Analysis of the microporosity shrinkage upon thermal post-treatment of H3PO4 activated carbons Original Research Article

Starting from a commercial pelletized phosphoric acid based activated carbon, with a typical opened and developed micro and mesoporosity, a post-heat-treatment in KOH, at different KOH/activated carbon ratios, has been studied. In all the cases, a pore size shrinkage has been observed. To find an explanation for the reason of this micropore size distribution shrinkage different factors have been studied, among them: (a) effect of the presence of impurities coming from the activation process with phosphoric acid; (b) effect of the KOH post-treatment temperature; (c) heat-treatment temperature of the precursor (without chemical agent); (d) effect of the reagent nature (NaOH, NaCl and KCl vs. KOH). The variable that produces the most intense shrinkage effect, and the disappearance of the mesoporosity, is the heat-treatment in presence of hydroxide, which affects even using a low hydroxide/activated carbon ratio. Such a low hydroxide/activated carbon ratio does not produce activation, nor porosity development of the starting activated carbon during the treatment. This shrinkage phenomenon, which seems to be independent of the method of preparation used to prepare the activated carbon, can be understood considering our previous studies about the reactions involved during chemical activation by hydroxides.