Effect of surface chemistry on gas-phase adsorption by activated carbon prepared from oil-palm stone with pre-impregnation

Abstract

Effects of surface chemistry (acidity or basicity) on gas-phase adsorption (NO2 or NH3) by activated carbons prepared from oil-palm stones pre-impregnated with various solutions (ZnCl2, H3PO4 and KOH) were studied in this paper. Textural and chemical characterizations of these activated carbons were carried out. High solid density and hardness, fairly high BET surface area and predominant microporosity (as shown in the pore size distributions) of the oil-palm-stone activated carbons suggested their potential applications in gas-adsorbing processes. Chemical characterization showed that impregnation affected significantly the surface chemistry, i.e. surface functional group. The samples pre-treated with H3PO4 presented acidic groups such as phenols and carboxylic acids, whereas those with KOH impregnation showed basic groups likely to be pyrones (cyclic ketone) and other keto-derivatives of pyran. From adsorption tests of NO2 and NH3, it was found that the activated carbons pre-treated with KOH could adsorb more NO2 but less NH3 than those pre-treated with H3PO4, even though they had almost identical BET surface areas. This indicated that the adsorptive capacity of the activated carbon was not only determined by its textural characteristics, but also related to the surface chemistry, which was relevant to the type of impregnating agent, concentration of impregnation solution and activation temperature. In general, the activated carbons prepared from oil-palm stones impregnated with H3PO4 and KOH are suitable for adsorbing basic (NH3) and acidic (NO2) gases, respectively.