Exploring a New Method to Study the Effects of Surface Functional Groups on Adsorption of CO₂ and CH₄ on Activated...

The commercial coconut shell-activated carbon was modified to change the number of oxygen-containing functional groups. N₂ 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 CO₂ and CH₄ on both the unmodified and modified samples were measured. To better understand the effects of surface oxygen-containing functional groups on adsorption of CO₂ and CH₄, 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 CO₂ 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 CO₂ and CH₄ and can be even promoted in studying the adsorption mechanism of other adsorbates.