Inorganic and organic compounds pollutants in addition to others heavy metals and toxic like SOx, H2S, NOx, CO, I and NH3 caused by industrialization, urbanization at large scale and increase in world population have become a source of soil, air and water pollution badly effecting human life and environment [1]. Controlling such pollution has becoming a challenge to keep the human society environmentally clean. To circumvent pollution effectively, among the others, adsorption process is simple and easy way to employ, does not produce any un-wanted substances and is thus attractive and used widely [2, 3].
The adsorbent must be competent enough of having large surface area, pore volume, and appropriate functionalities to circumvent the pollutants effectively and successfully. The advancement in this field has caused to develop a variety of such materials with varying desired efficiency in removing the toxic pollutants from soil, water and air [2, 4–6] but out of them activated carbon (AC), carbon nanotubes, and fullerene are thermally stable and show good adsorption properties [1, 7–10]. The source of AC is solid, porous and black carbonaceous martial. AC is obtained after processing these materials. Precursors of AC include wood, coconut shells and nut shells or of all ranks of coal i-e all have botanical origin. AC is a favorable material for adsorption of noxious gases owing to its remarkable characteristics of large surface area (500 to 3000 meter square per gram), surface chemistry, active surface and pore structure. [11].
As far as the structure of Ac is concerned, the recent experimental observations report it like fullerene construction. Its curved fragments contain hexagons, pentagons and other non-hexagonal rings. The first observations in this regard were understood and reported by Rosalind Franklin in 1950. [12].
The AC is a very useful material with potential applications in different areas including industry. The AC play vital role for gas storage, noxious waste and odor removal due to its large surface area and pore structure. Further applications include its use in nuclear industry, gas separation, catalysis, gas cleaning, metal extraction, water distillation and chemical purification. It has also uses in fuel cells and medicinal area [13, 14]. The most of its applications for practical purposes route through adsorption process [10]. The AC can be more useful if its adsorption properties are enhanced. Various atoms like O, H, N, S can be employed as functional material to serve the purpose. Different experimental techniques are used for accomplishment [14–18]. As reported in reference [19], the Eads of CO, NO and NO2 molecules on graphene enhanced due to creation of defects in adsorbent surface. The structure of graphene is similar to AC.
CO, NO, I and NH3 have side effects and effect the human healthy life in one or the other way. To have detailed overview about the adsorption properties of CO, NO, I and NH3 pollutants, we carried out investigations on different activated carbon structures. These AC surfaces of varied size i.e. 2–6 ring models have been considered for computation. These models have been constructed keeping in view the experimental observations reported by Harris et. al [12]. In order to have investigation on such models close to experimentally determined ones, we created defects to have pentagonal, squarely and heptagonal configurations in addition to hexagonal surfaces. We have functionalized the surfaces with H to compute the impact of hydrogen on adsorption properties of these noxious gases.
The investigations have been carried out in order circumvent the noxious gases CO, NO, I and NH3 by the process of adsorption on activated carbon. After experimental determination and reporting the evidence regarding the hexagon, pentagon, heptagon angular rings and other structures [12], we have computationally observed the adsorption properties. These configurations mimic graphene like structures. For the purpose of adsorption of gases, surface area and surface chemistry play a crucial role for efficient and effective adsorption and in case of AC surfaces, we have these. In order to have comprehensive view, we have considered the systems size, effect of creation of defects, mode of adsorption (planner, non-planner), adsorption site and functionalization of the surfaces with H-atoms. The results have clearly shown that some configuration are highly reactive and adsorb the noxious gases chemically. DFT can be effectively employed to compute the adsorption energies of gases on micro porous material like activated carbon. The results obtained are discussed in the following sections.