A density functional theory study of molecular H2S adsorption on (4,0) SWCNT doped with Ge, Ga and B
Graphical abstract
Introduction
Hydrogen sulfide (and other sulfur compounds) originates from various industries such as petrochemical, coke production, iron-steel, cement, paper, anaerobic treatment, leather, starch hydrolysis products and the nylon industry [1], [2], [3]. It is used in several chemical industries as a by-product or intermediate product. Hydrogen sulfide is an important pollutant due to its toxicity, corrosivity and rotten egg-like odor [1], [2], [3], [4], [5]. It can also damage transportation equipment and poison many catalysts in even low levels. It is also significantly effective on the nervous system [5]. If a person is exposed to low concentrations of H2S, there is a loss of consciousness and death can occur. When H2S is oxidized to SO2, it causes acid rains and because of this reason, it needs to be removed from the environment. In addition to these, natural gas includes an important amount of hydrogen sulfide which must be decreased to concentrations less than 20 ppm in order to meet fuel gas specifications and environmental regulations for pipeline transportation [2]. Thus, the removal of this gas is of great importance. There are several methods for the removal of H2S such as adsorption, biological treatment and scrubbing. These methods have both advantages and disadvantages. Biological treatment is proper to remove H2S and it also reduces chemical, energy and operating costs, while it has higher capital cost [6]. Adsorption is another process to remove H2S which two different adsorbents can be used separately. The first one consists of carbonaceous material that adsorbs H2S physically at room temperature. The second one is metal oxide adsorbents that can adsorb low concentration of H2S. Activated carbon has been studied by many researchers due to its high surface area, good surface reactivity and high porosity. Although it can show high adsorption capacity, it has some limitations to suffer low mechanical stability which causes high tortuosity [7], [8], [9]. Adsorption process has been utilized in petroleum and gas industries for the removal of H2S. H2S is transferred from a gaseous phase to solid phase. This method has high efficiency, while it has high investment and operation cost, and a high amount of heat required for regeneration and corrosion [7,10]. Membrane technology to remove H2S is mainly focused on purifying the biogas and upgrading to the natural gas standards. Membrane process is easy to use and there is no increase in cost, but its selectivity is low and it requires several processes to gain high purity [8,11].
Since carbon nanotubes (CNTs) were discovered by Iijima in 1991, they have been the focus of intense research for many application areas such as environmental monitoring, biomedical, fuel cells space and pharmaceuticals due to their remarkable structural, mechanical and electrical properties [12], [13], [14], [15], [16]. This unique carbonaceous material is a cylinder form of the graphene sheet. If one graphene sheet is enrolled, it is called as single-walled carbon nanotubes (SWCNT), or it is known as multi-walled carbon nanotubes (MWCNT) when consisted of several concentric graphene sheets [17,18]. Based on their small size, low density, high aspect ratio and high specific surface area that offer strong adsorption capability, many adsorption applications have been proposed for many toxic molecules [19], [20], [21], [22]. In comparison with other adsorbents like activated carbons and zeolites, carbon nanotubes suggest a great promise for the storage and separation of gases [23]. Gas molecules can find effective binding sites on the carbon nanotubes because of their highly accessible adsorption sites due to the presence of defects and porous structure [24,25]. So far, many theoretical and experimental studies were presented for the adsorption of different gases on CNTs [21]. However, SWCNT can only adsorb molecules like NO, NOx, SO2, CH4 and NH3 [20,23,26], they are not capable of adsorbing toxic molecules unless they are doped with atoms like oxygen, nitrogen or boron that means doping is an effective method in order to alter the chemical properties of carbon nanotubes according to the desired application [15,17]. Band structure, adsorption and sensor properties of the carbon nanotube can also change with the addition of dopant atom or surface modification [24]. Boron doped CNT was found to be highly effective for the HCN and HCOH adsorption. Since the pure CNT can only detect the Cl2 molecules, B and N were added as a dopant atom in order to adsorb H2S molecules [26]. Replacement of carbon atoms of the nanotube with boron or nitrogen changes the chemical reactivity which makes easier interaction between gas molecules and nanotube via charge transfer [27]. It was found that physical adsorption occurs between H2S molecule and boron or nitrogen doped CNT by using ab initio based approach [15]. Secondary structures are formed with dopant atom as a result of charge distributions and hence this improves the interaction of polar and non-polar molecules [28]. Carbon matrix of the nanotube changes from electron-deficient type into electron-donating type with boron doping [29]. One of the most effective metals which help strong connection of gas molecule on the nanotube surface is gold nanoparticle. Modification of CNT with gold metal makes it highly sensitive to H2S even at room temperature and further Pt addition into Au makes CNT available for the adsorption of NH3 and NO2 gases by increasing the binding energy between Au/Pt-CNT and these gases [5]. Additionally, using aluminum as a dopant for CNT has been widely used for the adsorption of different gases like CO, NO, NO2, H2S and cisplatin molecule by using DFT method because of the high porosity and rich active sites of Al based materials [12,17,20,22]. Since the charge transfer between CNT and NO/NO2 molecules is enhanced by Al doping, even the adsorption of small gas molecules like NO on nanotube can be possible [22]. While the chlorobenzene (CB) adsorption takes place via physisorption on pure SWCNT, Al doped ones improve the adsorption energies of the CB molecule as well as the CO molecule [30]. In addition to the B and Al atoms, Ga atom has been utilized as a dopant atom for carbon nanotube because of its lowest binding energy with fluorouracil molecule and formation of deficient sites which has improved the adsorption of fluorouracil (5-FU) molecule [31]. Another dopant atom which makes strong covalent bonds with carbon atoms of the nanotube is germanium. However, weak interaction of cyanide (CN) adsorption on Ge-CNT was observed which means adsorption of cyanide molecule on Ge-CNT is very low. [32]. On the other hand, thiophene and benzothiophene adsorptions were promising on Ge doped CNTs with increased reactivity of pure CNT for the removal process of these aromatic sulfur compounds from oil-hydrocarbons [33].
There are several studies related to vacancy that is formed easily on the carbon nanotube structure [34], [35], [36], [37], [38], [39]. Formation of vacancy can be improved by the adsorption capacity of CNT due to the changing physical and chemical properties of CNT [35]. Vasylenko et. al investigated the effect of vacancy in the single wall carbon nanotube on He and NO adsorption. It has been demonstrated that the formation of vacancy on CNT encourages the surface reconstruction and enables chemisorption of NO molecules. [34] Another study is related to Pb ions adsorption on (n,0) CNTs (n = 4,5,6). In this study, when there is a single gap, the adsorption capacity for the Pb ion increases significantly and the band gap changes before and after adsorption [36]. CNTs are not defect-free and single vacancies are usually present in these nanomaterials as native defects [40,41] The chemical doping of SWCNTs can occur by charge transfer between the metal (guest) and the nanotube (host). [42] The type or the diameter of the nanotube does not effect this interaction between the dopant and nanotube. Doping CNTs can be implemented by substitution of the carbon atoms with metals or intercalation of the metal atoms into the vacancies in the carbon nanotube structure [43]. Substitution reactions have been performed both experimentally and theoretically for the formation of metal doped CNTs [44], [45], [46]. Besides, Liu et. al studied the effects of vacancies on gas sensing properties of CNT. If vacancy defects are formed on the surface of CNTs, electron activity of the CNTs and hence the sensitivity to gases would increase [47].
Previous energetic evaluations have shown that 4 A° is the smallest diameter of carbon nanotubes that can be obtained experimentally with mechanically stable up to 1100 °C [48,49]. It is also the smallest superconducting nanotube [50]. SWCNTs have different diameter (n,0), length and chirality (n,n). n is the number which represents 3,4,5,6,7,8,9,10. This study related to the diameter, length and chirality of single walled carbon nanotubes on their free radical scavenging capability has shown that length and tube diameter have small influence for free radical trapping efficiency [51]. Additionally, the curvature limit of the CNTs has also been found to be ineffective regarding the gas adsorption on doped carbon nanotubes [52,53].
In the lights of these findings mentioned about carbon nanotubes, they can be considered as effective adsorbents for H2S removal as well. Although some researches have been done for the detection of H2S molecules, adsorption studies on carbon nanotube materials are not adequate for the removal of this poisonous gas molecule which is an environmental concern. In this work, H2S adsorption on metal doped carbon nanotubes has been investigated by using Density Functional Methods. This is the first study that demonstrates the Ge-doped SWCNT on H2S adsorption. According to the presented studies, while boron, gallium and germanium atoms were found to enhance the chemical and adsorption properties of SWCNT for different gases, comparison of H2S adsorption on these types of metal doped (4,0) SWCNTs have not been studied entirely.
Section snippets
Computational method
In this research, theoretical calculation is based on Density Functional Theory [54]. The method utilized for calculation applied in Gaussian 09 software is B3LYP method [55]. 6-31G(d,p) basis set was utilized for C, H and S atoms. LANL2DZ basis set was used for Ga, Ge and B atoms. The usage of different basis sets (LANL2DZ basis set for metal dopant atoms and 6-31G(d,p) basis set for non-metal atoms) has been also used in literature [14,[56], [57], [58], [59], [60], [61], [62], [63], [64]].
Results and discussion
Firstly, single point energy (SPE) calculation was done to find the spin multiplicities (SMs) for all of the metal doped carbon nanotubes and hydrogen sulfide molecule. Then all doped carbon nanotube clusters and hydrogen sulfide molecule were optimized by utilizing equilibrium geometry calculations. Hydrogen sulfide molecule was taken as neutral charge and state of singlet. The vibrational frequency for H-S stretching mode after optimization has been found as 2696.75 cm−1 which is coherent
Conclusions
In this work, the molecular adsorption of hydrogen sulfide on gallium, germanium and boron doped (4,0) single-walled carbon nanotubes (SWCNTs) were investigated by using Density Functional Theory calculations. For the adsorption of hydrogen sulfide, B3LYP method with 6-31G(d,p) and LANL2DZ basis sets have been utilized. The structural properties and adsorption behaviors of these clusters were described by using the values of adsorption energy, bond distances and vibrational frequencies and
CRediT authorship contribution statement
Gozde Gecim: Conceptualization, Methodology, Validation, Formal analysis, Writing - review & editing, Investigation. Mehtap Ozekmekci: Conceptualization, Methodology, Writing - original draft, Validation, Investigation, Formal analysis, Writing - review & editing.
Declaration of Competing Interest
None.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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