Understanding chemical reactions between carbons and NaOH and KOH: An insight into the chemical activation mechanism

Direct mixing of an anthracite with hydroxides (KOH or NaOH) and heat treatment up to 730 °C has shown to be a very good activation procedure to obtain activated carbons with very high surface areas and high micropore volumes. The reactions involved during the heat treatment of these hydroxide/anthracite mixtures have been analysed to deep into the fundamental of the knowledge of this chemical activation process, that has not been studied before. For this purpose, the present paper analyses the drying process, the atmosphere during the carbonisation, the chemical state of the activating agents (NaOH, KOH and Na₂CO₃) and the chemical reactions occurring during the heat treatment which have been followed by FTIR and TPD. The analysis of our results allows us to conclude that steam is a good atmosphere for the carbonisation process, alone or joined with nitrogen, but not as good as pure nitrogen. On the other hand, during the activation process, the presence of CO₂ should be avoided because it does not develop porosity. The reactions, and chemical changes, involved during this chemical process are discussed both from a thermodynamical point of view as well as identifying the reaction products (H₂ by TPD and Na₂CO₃ by FTIR). As a result, this paper helps to cover the present lack of understanding of the fundamentals of the reactions of an anthracite with hydroxides which are necessary to understand the activation of the material.