Structural analysis of char by Raman spectroscopy: Improving band assignments through computational calculations from first principles

Abstract

The complex heterogeneous nature of chars has confounded the complete analysis of the Raman spectra of these materials. The additional shoulders observed on the defect (D)-band and high intensity valley between the D and graphitic (G)-bands represent the primary regions of uncertainty. In this paper the effects of various vacancy and substitution defects in a coronene parent molecule have been systematically analyzed using density functional theory (DFT). The impacts of these defects are best understood in terms of a reduced symmetry as compared to a "parent" coronene molecule. Based on simulation results, a total of ten potential bands have been assigned between 1000 cm-1 and 1800 cm-1. These bands have been used to deconvolute a thermoseries of cellulose chars produced by pyrolysis at 300-700 °C. The shoulder observed in chars near 1200 cm-1 has been assigned to the symmetric breathing mode of various small polyaromatic hydrocarbons (PAH) as well as rings containing seven or more carbons. Intensity between 1400 cm-1 and 1550 cm-1 results from a range of coupled vibrational modes from various defect structures. The deconvolution of cellulose derived chars shows consistent growth of PAH clusters, loss of oxygen, and development of non-hexoganal ring systems as pyrolysis temperature increased.

Biomass Chars: The Effects of Pyrolysis Conditions on Their Morphology, Structure, Chemical Properties and Reactivity

“…This tendency was also observed previously during the characterization of chars prepared by cellulose slow pyrolysis at different temperatures [28]. This "red shift" in the D band peak position is more pronounced for low temperature chars, which have the highest contents of oxygenated defects structures [29]. Increasing the pyrolysis temperature strongly affects the char structure.…”

“…In fact, analysis of the Raman Spectra shows that char-600 has a lower intensity in the regions between 800 and Energies 2017, 10, 796 14 of 18 1200 cm −1 and 1700-1900 cm −1 when compared to the samples of char-800 and char-1000, respectively. Raman signal in these regions can be associated with the highly reactive structure in the char [29]. As shown in Figure 10, although the evolution of ( %) is somewhat chaotic between 500 °C and 1000 °C, it increases following a perfect linear relation (R 2 = 0.9999) with the char yield for the chars obtained between 1000 °C and 1400 °C.…”

Raman spectroscopy for analysis of thorium compounds

“…A photograph of the system is seen in Figure 1, and its sample compartment and interferometer core are similar to instruments that have been described previously. [15][16][17] The MultiRam is a dedicated FT-Raman instrument, but instead of a linear air-bearing interferometer, it has a flex-pivot mechanical bearing interferometer. The MultiRam 27 was used with an internal aperture setting of 3.5 mm and a dedicated beamsplitter with a silicon refractive surface on a quartz substrate.…”

“…This particular system was equipped with a (custom) Stokes/Anti-Stokes bandpass filter that passes light on both the low-and high-frequency side of the notch filter as described below. 16,17 All data were recorded at 2.0 cm -1 resolution with the samples at room temperature in the sample holder described above. The Raman excitation laser power was varied as needed, but most data were recorded with 70-100 mW power.…”

“…17,18,22 Using a series of such lines makes it possible to calibrate the interferometer HeNe laser frequency to approximately ±0.02 cm -1 on a daily basis. A second calibration to account for the temperature-induced Raman laser frequency drifts has been found to be very important for the near-infrared lasing crystals used in many modern spectrometer systems.…”