Obtenção e caracterização de carbono grafítico a partir do resíduo torta de café

Abstract

Large amounts of industrial waste have been generated annually, negatively contributing to the pollution and degradation of the environment. As a result, agro-industrial residues such as coffee husks and defective coffee have been studied as precursors for the production of value- added products, such as activated carbon, ethanol, biogas, among others. These residues can also be used as raw materials for the production of graphite, which shows high added value and applicability. Therefore, in this study, a residue named “coffee pie” (TC) was used as a precursor to obtain graphitic carbon through its carbonization and thermal treatment, using iron nitrate (III) and nickel nitrate (II) as catalysts. The graphitic carbons prepared with Fe(NO3)3 and Ni(NO3)2 were thermally treated at 800 and 1000°C, respectively, for 30 minutes in the absence of oxygen, by using N2 flow. The characterization of the materials was performed by Raman spectroscopy, X-ray diffraction (XRD), thermal analysis (TG/DTG) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). The Raman spectra showed bands that are characteristic of graphitic carbon for the samples containing nickel and iron. The ratio between the areas of the signals of the D/G bands decreased when coffee pie was thermally treated in the presence of the catalysts, indicating the formation of graphitic carbon. However, the formation of graphitic carbon was more efficient when nickel nitrate was used as catalyst. This higher content (29%) may be related not only to the type of catalyst used, but also to the higher heat treatment temperature. In the XRD analysis, well-defined diffraction lines were attributed to graphitic carbon, mainly for the sample containing nickel. Diffraction lines related to metallic iron, iron carbide and metallic nickel were also observed. It indicates that part of these metals remained in the samples after acid washing. The XRD results also allowed the calculation of the degree of graphitization (29%) and the number of graphite layers formed (15.78) for the sample obtained with nickel (II) nitrate. Thermal analysis showed the loss of mass of the samples as well as the greater thermal stability of the graphite carbon. SEM-EDS images showed that there were no significant changes in the morphology of the material prepared in relation to the carbonized coffee pie and the presence of iron and nickel particles present on the surface of the samples prepared with Fe(NO3)3 and Ni(NO3)2, respectively. Therefore, the results showed that it was possible to obtain graphitic carbon from raw materials derived from agro- industrial waste, which is a way to contribute to circular economy and to reduce environmental contamination.