Ultraviolet Raman spectra: The reasonable method of evaluating coal pyrolysis graphitization

Abstract

Raman spectrum is a common method of evaluating the graphitization degree of carbon materials. However, there is still some controversy over its evaluation for coal graphitization, i.e., whether large ID/IG means a high graphitization degree of coal. This work uses an ultraviolet (UV) Raman spectrum to find a better index of graphitization degree. The T peak (sp3 sites) of coal was first directly detected. The IT/IG and ID/IG decrease after pyrolysis in UV Raman spectra at 266 nm, which shows that the sp3 site relative content decreases and the sp2 site relative content increases. Moreover, the absolute value of the D band dispersion slopes as the excitation wavelength increases and the D peak width decreases, which shows that the coal gets more orderly after pyrolysis. The position difference (G–D) decreases and the G peak width increases after pyrolysis in UV–Vis Raman spectra, indicating the decomposition of vitrinite in coal. The 2D′ peak decreased after pyrolysis. These parameters in multi-wavelength Raman spectra can be used to evaluate the structure of coal.