Adsorption Mechanism of Oxide Gas Pollutants by Single-walled Carbon Nanotubes

Carbon nanotubes (CNTs) have exhibited great potentials in removal of toxic gases due to their large-specific surface area, high porosity, hollow structure, and light density. In this paper, adsorption mechanisms of single-walled carbon nanotubes (SWCNTs) trapping oxide gas pollutants are examined using Density Functional Theory (DFT). Both physisorption and chemisorption are determined through investigation of the preferred adsorption sites of the gases at specific nanotubes. According to the calculated electrical field, the physisorption is found relating to the potential well in the internal channel of the nanotube as well as to the rapid switch of the electrical field at the surface of the nanotube. It is further found that a larger diameter of the nanotube leads to a stronger adsorption capacity of the internal channel, but a weaker adsorption ability of the external surface. The chemisorption process is determined from interactions between the frontier orbitals of the gas molecules and those of the nanotubes. Quantitative analysis tells that molecules with a frontier-orbital energy ranging from -0.23 Ha to -0.15 Ha are more likely to be absorbed by the nanotube. A further examination shows that overlap of electron clouds and transfer of electrons occur during chemisorption, and that change of bond length of the gas molecules depends on the number of transferred electrons per bond. The order of adsorption strength is predicted as SO2, SO3, NO, and NO2 sequentially from the view of the binding energy.