Formation of oxygen structures by air activation. A study by FT-IR spectroscopy

Using cherry stones (CS) as starting material and commonly air as activating agent, formation of oxygen structures in activated carbon is investigated. In the preparation of samples, CS was first heated at temperatures between 450 and 900°C in N2 atmosphere. Then, in a successive activation stage, the product carbonized at 600°C was maintained in contact with an air stream at 25–325°C for 24 h, 300–600°C for 1 h, and 250°C for 1–96 h. The rest of the carbonization products of CS were also heated at 250°C in air for 24 h. Moreover, the product carbonized at 900°C was activated at 750 or 900°C in CO2 for 1 h. Furthermore, in a second activation stage, the products activated at such temperatures in CO2 and those at 300–600°C in air were heated at 250°C in air for 24 h. The starting material, carbonized products, and activated carbons were examined by FT-IR spectroscopy. A number of carbon–hydrogen atomic groupings and of oxygen groups and structures, i.e., OH, CO, and C–O–C) have been identified in CS. The yield of the activation and carbonization processes and also the chemical structure of the resultant products are strongly dependent on the carbonization temperature. In the products carbonized at 600–900°C, only ether type structures are detected. The activation at 250°C in air results in activated carbons that contain different oxygen structures when CS is carbonized at 450 or 600°C. At 750 or 900, by contrast, oxygen structures are not formed as a result of the activation treatment. This also applies when the carbonization product of CS at 900°C is activated solely in CO2 or first in CO2 and then in air. The heating conditions in air greatly influence the formation of oxygen structures (specifically, of lactonic and ion-radical types) to a large extent. It only occurs when activating at relatively low temperatures for a long time; at 300–600°C for 1 h, however, the oxygen structures are not formed.