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Density functional study on the adsorption and dissociation of nitroamine over the nanosized tube of MgO
Physica E 62 (2014) 48–54
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Density functional study on the adsorption and dissociation of nitroamine over the nanosized tube of MgO Maryam Nayebzadeh a, Ali Ahmadi Peyghan b,n, Hamed Soleymanabadi c a
Department of Chemistry, Khajeh Nasir Toosi University of Technology, Tehran, Iran Young Researchers and Elite club, Central Tehran Branch, Islamic Azad University, Tehran, Iran c Department of Chemistry, College of Science, Central Tehran Branch, Islamic Azad University, Tehran, Iran b
H I G H L I G H T S
G R A P H I C A L
The adsorption of nitroamine molecule was investigated on an MgO nanotube (MgONT). Dissociation of nitroamine at the open ends of MgONT is thermodynamically feasible. Electronic properties of MgONTs were slightly changed after the adsorption process.
The adsorption of nitroamine molecule was investigated on an MgO nanotube using density functional theory.
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 13 April 2014 Accepted 21 April 2014 Available online 29 April 2014
The adsorption of nitroamine (NH2NO2) molecule was investigated on an MgO nanotube (MgONT) using density functional theory in terms of energetic, electronic and geometric properties. It was found that adsorption and dissociation energies of NH2NO2 on the tube are about 20.7–47.7 and 24.9–49.6 kcal/mol, respectively. We found that the dissociation of nitroamine at the open ends of MgONT is thermodynamically feasible. Density of states analysis shows that the electronic properties of the MgONT were slightly changed after the adsorption and dissociation of NH2NO2 molecule. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanotube DFT Electronic property Dissociation
A B S T R A C T
1. Introduction Since the last two decades, carbon nanotubes (CNTs) [1] have attracted many research scholars from all over the world because of their exciting mechanical, electronic, magnetic, and thermodynamic properties in the various fields of nanoelectronic devices, energy storage, chemical probes, biosensors, field emission displays and medical monitoring [2,3]. Because of high surface to
n
Corresponding author. Tel.: þ 98 9125061827. E-mail address: [email protected] (A.A. Peyghan).
http://dx.doi.org/10.1016/j.physe.2014.04.016 1386-9477/& 2014 Elsevier B.V. All rights reserved.
volume ratio, CNTs are suitable for the adsorption of atmospheric gas molecules. The change in electronic and structural properties of CNTs and their exposure to environmental gases have been studied by experimental and theoretical methods [4,5]. Nanotube structures are not limited to carbon; numerous inorganic nanotubes have been prepared as well [6–10]. Recently, magnesium oxide nanotubes (MgONTs) have attracted considerable interest because of the development of their synthesis methods and the study of their remarkable properties [11,12]. Solid MgO is known as an inert material with a high melting point, consistent with strong ionic bonding, as a typical wideband gap insulator. Pure MgONTs are attractive as important metal-oxide
M. Nayebzadeh et al. / Physica E 62 (2014) 48–54
quasi-one-dimensional nanostructures for applications in catalysis, as an additive in refractory and superconductor products [13]. Previous theoretical simulations [14,15] of MgONTs have shown that the morphology of MgONTs turns out to be very sensitive to their size and to the number of atomic layers forming the tube walls. For example, single-walled tubes adopt a cylindrical shape, whereas triple-walled MgO tubes retain the shape of square prisms with smoothed edges (as MgONTs synthesized) and their bond lengths and thermal stability are close to the corresponding values of crystalline magnesium oxide [15]. Understanding and controlling the physical and chemical mechanisms behind reactions in heterogeneous catalysis stand as one of the long-term goals for surface science, and also, a sound understanding of the chemical reaction is a fundamental aim of chemistry. The MgO has been considered as an ideal system in order to study the catalytic properties of oxides, particularly because of its very simple cubic crystalline structure. It comes mainly from the strong Lewis basicity of surface oxygen anions. It is believed that several reactions of catalytic interest comprise primarily the rupture of a heterolytic bond where the basic character of an O2 anion predominates over the acid character of an Mg2 þ cation. This behavior can be observed in relatively simple reactions such as the H2 dissociation [16]. Nitroamine (NH2NO2) is the simplest prototype of nitramine energetic materials. In particular, the reaction of NH2NO2 with metal oxide surfaces is a catalytically important reaction [17]. By studying this reaction one can enhance the designing of NH2NO2based catalysis, and provide fundamental insights into the bond making/breaking involved, which will contribute to a better understanding of the properties of nitro-containing compounds of energetic materials on metal oxide surfaces. To clarify some of the fundamental issues related to the interaction of energetic materials, especially nitro compounds with the MgO surface, our work focuses on the atomic-level description of the interaction between the energetic compound of nitroamine and the MgO surface. In the current study, the interaction of NH2NO2 with an MgONT will be investigated through density functional theory (DFT) based on analyses of structure, energies, electronic properties, stability, etc.
49
molecule and the tube is calculated by using NBO analysis, which is defined as the charge difference between the NH2NO2 molecule adsorbed on the MgONT and an isolated NH2NO2 molecule.
3. Results and discussion We first optimized the structures of (6,5) MgONT as a model. As shown in Fig. 1, in the nomination of the tube, number 6 is referred to the number of atoms locating at the open end of the tube (as a six-membered ring) and number 5 is referred to the number of atom layers in the nomination of the tube. The calculated Mg–O bond length of the MgONT was found to be about 1.95 Å and the average diameter was about 3.95 Å. We obtained a rippled surface similar to that of single-walled boron nitride nanotubes [24]: the more the electronegative atoms (O atoms) move outward, the more the electronegative atoms (Mg atoms) move inward. It should be mentioned that a small deviation from planarity of the initial polygons is observed. Wilson has obtained similar but slightly more distorted structures using an empirical compressible ion potential model [25]. There are two types of atoms in MgONT; terminal atoms (MgT and OT) are at the open ends of tube, while central atoms (MgC and OC) are in center of the tube. Two types of Mg–O bonds can be found: one of them in parallel with the tube axis, and another not in parallel with the tube axis (diagonal). 3.1. Adsorption of NH2NO2 on MgONT There are two kinds of N atoms in the molecule: one is attached to two H atoms (N1, amino group) and another bonded to two oxygen atoms (N2, nitro group). In order to obtain stable configurations (local minima) of single NH2NO2 adsorbed on the tube, various possible initial adsorption geometries including single (hydrogen, N1, N2 or oxygen), double (H–N1, O–N2 or N1–N2) and triple (H–N1–N2 or O–N2–N1) bonded atoms close to central and terminal atoms are considered. For the sake of simplicity, we have considered three most stable configurations (Fig. 2) in which the NH2NO2 molecule is as near as possible to the MgONT
2. Computational methods Geometry optimizations, energy calculations, density of states (DOS), frontier molecular orbitals (FMO), natural bond orbitals (NBO) and molecular electrostatic potential (MEP) analyses were performed on an MgONT and different NH2NO2/MgONT complexes. The B3LYP functional augmented with an empirical dispersion term (B3LYP-D) with 6–31G (d) basis set was used as implemented in GAMESS suite of program [18]. The B3LYP, which is a combination of HF with a DFT based on the Becke threeparameter exchange coupled with the Lee–Yang–Parr (LYP) correlation potential [19], is one of the most popular hybrid density functional methods used in nanostructure studies [20–22]. GaussSum program has been used to obtain the DOS results [23]. With the optimized structures, the adsorption energy (Ead) of the NH2NO2 on the pure nanotube is obtained using the following equation: Ead ¼ EðNH2 NO2 Þ þ EðMgONTÞ–EðNH2 NO2 =MgONTÞ
ð1Þ
where E (NH2NO2/MgONT) is the total energy of the NH2NO2 adsorbed form of MgONT and E (NH2NO2) is referred to the energy of an isolated NH2NO2. E (MgONT) is the energy of MgONT. The positive value of Ead indicates the exothermic character of the adsorption. To investigate the electronic charge changes through the MgONT, the net charge-transfer (QT) between NH2NO2
Fig. 1. (a) Geometrical parameters of the optimized MgONT and its density of states (DOS) plot. Bonds are in angstrom.
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M. Nayebzadeh et al. / Physica E 62 (2014) 48–54
including: (A) the NH2NO2 molecule is bonded via oxygen atom to a MgC atom, also an H–bonding is formed between OC and hydrogen atom of amino group in diagonal MgC–OC bond, (B) the NH2NO2 molecule is bonded via oxygen atom to a MgC atom, also an H-bonding is formed between OC and hydrogen atom of amino group in parallel MgC–OC bond and (C) the NH2NO2 molecule is bonded via oxygen atom to a MgT atom, also an H-bonding is formed between OT and hydrogen atom of amino group. More detailed information including values of Ead and the charge transfer (QT) is listed in Table 1. Notice that the adsorption is site-selective so that in all cases the molecule is oriented in such a way that the H atom of amino group is directly linked to an O anion, while the amino O atom is directly linked with an Mg cation. The adsorption of the O atom of nitro group preferably on the Mg atom of the tube surface rather than the O site can be attributed to low ionization potential of Mg and, therefore, having the tendency of losing its valence electron to the electro-negative oxygen. However, in the bare tube some charges are transferred from the Mg atoms to the O atoms; when an O atom is placed in the vicinity of an Mg atom of the tube surface, it competes with the O atoms of surface for receiving the
electron from the Mg atom. Based on NBO population analysis, the point charge of magnesium and oxygen in MgONT is þ0.923 e and 0.923 e, respectively. As shown by the mapped-MEP of NH2NO2 in Fig. 3, the amino group is positively charged (blue colors), while the nitro group is negatively charged (red colors). So it seems that Mg and O atoms of the tube are suitable sites for nucleophilic and electrophilic attack of –NO2 and –NH2 groups, respectively. As shown in Table 1, the Ead values corresponding to various adsorption configurations are in the range of 20.7–47.7 kcal/mol. Table 1 Adsorption energy (Ead, kcal/mol), HOMO energies (EHOMO), LUMO energies (ELUMO) and HOMO–LUMO energy gap (Eg) of NH2NO2 adsorption on MgONT. Energies are in eV. Configuration
Ead
EHOMO
ELUMO
Eg
a
MgONT A B C
– 20.7 23.1 47.7
6.15 6.36 6.14 6.44
1.64 1.87 1.88 1.90
4.51 4.49 4.26 4.54
– 0.4 5.5 0.6
a
Change of Eg of MgONT after NH2NO2 adsorption.
Fig. 2. Models for stable adsorption of single NH2NO2 on the MgONT and their density of states (DOS) plot. Bonds are in Å.
ΔEg (%)
M. Nayebzadeh et al. / Physica E 62 (2014) 48–54
51
Fig. 4. Two possible geometries for the adsorption of NHNO2 anion on MgONT.
Table 2 Adsorption and activation energies (Ead and Eact, kcal/mol), HOMO energies (EHOMO), LUMO energies (ELUMO) and HOMO–LUMO energy gap (Eg) of NH2NO2 dissociation over MgONT, in eV. Fig. 3. Computed electrostatic potential on the molecular surface of a single NH2NO2 molecule. Color ranges, in a.u.: blue, more positive than 0.010; green, between 0.010 and 0; yellow, between 0 and 0.015; red, more negative than 0.015. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Configuration C gives rise to an Ead of 47.7 kcal/mol, which is more than the Ead values for A (20.7 kcal/mol) and B (23.1 kcal/mol) configurations. Corresponding newly formed O–MgC bond lengths between the molecule and the tube in these configurations ranged between 1.97 and 2.18 Å (Fig. 2). The more exothermic behavior of configuration C may be rationalized by the fact that, in the MgONT, 3-fold atoms at the end of the tube have more tendency to react with additional species compared to 4-fold atoms at the center of the tube. This finding is generally in good agreement with the previous results reported by Huang et al., in which they have investigated CO adsorptions on an MgONT surface [26]. The interaction distances of O…MgT and H…OT in configuration C are about 1.97 and 1.11 Å, respectively, and a net charge of 0.091 e is transferred from the tube to the NH2NO2 (Table 1). On the other hand, the adsorption through configuration C induces local structural deformation on both the NH2NO2 molecule and the MgONT molecule. In particular, the O–H bond length and H–N1– N2–O dihedral angle of NH2NO2 attached to the tube increase from 1.01 Å to 1551 in free NH2NO2 to 1.54 Å and 1791, respectively. To verify the effects of NH2NO2 adsorption on the electronic properties of the tube, DOS plots for different models of the pristine and NH2NO2/MgONT complexes have been shown in Figs. 1 and 2. For the bare MgONT (Fig. 1), it can be concluded that it is a semiconductor material with a difference in energies between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), Eg, of 4.51 eV. However, when the NH2NO2 molecule is adsorbed on the MgONT, the electronic properties of the tube did not change significantly and Eg of the tube slightly decreased to 4.54–4.26 eV after NH2NO2 adsorption. Therefore, it can be concluded that the associative adsorption of NH2NO2 molecule cannot essentially change the electronic properties of the MgONT. 3.2. Chemical dissociation of NH2NO2 on the MgONT In order to consider the favorability of NH2NO2 dissociation on the MgONT, we first investigated energetic possibility of the hypothetical NH2NO2 dissociation into different fragments on various double atoms of the tube surface including MgT–OT, MgT–OC, MgC–OT and MgC–OC. As candidates for stable dissociation geometries, we examined the bidentate and bridging
Configuration
Ead
Eact
EHOMO
ELUMO
Eg
a
MgONT P Q R S Z
– 31.7 24.9 39.2 49.6 47.4
– 17.2 15.1 12.3 4.2 3.8
6.15 6.41 6.25 6.39 6.37 6.50
1.64 1.80 1.93 1.93 1.80 1.93
4.51 4.41 4.32 4.46 4.57 4.57
– 2.2 4.2 1.1 1.3 1.3
a
ΔEg (%)
Change of Eg of MgONT after NH2NO2 adsorption.
structures shown in Fig. 4. In the bridging format, one oxygen atom and one nitrogen atom interact with two Mg atoms, while one oxygen atom and one nitrogen atom simultaneously interact with only one Mg atom in the bidentate formats. More detailed information from the simulation of the NH2NO2 dissociation over MgONT, including values of Ead, Eg and the charge transfer for these configurations, is listed in Table 2. The Ead values corresponding to various dissociation configurations are in the range of 24.9–49.6 kcal/mol. Although in all of the dissociation configurations a new OC–H bond was formed, only the geometries of the bridging structure (configuration P) are almost the same as that of the free NHNO2 anion. The N1–N2–O angle in the bidentate structures (configurations Q, R and S) is less than that in the bridging structure or the free anion (Fig. 5). Both the bridging and bidentate structures are stabilized by the overlap between the unoccupied s orbital of the Mg atom and the occupied 2pz orbital of the O and N1 atoms of the NHNO2 anion. Each 2pz orbital of the O and N1 atoms of the NHNO2 anion interacts with each s orbital of the Mg atom in the bridging structure, while the one pz orbital of the O and N1 atoms of the NHNO2 anion interacts with only one s orbital of the Mg atom in the bidentate structure. The N1–N2–O angle in the bidentate structures is decreased to achieve a greater overlap of these orbitals. The most interesting case, however, is the dissociation of the NH2NO2 molecule on the terminal atoms through configuration Z with corresponding Ead of 47.4 kcal/mol. Based on the NBO results and geometry analysis, the H–N1 bond of the molecule is broken after the adsorption process (Fig. 6). Interesting phenomenon in this configuration is the migration of one of the hydrogen atoms to nitro group. Also, two new bonds are formed: H–OT and N1–MgT, with corresponding lengths of 1.11 and 2.06 Å, respectively. The kinetic favorability of the all energetically possible configurations was investigated using a synchronous transit-guided quasi-Newton (STQN) method. This method, developed by Schlegel and co-workers [27,28], uses a quadratic synchronous transit approach to get closer to the quadratic region of the transition state and then uses a quasi-Newton or eigenvector-following
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Fig. 5. Models for stable dissociation of single NH2NO2 on the MgONT. Bonds are in Å.
Fig. 6. Density of states (DOS) plots for different models of NH2NO2 dissociated MgONT.
M. Nayebzadeh et al. / Physica E 62 (2014) 48–54
53
Fig. 7. Calculated HOMO and LUMO profiles for different configurations of NH2NO2 adsorption and dissociation on MgONT.
algorithm to complete the optimization. The results show that nitroamine adsorption occurs barrierless through configurations A, B, and C. As shown in Table 2, an energy barrier must be overcome to get into a final configuration in the configurations P, Q, R, S, and Z. It can be seen that the energy barriers for nitroamine dissociation at the end of the tube are significantly smaller than the others. However, we think that the activation energies are so small that the dissociation process may overcome them, even at room temperature, especially in the cases of S and Z. In the following, we have studied the influence of NH2NO2 dissociation on the electronic properties of the nanotube. DOS plots have been shown in Fig. 6, indicating that after the dissociation, the electronic properties of the tube are slightly changed compared to the pristine tube and all the chemically dissociated MgONT are still semiconductors with a wide Eg close to that of the pristine MgONT. For example, in the most stable case (configuration S), the DOS near the conduction and valence levels has a slight change compared to that of the pristine tube, which would result in an Eg enhancement from 4.51 to 4.57 eV. The DOS plots indicate that the contribution of the adsorption molecule is largely away from the Fermi level. Calculated profiles of HOMO and LUMO (Fig. 7) demonstrate that the LUMO has yet remained on the
nanotube, while the HOMO has shifted on the NH2NO2 molecule in all configurations.
4. Conclusion DFT calculations were employed to investigate the adsorption of NH2NO2 on an MgONT. We determined site preferences, binding geometries and adsorption, and dissociation energies of the NH2NO2. The energies corresponding to the adsorption and dissociation of NH2NO2 on the MgONTs were calculated to be in the range of 20.7–47.7 kcal/mol and 24.9–49.6 kcal/mol, respectively. The NH2NO2 adsorption and dissociation at the open end of the tube are facile thermodynamically. Electronic properties of the MgONT have limited change after the adsorption and dissociation of NH2NO2 molecule.
References [1] S. Iijima, Nature 354 (1991) 56. [2] A.D. Zhang, D.L. Wang, D.Y. Hou, Comput. Theor. Chem. 999 (2012) 121.
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[3] B.D. Chen, C.L. Yang, J.S. Yang, M.S. Wang, X.G. Ma, Comput. Theor. Chem. 991 (2012) 93. [4] J.M. Bonard, M. Croci, C. Klinke, R. Kurt, O. Noury, N. Weiss, Carbon 40 (2002) 1715. [5] S. Motavas, A. Ivanov, A. Nojeh, Comput. Theor. Chem. 1020 (2013) 32. [6] A. Ahmadi, J. Beheshtian, M. Kamfiroozi, J. Mol. Model. 18 (2012) 1729. [7] A. Ahmadi Peyghan, N. Hadipour, Z. Bagheri, J. Phys. Chem. C 117 (2013) 2427. [8] J. Beheshtian, M.T. Baei, A.A. Peyghan, Z. Bagheri, J. Mol. Model. 18 (2012) 4745. [9] A. Soltani, A. Ahmadi Peyghan, Z. Bagheri, Physica E 48 (2013) 176. [10] M.T. Baei, A.A. Peyghan, Z. Bagheri, Chin. Chem. Lett. 23 (2012) 965. [11] I.R. Shein, A.N. Enyashin, A.L. Ivanovskii, Phys. Rev. B 75 (2007) 245404. [12] G. Bilalbegović, Phys. Rev. B 70 (2004) 045407. [13] J. Zhan, Y. Bando, J. Hu, D. Golberg, Inorg. Chem. 43 (2004) 2462. [14] R. Kakkar, P.N. Kapoor, J. Phys. Chem. B 108 (2004) 18140. [15] R. Kakkar, P.N. Kapoor, K.J. Klabunde, J. Phys. Chem. B 110 (2006) 25941. [16] T. Ito, M. Kuramoto, M. Yoshioka, T. Tokuda, J. Phys. Chem. 87 (1983) 4411.
[17] J.Y. Zhao, F.Q. Zhao, X.H. Ju, H.X. Gao, S.Q. Zhou, J. Clust. Sci. 23 (2012) 395. [18] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347. [19] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [20] L. Chen, C. Xu, X.-F. Zhang, T. Zhou, Physica E 41 (2009) 852. [21] L.-H. Gan, J.-Q. Zhao, Physica E 41 (2009) 1249. [22] H. Kö kten, Ş. Erkoç, Physica E 44 (2011) 215. [23] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner, J. Comput. Chem. 29 (2008) 839. [24] A. Ahmadi, J. Beheshtian, N.L. Hadipour, Struct. Chem. 22 (2011) 183. [25] M. Wilson, J. Phys. Chem. 101 (1997) 4917. [26] M. Yang, Y. Zhang, S. Huang, H. Liu, P. Wang, H. Tian, Appl. Surf. Sci. 258 (2011) 1429. [27] C. Peng, H.B. Schlegel, Isr. J. Chem. 33 (1993) 449. [28] C. Peng, P.Y. Ayala, H.B. Schlegel, M.J. Frisch, J. Comput. Chem. 17 (1996) 49.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Density functional study on the adsorption and dissociation of nitroamine over the nanosized tube of MgO Maryam Nayebzadeh a, Ali Ahmadi Peyghan b,n, Hamed Soleymanabadi c a
Department of Chemistry, Khajeh Nasir Toosi University of Technology, Tehran, Iran Young Researchers and Elite club, Central Tehran Branch, Islamic Azad University, Tehran, Iran c Department of Chemistry, College of Science, Central Tehran Branch, Islamic Azad University, Tehran, Iran b
H I G H L I G H T S
G R A P H I C A L
The adsorption of nitroamine molecule was investigated on an MgO nanotube (MgONT). Dissociation of nitroamine at the open ends of MgONT is thermodynamically feasible. Electronic properties of MgONTs were slightly changed after the adsorption process.
The adsorption of nitroamine molecule was investigated on an MgO nanotube using density functional theory.
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 13 April 2014 Accepted 21 April 2014 Available online 29 April 2014
The adsorption of nitroamine (NH2NO2) molecule was investigated on an MgO nanotube (MgONT) using density functional theory in terms of energetic, electronic and geometric properties. It was found that adsorption and dissociation energies of NH2NO2 on the tube are about 20.7–47.7 and 24.9–49.6 kcal/mol, respectively. We found that the dissociation of nitroamine at the open ends of MgONT is thermodynamically feasible. Density of states analysis shows that the electronic properties of the MgONT were slightly changed after the adsorption and dissociation of NH2NO2 molecule. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanotube DFT Electronic property Dissociation
A B S T R A C T
1. Introduction Since the last two decades, carbon nanotubes (CNTs) [1] have attracted many research scholars from all over the world because of their exciting mechanical, electronic, magnetic, and thermodynamic properties in the various fields of nanoelectronic devices, energy storage, chemical probes, biosensors, field emission displays and medical monitoring [2,3]. Because of high surface to
n
Corresponding author. Tel.: þ 98 9125061827. E-mail address: [email protected] (A.A. Peyghan).
http://dx.doi.org/10.1016/j.physe.2014.04.016 1386-9477/& 2014 Elsevier B.V. All rights reserved.
volume ratio, CNTs are suitable for the adsorption of atmospheric gas molecules. The change in electronic and structural properties of CNTs and their exposure to environmental gases have been studied by experimental and theoretical methods [4,5]. Nanotube structures are not limited to carbon; numerous inorganic nanotubes have been prepared as well [6–10]. Recently, magnesium oxide nanotubes (MgONTs) have attracted considerable interest because of the development of their synthesis methods and the study of their remarkable properties [11,12]. Solid MgO is known as an inert material with a high melting point, consistent with strong ionic bonding, as a typical wideband gap insulator. Pure MgONTs are attractive as important metal-oxide
M. Nayebzadeh et al. / Physica E 62 (2014) 48–54
quasi-one-dimensional nanostructures for applications in catalysis, as an additive in refractory and superconductor products [13]. Previous theoretical simulations [14,15] of MgONTs have shown that the morphology of MgONTs turns out to be very sensitive to their size and to the number of atomic layers forming the tube walls. For example, single-walled tubes adopt a cylindrical shape, whereas triple-walled MgO tubes retain the shape of square prisms with smoothed edges (as MgONTs synthesized) and their bond lengths and thermal stability are close to the corresponding values of crystalline magnesium oxide [15]. Understanding and controlling the physical and chemical mechanisms behind reactions in heterogeneous catalysis stand as one of the long-term goals for surface science, and also, a sound understanding of the chemical reaction is a fundamental aim of chemistry. The MgO has been considered as an ideal system in order to study the catalytic properties of oxides, particularly because of its very simple cubic crystalline structure. It comes mainly from the strong Lewis basicity of surface oxygen anions. It is believed that several reactions of catalytic interest comprise primarily the rupture of a heterolytic bond where the basic character of an O2 anion predominates over the acid character of an Mg2 þ cation. This behavior can be observed in relatively simple reactions such as the H2 dissociation [16]. Nitroamine (NH2NO2) is the simplest prototype of nitramine energetic materials. In particular, the reaction of NH2NO2 with metal oxide surfaces is a catalytically important reaction [17]. By studying this reaction one can enhance the designing of NH2NO2based catalysis, and provide fundamental insights into the bond making/breaking involved, which will contribute to a better understanding of the properties of nitro-containing compounds of energetic materials on metal oxide surfaces. To clarify some of the fundamental issues related to the interaction of energetic materials, especially nitro compounds with the MgO surface, our work focuses on the atomic-level description of the interaction between the energetic compound of nitroamine and the MgO surface. In the current study, the interaction of NH2NO2 with an MgONT will be investigated through density functional theory (DFT) based on analyses of structure, energies, electronic properties, stability, etc.
49
molecule and the tube is calculated by using NBO analysis, which is defined as the charge difference between the NH2NO2 molecule adsorbed on the MgONT and an isolated NH2NO2 molecule.
3. Results and discussion We first optimized the structures of (6,5) MgONT as a model. As shown in Fig. 1, in the nomination of the tube, number 6 is referred to the number of atoms locating at the open end of the tube (as a six-membered ring) and number 5 is referred to the number of atom layers in the nomination of the tube. The calculated Mg–O bond length of the MgONT was found to be about 1.95 Å and the average diameter was about 3.95 Å. We obtained a rippled surface similar to that of single-walled boron nitride nanotubes [24]: the more the electronegative atoms (O atoms) move outward, the more the electronegative atoms (Mg atoms) move inward. It should be mentioned that a small deviation from planarity of the initial polygons is observed. Wilson has obtained similar but slightly more distorted structures using an empirical compressible ion potential model [25]. There are two types of atoms in MgONT; terminal atoms (MgT and OT) are at the open ends of tube, while central atoms (MgC and OC) are in center of the tube. Two types of Mg–O bonds can be found: one of them in parallel with the tube axis, and another not in parallel with the tube axis (diagonal). 3.1. Adsorption of NH2NO2 on MgONT There are two kinds of N atoms in the molecule: one is attached to two H atoms (N1, amino group) and another bonded to two oxygen atoms (N2, nitro group). In order to obtain stable configurations (local minima) of single NH2NO2 adsorbed on the tube, various possible initial adsorption geometries including single (hydrogen, N1, N2 or oxygen), double (H–N1, O–N2 or N1–N2) and triple (H–N1–N2 or O–N2–N1) bonded atoms close to central and terminal atoms are considered. For the sake of simplicity, we have considered three most stable configurations (Fig. 2) in which the NH2NO2 molecule is as near as possible to the MgONT
2. Computational methods Geometry optimizations, energy calculations, density of states (DOS), frontier molecular orbitals (FMO), natural bond orbitals (NBO) and molecular electrostatic potential (MEP) analyses were performed on an MgONT and different NH2NO2/MgONT complexes. The B3LYP functional augmented with an empirical dispersion term (B3LYP-D) with 6–31G (d) basis set was used as implemented in GAMESS suite of program [18]. The B3LYP, which is a combination of HF with a DFT based on the Becke threeparameter exchange coupled with the Lee–Yang–Parr (LYP) correlation potential [19], is one of the most popular hybrid density functional methods used in nanostructure studies [20–22]. GaussSum program has been used to obtain the DOS results [23]. With the optimized structures, the adsorption energy (Ead) of the NH2NO2 on the pure nanotube is obtained using the following equation: Ead ¼ EðNH2 NO2 Þ þ EðMgONTÞ–EðNH2 NO2 =MgONTÞ
ð1Þ
where E (NH2NO2/MgONT) is the total energy of the NH2NO2 adsorbed form of MgONT and E (NH2NO2) is referred to the energy of an isolated NH2NO2. E (MgONT) is the energy of MgONT. The positive value of Ead indicates the exothermic character of the adsorption. To investigate the electronic charge changes through the MgONT, the net charge-transfer (QT) between NH2NO2
Fig. 1. (a) Geometrical parameters of the optimized MgONT and its density of states (DOS) plot. Bonds are in angstrom.
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including: (A) the NH2NO2 molecule is bonded via oxygen atom to a MgC atom, also an H–bonding is formed between OC and hydrogen atom of amino group in diagonal MgC–OC bond, (B) the NH2NO2 molecule is bonded via oxygen atom to a MgC atom, also an H-bonding is formed between OC and hydrogen atom of amino group in parallel MgC–OC bond and (C) the NH2NO2 molecule is bonded via oxygen atom to a MgT atom, also an H-bonding is formed between OT and hydrogen atom of amino group. More detailed information including values of Ead and the charge transfer (QT) is listed in Table 1. Notice that the adsorption is site-selective so that in all cases the molecule is oriented in such a way that the H atom of amino group is directly linked to an O anion, while the amino O atom is directly linked with an Mg cation. The adsorption of the O atom of nitro group preferably on the Mg atom of the tube surface rather than the O site can be attributed to low ionization potential of Mg and, therefore, having the tendency of losing its valence electron to the electro-negative oxygen. However, in the bare tube some charges are transferred from the Mg atoms to the O atoms; when an O atom is placed in the vicinity of an Mg atom of the tube surface, it competes with the O atoms of surface for receiving the
electron from the Mg atom. Based on NBO population analysis, the point charge of magnesium and oxygen in MgONT is þ0.923 e and 0.923 e, respectively. As shown by the mapped-MEP of NH2NO2 in Fig. 3, the amino group is positively charged (blue colors), while the nitro group is negatively charged (red colors). So it seems that Mg and O atoms of the tube are suitable sites for nucleophilic and electrophilic attack of –NO2 and –NH2 groups, respectively. As shown in Table 1, the Ead values corresponding to various adsorption configurations are in the range of 20.7–47.7 kcal/mol. Table 1 Adsorption energy (Ead, kcal/mol), HOMO energies (EHOMO), LUMO energies (ELUMO) and HOMO–LUMO energy gap (Eg) of NH2NO2 adsorption on MgONT. Energies are in eV. Configuration
Ead
EHOMO
ELUMO
Eg
a
MgONT A B C
– 20.7 23.1 47.7
6.15 6.36 6.14 6.44
1.64 1.87 1.88 1.90
4.51 4.49 4.26 4.54
– 0.4 5.5 0.6
a
Change of Eg of MgONT after NH2NO2 adsorption.
Fig. 2. Models for stable adsorption of single NH2NO2 on the MgONT and their density of states (DOS) plot. Bonds are in Å.
ΔEg (%)
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Fig. 4. Two possible geometries for the adsorption of NHNO2 anion on MgONT.
Table 2 Adsorption and activation energies (Ead and Eact, kcal/mol), HOMO energies (EHOMO), LUMO energies (ELUMO) and HOMO–LUMO energy gap (Eg) of NH2NO2 dissociation over MgONT, in eV. Fig. 3. Computed electrostatic potential on the molecular surface of a single NH2NO2 molecule. Color ranges, in a.u.: blue, more positive than 0.010; green, between 0.010 and 0; yellow, between 0 and 0.015; red, more negative than 0.015. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Configuration C gives rise to an Ead of 47.7 kcal/mol, which is more than the Ead values for A (20.7 kcal/mol) and B (23.1 kcal/mol) configurations. Corresponding newly formed O–MgC bond lengths between the molecule and the tube in these configurations ranged between 1.97 and 2.18 Å (Fig. 2). The more exothermic behavior of configuration C may be rationalized by the fact that, in the MgONT, 3-fold atoms at the end of the tube have more tendency to react with additional species compared to 4-fold atoms at the center of the tube. This finding is generally in good agreement with the previous results reported by Huang et al., in which they have investigated CO adsorptions on an MgONT surface [26]. The interaction distances of O…MgT and H…OT in configuration C are about 1.97 and 1.11 Å, respectively, and a net charge of 0.091 e is transferred from the tube to the NH2NO2 (Table 1). On the other hand, the adsorption through configuration C induces local structural deformation on both the NH2NO2 molecule and the MgONT molecule. In particular, the O–H bond length and H–N1– N2–O dihedral angle of NH2NO2 attached to the tube increase from 1.01 Å to 1551 in free NH2NO2 to 1.54 Å and 1791, respectively. To verify the effects of NH2NO2 adsorption on the electronic properties of the tube, DOS plots for different models of the pristine and NH2NO2/MgONT complexes have been shown in Figs. 1 and 2. For the bare MgONT (Fig. 1), it can be concluded that it is a semiconductor material with a difference in energies between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), Eg, of 4.51 eV. However, when the NH2NO2 molecule is adsorbed on the MgONT, the electronic properties of the tube did not change significantly and Eg of the tube slightly decreased to 4.54–4.26 eV after NH2NO2 adsorption. Therefore, it can be concluded that the associative adsorption of NH2NO2 molecule cannot essentially change the electronic properties of the MgONT. 3.2. Chemical dissociation of NH2NO2 on the MgONT In order to consider the favorability of NH2NO2 dissociation on the MgONT, we first investigated energetic possibility of the hypothetical NH2NO2 dissociation into different fragments on various double atoms of the tube surface including MgT–OT, MgT–OC, MgC–OT and MgC–OC. As candidates for stable dissociation geometries, we examined the bidentate and bridging
Configuration
Ead
Eact
EHOMO
ELUMO
Eg
a
MgONT P Q R S Z
– 31.7 24.9 39.2 49.6 47.4
– 17.2 15.1 12.3 4.2 3.8
6.15 6.41 6.25 6.39 6.37 6.50
1.64 1.80 1.93 1.93 1.80 1.93
4.51 4.41 4.32 4.46 4.57 4.57
– 2.2 4.2 1.1 1.3 1.3
a
ΔEg (%)
Change of Eg of MgONT after NH2NO2 adsorption.
structures shown in Fig. 4. In the bridging format, one oxygen atom and one nitrogen atom interact with two Mg atoms, while one oxygen atom and one nitrogen atom simultaneously interact with only one Mg atom in the bidentate formats. More detailed information from the simulation of the NH2NO2 dissociation over MgONT, including values of Ead, Eg and the charge transfer for these configurations, is listed in Table 2. The Ead values corresponding to various dissociation configurations are in the range of 24.9–49.6 kcal/mol. Although in all of the dissociation configurations a new OC–H bond was formed, only the geometries of the bridging structure (configuration P) are almost the same as that of the free NHNO2 anion. The N1–N2–O angle in the bidentate structures (configurations Q, R and S) is less than that in the bridging structure or the free anion (Fig. 5). Both the bridging and bidentate structures are stabilized by the overlap between the unoccupied s orbital of the Mg atom and the occupied 2pz orbital of the O and N1 atoms of the NHNO2 anion. Each 2pz orbital of the O and N1 atoms of the NHNO2 anion interacts with each s orbital of the Mg atom in the bridging structure, while the one pz orbital of the O and N1 atoms of the NHNO2 anion interacts with only one s orbital of the Mg atom in the bidentate structure. The N1–N2–O angle in the bidentate structures is decreased to achieve a greater overlap of these orbitals. The most interesting case, however, is the dissociation of the NH2NO2 molecule on the terminal atoms through configuration Z with corresponding Ead of 47.4 kcal/mol. Based on the NBO results and geometry analysis, the H–N1 bond of the molecule is broken after the adsorption process (Fig. 6). Interesting phenomenon in this configuration is the migration of one of the hydrogen atoms to nitro group. Also, two new bonds are formed: H–OT and N1–MgT, with corresponding lengths of 1.11 and 2.06 Å, respectively. The kinetic favorability of the all energetically possible configurations was investigated using a synchronous transit-guided quasi-Newton (STQN) method. This method, developed by Schlegel and co-workers [27,28], uses a quadratic synchronous transit approach to get closer to the quadratic region of the transition state and then uses a quasi-Newton or eigenvector-following
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Fig. 5. Models for stable dissociation of single NH2NO2 on the MgONT. Bonds are in Å.
Fig. 6. Density of states (DOS) plots for different models of NH2NO2 dissociated MgONT.
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Fig. 7. Calculated HOMO and LUMO profiles for different configurations of NH2NO2 adsorption and dissociation on MgONT.
algorithm to complete the optimization. The results show that nitroamine adsorption occurs barrierless through configurations A, B, and C. As shown in Table 2, an energy barrier must be overcome to get into a final configuration in the configurations P, Q, R, S, and Z. It can be seen that the energy barriers for nitroamine dissociation at the end of the tube are significantly smaller than the others. However, we think that the activation energies are so small that the dissociation process may overcome them, even at room temperature, especially in the cases of S and Z. In the following, we have studied the influence of NH2NO2 dissociation on the electronic properties of the nanotube. DOS plots have been shown in Fig. 6, indicating that after the dissociation, the electronic properties of the tube are slightly changed compared to the pristine tube and all the chemically dissociated MgONT are still semiconductors with a wide Eg close to that of the pristine MgONT. For example, in the most stable case (configuration S), the DOS near the conduction and valence levels has a slight change compared to that of the pristine tube, which would result in an Eg enhancement from 4.51 to 4.57 eV. The DOS plots indicate that the contribution of the adsorption molecule is largely away from the Fermi level. Calculated profiles of HOMO and LUMO (Fig. 7) demonstrate that the LUMO has yet remained on the
nanotube, while the HOMO has shifted on the NH2NO2 molecule in all configurations.
4. Conclusion DFT calculations were employed to investigate the adsorption of NH2NO2 on an MgONT. We determined site preferences, binding geometries and adsorption, and dissociation energies of the NH2NO2. The energies corresponding to the adsorption and dissociation of NH2NO2 on the MgONTs were calculated to be in the range of 20.7–47.7 kcal/mol and 24.9–49.6 kcal/mol, respectively. The NH2NO2 adsorption and dissociation at the open end of the tube are facile thermodynamically. Electronic properties of the MgONT have limited change after the adsorption and dissociation of NH2NO2 molecule.
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