¹³C High-Resolution Solid-State NMR Study of Peat Carbonization

Abstract

Peat char samples produced by heat treatments under an inert atmosphere of nitrogen were characterized through 13 C 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°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°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.

Studies of the Compositions of Humic Acids from Amazonian Dark Earth Soils

“…The up-field shift of the aryl peak to 126 ppm is typical of charred residues attributable to polycyclic aromatic structures (Freitas et al 1999 and2001;Knicker et al, 2005b;Smernik et al, in press). The scores of this PC were smaller for NAS, and varied within the anthropogenic soils group (Figure 4).…”

“…The presence of graphitic-like structures can induce local magnetic susceptibility heterogeneities (Freitas et al, 1999(Freitas et al, , 2001) and lead to a "reflection" of the excitation pulse -the "skin effect" (Fernandes et al, 2003;Knicker et al, 2005a). The problem is enhanced when carrying out the CSA filter and TOSS experiments required for the quantification method of Mao and Schmidt-Rohr (2004) because these experiments involve the application of many cascaded π pulses that enhance the effect of the pulse imperfections.…”

“…This evidence is provided by the improved quality of the CSA spectra from applications of composite pulses, indicating magnetic susceptibility heterogeneities, probably due to graphitic like structures (Freitas et al, 1999), and the identification of aromatic carboxylated structures with the typical shifted aryl peak at 126 ppm by PCA analysis. Etelv.…”

Biochar: Pyrogenic Carbon for Agricultural Use - A Critical Review

“…direct polarization), when performed with high magnetic field equipment (>1.76 T), as is usual, underestimates highly condensed aromatic structures (soot and graphitic structures). This is a result of the high heterogeneity in local magnetic susceptibility and/or chemical shift anisotropy, which is not completely averaged out at the usual rates of magic angle sample spinning (Freitas et al, 1999(Freitas et al, , 2001Novotny et al, 2006). The underestimation is even worse for the cross-polarization technique (Figure 4), due to the inefficient H-C cross polarization (very low H content).…”

Condensation degree of burnt peat and plant residues and the reliability of solid-state VACP MAS 13C NMR spectra obtained from pyrogenic humic material

“…However, for carbons with large C-H internuclear distances and for C in groups with high molecular mobility the proton-dipolar interactions are weakened, which increases their T CH . Organic matter subjected to combustion transforms into condensed structures (Freitas et al, 1999;Schmidt and Noack, 2000;Baldock and Smernik, 2002). At high heat intensities, condensation is expected to result in aromatic clusters with a considerable proportion of core C for which the distance to the next proton exceeds the length of three to four bonds.…”