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Nanoporous carbons from sisal residues and their application in hybrid TiO2/carbon photocatalysts for the removal and degradation of phenol in solution
Publication . Andrade, Marta Amaral; Carvalho, Ana Paula Baptista de, 1959-; Ovín Ania, Maria Concepción
The main aim of this PhD thesis was the development of nanoporous adsorbents and catalysts using industrial by-products of low commercial value as precursors and to investigate their application in advanced remediation technologies for the removal and/or degradation of aromatic pollutants in aqueous solution. The first stage of this study consisted on investigating the valorization of rope industry´s wastes through the analysis of their pyrolytic behaviour. The thermal characterization of the residues confirmed that although most decomposition occurs at 400 ◦C, some pyrolytic reactions take place above 550 ◦C. The yields of the different fractions were 22 wt. % of a carbonaceous residue (char), 50 wt. % tars and a gas fraction at 800 ◦C. From the analysis of the different fractions, it was possible to conclude that the produced oil was rich in hydrocarbons and alcohols, while the gas fraction is mainly composed of CO2, CO and CH4, and the carbonaceous solid residue displayed somewhat porous features, with a more developed porous structure as the pyrolysis temperature increased. Further on, the synthesis of copper-doped activated carbons from sisal residues was explored, using a wet impregnation and low temperature calcination procedure. The incorporation of copper was also performed in a bituminous coal for comparison purposes. The role of copper on the physicochemical and structural features of the materials has shown to be strongly dependent on the nature of the carbon matrix. The dual role of copper on the reactivity of the carbons was observed; on one hand, favouring the development of microporosity in the case of the coal-derived activated carbon, due to the catalysed air gasification of the material at a very low temperature (i.e., 325 °C); on the other hand, the immobilization of copper on the sisal-derived carbon, acted as a combustion retardant during the calcination step, protecting the carbon matrix. In both cases, a homogenous distribution of copper within the carbon matrix, and a good preservation of large textural properties were observed. The incorporation of copper on the carbon material was also carried out through a different approach, via impregnation of the carbon precursor followed by activation. This allowed to obtain carbon materials displaying a well-developed nanoporous texture (although comparatively with a marked inhibition of the textural development), and a homogeneous dispersion of copper particles, predominantly as Cu(II) species. These materials were used in the photocatalytic degradation of phenol from solution under visible light, as hybrid titania/Cu-carbon composites. The photo-oxidation tests showed the outstanding role of copper under visible light, in terms of increased phenol conversion, mineralization degree and degradation rate. Similar overall conversions were obtained with half of the amount of the photoactive semiconductor (1:1 composites). The beneficial effect of copper loading was also observed in the marked regioselectivity of the intermediates, towards the preferential formation of catechol. Furthermore, the copper-loaded photocatalyst was found to be stable upon long irradiation exposure. The nanoporous carbons prepared from the activation of sisal wastes were also used as additives to TiO2 powders. The incorporation of the carbon material in the formulation of the photocatalyst (TiO2/carbon) proved to increase the photocatalytic performance of TiO2 regardless the studied illumination conditions, although the effect was more pronounced at λ > 200 nm. The photocatalytic runs performed using the carbon alone as catalyst confirmed a certain level of self photoactivity under different irradiation conditions (λ > 200 nm or λ > 360 nm). An evident deactivation of the carbon photocatalyst was observed after 60 min of irradiation, most likely due to the consumption of the photoactive sites. As for the photo-oxidation mechanism for the carbon component, a marked regioselectivity towards the ortho-substitution was also observed at high energy photons, confirming the strong effect of both the composition of the catalyst and the illumination conditions on the nature of the degradation intermediates of phenol.
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Fundação para a Ciência e a Tecnologia
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SFRH
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SFRH/BD/71673/2010