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Borane and alane mediated hydrogen release from silane and methylsilane
Chemical Physics Letters 620 (2015) 38–42
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Borane and alane mediated hydrogen release from silane and methylsilane Huyen Thi Nguyen a , D. Majumdar b , Jerzy Leszczynski b , Minh Tho Nguyen a,∗ a b
Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium Interdisciplinary Center for Nanotoxicity, Department of Chemistry, Jackson State University, Jackson, MS 39217, USA
a r t i c l e
i n f o
Article history: Received 25 September 2014 In final form 5 December 2014 Available online 11 December 2014
a b s t r a c t The dehydrogenations of silanes SiH4 and CH3 SiH3 in the presence of borane and alane were investigated using density functional (B3LYP) and coupled-cluster (CCSD(T)/aug-cc-pVnZ) theories. The calculated results showed that the hydrogen release reactions are more favorable in presence of BH3 . Our theoretical analyses have further revealed that the addition of an extra BH3 can lead to several low energy barrier pathways. This observation is important to understand the catalytic role of BH3 in such reactions (depending on its release mechanism). Overall, silane and its alkyl derivatives can be used as effective starting materials for H2 production. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered as the lightest, simplest, and cleanest alternative fuel for environment-friendly vehicles. However, efficient on-board storage of hydrogen remains a critical issue. Different strategies for hydrogen storage include conventional hydrogen storage in high-pressure tanks, hydrogen adsorption on materials with large surface areas, metal hydrides, complex hydrides, and chemical hydrogen storage (CHS) [1–5]. It is known that complex hydrides, which are complexes between morecovalent light-weight hydrides (e.g. BH3 and AlH3) , with more-ionic hydrides (e.g. LiH and NaH) are good candidates for hydrogen storage [5–12]. However, these complex hydrides only release hydrogen at high temperature and high pressure conditions which are sometimes not suitable for practical usages. Milder conditions can be achieved by the CHS strategy, in which hydrogen is released (from hydrides, for instance) through various chemical reactions with minimal energy requirements. Dehydrogenation reaction of ammonia borane NH3 BH3 , for example, already occurs at the temperature range of ∼80 to 130 ◦ C [13], whereas the complex hydrides such as LiAlH4 only decomposes to release H2 at the temperature above 200 ◦ C [5]. Therefore, the CHS could be considered as competitive alternative to other hydrogen storage systems. The catalytic role of BH3 and AlH3 on the dehydrogenation barrier of several hydrides has been reported, and AlH3 is slightly
better in this respect [6,11]. H2 release from BH3 NH3 in the presence of BH3 and AlH3 are much more favorable with respect to the non-catalytic one due to the lower energy barriers (12.5 and 15.8 kcal/mol, respectively) [6,11]. BH3 and AlH3 are also known to induce a significant reduction in the dehydrogenation barriers of CH3 CH3 (53.7 and 55.6 kcal/mol for the respective catalysts) [11]. High volatility of SiH4 has made it an important gaseous precursor to generate various reactive species (like, SiH3 , SiH2 , etc.) through various chemical vapor deposition (CVD) techniques [14]. These reactions are important to deposit n-type active transistor layer with further processing [12]. SiH4 is also used to generate boron-doped thin film (in the CVD process) through reactions with borane (BH3 ), diborane (B2 H6 ) and boron trichloride (BCl3 ) [15,16]. SiH4 has also importance in hydrogen storage and this property was investigated using high-level computational techniques to study the reactions between SiH4 and BH3 /AlH3 [9]. The results showed that BH3 yields energetically favorable reaction channel during H2 release from SiH4 . In the present work, quantum chemical calculations are carried out to further investigate the role of BH3 /AlH3 on the energetics and mechanisms of the hydrogen-release from SiH4 and CH3 SiH4 . The catalytic effect of BH3 is also explored for the reactions between SiH4 and BH3 in the presence of an extra BH3 molecule. 2. Computational details
∗ Corresponding author. E-mail address: [email protected] (M.T. Nguyen). http://dx.doi.org/10.1016/j.cplett.2014.12.010 0009-2614/© 2014 Elsevier B.V. All rights reserved.
Density functional theory (DFT) [17] was used for geometry optimizations and vibrational analyses. The popular hybrid
H.T. Nguyen et al. / Chemical Physics Letters 620 (2015) 38–42
39
Figure 1. Schematic potential energy profile for the dehydrogenation from SiH4 and BH3 . The labels of the hydrogen atoms connected to the Si and B atoms are indicated in dark blue and pink numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
B3LYP exchange-correlation functional [18,19] was used in DFT calculations together with 6-311G(d,p) basis set. Single point electronic energy computations were performed using the coupled-cluster theory (including triple excitations, CCSD(T)) on the DFT-optimized structures to obtain more accurate energetics. Larger correlation consistent aug-cc-pVTZ (abbreviated as aVTZ) basis set of atoms was used in such calculations. Relative energies, reported in the manuscript, were calculated from these CCSD(T)/aVTZ energies and corrected for zero-point energies (ZPE) that were extracted from harmonic vibrational frequency calculations at the B3LYP/6-311G(d,p) level. The computational strategy as discussed above has been gauged against the earlier results of computed geometries and interaction energies of the reactant complex RC1 between SiH4 and BH3 (Figure 1) at the second-order Møller–Plesset perturbation level (MP2) [15,16]. In the present work these parameters were further computed using MP2/aVTZ, DFT/B3LYP/6-311(d,p) and CCSD(T)/aVTZ methods (Table S1 in the ESI). Single point CCSD(T)/aVTZ calculations were also carried out to compute the interaction energies using the optimized geometries from both MP2 and DFT/B3LYP methods and these interaction energies are corrected for basis set superposition (BSSE) error. The core electron correlation effects were further evaluated for the MP2 and CCSD(T) calculations. All the calculations were carried out using the Gaussian 09 code [20]. For the complex RC1, the MP2 method yields the shortest bond distance while the DFT/B3LYP method yields the longest one (Table S1 in the ESI). The Si B bond distance obtained from CCSD(T)/aVTZ ˚ than that at the MP2 level. calculation is slightly larger (by 0.02 A) Inclusion the core-electron correlation into the MP2 calculation ˚ This reduction is in agreereduces the bond distance by 0.04 A. ment with the earlier results [16]. The effect of geometry change in different methods, on the other hand, has no marked effect on the computed interaction energies at the CCSD(T)/aVTZ level (Table S1 in the ESI). The same agreement is found for the interaction energies also (with ZPE corrections). In addition, the BSSE-corrected interaction energies of RC1 at different levels of theory also show that the inclusion of the core-electron correlation is not necessary because it only results in larger BSSE values. The full-core and frozen core interaction energies are nearly equal after corrected for BSSE. These results thus justify the use of the computational strategy, as discussed above, to investigate the proposed H2 -eliminations reactions of SiH4 and CH3 SiH4 .
Table 1 Interaction energies (E), BSSE-corrected energies (EBSSE ) and BSSE energies (kcal/mol) of different reactant complexes obtained at CCSD(T)/aVTZ level.
RC1 RC4 RC7 RC10 RC12 RC14 RC16
−E
−EBSSE
BSSE energies
6.4 3.8 9.0 8.0 6.0 18.1 12.6
5.5 3.4 7.9 7.0 5.5 15.5 10.9
1.1 0.4 1.1 1.0 0.5 2.6 1.7
The BSSE errors (of RC1 and other reactant complexes) are calculated at CCSD(T)/aVTZ level using the counterpoise (CP) method [21] as implemented in GAUSSIAN 09 suite of program. The data listed in Table 1 show that the BSSE energies are in the range of 0.4–2.6 kcal/mol. Relatively large BSSE values are those of the reactant complexes between SiH4 and 2 BH3 molecules. Compared to the BSSE energy obtained with the same CCSD(T) method but smaller aVDZ basis set [15], the use of the larger aVTZ basis set significantly reduce the BSSE value for RC1, from 3 kcal/mol down to 1 kcal/mol. 3. Results and discussion 3.1. Reactions between SiH4 and BH3 /AlH3 The potential energy surface for the dehydrogenation of the reactant complex RC1 is shown in Figure 1. The H5 of SiH4 (Figure 1) behaves as Lewis-base, while the B atom of BH3 behaves as Lewisacid. In the RC1 complex they interact with each other (B· · ·H5 ˚ through such acid–base interactions interaction distance: 1.32 A) (similar to the previous report [9,15,16]). Because of this interaction, the Si1 H5 bond becomes much weaker as compared to the ˚ other Si H bonds (bond distance elongation of >0.1 A). The removal of H5 and H8 atoms from RC1 via TS-1,3 has a small energy barrier (E: 9.7 kcal/mol with respect to RC1) and yields a product complex PC1 in which the H2 molecule still attaches to the P3 molecule. The structure of PC1 is very similar to that of TS-13 (cf., Figure 1). In addition, the natural charge distributions obtained from an NBO analysis (Table S2 of the ESI) also show a small change between the TS-13 and PC1. These similarities result in the very small difference in their relative energies, being 0.4 kcal/mol. It
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Figure 2. Schematic potential energy profile for the dehydrogenation from SiH4 and AlH3 . The labels of the hydrogen atoms connected to the Si and Al atoms are indicated in dark blue and pink numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
should be noted that this value is in general smaller than the error of the computational method. Therefore, the formation of P3 from RC1 might be considered as a barrier-free process [22]. The H2 release from this complex requires an additional 3.9 kcal/mol and thus makes this process become more endothermic. This H2 elimination is more favorable than the alternative path through TS-1,2 (cf. Figure 1), as the energy barrier in this case is 42.5 kcal/mol. The product P2 is also less stable than P3 due to the presence of strained triangular structure. Previous studies [9] on the RC1 complexes also produced similar results. The only difference is that in the present studies the B6 H9 bond in RC1 lies in the B6 Si1 H5 plane, while in the previous study [9] this bond rotates out of the plane forming partially eclipsed conformation with higher electrostatic and steric repulsions. As a result, the present RC1 complex shows stronger interactions (by 2.6 kcal/mol) with respect to the previously computed eclipsed conformation. In both cases, the interactions were computed using single point CCSD(T)/aVTZ method at the optimized structures at a lower level, and thus such a direct comparison is justified. Replacing borane BH3 with alane AlH3 molecule yields a less stable reactant complex RC4 with the interaction energy of −3.8 kcal/mol (Figure 2). Although the product energies (P5 and H2 ) are lower with respect to P3 + H2 (from RC1, Figure 1), the removal of hydrogen, in this case, has a transition structure (TS-4,5) with a barrier height of 25.5 kcal/mol. Thus this reaction is less favored with respect to the H2 removal via TS-1,3 in Figure 1.
3.2. Reactions between CH3 SiH3 and BH3 /AlH3 The compound CH3 SiH3 has slightly higher weight-percent of hydrogen than SiH4 . The two extra hydrogen atoms can take part in the dehydrogenation reaction. The BH3 /AlH3 can either interact weakly with SiH3 fragment before the dehydrogenation, or it can directly attack the CH3 fragment and release H2 molecule. The reaction pathways between CH3 SiH3 and BH3 are shown in Figure 3. The two reactant complexes RC7 and RC10 are different only in the orientation of the two interacting molecules. Their interaction energies are very close to each other and RC7 is slightly more stable (1 kcal/mol). From RC7, two different reaction pathways are possible. The first one involves the removal of two hydrogen atoms of SiH3 fragment (via TS: TS-7,8), while the second path involves removal two hydrogen atoms of the BH3 molecule and CH3 fragment (via TS: TS-7,9). The former pathway (involving TS-7,8) has a lower energy barrier (34.5 kcal/mol) with respect to the reactants. The H2 and H3 atoms, connected to Si atom, are removed during the dehydrogenation process through this TS. The H atom of BH3 then forms a bridged hydrogen bond between B and Si atoms in the product complex (P8). This product is less stable than the one formed in the reaction pathway via TS-7,9, in which the removal of H10 of BH3 and H8 of CH3 fragment is accompanied by the breaking of interactions between BH3 and SiH3 fragments. This process yields the product P9 (E: −3.9 kcal/mol with respect to RC7). However, the transition structure TS-7,9 is 21 kcal/mol less stable than TS-7,8 and thus this pathway is unfavorable. In the reactant complex RC10, the
Figure 3. Schematic potential energy profile for the dehydrogenation from CH3 SiH3 and BH3 . The labels of the hydrogen atoms connected to the Si, B, and C atoms are indicated in dark blue, pink and brown numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Figure 4. Schematic potential energy profile for the dehydrogenation from CH3 SiH3 and AlH3 . The labels of the hydrogen atoms connected to the Si, Al, and C atoms are indicated in dark blue, pink and brown numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
H10 of BH3 and H4 of SiH4 are in closer proximity than in RC7. These two hydrogen atoms can be removed through the TS TS-10,8 with the formation of the product P8. This TS also has quite high energy barrier (49.4 kcal/mol). The direct attack of BH3 to the CH3 fragment, yielding the product P9, has much lower energy barrier (TS-9, 18.0 kcal/mol). This extra stabilization effect in TS-9 is due to the electrostatic interactions between the positively charged B (atom 9 with a Mulliken atomic charge of 0.30 electron) and the negatively charged C (atom 5 with a Mulliken atomic charge of −0.69 electron) ˚ atoms (Figure 3) for their close proximity of 1.781 A. Similar potential energy surfaces were found for the reactions between CH3 SiH3 and AlH3 (Figure 4). Two dehydrogenation pathways from RC12 via TS-12,11 and TS-12,13 were found. The TS-12,11 for the removal of hydrogen atoms from AlH3 and CH3 is more stable with a barrier height of 25.9 kcal/mol. The direct attack of AlH3 to the CH3 fragment, on the other hand, has the less favorable transition structure TS-11 (barrier height: 27.8 kcal/mol). As a result, the dehydrogenation reaction via TS-12,11 is the most
favored one. Nevertheless, the energy barrier for this reaction is 10 kcal/mol higher with respect to the case of BH3 . Thus BH3 mediated reaction is more favored for this dehydrogenation process. 3.3. Reactions between SiH4 and BH3 in the presence of an additional BH3 (∼29% weight percent of hydrogen) Addition of second BH3 to the RC1 forms two reactant complexes RC14 and RC16. The former species is 5.5 kcal/mol more stable. The potential energy surfaces of the dehydration pathways from these reactant complexes are shown in Figure 5. The dehydrogenation reactions from RC14 and RC16 have the energy barriers in the range of 14.1–19.1 kcal/mol. The complex RC14 is formed from the interaction between SiH4 and two separated BH3 molecules, in which two hydrogen atoms of SiH4 (H2 and H3 ) form hydrogen bridges with the BH3 molecules. Furthermore, the H2 atom of SiH4 interacts with a BH3 molecule (pink labels, Figure 5) which in turn interacts with the additional
Figure 5. Schematic potential energy profile for the dehydrogenation from SiH4 and two BH3 . The labels of the hydrogen atoms connected to the Si atom and B atoms of first and second BH3 are indicated in dark blue, pink and light purple numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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BH3 molecule (light purple labels, Figure 5) forming a bridged hydrogen bond via H8 atom. The RC16 can also be considered as the reactant complex between SiH4 and B2 H6 molecule, where one of the two hydrogen bridges between two BH3 molecules is broken and replaced by a new bridged hydrogen bond between BH3 and SiH4 . Although this process has a high energy barrier (E: 30.7 kcal/mol with respect to SiH4 + B2 H6 ), the relative energy of the TS TS-16 is still lower than the total energies of SiH4 + 2BH3 . Therefore, this formation pathway of RC16 is comparable to the barrier-free step-by-step addition of BH3 to SiH4 . The TS TS-14,15 in the reaction pathway from the RC14 lies below the total energies of the reactants (SiH4 + 2BH3 ). This pathway is therefore the most probable dehydrogenation pathway as compared to the other pathways. In the TS-14,15 the H2 atom of SiH4 is removed along with the H13 atom of BH3 (Figure 5). The H7 atom of the first BH3 molecule then forms a hydrogen bridge between two B atoms in the product P15. The destabilization effect from the ring strain in P15 is compensated due to such interactions. The relative energy of products (P15 + H2 ) with respect to SiH4 + 2BH3 and RC1 + BH3 shows that the overall reaction is exothermic. It is to be noted that if the second BH3 molecule is released from P15 with its two original hydrogen atoms and the H7 atom, it could be considered as a catalyst for the hydrogenation of the RC1. Similarly, the first BH3 molecule could only be considered as a catalyst if it is released with the hydrogen atom attaching Si and B. The two dehydrogenation pathways from RC16 have slightly less stable transition structures TS-16,17 and TS-16,18 with the respective barrier heights of only 1.5 and 6.5 kcal/mol. The transition structures related to these processes were also reported earlier [23]. The two hydrogen atoms removed in TS-16,17 belong to two BH3 molecules. In the final product (P17, after H2 release), the H2 atom of SiH4 molecule moves closer to the B B bond forming two hydrogen bridges between the BH2 fragments. This reaction pathway can be considered as a dehydrogenation reaction of B2 H6 in the presence of SiH4 . In this case, additional BH3 remains attached to the product P17. When this BH3 is released, it takes away either the H2 atom of SiH4 or the H7 atom of the first BH3 molecule and thus plays the role of a catalyst for the dehydrogenation of RC1. The dehydrogenation reaction through TS-16,18 occurs via the removal of the H12 atom of the additional BH3 and H2 atom of SiH4 (Figure 5). The remaining BH2 and SiH3 fragments interact with each other forming a weak bond between Si1 and B9 in the product P18. Although the product P15 is much less stable than the products P17 and P18, this kinetically controlled pathway (through low-barrier transition state TS-14,15) seems to be more preferable. The additional BH3 could be considered as a catalyst (in the product formations P17 and P18) if it is released by the removal of H8 (or H7) atom of the first BH3 . In summary, the calculation data showed that BH3 and AlH3 mediated dehydrogenations of SiH4 are more favorable as compared to those of CH3 SiH3 . The differences in relative energies of the products and the reactants of these reactions are relatively small. It favors the possibility of reverse reactions. Furthermore, the double BH3 catalyzed dehydrogenations of SiH4 have very low energy barriers and thus could take place under mild conditions. The studied systems in this letter have potentiality to release H2 at low temperatures together with thermodynamic reversibility. These features show that they could be good candidates for chemical hydrogen storage.
probable pathway for H2 release. A detailed study was carried out using DFT/B3LYP/6-311G(d,p) method to obtain the optimized geometries of the reactants, products and TSs in the reaction paths. The CCSD(T)/aVTZ single point electronic energies with B3LYP/6311G(d,p)-ZPE corrections were used to generate final energetics of the reactions and interaction energy parameters involving the various weak complexes on the reaction surfaces. The approach was compared with more rigorous techniques and proved to be quite satisfactory. The calculated results for the reactions of SiH4 and CH3 SiH3 with BH3 and AlH3 showed that the reactions with BH3 are more favorable. The dehydrogenation reaction between SiH4 and BH3 follows a barrier-free pathway and the overall reaction is endothermic (by 6.8 kcal/mol with respect to the reactants). The reaction between SiH4 and AlH3 has a barrier height of 25.5 kcal/mol and consequently less favorable. Similarly, the reactions of CH3 SiH3 with BH3 also have lower transition structures. The most probable reaction from CH3 SiH3 + BH3 requires the system to have an extra 18.0 kcal/mol to overcome the energy barrier. Whereas for the reaction between CH3 SiH3 + AlH3 , the energy requirement is 25.9 kcal/mol. Addition of one extra BH3 molecule results in several low-energy-barrier pathways with the relative energies of the transition structures being −3.1, 1.5, and 6.5 kcal/mol. Further dehydrogenations are possible from the stable products of these pathways. Acknowledgements This work has been supported by the PREM (Award no. DMR1205194) and ONR (Award no. N00014-13-1-0501) grants. The Leuven group thanks the KU Leuven Research Council for continuing support via GOA and IDO programs. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2014.12.010. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
4. Concluding remarks [23]
We have investigated the dehydrogenations of SiH4 and CH3 SiH3 in the presence of BH3 and AlH3 to determine the most
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Borane and alane mediated hydrogen release from silane and methylsilane Huyen Thi Nguyen a , D. Majumdar b , Jerzy Leszczynski b , Minh Tho Nguyen a,∗ a b
Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium Interdisciplinary Center for Nanotoxicity, Department of Chemistry, Jackson State University, Jackson, MS 39217, USA
a r t i c l e
i n f o
Article history: Received 25 September 2014 In final form 5 December 2014 Available online 11 December 2014
a b s t r a c t The dehydrogenations of silanes SiH4 and CH3 SiH3 in the presence of borane and alane were investigated using density functional (B3LYP) and coupled-cluster (CCSD(T)/aug-cc-pVnZ) theories. The calculated results showed that the hydrogen release reactions are more favorable in presence of BH3 . Our theoretical analyses have further revealed that the addition of an extra BH3 can lead to several low energy barrier pathways. This observation is important to understand the catalytic role of BH3 in such reactions (depending on its release mechanism). Overall, silane and its alkyl derivatives can be used as effective starting materials for H2 production. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered as the lightest, simplest, and cleanest alternative fuel for environment-friendly vehicles. However, efficient on-board storage of hydrogen remains a critical issue. Different strategies for hydrogen storage include conventional hydrogen storage in high-pressure tanks, hydrogen adsorption on materials with large surface areas, metal hydrides, complex hydrides, and chemical hydrogen storage (CHS) [1–5]. It is known that complex hydrides, which are complexes between morecovalent light-weight hydrides (e.g. BH3 and AlH3) , with more-ionic hydrides (e.g. LiH and NaH) are good candidates for hydrogen storage [5–12]. However, these complex hydrides only release hydrogen at high temperature and high pressure conditions which are sometimes not suitable for practical usages. Milder conditions can be achieved by the CHS strategy, in which hydrogen is released (from hydrides, for instance) through various chemical reactions with minimal energy requirements. Dehydrogenation reaction of ammonia borane NH3 BH3 , for example, already occurs at the temperature range of ∼80 to 130 ◦ C [13], whereas the complex hydrides such as LiAlH4 only decomposes to release H2 at the temperature above 200 ◦ C [5]. Therefore, the CHS could be considered as competitive alternative to other hydrogen storage systems. The catalytic role of BH3 and AlH3 on the dehydrogenation barrier of several hydrides has been reported, and AlH3 is slightly
better in this respect [6,11]. H2 release from BH3 NH3 in the presence of BH3 and AlH3 are much more favorable with respect to the non-catalytic one due to the lower energy barriers (12.5 and 15.8 kcal/mol, respectively) [6,11]. BH3 and AlH3 are also known to induce a significant reduction in the dehydrogenation barriers of CH3 CH3 (53.7 and 55.6 kcal/mol for the respective catalysts) [11]. High volatility of SiH4 has made it an important gaseous precursor to generate various reactive species (like, SiH3 , SiH2 , etc.) through various chemical vapor deposition (CVD) techniques [14]. These reactions are important to deposit n-type active transistor layer with further processing [12]. SiH4 is also used to generate boron-doped thin film (in the CVD process) through reactions with borane (BH3 ), diborane (B2 H6 ) and boron trichloride (BCl3 ) [15,16]. SiH4 has also importance in hydrogen storage and this property was investigated using high-level computational techniques to study the reactions between SiH4 and BH3 /AlH3 [9]. The results showed that BH3 yields energetically favorable reaction channel during H2 release from SiH4 . In the present work, quantum chemical calculations are carried out to further investigate the role of BH3 /AlH3 on the energetics and mechanisms of the hydrogen-release from SiH4 and CH3 SiH4 . The catalytic effect of BH3 is also explored for the reactions between SiH4 and BH3 in the presence of an extra BH3 molecule. 2. Computational details
∗ Corresponding author. E-mail address: [email protected] (M.T. Nguyen). http://dx.doi.org/10.1016/j.cplett.2014.12.010 0009-2614/© 2014 Elsevier B.V. All rights reserved.
Density functional theory (DFT) [17] was used for geometry optimizations and vibrational analyses. The popular hybrid
H.T. Nguyen et al. / Chemical Physics Letters 620 (2015) 38–42
39
Figure 1. Schematic potential energy profile for the dehydrogenation from SiH4 and BH3 . The labels of the hydrogen atoms connected to the Si and B atoms are indicated in dark blue and pink numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
B3LYP exchange-correlation functional [18,19] was used in DFT calculations together with 6-311G(d,p) basis set. Single point electronic energy computations were performed using the coupled-cluster theory (including triple excitations, CCSD(T)) on the DFT-optimized structures to obtain more accurate energetics. Larger correlation consistent aug-cc-pVTZ (abbreviated as aVTZ) basis set of atoms was used in such calculations. Relative energies, reported in the manuscript, were calculated from these CCSD(T)/aVTZ energies and corrected for zero-point energies (ZPE) that were extracted from harmonic vibrational frequency calculations at the B3LYP/6-311G(d,p) level. The computational strategy as discussed above has been gauged against the earlier results of computed geometries and interaction energies of the reactant complex RC1 between SiH4 and BH3 (Figure 1) at the second-order Møller–Plesset perturbation level (MP2) [15,16]. In the present work these parameters were further computed using MP2/aVTZ, DFT/B3LYP/6-311(d,p) and CCSD(T)/aVTZ methods (Table S1 in the ESI). Single point CCSD(T)/aVTZ calculations were also carried out to compute the interaction energies using the optimized geometries from both MP2 and DFT/B3LYP methods and these interaction energies are corrected for basis set superposition (BSSE) error. The core electron correlation effects were further evaluated for the MP2 and CCSD(T) calculations. All the calculations were carried out using the Gaussian 09 code [20]. For the complex RC1, the MP2 method yields the shortest bond distance while the DFT/B3LYP method yields the longest one (Table S1 in the ESI). The Si B bond distance obtained from CCSD(T)/aVTZ ˚ than that at the MP2 level. calculation is slightly larger (by 0.02 A) Inclusion the core-electron correlation into the MP2 calculation ˚ This reduction is in agreereduces the bond distance by 0.04 A. ment with the earlier results [16]. The effect of geometry change in different methods, on the other hand, has no marked effect on the computed interaction energies at the CCSD(T)/aVTZ level (Table S1 in the ESI). The same agreement is found for the interaction energies also (with ZPE corrections). In addition, the BSSE-corrected interaction energies of RC1 at different levels of theory also show that the inclusion of the core-electron correlation is not necessary because it only results in larger BSSE values. The full-core and frozen core interaction energies are nearly equal after corrected for BSSE. These results thus justify the use of the computational strategy, as discussed above, to investigate the proposed H2 -eliminations reactions of SiH4 and CH3 SiH4 .
Table 1 Interaction energies (E), BSSE-corrected energies (EBSSE ) and BSSE energies (kcal/mol) of different reactant complexes obtained at CCSD(T)/aVTZ level.
RC1 RC4 RC7 RC10 RC12 RC14 RC16
−E
−EBSSE
BSSE energies
6.4 3.8 9.0 8.0 6.0 18.1 12.6
5.5 3.4 7.9 7.0 5.5 15.5 10.9
1.1 0.4 1.1 1.0 0.5 2.6 1.7
The BSSE errors (of RC1 and other reactant complexes) are calculated at CCSD(T)/aVTZ level using the counterpoise (CP) method [21] as implemented in GAUSSIAN 09 suite of program. The data listed in Table 1 show that the BSSE energies are in the range of 0.4–2.6 kcal/mol. Relatively large BSSE values are those of the reactant complexes between SiH4 and 2 BH3 molecules. Compared to the BSSE energy obtained with the same CCSD(T) method but smaller aVDZ basis set [15], the use of the larger aVTZ basis set significantly reduce the BSSE value for RC1, from 3 kcal/mol down to 1 kcal/mol. 3. Results and discussion 3.1. Reactions between SiH4 and BH3 /AlH3 The potential energy surface for the dehydrogenation of the reactant complex RC1 is shown in Figure 1. The H5 of SiH4 (Figure 1) behaves as Lewis-base, while the B atom of BH3 behaves as Lewisacid. In the RC1 complex they interact with each other (B· · ·H5 ˚ through such acid–base interactions interaction distance: 1.32 A) (similar to the previous report [9,15,16]). Because of this interaction, the Si1 H5 bond becomes much weaker as compared to the ˚ other Si H bonds (bond distance elongation of >0.1 A). The removal of H5 and H8 atoms from RC1 via TS-1,3 has a small energy barrier (E: 9.7 kcal/mol with respect to RC1) and yields a product complex PC1 in which the H2 molecule still attaches to the P3 molecule. The structure of PC1 is very similar to that of TS-13 (cf., Figure 1). In addition, the natural charge distributions obtained from an NBO analysis (Table S2 of the ESI) also show a small change between the TS-13 and PC1. These similarities result in the very small difference in their relative energies, being 0.4 kcal/mol. It
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Figure 2. Schematic potential energy profile for the dehydrogenation from SiH4 and AlH3 . The labels of the hydrogen atoms connected to the Si and Al atoms are indicated in dark blue and pink numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
should be noted that this value is in general smaller than the error of the computational method. Therefore, the formation of P3 from RC1 might be considered as a barrier-free process [22]. The H2 release from this complex requires an additional 3.9 kcal/mol and thus makes this process become more endothermic. This H2 elimination is more favorable than the alternative path through TS-1,2 (cf. Figure 1), as the energy barrier in this case is 42.5 kcal/mol. The product P2 is also less stable than P3 due to the presence of strained triangular structure. Previous studies [9] on the RC1 complexes also produced similar results. The only difference is that in the present studies the B6 H9 bond in RC1 lies in the B6 Si1 H5 plane, while in the previous study [9] this bond rotates out of the plane forming partially eclipsed conformation with higher electrostatic and steric repulsions. As a result, the present RC1 complex shows stronger interactions (by 2.6 kcal/mol) with respect to the previously computed eclipsed conformation. In both cases, the interactions were computed using single point CCSD(T)/aVTZ method at the optimized structures at a lower level, and thus such a direct comparison is justified. Replacing borane BH3 with alane AlH3 molecule yields a less stable reactant complex RC4 with the interaction energy of −3.8 kcal/mol (Figure 2). Although the product energies (P5 and H2 ) are lower with respect to P3 + H2 (from RC1, Figure 1), the removal of hydrogen, in this case, has a transition structure (TS-4,5) with a barrier height of 25.5 kcal/mol. Thus this reaction is less favored with respect to the H2 removal via TS-1,3 in Figure 1.
3.2. Reactions between CH3 SiH3 and BH3 /AlH3 The compound CH3 SiH3 has slightly higher weight-percent of hydrogen than SiH4 . The two extra hydrogen atoms can take part in the dehydrogenation reaction. The BH3 /AlH3 can either interact weakly with SiH3 fragment before the dehydrogenation, or it can directly attack the CH3 fragment and release H2 molecule. The reaction pathways between CH3 SiH3 and BH3 are shown in Figure 3. The two reactant complexes RC7 and RC10 are different only in the orientation of the two interacting molecules. Their interaction energies are very close to each other and RC7 is slightly more stable (1 kcal/mol). From RC7, two different reaction pathways are possible. The first one involves the removal of two hydrogen atoms of SiH3 fragment (via TS: TS-7,8), while the second path involves removal two hydrogen atoms of the BH3 molecule and CH3 fragment (via TS: TS-7,9). The former pathway (involving TS-7,8) has a lower energy barrier (34.5 kcal/mol) with respect to the reactants. The H2 and H3 atoms, connected to Si atom, are removed during the dehydrogenation process through this TS. The H atom of BH3 then forms a bridged hydrogen bond between B and Si atoms in the product complex (P8). This product is less stable than the one formed in the reaction pathway via TS-7,9, in which the removal of H10 of BH3 and H8 of CH3 fragment is accompanied by the breaking of interactions between BH3 and SiH3 fragments. This process yields the product P9 (E: −3.9 kcal/mol with respect to RC7). However, the transition structure TS-7,9 is 21 kcal/mol less stable than TS-7,8 and thus this pathway is unfavorable. In the reactant complex RC10, the
Figure 3. Schematic potential energy profile for the dehydrogenation from CH3 SiH3 and BH3 . The labels of the hydrogen atoms connected to the Si, B, and C atoms are indicated in dark blue, pink and brown numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
H.T. Nguyen et al. / Chemical Physics Letters 620 (2015) 38–42
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Figure 4. Schematic potential energy profile for the dehydrogenation from CH3 SiH3 and AlH3 . The labels of the hydrogen atoms connected to the Si, Al, and C atoms are indicated in dark blue, pink and brown numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
H10 of BH3 and H4 of SiH4 are in closer proximity than in RC7. These two hydrogen atoms can be removed through the TS TS-10,8 with the formation of the product P8. This TS also has quite high energy barrier (49.4 kcal/mol). The direct attack of BH3 to the CH3 fragment, yielding the product P9, has much lower energy barrier (TS-9, 18.0 kcal/mol). This extra stabilization effect in TS-9 is due to the electrostatic interactions between the positively charged B (atom 9 with a Mulliken atomic charge of 0.30 electron) and the negatively charged C (atom 5 with a Mulliken atomic charge of −0.69 electron) ˚ atoms (Figure 3) for their close proximity of 1.781 A. Similar potential energy surfaces were found for the reactions between CH3 SiH3 and AlH3 (Figure 4). Two dehydrogenation pathways from RC12 via TS-12,11 and TS-12,13 were found. The TS-12,11 for the removal of hydrogen atoms from AlH3 and CH3 is more stable with a barrier height of 25.9 kcal/mol. The direct attack of AlH3 to the CH3 fragment, on the other hand, has the less favorable transition structure TS-11 (barrier height: 27.8 kcal/mol). As a result, the dehydrogenation reaction via TS-12,11 is the most
favored one. Nevertheless, the energy barrier for this reaction is 10 kcal/mol higher with respect to the case of BH3 . Thus BH3 mediated reaction is more favored for this dehydrogenation process. 3.3. Reactions between SiH4 and BH3 in the presence of an additional BH3 (∼29% weight percent of hydrogen) Addition of second BH3 to the RC1 forms two reactant complexes RC14 and RC16. The former species is 5.5 kcal/mol more stable. The potential energy surfaces of the dehydration pathways from these reactant complexes are shown in Figure 5. The dehydrogenation reactions from RC14 and RC16 have the energy barriers in the range of 14.1–19.1 kcal/mol. The complex RC14 is formed from the interaction between SiH4 and two separated BH3 molecules, in which two hydrogen atoms of SiH4 (H2 and H3 ) form hydrogen bridges with the BH3 molecules. Furthermore, the H2 atom of SiH4 interacts with a BH3 molecule (pink labels, Figure 5) which in turn interacts with the additional
Figure 5. Schematic potential energy profile for the dehydrogenation from SiH4 and two BH3 . The labels of the hydrogen atoms connected to the Si atom and B atoms of first and second BH3 are indicated in dark blue, pink and light purple numbers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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BH3 molecule (light purple labels, Figure 5) forming a bridged hydrogen bond via H8 atom. The RC16 can also be considered as the reactant complex between SiH4 and B2 H6 molecule, where one of the two hydrogen bridges between two BH3 molecules is broken and replaced by a new bridged hydrogen bond between BH3 and SiH4 . Although this process has a high energy barrier (E: 30.7 kcal/mol with respect to SiH4 + B2 H6 ), the relative energy of the TS TS-16 is still lower than the total energies of SiH4 + 2BH3 . Therefore, this formation pathway of RC16 is comparable to the barrier-free step-by-step addition of BH3 to SiH4 . The TS TS-14,15 in the reaction pathway from the RC14 lies below the total energies of the reactants (SiH4 + 2BH3 ). This pathway is therefore the most probable dehydrogenation pathway as compared to the other pathways. In the TS-14,15 the H2 atom of SiH4 is removed along with the H13 atom of BH3 (Figure 5). The H7 atom of the first BH3 molecule then forms a hydrogen bridge between two B atoms in the product P15. The destabilization effect from the ring strain in P15 is compensated due to such interactions. The relative energy of products (P15 + H2 ) with respect to SiH4 + 2BH3 and RC1 + BH3 shows that the overall reaction is exothermic. It is to be noted that if the second BH3 molecule is released from P15 with its two original hydrogen atoms and the H7 atom, it could be considered as a catalyst for the hydrogenation of the RC1. Similarly, the first BH3 molecule could only be considered as a catalyst if it is released with the hydrogen atom attaching Si and B. The two dehydrogenation pathways from RC16 have slightly less stable transition structures TS-16,17 and TS-16,18 with the respective barrier heights of only 1.5 and 6.5 kcal/mol. The transition structures related to these processes were also reported earlier [23]. The two hydrogen atoms removed in TS-16,17 belong to two BH3 molecules. In the final product (P17, after H2 release), the H2 atom of SiH4 molecule moves closer to the B B bond forming two hydrogen bridges between the BH2 fragments. This reaction pathway can be considered as a dehydrogenation reaction of B2 H6 in the presence of SiH4 . In this case, additional BH3 remains attached to the product P17. When this BH3 is released, it takes away either the H2 atom of SiH4 or the H7 atom of the first BH3 molecule and thus plays the role of a catalyst for the dehydrogenation of RC1. The dehydrogenation reaction through TS-16,18 occurs via the removal of the H12 atom of the additional BH3 and H2 atom of SiH4 (Figure 5). The remaining BH2 and SiH3 fragments interact with each other forming a weak bond between Si1 and B9 in the product P18. Although the product P15 is much less stable than the products P17 and P18, this kinetically controlled pathway (through low-barrier transition state TS-14,15) seems to be more preferable. The additional BH3 could be considered as a catalyst (in the product formations P17 and P18) if it is released by the removal of H8 (or H7) atom of the first BH3 . In summary, the calculation data showed that BH3 and AlH3 mediated dehydrogenations of SiH4 are more favorable as compared to those of CH3 SiH3 . The differences in relative energies of the products and the reactants of these reactions are relatively small. It favors the possibility of reverse reactions. Furthermore, the double BH3 catalyzed dehydrogenations of SiH4 have very low energy barriers and thus could take place under mild conditions. The studied systems in this letter have potentiality to release H2 at low temperatures together with thermodynamic reversibility. These features show that they could be good candidates for chemical hydrogen storage.
probable pathway for H2 release. A detailed study was carried out using DFT/B3LYP/6-311G(d,p) method to obtain the optimized geometries of the reactants, products and TSs in the reaction paths. The CCSD(T)/aVTZ single point electronic energies with B3LYP/6311G(d,p)-ZPE corrections were used to generate final energetics of the reactions and interaction energy parameters involving the various weak complexes on the reaction surfaces. The approach was compared with more rigorous techniques and proved to be quite satisfactory. The calculated results for the reactions of SiH4 and CH3 SiH3 with BH3 and AlH3 showed that the reactions with BH3 are more favorable. The dehydrogenation reaction between SiH4 and BH3 follows a barrier-free pathway and the overall reaction is endothermic (by 6.8 kcal/mol with respect to the reactants). The reaction between SiH4 and AlH3 has a barrier height of 25.5 kcal/mol and consequently less favorable. Similarly, the reactions of CH3 SiH3 with BH3 also have lower transition structures. The most probable reaction from CH3 SiH3 + BH3 requires the system to have an extra 18.0 kcal/mol to overcome the energy barrier. Whereas for the reaction between CH3 SiH3 + AlH3 , the energy requirement is 25.9 kcal/mol. Addition of one extra BH3 molecule results in several low-energy-barrier pathways with the relative energies of the transition structures being −3.1, 1.5, and 6.5 kcal/mol. Further dehydrogenations are possible from the stable products of these pathways. Acknowledgements This work has been supported by the PREM (Award no. DMR1205194) and ONR (Award no. N00014-13-1-0501) grants. The Leuven group thanks the KU Leuven Research Council for continuing support via GOA and IDO programs. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2014.12.010. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
4. Concluding remarks [23]
We have investigated the dehydrogenations of SiH4 and CH3 SiH3 in the presence of BH3 and AlH3 to determine the most
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