On how mild oxidation affects the structure of carbons: Comparative analysis by different techniques

Understanding how the structure of carbonaceous materials changes upon oxidation at mild temperature as a function of the graphitic order is of great importance for the development of clean combustion technologies, such as carbon fuel cells. The micro- and nanostructures of a range of carbon materials at room temperature and upon mild oxidation at 300 °C have been analysed by means of mercury porosimetry, Nitrogen adsorption, X-Ray Diffraction, Raman spectroscopy and Electron Paramagnetic Resonance. The samples included carbons with increasing level of graphitic order: three chars derived from two bituminous coals and a lignite, a synthetic carbon and a graphitized coke. The experimental characterization allows to classify the materials according to different structural parameters, including porosity, surface area, degree of graphitization and paramagnetic activation of the carbonaceous surface. A correlation with the rank of the analysed materials is observed. For the graphitized coke, oxidation leads to more crystalline order and enhancement of the paramagnetic signal. A similar increase in the paramagnetic activity is observed for the Lignite char. On the other hand, for the higher rank, bituminous and synthetic chars, mild oxidation leads to a slight expansion of the amorphous carbon and loss of paramagnetic activity. The differences are rationalized in terms of formation of new carbon-oxygen complexes on the graphitized coke and on the Lignite char, and redistribution of already existing complexes for the higher-rank coals. This investigation complements previous X-ray photoelectron spectroscopy measurements.