se a actividade catalítica dos heteropoliácidos (HPAs) do tipo Keggin em fase homogénea e heterogénea. O ácido dodetungstofosfórico (PW) foi o HPA mais interessante. O PW foi suportado em sílicas mesoporosas, obtendo-se materiais mais activos do que o PW livre e equiparáveis ao ácido sulfúrico. A lixiviação de PW durante a reacção resultou na perda de actividade nas reutilizações. A actividade de sais de césio de PW livres ou suportados em sílicas mesoporosas foi também estudada na reacção da xilose, não se tendo observado vantagens significativas destes materiais relativamente aos correspondentes de PW. No Capítulo 5 descreve-se a preparação e caracterização de sílicas mesoporosas e um material híbrido contendo grupos ácidos organo-sulfónicos imobilizados. O desempenho catalítico destes materiais foi superior ao obtido com zéolitos comerciais...

EELS study of Fe- or Co-doped titania nanosheets

Ohwada, Megumi; Kimoto, Koji; Ebina, Yasuo; Sasaki, Takayoshi
Fonte: Oxford University Press Publicador: Oxford University Press
Tipo: Artigo de Revista Científica Formato: text/html
EN
Relevância na Pesquisa
27.612117%
Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe3+ and Co2+ ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti4+ and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe3+ ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe2+ ions in the reduction process by electron irradiation. In contrast...

Estudo de titanatos nanoestruturados obtidos por tratamento hidrotérmico de óxido de titânio em meio alcalino; Studies on nanostructured titanates obtained by alkali hydrothermal treatment of titanium oxide

Morgado Júnior, Edisson
Fonte: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química Publicador: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química
Tipo: Tese de Doutorado Formato: application/pdf
POR
Relevância na Pesquisa
48.59166%
TiTanate NanoTubes (TTNT) were synthesized by hydrothermal alkali treatment of TiO2 anatase followed by repeated washings with distinct degrees of proton exchange. TTNT samples with different sodium contents were characterized, as synthesized and after heattreatment (200-800ºC), by X-ray diffraction, scanning and transmission electron microscopy, electron diffraction, thermal analysis, nitrogen adsorption and spectroscopic techniques like FTIR and UV-Vis diffuse reflectance. It was demonstrated that TTNTs consist of trititanate structure with general formula NaxH2−xTi3O7·nH2O, retaining interlayer water in its multiwalled structure. The removal of sodium reduces the amount of water and contracts the interlayer space leading, combined with other factors, to increased specific surface area and mesopore volume. TTNTs are mesoporous materials with two main contributions: pores smaller than 10 nm due to the inner volume of nanotubes and larger pores within 5-60 nm attributed to the interparticles space. Chemical composition and crystal structure of TTNTs do not depend on the average crystal size of the precursor TiO2-anatase, but this parameter affects significantly the morphology and textural properties of the nanostructured product. Such dependence has been rationalized using a dissolution-recrystallization mechanism...

Síntese e caracterização de catalisadores de óxido de ferro suportado em titanatos lamelares; Synthesis and characterization of layered titanates supported iron oxide catalysts

BRASIL, Valéria Bacelar
Fonte: Universidade Federal de Goiás; BR; UFG; Mestrado em Química; Educação em Química Publicador: Universidade Federal de Goiás; BR; UFG; Mestrado em Química; Educação em Química
Tipo: Dissertação Formato: application/pdf
POR
Relevância na Pesquisa
27.612117%
Sodium trititanate and silica - trititanate supported iron oxide catalysts were prepared by coprecipitation of iron ions and by the deposition of iron oxide nanoparticles previously prepared. The catalysts and the supports were characterized regarding their composition, structure, thermal behavior and textural properties. The amount of iron in catalysts ranged from 0.98% to 5.9%, and the lowest percentages were observed for catalysts prepared by deposition of nanoparticles. The catalysts and supports are macroporous with limited mesoporosity. The catalysts and supports were evaluated in the reaction of dehydrogenation of ethylbenzene for the production of styrene. During tests, the structure of the solids did not chang. Both the supports and the catalysts were active in the dehydrogenation of ethylbenzene in the absence of steam, allowing the economy of energy. The results of verification of the ionic exchange process/ adsorption of Cu+2, showed that the solids containing iron oxide in the composition presented results higher than the solids without iron oxide. The exception was observed for the solids that have acid surface.; Os catalisadores de óxido de ferro suportados em trititanato de sódio e trititanato com sílica foram preparados por coprecipitação de íons ferro e por deposição de nanopartículas de óxido de ferro previamente preparadas. Os catalisadores e os suportes foram caracterizados quanto à composição...

The swelling transition of lepidocrocite-type protonated layered titanates into anatase under hydrothermal treatment

Yuan, Huiyu; Besselink, Rogier; Liao, Zhaoliang; ten Elshof, Johan E.
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
Publicado em 03/04/2014 EN
Relevância na Pesquisa
38.18185%
The common facets of anatase crystals are the (001) and (101) planes. However, the phase transformation from lepidocrocite-type titanate into anatase by hydrothermal processing yields an anatase microstructure with high concentration of exposed (010) planes. The phase transformation of a lepidocrocite-type protonated layered titanate (HTO) into anatase was studied using XRD, TEM, FTIR, and measurement of pH and zeta potential. It was found that HTO is proton-deficient. The phase transformation process begins after uptake of a sufficient number of protons into the lepidocrocite-type structure. With the uptake of protons new hydroxyl groups form on the internal surfaces of the layered titanate and result in a bilayer state of HTO. The phase transformation reaction is a topotactic dehydration reaction in which anatase forms and water is expelled by syneresis.

Ferromagnetism induced by entangled charge and orbital orderings in ferroelectric titanate perovskites

Bristowe, N. C.; Varignon, J.; Fontaine, D.; Bousquet, E.; Ghosez, Ph.
Fonte: Nature Pub. Group Publicador: Nature Pub. Group
Tipo: Artigo de Revista Científica
Publicado em 25/03/2015 EN
Relevância na Pesquisa
27.986006%
In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting spontaneous spin alignment is a fairly rare behaviour, but once materialized is often associated with itinerant electrons in metals. Here we predict and rationalize robust ferromagnetism in an insulating oxide perovskite structure based on the popular titanate series. In half-doped layered titanates, the combination of Jahn–Teller and oxygen breathing motions opens a band gap and creates an unusual charge and orbital ordering of the Ti d electrons. It is argued that this intriguingly intricate electronic network favours the elusive inter-site ferromagnetic (FM) ordering, on the basis of intra-site Hund's rules. Finally, we find that the layered oxides are also ferroelectric with a spontaneous polarization approaching that of BaTiO3. The concepts are general and design principles of the technologically desirable FM ferroelectric multiferroics are presented.