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CO catalytic oxidation over C59X heterofullerenes (X = B, Si, P, S): A DFT study
Computational and Theoretical Chemistry 1151 (2019) 50–57
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CO catalytic oxidation over C59X heterofullerenes (X = B, Si, P, S): A DFT study
T
Mehdi D. Esrafili , Safa Heidari ⁎
Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran
ARTICLE INFO
ABSTRACT
Keywords: Fullerene CO oxidation DFT Catalyst Doping
The aim of this study is to explore the mechanisms of CO oxidation to CO2 over some C59X heterofullerenes (X = B, P, Si and S) through systematic dispersion-corrected density functional theory calculations. The adsorption energy of O2 over these fullerenes becomes more negative in the order of C59S < C59B < C59P < C59Si. According to our results, the CO oxidation reaction cannot take place over C59B and C59S due to the poisoning of the active site of these surfaces by CO. The oxidation of first CO molecule over C59Si and C59P proceeds via the Langmuir–Hinshelwood mechanism.
1. Introduction Carbon monoxide (CO) is one of the most important air pollutants produced mainly by incomplete burning of carbon-based fuels in various combustion sources, such as vehicles and electric power plants [1,2]. The health significance of CO is due to the fact that it bonds strongly to the iron atom of hemoglobin and thus impairs the oxygencarrying capacity of the blood. Therefore, development of efficient techniques for capture CO from the atmosphere along with other direct methods such as its low temperature oxidation has attracted a great deal of attention in recent years [3–6]. Among them, the catalytic oxidation of CO by molecular oxygen (O2) is one of the most useful techniques, since O2 is costless, readily available, and its reaction with CO produces a less harmful product, i.e., CO2. Various noble metals [7–9] or their alloys [10,11] have been frequently used to catalyze this reaction. In many cases, these noble metal-based catalysts exhibit outstanding catalytic activity and selectivity towards the oxidation of CO. However, despite intensive research in this area, the high price, scarcity and limited sources of these catalysts have limited their mass-scale uses. Accordingly, considerable attention has been recently turned towards the replacement of these catalysts with their costless alternatives [12–23]. In the past decade, carbon-based nanostructures including fullerenes, graphene and nanotubes have attracted considerable attention due to their promising physical and chemical properties [24–27]. Various applications have been proposed for these nanomaterials including in hydrogen storage [28,29], ion batteries [30,31] and sensor devices [32,33]. In particular, the high surface-to-area ratio of these
⁎
materials make them as an ideal support for use in catalytic applications [34,35]. Numerous studies [36–38] have indicated that small carbon fullerenes, especially those chemically-doped with heteroatoms, exhibit superior catalytic activity towards the oxygen reduction reaction (ORR). For instance, a recent density functional theory (DFT) study [37] has indicated that N-doped C40 and C60 fullerenes, namely, C39N and C59N, show outstanding catalytic activity towards the ORR with activation energies close to those on Pt(1 1 1). Lin and coworkers [39] have found that C59N heterofullerene can act as a metal-free catalyst for CO oxidation using the DFT calculations. According to these studies, the presence of a nitrogen atom in C60 or other carbon fullerenes can provide active sites in these materials due to electronegative difference between the N (3.04) and C (2.52) atom. A similar situation can be also found in boron-doped carbon fullerenes [40], where boron dopant acts as a potential Lewis acid site for the adsorption of incoming electronrich molecules. The introduction of boron atom in these materials can also induce asymmetric spin densities that are mostly localized over the B and its neighboring C atom. As a result, B-doping of carbon fullerenes leads to an enhanced adsorption energy of the paramagnetic electronrich O2 molecule [36]. For instance, C59B is found to promote O2 dissociation reaction both kinetically and thermodynamically [41]. Similarly, it is expected that metal-free C59B can serve as a highly active catalyst in the CO oxidation reaction. However, to the best of our knowledge, there is neither theoretical or experimental studies about the reactivity of C59B fullerene towards CO oxidation. Inspired by the above reports, the aim of the present study is to provide theoretical insight into the reaction mechanisms for the oxidation of CO over different hetero-doped C60 fullerene (C59X; X = B, Si,
Corresponding author. E-mail address: [email protected] (M.D. Esrafili).
https://doi.org/10.1016/j.comptc.2019.02.007 Received 6 December 2018; Received in revised form 9 February 2019; Accepted 10 February 2019 Available online 11 February 2019 2210-271X/ © 2019 Elsevier B.V. All rights reserved.
Computational and Theoretical Chemistry 1151 (2019) 50–57
M.D. Esrafili, S. Heidari
The C59X doped species were prepared by substituting one C atom in C60 with a B, Si, P or S atom. A periodic supercell with a 25 × 25 × 25 Å cubic box was used for the calculations. The Γ point was considered for the summation in the Brillouin zone. The minimumenergy pathway (MEP) for each reaction step was obtained by linear synchronous transit (LST)/quadratic synchronous transit (QST) and nudged elastic band (NEB) methods. To investigate the stability of C59X systems, the formation energy (Eform) was calculated by the following equation
Eform = EC59X + EC
EC60
(1)
EX
where EC59X, EC60, EC and EX are the total energy of C59X and C60, the energy of atomic C in C60, and the energy of atomic X, respectively. With this definition, a negative formation energy indicates that the formation of C59X species is thermodynamically favorable. The adsorption energy (Eads) of each adsorbate was obtained by
Eads = Eadsorbate/cage
Eadsorbate
Ecage
(2)
where Eadsorbate/cage, Eadsorbate and Ecage are the total energies of the adsorbate/cage complex, the adsorbate and nanocage, respectively. To evaluate the atomic charges and charge-transfer values, the atomic charges were calculated by using Hirshfeld analysis [47]. 3. Results and discussion 3.1. Heteroatom-doped fullerenes (C59X)
Fig. 1. Optimized structure of C59X heterofullerenes. All bond distances are in Å.
As a primary prerequisite to study catalytic activity of C59X heterofullerenes, we firstly investigate the geometry and electronic structure of these systems. Fig. 1 shows the most stable geometry of C59X optimized at the PBE/DNP level of theory. All these structures are obtained via the substitution of a carbon atom of C60 with a X atom, and then allowing the system to relax. Since all carbon atoms in C60 are symmetrically equivalent, replacement of one carbon by the X atom results in a single isomer in C59X systems. As seen from Fig. 1, doping of a B atom leads to small structure deformation in C60, which is consistent with the results of earlier studies [36,40,41]. This can be mainly related to the similar size of B and C atoms. Meanwhile, the triple-coordination characteristic of B atom allows it to be comfortably incorporated into the C60 structure. As a result, the CeC bond distances around the dopant atom in C59B deviate slightly from those in the pure C60. According to Table 1, the Eform value of C59B is calculated to be −5.23 eV, which indicates the formation of this structure is thermodynamically favorable. We note that this Eform value is in good agreement with those of earlier studies [36], verifying the reliability of the model and the density functional used here. On the other hand, due to the larger size of Si, P and S atoms compared to the C, it is found that the dopant atom is pushed outward and forms a tetrahedral structure with its three adjacent C atoms. This leads to a large structural deformation in C60 and hence results in a small formation energy (Table 1). According to the Hirshfeld analysis, the introduction of X impurity in C60 can induce some electron density redistribution especially around the dopant atom. This charge redistribution can regulate the electronic structure and surface reactivity of C60 mainly due to the difference in electronegativity of X atom and neighboring C atoms. For instance, the carbon atom around the B in C59B are negatively charged about −0.06 |e|, which is larger than the corresponding value in the pure C60. In all cases, our results indicate that a positive charge is localized over the dopant atom (Table 1). Such a positively charge region may serve as a potential site for adsorbing O2 and CO molecules as discussed below.
Table 1 Calculated formation energies and atomic charge on the X atom of C59X heterofullerenes. Fullerene
Eform (eV)
qX (|e|)
C59B C59Si C59P C59S
−5.23 −2.39 −2.49 −0.99
0.09 0.26 0.21 0.33
P and S). We perform systematic DFT calculations to find the most stable adsorption configuration of O2 and CO molecules over these systems. To assess the catalytic performance of these C59X fullerenes, the obtained activation energies are compared with those of other catalysts from the literature. The results of this study can be useful for developing low cost catalysts for pollutant gas abatement applications. 2. Computational details The spin-polarized DFT calculations were performed with the DMol3 [42,43]. The generalized gradient approximation with the PerdewBurke-Ernzernhof (PBE) [44] functional was adopted to treat the exchange-correlation interactions. To accurately describe the van der Waals effects, the PBE + D2 method within the Grimme’s scheme [45,46] was used in the all calculations. A double-numerical basis set with the polarization function (DNP) was chosen as the basis set with a real-space cutoff radius of 4.6 Å. A Fermi smearing parameter of 0.005 Ha was used in the calculations. The convergence tolerance for energy change, maximum force, and maximum displacement were 1 × 105 Ha, 0.001 Ha Å−1, and 0.005 Å, respectively. Vibrational frequencies were calculated on the optimized geometries to identify the nature of the optimized structures.
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Fig. 2. Optimized structure and corresponding PDOS plots of O2 adsorbed over (a) C59B, (b) C59Si, (c) C59P and (d) C59S. All bond distances are in Å. In the PDOS plots, the dashed line indicates the Fermi level, which set to be zero.
Fig. 2(a) also supports this finding, where there is an important orbital hybridization between the B and O2 states around the Fermi level. In comparison, our results indicate that O2 adopts a side-one configuration over C59Si and C59P, and forms two chemical bonds with the dopant atom. This is roughly similar to those finding over transition-metal doped graphene sheets [48–51]. Consequently, the calculated Eads value for the adsorption of O2 over these systems is quite larger than that of over C59B. As Table 1 indicates, there also are more electrons transferred from the fullerene to the adsorbed O2, which confirms the stronger adsorption of O2 molecule over these surfaces. As a result, the 2π* state of O2 molecule is partially populated and hence shifts downward with respect to the Fermi level. Note also that the large adsorption energy of O2 over C59Si can be mainly attributed to the significant contribution of Si atom in the highest occupied molecular orbital (HOMO) of C59Si (Fig. S1 of Supporting Information). In the case of C59S, one can see that the nearest distances between the S and O2 molecule is 2.76 Å, which indicates O2 is physisorbed over this surface. There is a negligible charge-transfer from C59S to O2, which is well consistent with the small adsorption energy (−0.07 eV). Such weak interaction can be attributed to the relatively large energy difference between the HOMO of C59S (−5.43 eV) and 2π* state of O2 (−4.67 eV) molecule (Fig. S1). In fact, we found a good correlation between the net charge-transfer values and adsorption energies in Table 2, which clearly shows the important role of charge-transfer effects in the formation of these complexes. As shown in Fig. 3, an end-on configuration is found for the adsorption of CO molecule over the C59X systems. The adsorbed CO is almost vertical on the C59B and C59Si surfaces with a binding distance of 1.55 and 2.01 Å, respectively. We note that the binding distance of CO over C59B is much shorter than that of over B-doped graphene [52], which can be related to the curvature effects in the former system. One can also see that C59B exhibits a larger tendency towards CO molecule than C59Si, as evidenced by the calculated adsorption energies (Table 2). According to the Hirshfeld analysis, there are 0.22 and 0.12
Table 2 Calculated adsorption energies and net Hirshfeld charge-transfer values for the adsorption of O2 and CO over C59X heterofullerenes.a Cage
Molecule
Eads (eV)
qCT (e)
C59B
O2 CO O2 CO O2 CO O2 CO
−0.33 −0.91 −1.63 −0.33 −0.63 −0.11 −0.07 −0.16
−0.10 0.22 −0.31 0.12 −0.30 0.02 −0.15 0.02
C59Si C59P C59S
a The negative qCT value indicates the charge-transfer from the fullerene to the adsorbate, while positive qCT corresponds to the charge-transfer from the adsorbate to fullerene.
3.2. Adsorption of O2 and CO As noted in earlier theoretical studies [20,48–50], the preferred reaction mechanism for the oxidation of CO depends considerably on the configuration and adsorption energy of O2 and CO. Hence, we study the most stable configurations of these molecules over the title systems. Fig. 2 shows the optimized structures as well the corresponding partial density of states (PDOS) plots of O2 and CO adsorbed on C59X cages. The adsorption energies and net Hirshfeld charge-transfer values due to formation of these complexes are listed in Table 2. As Fig. 2(a) indicates, O2 molecule adopts an end-on configuration over C59B with the B-O binding distance of 1.57 Å. The adsorption energy of the O2 is calculated to be −0.33 eV, which is consistent with that of obtained value by Li and coworkers (−0.35 eV) [41]. There is about −0.10 e charge-transfer from C59B to O2, which leads to elongating of the OeO bond from 1.23 Å in the isolated O2 to 1.30 Å in the adsorbed form. Meanwhile, the positive charge on the B atom is decreased by 0.06 e upon the adsorption of O2. The PDOS analysis in
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Fig. 3. Optimized structure and corresponding PDOS plots of CO adsorbed over (a) C59B, (b) C59Si, (c) C59P and (d) C59S. All bond distances are in Å. In the PDOS plots, the dashed line indicates the Fermi level, which set to be zero.
electrons transferred from CO to C59B and C59Si, respectively, which indicates CO serves as an electron donor over these systems. In fact, the PDOS analysis in Fig. 3 shows that the CO-5σ state below the Fermi level almost disappears upon the adsorption of CO molecule. On the other hand, our results indicate that CO is somewhat tilted over C59P and C59S and has a small adsorption energy. Compared with the C59B and C59Si, the adsorption of CO over these systems is also accompanied with a fewer charge-transfer (0.02 e). Moreover, there is a negligible hybridization between the PDOS of the dopant atom and the CO-5σ around the Fermi level, which clearly confirms the weak interaction of adsorbed CO on the C59P and C59S surface.
3.3.1. ER mechanism The energy profile and local configurations for the initial state (IS), transition state (TS) and intermediate state (IM) and final state (FS) involved in the CO oxidation reaction over C59Si are shown in Fig. 4(a). As evident, the oxidation of the first CO molecule starts with the chemisorption of O2 on the Si atom. Then, CO approaches the O2 molecule and the IS-1 state is obtained. At the IS-1, CO lies about 3.4 Å above the O2 with CeO bond distances of 1.14 Å. This indicates that CO is weakly adsorbed over O2. Meanwhile, the OeO bond length of the chemisorbed O2 remains almost unchanged upon the adsorption of CO molecule. Reaching the TS-1, the OeO bond distance is elongated by 0.47 Å and the CeO bond distance of CO is stretched to 1.15 Å. Passing over the TS1 with an activation energy of 1.78 eV, a carbonate-like intermediate (CO3) is formed. This process is highly exothermic due to strong adsorption of CO3 moiety over the Si atom. As the CO oxidation reaction proceeds, the CO3 intermediate is dissociated into a CO2 molecule, and an activated oxygen atom (Oads) attached to the Si atom. The produced CO2 has a small adsorption energy of −0.19 eV, suggesting that it can be easily released from the surface at ambient condition. For the CO oxidation over C59P, the gas-phase CO directly reacts with the preadsorbed O2 molecule. The corresponding atomic configurations at various states along the reaction are indicated in Fig. 4(b). The adsorption energy of CO in IS-3 is calculated to be −0.23 eV, which suggests that this molecule weakly interacts with the adsorbed O2. Based on the Hirshfeld analysis, the formation of IS-3 is accompanied with about 0.02 e charge-transferred from the O2 to CO. Passing over TS-3 with a large activation energy of 1.93 eV, a CO2 molecule is directly formed with Oads adsorbed over the P atom. Note that unlike the CO oxidation over C59Si, we have not find any carbonate-like CO3 intermediate over C59P. This is consistent with our earlier finding about CO oxidation on P-doped graphene [6], and simply indicates that this intermediate is not stable over this surface. The released energy for this process is −4.78 eV, which is larger than the corresponding reaction energy over C59Si. Overall, the obtained large energy barriers for both C59Si and C59P suggest that the ER mechanism is hardly possible for the CO oxidation over these surfaces.
3.3. Reaction mechanisms for CO oxidation We now study the possible reaction mechanisms for the oxidation of CO molecule over C59X. According to the earlier studies [49–51,53,54], CO oxidation over a catalyst surface can proceed via either two wellknown mechanisms, i.e. the Langmuir–Hinshelwood (LH) or the Eley–Rideal (ER). In the LH mechanism, O2 molecule is first chemisorbed over the catalyst surface and then CO molecule attacks the O2 and a peroxo-type OCOO intermediate is obtained. Later, this intermediate is dissociated into CO2, and an O species adsorbed on the surface. For the ER mechanism, O2 molecule is first activated over the surface and then reacts with nearby CO molecule. Comparing the Eads values in Table 2 clearly shows that the adsoprption of CO is stronger than that of O2 over C59B and C59S. This means that when exposed to the O2 and CO gases, the mentioned surfaces are poisoned by CO molecule. Consequently, the CO oxidation reaction cannot proceed over these surfaces. Moreover, the small Eads value of O2 over C59S suggests that this molecule is not sufficiently activated over this surface and thermodynamically unstable. Thus, in the following section, we investigate the CO oxidation reaction over C59Si and C59P due to their more favorable adsorption energies. Both the ER and LH mechanisms are examined comparably in order find the most energetically favorable one.
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Fig. 4. The potential energy profile and relaxed stationary points for the oxidation of CO via the ER mechanism over (a) C59Si, and (b) C59P. All bond distances are in Å.
3.3.2. LH mechanism The energy profile and relaxed stationary points for the CO oxidation over C59Si via the LH mechanism is depicted in Fig. 5(a). As evident, the CO oxidation over this surface begins with the coadsorption of O2 and CO molecules (IS-4). However, due to the stronger interaction, O2 is attached to the Si atom and CO molecule is weakly physisorbed over the surface. The distance between CO and Si atom in IS-4 is about 3.20 Å and the Si-O bond lengths are 1.73 Å. Next, CO approaches the preadsorbed O2 to form TS-4 with the Si–CO binding distance to be 2.49 Å. By overcoming an activation energy of 0.32 eV, CO finally binds to the Si atom and an epoxo-type OOCO intermediate is obtained. This process is exothermic by 0.36 eV. Subsequently, the OOCO intermediate
is dissociated into CO2 and Oads moieties. The formed CO2 can be easily desorbed from the surface due to its small adsorption energy. Likewise, the CO oxidation over C59P involves the coadsorption of O2 and CO molecules and then the formation of an epoxo-type OOCO intermediate (Fig. 5(b)). The energy barrier to reach this intermediate is calculated to be 0.38 eV, which indicates this process is also likely to take place at normal temperature. Meanwhile, the corresponding reaction energy is −0.54 eV, suggesting that the formation of OOCO is a thermodynamically favorable process. To proceed, a physisorbed CO2 molecule is formed over the surface, leaving an Oads moiety adsorbed on the P atom. Compared with the ER mechanism, the CO oxidation over both
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Fig. 5. The potential energy profile and relaxed stationary points for the oxidation of CO via the LH mechanism over (a) C59Si, and (b) C59P. All bond distances are in Å.
C59Si and C59P surfaces via the LH mechanism has a much smaller energy barrier. This indicates that the LH mechanism is the more favorable mechanism and should be considered as the starting point for the CO oxidation over these surfaces. In the next step, another CO molecule attacks the chemisorbed Oads, for which the local configurations and corresponding energy profile are given in Fig. 6. One can see that as the CO molecule approaches the Oads, the SieOads or PeOads bond distance is continuously elongated until the second CO2 molecule is formed. Although this reaction step is exothermic over both surfaces, however, our DFT calculations reveal that the energy barrier for the CO + Oads reaction over C59P is much larger than that of over C59Si. To explain this finding, we calculated the adsorption energy of the Oads over these surfaces. It is found that the Eads value of Oads over C59P is −5.36 eV, which is larger than that of over C59Si (−5.08 eV). Consequently, the second step of CO oxidation cannot proceed over C59P due to the large estimated activation energy. Moreover, the energy barrier
obtained for the oxidation of second CO molecule over C59Si is almost similar to that of reported value over Si-doped graphene (0.07 eV) [55], but much smaller than those of reported over noble metals like Pd (0.91 eV) and Pt (0.79 eV) [56]. It is also comparable with those of values reported for a single noble-metal atom catalysts, like Pd(0.26 eV) [49] or Au-doped (0.18 eV) [57] graphene, which clearly reveals the large catalytic activity of C59Si towards the CO oxidation. 4. Conclusion We performed a detailed DFT study about the CO oxidation reaction over heterofullerenes C59X (X = B, Si, P and S), finding that the catalytic performance of these systems depends considerably on the nature of the dopant atom. According to our results, the adsorption energy of CO is larger than that of O2 over C59B and C59S. This indicates that when exposed to the O2 and CO gases, these surfaces are poisoned by
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Fig. 6. The potential energy profile and relaxed stationary points for the CO + Oads → CO2 over (a) C59Si and (b) C59P. All bond distances are in Å.
that the corresponding energy barrier over C59P is much larger than that of over C59Si. The activation energies for the oxidation of CO molecule over C59Si is comparable to or even smaller than that of over some noble-metal based catalysts.
CO molecule. Hence, the CO oxidation reaction cannot proceed over these surfaces. Comparing the calculated activation energies indicated that the oxidation of first CO molecule over C59Si and C59P surfaces proceeds via the LH mechanism. Although the oxidation of second CO molecule is exothermic over both surfaces, however, our results show
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Appendix A. Supplementary material
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
CO catalytic oxidation over C59X heterofullerenes (X = B, Si, P, S): A DFT study
T
Mehdi D. Esrafili , Safa Heidari ⁎
Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, Maragheh, Iran
ARTICLE INFO
ABSTRACT
Keywords: Fullerene CO oxidation DFT Catalyst Doping
The aim of this study is to explore the mechanisms of CO oxidation to CO2 over some C59X heterofullerenes (X = B, P, Si and S) through systematic dispersion-corrected density functional theory calculations. The adsorption energy of O2 over these fullerenes becomes more negative in the order of C59S < C59B < C59P < C59Si. According to our results, the CO oxidation reaction cannot take place over C59B and C59S due to the poisoning of the active site of these surfaces by CO. The oxidation of first CO molecule over C59Si and C59P proceeds via the Langmuir–Hinshelwood mechanism.
1. Introduction Carbon monoxide (CO) is one of the most important air pollutants produced mainly by incomplete burning of carbon-based fuels in various combustion sources, such as vehicles and electric power plants [1,2]. The health significance of CO is due to the fact that it bonds strongly to the iron atom of hemoglobin and thus impairs the oxygencarrying capacity of the blood. Therefore, development of efficient techniques for capture CO from the atmosphere along with other direct methods such as its low temperature oxidation has attracted a great deal of attention in recent years [3–6]. Among them, the catalytic oxidation of CO by molecular oxygen (O2) is one of the most useful techniques, since O2 is costless, readily available, and its reaction with CO produces a less harmful product, i.e., CO2. Various noble metals [7–9] or their alloys [10,11] have been frequently used to catalyze this reaction. In many cases, these noble metal-based catalysts exhibit outstanding catalytic activity and selectivity towards the oxidation of CO. However, despite intensive research in this area, the high price, scarcity and limited sources of these catalysts have limited their mass-scale uses. Accordingly, considerable attention has been recently turned towards the replacement of these catalysts with their costless alternatives [12–23]. In the past decade, carbon-based nanostructures including fullerenes, graphene and nanotubes have attracted considerable attention due to their promising physical and chemical properties [24–27]. Various applications have been proposed for these nanomaterials including in hydrogen storage [28,29], ion batteries [30,31] and sensor devices [32,33]. In particular, the high surface-to-area ratio of these
⁎
materials make them as an ideal support for use in catalytic applications [34,35]. Numerous studies [36–38] have indicated that small carbon fullerenes, especially those chemically-doped with heteroatoms, exhibit superior catalytic activity towards the oxygen reduction reaction (ORR). For instance, a recent density functional theory (DFT) study [37] has indicated that N-doped C40 and C60 fullerenes, namely, C39N and C59N, show outstanding catalytic activity towards the ORR with activation energies close to those on Pt(1 1 1). Lin and coworkers [39] have found that C59N heterofullerene can act as a metal-free catalyst for CO oxidation using the DFT calculations. According to these studies, the presence of a nitrogen atom in C60 or other carbon fullerenes can provide active sites in these materials due to electronegative difference between the N (3.04) and C (2.52) atom. A similar situation can be also found in boron-doped carbon fullerenes [40], where boron dopant acts as a potential Lewis acid site for the adsorption of incoming electronrich molecules. The introduction of boron atom in these materials can also induce asymmetric spin densities that are mostly localized over the B and its neighboring C atom. As a result, B-doping of carbon fullerenes leads to an enhanced adsorption energy of the paramagnetic electronrich O2 molecule [36]. For instance, C59B is found to promote O2 dissociation reaction both kinetically and thermodynamically [41]. Similarly, it is expected that metal-free C59B can serve as a highly active catalyst in the CO oxidation reaction. However, to the best of our knowledge, there is neither theoretical or experimental studies about the reactivity of C59B fullerene towards CO oxidation. Inspired by the above reports, the aim of the present study is to provide theoretical insight into the reaction mechanisms for the oxidation of CO over different hetero-doped C60 fullerene (C59X; X = B, Si,
Corresponding author. E-mail address: [email protected] (M.D. Esrafili).
https://doi.org/10.1016/j.comptc.2019.02.007 Received 6 December 2018; Received in revised form 9 February 2019; Accepted 10 February 2019 Available online 11 February 2019 2210-271X/ © 2019 Elsevier B.V. All rights reserved.
Computational and Theoretical Chemistry 1151 (2019) 50–57
M.D. Esrafili, S. Heidari
The C59X doped species were prepared by substituting one C atom in C60 with a B, Si, P or S atom. A periodic supercell with a 25 × 25 × 25 Å cubic box was used for the calculations. The Γ point was considered for the summation in the Brillouin zone. The minimumenergy pathway (MEP) for each reaction step was obtained by linear synchronous transit (LST)/quadratic synchronous transit (QST) and nudged elastic band (NEB) methods. To investigate the stability of C59X systems, the formation energy (Eform) was calculated by the following equation
Eform = EC59X + EC
EC60
(1)
EX
where EC59X, EC60, EC and EX are the total energy of C59X and C60, the energy of atomic C in C60, and the energy of atomic X, respectively. With this definition, a negative formation energy indicates that the formation of C59X species is thermodynamically favorable. The adsorption energy (Eads) of each adsorbate was obtained by
Eads = Eadsorbate/cage
Eadsorbate
Ecage
(2)
where Eadsorbate/cage, Eadsorbate and Ecage are the total energies of the adsorbate/cage complex, the adsorbate and nanocage, respectively. To evaluate the atomic charges and charge-transfer values, the atomic charges were calculated by using Hirshfeld analysis [47]. 3. Results and discussion 3.1. Heteroatom-doped fullerenes (C59X)
Fig. 1. Optimized structure of C59X heterofullerenes. All bond distances are in Å.
As a primary prerequisite to study catalytic activity of C59X heterofullerenes, we firstly investigate the geometry and electronic structure of these systems. Fig. 1 shows the most stable geometry of C59X optimized at the PBE/DNP level of theory. All these structures are obtained via the substitution of a carbon atom of C60 with a X atom, and then allowing the system to relax. Since all carbon atoms in C60 are symmetrically equivalent, replacement of one carbon by the X atom results in a single isomer in C59X systems. As seen from Fig. 1, doping of a B atom leads to small structure deformation in C60, which is consistent with the results of earlier studies [36,40,41]. This can be mainly related to the similar size of B and C atoms. Meanwhile, the triple-coordination characteristic of B atom allows it to be comfortably incorporated into the C60 structure. As a result, the CeC bond distances around the dopant atom in C59B deviate slightly from those in the pure C60. According to Table 1, the Eform value of C59B is calculated to be −5.23 eV, which indicates the formation of this structure is thermodynamically favorable. We note that this Eform value is in good agreement with those of earlier studies [36], verifying the reliability of the model and the density functional used here. On the other hand, due to the larger size of Si, P and S atoms compared to the C, it is found that the dopant atom is pushed outward and forms a tetrahedral structure with its three adjacent C atoms. This leads to a large structural deformation in C60 and hence results in a small formation energy (Table 1). According to the Hirshfeld analysis, the introduction of X impurity in C60 can induce some electron density redistribution especially around the dopant atom. This charge redistribution can regulate the electronic structure and surface reactivity of C60 mainly due to the difference in electronegativity of X atom and neighboring C atoms. For instance, the carbon atom around the B in C59B are negatively charged about −0.06 |e|, which is larger than the corresponding value in the pure C60. In all cases, our results indicate that a positive charge is localized over the dopant atom (Table 1). Such a positively charge region may serve as a potential site for adsorbing O2 and CO molecules as discussed below.
Table 1 Calculated formation energies and atomic charge on the X atom of C59X heterofullerenes. Fullerene
Eform (eV)
qX (|e|)
C59B C59Si C59P C59S
−5.23 −2.39 −2.49 −0.99
0.09 0.26 0.21 0.33
P and S). We perform systematic DFT calculations to find the most stable adsorption configuration of O2 and CO molecules over these systems. To assess the catalytic performance of these C59X fullerenes, the obtained activation energies are compared with those of other catalysts from the literature. The results of this study can be useful for developing low cost catalysts for pollutant gas abatement applications. 2. Computational details The spin-polarized DFT calculations were performed with the DMol3 [42,43]. The generalized gradient approximation with the PerdewBurke-Ernzernhof (PBE) [44] functional was adopted to treat the exchange-correlation interactions. To accurately describe the van der Waals effects, the PBE + D2 method within the Grimme’s scheme [45,46] was used in the all calculations. A double-numerical basis set with the polarization function (DNP) was chosen as the basis set with a real-space cutoff radius of 4.6 Å. A Fermi smearing parameter of 0.005 Ha was used in the calculations. The convergence tolerance for energy change, maximum force, and maximum displacement were 1 × 105 Ha, 0.001 Ha Å−1, and 0.005 Å, respectively. Vibrational frequencies were calculated on the optimized geometries to identify the nature of the optimized structures.
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Fig. 2. Optimized structure and corresponding PDOS plots of O2 adsorbed over (a) C59B, (b) C59Si, (c) C59P and (d) C59S. All bond distances are in Å. In the PDOS plots, the dashed line indicates the Fermi level, which set to be zero.
Fig. 2(a) also supports this finding, where there is an important orbital hybridization between the B and O2 states around the Fermi level. In comparison, our results indicate that O2 adopts a side-one configuration over C59Si and C59P, and forms two chemical bonds with the dopant atom. This is roughly similar to those finding over transition-metal doped graphene sheets [48–51]. Consequently, the calculated Eads value for the adsorption of O2 over these systems is quite larger than that of over C59B. As Table 1 indicates, there also are more electrons transferred from the fullerene to the adsorbed O2, which confirms the stronger adsorption of O2 molecule over these surfaces. As a result, the 2π* state of O2 molecule is partially populated and hence shifts downward with respect to the Fermi level. Note also that the large adsorption energy of O2 over C59Si can be mainly attributed to the significant contribution of Si atom in the highest occupied molecular orbital (HOMO) of C59Si (Fig. S1 of Supporting Information). In the case of C59S, one can see that the nearest distances between the S and O2 molecule is 2.76 Å, which indicates O2 is physisorbed over this surface. There is a negligible charge-transfer from C59S to O2, which is well consistent with the small adsorption energy (−0.07 eV). Such weak interaction can be attributed to the relatively large energy difference between the HOMO of C59S (−5.43 eV) and 2π* state of O2 (−4.67 eV) molecule (Fig. S1). In fact, we found a good correlation between the net charge-transfer values and adsorption energies in Table 2, which clearly shows the important role of charge-transfer effects in the formation of these complexes. As shown in Fig. 3, an end-on configuration is found for the adsorption of CO molecule over the C59X systems. The adsorbed CO is almost vertical on the C59B and C59Si surfaces with a binding distance of 1.55 and 2.01 Å, respectively. We note that the binding distance of CO over C59B is much shorter than that of over B-doped graphene [52], which can be related to the curvature effects in the former system. One can also see that C59B exhibits a larger tendency towards CO molecule than C59Si, as evidenced by the calculated adsorption energies (Table 2). According to the Hirshfeld analysis, there are 0.22 and 0.12
Table 2 Calculated adsorption energies and net Hirshfeld charge-transfer values for the adsorption of O2 and CO over C59X heterofullerenes.a Cage
Molecule
Eads (eV)
qCT (e)
C59B
O2 CO O2 CO O2 CO O2 CO
−0.33 −0.91 −1.63 −0.33 −0.63 −0.11 −0.07 −0.16
−0.10 0.22 −0.31 0.12 −0.30 0.02 −0.15 0.02
C59Si C59P C59S
a The negative qCT value indicates the charge-transfer from the fullerene to the adsorbate, while positive qCT corresponds to the charge-transfer from the adsorbate to fullerene.
3.2. Adsorption of O2 and CO As noted in earlier theoretical studies [20,48–50], the preferred reaction mechanism for the oxidation of CO depends considerably on the configuration and adsorption energy of O2 and CO. Hence, we study the most stable configurations of these molecules over the title systems. Fig. 2 shows the optimized structures as well the corresponding partial density of states (PDOS) plots of O2 and CO adsorbed on C59X cages. The adsorption energies and net Hirshfeld charge-transfer values due to formation of these complexes are listed in Table 2. As Fig. 2(a) indicates, O2 molecule adopts an end-on configuration over C59B with the B-O binding distance of 1.57 Å. The adsorption energy of the O2 is calculated to be −0.33 eV, which is consistent with that of obtained value by Li and coworkers (−0.35 eV) [41]. There is about −0.10 e charge-transfer from C59B to O2, which leads to elongating of the OeO bond from 1.23 Å in the isolated O2 to 1.30 Å in the adsorbed form. Meanwhile, the positive charge on the B atom is decreased by 0.06 e upon the adsorption of O2. The PDOS analysis in
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Fig. 3. Optimized structure and corresponding PDOS plots of CO adsorbed over (a) C59B, (b) C59Si, (c) C59P and (d) C59S. All bond distances are in Å. In the PDOS plots, the dashed line indicates the Fermi level, which set to be zero.
electrons transferred from CO to C59B and C59Si, respectively, which indicates CO serves as an electron donor over these systems. In fact, the PDOS analysis in Fig. 3 shows that the CO-5σ state below the Fermi level almost disappears upon the adsorption of CO molecule. On the other hand, our results indicate that CO is somewhat tilted over C59P and C59S and has a small adsorption energy. Compared with the C59B and C59Si, the adsorption of CO over these systems is also accompanied with a fewer charge-transfer (0.02 e). Moreover, there is a negligible hybridization between the PDOS of the dopant atom and the CO-5σ around the Fermi level, which clearly confirms the weak interaction of adsorbed CO on the C59P and C59S surface.
3.3.1. ER mechanism The energy profile and local configurations for the initial state (IS), transition state (TS) and intermediate state (IM) and final state (FS) involved in the CO oxidation reaction over C59Si are shown in Fig. 4(a). As evident, the oxidation of the first CO molecule starts with the chemisorption of O2 on the Si atom. Then, CO approaches the O2 molecule and the IS-1 state is obtained. At the IS-1, CO lies about 3.4 Å above the O2 with CeO bond distances of 1.14 Å. This indicates that CO is weakly adsorbed over O2. Meanwhile, the OeO bond length of the chemisorbed O2 remains almost unchanged upon the adsorption of CO molecule. Reaching the TS-1, the OeO bond distance is elongated by 0.47 Å and the CeO bond distance of CO is stretched to 1.15 Å. Passing over the TS1 with an activation energy of 1.78 eV, a carbonate-like intermediate (CO3) is formed. This process is highly exothermic due to strong adsorption of CO3 moiety over the Si atom. As the CO oxidation reaction proceeds, the CO3 intermediate is dissociated into a CO2 molecule, and an activated oxygen atom (Oads) attached to the Si atom. The produced CO2 has a small adsorption energy of −0.19 eV, suggesting that it can be easily released from the surface at ambient condition. For the CO oxidation over C59P, the gas-phase CO directly reacts with the preadsorbed O2 molecule. The corresponding atomic configurations at various states along the reaction are indicated in Fig. 4(b). The adsorption energy of CO in IS-3 is calculated to be −0.23 eV, which suggests that this molecule weakly interacts with the adsorbed O2. Based on the Hirshfeld analysis, the formation of IS-3 is accompanied with about 0.02 e charge-transferred from the O2 to CO. Passing over TS-3 with a large activation energy of 1.93 eV, a CO2 molecule is directly formed with Oads adsorbed over the P atom. Note that unlike the CO oxidation over C59Si, we have not find any carbonate-like CO3 intermediate over C59P. This is consistent with our earlier finding about CO oxidation on P-doped graphene [6], and simply indicates that this intermediate is not stable over this surface. The released energy for this process is −4.78 eV, which is larger than the corresponding reaction energy over C59Si. Overall, the obtained large energy barriers for both C59Si and C59P suggest that the ER mechanism is hardly possible for the CO oxidation over these surfaces.
3.3. Reaction mechanisms for CO oxidation We now study the possible reaction mechanisms for the oxidation of CO molecule over C59X. According to the earlier studies [49–51,53,54], CO oxidation over a catalyst surface can proceed via either two wellknown mechanisms, i.e. the Langmuir–Hinshelwood (LH) or the Eley–Rideal (ER). In the LH mechanism, O2 molecule is first chemisorbed over the catalyst surface and then CO molecule attacks the O2 and a peroxo-type OCOO intermediate is obtained. Later, this intermediate is dissociated into CO2, and an O species adsorbed on the surface. For the ER mechanism, O2 molecule is first activated over the surface and then reacts with nearby CO molecule. Comparing the Eads values in Table 2 clearly shows that the adsoprption of CO is stronger than that of O2 over C59B and C59S. This means that when exposed to the O2 and CO gases, the mentioned surfaces are poisoned by CO molecule. Consequently, the CO oxidation reaction cannot proceed over these surfaces. Moreover, the small Eads value of O2 over C59S suggests that this molecule is not sufficiently activated over this surface and thermodynamically unstable. Thus, in the following section, we investigate the CO oxidation reaction over C59Si and C59P due to their more favorable adsorption energies. Both the ER and LH mechanisms are examined comparably in order find the most energetically favorable one.
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Fig. 4. The potential energy profile and relaxed stationary points for the oxidation of CO via the ER mechanism over (a) C59Si, and (b) C59P. All bond distances are in Å.
3.3.2. LH mechanism The energy profile and relaxed stationary points for the CO oxidation over C59Si via the LH mechanism is depicted in Fig. 5(a). As evident, the CO oxidation over this surface begins with the coadsorption of O2 and CO molecules (IS-4). However, due to the stronger interaction, O2 is attached to the Si atom and CO molecule is weakly physisorbed over the surface. The distance between CO and Si atom in IS-4 is about 3.20 Å and the Si-O bond lengths are 1.73 Å. Next, CO approaches the preadsorbed O2 to form TS-4 with the Si–CO binding distance to be 2.49 Å. By overcoming an activation energy of 0.32 eV, CO finally binds to the Si atom and an epoxo-type OOCO intermediate is obtained. This process is exothermic by 0.36 eV. Subsequently, the OOCO intermediate
is dissociated into CO2 and Oads moieties. The formed CO2 can be easily desorbed from the surface due to its small adsorption energy. Likewise, the CO oxidation over C59P involves the coadsorption of O2 and CO molecules and then the formation of an epoxo-type OOCO intermediate (Fig. 5(b)). The energy barrier to reach this intermediate is calculated to be 0.38 eV, which indicates this process is also likely to take place at normal temperature. Meanwhile, the corresponding reaction energy is −0.54 eV, suggesting that the formation of OOCO is a thermodynamically favorable process. To proceed, a physisorbed CO2 molecule is formed over the surface, leaving an Oads moiety adsorbed on the P atom. Compared with the ER mechanism, the CO oxidation over both
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Fig. 5. The potential energy profile and relaxed stationary points for the oxidation of CO via the LH mechanism over (a) C59Si, and (b) C59P. All bond distances are in Å.
C59Si and C59P surfaces via the LH mechanism has a much smaller energy barrier. This indicates that the LH mechanism is the more favorable mechanism and should be considered as the starting point for the CO oxidation over these surfaces. In the next step, another CO molecule attacks the chemisorbed Oads, for which the local configurations and corresponding energy profile are given in Fig. 6. One can see that as the CO molecule approaches the Oads, the SieOads or PeOads bond distance is continuously elongated until the second CO2 molecule is formed. Although this reaction step is exothermic over both surfaces, however, our DFT calculations reveal that the energy barrier for the CO + Oads reaction over C59P is much larger than that of over C59Si. To explain this finding, we calculated the adsorption energy of the Oads over these surfaces. It is found that the Eads value of Oads over C59P is −5.36 eV, which is larger than that of over C59Si (−5.08 eV). Consequently, the second step of CO oxidation cannot proceed over C59P due to the large estimated activation energy. Moreover, the energy barrier
obtained for the oxidation of second CO molecule over C59Si is almost similar to that of reported value over Si-doped graphene (0.07 eV) [55], but much smaller than those of reported over noble metals like Pd (0.91 eV) and Pt (0.79 eV) [56]. It is also comparable with those of values reported for a single noble-metal atom catalysts, like Pd(0.26 eV) [49] or Au-doped (0.18 eV) [57] graphene, which clearly reveals the large catalytic activity of C59Si towards the CO oxidation. 4. Conclusion We performed a detailed DFT study about the CO oxidation reaction over heterofullerenes C59X (X = B, Si, P and S), finding that the catalytic performance of these systems depends considerably on the nature of the dopant atom. According to our results, the adsorption energy of CO is larger than that of O2 over C59B and C59S. This indicates that when exposed to the O2 and CO gases, these surfaces are poisoned by
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Fig. 6. The potential energy profile and relaxed stationary points for the CO + Oads → CO2 over (a) C59Si and (b) C59P. All bond distances are in Å.
that the corresponding energy barrier over C59P is much larger than that of over C59Si. The activation energies for the oxidation of CO molecule over C59Si is comparable to or even smaller than that of over some noble-metal based catalysts.
CO molecule. Hence, the CO oxidation reaction cannot proceed over these surfaces. Comparing the calculated activation energies indicated that the oxidation of first CO molecule over C59Si and C59P surfaces proceeds via the LH mechanism. Although the oxidation of second CO molecule is exothermic over both surfaces, however, our results show
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Appendix A. Supplementary material
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