The study aimed to assess how the crystallization and ageing times in SBA-15 synthesis influence the reproducibility and particle size of mesoporous molecular sieves. X-ray diffraction showed that higher crystallization times provide greater reproducibility of mesoporous silica, due to the fact that there is no significant variation in the width and intensity of peaks, while the same is not true for shorter crystallization times. Ageing time does not seem to exercise a significant influence when higher crystallization times are used in the synthesis. Scanning electron microscopy observations revealed that shorter crystallization times led to larger crystals in addition to the formation of large agglomerates, whereas with higher crystallization times the trend is to obtain smaller and more uniform grains. It is proposed that hydrogen bonds may be responsible for the formation of smaller particles and it is also established that depending on the synthesis conditions the reproducibility of the process can become low.
O presente artigo apresenta um recorte de uma pesquisa maior acerca das contribuições de uma intervenção didática desenvolvida, por uma professora participante do Programa de Desenvolvimento Educacional (PDE), junto a 48 estudantes da 3ª série do Ensino Médio de uma escola estadual de Maringá-PR. Cerca de um mês após a realização das atividades os alunos foram convidados a dizer o que lhes chamou mais atenção no período de estudos sobre o tema. As respostas dos alunos foram tratadas pela Análise Textual Discursiva, e revelaram a presença de elementos da perspectiva CTSA, no que diz respeito a compreensão de aspectos químicos relacionados à produção dos Plásticos; e o despertar da consciência social e ambiental à medida que relacionaram o tema à questões científicas e tecnológicas.
In the present work, cobalt nanoparticles were synthesised in the range of 4 to 72 nm through the chemical reduction method, in which cobalt nitrate, oleic acid and sodium borohydride were used as the precursor, stabiliser and reducing agent, respectively. The syntheses were carried out by varying the synthesis parameters, such as the amount of reducing agent, amount of stabiliser and turbulence level of the reaction medium, with the aim of evaluating their influence on the control of cobalt nanoparticle size distribution. The nanoparticles size decrease with high concentrations of reducing agent and stabiliser, due to an increase in the reaction rate and viscosity of the medium, respectively. It was also verified that the high level of turbulence used during the synthesis could provide anomalous diffusion of cobalt nanoparticles in the reaction medium. Under this condition, the growth kinetics may vary significantly with regards to two distinct reaction paths, causing opposite growth behaviour in relation to the synthesis occurring in the normal distribution of the nanoparticles.
Resumo Neste artigo trazemos os resultados de uma investigação que objetivou elaborar uma unidade didática temática (UDT) acerca dos plásticos, tendo como eixo norteador os pressupostos do Pensamento Crítico (PC) e sua posterior aplicação em uma turma do Ensino Médio. Após a aplicação, investigou-se o potencial deste material em mobilizar as capacidades do PC nos estudantes e quais foram seus possíveis indicadores. Este artigo está centrado na releitura e reorganização de dados oriundos da aplicação da UDT, delimitando-nos na análise de uma etapa que intentava mobilizar a capacidade de fazer e responder a questões de clarificação. Tal processo conduziu-nos a assumir duas categorias a priori, intituladas Fatos e Exemplos e a emergência de 19 subcategorias, as quais foram compreendidas como indicadores da mobilização da capacidade do PC. A UDT revelou possuir potencial em mobilizar tal capacidade do PC em razão do material didático-pedagógico ter sido intencionalmente planejado para este fim.
In the present work, the synthesis of Co/SBA-15 catalysts intended for Fischer-Tropsch (FT) synthesis was performed. During the synthesis of catalysts there was contamination of samples with sodium nitrate due to the nature of the reducing agent used for the synthesis of metallic phase. This kind of impurity is not advantageous for the FT reaction. Attempts at removal of sodium compounds were carried out by means of simple leaching treatments, using ethanol and acid ethylenediaminetetraacetic (EDTA) as solvent and complexing agent, respectively, followed by heat treatment at temperatures of 150 or 300°C. It was possible to conclude that the treatment using EDTA was more effective in removing almost all the alkaline phase of samples, despite the occurrence of oxidation, agglomeration, and removal of the metal nanoparticles in this process. In addition, there were no significant differences in the product selectivity of FT synthesis of the catalysts after sodium removal, although the nanoparticles were larger than 10 nm.
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