Effects of ferromagnetic inclusions on 13C MAS NMR spectra of heat-treated peat samples

Peat char samples produced by heat treatments under inert atmosphere were characterized through C-13 solid-state NMR, Mossbauer spectroscopy, and magnetization measurements. For samples prepared above 700 degreesC a set of strong spinning sidebands (SSB) were detected in NMR spectra acquired with direct polarization (I)P) and magic-angle spinning (MAS), an effect that was shown to be consequence of the presence of ferromagnetic impurities associated with the mineral content of the studied peat samples. A detailed investigation on the nature and thermal transformations of the magnetic particles present in the natural and heat-treated peat samples was carried out by using magnetization measurements and Mossbauer spectroscopy. A pronounced increase in the ferromagnetic character of the heat-treated samples was verified from 600 degreesC upward, with the development of Fe3+ compounds, Fe3C, and Fe clusters. The effect of these ferromagnetic inclusions was the creation of magnetic Field inhomogeneities throughout the sample, which lead to the production of an anisotropic broadening of the NMR lines. A simplified model is presented to explain the influence of these particles on the C-13 DP-MAS NMR spectra of the heat-treated peat samples, showing their capability to generate SSB much stronger than the ordinary SSB due to chemical shift anisotropy in pure carbon materials.