Structural Characterization of Selected Indian Coals by X-ray Diffraction and Spectroscopic Techniques

The structural parameters of three Indian coals were determined by X-ray diffraction, Raman, UV-Vis-NIR, FTIR spectroscopy and SEM-EDS. This study reveals that the coals contain crystalline carbon of turbo- stratic structure with some amount of amorphous carbon. The average lateral sizes (La), stacking heights (Lc) and interlayer spacing (d002) of the crystallite structures calculated from the X-ray intensities range from 28.15-38.43 A°, 22.64-20.16A° and 3.52-3.34 A°, respectively which is higher than that of pure graphite, suggesting a low degree of crystalline order in the studied sample. The decrease in La with rank shows that the carbon in coal is nano crystalline in structure. Raman spectroscopic study confirms the presence of multi layer formation of graphite layer. The defect band at 1355 cm-1 is due to benzene or condensed benzene rings present in amorphous carbon. The FTIR spectra of the coals show the presence of stretching vibrations of -OH bonds, aliphatic -CH, -CH2 and -CH3 absorptions, C = C and -CH of aromatic structure. The C-H bending frequencies have higher intensity than the C-H stretching band. This is due to the intense broad absorption produced in this region by graphite components. A strong linear relationship exists between the coal structural parameters (fa,d002,Lc) and the elemental carbon and volatile contents of the coal which reflects the dependency of the coals structure on their ranks. The SEM image shows the presence of layered structure on the surface. The EDS analysis confirms the presence of Carbon and Oxygen on the surface. The absorption maximum of benzene-oxygen system was found between 235-270 nm. The weak band at the 680 nm is attributed to the ∏-∏* electronic transitions of the polynuclear aromatic hydrocarbons.