Effect of functional groups on VOCs adsorption by activated carbon: DFT study

Volatile organic compounds (VOCs) have attracted increasing attention in the field of environmental control due to their harmful effects on human health and the environment. Activated carbon is promising as an adsorbent for VOCs. The physisorption of six VOCs (methane (CH4), dichloromethane (CH2Cl2), ethylene (C2H4), formaldehyde (HCHO), ethanol (C2H5OH), and toluene (C7H8)) by four activated carbon models (without functional group (NF), with hydroxyl (HD), pyridine (PD), and pyrrole (PR) group, respectively) was studied in combination with their electrostatic potential (ESP). The results show that ESP plays a decisive role in the adsorption of CH4, CH2Cl2, C2H4, HCHO, and C2H5OH, as their adsorption is mainly carried out by H-bonds. The functional group affects the adsorption process by changing the ESP distribution on the activated carbon surface. The orders of the absolute ESP values on the plane and edge of activated carbon are PR>HD>NF>PD and HD>PR>PD>NF, respectively, so that the order of the interaction strength and the physisorption energy are almost the same. Meanwhile, dispersion can enhance adsorption and change the order. For example, the order of CH2Cl2 adsorption at the activated carbon edge is PD>HD>PR>NF. C7H8 is difficult to be adsorbed at the activated carbon edge, but it can be adsorbed on the plane by a π-π stacking interaction whose strength is controlled by the relative molecular mass of activated carbon. This study will guide the design of activated carbon for efficient adsorption of VOCs.