Efeito da dopagem de Mn no espinélio MgAl2O4, do tratamento térmico e da adição de Aloe vera na síntese de Nanotubos de Carbono

Abstract

Discovered in the mid-1990s, carbon nanotubes (CNT) have special properties that expand their range of applications. CNT's can be produced from the methane decomposition reaction in the presence of catalysts. However, the quality and quantity of CNTs produced strongly depends on the structure and properties of the catalysts. Thus, the objective of this thesis was to evaluate the relationship of the synthesis parameters of Ni catalysts supported on Mn-doped MgAl2O4, obtained using the green extract of Aloe vera for the production of carbon nanotubes. For that, the catalyst supports were synthesized via microwave-assisted combustion, achieving two routes. The first was using only water; and the second using Aloe vera as a chelating agent. Both routes had the objective of doping the A and B sites with Mn ions in the MgAl2O4 for the evaluation of their characteristics. Part of the synthesized support was subjected to calcination at 900 °C for 3 hours and part remained without heat treatment. Then, the materials were impregnated with 50% nickel and subjected to heat treatment (550 °C/3h). The formed catalyst was taken to the catalytic unit for activation and then for the synthesis of carbon nanotubes. The supports were characterized by X-ray diffraction (XRD). The catalysts were also subjected to XRD and thermal analysis (TG). The nanotubes were characterized by Raman, SEM-FEG and TG. The XRD results indicated that the ions of the supports were replaced in different proportions, as well as the same analysis showed the successful impregnation of nickel. The NTC along with the catalysts were analyzed by Raman spectroscopy, thermal analysis (TG) and scanning electron microscopy (SEM). The Raman results pointed to the formation of carbon nanotubes in all catalysts, with an IG/ID ratio > 1, indicating crystallinity of the formed material. The relationship between the results of thermal analysis (TG) and Raman spectroscopy indicates that the nickel catalyst impregnated in MgAl2O4 with Mn2+ substituted in the A site, synthesized using Aloe vera and calcined at 900 °C (Ni/Mn-SAAV900) produced Less defective CNT's (IG/ID = 1.42), but with lower CNT production (4%p). In contrast, the nickel catalyst impregnated in MgAl2O4 with Mn2+ substituted at the B site, synthesized using Aloe vera and calcined at 900 °C (Ni/Mn-SBAV900) showed an IG/ID ratio = 0.77, revealing more defective CNTs, but with a little more than twice as many NTC's formed. The micrographs corroborated the other analyses, indicating the formation of carbon nanotubes. The methodology adopted using green synthesis proved to be a way to produce carbon nanotubes.