Methanol Activation Catalyzed by Small Earth ... - ACS Publications

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Methanol Activation Catalyzed by Small Earth-Alkali Mixed Silicon Clusters Sim−nMn with M = Be, Mg, Ca and m = 3−4, n = 0−1 Tran Dieu Hang, Huyen Thi Nguyen, and Minh Tho Nguyen* Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium S Supporting Information *

ABSTRACT: The methanol activation pathways occurring on small pure and mixed silicon clusters Sim−nMn with M = Be, Mg, Ca and m = 3−4, n = 0−1 were investigated using quantum chemical computations (density functional theory B3LYP/aug-cc-pVTZ and coupled-cluster theory CCSD(T)/CBS extrapolated from energies with the aug-cc-pVnZ basis sets, n = D, T, Q) to examine their thermodynamic and kinetic feasibilities. In all cases considered, the cleavage of the O−H bond is favored over that of the C−H bond. The O−H bond cleavage in the presence of the singlet Si3 cluster is thermodynamically less preferred than on mixed Si2M clusters, even though it becomes more kinetically favored. Most importantly, the energy barriers for the O−H bond breaking on the singlet Si3, Si2Ca, and Si3Ca clusters are found to be lower than the previously reported results for metal clusters, catalytic metal surfaces, metal oxides, etc. The small mixed Si clusters thus appear to be good catalysts for methanol activation and most probably in other dehydrogenation processes from the X−H bonds of organic compounds. These findings suggest further extensive searches for doped silicon clusters as realistic catalysts that can experimentally be prepared, for methanol activation particularly and dehydrogenation processes generally.

1. INTRODUCTION Methanol, the simplest and lightest alcohol, has been attracting considerable interest in both experimental and theoretical studies owing to its applications in diverse industrial fields. The most common use of methanol is by far in the preparation of other chemicals, especially formaldehyde, an important ingredient in organic synthesis.1 Being a polar liquid within a large range of temperatures (−98 to 65 °C), methanol is thus used as an antifreeze, solvent, and fuel for conveyance.2 In addition, methanol acts as a hydrogen carrier and is transformed to pure hydrogen via steam reforming.3−6 Last but not least, it is used as the fuel in the direct methanol fuel cells7 that are a prototype of proton-exchange fuel cells. It is therefore considered as a potential candidate in the next generation of renewable green fuels.8 An essential stage in nearly all applications stated above involving methanol is to activate its strong C−H (96.1 kcal/mol) and O−H (104.6 kcal/ mol) bonds. However, breaking of these strong bonds is still a main challenge and thus continues to receive much attention from academic and industrial researchers. Indeed, investigations on the selective rupture of the C−H and O−H bonds of methanol as well as determination of the related activation barriers in the presence of a wide variety of compounds including metal oxides,9−11 transition metal (TM) surfaces,12−17 metal clusters,18−32 and bioinspired metal compounds33 have been conducted. Let us briefly summarize some interesting features of these studies. Methanol activation barriers in the presence of various catalysts are given in Table 1. Accordingly, the O−H bond © XXXX American Chemical Society

dissociation process on the TiO2(101) surface is characterized by a small barrier of ∼14 kcal/mol,10 whereas a benchmark study11 on methanol activation on bare [FeIVO]2+ revealed that cleavage of C−H bond is predominant. Along with TM oxides, noble metal surfaces were also suggested as ideal catalysts for decomposition reactions of methanol. For instance, the energy barrier for the rupture of the C−H bond is larger than that of the O−H bond on the surface Pt(111).12 Meanwhile, the methanol activation reactions occur on the Pd(111) surface13 with a reversed ordering, and the cleavage of the O−H bond now becomes prominent. A similar result on the Pd(211) surface was reported in a recent study14 (see Table 1). Furthermore, methanol activation reactions were also conducted on other metal surfaces, namely, Cu and Ir. Again, the energy barrier for O−H dissociation was reported to be smaller than that of C−H bond cleavage on both Cu(111)13 and Cu(110) surfaces,16 but the Cu(110) surface induces lower barriers (cf. Table 1). On the other hand, both O−H and C−H bond breaking pathways on Ir(111) surfaces15 turn out to be competitive with similar energy barriers (ΔE#) of around 12 kcal/mol. Widely known as excellent catalysts, small metal clusters also emerged as significant agents for methanol dissociation reactions. Previous studies18,25 on methanol decompositions in the presence of Cu4, Co4, and Pd4 clusters demonstrated that Received: March 2, 2016 Revised: April 29, 2016

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DOI: 10.1021/acs.jpcc.6b02215 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

bonds, bare silicon clusters are chemically too active and thus have limited applications. Such a limitation can be overcome by doping silicon clusters with metal atoms. A suitable dopant can improve dramatically the electronic, optical, and chemical properties of the resulting doped cluster, as compared to the corresponding pure cluster. Our recent study42 revealed that, among others, mixing earth-alkaline metals into the silicon trimer increases the singlet−triplet energy separation of the Si3 cluster and reinforces the stability of the closed-shell singlet state. However, there are some similarities in geometric, electronic, and aromatic properties between these binary clusters Si3M and the silicon tetramer Si4. Moreover, earthalkaline metals have been reported to be useful and efficient catalysts in many different processes.43−47 In the group 2 metals, Be and Mg exhibit different behavior in comparison to other metals of the same group. They are analogous to the group 12 metals, namely, Zn and Cd, whereas the Ca, Sr, and Ba have similar chemical properties to the lanthanoid elements. Some basic questions emerge as follows: (1) whether the earth alkaline metals can improve the reactivity of small silicon clusters toward methanol activation, and (2) how large the extent of enhancement varies with the mixture of different metals including Be, Mg, and Ca. In spite of many experimental and theoretical investigations on methanol activation summarized above (cf. Table 1), data neither on silicon clusters nor on earth alkaline metal clusters are actually available for this process. Methanol decomposition reactions in the presence of these clusters therefore merit detailed studies. In this context, we set out to investigate, by means of quantum chemical computations, the catalytic effects of both small pure and mixed silicon clusters Sim−nMn with M = Be, Mg, Ca, and m = 3−4, n = 0−1, toward the initial bond dissociations of methanol.

Table 1. Adsorption Energies (Eads) and Activation Barriers for Reaction Pathways Involving O−H and C−H Bond Rupture in Methanol on Various Materials

catalyst TiO2(101) [FeIVO]2+ Pt(111) Pd(111) Pd(211) Cu(111) Cu(110) Ir(111) Cu3 Cu4 Co4 Pd4 Pt3 PtAu2 Rh6 Rh4Ru2 (Ru) Rh4Ru2 (Rh) CunAum+

Eads (kcal/mol) − − −10.3 −5.8 −9.2 −3.9 −12.7 −7.8 − −17.9 −25.5 − −17.5 −17.4 −17.9 −12.2 −12.4 ± −40.2

O−H activation barrier (kcal/mol)

C−H activation barrier (kcal/mol)

ref

13.6 61.9 22.0 24.6 18.9 24.6 14.5 12.4 34.4 29.9 10.8 13.3 24.9 21.4 13.9 9.9 6.0 −

− 43.4 34.0 12.7 14.7 32.2 30.8 12.0 − 28.5 14.3 19.8 14.4 10.6 13.6 11.9 10.9 −

10 11 12 13 14 13 16 15 21 25 25 18 28 28 32 32 32 31

both C−H and O−H breaking pathways occurring on Co4 clusters are most favored and characterized by lower barriers of about 14 and 11 kcal/mol, respectively. In a combined experimental and theoretical study on neutral iron oxide clusters, the O−H breaking was identified to be dominant.23 The catalytic ability of clusters has been known to be enhanced following doping. Therefore, the hydrogenation reactions of methanol on pure and bimetallic clusters have been the subject of continuing theoretical and experimental studies. Binary Rh4Ru2 clusters have in fact been found to induce lower energy barriers for both C−H and O−H bond breaking than the pure Rh6 cluster (Table 1).32 Note that the barriers for bond dissociation calculated in the latter study with bimetallic clusters as catalysts are lower than those with pure metal clusters (Co4, Pd4, and Cu3, ...) and on metal surfaces (Cu(111), Pd(111), and Ir(111), ...). Another important factor to be considered for the understanding of the catalytic mechanism is the adsorption ability of methanol on materials because adsorption activates the decisive chemical bonds of the reactant. This is characterized by the adsorption energy which shows the strength of the interactions between methanol and the catalysts and is usually calculated as the difference between the energies of the complex and separated fragments. The stronger the binding between methanol and clusters is, the larger the reactants’ activation is and the more easily the reaction happens. The adsorption energies of methanol on the different compounds mentioned above are also listed in Table 1, ranging from −4 to −40 kcal/mol. By definition, a negative value of adsorption energy indicates an attractive interaction. Small silicon-based clusters have attracted continuing attention in part due to their abundant applications in various fields, especially in semiconductor industries and nanomaterial science. From the reactivity point of view, silicon clusters are different from the bulk silicon, as shown in both experimental and theoretical investigations.34−40 Besides, the enhanced reactivity of noble metal clusters such as Aun with O2 upon doping a silicon atom was reported.41 Because of their dangling

2. COMPUTATIONAL METHODS All electronic structure calculations are carried out using density functional theory (DFT) method implemented in the Gaussian 09 suite of program.48 The hybrid B3LYP functional49,50 in conjunction with the 6-311+G(d) basis set is initially used to optimize geometries of all stationary structures considered. Improved geometrical parameters are then reoptimized using the same functional but with the larger aug-cc-pVTZ (denoted hereafter as aVTZ) basis set. Harmonic vibrational frequency analyses are performed at the same level to confirm the nature of the stationary structures and to obtain the zero-point correction energies (ZPEs). The popular B3LYP functional is widely used in previous investigations for both pure and doped silicon clusters51−55 and the reactions of methanol.11,23,56,57 In our recent study,42 the mixed alkaline-earth silicon clusters Si3M with M = Be, Mg, Ca were investigated using the exchange-correlation B3LYP functional which provides the geometries for the composite G4 method. Intrinsic reaction coordinate (IRC)58 profiles are computed at the B3LYP/aVTZ level to ensure that the transition structures (TSs) located are correctly connected. As for a convention, each TS described in the following sections is denoted by a label of ts-A,B, where A and B are the two connecting energy minima. In order to obtain more accurate energetics, B3LYPoptimized structures are used for a series of single-point electronic energy computations using the coupled-cluster theory CCSD(T) with the correlation consistent basis set, B


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Table 2. CCSD(T)/CBS + ZPE Relative Energies (ΔE, kcal/mol) of the Most Stable Complexes Obtained through Interaction of Methanol (Met) with the Sim−nMn with M = Be, Mg, Ca and m = 3−4, n = 0−1 Clusters (Sim−nMn−Met) complexes (Si3−Met)

RE

complexes (Si2Be−Met)

RE (kcal/mol)

complexes (Si2Mg−Met)

RE

complexes (Si2Ca+Met)

RE

cp-3-s1 cp-3-s2 cp-3-s3 cp-3-t4

0.0 0.5 3.6 4.3

0.0 21.4 23.6 30.2

cp-2Ca-s1 cp-2Ca-t2 cp-2Ca-t3

0.0 26.0 40.0

RE

0.0 18.0 33.8 49.5 55.3 RE (kcal/mol)

cp-2Mg-s1 cp-2Mg-s2 cp-2Mg-t3 cp-2Mg-t4

complexes (Si4−Met)

cp-2Be-s1 cp-2Be-t2 cp-2Be-s3 cp-2Be-t4 cp-2Be-t5 complexes (Si3Be−Met)

complexes (Si3Mg−Met)

RE

complexes (Si3Ca−Met)

RE

cp-4-s1 cp-4-t2 cp-4-t3

0.0 27.9 40.0

cp-3Be-s1 cp-3Be-t2 cp-3Be-s3 cp-3Be-t4

0.0 31.3 37.0 56.0

cp-3Mg-s1 cp-3Mg-s2 cp-3Mg-t3 cp-3Mg-t4

0.0 19.1 27.0 47.8

cp-3Ca-s1 cp-3Ca-t2 cp-3Ca-t3

0.0 22.8 38.8

aug-cc-pVnZ with n = D, T and Q (denoted as aVnZ), and in the case of SixCa (x = 2, 3), the cc-pVnZ basis sets. The CCSD(T) electronic energies are then extrapolated to the complete basis set (CBS) energies using the expression (eq 1)59

complex at each size can be also found in its schematic potential energy profiles (Figure 1−Figure 4). Each complex is marked

E(n) = ECBS + B exp[− (n − 1)] + C exp[− (n − 1)2 ] (1)

where n = 2, 3, and 4 stand for the aVnZ basis sets and E(n) and ECBS are the CCSD(T)/aVnZ and the CCSD(T)/CBS energy and B, C fitting parameters, respectively. In the tables and figures displayed in the text, energetic values are given in kcal/mol with a decimal figure. In an attempt to facilitate the reading, all energetic values, obtained from CCSD(T)/CBS + ZPE computations, cited in the following sections are rounded up (without decimal figure).

3. RESULTS AND DISCUSSION This part is organized into three sections. The first section is devoted to a description of the structural characteristics, electronic states and relative energies of the most stable complexes obtained through interaction of methanol with the pure and mixed silicon clusters considered. The two remaining sections will focus on the methanol decomposition reactions in the presence of these clusters. 3.1. Structures and Electronic Properties of the Lowest-Lying Sim−nMn−CH3OH Complexes with M = Be, Mg, Ca, and m = 3−4, n = 0−1. The most stable structures of the pure silicon Si3 and Si4 and the mixed Si3M including M = Be, Mg, Ca clusters were reported in much detail in our previous studies.42,51,52 Let us briefly summarize a few striking points. The low-spin (1A1, C2v) and high-spin (3A2′, D3h) states of the trimer were determined to be degenerate, whereas the singlet rhombic structure (1Ag, D2h) of the tetramer is much more stable, which lies ∼0.9 eV above the triplet counterpart. Attachment of an alkaline-earth-metal atom to Si3 to form the rhombic four-membered binary clusters Si3M has a tendency to increase the stability of the singlet spin states and therefore enlarge the singlet−triplet energy separations (ΔEST) of the mixed clusters Si3M. From the ground state of these clusters, their interactions with the methanol molecule are investigated in both singlet and triplet states. The lowest-lying geometries, electronic states, and CCSD(T)/CBS relative energies of all stable complexes formed following interaction of methanol and silicon clusters are displayed from Figures S1 to S6 in the Supporting Information (SI). Relative energies of all species are summarized in Table 2. The global minimum geometry of

Figure 1. Schematic potential energy profiles for the methanol dissociation reaction with the singlet Si3 cluster as catalyst. Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations. The white, dark blue, red, and gray balls denote H, Si, O, and C atoms, respectively. The other atoms are Be, Mg, and Ca.

by a label “cp-aM-bc”, in which cp stands for a complex; a denotes the number of Si atoms; M is alkaline-earth-metal (if present); b = s, t for singlet and triplet, respectively; and c = 1, 2, ... numbers the isomers in an increasing relative energy order. The complexes are displayed in the figures starting with the

Figure 2. Schematic potential energy profiles for the methanol dissociation reaction when interacting with the triplet Si3 cluster. Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations. C


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Figure 3. Schematic potential energy profiles illustrating the O−H cleavage reaction (a) and C−H rupture reaction (b) of methanol in the presence of the singlet Si2M clusters (M = Be, Mg, Ca). Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations.

kcal/mol more stable than the latter, and they thus compete to become the ground state of the Si3−CH3OH complex. The third complex cp-3-s3 is formed through the attachment of the C atom in methanol to the Si−Si bond of Si3 and lies higher in energy than cp-3-s1 by only 3.6 kcal/mol. Formed in a similar way as cp-3-s2, the high spin cp-3-t4 (3A, C1) is 4.3 kcal/mol less favored than cp-3-s1. Complexes with Si2M (M = Be, Mg, Ca). This triatomic species has a singlet ground state (C2v, 1A1) with a S-T energy difference of 19, 27, and 30 kcal/mol corresponding to M = Be, Mg, and Ca, respectively. All complexes are formed with these mixed clusters in two different ways. Their relative energies are given in Table 2. Geometries of these complexes can be found in Figures S2−S4 (SI). The most stable complexes ending with 1 are shown in Figure 3. For Si2Be, both complexes cp-2Be-s1 (1A, C1) and cp-2Be-t2 (3A, C1), with a large S−T energy separation of 18 kcal/mol in favor of the low-spin state, result from interaction between the O and Be atoms (cf. Figure S2, SI). The value increases to 21 and 26 kcal/mol in the cases of Si2Mg and Si2Ca, respectively. On the contrary, in three remaining complexes, namely, cp-2Be-s3, cp-2Be-t4, and cp2Be-t5, the CH3OH−Si2Be interaction occurs through an O− Si bond (cf. Figure S2, SI). The Be−O bond in cp-2Be-s1 is significantly shorter than those found for O−Si in cp-2Be-s3 (up to 1.21 Å), cp-2Be-s3 s3 (0.60 Å), and cp-2Be-s3 (0.72 Å). Consequently, these complexes are very less stable and located at least ∼34 kcal/mol above cp-2Be-s1 given in Table 2. Furthermore, the net charge of the Be atom is more positive

Figure 4. Schematic potential energy profiles illustrating the methanol dissociation reaction of methanol in the presence of the singlet Si4. Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations.

most stable complex which always ended with 1. The relative energies of the less favored complexes 2, 3, etc. are determined with respect to the corresponding most stable structure 1. Complexes with Si3. Due to a large number of complexes generated from its interaction with methanol, we only report here some of the most stable complexes involved in the methanol activation process. Geometries of these complexes in both electronic states are shown in Figures 1 and 2 and Figure S1 (SI). Table 2 lists their relative energies. Similar to the pure trimer, four complexes in the singlet and triplet states are very close in energy. The O atom in CH3OH is bridged to a Si−Si bond of Si3 in cp-3-s1, whereas it is linked with only a Si atom in cp-3-s2. They are close in energy, with the former being 0.5

Table 3. Mulliken Net Charges q (Electrons) on the Metal and Si Atoms of Neutral SinM (M = Be, Mg, Ca; n = 2, 3) Clusters Obtained Using the B3LYP Functional

D


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prefer a Mg−O connection over the Si−O bond in both spin states (cf. Figure S6). Si3Ca. Similar to the lower homologue Si2Ca, no singlet Si− O interaction complex is found. This could be understood by the existence of 3d electrons in the Ca atom, whose shape is more easily distorted and giving rise to some ionic polarization. This effect tends to increase the bond constituents and therefore reinforces the polar Ca−O bond and weakens the Si− O bond in the complexes. Moreover, the Ca atom in Si3Ca is more positively charged than the M atom in Si3Mg and Si3Be, and the charge of Si in the latter is less negative than that of the former (Table 3). The oxygen thereby prefers to link Ca rather than Si in comparison with Si 3 Be and Si 3 Mg. The corresponding triplet complex cp-3Ca-t3 is energetically less favorable by nearly 39 kcal/mol. Thus, the S−T energy separation is slightly enlarged with respect to that of 29 kcal/ mol of the mixed Si3Ca cluster. The singlet Ca−O bondcontaining complex cp-3Ca-s1 turns out to be 23 kcal/mol below the triplet counterpart cp-3Ca-t2. Overall, the most stable complexes of pure silicon clusters (Si3, Si4) and binary clusters SixM (x = 2, 3; M = Be, Mg, Ca) with methanol arise, in all cases, from the ground singlet state structures of the corresponding clusters. Interaction of methanol with these clusters does not substantially change the S−T energy separations of the isolated clusters. In most cases, the singlet manifold is much more favored than the triplet. The pure silicon trimer Si3 has a very small S−T energy separation but still results in the low spin complex. In the case of binary clusters, all lowest-lying complexes arise from an M− O interaction in preference over a Si−O connection. Such an intermolecular interaction is due to the net positive charge of the M atom in the mixed cluster, which induces a larger electrostatic attraction. 3.2. Methanol Activation on Silicon Trimers Si3 and Si2M (M = Be, Mg, Ca). 3.2.1. Methanol Dissociation in the Presence of Si3. Possible dissociation pathways of three stable complexes between methanol and the singlet Si3 cluster (Si3(s)), namely, cp-3-s1, cp-3-s2, and cp-3-s3, are explored. These complexes have small interaction energies of −6, −5, and −2 kcal/mol, respectively. Schematic potential energy profiles showing such channels are plotted in Figure 1. Four TSs found for methanol activation reactions from three complexes stated above belong to two kinds of dissociation channels, namely, both O−H bond and C−H bond cleavages, with barrier heights ranging from 6 to 51 kcal/mol. Two O−H bond detachment pathways of cp-3-s1 and cp-3-s2 via ts-3s1,5 and ts-3s2,6 turn out to have the lowest energy barriers, which are 6 and 7 kcal/ mol, respectively. The lower barrier heights of these TSs are likely due to the formation of Si−H−O hydrogen bonds. These values are smaller than those of the reactions involving Rh6 (14 kcal/mol) and Rh4Ru2 clusters (10 and 6 kcal/mol),32 where the atoms linking to the O atom in methanol correspond to the Ru and Rh atoms. Meanwhile, the recently reported barrier energy of 6 kcal/mol on the Rh4Ru2 cluster is lower than those of previous results on transition metal clusters (Co4, Cu3, Pd4)21,25 as well as on metal surfaces (Ir(111), Cu(111, Pd(211), Pd(111)...),12−15,17,18 which are known as potential catalysts (Table 1). It is noteworthy that ts-3s1,5 is placed below the zero energy level of the separated reactants (cf. Figure 1). The product 5 with the O−H hydrogen being transferred to a Si atom can thus be readily formed from cp-3-s1. The product 6 formed from cp-3-s2 via ts-3s2,6 is 13 kcal/mol higher in energy than 5. The

than that of the Si atom in the Si2Be (cf. Table 3 listing the Mulliken atomic charges of the neutral Si2M). Accordingly, the O atom possessing a large electronegativity in methanol preferentially links to the Be atom of Si2Be. Similarly, the M−O interaction turns out to be stronger than the Si−O counterpart in both low- and high-spin states for Si2Mg and Si2Ca. Indeed, cp-2Mg-s2 and cp-2Mg-t4 (Si−O bond) are less favored, lying 21 and 7 kcal/mol above cp-2Mg-s1 and cp2Mg-t3 (Mg−O bond), respectively (Table 2 and Figure S3). Such energetic differences in both singlet and triplet states can be explained by the difference in distance of the Si−O and Mg−O in complexes. In other words, the Si−O distance is 0.51 Å longer than that of Mg−O in the singlet states, and the value decreases to 0.14 Å for the triplet complexes. It is interesting to note that the singlet Si−O complex is not found in the case of Si2Ca (but found in Si2Be and Si2Mg cases). The most remarkable reason here is the large positive charge of the Ca atom (0.58 electron) in the Si2Ca as compared to those of the Be (0.23 electron) and Mg atoms (0.25 electron) in the mixed clusters (Table 3). In addition, the net charge of Si in Si2Ca (−0.29 electron) is more negative than those in Si2Be (−0.11 electron) and Si2Mg (−0.12 electron). As a consequence, the O atom in methanol would connect the Ca atom rather than the Si atom in Si2Ca. In the high spin state, the triplet cp-2Ca-t3 with a Si−O bond is found but by 40 kcal/mol higher in energy than the singlet cp-2Ca-s1 (Ca−O bond) (cf. Figure S4 of SI). Complexes with Si4. Methanol adsorbs at two different Si positions of the pure tetramer through the O atom, as displayed in Figure S5 (SI). The most stable complex is, as expected, the singlet complex cp-4-s1 (1A, C1), analogous to the case of the pure silicon cluster Si4. Two remaining complexes cp-4-t2 (3A, C1) and cp-4-t3 (3A, C1), with different Si−O interacting sites from cp-4-s1, are much less favored with S−T energy separations of ∼28 and 40 kcal/mol, respectively (Table 2). Complexes with Si3M (M = Be, Mg, Ca). Relative energies of lowest-lying complexes between Si3M and methanol are shown in Table 2. Their geometries are displayed in Figure S6 of the SI. Si3Be. The interactions bear a strong similarity to those in the smaller binary cluster Si2Be. The singlet complex cp-3Be-s1 shaped by connecting the oxygen to the Be atom is predicted to be the ground state structure. The corresponding triplet cp3Be-t2 (3A, C1) turns out to be nonplanar and less favored, being 31 kcal/mol above cp-3Be-s1. In other words, upon adsorption of methanol on Si3Be, the S−T energy difference is extended from 27 (for Si3Be)42 to 31 kcal/mol (for Si3Be− CH3OH). The singlet complex cp-3Be-s3 with a Si−O bond is much less stable, lying 37 kcal/mol higher in energy. The other triplet cp-3Be-t4 formed through Si−O interaction is even less preferred, being 56 kcal/mol higher in energy. Clearly, the Be− O interaction, with an intermolecular distance of 1.59 Å, tends to stabilize the resulting complexes more than the Si−O connection (2.50 Å) in both low and high spin states. The reason is that Be in the Be−O bond is more positively charged than Si in the Si−O interaction, thus giving rise to a larger electrostatic interaction force (Table 3). Si3Mg. A similar behavior can be observed in this system, as presented in Figure S6 (SI). The main distinction with respect to the adsorption of methanol on Si3Be is that the Si−O bond in cp-3Mg-s2, with relative energy of 19 kcal/mol, is elongated by 1.4 Å as compared to cp-3Be-s2 (Table 2). Regarding the lowest-lying complex cp-3Mg-s1, the S−T separation is now reduced to ∼28 kcal/mol. The most stable complexes again E


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replacement of one Si atom in the Si3 cluster with an alkaline-earth metal greatly facilitates the interaction between methanol and the cluster. As demonstrated in Figure 3a, the energy barriers of 9 and 14 kcal/mol for O−H cleavage on the Si2Be and Si2Mg clusters via ts-2Be-s1,10 and ts-2Mg-s1,11 are higher with respect to the Si2Ca case with barrier height of 6 kcal/mol. This indicates that the O−H dissociation via ts-2Ca-s1,12 on the Si2Ca cluster is the most kinetically favorable one among the three pathways. All these TSs are lower in energy than the total energy of the separate reactants and can thus be easily achieved without the need of adding an additional energy to the reactants. With the ΔE# being 9 and 6 kcal/mol, the Si2Be and Si2Ca clusters again show better activation ability than the TM clusters and surfaces mentioned above with the ΔE# of the latter cases ranging from 10 to 30 kcal/mol12−16,18,25 (cf. Table 1). In comparison to Rh4Ru2 with the Rh−O bond in a recent study,32 the energy barrier in the Si2Ca case is slightly smaller. The formation of corresponding products 10, 11, and 12 from methanol and Si2M (M = Be, Mg, Ca) can be considered as exothermic processes with released energies of −37, −29, and −23 kcal/ mol, respectively. Compared to the singlet Si3, substitution of alkaline-earth metals for a Si atom on a Si3 cluster insignificantly changes the energy barriers for the O−H rupture in methanol. Nevertheless, the overall energy barriers, i.e., the relative energy of the least stable TS on a specific reaction pathway with respect to the total energy of the separate reactants, obtained for binary clusters Si2M through breaking the O−H bond are within the range of −9 to −27 kcal/mol. These values are much smaller than those of the pure silicon trimer whose overall energy barriers are calculated to be −0.3 and 1.6 kcal/mol via ts-3s1,5 and ts-3s2,6, respectively. The smaller the overall energy barrier is, the more easily the reaction happens. Figure 3b shows the schematic potential energy profile illustrating the cleavage of the C−H bond of methanol when interacting with Si2Be. This pathway is characterized by a relatively high energy barrier of 35 kcal/mol. However, the available energy of the system is still sufficient to break the C− H bond due to the large absorption energy of −36 kcal/mol. It is interesting to note that unlike the Si3 case ts-2Be-s1,13 found for C−H dissociation on Si2Be turns out to be located below the zero reference level, which would facilitate formation of the corresponding product 13. The rupture of the C−H bond via ts-2Be-s1,13 to yield 13 remains exothermic, similar to the case of Si3 but with a lower released energy of −5 kcal/mol. Two different C−H dissociation pathways on other binary clusters, namely, Si2Mg and Si2Ca, are also displayed in Figure 3b. Replacement of the Be atom with the Mg or Ca atom induces a slight increase in energy barriers via two TSs ts-2Mgs1,14 and ts-2Ca-s1,15, which are now of 40 kcal/mol in both cases. Even though such barriers are still quite high, the overall energy barriers are lowered significantly due to the large interaction energies of −23 and −17 kcal/mol obtained from the formation of the complexes cp-2Mg-s1 and cp-2Ca-s1. Unlike the previous cases, these two reactions forming the products 14 and 15 are endothermic. In other words, despite the fact of possessing a large energy barrier for the bond cleavage step, the dissociation of the C−H bond via ts-2Bes1,13 on the Si2Be cluster is still the most favorable channel, thanks to the large interaction energy between methanol and this cluster. The lowest overall energy barrier of this channel remains −1 kcal/mol below the reference state. The much higher energy barriers of C−H bond dissociation pathways, as

two other possible pathways for C−H breaking via TSs ts-3s1,4 and ts-3s3,7, on the other hand, have much higher energy barriers being 51 and 19 kcal/mol, respectively. Although the difference between the two latter TSs is considerable (ΔE# ∼ 32 kcal/mol), their corresponding products involving 4 and 7 are very close in energy (∼4 kcal/mol) in favor of the latter. As a result, the C−H bond dissociation reaction via ts-3s3,7 turns out to be the most preferred one among C−H bond cleavage pathways of methanol by Si3. To sum up, all reactions catalyzed by Si3 are exothermic, and the O−H breaking reaction via ts-3s1,5 to yield the stable product 5 is the most favorable channel in all reaction pathways happening under the effects of the singlet Si3 cluster. As discussed above, because of the small difference in energy between the singlet cp-3-s1 and the triplet cp-3-t4 complexes, it is of interest to explore the reaction pathways of methanol in the high spin state. Two possible channels for cp-3-t4 involving two different TSs, namely, ts-3t4,8 and ts-3t4,9, are displayed in Figure 2. While the reaction via ts-3t4,8 is exothermic with a net energy release of −23 kcal/mol upon a partial breaking of the O−H bond, the C−H dissociation via ts-3t4,9 is endothermic by an amount of 31 kcal/mol. The major reason for such a difference is that the former produces a stable product 8, whereas the latter yields a less stable set of products 9, involving a Si3 H and a hydroxymethyl (CH 2 OH) intermediate. Taking the weak hydrogen interaction into account, the barrier height of the latter is much higher than that of the former with an energy gap of 18 kcal/mol. Consequently, the O−H dissociation pathway is still dominant. Nevertheless, its barrier height is still much higher with respect to the Si3 singlet case. Overall, the present results reveal that methanol activation is more efficient in the presence of the singlet Si3 cluster. 3.2.2. Methanol Dissociation in Interacting with Si2M Clusters, M = Be, Mg, Ca. As analyzed above, the S−T energy separation of complexes Si2M−CH3OH is considerable with at least 18 kcal/mol in favor of the singlet state. Therefore, all possible dissociation pathways of methanol on the mixed clusters Si2M are only reported here on the singlet surface. For the purpose of comparison, potential energy profiles of each dissociation channel of methanol in the presence of Si2M with M = Be, Mg, and Ca are illustrated together in the same figure. The three O−H activation channels of methanol are presented in Figure 3a. Methanol is strongly bound to the Si2M clusters (M = Be, Mg, and Ca) with interaction energies of −36, −23, and −17 kcal/mol, respectively. The declining interaction energies in going from Be to Ca is consistent with the increase in M−O bond lengths of 1.59, 2.06, and 2.31 Å, respectively. The interaction of methanol on several TM clusters including Cu3, Cu4, Co4, and single-crystal surfaces involving Cu(111), Cu(110), Ir(111), Pd(111), Co(0001) or Au2Pd (111) and Au2Pd/(111), CunAum+... were extensively investigated in both experimental and theoretical studies.13,15−17,21,25,60−62 Their interaction energies are reported, as mentioned above, in the range of −4 to −40 kcal/mol. In general, the interaction energies between methanol and the Si2M clusters, especially with M = Be, are stronger than most previously reported values (cf. Table 1), except for the case of CuAu+ with the binding energy of −40 kcal/mol. As compared to the corresponding pure Si3 clusters, interactions between methanol and Si2M clusters are also stronger. Clearly, F


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cp-3Mg-s1 and cp-3Ca-s1, but with smaller adsorption energies of −22 and −19 kcal/mol. The reduction of the adsorption energy is in accord with the increase of M−O bond lengths. Accordingly, the bond lengths of M−O extend from 1.59 (Be−O) to 2.06 (Mg−O) and 2.33 Å (Ca−O). As expected, the adsorption energies on the corresponding binary clusters Si3M are much stronger than that of the pure Si4 cluster. However, interactions are only slightly stronger than those of the Si2M cases, apart from Si3Mg. It would seem that the larger the binary cluster size is, the stronger the adsorption energy becomes in the cluster sizes considered. By comparison with data given in Table 1, methanol binds more strongly to Si3Be than the other compounds apart from the diatomic cation CuAu+. Calculated results obviously show that the O−H dissociation channels have much smaller energy barriers as compared to the C−H breaking cases as expected (cf. Figure 5). Indeed, all the relevant TSs in the former case are lying below the reference points, which greatly facilitate the generation of resulting products, whereas all TSs of the latter are calculated to be located above the zero level. As a result, while formations of all resulting products in the former processes are exothermic with the released energies of −24 (Si3Be), −19 (Si3Mg), and −16 kcal/mol (Si3Ca), all the latter processes are endothermic. The stability of the products generated in both cases tends to decline gradually in going from Si3Be to Si3Ca, in accordance with the increasing M−O distances. The effect of a hydrogen bond on the energy barrier heights can be clearly seen here. The O−H dissociation pathways on Si3Be and Si3Mg via ts-3M-s1,18 have energy barriers of 13 and 9 kcal/mol, whereas the rupture of the O−H bond on Si3Ca takes place easily, characterized by a small energy barrier of ∼2 kcal/mol. It is due to the strengths of hydrogen interaction present within these TSs. It is well established that the electronegativity lessens upon descending from Be to Ca. The smaller the electronegativity of alkaline-earth metals M is, the more electronegative the oxygen is through the M−O bond. This leads to an increase in the stability of the resulting hydrogen interaction. Additionally, the presence of 3d electrons in the Ca atom makes a relatively valuable contribution to strengthen the Ca−O bond thanks to its ionic polarization. That is also the reason why the lowest-lying energy barriers of the O−H breaking pathway are found for both Si2Ca and Si3Ca clusters. A similar trend is also observed for the C−H dissociation paths. Indeed, the barrier height for this channel gradually declines from 41 kcal/mol on Si3Be to 39 and 35 kcal/mol on Si3Mg and Si3Ca, respectively. Despite the fact that the adsorption of methanol on the Si3M clusters is an energetically favored process with large interaction energies as discussed above, the reactants still need to overcome additional energies of 1, 17, and 17 kcal/mol corresponding with M = Be, Mg, and Ca to generate corresponding products 19-M. In summary, the O−H activation of methanol in the presence of Si3Ca is the most kinetically favored pathway as compared to the other ones. The calculated barriers ΔE# on binary clusters Si3M are generally smaller than those of other compounds given in Table 1. The most remarkable finding here is that our calculated O−H energy barrier on the Si3Ca cluster turns out to be smaller than all previous results collected in Table 1. These features indicate that earth-alkaline silicon clusters, especially the Si3Ca, behave as excellent catalysts toward methanol activation. They would be therefore

compared to O−H bond cleavage channels, on binary silicon trimers again indicate an important role of hydrogen interaction in stabilizing the O−H bond breaking TSs. Evidently, the O−H bond dissociation pathways are again confirmed to be more favored than the other ones. 3.3. Methanol Activation in the Presence of the Tetrameric Clusters Si4 and Si3M, M = Be, Mg, Ca. As for the Si2M cases, we only report in this section the possible pathways for dehydrogenation of methanol in the presence of the singlet Si4 and Si3M clusters due to their large singlet− triplet gap (ΔEST). 3.3.1. Methanol Dissociation in the Presence of Si4. Two hydrogen dissociation channels through O−H and C−H bonds from cp-4-s1 are shown in Figure 4. Although the interaction between methanol and the singlet Si4 in the complex cp-4-s1 is quite strong with an adsorption energy of −10 kcal/mol, the reactants still need to overcome the overall energy barriers of 12 and 45 kcal/mol, via ts-4s1,17 and ts-4s1,16, to produce the products. In this case, the TS connecting cp-4-s1 and the product 17 is much more stable than that between cp-4-s1 and 16 with a relative energy of 32 kcal/mol. The main reason for the difference is that the latter does not benefit from a stabilization effect owing to the absence of any hydrogen interaction. As a consequence, the C−H bond dissociation channel is not as favorable as the O−H bond breaking one. Both reactions are exothermic with released energies of −9 and −14 kcal/mol for the rupture of the O−H and C−H bonds, respectively. In general, the dissociation reactions of methanol happening on the Si4 cluster are less preferred in comparison with the most favorable pathways on the singlet Si3 cluster. 3.3.2. Methanol Dissociation with the Help of Binary Clusters Si3M (M = Be, Mg, Ca). Because the potential energy profiles for the dissociation pathways of methanol and the structures of TSs and products found on Si3Be, Si3Mg, and Si3Ca are rather similar, only profiles for the reactions of methanol in the presence of Si3Be are depicted in Figure 5. The

Figure 5. Schematic potential energy profiles illustrating the methanol dissociation reaction of methanol in the presence of the singlet Si3M cluster (M = Be, Mg, Ca). Relative energies in kcal/mol were obtained from CCSD(T)/CBS + ZPE computations.

corresponding energetic data for the two remaining clusters are given in purple and red in parentheses in the same figure. Structures of relevant TSs and resulting products are given in Figure S7 (SI). Analogous with the previous smaller binary clusters Si2M, the generation of stable complexes cp-3Be-s1 has a substantial exothermicity of −40 kcal/mol. This characteristic is also found for the formation of the other complexes, namely, G


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The Journal of Physical Chemistry C anticipated as being good catalysts for activating molecules similar to CH3OH, such as C2H5OH, NH3BH3, SiH4, ....

4. CONCLUDING REMARKS The first step of methanol activation reaction in the presence of small pure silicon clusters (Si3, Si4) and mixed alkaline-earth metal silicon clusters (Si2M, Si3M with M = Be, Mg, and Ca) was explored using the DFT method (the hybrid functional B3LYP with aug-cc-pVTZ (aVTZ) basis set). More accurate energies were obtained from single-point electronic energy calculations at the coupled-cluster theory at complete basis set CCSD(T)/CBS, with ZPE (B3LYP/aVTZ) corrections. The main conclusions emerging from our calculated results are as follows: (i) Interaction of methanol with pure and mixed alkalineearth metal silicon clusters insignificantly changes the S−T energy separation of the resulting clusters. All the most stable complexes have the singlet ground state. In the case of binary clusters, all lowest-lying complexes arise from an M−O interaction rather than the Si−O connection. (ii) The O−H dissociation pathways are predominant in all cases considered. (iii) In the case of Si3 and Si2M, although the O−H activation on the singlet Si3 cluster is kinetically preferred over those with the mixed Si 2 M clusters, it becomes less thermodynamically favored because methanol weakly binds to Si3. The energy barriers for O−H bond cleavage with the catalytic effect of the singlet Si3 and Si2Ca clusters were calculated to be lower with respect to previously reported results on different types of elemental clusters. (iv) Replacement of one Si atom in the Si4 cluster by an earth alkaline metal induces a considerable decrease in both O−H and C−H dissociation barriers, especially in the case of the Si3Ca cluster. The O−H dissociation pathway occurring with involvement of Si3Ca turns out to be the most kinetically favored channel as compared to all reaction pathways considered. The most remarkable result here is that the energy barrier computed for Si3Ca appears to be much lower than all values previously reported for catalytic effects of atomic clusters and surfaces. Besides Si3 and Si2Ca, the methanol activation happens efficiently in the presence of Si3Ca, and it is thus expected to induce a similar effect for the activation for the X− H bonds of other organic molecules. The catalysis by sizeselected elemental clusters in the gas phase needs further experimental and theoretical investigations to gain practical advances.62 This study can be considered as crucial proof for the catalytic ability of pure and mixed silicon clusters. These bestowed findings open a new avenue for seeking promising alternatives for the currently metal-based catalysts toward methanol activation and would stimulate experimental studies on the realistic reaction conditions for the X−H bond cleavages using small silicon clusters.

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methanol and silicon clusters are displayed from Figures S1 to S6. Figure S7 displays the shapes of transition structures and corresponding products of methanol dissociation on Si3Mg and Si3Ca clusters (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +32 16 32 73 61. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to the KU Leuven Research Council (GOA program, IRO and PDM fellowships) and FWOVlaanderen, and the Vietnam Ministry of Education and Training (911 scholarship).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02215. Tables containing the Cartesian coordinates of the lowest-lying complexes, transition structures, and products considered. The lowest-lying geometries, electronic states, and the CCSD(T)/CBS relative energies of all stable complexes formed following interaction of H


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