Síntese e caracterização de compósitos mesoporosos de sílica-óxido de cobalto e avaliação catalítica na reação de redução de no com CO

• Autor(a) principal: SANTOS, Gustavo Amorim
• Data de Publicação: 2010
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Repositório Institucional da UFG
• dARK ID: ark:/38995/0013000007jn2
• Texto Completo: http://repositorio.bc.ufg.br/tede/handle/tde/1039
• topic: óxido de cobalto; sílica; catálise; redução de NO; cobalt oxide; silica; catalysis; NO reduction; CNPQ::CIENCIAS EXATAS E DA TERRA::QUIMICA

In this work, a methodology for the synthesis of silica-cobalt oxide-based composites was developed by modifying the well known Stöber sol-gel method. The main novelty of the procedure was the use of colloidal suspensions of cobalt oxide or cobalt hydroxide nanoparticles (Co3O4,Co(OH)2), which were previously prepared from the precipitation of Co2+ ions in alkaline medium. The silica-cobalt composites having cobalt contents (w/w) around 8.4 to 11% were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermogravimetric and differential thermal analysis (TG/DTA), temperature programmed reduction (TPR) and adsorption /desorption of nitrogen. The characteristics of the composites which were prepared by employing the colloidal suspensions were compared to those exhibited by other composites which were obtained from cobalt nitrate solution. The influence of cerium nitrate and citric acid was also evaluated. The methodology employed in the synthesis of nanoparticles and composites were reproducible and the prepared materials showed surface areas (300 to 567 cm3/g) and pore diameters (3.8 to 8.1 nm) values which were considered appropriate to use them as catalysts. The calcined composites which were prepared using colloidal suspensions of Co3O4 nanoparticles showed TPR curves with maximum picks at relatively low temperatures, in the range of 275 to 520 oC, suggesting the existence of weak silica-cobalt interactions. The average diameter of Co3O4 crystallites varied from 22 to 30 nm. However, the use of citric acid as a pore-forming agent in the sol-gel synthesis led to composites with enhanced silica-cobalt interactions as a result of the dissolution of the cobalt oxide particles and formation of cobalt silicate. The incorporation of cerium nitrate in the sol-gel mixture did not modify the textural properties of the composites but the reduction of cobalt oxide to metallic cobalt was retarded. The composite which was obtained from the cobalt hydroxide, β-Co(OH)2, colloidal suspension showed weak silica-cobalt interactions as well as intermediate ones, the average diameter of Co3O4 crystallites being 12 nm. Differently from the composites obtained from colloidal suspensions, those which were synthesized using a cobalt nitrate aqueous solution presented mainly cobalt silicate and a low quantity of Co3O4 particles of 11 and 17 nm. The TPR curves of these composites indicated very strong silica-cobalt interactions, as expected. The composites were evaluated as catalysts in the reduction reaction of NO to N2 with the oxidation of CO to CO2. The composites which were prepared from Co3O4 colloidal suspensions without the use of citric acid as pore-forming agent showed maximum conversions as high as 90% for both NO to N2 and CO to CO2, at temperatures above 600°C. On the other hand, the composite prepared using critic acid showed much lower conversions (60 %) at the same temperature.