Mechanistic Study on Water Splitting Reactions by Small Silicon

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Mechanistic Study on Water Splitting Reactions by Small Silicon Clusters SiX, X = Si, Be, Mg, Ca 3

Tran Dieu Hang, and Minh Tho Nguyen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02237 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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The Journal of Physical Chemistry

Mechanistic Study on Water Splitting Reactions by Small Silicon Clusters Si3X, X = Si, Be, Mg, Ca Tran Dieu Hang#,% and Minh Tho Nguyen≠,ǂ,%,* ≠ Computational

Chemistry Research Group, Ton Duc Thang University, Ho Chi Minh City,

700000 Vietnam ǂ

Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, 700000 Vietnam

# Department %

of Chemistry, Quy Nhon University, Quy Nhon, Vietnam

Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

(Abstract) Interaction, dissociation and dehydrogenation reactions of water monomer and dimer with pure and mixed tetrameric silicon clusters Si3X with X = Si, Be, Mg, Ca were investigated using high accuracy quantum chemical calculations. While geometries were optimized using the DFT/B3LYP functional with the aug-cc-pVTZ basis set, reaction energy profiles were constructed making use of the coupled-cluster theory with extrapolation to complete basis set, CCSD(T)/CBS. Cleavage of the O-H bond in water dimer is found to be more favored than that of water monomer in the reaction with Si4. The water acceptor monomer in water dimer performs as an internal catalyst facilitating H atoms transfer to form H2. Adsorption of water dimer on Si3X clusters mostly takes place upon interaction of the donor water molecule with Si cluster. Water dimer adsorbs more strongly on Si3M than on Si4. The most stable complexes obtained upon interaction of water dimer with Si3M mainly arise from M – O interaction in preference over a Si – O connection. Substitution 1

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of a Si atom in Si4 by an earth alkaline metal induces a substantial reduction of the energy barrier for the (rate-limiting) first O – H bond cleavage of water dimer. The most remarkable achievement upon doping is a disappearance of the overall energy barrier for the initial O-H bond cleavage in water dimer. Of the three binary Si3M clusters considered, dehydrogenation of water dimer driven by Si3Be is the most kinetically and thermodynamically favorable pathway. In comparison to another cluster such as Al6 and nanoparticles Ru55, energy barriers for water dimer dissociation on Si3M are much lower. The mixed clusters Si3M turn out to be as efficient alternative reagents for O – H dissociation and hydrogen production from water dimer. This study proposes further searches for other mixed silicon clusters as realistic gas phase reagents for crucial dehydrogenation processes in such a way they can be prepared and conducted in experiment.

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1. Introduction Global energy consumption grows speedily along with rapid increase in the world population and standards of living.1 Rising concern about the looming crisis of fossil based energy, and the seriousness of environmental consequences require a continuing development of viable, sustainable, clean and economic alternatives. Possessing the high energy density and environmentally friendly features, hydrogen has been known as energy carrier to meet future global energy demands.2,3 Nevertheless, there is only a small amount of molecular hydrogen (H2) in nature.4 Nowadays, hydrogen gas used for, among others, rockets and automobiles is principally produced in industry from steam reforming of fossil fuels. An issue is that the steam reforming processes induce a large number of COx byproducts. Therefore, it is actually necessary to find and develop new environment-friendly sources and methods, which are independent of fossil fuels for production of hydrogen. Water has long been emerged to be the best source for hydrogen production.4 In the troposphere and the upper stratosphere, water vapor exist in the form of monomers, and smaller quantities of dimers and oligomers.5 Nevertheless, by using near-infrared absorption spectroscopy, Pfeilsticker et al.6 detected water dimers in the atmosphere with a concentration of 6 x 1014 molecules.cm-3 at 292 K, and this number increases up to 9 x 1014 molecules.cm-3 in a study carried out by Dunn.7 Meanwhile, Goldman and co-workers8 determined the concentration of water dimers at high relative humidity up to 1.7 x 1015 molecules.cm-3. Overall, the finding of efficient catalysts to enhance the water splitting is a crucial step for production of molecular hydrogen, and remains a great challenge.9

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Clusters of the elements possess a variety of unique and often unexpected properties, which are not observed in the corresponding bulk form. They have been attracting particular interest in the gas phase catalysis since they can act as individual active sites, and minor changes in both the size and composition, including the addition or removal of a single atom and/or doping, can induce a significant impact on the properties of the resulting clusters. When used as catalysts, clusters tend to modify the activity and selectivity of a reaction. In fact, the reduced size of elemental clusters rises the number of under-coordinated atoms, as compared to that in larger nano-catalysts, and they can thus potentially act as catalysts with improved activities. Moreover, as many of the most efficient catalysts are now produced from precious metals, the use of pure and mixed elemental clusters could also have a significant economic impact by reducing the amount of precious metals demanded for catalytic reactions. Nowadays, the use of size-selected clusters in catalysis in the gas phase has made substantial progress thanks to initial investigations on modelling catalytic active sites. Recent experimental developments have emphasized the working of well-defined clusters under realistic reaction conditions, and have discovered some remarkable catalytic properties of sub-nanometre clusters.10 The reactions of gaseous transition metal clusters with small molecules have formed the subject of numerous studies to elucidate the dependence of reactivity on cluster size.11,12 In addition, with the catalyst particles getting smaller, free clusters in the gas phase also provide us with an outstanding testing ground for examining the basic adsorption and/or chemisorption phenomena. Of the many possible clusters as catalysts, small silicon-based clusters continues to attract great attention in part owing to their abundant applications in various fields. From the perspective 4


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of chemical reactivity, silicon clusters act differently from the bulk silicon, as indicated by both theoretical and experimental studies.13–19 The reactivity of noble clusters such as Aun with O2 was found to be enhanced upon doping of a silicon atom.20 Due to their dangling bonds, pure silicon clusters are however destabilizing, and hence have limited applications. Such a drawback can be overcome upon doping bare silicon clusters with metal atoms. In fact, an appropriate dopant can dramatically change some properties of the resulting doped clusters, namely electronic, chemical and optical features.21 Earth-alkaline metals have been suggested to be efficient catalyst in many processes.22–25 Our recent studies26,27 also revealed that small earth-alkali mixed silicon clusters SinM with n = 2,3 and M = Be, Mg, Ca, can be regarded to be promising alternatives for the expensive metal-based catalysts used for methanol dissociation and other dehydrogenation processes of organic compounds. In particular, the energies barriers for the cleavage of O – H bond in the presence of doped Si clusters has been found to be lower than all previously reported values on various types of elemental clusters, metal surfaces or metal oxides. Despite many previous studies on H2 production reactions carried out on different materials, results either on small pure silicon clusters or on doped silicon clusters for water splitting processes are not available yet. In this context, we set out to investigate the interactions and reactions of water monomer and dimer with tetratomic pure and doped silicon clusters Si3X with X = Si, Be, Mg, Ca by means of quantum chemical computations. For the reason stated above, we select the water monomer and dimer as the simplest reagent for gas phase splitting reactions. Our computed results on the detailed mechanisms shed new light into the catalytic abilities of the mixed silicon clusters for hydrogen production from water, and thereby suggest directions for the search for other stable doped silicon clusters as potential catalysts for dehydrogenation processes.

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2. Computational Methods All electronic structure computations are conducted using both density functional theory (DFT) and wave-function methods. DFT computations are carried out using the Gaussian 09 suite of program.28 The hybrid B3LYP functional29,30 in conjunction with the 6-311+G(d) basis set is used to optimize the geometries of all stationary structures located. Improved geometrical parameters are then re-optimized using the same functional but with the larger correlation consistent aug-cc-pVTZ (denoted hereafter as aVTZ) basis set. The B3LYP functional has been shown to provide energetic data in line with experiment, or high accuracy wave-function methods such as the coupled-cluster theory CCSD(T) in previous investigations for both pure and doped silicon clusters,26,27,31–36 and the reactions of water dimer.37,38 Harmonic vibrational frequency analyses are performed at the same level to confirm the nature of the stationary structures and to evaluate the zero-point correction energies (ZPE). Intrinsic reaction coordinate (IRC)39 profiles are computed at the B3LYP/aVTZ level to make sure that each transition structure (TS) located is correctly connected with two energy minima. As for a convention, each TS mentioned in the following sections is labeled by ts-x,y where x and y are the two connecting energy minima With the aim of obtaining more accurate energetics, all B3LYP optimized structures are subsequently used for a series of single-point electronic energy computations making use of the coupled-cluster theory CCSD(T) in conjunction with the correlation consistent basis sets aug-ccpVnZ with n = D, T and Q (denoted as aVnZ). For Ca, the basis set cc-pVnZ (VnZ) with n = D, T and Q are used. The total CCSD(T) electronic energies are then extrapolated to the complete basis set (CBS) energies using the expression (1):40 E(n) = ECBS + B exp[– ( n – 1)] + C exp[– ( n – 1)2]

(1) 6


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where n = 2, 3 and 4 stand for the aVnZ or VnZ basis sets, respectively, E(n) and ECBS are the CCSD(T)/aVnZ and CCSD(T)/CBS energy, respectively, and B, C are fitting parameters. Relative energy values reported in different Figures are given in kcal/mol with a decimal figure. The electron localization function (ELF) maps,41 describing a partition of the total electron density into basins used to analyze the electron distribution, are constructed using the B3LYP/aVTZ densities. 3. Results and Discussion 3.1. Reaction of Water Monomer and Dimer with Si4 3.1.1. Si4-(H2O)n complexes with n = 1,2 It is predictable that the reaction between tetramer Si4 cluster and both water monomer and dimer proceeds through formation of pre-associative complexes Si4-(H2O)n with n = 1, 2. Therefore, these complexes formed from interaction of water monomer and dimer with Si cluster are first explored. Figure 1 illustrates the geometrical shapes of the most stable complexes and their corresponding CCSD(T)/CBS relative energies. The complex obtained is denoted by cp.wx.n where cp and w stand for complex and water, x = m, d for monomer and dimer, n = 1, 2, … numbers the isomers in a growing relative energy order. Accordingly, the cp.wx.1 structure is the most stable complex. The structure of Si4 has extensively been reported in previous papers26,32 and thus warrants no further comments. The singlet rhombic structure (1Ag, D2h) was found to be much more favored than the triplet counterpart and becomes the ground state structure of tetramer Si4.

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cp.wm.1

cp.wd.1

cp.wd.2

cp.wd.3

cp.wd.4

–4.8

0.0, –12.7

1.8, –10.9

6.6, –6.1

7.4, –5.0

Figure 1. Optimized geometrical shapes and CCSD(T)/CBS + ZPE relative energies (ΔE, values in kcal/mol) of complexes obtained upon adsorption of water monomer and dimer on Si4. The white, dark blue, red denote H, Si, and O atoms, respectively. The values given in italic are the binding energies of water monomer and dimer with Si4 cluster.

Complex with Water Monomer. When water monomer is adsorbed on the Si4 cluster, as shown in Figure 1, only one complex cp.wm.1 is formed upon attachment of the O atom to one Si atom in cluster. The interaction energy between water monomer and Si4 is found to be –4.8 kcal/mol (Figure 1). The main reason for such interaction is the difference in net charges of Si atoms in the tetramer. In fact, the Si atom connecting with O atom has a positive charge whereas the net charges of next two Si atoms are negative (Mulliken net charges of all Si atoms in Si4 are given in Figure S1 in the Supporting Information, ESI). Possessing a large electronegativity, the O atom thus tends to connect to the positively charged Si atom.

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Figure 2. Shape, bond distances (angstrom) and net charges (electron) of water dimer Complex with Water Dimer. Let us first remind the shape of water dimer. Known as an archetype for hydrogen bonding, the structure of the water dimer was extensively and continuously studied using both theoretical and experimental approaches. As shown in Figure 2, the water dimer structure consists of a near-linear O-H--O hydrogen bond between two water monomers. The monomer on the right side behaves as a single H-bond donor, and the other acts as an H-bond acceptor. Unlike the case of water monomer, adsorption of water dimer on silicon tetramer Si 4 is taken place at two different Si positions leading to formation of four different complexes from cp.wd.1 to cp.wd.4 in which water dimer is adsorbed mostly through the oxygen atom of the donor water monomer (cf. Figure 1). The first two complexes including cp.wd.1 and cp.wd.2 are formed upon attachment of oxygen atoms to the same silicon atom in pure Si4, but in two different directions. They are quite close in energy, with the latter being ~2 kcal/mol less stable than the former, and they therefore could compete to become the ground state of the Si4-(H2O)2 complex. As compared to water monomer, water dimer is more strongly bound to Si4 with binding energies of –12.7 and –10.9 kcal/mol to form cp.wd.1 and cp.wd.2, respectively (cf. Figure 1). Meanwhile, water dimer is adsorbed on Si4 through another Si atom resulting in formation of other complexes 9



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cp.wd.3 and cp.wd.4, whose energy difference is only ~1 kcal/mol. These complexes are less preferred and located at ~7 kcal/mol above cp.wd.1 (Figure 1). The ELF map at bifurcation value of ELF = 0.85 (Figure 3) shows one basin in the region between both the O2 atom in water dimer and the Si2 atom in Si tetramer, confirming the Si – O interaction is actually formed upon adsorption. Population analysis also illustrates the presence of disynaptic basin V(O2,Si2) having 2.2 electrons and disappearance of monosynatic V(O2) lone pair. In summary, stable complexes are formed upon adsorption of water monomer and dimer on tetramer silicon cluster Si4 mostly via attachment of oxygen atom of donor water monomer to the positively charged Si atom of cluster.

cp.wd.1

TS.d2,4

10


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Figure 3. Plots of ELF of complex cp.wd.1 and TS TS.2,4 at bifurcation values of ELF = 0.85 and 0.83, respectively (B3LYP/aVTZ). 3.1.2. Reaction between Si4 and Water Monomer Two different dehydrogenation pathways are found in the Si4 + water monomer reaction. Figure 4 plots potential energy profiles illustrating H2 release pathways starting from complex cp.wm.1. The first pathway coming from cp.wm.1 via TS.m1,m2 undergoes dissociation of the O – Ha bond in water monomer, with a barrier of 22.3 kcal/mol. To overcome this step from the isolated reactants, the required overall barrier is 17.5 kcal/mol. Let us note that the overall energy barrier is defined in this work by the relative energy of a transition structure on a particular reaction pathway with respect to the zero energy reference of the corresponding isolated reactants. Formation of resulting intermediate m2 whose Ha atom now links to S1 atom of the cluster induces an exothermic reaction energy of –15.8 kcal/mol. In the next step, the O – Hb bond is transferred from Si2 to Si1 atom via TS.m2,m4 with a barrier of 7.7 kcal/mol, yielding stable intermediate m4. Then, H2 detachment from m4 leaves the Si4O oxide product Pm6. However, this stage is characterized by a very high barrier of 63.1 kcal/mol and endothermic. Another alternative H2 – loss pathway from cp.wm.1 also starts by a cleavage of O – H bond in water monomer via TS.m1,m3, in which Hb atom is bridged to O and Si2 atoms. In contrast of Si1atom, Si2 atom is positively charged. Accordingly, the O – Hb dissociation happens in a difficult way as compared to the breaking of the O – Ha bond. Thus, this step is characterized by a high barrier of 28 kcal/mol. Reaction energy profile in Figure 4 indicates that initial O – H breaking is the rate-determining step with an overall barrier of 23.1 kcal/mol. The next step takes place with isomerization from trans-configuration m3 to cis-configuration m5 via TS.m3,m5 with a tiny 11



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barrier of only 2.4 kcal/mol. From m5, both hydrogen atoms Ha and Hb can be released simultaneously via a four-membered Si2…Ha…Hb…O ring structure in TS.m5,m7 to form product Pm7 and H2, with a high barrier of ~59 kcal/mol (Figure 4).

Figure 4. Schematic potential energy profiles illustrating the reaction of water monomer with Si4. Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations. 3.1.3. Reaction Between Si4 and Water Dimer Schematic potential energy profiles associated with possible pathways for dehydrogenation upon water dimer splitting by pure silicon tetramer Si4 are displayed in Figure S2 of ESI file. Geometrical shapes of all species shown in Figure S3 are displayed in Figures S3 and S4 (ESI). 12


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Because there are an abundant number of channels found for this reaction, we only report here some important pathways, the remaining parts are discussed in the ESI. In order to facilitate the reading, only the main results are shown in Figure 5 and 6. As demonstrated in Figure 5, generation of two stable complexes cp.wd.1 and cp.wd.2 from initial separated reactants are exothermic with adsorption energies of –12.7 and –10.9 kcal/mol, respectively. The first H2 release pathway coming from complex cp.wd.1 via TS TS.d1,1 in which two hydrogen atoms of two different water monomers tend to dissociate concurrently forming product P1 and one H2 molecule. This channel is characterized by the highest energy barrier of 55.7 kcal/mol with respect to the complex, and 43 kcal/mol with respect to the separated reactants, even it induces a substantial exothermicity of –46.4 kcal/mol. Another channel going from complex cp.wd.1 involves TS.d1,2 in which the O2 – Hc bond in the donor water molecule begins to be dissociated with an overall barrier of 18.6 kcal/mol to form the more favored intermediate 2. The overall barrier in this step turns out to be lower as compared to the corresponding step via TS.m1,m3 in water monomer reaction. Thanks to the presence of water acceptor molecule, the net charge of Hc is less positive than that of Hb in the water monomer reaction, and attachment of Hc to Si2 in TS.d1,2 becomes easier. Similar to the case of water dimer, this stage is also a rate-limiting step. From 2, cleavage of O – H bond in the acceptor water continues to come up via TS.2,6 to generate a more stable intermediate 6 whose H and O atoms now connects a Si atom in Si4 (cf. Figure 5). Although this step needs to overcome a high energy barrier of 42.0 kcal/mol, formation of stable intermediate 2 in the previous stage is energetically favored with an overall exothermic energy of –50.8 kcal/mol, which is large enough to compensate in keeping a favorable profile. In 13



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fact, TS.2,6 is located well below the zero-energy level, and intermediate 6 can thus be spontaneously obtained without additional activation energy.

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Figure 5. Schematic potential energy profiles illustrating the reaction of water dimer with Si 4, forming products P1 and P30. Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations. Intermediate 6 can also be obtained through another channel emanating from complex cp.wd.2 via two distinct TSs TS.d2,4 and TS.4,6 (cf. the red line is Figure 5). This first step occurs via TS.d2,4 describing an O2– Hd bond breaking and transfer of one Hd atom from the donor water molecule to the acceptor counterpart which in turn transfers one of its H atoms Ha to Si2 atom in Si4 with a barrier height of 11.9 kcal/mol. The ELF localization domains located at bifurcation values ELF = 0.83 of TS.d2,4 (also shown in Figure 3) indicate the H-transfer process. Particularly, along with the electron reduction in disynaptic V(O1,Ha) = 0.6 electron, one basin V(Si2,Ha) = 1.1 electron is observed in region between Si2 and Ha atoms, proving an almost broken O1 – Ha bond in the acceptor water monomer and formation of the new Si2 – Ha bond. Moreover, the disappearance of V(O2, Hd) basin and emergence of basin V(O1, Hd) = 1.5 electron also confirm the cleavage of O2 – Hd bond and formation of O1 – Hd. Clearly, the acceptor water monomer induces a catalytic effect in significantly reducing the O – H cleavage barrier in donor water monomer. It is well known that water molecules, present in the form of oligomer, often play not only as reagents but also the role of catalysts favorizing the transfer of hydrogen atoms, and reducing the overall barriers. Accordingly, the overall energy barrier now turns out to be only 1 kcal/mol and becomes the most preferred pathway of all these pathways considered. The second channel via TS.4,6 also produces 6 and is associated with a high barrier of 50.8 kcal/mol, but with an overall barrier is only 3.8 kcal/mol, due to formation of stable intermediate 4 in the previous step. Both steps are exothermic processes with released energies of –47.0 and – 15



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68.2 kcal/mol, respectively. There are five possible channels to generate hydrogen molecule from 6 (cf. Figure S2) leading to formation of 11 different products. Detailed discussion of these pathways is given in ESI. We only report here the most favored pathways illustrated in Figure 5.

The pathway starts by a free rotation of Hd atom (O2 – Hd bond) to transform the transconfiguration 6 into cis-configuration 12 in turn involves an out of plane O1 – Hb rotation, leading to less stable intermediate 24, due to a less favored spatial orientation between Ha and Hb (cf. Figure 5). Each step is characterized by a tiny barrier of ~2 kcal/mol, suggesting a rather free isomerization process. Then both hydrogen atoms Ha and Hc can be released simultaneously via the Si1…Ha…Hc…Si2 four-membered TS.24,30, yielding P30 with an energy barrier of 46.6 kcal/mol. Even though the energy barrier found in this route is relatively high from the respective intermediate, it still has an advantage over the separate reactants, as no additional internal energy requirement due to formation of very stable intermediates 6, 12 and 24. The associated potential energy profile for another H2 release pathway from cp.wd.1 via TS TS.d1,3 is demonstrated in Figure 6. This pathway is initiated by a cleavage of O1 – Hb bond in the acceptor water via an attack of the Hb atom to the opposite Si3 – Si4 bond in TS.d1,3 forming intermediate 3 with an energy release of –19.8 kcal/mol. This pathway happens in a much easier way, involving a barrier of 23.7 kcal/mol with respect to the channel via TS.d1,2. The main reason for such a difference lies in the net charges of Si atoms in the four-membered Si4. The net charges of two Si3 and Si4 atoms are in fact more negative than that of Si1 (see Figure S5). Being positively charged, the H atom inevitably prefers to connect to Si3 and Si4 than to Si1. Owing to an adsorption of water dimer on Si4 to form cp.wd.1 with a complexation energy of –12.7 kcal/mol, the overall 16


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barrier associated to this route is now reduced to 11 kcal/mol. This step is also considered as the rate-determining step for the whole process. The next step is a breaking of O2 – Hc bond in the donor water monomer via TS.3,5 with an overall barrier of ~8 kcal/mol and forms stable intermediate 5 having a considerable exothermicity of –70.6 kcal/mol, which ultimately facilitates the occurrence of the following processes. Indeed, all TSs for pathways found from 5 are located below the zero energy reference, in such a way that the pathways can be achieved without the need of extra energy to the isolated reactants (Figure 6).

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Figure 6. Schematic potential energy profiles illustrating the reaction of water dimer with Si4 from cp.wd.1 via TS TS.d1,3, forming product P25. Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations.

Most H2-loss pathways from intermediate 5 involve a rupture of O2 – Hd bond in the second water molecule via TS.5,9 forming the more stable intermediate 9 with a barrier of 35.2 kcal/mol with respect of the same reference. This step is energetically favored with a large exothermic energy of –71.7 kcal/mol. Formation of P25 + H2 takes place upon direct removal of two hydrogen atoms Ha and Hb from O1 – Ha and Si4 – Hb bonds via TS.9,25 (Figure 6). Overall, the first O – H bond dissociation is the rate-determining step in the reaction of water monomer and dimer with the silicon tetramer Si4. Our results point out that the rupture of the O-H bond in the case of water dimer is more favored than that of water monomer. Furthermore, recent investigations37,42 on dehydrogenation reaction of water monomer and water dimer on elemental clusters reported that the energy barrier for water dimer dissociation is significantly smaller with respect to that for water monomer. This shows the catalytic effect of the acceptor water molecule on the dissociation of the O – H bond of donor water molecule. In the reaction of water dimer with Si4, the cleavage of O – H bond in the acceptor water molecule happens in a much easier way with respect to that involving the donor water molecule. Accordingly, starting from cp.wd.1, the pathway via TS.Sia,3 is preferential with regard to the H2 release via TS.Sia,2. In Si4 + water dimer reaction, the channel initiating from cp.wd.1 is kinetically less favored than those connected to complex cp.wd.2, even though the latter is slightly less stable. 3.2. Reaction Between Si3M with M = Be, Mg, Ca and Water Dimer 18


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As indicated in the previous section, the H2 release reaction of water dimer is more preferred than that of water monomer, hence we only focus in this section on investigating the reaction of water dimer with alkaline earth metal silicon tetramer Si3M with M = Be, Mg, Ca. 3.2.1. Si3M-(H2O)2 Complexes with M = Be, Mg, Ca Si3Be. Stable complexes between water dimer and Si3Be cluster are formed following two different ways, as given in Figure 7. The most stable cp.Be.a is shaped by connection of the donor monomer O atom with Be with a significant adsorption energy of –46.6 kcal/mol. Obviously, substitution of one Si in Si4 by a Be atom substantially facilitates the interplay between water dimer and the cluster.” (page 19) Three remaining complexes cp.Be.b, cp.Be.c, cp.Be.d, obtained through O – Si interaction turn out to be much less favored, being around ~42 kcal/mol higher in energy than cp.Be.a. Such a trend can be elucidated by the difference in the net charges of Si and Be atoms in Si3Be. In other words, the atomic charge of Be is more positive than that of the Si atom. Holding a large electronegativity, the O atom inevitably prefers to attach to Be rather than Si. Consequently, the Be – O interaction is more stabilized than the Si – O counterpart.

cp.Be.a

cp.Be.b

cp.Be.c

cp.Be.d 19



0.0, –46.6

42.2, –4.4

42.8, –3.8

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43.3, –3.3

`

cp.Mg.a

cp.Mg.b

cp.Mg.c

cp.Mg.d

0.0, –30.5

3.0, –27.5

3.5, –27.0

27.0, –3.5

(cp.Ca.a,

(cp.Ca.a,

0.0, –26.7)

1.1, –25.6)

Figure 7. Shapes of optimized geometries and CCSD(T)/CBS + ZPE relative energies (ΔE, kcal/mol) of the most stable complexes obtained upon adsorption of water dimer on Si3M clusters with M = Be, Mg, Ca. The values given in italic are the binding energies of water dimer with mixed clusters. Si3Mg. An analogous trend is observed in the case of Si3Mg. Four complexes generated upon adsorption of water dimer to Si3Mg shown in Figure 7 also arise from two different interactions. As expected, the first three complexes cp.Mg.a, cp.Mg.b and cp.Mg.c shaped by connection of oxygen in donor water monomer with the Mg atom is again much more stable than the last one cp.Mg.d formed via the O – Si interaction. Of the first three complexes, cp.Mg.a formed by inplane adsorption of water dimer on the cluster is more favored than out-of-plane complexes cp.Mg.b and cp.Mg.c. Their relative energies are given in Figure 7. 20


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Si3Ca. It is interesting to note that no Si – O bonding complex is found in the case of Si3Ca. This could be explained by the presence of 3d electrons in the Ca atom, whose shaped is more easily distorted and disposed by ionic polarization. Such a phenomenon was also observed in the methanol plus Si3Ca system.26 The complexes found for Si3Ca have strongly similar structures as the two first Si3Mg – (H2O)2 complexes described above; their relative energies are also given in Figure 7 (values in italics). As energy difference of both complexes amounts only to ~1 kcal/mol, they are competitive to be the ground state structure of the Si3Ca – (H2O)2 complexes. Similar to Si3Be, formation of both cp.Mg.a and cp.Ca.a complexes have a considerably large complexation energies of –30.5 and –26.7 kcal/mol, respectively (cf. Figure 7).. The decreasing adsorption energy in going from Be to Mg and Ca is correlated with the lengthening of M – O bonds from 1.58 to 2.02 and then 2.33 Å, respectively. As predicted, water dimer is more strongly adsorbed on Si3M with respect to pure Si4. In comparison to Al6 (adsorption energies being – 13 and – 16 kcal/mol), adsorption of water dimer on Si3M is clearly stronger. In summary, adsorption of water dimer on pure and doped silicon clusters Si 3X happens mostly upon attachment of the oxygen atom of the donor water monomer to one specific atom of clusters. In the earth-alkali doped silicon clusters Si3M, the most stable complex cp.M.a stems from an M – O bonding in preference over a Si – O connection. 3.3.1. Si3Be + Water Dimer Possible pathways for H2 release from the Si3Be + water dimer reaction are identified involving three complexes cp.Be.a, cp.Be.b and cp.Be.d. As discussed above, complex cp.Be.a is much more stable than the others, and the H2 dissociation reaction from cp.Be.a is the most favored 21



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one. Therefore, we only focus here on the pathways of cp.Be.a whose potential energy profiles for H2 release are illustrated in Figure 8. Relevant optimized geometrical shapes of the species outlined in Figure 8 are described in Figure 9.

Figure 8. Schematic potential energy profiles from cp.Be.a in reaction of water dimer and Si3Be. Relative energies (kcal/mol) were obtained from CCSD(T)/CBS + ZPE computations.

The most favored channel starts by a physisorption of the water dimer on the Si3Be. Obviously, as compared to the case of Si4, water dimer is more strongly bound to Si3Be to form cp.Be.a with a considerable adsorption energy of –46.6 kcal/mol, favorizing the next processes to happen. Subsequently, the O2 – Hb bond in acceptor water monomer breaks to produce intermediate 34 via 22


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a six-membered ring TS.Be.a,34 with an energy obstruction of only 3 kcal/mol relative to the complex, and a reaction exothermicity of –64.1 kcal/mol. In this step, the Hc atom of donor water monomer, which has the same net charge as the Hb, is now transferred to the acceptor water molecule to form again a water monomer (Figure 10a). In this context, thus the hydrogen bond acceptor water again plays a role as an internal catalyst to dissociate the O – H bond of the hydrogen bond donor water monomer. One water molecule is indeed regenerated at the end of the process. Unlike the case of Si4, TS.Be.a,34 for the initial O – H cleavage lies now –43.1 kcal/mol below the separated reactants Si3Be + water dimer. The process is seemingly spontaneous with the absence of an overall energy barrier. Clearly, replacement of one Si atom in Si4 by one Be atom induces a substantial reduction in the energy barrier for initial O – H bond rupture.

TS.Bea,34

34

TS.34,35

35

23



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TS.35,36

TS.34,36

36

TS.36,37

P37

TS.36,38

38

TS.38,40

TS.35,39

39

TS.39,41

P41

Figure 9. Shapes of transition structures, intermediates and products appeared in the pathways displayed in Figure 8. In order to obtain more insight into the electron distribution of TS.Be.a,34, net atomic charges and the ELF map of this TS at a bifurcation value of ELF = 0.86 are presented in Figure 24


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10b. The high bifurcation value provides us with a better view on the electron densities at basins. Apparently, when replacing a Si in tetrameric silicon cluster by a Be atom, a charge transfer is, as expected, operated from the Be atom to the trimeric silicon.

(a)

25



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(b) Figure 10. Charge distribution in TS.Bea,34: Mulliken met charges (electron) and ELF maps at a bifurcation value ELF = 0.86 (B3LYP/aVTZ) As a result, the positive charge of +0.5 electron is now centered on the Be atom (Figure 10a). The Be – O bond is thus more polarized than the Si – O counterpart. Such polarized bonds tend to stabilize this TS. In the ELF map shown in Figure 10b, one basin is viewed in the region between both O and Be atoms, showing that a Be – O bond is actually formed upon adsorption. Population analysis also indicates the existence of a disynaptic basin V(O,Be) containing 2.6 electron. In comparison to the disynaptic basin V(Be,Si) of 2.2 electrons in the free Si3Be,43 the electron 26


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population in the Be – Si basin is now reduced by 1.7 electron. In particular, a trisynaptic basin V(Si1,Si3,Be) located demonstrates formation of a certain three-center bond covering Be and Si atoms. Adsorption of water dimer on Si3Be apparently gives rise to an electron loss in Si−Be bonds. Similarly, attachment of Hb to Si1 leads to a decreased population in monosynaptic V(Si1) lone pair as compared to V(Si3) lone pair (Figure 10b). Electron population of disynaptic basin V(O2, Hc) in TS.Be.a,34 has a large value of 1.8 electron, confirming a nearly complete hydrogen transfer from the donor water monomer to the acceptor water monomer, as discussed above. Transformation of 34 to 36 can undergo by two different routes. The first involving an interaction between the O atom in the acceptor water molecule and the Be via TS-34,36, with a very tiny barrier of 0.4 kcal/mol, is clearly favored over the two-step route via TS-34,35 and TS35,36 (Figure 8). From 36, two channels are opened to generate H2. In the first via TS.36,37, two hydrogen atoms of the O2 join together to form H2 and product P37 with a substantial reaction energy of –131 kcal/mol (cf. Figure 9). This pathway is however characterized by a high barrier of 55 kcal/mol, even though TS.36,37 is still located below the zero-reference level. The second channel generating product P40 + H2 occurs via TS.36,38 (at 13 kcal/mol) results from an interplay between the O1 atom to the Si1 atom yielding 38. Then, both H atoms bonded to O1 and Si1 atoms are concurrently dissociated via TS.38,40 to form H2 with an energy barrier of 30 kcal/mol. The last H2-loss pathway originating from 35 experiences two stages. First, the H atom connecting Si1 atom interacts with Si2 leading to a cleavage of the Be – Si1 bond to form 39 via TS.35,39, with a low energy obstruction of 6 kcal/mol. Subsequently, a simultaneous release of both Ha and Hb atoms via TS.39,41 needs to overcome an energy barrier of 25 kcal/mol to generate 27



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product P41 + H2. As depicted in Figure 8, this pathway is characterized by the lowest barrier among those considered. As a consequence, the H2 release via TS.39,41 turns out to be the most favored one of the reaction between Si3Be and water dimer. Again, the most noteworthy feature here is that all TSs in the potential energy profiles for dehydrogenation coming from cp.Be.a are lying lower in energy than the separated reactants. This means that the pathways can spontaneously occur starting from the reactants. Potential energy profiles describing the H2-loss from other complexes, namely cp.Be.b and cp.Be.d are displayed in Figures S6 and S7, S8, S9 (ESI), respectively. Detailed discussion of these pathways are also given in ESI. Different from the cp.Be.a case, the first rate-determining O – H bond breaking from cp.Be.b and cp.Be.d takes place in the donor water monomer via TS.Be.b,b1 and TS.Be.d,d1 with energy barriers of 22.9 and 34.4 kcal/mol, respectively. Again, rupture of the O – H bond in the donor water molecule occurs in a harder way as compared to that involving the acceptor water. 3.3.2. Si3M with M = Mg, Ca + Water Dimer The dehydrogenation pathways in the reactions of two clusters Si3Mg and Si3Ca with water dimer arise from the in-plane complex cp.M.a. Potential energy profiles for both systems are described in Figure S7 (ESI). Such reaction energy profiles and geometries of TSs, intermediates and products found for Si3Mg and Si3Ca are quite similar to those of the Si3Be system. For purpose of comparison, relative energies of the complexes, TSs, intermediates and products on the main dehydrogenation pathways of Si3Mg and Si3Ca are also given in Figure S10 (ESI).

28


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The first step in dehydrogenation paths of both Si3Mg and Si3Ca is also initiated by dissociation of the O – H bond in the acceptor water molecule via TS.Mg.a,a1 and TS.Ca.a,a1 with energy barriers of 7.1 and 21.7 kcal/mol, respectively. Geometries of both TSs are analogous to that of TS.Be.a,34. The barrier increase in going from Be to Ca is consistent with the rise in M – O bond length in TSs. As expected, both TSs are situated below the zero-reference level, and can therefore be attained without addition of extra energy to the reactants. Accordingly, these pathways are more favored as compared to the corresponding path of Si4. Substitution of one Si atom in pure tetramer silicon cluster Si4 by an earth alkaline metal induces a significant reduction in the O – H cleavage barrier. In comparison to similar reaction of water dimer with the Ru55 nanoparticle, such a hydrogen release pathway with the assistance of Si3M is much more preferred. The O – H bond dissociation barriers calculated in the case of Si3Be are still lower with respect of those of dehydrogenation reaction of water dimer on the Al6 cluster (6 and 10 kcal/mol).37 The H2 loss pathway via TS.Mg.a,a1 resulting in formation of intermediate a1 whose structure is similar to intermediate 35, is significantly exothermic. The following steps from a1 happen similarly to those of cp.Be.a from 35 to form product P40. Along with H2 molecule, product Pa4 obtained in this path has a similar shape as P40. Overall, all TSs in these steps are invariably located below the energy of separated reactants. In this pathway, our results point out again that the H bond acceptor water monomer exhibits a catalytic effect on the cleavage of O – H bond of the H bond donor water monomer. In general, interaction of water dimer with Si3M is much stronger than that of the pure silicon tetramer. Of the three doped clusters studied, the H2 release reaction of water dimer with the assistance of Si3Be is the most kinetically and thermodynamically favored pathway. The most 29



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remarkable feature in the H2 loss pathway of water dimer using Si3M is the absence of an overall barrier for the O – H bond rupture step. Accordingly, the dehydrogenation reaction of water dimer on doped silicon clusters Si3M with M = Be, Mg, Ca is more favorable with respect to the corresponding pure silicon cluster Si4. 4. Concluding Remarks In the present theoretical study, the molecular mechanism of H2 release from water dimer with the assistance of the pure and doped silicon clusters Si3X with X = Si, Be, Mg, Ca were explored using density functional theory (B3LYP) and wavefunction method (CCSD(T)). Geometries of relevant stationary points were optimized using the B3LYP/aVTZ method, while more accurate energies of the complexes, intermediate, transition structures and products were determined from single-point electronic energy computations using the coupled-cluster theory at extrapolated complete basis set CCSD(T)/CBS. The main findings emerged from our computations are as follows: i) Adsorption of water monomer on Si4 cluster occurs upon connection of oxygen atom. In water dimer, this involves the oxygen of donor water monomer to a positively charged silicon in cluster. In the mixed alkaline-earth metal silicon Si3M with M = Be, Mg, Ca, the complexes formed upon M – O interaction is more stable than Si – O bond. Water dimer is bound more strongly to the doped Si3M than to the pure Si4 cluster. ii) The rate-limiting step in the reaction of water monomer and dimer with Si4 arises from an initial O – H bond dissociation. Our results reveal that the dissociation of the O – H bond in the case of water dimer is more favored than that of water monomer. As a water molecule is regenerated, the 30


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water acceptor monomer thus plays the role of an internal catalyst facilitating the transfer of H atoms to form hydrogen molecule. iii) Replacement of one Si atom in the tetramer Si4 by an earth alkaline metal M gives rise to a substantial reduction in the energy barrier for the first O – H bond cleavage of water dimer. The most remarkable result is the lack of an overall energy barrier for the (rate-determining) O – H bond cleavage in water dimer when Si3M is involved. Indeed, all transition structures are found to be located below the starting reference level. Consequently, the binary clusters Si3M appear as good candidates for assisting O – H dissociation and H2 release reaction of water dimer, and are thus expected to induce similar effects for other dehydrogenation processes. iv) Of the mixed clusters Si3M considered, dehydrogenation reaction of water dimer with assistance of Si3Be is the most favorable one in both kinetic and thermodynamic aspects. The energy barriers for the first initial O – H bond breaking pathway of water dimer in the presence of clusters studied turn out to be smaller than the corresponding values previously reported for nanoparticles such as Ru55 (Si3M), and another cluster such as Al6 (Si3Be). The present study provides an essential proof for the high reactivity of small earth alkaline mixed silicon clusters for water splitting to produce H2. Along with our recent investigations which illustrated that mixed earth alkaline and alkaline silicon clusters are efficient alternative catalysts for methanol activation, the bestowed findings in this context continue to open a new way for seeking auspicious alternatives for H2 production using small clusters in the gas phase.

31



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Acknowledgments The authors are indebted to the KU Leuven Research Council (GOA program and IRO scholarship). We also thank Ton Duc Thang University (Demasted) for support. Supporting Information Potential energy profiles for various reactions: water dimer + Si4 (S1), water dimer + Si3Be (S3, S4), water dimer + Si3Mg and Si3Ca (S7 and S10). Shapes of TSs and products (S5, S6, S8 and S9) and net charges in TS (S2). Texts describing the potential energy profiles in Section 3.1.3 and in Figures S6, S7, S8 and S9. This material is available free of charge via the Internet at http://pubs.acs.org.

Authors Information Emails: [email protected], [email protected] ORCID: Minh Tho Nguyen: 0000-0002-3803-0569; Tran Dieu Hang: 0000-0002-1487-0686

Note The authors declare no competing financial interest.

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76x45mm (300 x 300 DPI)


Mechanistic Study on Water Splitting Reactions by Small Silicon

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