Exploring a New Method to Study the Effects of Surface Functional Groups on Adsorption of CO 2 and CH 4 on Activated Carbons

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The commercial coconut shell-activated carbon was modified to change the number of oxygen-containing functional groups. N 2 adsorption/desorption isotherms, Fourier transform infrared (FT-IR), and Boehm titration were adopted to describe the physical and chemical properties of the samples. The adsorption isotherms of CO 2 and CH 4 on both the unmodified and modified samples were measured. To better understand the effects of surface oxygen-containing functional groups on adsorption of CO 2 and CH 4, the overall adsorption could be considered as the result of adsorption within the pores and adsorption onto the oxygen-containing functional groups. Thus, a new way to understand different adsorption mechanisms by calculation was proposed. On the basis of the results, there is a significant correlation between the saturation adsorption capacity of CO 2 and the number of oxygen-containing functional groups, especially carboxyl and hydroxyl. According to the values of enthalpy (−12.2 to −20 kJ/mol), it can be known that the adsorption caused by oxygen-containing functional groups is exothermic and belongs to physisorption. A semiempirical relationship between the variation of the surface oxygen-functional groups and the variation of the adsorbed amount was established. The method proposed in this paper provides a new way to study the effects of surface functional groups on the adsorption of CO 2 and CH 4 and can be even promoted in studying the adsorption mechanism of other adsorbates.