Percorrer por autor "Miranda, Sandra"
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- Activated carbons functionalized with thiol and sulfonic acid groups for catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Silva, Adrián; Faria, JoaquimActivated carbons are known catalysts for hydrogen peroxide decomposition through a pathway involving the formation of hydroxyl radicals. Thus, it seems logical to take advantage of the strong oxidizing properties of these radicals for the degradation of organic pollutants adsorbed and concentrated on the surface of activated carbons. In a recent work, we established the unprecedented performance of acidic activated carbons functionalized with sulphuric acid in the catalytic wet peroxide oxidation of Chromotrope 2R, an anionic azo dye. In the present work, we explore the influence of the amounts and types of surface groups with sulphur on the catalytic efficiency of the activated carbons.
- Activated carbons treated with sulphuric acid: catalysts for catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Silva, Adrián; Faria, JoaquimDifferent liquid phase thermal treatments were applied to a commercial activated carbon (Norit ROX 0.8) in order to produce modified activated carbons with varying surface chemistry and increased acidic character. Chemical characterization of the prepared materials includes determination of the point of zero charge and evaluation of the concentration and nature of acidic and basic surface functionalities by acid/base titrations and temperature programmed desorption. The prepared materials were used as catalysts in the catalytic wet peroxide oxidation of the acid dye Chromotrope 2R in order to assess their removal efficiency. The relationship between the surface chemistry and efficiency for dye removal is discussed. As expected, decreasing acidity of the catalysts surface will correlate with increasing dye conversion. Unexpectedly, treatment with sulphuric acid leads to a very high yield of dye removal which falls out of the previous correlation. This was explained in terms of the introduction of sulphur containing groups on the carbon surface, which promotes the surface interaction between the pollutant and hydrogen peroxide: higher production of hydroxyl radicals close to the pollutant leads to improved dye removal. In addition, reutilization studies show that the catalyst prepared by sulphuric acid treatment is able to keep its performance in successive runs.
- Activated carbons treated with sulphuric acid: catalysts for catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Silva, Adrián; Faria, JoaquimThe commercial activated carbon Norit ROX 0.8 was subjected to different liquid phase treatments in order to produce activated carbons with varying surface chemistry and acid/base character. The prepared materials were used as catalysts in the catalytic wet peroxide oxidation of the acid dye Chromotrope 2R in order to assess their removal activity. Chemical characterization of the prepared materials includes determination of the point of zero charge and evaluation of the concentration and nature of acidic and basic surface functionalities by acid/base titrations and temperature programmed desorption. The relationship between the surface chemistry and efficiency for dye removal is discussed. In general, dye removal is increased when using catalysts with higher basic character, due to enhanced electrostatic attraction between the acid dye and the catalyst surface when compared with catalysts presenting acidic character. In addition, treatment with sulphuric acid introduces on the catalysts surface sulphur containing groups, which promotes the surface interaction between the pollutant and hydrogen peroxide. This leads to improved dye removal resulting from higher production of hydroxyl radicals close to the pollutant. Reutilization studies show that the catalyst prepared by sulphuric acid treatment is able to keep its performance in successive runs.
- Mechanistic insights into catalytic wet peroxide oxidation over activated carbons treated with sulfuric acidPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Figueiredo, José; Silva, Adrián; Faria, Joaquim
- The role of activated carbons functionalized with thiol and sulfonic acid groups in catalytic wet peroxide oxidationPublication . Gomes, Helder; Miranda, Sandra; Sampaio, Maria; Figueiredo, José; Silva, Adrián; Faria, JoaquimNorit ROX 0.8 commercial activated carbon was subjected to liquid phase treatments with sulfuric acid at different concentrations (5-18 M) and temperatures (353 and 423 K), to generate different amounts (760-1000 mu mol g(-1)) of sulfur groups (thiol and sulfonic acid), with varying surface acidity. The prepared materials were chemically characterized by the determination of the point of zero charge (2.0-5.8) and by mass titration, temperature programmed desorption and X-ray photoelectron spectroscopy, to evaluate the nature and concentration of acidic surface functionalities. The materials were tested as catalysts in the catalytic wet peroxide oxidation of aqueous solutions of the acid dye Chromotrope 2R. A correlation between the surface acidic strength and the efficiency for dye removal from solution was found, which was ascribed both to adsorption and to reaction. In general, dye removal by adsorption is increased when using adsorbents with lower acidic character, due to enhanced electrostatic attraction between the acid dye and the catalyst surface. In addition, the introduction of sulfur containing groups promotes the decomposition of hydrogen peroxide, leading to increased production of hydroxyl radicals close to the pollutant and subsequent improved dye degradation.
- Tratamento de efluentes corados por oxidação catalítica com peróxido de hidrogénioPublication . Miranda, Sandra; Gomes, HelderO principal objectivo deste trabalho consistiu no desenvolvimento de catalisadores heterogéneos sem metal impregnado (carvões activados com química superficial variável), para o tratamento de efluentes corados por oxidação química com peróxido de hidrogénio. O carvão activado Norit Rox 0.8 foi utilizado como material de partida e submetido a vários tratamentos químicos (com ácido nítrico, com peróxido de hidrogénio e com ácido sulfúrico) para obter materiais com diferentes propriedades superficiais. Da caracterização química efectuada verifica-se que o carvão activado tratado com ácido nítrico apresenta uma maior concentração de ácidos carboxílicos e fenóis, conferindo-lhe por contacto directo uma elevada acidez. Os carvões tratados com ácido sulfúrico apresentam uma grande concentração de grupos contendo enxofre. Os catalisadores produzidos foram testados na degradação de soluções aquosas de corante Chromotrope 2R por oxidação catalítica com peróxido de hidrogénio num reactor batch, definindo-se como condições padrão, uma temperatura de 323 K, um pH de 3, uma concentração de corante de 100 mg/L, uma concentração de peróxido de hidrogénio de 34.6 mM e uma concentração de catalisador de 0.5 g/L. Os melhores resultados foram obtidos com os carvões activados tratados com ácido sulfúrico, que permitiram uma remoção de cor entre 30 a 37% após 150 min de reacção. A estabilidade do catalisador mais promissor, avaliada pela realização de ensaios consecutivos com o material recuperado após reacção, mostrou possuir características adequadas para a sua utilização em processos contínuos. As variáveis pertinentes para o processo foram também estudadas, verificando-se que as condições óptimas do processo que permitem uma maior remoção de corante, dentro da gama de valores estudados são: concentração de peróxido de hidrogénio de 17.3 mM, concentração de catalisador de 2 g/L e tamanho de partícula entre 0.106 e 0.250 mm. Nestas condições é possível uma remoção de corante de cerca de 99% após 150 min de reacção. The main objective of this work consisted on the development of heterogeneous catalysts without impregnated metals (activated carbons with variable surface chemistry), for the treatment of wastewater by chemical oxidation with hydrogen peroxide. Activated carbon Norit Rox 0.8 was used as a starting material and submitted to several chemical treatments (nitric acid, hydrogen peroxide and sulphuric acid) to obtain materials with different surface properties. From chemical characterization it was observed that the activated carbon treated with nitric acid by direct contact presents the highest concentration of carboxylic acids and phenols, justifying its higher acidic character, when compared with the other materials. Activated carbons treated with sulphuric acid presents a large concentration of groups containing sulphur. The produced catalysts were tested in the catalytic wet peroxide oxidation of the acid dye Chromotrope 2R in order to assess their removal activity. The reaction was performed in batch reactor, considering as test conditions the following: temperature of 323 K, pH = 3, concentration of Chromotrope 2R of 100 mg/L, concentration of hydrogen peroxide of 34,6 mM and concentration of catalyst of 0.5 g/L. The best results were obtained with activated carbons treated with sulphuric acid, which allowed a colour removal between 30 and 37% after 150 min of reaction. The stability of the most promising catalyst, evaluated by successive experiments with recovered material after reaction, showed that it is able to keep the reaction removal efficiency at least up to four runs The most relevant variables for this process were also studied, showing that the optimal process conditions that allow a higher dye removal, in the range of studied values are: concentration of hydrogen peroxide of 17.3 mM, concentration of catalyst of 2 g/L and particle size from 0.106 to 0.250 mm. With these conditions it is possible to achieve a dye removal of almost 99% after 150 min of reaction.