The thermal conversion of fossil solid fuels, car tires, biomass, or sewage sludge leads to significant amounts of waste, for which developing a direction for safe disposal can be a complex issue. This group is characterized above all by a high concentration of carbon in its chemical composition and, depending on its origin, by the presence of oxygen, hydrogen, and nitrogen heteroatoms, which, being part of surface complexes, may constitute active sites capable of forming bonds with the reactant molecule in a given chemical reaction. The groups that determine the practical utility of the carbon materials are those consisting of carbon and/or hydrogen atom bonded to an oxygen atom. These groups determine the adsorption, catalytic, oxidation-reducing, and acid-base properties, which are key e.g. for adsorption-catalytic gas purification.
Despite extensive work, the nature of surface functional groups has not been fully elucidated. Difficulties in interpreting the obtained test results are mainly due to the complexity of the structure of the carbon substance. The range of techniques available is also debatable. Moreover, the surface carbon-oxygen groups, formed as a result of the interaction of the carbon material with the oxidant, are characterized by different thermal stability. Consequently, under inert atmosphere conditions and at high temperatures, their conversion to specific gaseous products occurs. As carbon materials have high reactivity with oxygen, these groups are typically released into carbon monoxide and dioxide, and water vapor. The thermal stability of the functional groups is very important because many of the high carbonaceous waste management directions require thermal treatment. Although the mechanism of thermal decomposition of functional groups is well understood, the kinetics is already difficult to reveal due to the overlap of intermediate reactions.
Considering the above, to complement the characterization of surface groups obtained by titration techniques, infrared spectroscopic analysis in KBr and Raman modes was carried out. The use of different types of electromagnetic radiation gave the possibility to observe in the spectra bands characteristic for particular functional groups (with single and multiple bonds between atoms). The proposed procedure, thanks to the complementarity of the techniques, enabled to significantly extend the range of obtained information on the surface functional groups. The subjects of the research were high carbonaceous waste of the combustion process, i.e.: selected fractions of unburned carbon from lignite and coal fly ash, petroleum coke, char formed by pyrolysis of MDF boards, and zeolite - as precursors of carbon adsorbents/catalysts in the process of removal/reduction of NOx, SO2, NH3, CO2 and commercial activated carbons from coal dust, dedicated to the purification of flue gases precisely. In addition, an attempt was made to describe the chemical kinetics of the decomposition of functional groups by TGA, with identification of gaseous products by FTIR and analysis of the thermal character of the separated products by DSC. A one-step model was used to separate the steps of the different individuals, taking into account the mechanism of multiple parallel reactions. The desorption rate of the oxide layer was described by the Arrhenius equation.
- Complementing the titration analysis with an instrumental analysis provides a reliable picture of the chemical structure;
- The use of various types of electromagnetic radiation makes it possible to observe in the spectra the bands characteristic of particular functional groups (with single and multiple bonds between atoms);
- The thermal stability of the functional groups is very important, as many of the management directions of high carbonaceous waste require thermal treatment or are carried out at a temperature higher than 150 °C (for which the decomposition of functional groups is observed).