Optimized adsorption of volatile organic compounds on graphene oxide and nanoporous graphene activated with ZnCl2: a combined experimental and computational study

Abstract

The present research is a comparison study of adsorption capacity of graphene oxide (GO) and nanoporous graphene (NPG) for volatile organic compounds' vapor (here gasoline vapor) adsorption. GO was synthesized using the modified Hummers method. For the synthesis of NPG, a low-cost precursor with unique properties (camphor) was used by the chemical vapor deposition (CVD) method. The effect of reaction temperature parameter, the ratio of camphor to zinc oxide nanocatalyst and reaction time was investigated. The physicochemical properties of the samples were characterized using XRD, FT-IR, Raman, FE-SEM, TEM and BET techniques. It was observed that activation by ZnCl₂ at 600 °C and 180 min (i.e. NPG2) gives a surface area of 181.61 m² g⁻¹. NPG2 showed high adsorption capacity for VOC adsorption (559 mg g⁻¹), which was about 1.34–2.58 times more than other synthesized samples (adsorption capacities of PG1, PG3, GO1, GO2, and GO3 were 415, 310, 300, 367, and 216 mg. g⁻¹, respectively). The high VOC adsorption capacity of PG was due to its π–π interactions with the NPG surface. Therefore, the NPG2 sample was selected as the best sample. In general, all synthesized samples showed rapid kinetic behaviors for gasoline vapor adsorption, and their maximum adsorption capacity was obtained in the first 35 min. To shed insight on the adsorption process of gasoline on NPG and GO, density functional theory (DFT) calculations were performed. According to the DFT calculations, the adsorption strength of an isobutane (ISO) molecule improved as the pore size on the NPG increased. The adsorption energy of ISO on GO was less than that on NPG, most likely due to steric repulsion between the ISO and O moieties on the GO. The negative enthalpy and Gibbs free energy changes caused by ISO adsorption on NPG and GO showed that the process is thermodynamically favorable at room temperature and pressure.