2.4. FTIR Spectroscopy Experiment

Each group in the coal molecule has different vibration modes when subject to a light source with a continuous wavelength, and the same groups also have varying vibration forms. Therefore, FTIR spectroscopy was used to analyze the content of the functional groups. First, 1 mg of the sample with 150 mg of KBr were uniformly ground for 2 min and pressed into a pellet. Subsequently, the samples were dried in a vacuum oven under 60 °C for 12 h. Finally, FTIR spectra of the samples were obtained by a FTIR spectrometer (FTIR-8400, Shimadzu, Japan) with spectral region 400–4000 cm^–1and resolution 0.4 cm^–1, which are displayed in Figure ​Figure11.

FTIR spectra of the samples.

In order to better analyze the content of functional groups in the FTIR diagram, Peak Fit v4.12 was used for the peak fitting of the FTIR sample data. First, after the infrared data is imported, automatic smoothing correction is performed. Next, AutoFit Peaks II Second Derivative method is chosen for peak fitting. Finally, we manually repeat this until r² > 0.99 stops fitting and save the corresponding data. The peak-fitting figures of the FTIR are displayed in Figures ​Figures22–8.

Peak-fitting FTIR figure of S₁.

Peak-fitting FTIR figure of S₇.

Peak-fitting FTIR figure of S₂.

Peak-fitting FTIR figure of S₃.

Peak-fitting FTIR figure of S₄.

Peak-fitting FTIR figure of S₅.

Peak-fitting FTIR figure of S₆.

For analyzing the content of functional groups after peak fitting more intuitively, the representatives of the main functional groups with the most reactivity were selected to calculate the content relative of peak area,⁵⁶⁻⁵⁸ and the results are shown in Table 4.