Browsing by Author "Freitas, Isabel Cristina de"

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    Combining active phase and support optimization in MnO2-Au nanoflowers : enabling high activities towards green oxidations.
    (2018) Silva, Anderson Gabriel Marques da; Rodrigues, Thenner Silva; Candido, Eduardo Guimarães; Freitas, Isabel Cristina de; Silva, Alisson Henrique Marques da; Fajardo, Humberto Vieira; Balzer, Rosana; Gomes, Janaina Fernandes; Assaf, Jose Mansur; Oliveira, Daniela Coelho de; Oger, Nicolas; Paul, Sebastien; Wojcieszak, Robert; Camargo, Pedro Henrique Cury
    Among the several classes of chemical reactions, the green oxidation of organic compounds has emerged as an important topic in nanocatalysis. Nonetheless, examples of truly green oxidations remain scarce due to the low activity and selectivity of reported catalysts. In this paper, we present an approach based on the optimization of both the support material and the active phase to achieve superior catalytic performances towards green oxidations. Specifically, our catalysts consisted of ultrasmall Au NPs deposited onto MnO2 nanoflowers. They displayed hierarchical morphology, large specific surface areas, ultrasmall and uniform Au NPs sizes, no agglomeration, strong metal-support interactions, oxygen vacancies, and Auδ+ species at their surface. These features led to improved performances towards the green oxidations of CO, benzene, toluene, o-xylene, glucose, and fructose relative to the pristine MnO2 nanoflowers, commercial MnO2 decorated with Au NPs, and other reported catalysts. We believe that the catalytic activities, stabilities, and mild/green reaction conditions described herein for both gas and liquid phase oxidations due to the optimization of both the support and active phase may inspire the development of novel catalytic systems for a wealth of sustainable transformations.
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    Sub-15 nm CeO2 nanowires as an efficient nonnoble metal catalyst in the room-temperature oxidation of aniline.
    (2018) Silva, Anderson Gabriel Marques da; Batalha, Daniel Carreira; Rodrigues, Thenner Silva; Candido, Eduardo Guimarães; Luz, Sulusmon Cesar; Freitas, Isabel Cristina de; Fonseca, Fabio Coral; Oliveira, Daniela Coelho de; Taylor, Jason Guy; Torresi, Susana Ines Cordoba de; Camargo, Pedro Henrique Cury; Fajardo, Humberto Vieira
    We described herein the facile synthesis of sub-15 nm CeO2 nanowires based on a hydrothermal method without the use of any capping/stabilizing agent, in which an oriented attachment mechanism took place during the CeO2 nanowire formation. The synthesis of sub-15 nm CeO2 nanowires could be achieved on relatively large scales (∼2.6 grams of nanowires per batch), in high yields (>94%), and at low cost. To date, there are only a limited number of successful attempts towards the synthesis of CeO2 nanowires with such small diameters, and the reported protocols are typically limited to low amounts. The nanowires displayed uniform shapes and sizes, high surface areas, an increased number of oxygen defects sites, and a high proportion of Ce3+/Ce4+ surface species. These features make them promising candidates for oxidation reactions. To this end, we employed the selective oxidation of aniline as a model transformation. The sub-15 nm CeO2 nanowires catalyzed the selective synthesis of nitrosobenzene (up to 98% selectivity) from aniline at room temperature using H2O2 as the oxidant. The effect of solvent and temperature during the catalytic reaction was investigated. We found that such parameters played an important role in the control of the selectivity. The improved catalytic activities observed for the sub-15 nm nanowires could be explained by: i) the uniform morphology with a typical dimension of 11 ± 2 nm in width, which provides higher specific surface areas relative to those of conventional catalysts; ii) the significant concentration of oxygen vacancies and high proportion of Ce3+/Ce4+ species at the surface that represent highly active sites towards oxidation reactions; iii) the crystal growth along the (110) highly catalytically active crystallographic directions, and iv) the mesoporous surface which is easily accessible by liquid substrates. The results reported herein demonstrated high activities under ambient conditions, provided novel insights into selectivities, and may inspire novel metal oxide-based catalysts with desired performances
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    Synergistic effect between CeO2 nanowires and gold NPs over the activity and selectivity in the oxidation of thioanisole.
    (2021) Rosado, Taíssa F.; Teixeira, Moisés Paulo; Moraes, Leonardo César de; Silva, Leonardo A. da; Silva, Augusto V. Pontes; Taylor, Jason Guy; Freitas, Isabel Cristina de; Oliveira, Daniela Coelho de; Gardener, Jules; Solorzano, Guilhermo; Alves, Tiago Vinicius; Venancio, Mateus Fernandes; Silva, Maria Isabel Pais da; Brocchi, Eduardo de Albuquerque; Fajardo, Humberto Vieira; Silva, Anderson Gabriel Marques da
    Gold nanoparticles incorporated on ceria nanowires have been employed as efficient nanocatalysts for the se lective oxidation of thioanisole. The control of both physical and chemical parameters as well as metal–support interactions are important factors that determine their performances. Considering their one-dimensional morphology with large surface area, thin diameters, high concentration of oxygen vacancies, and small Au NPs uniformly on the entire CeO2 surface with a high fraction of oxidized gold species, these characteristics make them favorable nanocatalysts for oxidation transformations. The CeO2-Au nanowires displayed improved per formances towards the oxidation of thioanisole when compared to the pure CeO2 nanowires and commercial CeO2-Au catalysts. The CeO2-Au nanowires catalyzed the selective synthesis of methyl phenyl sulfoxide with up to 100 % of selectivity and high conversion. The impact of solvent and temperature during the catalytic reaction was also experimentally and theoretically investigated by DFT calculations, indicating a key role in the observed activities.