Influence of carbonization conditions on porous structure of carbon materials

In this paper, changes in the porous structure of carbon material are investigated during chemical activation of apricot pit at different temperatures. The main purpose of the research has been to study the features of the internal micro- and mesostructure of the carbon surface, as well as to find patterns in the size distribution of pores dependent on the carbonization temperature of plant raw materials and subsequent chemical activation. The object of the study is porous carbon material, obtained from dried apricot fruit pits, pre-crushing, and cleaned from the core. This raw material has been carbonized at 300–900 °C with an interval of 100 °C and subjected to chemical activation by potassium hydroxide in the weight ratio XК, where ХК = m(KOH)/m(C). Thereby, a series of samples C3÷C9 has been obtained. The porous structure characteristics (specific surface area and total pore volume) of porous carbon material have been determined based on the analysis of nitrogen adsorption / desorption isotherms. It has been found that these materials have a frame structure with a large number of micropores. The analysis of literature data has revealed that the experimental curves belong to the isotherms that are characteristic of multilayer adsorption in micro- and mesopores of materials of organic origin. The hysteresis loop observed on these isotherms is related to the sorption processes in narrow pores. It has been found that carbonization of raw materials and chemical activation contribute to the cleaning of the frame structure, and there are acceptable modes of heat treatment of raw materials of plant origin, which determine the optimal pore size distribution and have a specific surface area S = (1042–1313) m2/g. With the help of scanning electron microscopy, the peculiarities of the formation of impurities on the surface of the original samples have been studied and their nature has been elucidated. It has been found that the largest total pore area has a sample carbonized at 600 ºC, and the largest pore volume Vtotal = 0.68 cm3/g for sample C3 is associated with a significant number of mesopores in the porous structure. As the carbonation temperature increases to 800-900 °C, narrow micropores degenerate and the carbon matrix transforms, resulting in a decrease in both the total pore area and the total porous volume.