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Preparação e Modificação de Materiais de Carbono para o Controlo das Emissões de NOx
| Content Provider | Semantic Scholar |
|---|---|
| Author | Sousa, Juliana P. S. |
| Copyright Year | 2013 |
| Abstract | Nitrogen oxides are involved in several environmental pollution phenomena, therefore the development of techniques for treating the gaseous effluents from combustion processes is important. Existing processes for the removal of NO, in addition to causing many problems, use expensive catalysts, which hinders their commercialization. Carbon materials are relatively inexpensive and have revealed significant activity at low temperatures, thus being an interesting alternative to the traditional catalysts. The surface chemistry is a key factor in the performance of carbon materials to control emissions of NO, in particular the nitrogen functionalities which show a promoting effect is on the removal of NO. The main objective of this work is to study the oxidation of NO on carbon materials with different textural and chemical properties. The surface chemistry will be studied in detail, in order to optimize the performance of the materials and to clarify the reaction mechanisms and NOx-carbon interactions. In the first part of the work, carbon materials (activated carbons and carbon nanotubes) were submitted to a series of treatments for the introduction of oxygen and nitrogen surface groups. Subsequently, a new series of materials was prepared, including carbon xerogels and activated carbon fibers, in which the nitrogen functional groups were introduced during the synthesis process. In order to evaluate the effect of nitrogen on the catalytic activity of the carbon materials, carbon xerogels without nitrogen were also prepared. The materials were extensively characterized by different techniques (N2 adsorption at -196 °C, temperature programmed desorption, pH of point of zero charge, X-ray photoelectron spectroscopy, scanning electron microscopy, elemental analysis, proximate analysis and selective neutralization), in order to correlate their performances with the respective chemical and textural properties. The prepared carbon materials were applied on adsorption and oxidation of NO. In the NO adsorption experiments, performed at room temperature without oxygen, NO practically was not adsorbed on carbon materials. In the oxidation reactions of NO at 25°C, it was found that the oxygen concentration is a very important parameter. The highest NO conversions were obtained in experiments carried out with 10% oxygen. The samples CX-5.3-900oC, CXE-900oC and CXEU-900oC showed the highest conversion of NO: 98%, 96% and 94%, respectively. Carbon fibers and carbon nanotubes showed, in general, the lowest NO conversions. Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com) The incorporation of nitrogen increased significantly the catalytic activity of carbon materials in the oxidation of NO. In the case of carbon xerogels, it was found that the nitrogen-containing surface groups that most contribute to their catalytic activity are the pyridine groups in combination with the pyrrole groups. With carbon materials without nitrogen, it was found that NO conversion increases with the surface area. The textural properties contribute to the catalytic activity of carbon materials provided that their surface is not submitted to any chemical modification, and in this situation the most important textural property is the mesopore surface area. The catalytic oxidation of NO is favored at low temperatures. With the increase of the reaction temperature the carbon materials lose their catalytic activity. In the long term and reutilization experiments, it was found that the changes of the chemical and textural properties of the catalyst, as a result of the reaction, did not cause any deactivation. The proposed mechanism for the oxidation of NO consists on the adsorption of oxygen on the surface of carbon materials, followed by the reaction of NO with the adsorbed oxygen producing adsorbed NO2, which is subsequently desorbed. It was found that the NO2 produced is retained in the micropores of carbon materials. In materials without micropores, NO2 is released from the beginning of the reaction. The carbon materials were also used in the reduction of NO with and without the presence of oxygen. The activated carbons with the best performance on NO reduction are the activated carbons with the largest amount of nitrogen. In experiments carried out with oxygen, it was found that the carbon materials begin to reduce NO at lower temperatures than in experiments without oxygen. Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com) |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://repositorio-aberto.up.pt/bitstream/10216/74716/2/27301.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
