Supported Sub-Nanometer Gold Cluster Catalyzed Transfer

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Supported Sub-Nanometer Gold Cluster Catalysed Transfer Hydrogenation of Aldehydes to Alcohols Rameswar Bhattacharjee, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07520 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Supported Sub-Nanometer Gold Cluster Catalysed Transfer Hydrogenation of Aldehydes to Alcohols

Rameswar Bhattacharjee, Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur – 700032, West Bengal (India). *Email: [email protected] Abstract The ability of sub-nanometer sized Au-clusters to activate small molecule is well known. Nevertheless, typical experimental situations involve loading of the bare Aun (n < 10) on oxide surfaces. Recent plethora of literature indicate that supported gold clusters are extremely potent in catalyzing molecular transformation. In this work, we examine the role of the supporting substrate namely TiO2 (110) in enhancing the activity of a small Au8 cluster for an industrially important reaction viz. conversion of benzaldehyde into benzyl alcohol. The barrier for rate limiting C-H bond activation of the solvent/H-donor gets reduced by ~4 kcal/mol over a TiO2(110) surface with respect to its unsupported analogue. The activation energy (Ea) for the catalytic transfer hydrogenation (CTH) involving transfer of two hydrogens, one each from the solvent and the hydrogenated Au-cluster simultaneously to the aldehyde is also reduced significantly. Strong Au8-TiO2(110) interactions result in charge transfer thereby making Aucluster electron deficient which assists by activating the rate limiting C-H bond cleavage. The present article provides a microscopic picture for catalysis of synthetically important complex reactions by supported Au-clusters.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

Introduction Metal nanoparticles because of their relatively large exposed surface and high effective charge have been explored as catalysts in a variety of industrial and chemical processes.1-2 Apart from an interest in structure and bonding of low-dimensional systems like bare metal clusters3-4 typical experimental conditions involve loading of such clusters on oxide surfaces.5-6 The commonly used surfaces on which the Au-clusters are supported are TiO2,7 Fe2O3,8 Al2O3,9 MgO,10 CeO211 and SiO2.12 Understanding the role of supporting oxide surface in enhancing the activity of bare metal clusters has been a subject of intense activity.13-14 The field was pioneered by Hutchings and Haruta .15 Based on synchrotron based photoemission spectroscopy and first-principles DFT slab calculations, Rodriguez et al. showed that Au/TiO2 (110) is chemically more active than metallic gold and bare TiO2 for dissociation of SO2 due to explicit bond formation between Au and titania.16 Hammer and co-workers have shown that the oxide substrate mediates in the charge-transfer process in CO → CO2 conversion over a Au7 cluster deposited over TiO2 (110) surface.17 Based on infrared-kinetic measurements, Green et al. showed that CO first reacts with TiO2 sites followed by Au and the catalytic activity occurs in the perimeter of the Au nanoparticle.18 Hu and co-workers demonstrated that the oxide activates O2 towards CO oxidation by transfer of electron to the anti-bonding molecular orbitals of O2.19 Similar electron transfer effects were also observed by Marx and co-workers for methanol oxidation over Aun (n=11-16)/TiO2(110) interface.20 Based on STM imaging, photoemission and DFT calculations, Sanz and co-workers concluded that Au…oxide interface is essential for dissociation of H2O in water-gas shift (WGS) reaction.21 A significant boost towards an understanding of the TiO2…Aun interaction has been provided by Jiang and co-workers which provided experimental


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


evidence for charge transfer from TiO2(110) surface to Au clusters through X-Ray photoelectron spectroscopy.22

Figure 1: Catalytic cycle for transfer hydrogenation of carbonyls over Au/TiO2.

Inspite of the mentioned efforts, examples of Au nanoparticle catalyzed chemical transformations assisted by TiO2(110) surface have been restricted to small molecule activation like CO oxidation, WGS reaction23 and methanol oxidation.24 Such reactions involve only a few barriers. An important addition to this list has been the mechanistically complicated reduction of aromatic aldehydes and ketone to their corresponding alcohol in presence of a hydrogen donor like iso-propanol as solvent.25 The catalytic loop consists of C-H activation by the Au-cluster followed concomitant transfer of two hydrogen (one from the cluster as hydride and one from the solvent as proton) simultaneously towards R-CO-R’ forming R-CH(OH)-R’. Figure 1 shows the


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

catalytic cycle. Similar transfer hydrogenation (TH) of aldehydes by Ru-nanoparticles on Fe3O4 and Au-nanocluster over CeO2 have been recently reported.26-27 It is worthwhile to mention that TH of C=O and C=N bonds by ruthenium complexes is well-known in homogeneous catalysis and has been utilized for asymmetric hydrogenation of ketones by Noyori and co-workers with great success.28-29 The mechanism for TH catalyzed by transition metal complexes have subjected to detailed kinetic and computational studies.30-32 Unfortunately however, the mechanism for TH assisted by metal clusters supported over oxide surfaces is yet to be understood. This becomes appealing considering that these heterogeneous catalysts have yields as high as 99%.22,

26

In the present letter for the first time, we provide a mechanistic

understanding of such extremely efficient TH of aldehydes by metal clusters supported over oxide surfaces.

Computational Methods and Models Periodic DFT calculations for the two layer rutile TiO2 (110) surface,33 having 72 atoms in each layer (24 Ti atoms and 48 O atoms), were performed using the PWSCF (plane-wave selfconsistent field) code with the generalized gradient approximation (GGA) and Perdew–Burke– Ernzerhof (PBE) functional34 as implemented in the QUANTUM ESPRESSO package.35 The ionic cores were taken care of by an ultrasoft pseudopotential.36 The upper layer of the surface were relaxed while the bottom layer kept fixed at their bulk position by taking a 2 × 2 × 1 k-point mesh.37 The cell parameters for the TiO2 (110) surface are a=17.87 Å, b=12.77 Å and c=30 Å and α=β=γ=900. An Au8 cluster of Td symmetry over the optimized TiO2 surface was also relaxed using the same k-point. The free Td Au8 cluster in gas-phase was optimized within a


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cubic box of dimension 25Å ×25Å× 25Å. The energy difference between the planar and Td structure in Au8 is very small (~ 3 kBT) and hence, a tetrahedral Au8 cluster over TiO2(110) should be reasonable model for a nanocluster.38 For further study of reaction we have frozen the TiO2 surface and used 2 × 2 × 1 k-point mesh. In all cases, a kinetic energy cutoff of 35 Ry39-40 and charge density cutoff of 330 Ry were used. Dispersion interactions were taken into account by using Grimme’s DFT-D2 empirical formalism.41 Nudged elastic band (NEB)42 with climbing image method was used to calculate the minimum energy path (MEP) and the energies of structures along the pathway that connects relevant species for CH activation and transfer hydrogenation. The NEB calculations were performed with 8 images between the initial and final structure at the Γ-point. For comparison, additional single point energy calculations were carried out on the structure of transition states (top image of the MEP computed through NEB) at 2 × 2 × 1 k-point mesh. For calculating charge density a Bader charge -population analysis43 as implemented in Vienna Ab Initio Simulation Package (VASP) was utilized.44 We set a higher kpoint grid of 5 × 5 × 1 and energy cutoff =400eV for this calculations. We have used a dispersion coefficient, C6=40.62 Jnm6mol-1 and R0= 1.772 Å for vdW radius of gold atom

45

within VASP 5.3 as gold is not included in the elements within the original DFT+D2 implementation.41

Results and Discussion Experiments clearly show that no reactions are observed on Au-free TiO2 catalysts and a synergetic influence of TiO2 surface on gold nanoparticle provided the maximum advantage for TH. We consider a rather large optimized 6×2 supercell of bilayer TiO2 (110) surface to properly


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

reproduce experimental Ti-Ti, Ti-O and O-O distances and eliminate effects of dangling bonds/unsatisfied valency on the surfaces46 (see Supp. Info. File). Although slab model of TiO2 (110) surfaces are known to exhibit oscillating behaviors with the number of layers, we have considered a bilayer model with a large surface area to make it computationally tractable.47 A large supporting surface ensures isolated Aun as expected for small concentration of Au on TiO2 as experiments indicate the lack of any agglomeration of clusters and the clusters remain as isolated clusters over the surfaces.25 The choice of a computationally tractable and experimentally relevant model of Au-cluster supported over TiO2 requires careful calibration. The lowest energy conformation of small Aun clusters (n ≤ 13) are known to be planar though the exact size at which 2D-3D transition occurs in Aun is debatable and expectedly, dependent on the methods of computations.48-51 Of particular interest is the Au8 cluster which reveals nonplanar D2d and Td symmetries at MP2 (Møller–Plesset perturbation theory of second order)/SBKJC(1f) (effective core potential developed by Stevens, Basch, Krauss, Jasien and Cundari) level and CCSD(T) (coupled cluster singles and doubles with noniterative perturbative triples correction )/SBKJC(1f) // MP2/SBKJC(1f) level respectively.52 It is noteworthy to mention that DFT calculations using B3LYP and PW91 functionals predict the planar isomers with D2h and C2v symmetries.52 However, the relative stability of the non-planar clusters is due to the contribution from the triplet excitations for each of the isomers considered in the study. More recent, implementations in DFT including the long-range dispersion corrected M06-L within the same ECP basis set predict planar “coverleaf” structure which remains consistent at the CCSD(T)/ SBKJC(1f) //PBE/LANL2DZ (Los Alamos National Laboratory 2-double-z) level of theory.53 Nevertheless, tetrahedral Au8 is slightly higher in energy compared to the planar one and can serve an efficient model of a three dimensional cluster as mentioned before in the


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


computational section.38 It is important to note that selection of a non-planar model for Au8 (Td with dAu-Au=2.70 Å at our level of computations) is essential to describe catalytic TH as the aldehyde and the hydrogen donor solvent interact along different faces of the Aun cluster. Similar tetrahedra like geometry of Au8 over MgO (001) has been used as a model to rationalize the low temperature oxidation CO by Landman and co-workers.54

Figure 2: (A) Structure of Au8 supported on TiO2 (110) surface; Inset: Optimized configuration of bare Au8 cluster (B) Isosurface of charge density difference, Δρ = ρ(Au8+TiO2) – ρ(Au8) – ρ(TiO2)] with isosuface value of 0.0009|e|/Bohr3. Olive and cyan surfaces represent electron deficient and rich regions respectively.

The optimized configuration of a gold octamer adsorbed on TiO2 is shown in Figure 2 (A). The tetrahedral bare Au8 cluster with dAu-Au=2.70 Å exhibits a structural distortion on being loaded on the surface which we attribute to the structural fluxtionality of metal clusters, in


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

general.48-53 The shortest distance between Au and the O atoms on the top layer of TiO2 (110) is ~ 2.22 A, implying rather strong covalent interactions between Au8 and TiO2 surface. Similarly, there are one more Au-O bonds with Au-O distances of ~ 2.80 Å resulting in considerable interaction. In fact, the binding energy of the gold cluster with titania surface is computed to be 3.87 eV supporting strong stabilization of the the gold particles on TiO2 and is consistent with previous reports.55 Clearly, the distortion undergone during the structural change of gold cluster over titania surface is well compensated by the Au…TiO2 interaction. Bader charge analyses for the charge density difference (Δρ) between the cluster and TiO2 surface revealed a charge transfer (CT) of 0.23e along Au8 → TiO2 (Fig. 2 (B)). Hence, chemisorption of gold nanoparticles on TiO2 is accompanied by a moderate CT that renders Au8 little electron-deficient and activates it for reaction with small molecules. Several previous studies have reported that slightly oxidized gold (Au+δ) instead of metallic gold (Au0) is essential for high catalytic activity towards C-H activation .56-57


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: Minimum energy path for transformation of benzaldehyde to benzyl alcohol over bare Au8 cluster. The structures of the transition-states for asymmetric transfer hydrogenation (TS2), transition state for hydride transfer from Au-cluster to benzaldehyde (TS3) and product (6) are shown in inset. Only the mechanistically relevant H-atoms are shown for clarity.

For a critical understanding of the assistance provided by the supporting surface in catalyzing the reaction, we perform a gedanken experiment by calculating the minimum energy path (MEP) over bare Au8 clusters. Fig. 3 shows the reaction profile along with structures of key intermediates and transition-states. Interaction of the solvent with the bare Au8 cluster (1) leads to a significantly stable reactant complex (2) with a binding energy of -23.0 kcal/mol resulting in


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

a distorted Au8 with pseudo tetrahedral shape. 2 undergoes C-H bond cleavage to form a (CH3)2C(OH)-Au8-H complex (3) where the solvent and the hydride are bonded to the same edge Auatom with solvent-Au-H angle= 85.6 o. The barrier for this C-H activation step (TS1) is 22.8 kcal/mol. Addition of aldehyde to 3 leads to barrierless formation an extremely stable 3…PhCHO complex, 4. The stability of 4 can be attributed into interaction of the solvent and the aromatic aldehyde with two different faces of Au8 clearly justifying the choice of a 3D model of Au8 to describe the reaction. Transfer hydrogenation in 4 occurs sequentially where a proton is initially transferred from the alcohol to the aldehyde through a very shallow barrier (TS2) of Ea=1.9 kcal/mol to form a proton transferred product, 5. Following this, a hydride from the Au-H bond attacks the carbonyl carbon as a nucleophile which results in formation of the reduced aldehyde (benzyl alcohol) and oxidized solvent (acetone) in 6. The barrier for this step through TS3 is 15.7 kcal/mol. The barrier for the rate determining step namely, C-H bond activation in TS1 is prohibitively large to be surmounted at ambient conditions. Hence, bare Au clusters would be insufficient to promote transfer hydrogenation. We have also explored the possibility of O-H bond activation by the Au-cluster subsequent to 2 and is found to have a much higher barrier of 48.0 kcal/mol (c.f. TS1 = 22.8 kcal/mol) and the process is computed to be highly exothermic (∆E= +37.2 kcal/mol). Clearly, it is the C-H bond activation which is favored over Au-cluster and any strategy to reduce this barrier should result in the transfer hydrogenation products. Presence of the TiO2 (110) support for Au8 provides a mechanistic benefit for the reaction. The reaction profile for the supported Au-catalyzed reaction is shown in Fig. 4. Binding of Au8 with 2-propanol stabilizes the 2-propanol…Au/TiO2 complex (8) by 15.1 kcal/mol which is 7.9 kcal/mol lower than that for the bare Au8…2-propanol complex (2). Clearly, an electron-


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


deficient Au8 cluster over TiO2 (110) binds weakly with the solvent compared to a neutral cluster. 8 undergoes C-H bond activation to form a 2-propanol-Au-H complex (9) similar to 3 with ∠solvent-Au-H = 87.8o through a low barrier transition-state (TS4) of 19.1 kcal/mol. Subsequent addition of benzaldehyde to 9 leads to formation a stable solvent…cluster…aldehyde complex (10) where again analogous to bare cluster (c.f. 4), the two molecules interact along different faces of the cluster. In marked contrast to the case for bare Au-clusters, 10 undergoes simultaneous transfer of proton from the hydroxyl group of the alcohol to carbonyl oxygen and hydride from the Au-cluster to carbon atom of the carbonyl group to form the product complex (11) via a concerted transition-state (TS5). The small barrier for TS5 (Ea=6.2 kcal/mol) may be traced to its cyclic structure which facilitates synchronous motion of the proton and hydride. As a result, insignificant structural reorganization occurs for the heavy atoms along reaction coordinate from the reactant to the transition-state (10 → TS5) and only the two hydrogens move. Therefore, a reactant-like TS5 resulting in exothermic product (ΔE(11 - 10)= -13.0 kcal/mol) involves a small barrier as expected from a simple Marcus-like description of two parabolic energy surfaces positioned at the reactant and the product which intersect at the transition-state.58 It is also worthwhile to mention that apart from classical Arrhenius type thermal activation, a small and narrow barrier (the proton and hydride move by less than 1 Å along 10 → 11) in TS5 should also favor the reaction through thermally activated quantum mechanical tunneling.59-60 This becomes particularly relevant since, the reaction coordinate entirely consists of motion of only the H-atoms and the heavier Au/C/O atoms remain almost stationary for this path. Therefore, it is predicted that primary H/D isotope effect for TS5 would be measurably large. Final workup of 11 with bases like KOH/K2CO3 or other phase – transfer catalysts (PTC) that may facilitate the rate of transfer product from one phase to another and


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

should regenerate the catalyst as observed experimentally.61 It is worthwhile to notice that removal of acetone and benzyl alcohol from Au8 cluster supported on TiO2 is less endothermic than for bare cluster due to reduced O…Au interactions. We have also further considered the possible processes at the Au-TiO2 interface area (perimeter site) which has been discussed previously.62-65 Although adsorption energy of isopropanol is much higher (32.7 kcal/mol) in the interface area, C-H activation is not favored as endothermicity of this process is very high (ΔE=+25.5 kcal/mol). The optimized geometry of reactant (I) and product (II) at interface is shown in Fig. 4(B).


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: (A) Minimum energy path for transformation of benzaldehyde to benzyl alcohol over TiO2(110) supported Au8 cluster. The structure of the transition-state for synchronous transfer hydrogenation through a cyclic transition state (TS5) and the structure of final product (11) are shown in inset. Only the mechanistically relevant H-atoms are shown for clarity. (B) Optimized structures of reactant and product for C-H activation at Au/TiO2 interface area (perimeter-site) and their relative energies.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

The rate determining state for reduction of aldehyde over both bare Au8 and Au8/TiO2 cluster remains C-H activation through TS1 and TS4 respectively. However, presence of a TiO2 surface reduces activation energy for TS4 by 3.7 kcal/mol which indeed enhances the rate of C-H activation by ~500 times over bare cluster (Au8) (within transition state theory model and with the approximation of same pre-exponential factor). Fig. 5 shows the optimized configuration of the respective reactant complexes and transition-states. For the bare Au-cluster, the C-H bond elongation (ΔdC-H) of 2-propanol from the reactant complex (2) to the transition-state (TS1) is 0.69 Å. However, for 8 → TS4, ΔdC-H = 0.63 Å indicating that in presence of a support, the transition-state is early (reactant-like) and requires smaller distortion. In other words, structural deformation of 8 in order to attain the transition state structure (TS4) for C-H activation is less relative to the deformation of 2 for achieving TS1. This is also supported by a distortioninteraction analyses of the respective barriers for which the activation energy (Ea) is described a combination of the distortion energy (ΔEdist) and interaction energy (ΔEint).66 Distortion energy represents the energy associated with the distortion of each of the fragment in the reactant complex in order to gain transition state geometry separately. For example, ΔEdist for 8 → TS4 estimates the energy associated with the deformation of both Au/TiO2 and 2-propanol independently [(ΔEdist(Au/TiO2) + ΔEdist(2-propanol), where ΔEdist(Au/TiO2) = E(Au/TiO2)TS4E(Au/TiO2)8 and ΔEdist(2-propanol) = E(2-propanol)TS4- E(2-propanol)8] to reach their structure in TS4. On the other hand, ΔEint display the additional interaction between the fragments in the transition state relative to the reactant complex. We have calculated ΔEint for 8 → TS4 by subtracting the binding energy (BE) of 2-propanol and Au/TiO2 in 8 from the same in TS4 [BE(Au/TiO2…2-propanol)TS4 – BE(Au/TiO2…2-propanol)8]. A similar approach has recently


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been used by our group for some critical understanding of the effect of triazole on regioselectivity of the C-C bond formation.67 In 2 → TS1, ΔEdist = 63.6 kcal/mol and ΔEint = 40.8 kcal/mol resulting in a Ea=22.8 kcal/mol. For 8 → TS4, ΔEdist = 60.3 kcal/mol; ΔEint = -41.4 kcal/mol leading to Ea=19.1 kcal/mol. Clearly, the smaller penalty incurred to distort (elongate) the C-H bond reduces the barrier for C-H activation over Au/TiO2 moderately. Structural comparison of TS1 vs TS4 shows that the three membered solvent…H…Au cyclic ring (Fig. 5(B) and Fig. 5 (E)) is more compact for the supported Au-cluster again indicating enhanced stability of TS4.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Figure 5: (A) Reactant complex (2) for C-H activation on bare Au-cluster, (B) Transition-state (TS1) for C-H activation on bare Au-cluster (C) Product after C-H activation on bare Au-cluster (3), (D) Reactant complex (8) for C-H activation on Au/TiO2 (E) Transition-state (TS4) for C-H activation on Au/TiO2, (F) Product after C-H activation on Au/TiO2 (9). Only the mechanistically relevant H-atoms are shown for clarity.


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For further investigations, we have analyzed charge density difference (CDD) for both reactant and transition state for the C-H activation on bare Au8 cluster as well as on Au8/TiO2. Such calculations reveal the direction and magnitude of electron shifts within certain regions of the system. Our results shows that minimal electron density change occurs upon adsorption of 2propanol over bare Au8 cluster (2) (0.04e, 2-propanol → Au8). However, at the transition state for C-H activation (TS1), 0.15e charge flows from 2-propanol to Au8. Clearly during C-H bond activation, electron density migrates along 2-propanol→Au8. In case of TiO2 supported Au8, the cluster is already little electron deficient (as shown earlier) and hence, it can accept more electron density making it more suitable for C-H activation. Bader charge analysis also displays that in 8 there is a 0.13e charge transfer along 2-propanol→Au8/TiO2 while in TS4 it is significantly larger (~0.3e) along the same direction. It is therefore found that TiO2 promotes the charge transfer from 2-propanol to Au8 and facilitates C-H activation. Figure 6 depicts all the CDD plots discussed here. Moreover, we have plotted the valence band maximum (VBM) and conduction band minimum (CBM) (Figure S4 and S5) in order to visualize the nature of orbital interaction within the reactant complex and transition state corresponding to the C-H activation on bare and TiO2 supported Au8 cluster. They show strong hybridization between the molecule and Au8 orbitals along reactant →transition-state for both bare and supported clusters.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

Figure 6: Charge density difference plots for 2, TS1, 8 and TS4.

Conclusions In conclusion, the present study shows that the oxide support plays a crucial role in anchoring the Au particles and in accepting partial electron density from Au which contribute towards the unique catalytic activity of Au/TiO2. The barriers for C-H activation of the secondary alcohol as well as concerted transfer hydrogenation get moderately reduced on supported Au-clusters. In


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fact, the activation energy for concerted transfer hydrogenation on Au/TiO2 (TS5) is lower than that for typical homogeneous Ru-complexes.68 It is predicted that heterogeneous transfer hydrogenation on Au/oxide support should be a method of choice for industrial scale reduction of variety of ketones. Interestingly, the Au/TiO2 surface is expected to bind selectively to different orientation of the reactant/product and should therefore prefer one particular enantiomer over others. Currently, we are exploring such asymmetric transfer hydrogenation catalyzed by supported Au-clusters which would be reported subsequently.

Acknowledgements RB thanks CSIR India for SRF. AD thanks INSA, DST, and BRNS for partial funding.

Author Information *Corresponding Authors: [email protected] Phone number: +91-33-24734971. Notes The authors declare no competing financial interest. Supporting Information Structures of intermediates and TiO2 (110) surface. Plots of VBM and CBM. This information is available free of charge via the internet at http://pubs.acs.org


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

References: (1) Cuenya, B. R. Metal Nanoparticle Catalysts Beginning to Shape-up. Acc. Chem. Res., 2013, 46, 1682–1691. (2) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. High-Surface-Area Catalyst Design: Synthesis, Characterization, and Reaction Studies of Platinum Nanoparticles in Mesoporous SBA-15 Silica. J. Phys. Chem. B, 2005, 109, 2192–2202. (3) Norris, D. J.; Sacra, A.; Murray, C. B.; Bawendi, M. G. Measurement of the Size Dependent Hole Spectrum in CdSe Quantum Dots, Phys. Rev. Lett. 1994, 72, 2612. (4) Datta, A.; Mallajosyula, S. M.; Pati, S. K. Nonlocal Electronic Distribution in Metallic Clusters: A Critical Examination of Aromatic Stabilization. Acc. Chem. Res., 2007, 40, 213–221. (5) Lee, S.; Fan, C.; Wu, T.; Anderson, S. L. CO Oxidation on Aun/TiO2 Catalysts Produced by Size-Selected Cluster Deposition. J. Am. Chem. Soc. 2004, 126, 5682-5683. (6) Tong, X.; Benz, L.; Kemper, P.; Metiu, H.; Bowers, M. T.; Buratto, S. K. Intact SizeSelected Aun Clusters on a TiO2(110)-(1 × 1) Surface at Room Temperature J. Am. Chem. Soc. 2005, 127, 13516-13518. (7) Liu, Z-P.; Gong, X-Q.; Kohanoff, J.; Sanchez, C.; Hu, P. Catalytic Role of Metal Oxides in Gold-Based Catalysts: A First Principles Study of CO Oxidation in TiO2 Supported Au. Phys. Rev. Lett. 2003, 91, 266102. (8) Hutchings, G. J.; Hall, M. S.; Carley, A. F.; Landon, P.; Solsona, B. E.; Kiely, C. J.; Herzing, A.; Makkee, M.; Moulijn, J. A.; Overweg, A.; Fierro-Gonzalez, J. C.; Guzman, J.; Gates, B. C. Role of Gold Cations in the Oxidation of Carbon Monoxide Catalyzed by Iron Oxide-Supported Gold. J. Catal. 2006, 242, 71-81


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Bus, E.; Miller, J. T.; van Bokhoven, J. A. Hydrogen Chemisorption on Al2O3-Supported Gold Catalysts. J. Phys. Chem. B 2005, 109, 14581– 14587. (10) Ricci, D.; Bongiorno, A.; Pacchioni, G.; Landman, U. Bonding Trends and Dimensionality Crossover of Gold Nanoclusters on Metal-Supported MgO Thin Films Phys. Rev. Lett. 2006, 97, 036106. (11) Guzman, J.; Carrettin, S.; Corma, A.Spectroscopic Evidence for the Supply of Reactive Oxygen During CO Oxidation Catalyzed by Gold Supported on Nanocrystalline CeO2 J. Am. Chem. Soc. 2005, 127, 3286– 3287. (12) Guczi, L.; Petö, G.; Beck, A.; Frey, K.; Geszti, O.; Molnár, G.; Daróczi, C.Gold Nanoparticles Deposited on SiO2/Si (100): Correlation Between Size, Electron Structure, and Activity in CO Oxidation J. Am. Chem. Soc. 2003, 125, 4332– 4337. (13) Howard, K. L.; Willock, D. J. A Periodic DFT Study of the Activation of O2 by Au Nanoparticles on α-Fe2O3. Faraday Discuss. 2011, 152, 135-151. (14) Conte, M.; Miyamura, H.; Kobayashi, S.; Chechik, V. Spin Trapping of Au-H Intermediate in the Alcohol Oxidation by Supported and Unsupported Gold Catalysis. J. Am. Chem. Soc. 2009, 131, 7189-7196. (15) Hutchings, G. J.; Haruta, M. A Golden Age of Catalysis: A perspective. Appl. Catal., A 2005, 291, 2−5. (16) Rodriguez, J. A.; Liu, G.; Jirsak, T.; Hrbek, J.; Chang, Z.; Dvorak, J.; Maiti, A. Activation of Gold on Titania: Adsorbtion and Reaction of SO2 on Au/TiO2(110). J. Am. Chem. Soc. 2002, 124, 5242-5250. (17) Molina, L. M.; Ramussen, M. D.; Hammer, B. Adsorption of O2 and Oxidation of CO at Au Nanoparticles Supported by TiO2(110). J. Chem. Phys. 2004, 120, 7673-7680.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

(18) Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Jr. Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst. Science, 2011, 333, 736739. (19) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. Identifying an O2 Supply Pathway in CO Oxidation on Au/TiO2(110): A Density Functional Theory Study on the Intrinsic Role of Water. J. Am. Chem. Soc. 2006, 128, 4017-4022. (20) Camellone, M. F.; Marx, D. Nature and Role of Activated Molecular Oxygen Species at the Gold/Titania Interface in the Selective Oxidation of Alcohols. J. Phys. Chem. C, 2014, 118, 20989-21000. (21) Rodriguez, J. A.; Evans, J.; Graciani, J.; Park, J-B.; Liu, P.; Hrbek, J.; Sanz, J. F. High Water-Gas Shift Activity in TiO2(110) Supported Cu and Au Nanoparticles: Role of the Oxide and Metal Particle Size. J. Phys. Chem. C, 2009, 113, 7364-7370. (22) Jiang, Z.; Zhang, W.; Li, J.; Xu, F.; Zhu, J.; Huang, W. Direct XPS Evidence for Charge Transfer from a Reduced Rutile TiO2(110) Surface to Au Clusters. . J. Phys. Chem. C, 2007, 111, 12434-12439. (23) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Andreeva, D.; Tabakova, T. FTIR Study of the Low-Temperature Water–Gas Shift Reaction on Au/Fe2O3 and Au/TiO2 Catalysts J. Catal. 1999, 185, 176-185. (24) Chang, F.-W.; Yu, H.-Y.; Selva Roselin, L.; Yang, H.-C.Production of Hydrogen via Partial Oxidation of Methanol Over Au/TiO2 Catalysts Appl. Catal. A: Gen. 2005, 290, 138– 147.


22

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Su, F-Z.; He, L.; Ni, J.; Cao, Y.; He, H-Y.; Fan, K-N. Efficient and Chemoselective Reduction of Carbonyl Compounds with Supported Gold Catalysts Under Transfer Hydrogenation Conditions. Chem. Comm. 2008, 3531-3533. (26) Baig, R. B. N.; Varma, R. S. Magnetic Silica-Supported Ruthenium Nanoparticles: An Efficient Catalyst for Transfer Hydrogenation of Carbonyl Compounds. ACS Sustainable Chem. Eng. 2013, 1, 805-809. (27) He, L.; Ji, N.; Wang, L-C.; Yu, F-J.; Cao, Y.; He, He-Yong, Fan, K-N. Aqueous RoomTemperature Gold-Catalyzed Chemoselective Transfer Hydrogenation of Aldehydes. Chem. Eur. J. 2009, 15, 11833-11836. (28) Noyori, R.; Hashiguchi, S. Asymmetric Transfer Hydrogenation Catalyzed by Chiral Ruthenium Complexes. Acc. Chem. Res. 1997, 30, 97-102. (29) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation of Aromatic Ketones Catalyzed by Chiral Ruthenium (II) Complexes. J. Am. Chem. Soc. 1995, 117, 7562-7563. (30) Samee, J. S. M.; Backvall, J-E.; Andersson, P. G.; Brandt, P. Mechanistic Aspects of Transition Metal-Catalyzed Hydrogen Transfer Reactions. Chem. Soc. Rev. 2006, 35, 237248. (31) Yamakawa, M.; Ito, H.; Noyori, R. The Metal-Ligand Bifunctional Catalysis: A Theoretical Study on the Ruthenium(II)-Catalyzed Hydrogen Transfer between Alcohols and Carbonyls. J. Am. Chem. Soc. 2000, 122, 1466-1478. (32) Sheeba, M. M.; Preethi, S.; Nijamudheen, A.; Tamizh, M. M.; Datta, A.; Farrugia, L. J.; Karvembu, R. Half-Sandwich Ru(η6-C6H6) Complexes with Chiral Aroylthioureas for


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Enhanced Asymmetric Transfer Hydrogenation of Ketones – Experimental and Theoretical Studies. Catal. Sci. Technol. 2015, 5, 4790-4799. (33) Li, L.; Gao, Y.; Li, H.; Zhao, Y.; Pei, Y.; Chen, Z. F.; Zeng, X. C. CO Oxidation on TiO2 (110) Supported Subnanometer Gold Clusters: Size and Shape Effects. J. Am. Chem. Soc. 2013, 135, 19336 (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (35) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. QUANTUM ESPRESSO: A Modular and OpenSource Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502 (36) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 7892. (37) Monkhorst, H. J.; Pack, J. D. Special Points for Brillonin-zone Integrations*. Phys. Rev. B 1976, 13, 5188-5192. (38) Diefenbach, M.; Kim, K. S. Spatial Structure of Au8: Importance of Basis Set Completeness and Geometry Relaxation J. Phys. Chem. B 2006, 110, 21639– 21642. (39) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Lægsgaard, E.;Besenbacher, F.; Hammer, B. Formation and Splitting of Paired Hydroxyl Groups on Reduced TiO2 (110) Phys. Rev. Lett. 2006, 96, 066107. (40) Martsinovich, N.; Jones, D. R.; Troisi, A. Electronic Structure of TiO2 Surfaces and Effect of Molecular Adsorbates J. Phys. Chem. C 2010, 114, 22659– 22670.


24

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a LongRange Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. (42) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978– 9985. (43)

Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm

for Bader Charge Allocation J. Comput. Chem. 2007, 28, 899-908. (44) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558– 561 (45) Amft, M.; Lebegue, S.; Eriksson, O.; Skorodumova, N. V. Adsorption of Cu, Ag, and Au Atoms on Graphene Including van der Waals Interactions. J. Phys.: Condens. Matter 2011, 23, 395001. (46) Marsinovich, N.; Jones, D. R.; Troisi, A. Electronic Structure of TiO2 Surfaces and Effects of Molecular Adsorbates Using Different DFT Implementations. J. Phys. Chem. C, 2010, 114, 22659-22670. (47) Bredow, T.; Giordano, L.; Cinquini, F.; Pacchini, G. Electronic properties of rutile TiO2 ultrathin films: Odd-Even Oscillations with the Number of Layers. Phys. Rev. B. 2004, 70 035419. (48) Hakkinen, H.; Landman, U. Gold Clusters (AuN, 2

Supported Sub-Nanometer Gold Cluster Catalyzed Transfer

Recommend Documents