Role of Carbon Support for Subnanometer Gold-Cluster-Catalyzed

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Role of Carbon Support for Subnanometer Gold-Cluster Catalyzed Disiloxane Synthesis from Hydrosilane and Water Rameswar Bhattacharjee, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04419 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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The Journal of Physical Chemistry

Role of Carbon Support for Subnanometer Gold-Cluster Catalyzed Disiloxane Synthesis from Hydrosilane and Water Rameswar Bhattacharjee, Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur – 700032, West Bengal (India). *Email: [email protected]

ACS Paragon Plus Environment

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Abstract We study the mechanism for the formation of disiloxane from hydrosilane precursor using plane wave density functional theory (DFT). Si-O-Si bond linkage of disiloxane is an essential framework of many useful materials. The reactions are catalyzed by the presence of subnanometer Au-cluster on graphitic support. Graphene support lowers the energy barrier of the rate limiting O-H bond activation of water by ~ 4.0 - 9.0 kcal/mol. The rate limiting step can be further accelerated by the presence of defects in graphene support. Such high activity of Aucluster over carbon support is attributed to the enhanced charge transfer from cluster to the adsorbed substrate in presence of the support. Regeneration of the catalyst through dehydrogenation of H2 from (H2)Aun•••Gn is shown to be facile.


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1. Introduction The exceptional activity of metal nanoparticles is one of the fundamental topics in heterogeneous catalysis. In last decade, gold has taken a considerable attention in catalysis community due to its excellent catalytic activity.1-6 A series of factors such as concentration of low coordinated gold atoms, quantum size effects, interaction with underlying support, and existence of cationic and anionic gold are few vital factors that control the catalytic performance of supported gold particles.7-11 Gold in bulk is known to be chemically inert and therefore regarded as a poor catalyst, however, works led by Haruta, and Hutchings expose the great importance gold nanostructures for various kind of industrially important chemical transformation. The wellexplored reaction that involves supported gold cluster is CO oxidation by Haruta and is considered as the landmark.12 Besides this, C-C bond formation,13 olefin epoxidation,14,15 alcohol oxidation,4,16 H2O2 synthesis17 and selective hydrogenation18 are of few particular interest in the chemical industry. Even, a highly dispersed gold catalyst is very active for environmental applications like sensing hazardous gas19,20 or by removing offensive odors.21 Recently, several reactions which are catalyzed by supported gold nanoparticles, have been carried out in water which therefore offers an environmentally benign alternative to the conventional route that utilizes costly and harmful organic solvent.5,22 In fact, sometimes water plays a crucial role by activating the catalyst.23 In heterogeneous catalysis, the underlying support plays a critical role by enhancing the catalytic performance of the loaded metal cluster. In particular, metal-Support interaction (MSI) effects causes unique changes in the structural and electronic property of the combined system which is absent in the individual material.24 Recently, we shown that the TiO2 (110) support plays a key role for the C-H activation by small gold cluster.25 Most preferred supports for gold cluster (Aun) are metal oxides such as TiO2,26 Fe2O3,27 CeO2,28 MgO29 and


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Al2O330. However more recently, gold cluster supported on carbon, boron nitride (BN) and silica (SiO2) are also being employed for several reactions.31-35

Carbon based material has attracted considerable attention as a support of metal nanocluster. Among them particularly, graphene, a one atom thick carbon sheet has been found to be more promising owing to its unusual electronic and mechanical property.36-38 Due to its high specific area, graphene can offer an ideal support for growing and anchoring Aunanoparticles and hence have been exploited in heterogeneous catalysis.39,40 In this regard the contributions of Wilcox and co-workers is noteworthy.41,42 Thermally, a single layer of graphene is stable up to 500 oC in the air, as suggested from Raman Spectroscopy.43 However, after 520 o

C, defects appear, comprising of stone-wales, monovacancy and divacancy defects, among

which the divacancy type is thermodynamically more favored and consequently, more abundant in nature.44 Nevertheless, in presence of water, some of the dangling bonds at the edge of graphene may induce edge-oxidation, leading to the generation of hydroxylated graphene oxide regions. Metal–embedded graphene has also been experimentally fabricated with high thermal stability.45 By employing DFT, Giovannetti et al. observed that the interaction in metal-graphene can arise due to either chemisorption or physisorption depending upon the choice of metal46. Several studies found that platinum cluster over carbon support is an excellent catalyst for many important reactions. Particularly, experiment of Yoo et al. showed that subnanometer platinum cluster have high catalytic activity on graphene nanosheet.47 Many theoretical investigations also illustrated that nanocluster placed on graphene displays excellent activity by transferring charge from cluster to substrate.48-50 In the case of gold, limited examples of catalysis is known in literature where carbon is used as the platform of gold particles. In 1991, Hutchings showed that


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Au/carbon is a preferred catalyst for the hydrochlorination of acetylene.34 Also, carbon was shown to be more beneficial than oxide materials in the liquid phase oxidation of alcohols such as oxidation of D-glucose to D-gluconic acid.51 Tang and coworkers have reported that Au/C served as an efficient catalyst for oxygen reduction reaction (ORR).52

In recent times, much research has been devoted for the oxidation of hydrosilane to construct silicon(Si)-oxygen(O) linkage which is a useful building block of many essential polymeric materials like liquid crystal, thermosets and some bioactive compounds.53-56 A variety of methods has been developed (including homogeneous and heterogeneous catalysis) for the oxidation of hydrosilanes to form silanols and disiloxane. Amongst them, oxidation of organosilane with water by using heterogeneous catalyst is of our particular interest. Although few heterogeneous transition metals such as Ag, Pt, Pd, and Ni have been employed for the catalytic oxidation of hydrosilane,57-60 heterogeneous catalyst based on gold has been exploited more widely compared to others. For example, Kaneda and coworkers have shown that Au nanoparticles supported on hydroxyapatite was an active catalyst for this type of reaction.61 Subsequently, Doris et al. reported the first AuNps- carbon nanotube based catalytic system for the oxidation of silanes in THF with a turnover frequency of 18000 h-1.62 Yongfeng and coworkers have reported similar conversions using nanotube-gold nanohybrid.63 In this context, very recently research by Sawama et. al. shows that gold/carbon based catalyst can selectively oxidize hydrosilane to disiloxane as a major product.64 They have also used H2O as the solvent and oxidant. Further, in their study, H2 gas was shown to evolve during the course of reaction at room temperature. However, the exact mechanism of this conversion is not understood properly. The present work focuses on the mechanistic investigation of all elementary steps associated


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with this conversion. To this regard, we have modeled a comparatively large supercell of pristine graphene as carbon support and a three-dimensional small gold cluster containing 16 atoms of gold as an efficient model for AuNPs. It is shown that Si-H bond cleavage is extremely facile over AuNPs. O-H bond cleavage of water is found to be the highest energetic step of the overall reactions. The possibility of formation of disilane is ruled out as this step is computed to be highly endothermic. We have further introduced a 5-8-5 type of defect in graphene and investigated the overall process. Very interestingly, on defected graphene, the reaction becomes more favorable than pristine one.

2. Computational Methodology: All calculations were based on density functional theory in which wave functions were expanded with a plane wave basis set. Generalized gradient approximation functional proposed by Perdew, Bruke and Ernzerhof, also known as GGA-PBE,65 were employed for all density functional calculation in Quantum Espresso.66 Vanderbilt ultarsoft pseudopotential was used to describe the ionic cores.67 In all cases, the wave function and charge density were limited by kinetic energies of 30 Ry and 300 Ry, respectively. The method of Gaussian smearing was used with a smearing width of 0.01 Ry. Brillouin zone integration was carried out with 3×3×1 kmesh.68 Dispersion interactions was incorporated by using Grimme’s DFT-D2 empirical formalism.69 The convergence criterion for the ionic relaxation was set such that the total force must be less than 0.001 au. The climbing image nudged elastic band (CI-NEB) method was adopted to find the minimum energy path and for locating transition state of all elementary step.70 This expensive calculation was performed with 8 images connecting the initial and final structure at the Γ-point. For comparison, further single point energy calculations were performed


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on the structure of transition states (top image of the MEP) at

3 × 3 × 1 k-point mesh. To

support the consistency of our obtained results, we performed additional single point calculation by employing PW91-GGA functional.71

The charge density differences were plotted using the following general relation: ∆𝜌 = 𝜌!!! − 𝜌! − 𝜌! where 𝜌!!! is the net charge density of the optimized composite system, 𝜌! and 𝜌! are the charge density of fragments A and B at that geometry as optimized for A-B . Bader method of charge separating was employed in order to quantify the net charge transfer upon adsorption of different adsorbate.72 For the charge density difference, Bader charge analysis and partial density of state (PDOS) analyses, single point energy calculations were done using PAW-PBE65 functional as implemented in Vienna Ab Initio Simulation Package (VASP).73 For that calculation, Brillouin zone was sampled with a higher k-point grid of 5 × 5 × 1. For DFT-D2 calculations in VASP, we have used dispersion coefficient (C6) and vdW radius (R0) as 40.62 J nm6/mol and 1.772 Å, respectively, for gold.74 The average energy of the d-band, also known as d-band center (εd),75 can be calculated using the following expression 𝜀! =

! 𝐸∙𝑁 𝐸 !! ! 𝑁 𝐸 ∙ !!

∙ 𝑑𝐸 𝑑𝐸

Graphene sheets were represented by applying a 7 × 7 supercell consists of 98 carbon atoms. The supercell was made using an optimized unit cell of the calculated C-C bond length of 1.42 Å and lattice parameter of 2.46 Å which is in consistent with the experimental value.76,77 A vacuum region of 30 Å in the Z-direction was used to avoid spurious interaction between the


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periodic images. Double vacancy defect was introduced by removing two adjacent carbon atoms form the large supercell of graphene. Herein, we considered a 5-8-5 type of defect which is one of the most common defects occurs in graphene and also known to be stable even at a higher temperature.78,79 We have taken a small gold cluster consist of 16 gold atoms which should be an efficient model of gold nanocluster and often chosen in the literature as a model of nanocatalysts.50,80 The initial geometry of Au16 cluster was taken from a previous report.81 All calculations regarding the catalysis on bare Au16 cluster were performed using a periodic box of dimension 30Å × 30Å× 30Å.

3. Result and Discussion: We considered a rather large supercell (7×7) of graphene (G) sheets to ensure the isolation of gold cluster over the surface. Figure 1(A) shows the optimized structure of Au16 cluster loaded over pristine graphene (2). The cluster is adsorbed through Au..π type of interaction which is also known as physisorption as no chemical bond is formed between gold and carbon atoms. The binding energy of Au16 with graphene is computed to be around 0.50 eV per adatom. The equilibrium distance of the gold cluster over graphene is around 2.8 Å as portrayed in the figure. The optimized structure of Au16 is hollow cage type as shown in the inset of Figure 1(A). However, while optimized over graphene, the gold cluster is reconstructed to achieve maximum stabilization over graphene. Moreover, a p-type doping of the graphene is predicted due to adsorption of the gold cluster as 0.1 e- net charge transfer occurs from graphene to gold. In fact, there are approximately 18 carbon atoms of graphene sheets which are covered by Au16 cluster; therefore around 0.006e- average charge transfer is estimated from each carbon


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atom. It is worth mentioning here that 18% of the carbon atoms are covered by gold atoms. Electron charge density difference (CDD) plot shows that the maximum charge distribution occurs at the contact region of Au16-graphene composite system. The charge transfer from graphene to the gold cluster was also confirmed experimentally by Liu and co-workers.82 Further, the total density of states (TDOS) of the Au16/graphene along with the partial density of state (PDOS) of all orbital of carbon atoms of graphene and the PDOS of the 5d orbitals of the adsorbed Au16-cluster are shown in Figure 1(C). The d-band center of Au 5d band is found to be at -2.67 eV which is often identified as a crucial parameter that characterize the activity of delectrons of metal. In fact, the position of d-band center estimates the ability of the metal to adsorb a substrate. Also, when no absorption of gold cluster on graphene occurs, the pattern of dbands of the cluster is different as shown in the supporting information (Figure S2).


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Figure 1: (A) Optimized structure of Au16/G, (B) Isosurface of charge density difference, ∆ρ = ρ(Au16/G) – ρ(Au16) – ρ(G), isosurface value= 0.0005|e|/Bohr red and green surface represents the positive and negative section, respectively. C) Total density of states (TDOS) of Au16/G (black dash line) along with projected density of states (PDOS) of 5d of Au atoms (red) and PDOS of carbon atoms (blue). In this section, we will decipher a full mechanistic picture of the disiloxane formation based on Si-H and O-H bond activation of dimethylphenylsilane (I) and water, respectively. The catalytic cycle starts by the adsorption of hydrosilane (I) on the top of the gold cluster to form a reactant complex (3). This adsorption is calculated to be 19.1 kcal/mol exothermic in nature and therefore thermodynamically favorable. We have also considered the absorption of I in the Au-


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graphene interface area which is energetically less favorable as the adsorption energy is computed to be -17.5 kcal/mol. The adsorbed hydrosilane then undergoes Si-H bond activation and it is found to be highly favorable as it requires a very small activation barrier of 2.0 kcal/mol. The structure of the transition state (TS1) is shown in figure 3 which looks very similar to the reactant complex (3) and therefore can be classified as an early transition state. In a similar study, Yoshizawa and co-workers found that the dissociation of Si-H bond is very facile on Pd nanoparticles and almost a barrierless process.83 It is noteworthy to mention here that after Si-H cleavage, Si-Au and H-Au bond formation occur at adjacent Au-center instead of on the same Au-atom which results in an intermediate 4. This step is computed to be highly exothermic (∆E= -29.5 kcal/mol). Hence, Si-H bond cleavage over graphene supported gold cluster is kinetically as well as thermodynamically favorable. Following this, the reaction can proceed via two plausible pathways from 4. In the first path, 4 can take another molecule of dimethylphenylsilane (I) and go through the second Si-H cleavage to form an intermediate 5 which can further undergo Si-Si bond elimination and form 6, a complex of 1,3-diphenyl-1,1,3,3-tetramethyldisilane (II). However, this choice is discarded as Si-Si elimination is observed to be highly endothermic (∆E=38.4 kcal/mol). The optimized geometry of 5 and 6 are depicted in figure 3. The high endothermicity of this particular step may be attributed to the fact that construction of one Si-Si bond cannot compensate the rupture of two strong Au-Si bonds. The second path involves the adsorption of a water molecule on 4 to form a weak complex (7) followed by the dissociation of an O-H bond of water to form an intermediate complex of dimethylphenylsilanol (8). Adsorption of the water molecule on 4 is calculated to be exothermic by 9.7 kcal/mol. We have computed the minimum energy path (MEP) of the O-H bond cleavage of water using NEB method. As shown in figure 3, O-H bond dissociation at Au-center adjacent to Au-Si bond goes through


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transition state structure TS2 where Au-H, Au-O and O-H bond distances are 1.58Å, 2.20Å and 1.73Å, respectively. The energy of the transition state is 28.7 kcal/mol higher than the corresponding reactant complex and this step is exothermic by 5.0 kcal/mol. For further verification, we again performed the CI-NEB calculation with 12 images for this step and found a negligible difference compared with the aforesaid result that indicates the accuracy of our chosen method. Another possibility suggested in the previous report such as backside attack by a water molecule to produce silanol was also considered.83 However, we found that backside attack by water to break the strong Si-Au bond in order to form silanol is impossible over the potential energy surface. It is noteworthy to mention here that silanol can be separated from 8 although O-H bond of silanol is found to be reactive for further reaction. The energy required for the isolation of silanol is 16 kcal/mol. Finally, adsorbed dimethylphenylsilanol (III) can undergo O-H bond dissociation further and generate 11 where Si-O-Si linkage in 1,3-diphenyl-1,1,3,3tetramethyldisiloxane (IV) is formed. This step is 10.1 kcal/mol exothermic and goes through an MEP that puts TS3 at the top of the energy surface with an energy barrier of 22.6 kcal/mol. The removal energy of disiloxane (IV) is computed to be ~30 kcal/mol. Here, some phase transfer catalyst might be useful for transferring the product from one phase to another that facilitate the regeneration of the catalyst.84 To verify the dependence of DFT functional, we have performed single point calculation on PBE optimized geometry using one more functional of Perdew and Wang (PW91) and confirmed that there is a negligible difference between the results obtained from PBE and PW91. In the overall process, O-H bond cleavage of water has the maximum energy barrier and therefore this elementary step is the most influential step and might contribute maximum to the overall rate of the reaction. In fact, according to energy span model developed by Kozuch and Shaik,85 7 and TS2 are identified as the TOF determining intermediate (TDI) and


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TOF determining transition state (TDTS) of the entire catalytic cycle. As this particular step is recognized as the slowest and most crucial one, we have computed the energetics of this elementary step over bare Au16 cluster in order to estimate the influence of the carbon support in this reaction. Interestingly, it is observed that in absence of graphene support, the activation barrier for O-H bond dissociation of water gets significantly elevated. The calculated barrier for O-H cleavage on the bare Au16 cluster is 32.6 kcal/mol, 4.3 kcal/mol higher compared to the similar process where graphene has been used as support. Hence, graphene is found to play a vital role in this conversion by decreasing the activation barrier of the rate limiting step.80,82,86 The energy profile diagram for the disiloxane formation on the bare Au16 cluster is shown in the supporting information (Figure S3).

Figure 2: Schematic energy profile for the transformation of hydrosilane to disiloxane using water. All energies are given in kcal/mol unit. ∆EPBE and ∆EPW91 represent the energies using PBE and PW91 DFT functional.


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Figure 3: Optimized structure of various intermediates, transition states (TS), along the potential energy surface as discussed in figure 2. Only relevant hydrogens are shown for clarity. Important bond distances are given in Å.


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It is observed that during the course of the reaction, the Au-cluster becomes hydrogenated. In the proposed mechanism, two hydrogenated-Au16/graphene intermediates are recognized (9 and 12). This chemisorbed hydrogen might reduce the activity of the catalytic system and therefore removal of hydrogen is an important aspect in order to reactivate the catalyst. For this purposes, we have computed the energetics of this process which is shown in figure 4. The hydrogenated intermediate 9 is taken under consideration for investigation of the reaction. In figure 4, the optimized geometry of 9 is depicted where one hydrogen atom is attached to the two adjacent gold and somewhat relaxes Au-Au bridge. The average bond distance of Au-H is found to be ~1.75Å. The energy required to reach the transition state (TS4) for H2 elimination is 19.1 kcal/mol. In TS4, the two hydrogens are bonded to one Au-atoms with Au-H bond distance of ~1.71Å and H-H bond distance of 1.04 Å. This step is computed to be endothermic by 13.2 kcal/mol. However, if free energy is considered, the endergonicity would be much lower as high entropy gain is expected due to the release of di-hydrogen (H2). Further works on the use of this hydrogenated species (9) for some essential hydrogenation reactions are going on in our group which will be reported in due course.

Figure 4: Optimized structure of hydrogenated intermediate 9, transition state for the H2 generation (TS4) and the corresponding product (13). The energy of 9 is taken as reference and


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the energy of TS4 and 13 are calculated accordingly. Energies given in bracket are calculated using PW91 functional.

Based on the above findings we have proposed a catalytic cycle of disiloxane (IV) formation from hydrosilane (I) and water. The catalytic cycle is considered to begin with the absorption of hydrosilane where the catalyst (A) activates the Si-H bond of I with the formation of an intermediate complex (B). The crucial step then take places wherein B interacts with water and dissociate its O-H bond to from first silanol (III) which would further react with the SiMe2Ph attached with gold cluster and generate the desired product disiloxane. The entire cycle is portrayed in Scheme 1.

Scheme 1: Schematic representation of disiloxane formation from water and hydrosilane using Au/C catalytic system. Our results showed that pristine graphene is a useful support for Au-cluster and provides a mechanistic advantage for disiloxane synthesis from silanes and water. To tune the reaction


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further, we carved a vacancy on graphene and computed the entire catalytic process. Note that the existence of a vacancy in graphene is confirmed experimentally87. Particularly, a vacancy pair was created by removing two adjacent carbon atoms. A divacancy is a stable defect in graphene even at a high temperature as mentioned earlier.78,79 The removal of two carbon atoms lead to form two five membered rings and a reconstructed eight membered ring also known as 58-5 type of defect, see figure 5 . The formation energy (Ef) of the defect is calculated to be 7.14 eV which is in fair agreement with the reported value.88 Ef is calculated using the following formula 𝐸! = 𝐸!"# −

𝑛−2 𝐸!"#$ 𝑛

Here, Evac and Epris are the total energies of the defective and perfect graphene, respectively. “n” stands for the number of carbon atoms in the supercell of pristine graphene which is 98 for the present situation. The optimized C-C bond lengths of the defected graphene (G2v) are in consistent with the reported parameter.89 Next, Au16 cluster is loaded over the defected graphene and optimized. The optimized structure consists of one gold atom directly forming a bond with carbon with an Au-C bond distance of 2.22 Å, see figure 5(A). Removing of carbons results breaking of sp2 bond in graphene leading to the hybridization of 2p orbitals of carbon and 5d orbitals of gold nearby the defect and thus a strong interaction is observed.90,91 Consequently, the binding energy for Au16/G2V system is greater (0.58 eV/adatom) than that of the obtained binding energy for Au16/G. Besides, a significant orbital interaction is seen from the visual inspection of the valence bond maxima (VBM) and conduction band minima (CBM) which further confirms bonding interaction between the gold cluster and graphene (figure S6). Moreover, the d-band center of Au16 over defected graphene is calculated to be at -2.79 eV which is slightly downshifted relative to εd of Au16 over pristine graphene. We have also provided the change of


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the d-band pattern of Au16 cluster while placed over perfect and defected graphene and compared this with the d-band of the free standing Au16 cluster, see figure S7. On the other hand, the total charge gathered on Au16 is very small while placed over G2V, however, a significant charge redistribution is observed from the CDD plot of the total system, see figure 5 (B).Thus, the electronic properties of Au16 are altered by small extent when placed on defected graphene compare to perfect one. In next paragraph, we will discuss the catalytic performance of the Au16/G2V for the reaction of our interest i.e. disiloxane (IV) preparation from silane(I) and water.

Figure 5: (A) Optimized structure of Au16/G2V, (B) Isosurface of charge density difference, ∆ρ = ρ(Au16/G2V) – ρ(Au16) – ρ(G2V), isosurface value= 0.0005|e|/Bohr red and green surface represents the positive and negative region, respectively. (C) Defective graphene sheet with two


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C vacant (G2V), D) Total density of states(TDOS) of Au16/G2V hybrid (black dash line) along with projected density of states (PDOS) of 5d of Au atoms (red) and PDOS of carbon atoms (blue).

For examining the similar mechanism on Au16/G2V, we followed the minimum energy route as established earlier in this article where Au16/G has been used as an active catalyst. The aim of this investigation is to find out the role of the defect in the graphene on the overall energetics the reaction. Energy profile diagram for the entire catalysis over Au16/G2V is shown in Figure 6. The first step of this reaction involves Si-H bond cleavage that initiates with the adsorption of hydrosilane (I) which is 11.1 kcal/mol exothermic. The stronger adsorption of Aucluster with defected graphene support contributes to the low adsorption energies of hydrosilane which is also reflected from the lower value of d-band center. In this case, we have not computed expensive MEP to calculate the energy barrier for Si-H bond dissociation as it has been found to be extremely facile over Au16/G (∆E‡ =2.0 kcal/mol) and the change in the activation energy in presence of defect should have a negligible effect on the energetics of the overall reaction. We have computed the exothermicity of this elementary step as 31.8 kcal/mol. The structure of the reactant (15) and product (16) associated with the Si-H bond cleavage are displayed in figure 7 which is similar to that obtained in the case of Au16/G. 16 would interact with water and form an intermediate 17 which would subsequently undergo O-H bond dissociation to build another intermediate named 18 (a weak complex of dimethylphenylsilanol) via transition state structure TS5. The activation energy for this O-H bond dissociation step is computed to be 23.5 kcal/mol. It is noteworthy to mention here that this step was identified as the rate limiting one and the activation barrier for the similar step are computed to be 28.7 kcal/mol and 32.6 kcal/mol over Au16/G and over bare Au16, respectively. Clearly, a significant reduction of the energy barrier for


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the O-H bond activation is observed in presence of defected graphene (G2V) as a support for the gold cluster. Finally, dimethylphenylsilanol would further react with Au-cluster to undergo O-H bond dissociation as well as Si-O-Si bond formation simultaneously that leads to from a highly stable intermediate 21 where disiloxane (IV) remains physisorbed on the gold cluster. Although the detachment energy of IV is computed to be ~35 kcal/mol, it would be smaller if free energy is considered and even can be facilitated by phase transfer catalyst (PTC) as mentioned earlier.84

Figure 6: Computed potential energy surface for the formation of disiloxane (IV) on Au/G2V.


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Figure 7: Optimized structure of all intermediates and transition state as discussed in figure 6.

For a critical interpretation of the assistance provided by the perfect and defected carbon support (G and G2V) for catalyzing the reaction, we have examined charge density difference (CDD) of both reactant complex and transition state for the O-H activation on Au16/G as well as on Au16/G2V in order to visualize the direction and magnitude of charge transfer between catalyst and adsorbates during the reaction (in this case adsorbates is water molecule). It is seen that when water gets adsorbed on intermediate 4 to build another intermediate 5, a slight charge of


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0.04 e- is transferred from 4 to water whereas in the corresponding transition state (TS2) the amount of charge transfer is comparatively greater that is about 0.28 e- along the same direction. Therefore, during O-H activation, gold cluster transfers electron density to the adsorbed water molecule. We have also performed similar kind of analyses for the corresponding reactant complex (17) and transition state (TS6) where defected graphene has been used as the platform for the gold cluster. Interestingly, in 17 a similar charge transfer of 0.04 e- is detected along 16 → H2O but in TS6 a net charge transfer of 0.52 e- is transferred along that direction. Thus, G2V is found to promote a greater amount of charge transfer from cluster to water and facilitates the OH bond cleavage of water. CDD plots of the concerned transition states (TS2 and TS5) are depicted in Figure 8. The change in the pattern of sp-bands of water upon adsorption on Au16/G and Au16/G2V are shown in the supporting information. Also, we have plotted electron density isosurfaces for the valence bond maximum (VBM) and conduction band minimum (CBM) of the TS2 and TS5 which is shown in the same figure. It is seen that VBM and CBM have a significant contribution from the O and H atoms of the water molecule. To gain a deeper insight, we have used distortion-interaction model which gives an estimation of activation energy (Ea) by dividing it into distortion energies (∆Edist) and interaction energies (∆Eint).92,93 We recently employed this model to explain the origin of reactivity and selectivity of various reactions.25,94-96 ∆Edist denotes the energy associated with the deformation of each fragment in the reactant complex in order to reach transition state structure on the other hand ∆Eint stands for the additional interaction energy (relative to the reactant complex) between fragments in the corresponding transition state. Along 7 → TS2, ∆Edist and ∆Eint are calculated as 76.0 kcal/mol and -47.3 kcal/mol, respectively resulting Ea=28.7 kcal/mol. Similarly, for 17 → TS5, ∆Edist =137.5 kcal/mol and ∆Eint = -114.0 kcal/mol leading to an activation energy of 23.5 kcal/mol.


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Therefore, although distortion energy is higher for the O-H activation on Au16/G2V, greater interaction in the transition state offering a lower activation barrier for the step compared with the similar process on Au16/G. Similar calculations are carried out for the O-H activation on the bare Au16 cluster and it is observed that the higher activation energy for the same is mostly due to less interaction of the distorted fragments in the transition state relative to the energy required to distort them to construct transition state. The details are given in the supporting information.

Figure 8: First row displays the isosurface plots of CDD, VBM and CBM of TS2 and the second row is for the similar plots of TS5. For charge density difference (CDD) isosurface value= 0.001|e|/Bohr is used, red and green surface represents the positive and negative region, respectively.


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4. Conclusion: In summary, we demonstrate a first principle investigation of the effects of carbon support on the catalytic activity of Au16 cluster for the formation of disiloxane from hydrosilane and water. Both perfect and defected graphene has been employed in the present study. We found that Si-H bond activation is highly facile on the gold cluster as it requires a little activation barrier and the step is computed to be highly exothermic. In the entire catalytic process, O-H bond activation of water comprises of highest energy barrier and therefore this step is identified as the maximum contributor to the rate of the entire transformation. It is observed that graphene supports greatly enhances the reactivity of Au16 cluster by reducing the reaction barrier of rate limiting O-H bond activation of water. Meanwhile, the formation of disilane is found to be highly endothermic and therefore possible route of the reaction through disilane formation has been ruled out. On the other hand, activation of O-H bond of silanol is found to be facile on supported gold cluster that leads to the formation disiloxane as a major product. Both pristine and defected graphene offer a useful support of the Au-cluster by reducing the barrier of O-H activation from 32.6 kcal/mol for the case of bare Au16 to 28.7 kcal/mol and 23.5 kcal/mol in the case of Au16/G and Au16/GV, respectively. It is further argued that defected graphene is superior over the pristine analog as it eases O-H bond dissociation by an amount of 5.2 kcal/mol. Also, we have established that graphene promotes the charge transfer from cluster to water molecule which is found to be crucial for the O-H activation. The higher activity of Au16 cluster over defected graphene is a manifestation of a greater amount of charge transfer from cluster to adsorbed water. Distortion-interaction analyses reveal that the lower energy barrier for O-H activation of water on gold cluster supported on defected graphene (G2V) is due to the higher interaction between the distorted fragments in the transition state. We gained a valuable insight


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from this investigation that would be useful in various perspectives. Water is a green solvent and therefore using water as the solvent as well as oxidant is a benign approach in organic synthesis. The present article clearly displays the role of underlying graphene support in water activation by transferring electron density to the metal cluster and accelerates the formation of silanol followed by disiloxane. The defects in graphene not only stabilize the cluster but also increase the reactivity of the same. Besides, with further modification, the hydrogenated cluster may be used as a hydrogen source for the different purpose. Our results may provide a unified understanding of the current experimental result and would be beneficial for designing new graphene based nanocatalysts in future.

Acknowledgements RB thanks, CSIR India for SRF. AD thanks, INSA, DST, and BRNS for partial funding. Author Information *Corresponding Authors: [email protected] Notes The authors declare no competing financial interest. Supporting Information Valence bond maxima (VBM) and conduction band minima (CBM) of some important intermediate and transition state. Partial density of state (PDOS) of Au16 -cluster 5d band in some relevant intermediate and transition state. Schematic energy profile for the transformation of hydrosilane to disiloxane on free standing Au16 cluster and structures of all intermediates. Distortion-Interaction analyses for the O-H activation on bare gold cluster. Total density of state


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(TDOS) along with partial density of state of TS2 and TS5 and change in the partial density of states (PDOS) of sp- bands of water due to adsorption.

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Role of Carbon Support for Subnanometer Gold-Cluster-Catalyzed

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