TEM CHARACTERISATION AND RAMAN SPECTROSCOPY OF VITRINITE

摘要

Despite of environmental concerns, coal is still a significant natural energy resource and is widely used for electricity generation and steel making, as well as for unconventional methane production (coal bed methane; CBM). Coal seams are furthermore evaluated for their carbon (CO_2) storage properties. Physicochemical and mechanical properties of coals still need to be investigated for the evaluation of long-term storage effects [1]. Coal properties are controlled by the individual distribution of its constituents, called macerals, which are divided into the main groups vitrinite, intertinite, and liptinite. According to previous studies, sorption capacity, structure and physical properties of vitrinite are more sensitive to experimental treatment with CO_2 compared to inertinite [1, 2]. Since vitrinite also seems to show a complex relationship of micromechanical properties [3] with increasing coal rank (maturity) and is usually the most abundant maceral in coal seams, our research focuses on this maceral group. Well investigated coal samples from the Ukrainian Donets Basin [4, 5], covering a maturity range from 0.62 to 1.47 %Rr (vitrinite reflectance), were selected for this study. The aim is to investigate nanopores and sub-micrometre scale mineral inclusions in organic matter via transmission electron microscopy (TEM), as these features likely have a significant impact on the material properties. Furthermore, high-resolution transmission electron microscopy (HRTEM) is employed in order to study aromatic structural features and stacking. As a complimentary characterisation method μ-Raman spectroscopy is conducted to further refine the maturity assessment and characterise changes in the aromatic vitrinite structure based on the G (graphitic) and D (disordered) bands [6]. To ensure correlation between the individual experimental techniques, vitrinite is identified based on optical microscopy under white reflected light from polished sections with randomly oriented coal fragments. Material for a single sample is obtained by careful drilling and milling out of vitrinite grains. The vitrinite powder is further diluted in isopropanol, dispersed by ultrasonification and finally dropped on a lacey carbon film for TEM investigation. Raman spectroscopy is performed on untreated specimen surfaces (Fig. 1). For a comprehensive understanding, CO_2 adsorption measurements will be conducted. This complementary approach yields valuable insights into the microstructural evolution of vitrinite with increasing maturity, as well as on micromechanics, adsorption capacity and physicochemical properties.