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A DFT study on SO3 capture and activation over Si- or Al-doped graphene
Chemical Physics Letters 658 (2016) 146–151
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
A DFT study on SO3 capture and activation over Si- or Al-doped graphene Mehdi D. Esrafili ⇑, Nasibeh Saeidi, Parisa Nematollahi Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran
a r t i c l e
i n f o
Article history: Received 30 May 2016 In final form 17 June 2016 Available online 18 June 2016 Keywords: DFT SO3 dissociation Si-doped graphene Al-doped graphene
a b s t r a c t This study reports the adsorption and favorable reaction mechanism of SO3 reduction by CO molecule over Si- or Al-doped graphene using DFT calculations. The adsorption energy of the most stable configuration of SO3 is calculated to be about 103 and 124 kcal/mol over the Si- and Al-doped graphene, respectively. The SO3 reduction over these surfaces proceeds through the following elementary steps (a) SO3 ? SO2 + Oads and (b) Oads + CO ? CO2. The estimated activation energy (Eact) for the dissociation of SO3 over the Si-doped graphene is about 9 kcal/mol smaller than that on the Al-doped graphene. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The development of highly sensitive and efficient materials for monitoring and removing of toxic gases are very important for environmental safety. Sulfur oxides (SOx) are a main group of air pollutants created mainly by the burning of sulfur-contaminated fossil fuels, diverse manufacturing processes and volcanoes [1,2]. Sulfur dioxide (SO2) and to a lesser extent, sulfur trioxide (SO3) can react with the atmospheric moisture to form acid rain and smog, which are harmful for the environment, equipment and industrial instruments [3]. So far, several experimental and theoretical studies have been reported on the adsorption of SOx molecules over different metal and metal oxide surfaces [4–6]. Although these surfaces can potentially remove the SOx molecules from the air, but it has been found that the adsorption of sulfur oxides can be better performed using metal-free surfaces such as graphene and carbon nanotubes [7–10]. Graphene is a novel form of carbon which can act as a support for metal atoms and clusters to realize new carbon–metal nanocomposite catalysts [11–13]. Due to its unique chemical and physical properties, such as high surface area, rich electronic states and good mechanical properties, graphene has been the subject of intense studies in recent years [14–16]. For example, numerous theoretical investigations have been performed to show the interaction of toxic gases with the graphene sheet [17,18]. To improve the surface reactivity of graphene, several strategies, including chemical doping with foreign atoms e.g. Si [19,20], N or P [21– 23], Sn [24] and some transition metals [25–27] have been also
⇑ Corresponding author. E-mail address: [email protected] (M.D. Esrafili). http://dx.doi.org/10.1016/j.cplett.2016.06.045 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
reported. Recently, there have been many theoretical or experimental studies carried out on the adsorption of SOx molecules over metal-doped graphene [28–32]. For example, Al-Sunaidi and AlSaadi [33] reported recently the adsorption of SO, SO2 and SO3 molecules over the Al-doped graphene by means of density functional theory (DFT) calculations. Their results indicated that the adsorption energies for these gases on Al-doped graphene are comparable to those of the Al- and Si-doped carbon nanotubes. In this Letter, the adsorption and subsequent reduction of SO3 by the CO molecule are studied over Si- or Al-doped graphene sheets using DFT calculations. The electronic structure and catalytic activity of both Si- and Al-doped graphene sheets are also investigated in detail. Although both SO2 and SO3 molecules are considered as a pollutant, but, the toxicity and corrosion of SO3 are more than those of SO2 molecule. So, the reduction of SO3 to SO2 can be viewed as an important reaction from both environmental and industrial points of view. The results of this study can be also helpful for designing and developing noble metal-free catalysts based on graphene.
2. Computational details In this Letter, all DFT calculations were carried out using the Gaussian 09 package [34]. The geometry optimization and the subsequent frequency calculation of complexes were performed at the M06-2X/6-31G⁄ computational level. A hexagonal graphene supercell (4 4 graphene unit cell), containing 48 carbon atoms, was chosen as the basic model for the calculations. The Si- and Algraphene were then modeled by substituting a single Si or Al atom with one C atom on the surface. The adsorption energies (Eads) of
M.D. Esrafili et al. / Chemical Physics Letters 658 (2016) 146–151
the molecules adsorbed over Si-/Al-doped graphene were calculated as the following equation:
EadsðAÞ ¼ EA-M EM EA
ð1Þ
where EA-M, EM and EA are the total energies of the adsorbate–substrate (A-M) system, the substrate (M) and adsorbate (A), respectively. Natural bond orbital (NBO) [35] analysis was performed at the M06-2X/6-31G⁄ level. To verify the thermodynamic feasibility of the adsorption process, the change of enthalpy (DH) and free-energy (DG) were calculated at 298.14 K and 1 atmosphere according to the following equations:
DH ¼ DG ¼
X
X
ðe0 þ Hcorr Þ
ðe0 þ Hcorr Þ
products
reactants
X
X
ðe0 þ Gcorr Þ
products
ðe0 þ Gcorr Þ
ð2Þ ð3Þ
reactants
Hcorr ¼ Etot þ kB T
ð4Þ
Gcorr ¼ Hcorr TStot
ð5Þ
where e0 is the total electronic energy at T = 0 K, while Hcorr and Gcorr are thermal corrections which should be added to e0 to obtain the enthalpy and Gibbs free energy, respectively. The internal thermal energy Etot is contributed from translational (Etr), rotational (Erot), vibrational (Evib), and electronic (Eel) energies, and Stot, Str, Srot, Svib, Sel are the corresponding entropies. The kB is the Boltzmann constant. 3. Results and discussion 3.1. Geometric structure of Si- and Al-doped graphene First of all, the optimized structures of Al- and Si-doped graphene are analyzed in detail. Fig. 1 shows the top and side views of the optimized Si- and Al-doped graphene sheets. As it is clear, the average bond length of Si (Al) atom with its neighboring C atoms is 1.73 (1.84) Å which is about 0.31 (0.42) Å larger than the average C–C bond length in the pristine graphene. This value is in good agreement with other related estimates for the Si- and Al-doped graphene [36–38]. In both cases, the dopant atom a little protrudes out of the graphene surface to get more space due to its
147
relatively larger atomic radius than that of C atom. The calculated adsorption energies of the Si and Al atoms over the vacancy site of the graphene are 249.4 and 156.7 kcal/mol, respectively, indicating a strong interaction between these dopant atoms and the surface. The NBO analysis shows that about 1.6 (1.7) electrons are transferred from the Si (Al) atom to its adjacent C atoms that lead to the strong bonding between the positively charged Si (Al) atom and its neighbors, due to the electronegativity difference between Si (Al) and C atoms. It should be noted that the aggregation of metal atoms, such as Al or Si, in their high concentrations over the catalyst surface is a considerable problem [37,38]. Therefore, to evaluate this possibility, the diffusion of Si and Al atom to their nearest position on graphene sheet is also considered. As Fig. 2 indicates, the calculated diffusion barrier is obtained to be about 68 and 80 kcal/mol for the Al and Si atoms, respectively. These values, together with the relatively large adsorption energies of these dopants over the vacancy site of the graphene indicate that both Si and Al atoms bind strongly at the defect site and these surfaces are stable enough to be utilized in removing and subsequent reduction of the SO3 molecule. 3.2. Adsorption of SO3 molecule over the Si- and Al-doped graphene To find the favorable configurations of a single SO3 molecule over the Si- and Al-doped graphene, several adsorption patterns were considered. The most stable configuration of SO3 adsorption on these surfaces (complexes A and B) along with their electron density difference (EDD) plots are depicted in Fig. 3. In the EDD plots, the blue and red colors are related to the electron density accumulation and depletion areas, respectively. In addition, the binding distances (R), adsorption energy (Eads), NBO charge transfer (qCT) along with the related enthalpy (DH298) and Gibbs free energy (DG298) changes of these complexes are listed in Table 1. In the complex A, the SO3 molecule is adsorbed above the Si atom of the Si-doped graphene, forming a four-membered ring in which the Si atom directly binds to two oxygen atoms of the SO3 with an average Si–O bond length of 1.79 Å (Fig. 3). The large Eads (103.80 kcal/mol) and the negative DG298 values confirm that this configuration is stable and there is a strong chemical binding between the SO3 molecule and the Si atom of the surface. Also, a sizable charge of about 1.27 e is transferred from the SO3 molecule to the surface (Table 1), which can also be seen from the blue and
Fig. 1. Optimized structure of Si- and Al-doped graphene from top and side views. All bond distances are in Å.
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Fig. 2. The energy profile for the diffusion of M = Si or Al atom on the graphene. IS, TS and FS represent initial structure, transition structure and final structure, respectively.
Fig. 3. The optimized structure and electron density difference (EDD) plot of the most stable configuration of SO3 adsorbed over the Si- (complex A) and Al-doped graphene (complex B). In the EDD plots, the electron density depletion and accumulation sites are displayed in red and blue, respectively. All bond distances are in Å. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
red regions in the corresponding EDD plot. Like the complex A, the SO3 molecule in the complex B adsorbs over the Al atom of the Al-
doped graphene with an average Al–O bond length of 1.80 Å. The calculated adsorption energy of the SO3 molecule over the Al-
M.D. Esrafili et al. / Chemical Physics Letters 658 (2016) 146–151 Table 1 Calculated binding distance (R), charge transfer (qCT), adsorption energy (Eads), change of enthalpy (DH298) and change of Gibbs free energy (DG298) for the most stable adsorption configuration SO3 over Si- and Al-doped graphene. Complex
R (Å)
Eads (kcal/mol)
qCT (e)
DH298 (kcal/mol)
DG298 (kcal/mol)
A
1.73/ 1.84 1.78/ 1.81
103.80
1.27
102.65
89.52
124.27
1.39
122.10
109.55
B
doped graphene is 124.27 kcal/mol, which is larger than those of over the Si-doped graphene or Al-doped carbon nanotubes [33]. In addition, the NBO analysis indicates that about 1.40 electrons are transferred from the SO3 molecule to the Al atom, which is clear from the blue regions around the Al–O and O–S bonds in the EDD map (Fig. 3).
3.3. Reduction of SO3 by CO molecule over the Si- and Al-doped graphene Fig. 4 indicates the reaction pathway (including of initial state (IS), transition state (TS), intermediate state (MS) and final state (FS)) along with their corresponding geometric values for the SO3 reduction by CO molecule over the Si-doped graphene. As Fig. 4 indicates, the reduction of the SO3 starts from complex A which is used as IS-1. Passing from this state, TS-2 is formed by an activation energy of 8.73 kcal/mol. It should be noted that in TS-1, the Si– O1 (Si–O2) bond length is reduced (increased) from 1.73 (1.84) Å in IS-1 to 1.68 (2.44) Å. As is clear, the SO2 molecule is going to release from the surface passing from the intermediate state MS-1. In this complex, the SO3 molecule is attached to the Si atom via its O1 atom (Si–O1 = 1.72 Å) while the Si–O2 bond is broken (Si– O2 = 3.04 Å). In the next step, to release the SO2 molecule, MS-1 should overcome the corresponding transition state (TS-2) in which the Si–O1 bond length decreases to 1.69 Å and the distance between the O1 and S atoms increases to 2.31 Å. The formed SO2 molecule over the surface is about 2.79 Å far from the O1 atom
149
and can easily desorb from the surface (Eads = 6.46 kcal/mol). Note that the relatively small adsorption energy for SO2 compared to SO3 over Si-graphene can be attributed to the strong covalent bonding between the O1 and Si atoms, which decreases the tendency of the dopant atom to interact with SO2 molecule. Besides, the main interaction between SO2 and the surface in FS-2 is due to a noncovalent S O interaction between the SO2 and O1 species. This is completely different from that of SO3 adsorption, which is due to strong covalent Si/Al O interactions between the oxygen atoms of SO3 and the dopant atom. Moreover, NBO calculations reveal that the oxygen atoms of SO2 have a smaller negative charge (0.821 e at M062X/6-31G⁄ level) than those of SO3 (0.824 e), which may provide another evidence for the smaller adsorption energy of SO2 compared to SO3 molecule. As Fig. 4 indicates, the activation energy (Eact) of the reaction MS-1 ? FS-2 is calculated to be 14.27 kcal/mol, which seems to be overcome at room temperature. Also, this calculated Eact for the dissociation of the SO3 molecule is smaller than those reported over single-walled carbon nanotubes [39]. Lastly, we examine whether the remained oxygen atom (Oads) can react with the CO molecule to produce the CO2 molecule. We also choose the most stable configuration as the IS-3, where the CO molecule and Oads are co-adsorbed over the surface (Fig. 4). In this structure, the CO molecule is about 2.64 Å far from the Oads. It is found that the reaction can proceed and reach to the second transition state (TS-3) passing via a small energy barrier of 2.07 kcal/mol. This value is smaller than those of reported by other studies [24,40,41]. In TS-3, the binding distance between the Oads and Si atom decreases to 2.03 Å and the CO2 molecule is formed in FS-3. In the FS-3 configuration, the CO2 molecule is about 3.88 Å far from the surface and can be easily desorb and renewed the surface. Fig. 5 indicates the SO3 reduction by the CO molecule over the Al-doped graphene. The process begins with the stable adsorption configuration B as IS-4. Passing from this state with a relatively large barrier energy of 17.33 kcal/mol, TS-4 is obtained. Note that the relatively large Eact value for this step can be attributed to the large (more negative) adsorption energy of the SO3 molecule over this surface. In this configuration, the Al–O1 (Al–O2) bond
Fig. 4. The reaction pathway along with the corresponding optimized structures for the reduction of SO3 by CO molecule over the Si-doped graphene. All bond distances and energies are in Å and kcal/mol, respectively.
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Fig. 5. The reaction pathway along with the corresponding optimized structures for the reduction of SO3 by CO molecule over the Al-doped graphene. All bond distances and energies are in Å and kcal/mol, respectively.
length is increased from 1.79 (1.81) Å in IS-4 to 1.88 (1.99) Å in TS4. The Al–O2 bond is going to broke in order to form the SO2 molecule. So, the intermediate state IM-2 is formed in which the Al–O2 bond increases 2.83 Å. In this configuration, the SO3 molecule is vertically located above the Al atom with the Al–O1 bond length of 1.82 Å. The SO2 molecule is finally achieved passing via TS-5 with a large Eact of 23.71 kcal/mol. In this state, the SO2 molecule is about 2.36 Å far from the Al atom. Finally, in FS-5, the formed SO2 molecule with O1–S bond length of 2.48 Å is released from the surface. In the next step, IS-6 ? FS-6, the remaining oxygen atom (Oads) atom which adsorbs right above the Al atom (Al–Oads = 1.77 Å) reacts with the CO molecule to form the CO2. In the coadsorption configuration IS-6, the distance between the CO molecule and the Oads is about 2.48 Å. In the next state (TS-6), the CO molecule gets closer to the Oads (C–Oads = 2.03 Å) in order to form the CO2 molecule in FS-6. The Eact of this process is 1.49 kcal/mol, which is in good agreement with other reported values over the Aldoped graphene [38]. The formed CO2 molecule is finally removed easily from the surface.
4. Conclusion In this Letter, the adsorption and subsequent reduction of the SO3 molecule by the CO molecule were studied over Si- and Aldoped graphene sheets using DFT calculations. It is found that both these surfaces are stable enough to remove the SO3 molecule at ambient conditions. Strong adsorption energy between SO3 and Si-/Al-doped graphene indicates the nature of chemical adsorption. The possible reaction pathway proposed for the reduction of SO3 with CO molecule are as follows: SO3 ? SO2 + Oads and Oads + CO ? CO2. Based on the results of this study, it can be proposed that the Si-doped graphene can act as more effective catalyst for the dissociation and reduction of SO3 molecule than the Al-doped one.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
A DFT study on SO3 capture and activation over Si- or Al-doped graphene Mehdi D. Esrafili ⇑, Nasibeh Saeidi, Parisa Nematollahi Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran
a r t i c l e
i n f o
Article history: Received 30 May 2016 In final form 17 June 2016 Available online 18 June 2016 Keywords: DFT SO3 dissociation Si-doped graphene Al-doped graphene
a b s t r a c t This study reports the adsorption and favorable reaction mechanism of SO3 reduction by CO molecule over Si- or Al-doped graphene using DFT calculations. The adsorption energy of the most stable configuration of SO3 is calculated to be about 103 and 124 kcal/mol over the Si- and Al-doped graphene, respectively. The SO3 reduction over these surfaces proceeds through the following elementary steps (a) SO3 ? SO2 + Oads and (b) Oads + CO ? CO2. The estimated activation energy (Eact) for the dissociation of SO3 over the Si-doped graphene is about 9 kcal/mol smaller than that on the Al-doped graphene. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The development of highly sensitive and efficient materials for monitoring and removing of toxic gases are very important for environmental safety. Sulfur oxides (SOx) are a main group of air pollutants created mainly by the burning of sulfur-contaminated fossil fuels, diverse manufacturing processes and volcanoes [1,2]. Sulfur dioxide (SO2) and to a lesser extent, sulfur trioxide (SO3) can react with the atmospheric moisture to form acid rain and smog, which are harmful for the environment, equipment and industrial instruments [3]. So far, several experimental and theoretical studies have been reported on the adsorption of SOx molecules over different metal and metal oxide surfaces [4–6]. Although these surfaces can potentially remove the SOx molecules from the air, but it has been found that the adsorption of sulfur oxides can be better performed using metal-free surfaces such as graphene and carbon nanotubes [7–10]. Graphene is a novel form of carbon which can act as a support for metal atoms and clusters to realize new carbon–metal nanocomposite catalysts [11–13]. Due to its unique chemical and physical properties, such as high surface area, rich electronic states and good mechanical properties, graphene has been the subject of intense studies in recent years [14–16]. For example, numerous theoretical investigations have been performed to show the interaction of toxic gases with the graphene sheet [17,18]. To improve the surface reactivity of graphene, several strategies, including chemical doping with foreign atoms e.g. Si [19,20], N or P [21– 23], Sn [24] and some transition metals [25–27] have been also
⇑ Corresponding author. E-mail address: [email protected] (M.D. Esrafili). http://dx.doi.org/10.1016/j.cplett.2016.06.045 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
reported. Recently, there have been many theoretical or experimental studies carried out on the adsorption of SOx molecules over metal-doped graphene [28–32]. For example, Al-Sunaidi and AlSaadi [33] reported recently the adsorption of SO, SO2 and SO3 molecules over the Al-doped graphene by means of density functional theory (DFT) calculations. Their results indicated that the adsorption energies for these gases on Al-doped graphene are comparable to those of the Al- and Si-doped carbon nanotubes. In this Letter, the adsorption and subsequent reduction of SO3 by the CO molecule are studied over Si- or Al-doped graphene sheets using DFT calculations. The electronic structure and catalytic activity of both Si- and Al-doped graphene sheets are also investigated in detail. Although both SO2 and SO3 molecules are considered as a pollutant, but, the toxicity and corrosion of SO3 are more than those of SO2 molecule. So, the reduction of SO3 to SO2 can be viewed as an important reaction from both environmental and industrial points of view. The results of this study can be also helpful for designing and developing noble metal-free catalysts based on graphene.
2. Computational details In this Letter, all DFT calculations were carried out using the Gaussian 09 package [34]. The geometry optimization and the subsequent frequency calculation of complexes were performed at the M06-2X/6-31G⁄ computational level. A hexagonal graphene supercell (4 4 graphene unit cell), containing 48 carbon atoms, was chosen as the basic model for the calculations. The Si- and Algraphene were then modeled by substituting a single Si or Al atom with one C atom on the surface. The adsorption energies (Eads) of
M.D. Esrafili et al. / Chemical Physics Letters 658 (2016) 146–151
the molecules adsorbed over Si-/Al-doped graphene were calculated as the following equation:
EadsðAÞ ¼ EA-M EM EA
ð1Þ
where EA-M, EM and EA are the total energies of the adsorbate–substrate (A-M) system, the substrate (M) and adsorbate (A), respectively. Natural bond orbital (NBO) [35] analysis was performed at the M06-2X/6-31G⁄ level. To verify the thermodynamic feasibility of the adsorption process, the change of enthalpy (DH) and free-energy (DG) were calculated at 298.14 K and 1 atmosphere according to the following equations:
DH ¼ DG ¼
X
X
ðe0 þ Hcorr Þ
ðe0 þ Hcorr Þ
products
reactants
X
X
ðe0 þ Gcorr Þ
products
ðe0 þ Gcorr Þ
ð2Þ ð3Þ
reactants
Hcorr ¼ Etot þ kB T
ð4Þ
Gcorr ¼ Hcorr TStot
ð5Þ
where e0 is the total electronic energy at T = 0 K, while Hcorr and Gcorr are thermal corrections which should be added to e0 to obtain the enthalpy and Gibbs free energy, respectively. The internal thermal energy Etot is contributed from translational (Etr), rotational (Erot), vibrational (Evib), and electronic (Eel) energies, and Stot, Str, Srot, Svib, Sel are the corresponding entropies. The kB is the Boltzmann constant. 3. Results and discussion 3.1. Geometric structure of Si- and Al-doped graphene First of all, the optimized structures of Al- and Si-doped graphene are analyzed in detail. Fig. 1 shows the top and side views of the optimized Si- and Al-doped graphene sheets. As it is clear, the average bond length of Si (Al) atom with its neighboring C atoms is 1.73 (1.84) Å which is about 0.31 (0.42) Å larger than the average C–C bond length in the pristine graphene. This value is in good agreement with other related estimates for the Si- and Al-doped graphene [36–38]. In both cases, the dopant atom a little protrudes out of the graphene surface to get more space due to its
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relatively larger atomic radius than that of C atom. The calculated adsorption energies of the Si and Al atoms over the vacancy site of the graphene are 249.4 and 156.7 kcal/mol, respectively, indicating a strong interaction between these dopant atoms and the surface. The NBO analysis shows that about 1.6 (1.7) electrons are transferred from the Si (Al) atom to its adjacent C atoms that lead to the strong bonding between the positively charged Si (Al) atom and its neighbors, due to the electronegativity difference between Si (Al) and C atoms. It should be noted that the aggregation of metal atoms, such as Al or Si, in their high concentrations over the catalyst surface is a considerable problem [37,38]. Therefore, to evaluate this possibility, the diffusion of Si and Al atom to their nearest position on graphene sheet is also considered. As Fig. 2 indicates, the calculated diffusion barrier is obtained to be about 68 and 80 kcal/mol for the Al and Si atoms, respectively. These values, together with the relatively large adsorption energies of these dopants over the vacancy site of the graphene indicate that both Si and Al atoms bind strongly at the defect site and these surfaces are stable enough to be utilized in removing and subsequent reduction of the SO3 molecule. 3.2. Adsorption of SO3 molecule over the Si- and Al-doped graphene To find the favorable configurations of a single SO3 molecule over the Si- and Al-doped graphene, several adsorption patterns were considered. The most stable configuration of SO3 adsorption on these surfaces (complexes A and B) along with their electron density difference (EDD) plots are depicted in Fig. 3. In the EDD plots, the blue and red colors are related to the electron density accumulation and depletion areas, respectively. In addition, the binding distances (R), adsorption energy (Eads), NBO charge transfer (qCT) along with the related enthalpy (DH298) and Gibbs free energy (DG298) changes of these complexes are listed in Table 1. In the complex A, the SO3 molecule is adsorbed above the Si atom of the Si-doped graphene, forming a four-membered ring in which the Si atom directly binds to two oxygen atoms of the SO3 with an average Si–O bond length of 1.79 Å (Fig. 3). The large Eads (103.80 kcal/mol) and the negative DG298 values confirm that this configuration is stable and there is a strong chemical binding between the SO3 molecule and the Si atom of the surface. Also, a sizable charge of about 1.27 e is transferred from the SO3 molecule to the surface (Table 1), which can also be seen from the blue and
Fig. 1. Optimized structure of Si- and Al-doped graphene from top and side views. All bond distances are in Å.
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Fig. 2. The energy profile for the diffusion of M = Si or Al atom on the graphene. IS, TS and FS represent initial structure, transition structure and final structure, respectively.
Fig. 3. The optimized structure and electron density difference (EDD) plot of the most stable configuration of SO3 adsorbed over the Si- (complex A) and Al-doped graphene (complex B). In the EDD plots, the electron density depletion and accumulation sites are displayed in red and blue, respectively. All bond distances are in Å. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
red regions in the corresponding EDD plot. Like the complex A, the SO3 molecule in the complex B adsorbs over the Al atom of the Al-
doped graphene with an average Al–O bond length of 1.80 Å. The calculated adsorption energy of the SO3 molecule over the Al-
M.D. Esrafili et al. / Chemical Physics Letters 658 (2016) 146–151 Table 1 Calculated binding distance (R), charge transfer (qCT), adsorption energy (Eads), change of enthalpy (DH298) and change of Gibbs free energy (DG298) for the most stable adsorption configuration SO3 over Si- and Al-doped graphene. Complex
R (Å)
Eads (kcal/mol)
qCT (e)
DH298 (kcal/mol)
DG298 (kcal/mol)
A
1.73/ 1.84 1.78/ 1.81
103.80
1.27
102.65
89.52
124.27
1.39
122.10
109.55
B
doped graphene is 124.27 kcal/mol, which is larger than those of over the Si-doped graphene or Al-doped carbon nanotubes [33]. In addition, the NBO analysis indicates that about 1.40 electrons are transferred from the SO3 molecule to the Al atom, which is clear from the blue regions around the Al–O and O–S bonds in the EDD map (Fig. 3).
3.3. Reduction of SO3 by CO molecule over the Si- and Al-doped graphene Fig. 4 indicates the reaction pathway (including of initial state (IS), transition state (TS), intermediate state (MS) and final state (FS)) along with their corresponding geometric values for the SO3 reduction by CO molecule over the Si-doped graphene. As Fig. 4 indicates, the reduction of the SO3 starts from complex A which is used as IS-1. Passing from this state, TS-2 is formed by an activation energy of 8.73 kcal/mol. It should be noted that in TS-1, the Si– O1 (Si–O2) bond length is reduced (increased) from 1.73 (1.84) Å in IS-1 to 1.68 (2.44) Å. As is clear, the SO2 molecule is going to release from the surface passing from the intermediate state MS-1. In this complex, the SO3 molecule is attached to the Si atom via its O1 atom (Si–O1 = 1.72 Å) while the Si–O2 bond is broken (Si– O2 = 3.04 Å). In the next step, to release the SO2 molecule, MS-1 should overcome the corresponding transition state (TS-2) in which the Si–O1 bond length decreases to 1.69 Å and the distance between the O1 and S atoms increases to 2.31 Å. The formed SO2 molecule over the surface is about 2.79 Å far from the O1 atom
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and can easily desorb from the surface (Eads = 6.46 kcal/mol). Note that the relatively small adsorption energy for SO2 compared to SO3 over Si-graphene can be attributed to the strong covalent bonding between the O1 and Si atoms, which decreases the tendency of the dopant atom to interact with SO2 molecule. Besides, the main interaction between SO2 and the surface in FS-2 is due to a noncovalent S O interaction between the SO2 and O1 species. This is completely different from that of SO3 adsorption, which is due to strong covalent Si/Al O interactions between the oxygen atoms of SO3 and the dopant atom. Moreover, NBO calculations reveal that the oxygen atoms of SO2 have a smaller negative charge (0.821 e at M062X/6-31G⁄ level) than those of SO3 (0.824 e), which may provide another evidence for the smaller adsorption energy of SO2 compared to SO3 molecule. As Fig. 4 indicates, the activation energy (Eact) of the reaction MS-1 ? FS-2 is calculated to be 14.27 kcal/mol, which seems to be overcome at room temperature. Also, this calculated Eact for the dissociation of the SO3 molecule is smaller than those reported over single-walled carbon nanotubes [39]. Lastly, we examine whether the remained oxygen atom (Oads) can react with the CO molecule to produce the CO2 molecule. We also choose the most stable configuration as the IS-3, where the CO molecule and Oads are co-adsorbed over the surface (Fig. 4). In this structure, the CO molecule is about 2.64 Å far from the Oads. It is found that the reaction can proceed and reach to the second transition state (TS-3) passing via a small energy barrier of 2.07 kcal/mol. This value is smaller than those of reported by other studies [24,40,41]. In TS-3, the binding distance between the Oads and Si atom decreases to 2.03 Å and the CO2 molecule is formed in FS-3. In the FS-3 configuration, the CO2 molecule is about 3.88 Å far from the surface and can be easily desorb and renewed the surface. Fig. 5 indicates the SO3 reduction by the CO molecule over the Al-doped graphene. The process begins with the stable adsorption configuration B as IS-4. Passing from this state with a relatively large barrier energy of 17.33 kcal/mol, TS-4 is obtained. Note that the relatively large Eact value for this step can be attributed to the large (more negative) adsorption energy of the SO3 molecule over this surface. In this configuration, the Al–O1 (Al–O2) bond
Fig. 4. The reaction pathway along with the corresponding optimized structures for the reduction of SO3 by CO molecule over the Si-doped graphene. All bond distances and energies are in Å and kcal/mol, respectively.
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Fig. 5. The reaction pathway along with the corresponding optimized structures for the reduction of SO3 by CO molecule over the Al-doped graphene. All bond distances and energies are in Å and kcal/mol, respectively.
length is increased from 1.79 (1.81) Å in IS-4 to 1.88 (1.99) Å in TS4. The Al–O2 bond is going to broke in order to form the SO2 molecule. So, the intermediate state IM-2 is formed in which the Al–O2 bond increases 2.83 Å. In this configuration, the SO3 molecule is vertically located above the Al atom with the Al–O1 bond length of 1.82 Å. The SO2 molecule is finally achieved passing via TS-5 with a large Eact of 23.71 kcal/mol. In this state, the SO2 molecule is about 2.36 Å far from the Al atom. Finally, in FS-5, the formed SO2 molecule with O1–S bond length of 2.48 Å is released from the surface. In the next step, IS-6 ? FS-6, the remaining oxygen atom (Oads) atom which adsorbs right above the Al atom (Al–Oads = 1.77 Å) reacts with the CO molecule to form the CO2. In the coadsorption configuration IS-6, the distance between the CO molecule and the Oads is about 2.48 Å. In the next state (TS-6), the CO molecule gets closer to the Oads (C–Oads = 2.03 Å) in order to form the CO2 molecule in FS-6. The Eact of this process is 1.49 kcal/mol, which is in good agreement with other reported values over the Aldoped graphene [38]. The formed CO2 molecule is finally removed easily from the surface.
4. Conclusion In this Letter, the adsorption and subsequent reduction of the SO3 molecule by the CO molecule were studied over Si- and Aldoped graphene sheets using DFT calculations. It is found that both these surfaces are stable enough to remove the SO3 molecule at ambient conditions. Strong adsorption energy between SO3 and Si-/Al-doped graphene indicates the nature of chemical adsorption. The possible reaction pathway proposed for the reduction of SO3 with CO molecule are as follows: SO3 ? SO2 + Oads and Oads + CO ? CO2. Based on the results of this study, it can be proposed that the Si-doped graphene can act as more effective catalyst for the dissociation and reduction of SO3 molecule than the Al-doped one.
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