13C High-resolution solid-state NMR study of peat carbonization

Feat char samples produced by heat treatments under an inert atmosphere of nitrogen were characterized through C-13 solid-state NMR, allowing the achievement of a detailed picture of the mechanism of carbonization. The treatments were realized at temperatures between 200 and 1000 degrees C, with a controlled heating rate and residence time. NMR spectra were obtained in a home-built spectrometer operating at a field of 2.0 T with the use of high-resolution techniques. The results showed the steps followed during the carbonization process. Initially we observed a degradation of the natural material, with preferential loss of carbohydrates and carboxylic groups. From a heat-treatment temperature of about 500 degrees C upward, the spectra were nearly constituted of only one resonance line with a chemical shift of about 130 ppm (from TMS), typical of aromatic structures. For the most severely treated samples, a decrease in the chemical shift of the resonance line was observed, indicating a deshielding associated with the increase in the electrical conductivity of the material. The structure of these samples was identified as consisting of planes of sp(2) carbon atoms, with a chemical environment similar to graphitic materials but with no long-range spatial order.