Mehdi D. Esrafili*,
Parisa Nematollahi and
Roghaye Nurazar
Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, P.O. Box: 5513864596, Maragheh, Iran. E-mail: esrafili@maragheh.ac.ir; Fax: +98 4212276060; Tel: +98 4212237955
First published on 2nd February 2016
In this work, we employ density functional theory calculations to investigate the CO oxidation mechanisms over B12N12 and B11N12C nanocages. Two possible reaction pathways can be proposed for the oxidation of CO with O2 molecule over these surfaces: CO + O2 → OOCO → CO2 + Oads and CO + Oads → CO2. Both Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms are considered for these two reaction pathways. Our calculations indicate that the CO oxidation reaction over the B11N12C nanocage proceeds via the ER mechanism followed by LH mechanism with an activation energy (Eact) of 0.58 eV. In the case of the B12N12 nanocage, it can be estimated that both reaction pathways go through the LH mechanism. The Eact of the first reaction step is about 2.5 eV, while it is negligible for the second route. Based on the present theoretical results, the catalytic activity of the B11N12C nanocage toward the CO oxidation reaction is more than that of the B12N12 cluster. This can be related to the presence of a C atom that plays a significant role in the activation of the whole cluster. Meanwhile, the performance of the B11N12C nanocage as a catalyst used for the oxidation of CO with O2 molecules may proceeded at near ambient temperatures. These results indicate that the B11N12C nanocage can be utilized as a favorable low-cost catalyst for the CO oxidation reaction.
Recently, scientists have found that different kinds of nanostructures such as single-walled or multi-walled carbon nanotubes (CNTs),12,13 graphene14 or boron-nitride nanotubes (BNNTs)15 have a great tendency toward the adsorption of toxic gas pollutants. These nanostructures are also a promising candidates to support metal atoms or clusters to realize new catalysts due to their large surface-to-volume ratio,14 and outstanding electrical, mechanical and thermal properties.16 Hence, many theoretical and experimental studies have been performed about the oxidation of CO on these nanostructures. For example, in one experimental study, Yoo et al. showed that the catalytic properties of graphene sheet enhances with the interaction of Pt nanoclusters.17 In another comparative theoretical investigation, Tang et al. reported that the surface activity of the Pt-embedded graphene is increased significantly toward the oxidation of CO molecule.18 Wang et al. found a strong chemical interaction between CO molecule and Si-doped BNNTs in comparison with pristine BNNTs.19
After the discovery of C60,20 numerous studies have been focused on this outstanding carbon-based material due to its unique electronic properties with potential applications in building nanodevices.21,22 C60 displays the promising ability for gas storage which makes it acts as a chemical sensor.23 In recent years, many theoretical and experimental studies have focused on the possible fullerene-like structures.24,25 They found that III–V compounds, especially the group III nitrides can be desirable nanoscaled materials due to their direct band gap which affords optical and magnetic characteristics. So, the III–V fullerene-like cages have been theoretically estimated and experimentally synthesized.26–28 On the other hand, different III–V nanostructures such as BNNTs,29,30 BN nanosheets31,32 and BN clusters,33,34 with the unique polarity of B–N bonds, have more improved surface reactivity than carbon nanostructures. In particular, (BN)n (n = 4–30) nanocages have been extensively investigated because of their high temperature stability, large thermal conductivity, oxidation resistance and low electric constant.35,36 For example, Nigam et al. have shown that a chemically inert (BN)36 cluster can be activated by incorporating magnetic nanoparticles inside it.37 Therefore, the adsorption of O2 molecule on these clusters results in an O–O bond elongation and then improves the CO oxidation reaction. Among these BN nanocages, the fullerene-like cage B12N12 as the smallest stable cage cluster is extensively investigated. Oku et al. experimentally synthesized the B12N12 nanocage by laser desorption time-of-flight mass spectrometry and reported that these clusters consists of 4- and 6-membered rings of BN with the band gap energy of about 5.1 eV.38 On the other hand, there are several interesting investigations about the adsorption of chemical toxic gases on this favorable nanocage. For instance, in our previous work,33 the adsorption and decomposition of methanol on B12N12 nanocage have studied in details. We found that the electrical conductivity of the B12N12 enhances upon the adsorption of CH3OH and this nanocage could be used as an efficient metal-free catalyst for the dehydrogenation of the methanol molecule.
Today, chemical doping with foreign atoms is a favorable approach to modify the properties of host materials. In comparison with the undoped B12N12 cluster, C-doped BN fullerene (B11N12C) displays more reactivity than B12N12.39–41 For example, Wu et al. showed that the hydrogenation reaction on B11N12C cluster is both thermodynamically and kinetically favored under ambient conditions in which C atom acts as an activation center to dissociate H2 molecule.25 On the other hand, since the carbon atom has one less valence electron than nitrogen, the B12N11C cluster exhibits the electron acceptor property around the carbon atom. Although this leads to an increase in the hydrogen chemisorption energy, but it makes a worse dehydrogenation property for the corresponding hydrogenated B12N11C cage.40 Furthermore, the adsorption and dehydrogenation of methanol on the B11N12C surface was recently studied.42 It is found that the B11N12C nanocage can effectively decompose the CH3OH molecule with the carbon atom as an activation site. The results also indicated that in contrast to the B12N11C, more electrons can be transferred from the electron-rich B11N12C to the CH3OH molecule and thus lower activation energy can be achieved for the dehydrogenation reaction of the methanol on this cluster.
In this study, the oxidation of CO by O2 molecule on the B12N12 and B11N12C nanocages is investigated. Density functional theory (DFT) calculations are performed to show the strength and nature of the CO and O2 interaction with the surface of these nanocages. The activation energies for the each reaction step on the B12N12 and B11N12C cages are discussed and the corresponding mechanisms are compared. The results of this study could be useful for designing and developing metal-free catalysts based on BN nanostructures. To the best of our knowledge, the oxidation of CO molecule over B12N12 and B12N11C nanocages is reported for the first time.
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Fig. 1 Optimized structures of pristine B12N12 and B11N12C clusters. All bond distances are in Å. Color code for each optimized structure: blue ball: N; brown ball: B; gray ball: C. |
Surface | R (Å) | Eads (eV) | qCT (e) | Spin density (au) | ΔG298 (eV) | ΔH298 (eV) |
---|---|---|---|---|---|---|
a The estimated spin density over O2 molecule.b The estimated spin density over carbon atom of B11N12C. | ||||||
B12N12 | — | |||||
A | 2.70 | −0.09 | 0.08 | 1.98a | 0.28 | −0.04 |
B | 1.78 | −0.21 | 0.30 | — | 0.24 | −0.17 |
C | 1.79 | −0.10 | 0.17 | — | 0.37 | −0.06 |
B11N12C | 1.06b | |||||
D | 1.44 | −1.78 | 0.19 | 0.97a | −1.20 | −1.71 |
E | 1.82 | −0.07 | 0.01 | — | 0.39 | −0.03 |
On the other hand, complex B is achieved when a singlet spin state of O2 molecule is considered. It is important to note that despite the larger Eads (−0.21 eV) value of this complex, its total energy is smaller (less negative) than that of complex A. In addition, like complex A, this adsorption configuration is formed via an exothermic reaction while it is not a favorable reaction from the thermodynamic point of view (Table 1). In contrast with complex A, the EDD isosurface shows that the much more pronounced electron density is accumulated between the O2 molecule and B12N12 (Fig. S1†). Also, a net charge of about 0.30e is transferred from the B12N12 surface to the O2 molecule which shows a stronger interaction between the B and O atoms than that in complex A.
Furthermore, CO molecule is found to be weakly adsorbed over the B12N12 cluster while it is placed in a vertical (end-on) position right above the B atom of the surface with the small Eads of −0.10 eV (complex C). The B–C bond length of this configuration is about 1.79 Å. Although the formation of this complex is almost exothermic (ΔH298 = −0.06 eV), this is not a thermodynamically favorable reaction at room temperature (ΔG298 > 0). There is also a small net charge (0.17e) transferred from the CO molecule to the B12N12 surface, indicated that the CO molecule can be easily desorbs from the nanocage. It is also clear from the EDD map that a small electron density exists between the B atom of the surface and C atom of the CO molecule which is originated from the large binding distance of B–C bond (Fig. S1†). Thus, it is predicted that the CO and triplet O2 molecules can be simultaneously adsorbed over the B12N12 surface due to their similar adsorption energies.
The adsorption of individual O2/CO molecules over the B11N12C cage is also studied in detail. It should be mentioned that both triplet and singlet spin states of O2 molecule are considered for the O2 adsorption configuration. However, in both cases a doublet spin state is achieved. Fig. S2† shows the most stable adsorption configuration of O2 (configuration D) on the B11N12C surface in which the O–O molecule is in parallel position to the surface right above the C atom and chemisorbs with the Eads of −1.78 eV (Table S1†). It is clear from the small C–O bond length (1.44 Å) and the large Eads that there is a significant interaction between the O2 molecule and the B11N12C surface. In contrast to the nearly unchanged O–O bond length after adsorption of triplet and singlet O2 over B12N12 cluster (complexes A and B), here, the O–O bond length of the adsorbed O2 molecule is elongated to 1.30 Å which is due to the net charge of 0.19e is transferred from the B11N12C to the 2π* orbitals of the O2 molecule. It is clearly demonstrated in the EDD map in Fig. S2† that a large electron density is accumulated around the C–O bond which confirms that there is a significant chemical interaction between the gas molecule and the C atom of the B11N12C surface. These results indicate that the transferred electrons move from the C dopant to the adsorbed O2 molecule. It should be noted that the adsorption of O2 molecule on B11N12C cluster is exothermic (ΔH298 = −1.71 eV). Also, it is a feasible reaction that can take place at room temperature (ΔG298 = −1.20 eV). Besides, the spin density distribution analysis shows that a large spin density is accumulated over the O2 molecule (0.98 au) rather than C atom (0.01 au) which provides an additional support for the activation of O2 molecule over the B11N12C cluster.
In the next step, the adsorption of a single CO molecule on the B11N12C cluster is investigated. Complex E in Fig. S2† shows that this molecule is adsorbed in the same geometric configuration with complex C, right above the B atom of the cluster with almost the same Eads and B–O bond length values (Table 1). Like complex C, the formation of configuration E is exothermic while it is thermodynamically impossible at room temperature (Table S1†). A net charge of about 0.01e is transferred from the CO molecule to the B11N12C cage. The EDD isosurface presented in Fig. S2† indicates that the electron density is depleted around the B–C bond which is caused from the weak interaction between the CO molecule and the B11N12C surface.
In summary, comparing the Eads values of the adsorbed O2 molecule over the B12N12 and B11N12C clusters clearly shows that the O2 molecule is adsorbed more effectively over the B12N12C cluster and has a larger effect on it. Also, the CO molecule is considered to be adsorbed on the B11N12C surface later than the O2 molecule due to its smaller (less negative) Eads value. Therefore, in a mixture of O2/CO molecules as the reaction gas, the B11N12C surface will be chiefly covered by adsorbed O2 molecules. So, the configuration D can be used as an initial state for the CO oxidation reaction.
O2 + CO → Oads + CO2 | (3) |
Oads + CO → CO2 | (4) |
The corresponding activation energies (Eact) and reaction energies are also reported in Table 2. The local configurations of the gas molecules over the B12N12 cluster and their initial state (IS), transition state (TS) and final state (P) are depicted in Fig. S4.† The co-adsorbed O2/CO molecules over the B12N12 surface forms the IS-1 in which both O2 and CO molecules are adsorbed over the surface with corresponding B–O1 and B–C bond lengths of 2.85 and 1.78 Å, respectively (Fig. S4†). It is important to note that the estimated Eads value for CO and O2 co-adsorption on B12N12 (−0.20 eV) is slightly more negative than that the sum of individual adsorption energies of these species (−0.19 eV). In addition, the relatively small difference between the adsorption energies of individual O2 and CO molecules indicates that there is a certain probability of having O2 and CO co-adsorbed on the B12N12. Then, the C atom of the CO molecule approaches to the O2 atom and a peroxo-type structure O1–O2–C–O is produced (TS-1) over the B12N12 surface with the bond lengths of 1.34, 1.42 and 1.17 Å for O1–O2, O2–C and C–O, respectively (Fig. S4†). The activation energy (Eact) along the reaction pathway IS-1 → P-1 is 2.47 eV which is too high to proceeds rapidly at room temperature. Passing over the transition state TS-1, the formed CO2 molecule with the Eads value of −0.18 eV, desorbs easily from the surface, leaving an atomic oxygen (Oads) adsorbed over the B atom of the B12N12 surface with the B–Oads bond length and adsorption energy of 1.43 Å and −5.3 eV, respectively (Fig. S4†).
Reaction | Eact (eV) | ΔE (eV) |
---|---|---|
B12N12 | ||
IS-1 → P-1 | 2.47 | −1.06 |
IS-2 → P-2 | — | −1.40 |
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B11N12C | ||
IS-5 → P-5 | 0.58 | −3.10 |
IS-6 → P-6 | 0.09 | −1.51 |
Considering the high energy barrier of the CO2 formation via the LH mechanism (Fig. S3†), the question arises whether this pathway (IS-1 → P-1) can be carried on via the ER mechanism. It is noteworthy that the Eads of co-adsorbed O2/CO molecules when they are proceeded via the ER mechanism in the triplet state of the O2 molecule is about +1.26 eV which is not actually possible. Therefore, because of this positive Eads value, the ER mechanism can't be considered for the first step.
In the following step, we check the activity of the atomic Oads (O1) for CO oxidation via the LH mechanism. According to Fig. S4,† the configuration in which the C atom of the CO molecule is about 3.06 Å far away from the atomic Oads is chosen as an IS-2. It is interesting to note that the P-2 is obtained via a barrier-less process. In TS-2, the O1–C–O complex is formed in a vertical position to the surface to facilitate the formation of CO2 molecule (O1–C = 1.32 Å). Finally, in P-2, the second CO2 molecule is formed which weakly interacted with the B12N12 (B–C = 2.99 Å) and hence can be easily released from the surface (Eads = 0.21 eV). Generally, according to Table 2 and Fig. S3,† the reaction energy of these reaction steps (IS-1 → P-1 and IS-2 → P-2) are about −1.06 and −1.40 eV, respectively. It should be noted that since the second pathway with the LH mechanism proceeds via a barrier-less reaction, the ER mechanism does not consider for this reaction route.
Furthermore, the oxidation of CO molecule over the B12N12 surface is studied when the singlet O2 is considered. The energy profile for the IS-3 → P-3 and IS-4 → P-4 reactions in which both LH and ER mechanisms are carefully studied is shown in Fig. S5.† Also, their local configurations along with some geometric values of the IS, TS and P are demonstrated in Fig. S6† and their corresponding thermodynamic values are listed in Table S1.† The stable configuration for IS-3 is achieved in which the CO and O2 molecules are adsorbed over the surface via the ER mechanism. In this configuration, a five-member ring is formed and the carbon atom of the CO molecule approaches to the O2 atom with the O2–C bond length of 1.35 Å. It should be noted that the Eads of CO + O2 in this complex (−4.12 eV) is more negative than that in complex A due to the formation of the stable five-member ring. So, this can be used as a favorable configuration in CO oxidation reaction. Because of the great stability of this complex, the TS-3 with the O2–C bond length of 2.03 Å is difficultly formed via the high Eact of about 3.35 eV which cannot be overcome at room temperature (Fig. S5, Table S1†). Passing from TS-3, the P-3 is obtained with the reaction energy of 2.41 eV. In this state, the atomic Oads (O1) remains in the surface with the B–O1 bond length of 1.38 Å while the first CO2 molecule is weakly interacted to the surface (N–C = 3.04 Å) that can be released easily at ambient conditions. In the next pathway, another CO molecule is adsorbed over the B12N12 surface via the LH mechanism and forms the IS-4. Herein, the B–C bond length is about 1.76 Å and the carbon atom is about 2.83 Å far from the atomic O1. The CO2 molecule is obtained passing from TS-4 with the small Eact of about 0.06 eV. It is clear from the P-4 configuration that the second CO2 molecule can be easily desorbed from the B12N12 surface (Fig. S6†).
In the following, the reaction between the CO and the atomic oxygen is carefully studied based on the LH mechanism via the IS-6 → P-6 pathway (Fig. S7 and S8†). Once the first formed CO2 molecule leaves from the surface of nanocage, the second CO is supposed to be co-adsorbed with the O1 at the reaction site. In the IS-6, the C–O1 bond is about 1.37 Å while the CO molecule is physically adsorbed over the B atom of the B11N12C cluster (B–C = 1.81 Å) while it is about 3.24 Å far away from the atomic O1. Attacking of O1 to the carbon atom of CO molecule generates TS-6 with the O1–C distance of 1.97 Å. The energy barrier equal to 0.09 eV is needed for the IS-6 → TS-6 reaction (Table 2). Like before, the CO2 molecule is weakly interacted with the surface (Eads = 0.1 eV and C–C = 3.04 Å), so, the CO2 molecule can effectively desorbs from the B11N12C surface and the B11N12C catalyst can thus be recovered for a new round of CO oxidation.
For the ER mechanism, the process IS-6 → P-6 is completely different. As shown in Fig. S9 and S10,† the CO molecule is weakly adsorbed over the B11N12C surface (O1–C = 2.73 Å) and considered as the IS-8. In this state, the C atom of the CO molecule attacks the O1 to generate TS-8 while the distance between the atomic O1 and C atom of the CO molecule is decreased from 2.73 to 1.97 Å. The Eact of this reaction is calculated about 0.21 eV which is significantly larger than that in LH mechanism (0.09 eV). At the end, the final product CO2 is formed over the B11N12C cluster and released from the surface at room temperature. Interestingly, it can be suggested that the oxidation of CO molecule over the B11N12C is preferred to proceed via the ER mechanism followed by LH mechanism.
Based on the present theoretical results, the catalytic activity of B11N12C nanocage toward the oxidation of CO molecule is more than that in B12N12 cluster due to the existence of C atom as an activation site. Moreover, the corresponding activation energy barriers of these mechanisms are significantly lowered compared with the undoped B12N12 case. Meanwhile, the performance of B11N12C nanocage as a catalyst used for the oxidation of CO with O2 molecule may be proceed at near ambient temperatures.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25069b |
This journal is © The Royal Society of Chemistry 2016 |