POROSITY IN GRANULAR CARBONS ACTIVATED WITH PHOSPHORIC-ACID

Three series of activated carbon have been prepared by heal treatment of peach stones impregnated with solutions of phosphoric acid, in order to analyze the effect of phosphoric acid on the yield, bulk density and porosity of the resultant activated carbons. The analysis of the adsorption isotherms of N-2 at 77 K, CO2 at 273 K and n-C4H10 at 273 K shows that the amount of phosphorus introduced into the material, X(p), is the main factor conditioning the porosity and pore size distribution of the activated carbon. In general terms, the increase in X(p) leads to an increase in the volumes of micro and mesopores of the carbon. Furthermore, there is a noticeable similarity between the volume of micropores and the volume occupied by the P2O5 . xH(2)O in the interior of the peach stones. This suggests that the microporosity is mainly caused by the phosphoric acid remaining in the impregnated material, which inhibits the contraction of the material during carbonization. On the other hand, it is suggested that the mesoporosity of the activated carbon, which is large only when the concentration of phosphoric acid is high, is mainly caused by the hydrolysis of the lignocellulosic material and subsequent partial extraction of some of its components during impregnation.