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Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study
Accepted Manuscript Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study Mehdi D. Esrafili PII:
S0038-1098(18)30664-1
DOI:
10.1016/j.ssc.2018.09.007
Reference:
SSC 13499
To appear in:
Solid State Communications
Received Date: 28 February 2018 Revised Date:
26 July 2018
Accepted Date: 11 September 2018
Please cite this article as: M.D. Esrafili, Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study, Solid State Communications (2018), doi: 10.1016/ j.ssc.2018.09.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study
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Mehdi D. Esrafili Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran Phone: (+98) 4212237955. Fax: (+98) 4212276060. P.O. Box: 5513864596. E-mail: [email protected]
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Abstract
Using first-principle calculations, we compare the catalytic activity of the experimentally available C- or Si-doped boron nitride nanosheet (C-/Si-BNNS) towards the epoxidation of
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ethylene by N2O molecule. The epoxidation reaction of ethylene over these surfaces includes the decomposing of N2O into N2 and an oxygen atom (Oads), formation of the ethyleneoxy intermediate, and the creation of ethylene oxide. Our results show that the catalytic activity of CBNNS for epoxidation of ethylene is better than Si-BNNS, due to more favorable charge-transfer effects. The results presented here suggest a green, low-cost, and metal-free approach for low-
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temperature epoxidation of ethylene using C-BNNS.
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Keywords: A. BN nanosheet; C. Ethylene oxidation; D. Reaction Mechanism; D. C-doping.
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1. Introduction The catalytic oxidation of ethylene to ethylene oxide (EO) has received great attention due its importance in the chemical industry. EO is a valuable intermediate for the production of various fine chemicals such as ethylene glycol, polyester, plastics, and ethoxylates [1,2]. EO is
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commercially produced by the silver-catalyzed partial oxidation of ethylene with air or oxygen. A commonly used support material for silver catalysts is α-alumina, mainly due to its inertness for the isomerization of EO to acetaldehyde [3-10]. Similar epoxidation reactions have also been reported on other noble metal-based catalysts such as gold clusters supported on titanosilicalite
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[11-13]. Despite their high reactivity and selectivity, however, the high price, poor durability and rapid deactivation of these noble-metal based catalysts severely limit their large-scale application
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in the ethylene epoxidation. Thus, alternative costless, stable and metal-free catalysts with high catalytic activities comparable to or even higher than noble metal-based catalysts are demanding. Since the discovery of graphene in 2004 [14], much attention has been devoted into novel analogous two-dimensional structures, like hexagonal boron nitride (h-BN) sheet [15-17]. Different from semimetallic graphene with zero energy gap, h-BN is always a semiconductor with a wide band gap (> 5.0 eV) [18,19]. Moreover, due to the large specific surface area and
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chemical inertness of h-BN, it is usually considered as an ideal support in heterogeneous catalysis [20-22]. It is also accepted that doping of h-BN with foreign atoms can greatly modulate its electronic structure and chemical properties [15,23-26]. Thus the inert h-BN nanosheet may be transformed into a very active catalyst through the incorporation of these
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impurities. For example, such heteroatom-doped h-BN sheets have been reported as a highly active catalyst for the CO oxidation [27-29]. A great potential has been also suggested for further
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application of these materials in the low-temperature reduction of N2O [30] or O2 [22,31-33]. To the best of our knowledge, the catalytic oxidation of ethylene to EO has not been reported yet over heteroatom-doped h-BN nanosheets. In this work, by using plane-wave density functional theory (DFT) calculations, we compare the potential of C- or Si-doped h-BN nanosheet (C-/Si-BNNS) as an active and metal-free catalyst for the epoxidation of ethylene by N2O molecule. Fortunately, both these metal-free BN-based materials have already been achieved experimentally [24,34]. The present methodology for the epoxidation of ethylene is a green approach, since N2 gas is the only by-product of this reaction, and N2O as a main greenhouse gas is removed at the same time. 2
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2. Computational details All the spin-polarized DFT calculations were carried out using the DMol3 [35,36]. The Perdew-Burke-Ernzerhof (PBE) density functional [37] within the generalized gradient approximation (GGA) was chosen for the DFT calculations. A double numerical plus
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polarization (DNP) was used as the basis set. To consider the van der Waals effects, the empirical correction in the Grimme scheme [38,39] was applied. A 4×4 supercell containing 16 B and 16 N atoms with the periodic boundary conditions on the x–y plane was employed as the pristine h-BN. A boron atom was then substituted with a C or Si atom to build C- or Si-BNNS.
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The 3 × 3 × 1 k points were used for calculating the Brillouin zone integration. The convergence tolerance of energy of 1.0×10-5 Ha, maximum force of 0.001 Ha/Å and maximum displacement
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of 0.005 Å were employed in all the geometry optimizations. In order to achieve accurate electronic convergence, we apply a smearing of 0.005 Ha to the orbital occupation. The minimum-energy pathway (MEP) for each reaction step was obtained by linear synchronous transit (LST)/quadratic synchronous transit (QST) and nudged elastic band (NEB) methods. The nature of transition states was verified by the vibrational frequencies, which exhibit only one imaginary frequency throughout the potential energy surface. The zero-point
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energy (ZPE) correction was considered for all adsorption energies and activation energies. The adsorption energy (Eads) of each adsorbate was obtained using the following equation: Eads(A) = EA–S - ES - EA
(1)
where EA–S, ES and EA are the total energies of the adsorbate-substrate (A–S) system, the
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substrate (S) and adsorbate (A), respectively. The formation energy (Ef) of Si-BNNS was calculated by the formula:
(2)
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Ef = (EX-BNNS + EB - EX) - EBNNS
where EBNNS is the total energy of the pristine h-BN, EX-BNNS is the total energy of the C- or SiBNNS, EB is the energy of an isolated boron atom and EX is the energy of a single C or Si dopant atom.
3. Results and discussion 3.1. Properties of C-BNNS and Si-BNNS Figure 1 compares the optimized structure, corresponding partial density of states (PDOS) plot and spin density map of C-BNNS and Si-BNNS. It is seen that after C-doping, h-BN sheet preserves its planar structure. The relaxed C-N bond lengths are calculated to be 1.40 Å, which 3
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are slightly different than those of B-N bonds in the pristine h-BN sheet (1.42 Å). This finding shows that in C-BNNS, the C atom is comfortably incorporated into the h-BN network. On the other hand, one can see that the Si atom binds with a tetrahedral-like configuration to its surrounding N atoms, with the Si-N bond distances of 1.73 Å. This finding is consistent with
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other related studies [40,41], and is related to the larger atomic radius of Si compared to B atom. Based on the Hirshfeld charge density analysis, there is 0.10 (0.38) |e| charge-transfer from the C (Si) atom into its neighboring nitrogen atoms, which may reveal the existence of slightly polar CN or Si-N bonds in these substrates. The calculated formation energy (Ef) of C-BNNS and Si-
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BNNS is 2.61 and 5.02 eV, respectively, which indicates that C atom might be more easily incorporated into the h-BN sheet than the Si atom. Note also that the Ef value for Si-BNNS is
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larger than that of Si-doped graphene (4.09 eV) [42] , demonstrating that a single Si atom is less stable over a monovacancy defect of BNNS than graphene. Besides, the density of states (DOS) analysis indicates that there is a distinct peak at the Fermi level of C-BNNS and Si-BNNS, which is attributed to the C-2p and Si-3p states, respectively. The appearance of the mentioned peak leads to a considerable decrease in the band-gap of h-BN. This finding together with the localized high spin density over the C or Si atom (Figure 1), show the enhanced chemical
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reactivity of the surface by the incorporation of the C or Si impurity. Furthermore, the adsorption energy of the C and Si atom at the vacancy site of h-BN is about -13.5 and -11.1 eV, respectively. This confirms the previous findings that indicate a single B-vacancy defect in h-BN is able to prevent the migration of the C or Si atom on the graphene surface [31,40]. Hence, both
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C-BNNS and Si-BNNS can serve as a stable catalyst to be used in the oxidation of C2H4. 3.2. Absorption of N2O and C2H4 over C-BNNS and Si-BNNS
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The most stable adsorption configurations of N2O and C2H4 molecules over C-BNNS and Si-BNNS are depicted in Figure 2. The corresponding adsorption energies (Eads) and net Hirshfeld charge-transfer (Qnet) values are listed in Table 1. For the both surfaces, it is found that N2O is able to attach the dopant atom through its O- or N-end. For the binding through the terminal N atom (configurations A and B), N2O is tilted over the surface with the C-N and Si-N binding distance of 2.29 and 3.37 Å, respectively. The calculated Eads value of N2O over CBNNS is -0.23 eV, which is larger than that of over Si-BNNS (-0.02 eV). Meanwhile, the obtained adsorption energy of N2O in the configuration B is slightly smaller than the corresponding value over BN nanotubes [40], due to the curvature effects. According to Table 1, 4
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the amount of net charge transfer from the surface into N2O molecule in A and B is 0.06 and 0.03 |e|, respectively. This finding together with the negligible hybridization of N2O-2π* orbital and the single-occupied molecular orbital (SOMO) of the surface confirms the weak interaction of N2O with the C or Si atom. On the other hand, when N2O approaches to the C-/Si-BNNS
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through its oxygen atom, it is spontaneously dissociated into N2 and O atom, and leads to the lowest-energy configuration with a large adsorption energy (configurations C and D). Note that this is in contrast with the absorption of N2O over other surfaces like Si-doped graphene [43], in which a physisorbed configuration is obtained. The latter can be related to the larger charge-
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transfer effects between the N2O and C-/Si-BNNS, which leads to a large activation of N-O bond of N2O over the surface. Meanwhile, the large positive charge on the Si atom of Si-BNNS is able
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to greatly polarize the oxygen atom of N2O, and this leads to a decrease of electron density over the N-O bond of this molecule and hence its dissociation. In fact, previous DFT calculations have found that N2O can be easily decomposed to N2 and O over Al- or Ge-doped graphene by applying a perpendicular electric field, which polarizes the charge density over this molecule [44]. The binding distance between N2 molecule and surface is about 3.5 Å, which indicates that it can be easily released from the surface. The Hirshfeld charge on the O atom over the C and Si
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atom is -0.20 and -0.35 |e|, respectively. In addition, the PDOS analysis clearly indicates the appearance of a local state just below the Fermi level, which should be due to the formation of a chemical C-O or Si-O bond.
The most energetically favorable configurations of C2H4 adsorbed over C-BNNS and
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Si-BNNS are depicted in Figure 2 (configurations E and F). The adsorption energy of C2H4 is calculated to be -0.27 and -1.12 eV over C-BNNS and Si-BNNS, respectively. For C2H4 on C-
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BNNS, the binding distance between ethylene and C atom of the surface is 2.70 Å. The C2H4 is aligned almost parallel to the nanosheet surface which is due to a favorable orbital interaction between the π orbital of C2H4 and the SOMO of the surface. The amount of charge-transfer is negligible (0.08 |e|), which leads to small elongation of C-C bond form 1.34 Å in the isolated C2H4 to the 1.35 Å in the absorbed form. On the other hand, when C2H4 adsorbs on Si-BNNS, a new Si-C chemical bond is formed. The Si-C binding distance is calculated to be 1.91 Å, which is much shorter than that of over C-BNNS. Meanwhile, there is about 0.07 |e| charge-transfer from the π orbital of C2H4 into the surface, which leads to 0.13 Å elongation of the C-C bond. Considering the PDOS analysis results, it is seen that there are some significant orbital mixing 5
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between the surface and C2H4 around the Fermi level. As expected, such orbital mixing is more important for Si-BNNS than C-BNNS one, which confirms the more favorable adsorption of C2H4 over the former surface. 3.3. Oxidation of C2H4 by N2O over C-BNNS and Si-BNNS
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Having characterized the most stable adsorption configurations of N2O and C2H4, we now consider the reaction mechanism of ethylene oxidation on C-/Si-BNNS surface. Since the adsorption energy of N2O is much larger than that of C2H4, it is expected that the oxidation of ethylene starts with the dissociative adsorption of N2O over both surfaces. Figure 3 shows the
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corresponding potential energy diagram and optimized stationary points along the reaction pathway.
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We first consider the oxidation of C2H4 over C-BNNS. As Figure 3(a) indicates, the reaction starts with the dissociation of N2O molecule into N2 and an activated oxygen atom (Oads). Due to the weak adsorption of N2 over the surface (Eads=-0.23 eV), it can easily detach from the surface. The remaining Oads atom on the surface carries a notable negative charge (-0.20 |e|), and can easily interact with the incoming C2H4 molecule to form initial state IS-1. The adsorption energy of the physisorbed C2H4 molecule in IS-1 is calculated to be -0.28 eV, with
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the Oads...C2H4 binding distance of about 2.40 Å. Passing a small activation energy (0.04 eV), the Csurface-Oads bond is continually elongated and the ethyleneoxy intermediate is consequently produced in IM-1. The reaction energy to obtain IM-1 is negative by 0.41 eV. Meanwhile, the chemisorption of C2H4 causes a decrease in the Hirshfeld charge on the Oads from -0.20 |e| in IS-
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1 to -0.16 |e| in IM-1. In the next step, the distance between the C atom of the surface and Oads is further elongated and a new C-O bond is formed between the C atom of C2H4 and Oads. By
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overcoming a small activation energy (0.22 eV), C2H4 is finally converted to EO, which can be easily desorbed from the surface due to its negligible adsorption energy (-0.10 eV). Note that the energy barrier to form EO over C-BNNS is much smaller than those of over noble-metal based catalysts like Au [45], Ag [46,47] or Pt-doped graphene [48], but very close to that of using Fe– BTC (BTC =1,3,5-benzentricarboxylate) catalyst [49]. The produced EO has an adsorption energy of -0.31 eV, so it can be easily released from the surface at ambient temperature. Figure 3(b) indicates the optimized configurations at various steps of the ethylene oxidation over Si-BNNS. Similar to the case of C-BNNS, the most stable N2O adsorption configuration was selected as the starting point for the oxidation of C2H4 over Si-BNNS. This 6
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leads to the spontaneous decomposition of N2O into N2 and Oads moiety. After the N2 molecule drifts away, C2H4 is adsorbed next to the Si-Oads site and the initial state IS-3 is obtained. The physisorption of C2H4 in IS-3 results in a slight decrease of the negative charge on the Oads atom due to the formation of two new H...O bonds. The ethyleneoxy intermediate is then formed over
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the surface through the transition state (TS-3) in which the formation of the C-O bond starts. To reach IM-3, an energy barrier of 0.87 eV must be overcome, which is much larger than that of over C-BNNS. As the reaction proceeds, the Si-Oads bond length is continually elongated, and finally EO is formed. Unlike the C-BNNS surface, the formation of EO over the Si-BNNS is
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highly endothermic and needs a quite large activation energy, which clearly indicates that the epoxidation of C2H4 is almost impossible or proceeds with great difficulty over Si-BNNS. The
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adsorption energy of EO is calculated to be -0.48 eV over Si-BNNS, which is larger than that of over C-BNNS.
To understand the origin of the high catalytic activity of C-BNNS surface compared to SiBNNS towards epoxidation of C2H4, we examined the electronic structures for the first step of this reaction progress. Figure 4 indicates the PDOS analysis results for the corresponding stationary points. It is clearly seen that there is some negligible orbital mixing between the
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SOMO states of C-BNNS and π state of C2H4 in IS1, which verifies the weak interaction of these moieties as noted above. As the reaction proceed from IS-1 to TS-1, the π of C2H4 moves towards the Fermi level and some orbital interactions occurs. This causes a charge-transfer from C2H4 into surface due to the proper orbital interaction (Figure S1 of Supporting Information).
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Note that such orbital interaction is negligible for the corresponding reaction over Si-BNNS. Hence, this should be the reason for the smaller catalytic activity of the this surface compared to
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C-BNNS.
As another possible reaction during the epoxidation of C2H4, we also studied the disproportion of N2O molecules (i.e. 2N2O → N2 + O2) over the C- and Si-BNNS surfaces. Figure S2 shows the optimized stationary points along with some important geometry parameters. As seen, the disproportion of N2O molecules reaction starts with the adsorption of the second N2O molecule over the surfaces. The O atom of N2O then starts to approach the Oads atom on the surface to reach the transition state (TS-5 or TS-6). With O continuing to approach, the N-O bond of N2O molecule breaks and an O2 molecule is finally obtained over the surface. However, this reaction needs to overcome a large energy barrier over both surfaces. This means 7
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that under normal condition, the disproportion of N2O molecules cannot proceed over C-BNNS or Si-BNNS, a result has been already obtained in the other related studies. 4. Conclusion In short, by performing dispersion-corrected DFT calculations, the epoxidation of C2H4
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molecule by N2O molecule was studied over the carbon or Si-embedded h-BN nanosheet. It is found that N2O has a larger adsorption energy over both surfaces than C2H4. Meanwhile, the SiBNNS showed a larger tendency to adsorb N2O or C2H4 molecules than the C-BNNS, mainly due to more favorable charge-transfer effects. According to our results, the energy barrier to
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reach EO over the C-BNNS is only 0.22 eV, which indicates this reaction can easily proceed over this surface at room temperature. On the other hand, a quite large activation energy was
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found for the epoxidation of C2H4 over Si-BNNS, which reveals that this reaction is almost impossible or proceeds with great difficulty over this surface. These findings suggest that C-
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BNNS may serve as a promising metal-free catalyst to low-temperature epoxidation of C2H4.
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Table 1. Adsorption energy (Eads, eV) and net Hirshfeld charge-transfer (Qnet, |e|) for the adsorption of N2O and C2H4 over the C-BNNS and Si-BNNS adsorbate
Eads
Qnet
C-BNNS
N2O (N-site)
-0.23
0.06
N2O (O-site)
-1.41
0.20
C2H4
-0.27
N2O (N-site)
-0.02
N2O (O-site)
-2.87
C2H4
-1.12
0.08
-0.03 0.33
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Si-BNNS
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surface
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Figure 1. Optimized structure (left), electronic density of states (middle) and spin density (isovalue = 0.01 au, right) of C-BNNS and Si-BNNS. The Fermi level in the density of states plots is set to be zero and is denoted by the dashed line.
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Figure 2. Optimized geometry and corresponding PDOS plots for the adsorption of N2O and C2H4 over C-BNNS and Si-BNNS. All bond distances are in Å. In the PDOS plots, the dashed line indicates the Fermi level, which set to be zero.
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Figure 3. The potential energy profile and optimized stationary points for the epoxidation of C2H4 over (a) C-BNNS and (b) Si-BNNS. All bond distances are in Å.
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Figure 4. PDOS analysis of different stationary points along the epoxidation of C2H4 over CBNNS and Si-BNNS
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Research highlights:
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1) The epoxidation of ethylene is studied over C- and Si-doped boron nitride nanosheet (C/Si-BNNS) for the first time. 2) C-BNNS shows a better catalyst activity than Si-BNNS. 3) The activation energy for the epoxidation of ethylene over C-BNNS is only 0.22 eV.
S0038-1098(18)30664-1
DOI:
10.1016/j.ssc.2018.09.007
Reference:
SSC 13499
To appear in:
Solid State Communications
Received Date: 28 February 2018 Revised Date:
26 July 2018
Accepted Date: 11 September 2018
Please cite this article as: M.D. Esrafili, Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study, Solid State Communications (2018), doi: 10.1016/ j.ssc.2018.09.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study
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Mehdi D. Esrafili Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran Phone: (+98) 4212237955. Fax: (+98) 4212276060. P.O. Box: 5513864596. E-mail: [email protected]
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Abstract
Using first-principle calculations, we compare the catalytic activity of the experimentally available C- or Si-doped boron nitride nanosheet (C-/Si-BNNS) towards the epoxidation of
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ethylene by N2O molecule. The epoxidation reaction of ethylene over these surfaces includes the decomposing of N2O into N2 and an oxygen atom (Oads), formation of the ethyleneoxy intermediate, and the creation of ethylene oxide. Our results show that the catalytic activity of CBNNS for epoxidation of ethylene is better than Si-BNNS, due to more favorable charge-transfer effects. The results presented here suggest a green, low-cost, and metal-free approach for low-
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temperature epoxidation of ethylene using C-BNNS.
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Keywords: A. BN nanosheet; C. Ethylene oxidation; D. Reaction Mechanism; D. C-doping.
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1. Introduction The catalytic oxidation of ethylene to ethylene oxide (EO) has received great attention due its importance in the chemical industry. EO is a valuable intermediate for the production of various fine chemicals such as ethylene glycol, polyester, plastics, and ethoxylates [1,2]. EO is
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commercially produced by the silver-catalyzed partial oxidation of ethylene with air or oxygen. A commonly used support material for silver catalysts is α-alumina, mainly due to its inertness for the isomerization of EO to acetaldehyde [3-10]. Similar epoxidation reactions have also been reported on other noble metal-based catalysts such as gold clusters supported on titanosilicalite
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[11-13]. Despite their high reactivity and selectivity, however, the high price, poor durability and rapid deactivation of these noble-metal based catalysts severely limit their large-scale application
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in the ethylene epoxidation. Thus, alternative costless, stable and metal-free catalysts with high catalytic activities comparable to or even higher than noble metal-based catalysts are demanding. Since the discovery of graphene in 2004 [14], much attention has been devoted into novel analogous two-dimensional structures, like hexagonal boron nitride (h-BN) sheet [15-17]. Different from semimetallic graphene with zero energy gap, h-BN is always a semiconductor with a wide band gap (> 5.0 eV) [18,19]. Moreover, due to the large specific surface area and
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chemical inertness of h-BN, it is usually considered as an ideal support in heterogeneous catalysis [20-22]. It is also accepted that doping of h-BN with foreign atoms can greatly modulate its electronic structure and chemical properties [15,23-26]. Thus the inert h-BN nanosheet may be transformed into a very active catalyst through the incorporation of these
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impurities. For example, such heteroatom-doped h-BN sheets have been reported as a highly active catalyst for the CO oxidation [27-29]. A great potential has been also suggested for further
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application of these materials in the low-temperature reduction of N2O [30] or O2 [22,31-33]. To the best of our knowledge, the catalytic oxidation of ethylene to EO has not been reported yet over heteroatom-doped h-BN nanosheets. In this work, by using plane-wave density functional theory (DFT) calculations, we compare the potential of C- or Si-doped h-BN nanosheet (C-/Si-BNNS) as an active and metal-free catalyst for the epoxidation of ethylene by N2O molecule. Fortunately, both these metal-free BN-based materials have already been achieved experimentally [24,34]. The present methodology for the epoxidation of ethylene is a green approach, since N2 gas is the only by-product of this reaction, and N2O as a main greenhouse gas is removed at the same time. 2
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2. Computational details All the spin-polarized DFT calculations were carried out using the DMol3 [35,36]. The Perdew-Burke-Ernzerhof (PBE) density functional [37] within the generalized gradient approximation (GGA) was chosen for the DFT calculations. A double numerical plus
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polarization (DNP) was used as the basis set. To consider the van der Waals effects, the empirical correction in the Grimme scheme [38,39] was applied. A 4×4 supercell containing 16 B and 16 N atoms with the periodic boundary conditions on the x–y plane was employed as the pristine h-BN. A boron atom was then substituted with a C or Si atom to build C- or Si-BNNS.
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The 3 × 3 × 1 k points were used for calculating the Brillouin zone integration. The convergence tolerance of energy of 1.0×10-5 Ha, maximum force of 0.001 Ha/Å and maximum displacement
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of 0.005 Å were employed in all the geometry optimizations. In order to achieve accurate electronic convergence, we apply a smearing of 0.005 Ha to the orbital occupation. The minimum-energy pathway (MEP) for each reaction step was obtained by linear synchronous transit (LST)/quadratic synchronous transit (QST) and nudged elastic band (NEB) methods. The nature of transition states was verified by the vibrational frequencies, which exhibit only one imaginary frequency throughout the potential energy surface. The zero-point
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energy (ZPE) correction was considered for all adsorption energies and activation energies. The adsorption energy (Eads) of each adsorbate was obtained using the following equation: Eads(A) = EA–S - ES - EA
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where EA–S, ES and EA are the total energies of the adsorbate-substrate (A–S) system, the
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substrate (S) and adsorbate (A), respectively. The formation energy (Ef) of Si-BNNS was calculated by the formula:
(2)
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Ef = (EX-BNNS + EB - EX) - EBNNS
where EBNNS is the total energy of the pristine h-BN, EX-BNNS is the total energy of the C- or SiBNNS, EB is the energy of an isolated boron atom and EX is the energy of a single C or Si dopant atom.
3. Results and discussion 3.1. Properties of C-BNNS and Si-BNNS Figure 1 compares the optimized structure, corresponding partial density of states (PDOS) plot and spin density map of C-BNNS and Si-BNNS. It is seen that after C-doping, h-BN sheet preserves its planar structure. The relaxed C-N bond lengths are calculated to be 1.40 Å, which 3
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are slightly different than those of B-N bonds in the pristine h-BN sheet (1.42 Å). This finding shows that in C-BNNS, the C atom is comfortably incorporated into the h-BN network. On the other hand, one can see that the Si atom binds with a tetrahedral-like configuration to its surrounding N atoms, with the Si-N bond distances of 1.73 Å. This finding is consistent with
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other related studies [40,41], and is related to the larger atomic radius of Si compared to B atom. Based on the Hirshfeld charge density analysis, there is 0.10 (0.38) |e| charge-transfer from the C (Si) atom into its neighboring nitrogen atoms, which may reveal the existence of slightly polar CN or Si-N bonds in these substrates. The calculated formation energy (Ef) of C-BNNS and Si-
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BNNS is 2.61 and 5.02 eV, respectively, which indicates that C atom might be more easily incorporated into the h-BN sheet than the Si atom. Note also that the Ef value for Si-BNNS is
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larger than that of Si-doped graphene (4.09 eV) [42] , demonstrating that a single Si atom is less stable over a monovacancy defect of BNNS than graphene. Besides, the density of states (DOS) analysis indicates that there is a distinct peak at the Fermi level of C-BNNS and Si-BNNS, which is attributed to the C-2p and Si-3p states, respectively. The appearance of the mentioned peak leads to a considerable decrease in the band-gap of h-BN. This finding together with the localized high spin density over the C or Si atom (Figure 1), show the enhanced chemical
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reactivity of the surface by the incorporation of the C or Si impurity. Furthermore, the adsorption energy of the C and Si atom at the vacancy site of h-BN is about -13.5 and -11.1 eV, respectively. This confirms the previous findings that indicate a single B-vacancy defect in h-BN is able to prevent the migration of the C or Si atom on the graphene surface [31,40]. Hence, both
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C-BNNS and Si-BNNS can serve as a stable catalyst to be used in the oxidation of C2H4. 3.2. Absorption of N2O and C2H4 over C-BNNS and Si-BNNS
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The most stable adsorption configurations of N2O and C2H4 molecules over C-BNNS and Si-BNNS are depicted in Figure 2. The corresponding adsorption energies (Eads) and net Hirshfeld charge-transfer (Qnet) values are listed in Table 1. For the both surfaces, it is found that N2O is able to attach the dopant atom through its O- or N-end. For the binding through the terminal N atom (configurations A and B), N2O is tilted over the surface with the C-N and Si-N binding distance of 2.29 and 3.37 Å, respectively. The calculated Eads value of N2O over CBNNS is -0.23 eV, which is larger than that of over Si-BNNS (-0.02 eV). Meanwhile, the obtained adsorption energy of N2O in the configuration B is slightly smaller than the corresponding value over BN nanotubes [40], due to the curvature effects. According to Table 1, 4
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the amount of net charge transfer from the surface into N2O molecule in A and B is 0.06 and 0.03 |e|, respectively. This finding together with the negligible hybridization of N2O-2π* orbital and the single-occupied molecular orbital (SOMO) of the surface confirms the weak interaction of N2O with the C or Si atom. On the other hand, when N2O approaches to the C-/Si-BNNS
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through its oxygen atom, it is spontaneously dissociated into N2 and O atom, and leads to the lowest-energy configuration with a large adsorption energy (configurations C and D). Note that this is in contrast with the absorption of N2O over other surfaces like Si-doped graphene [43], in which a physisorbed configuration is obtained. The latter can be related to the larger charge-
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transfer effects between the N2O and C-/Si-BNNS, which leads to a large activation of N-O bond of N2O over the surface. Meanwhile, the large positive charge on the Si atom of Si-BNNS is able
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to greatly polarize the oxygen atom of N2O, and this leads to a decrease of electron density over the N-O bond of this molecule and hence its dissociation. In fact, previous DFT calculations have found that N2O can be easily decomposed to N2 and O over Al- or Ge-doped graphene by applying a perpendicular electric field, which polarizes the charge density over this molecule [44]. The binding distance between N2 molecule and surface is about 3.5 Å, which indicates that it can be easily released from the surface. The Hirshfeld charge on the O atom over the C and Si
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atom is -0.20 and -0.35 |e|, respectively. In addition, the PDOS analysis clearly indicates the appearance of a local state just below the Fermi level, which should be due to the formation of a chemical C-O or Si-O bond.
The most energetically favorable configurations of C2H4 adsorbed over C-BNNS and
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Si-BNNS are depicted in Figure 2 (configurations E and F). The adsorption energy of C2H4 is calculated to be -0.27 and -1.12 eV over C-BNNS and Si-BNNS, respectively. For C2H4 on C-
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BNNS, the binding distance between ethylene and C atom of the surface is 2.70 Å. The C2H4 is aligned almost parallel to the nanosheet surface which is due to a favorable orbital interaction between the π orbital of C2H4 and the SOMO of the surface. The amount of charge-transfer is negligible (0.08 |e|), which leads to small elongation of C-C bond form 1.34 Å in the isolated C2H4 to the 1.35 Å in the absorbed form. On the other hand, when C2H4 adsorbs on Si-BNNS, a new Si-C chemical bond is formed. The Si-C binding distance is calculated to be 1.91 Å, which is much shorter than that of over C-BNNS. Meanwhile, there is about 0.07 |e| charge-transfer from the π orbital of C2H4 into the surface, which leads to 0.13 Å elongation of the C-C bond. Considering the PDOS analysis results, it is seen that there are some significant orbital mixing 5
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between the surface and C2H4 around the Fermi level. As expected, such orbital mixing is more important for Si-BNNS than C-BNNS one, which confirms the more favorable adsorption of C2H4 over the former surface. 3.3. Oxidation of C2H4 by N2O over C-BNNS and Si-BNNS
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Having characterized the most stable adsorption configurations of N2O and C2H4, we now consider the reaction mechanism of ethylene oxidation on C-/Si-BNNS surface. Since the adsorption energy of N2O is much larger than that of C2H4, it is expected that the oxidation of ethylene starts with the dissociative adsorption of N2O over both surfaces. Figure 3 shows the
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corresponding potential energy diagram and optimized stationary points along the reaction pathway.
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We first consider the oxidation of C2H4 over C-BNNS. As Figure 3(a) indicates, the reaction starts with the dissociation of N2O molecule into N2 and an activated oxygen atom (Oads). Due to the weak adsorption of N2 over the surface (Eads=-0.23 eV), it can easily detach from the surface. The remaining Oads atom on the surface carries a notable negative charge (-0.20 |e|), and can easily interact with the incoming C2H4 molecule to form initial state IS-1. The adsorption energy of the physisorbed C2H4 molecule in IS-1 is calculated to be -0.28 eV, with
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the Oads...C2H4 binding distance of about 2.40 Å. Passing a small activation energy (0.04 eV), the Csurface-Oads bond is continually elongated and the ethyleneoxy intermediate is consequently produced in IM-1. The reaction energy to obtain IM-1 is negative by 0.41 eV. Meanwhile, the chemisorption of C2H4 causes a decrease in the Hirshfeld charge on the Oads from -0.20 |e| in IS-
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1 to -0.16 |e| in IM-1. In the next step, the distance between the C atom of the surface and Oads is further elongated and a new C-O bond is formed between the C atom of C2H4 and Oads. By
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overcoming a small activation energy (0.22 eV), C2H4 is finally converted to EO, which can be easily desorbed from the surface due to its negligible adsorption energy (-0.10 eV). Note that the energy barrier to form EO over C-BNNS is much smaller than those of over noble-metal based catalysts like Au [45], Ag [46,47] or Pt-doped graphene [48], but very close to that of using Fe– BTC (BTC =1,3,5-benzentricarboxylate) catalyst [49]. The produced EO has an adsorption energy of -0.31 eV, so it can be easily released from the surface at ambient temperature. Figure 3(b) indicates the optimized configurations at various steps of the ethylene oxidation over Si-BNNS. Similar to the case of C-BNNS, the most stable N2O adsorption configuration was selected as the starting point for the oxidation of C2H4 over Si-BNNS. This 6
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leads to the spontaneous decomposition of N2O into N2 and Oads moiety. After the N2 molecule drifts away, C2H4 is adsorbed next to the Si-Oads site and the initial state IS-3 is obtained. The physisorption of C2H4 in IS-3 results in a slight decrease of the negative charge on the Oads atom due to the formation of two new H...O bonds. The ethyleneoxy intermediate is then formed over
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the surface through the transition state (TS-3) in which the formation of the C-O bond starts. To reach IM-3, an energy barrier of 0.87 eV must be overcome, which is much larger than that of over C-BNNS. As the reaction proceeds, the Si-Oads bond length is continually elongated, and finally EO is formed. Unlike the C-BNNS surface, the formation of EO over the Si-BNNS is
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highly endothermic and needs a quite large activation energy, which clearly indicates that the epoxidation of C2H4 is almost impossible or proceeds with great difficulty over Si-BNNS. The
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adsorption energy of EO is calculated to be -0.48 eV over Si-BNNS, which is larger than that of over C-BNNS.
To understand the origin of the high catalytic activity of C-BNNS surface compared to SiBNNS towards epoxidation of C2H4, we examined the electronic structures for the first step of this reaction progress. Figure 4 indicates the PDOS analysis results for the corresponding stationary points. It is clearly seen that there is some negligible orbital mixing between the
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SOMO states of C-BNNS and π state of C2H4 in IS1, which verifies the weak interaction of these moieties as noted above. As the reaction proceed from IS-1 to TS-1, the π of C2H4 moves towards the Fermi level and some orbital interactions occurs. This causes a charge-transfer from C2H4 into surface due to the proper orbital interaction (Figure S1 of Supporting Information).
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Note that such orbital interaction is negligible for the corresponding reaction over Si-BNNS. Hence, this should be the reason for the smaller catalytic activity of the this surface compared to
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C-BNNS.
As another possible reaction during the epoxidation of C2H4, we also studied the disproportion of N2O molecules (i.e. 2N2O → N2 + O2) over the C- and Si-BNNS surfaces. Figure S2 shows the optimized stationary points along with some important geometry parameters. As seen, the disproportion of N2O molecules reaction starts with the adsorption of the second N2O molecule over the surfaces. The O atom of N2O then starts to approach the Oads atom on the surface to reach the transition state (TS-5 or TS-6). With O continuing to approach, the N-O bond of N2O molecule breaks and an O2 molecule is finally obtained over the surface. However, this reaction needs to overcome a large energy barrier over both surfaces. This means 7
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that under normal condition, the disproportion of N2O molecules cannot proceed over C-BNNS or Si-BNNS, a result has been already obtained in the other related studies. 4. Conclusion In short, by performing dispersion-corrected DFT calculations, the epoxidation of C2H4
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molecule by N2O molecule was studied over the carbon or Si-embedded h-BN nanosheet. It is found that N2O has a larger adsorption energy over both surfaces than C2H4. Meanwhile, the SiBNNS showed a larger tendency to adsorb N2O or C2H4 molecules than the C-BNNS, mainly due to more favorable charge-transfer effects. According to our results, the energy barrier to
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reach EO over the C-BNNS is only 0.22 eV, which indicates this reaction can easily proceed over this surface at room temperature. On the other hand, a quite large activation energy was
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found for the epoxidation of C2H4 over Si-BNNS, which reveals that this reaction is almost impossible or proceeds with great difficulty over this surface. These findings suggest that C-
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BNNS may serve as a promising metal-free catalyst to low-temperature epoxidation of C2H4.
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Table 1. Adsorption energy (Eads, eV) and net Hirshfeld charge-transfer (Qnet, |e|) for the adsorption of N2O and C2H4 over the C-BNNS and Si-BNNS adsorbate
Eads
Qnet
C-BNNS
N2O (N-site)
-0.23
0.06
N2O (O-site)
-1.41
0.20
C2H4
-0.27
N2O (N-site)
-0.02
N2O (O-site)
-2.87
C2H4
-1.12
0.08
-0.03 0.33
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Si-BNNS
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Figure 1. Optimized structure (left), electronic density of states (middle) and spin density (isovalue = 0.01 au, right) of C-BNNS and Si-BNNS. The Fermi level in the density of states plots is set to be zero and is denoted by the dashed line.
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Figure 2. Optimized geometry and corresponding PDOS plots for the adsorption of N2O and C2H4 over C-BNNS and Si-BNNS. All bond distances are in Å. In the PDOS plots, the dashed line indicates the Fermi level, which set to be zero.
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Figure 3. The potential energy profile and optimized stationary points for the epoxidation of C2H4 over (a) C-BNNS and (b) Si-BNNS. All bond distances are in Å.
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Figure 4. PDOS analysis of different stationary points along the epoxidation of C2H4 over CBNNS and Si-BNNS
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Research highlights:
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1) The epoxidation of ethylene is studied over C- and Si-doped boron nitride nanosheet (C/Si-BNNS) for the first time. 2) C-BNNS shows a better catalyst activity than Si-BNNS. 3) The activation energy for the epoxidation of ethylene over C-BNNS is only 0.22 eV.