Interface Effects in Hydrogen Elimination Reaction from Isopropanol

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Interface Effects in Hydrogen Elimination Reaction from Isopropanol by Ni Cluster on #-AlO(010) Surface 13

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Andrey Lyalin, Ken-Ichi Shimizu, and Tetsuya Taketsugu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00839 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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The Journal of Physical Chemistry

Interface Effects in Hydrogen Elimination Reaction from Isopropanol by Ni13 Cluster on Θ-Al2O3(010) Surface Andrey Lyalin,∗,† Ken-ichi Shimizu,‡,¶ and Tetsuya Taketsugu∗,§,†,¶ †Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan ‡Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan ¶Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8245, Japan §Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan E-mail: [email protected]; [email protected]

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January 26, 2017 Abstract We present results of theoretical investigation on catalytic hydrogen elimination from isopropanol (C3 H8 O) by free and Θ-Al2 O3 (010)-supported Ni13 cluster. The specific role played by the perimeter interface between the nickel cluster and alumina support is discussed. It is demonstrated that dehydrogenation of C3 H8 O on the free Ni13 cluster is a two-step process with the first hydrogen elimination from the alcohol hydroxyl group followed by C-H bond cleavage. Our calculations show that H elimination from OH group of C3 H8 O to Ni13 cluster is the rate determining step with the barrier of 0.95 eV, while the C-H bond cleavage requires overcoming the barrier of 0.41 eV. In the case of Ni13 cluster supported on Θ-Al2 O3 (010) the isopropanol molecule adsorbs on top of the surface Al atom in the close vicinity of the nickel cluster, which results in considerable decrease in barrier for H elimination due to formation of the complementary adsorption sites at the metal/support interface. It is demonstrated that intermediate formation of the Ni-C bond considerably promotes C-H bond cleavage. The described mechanism provides fundamental understanding of the process of the oxidant-free catalytic hydrogen elimination from alcohols on supported nickel clusters and can serve as a tool for rational design of novel type of nanocatalysts based on abundant noble-metal-free materials.

Introduction Oxidant-free catalytic dehydrogenation of alcohols to carbonyl compounds is important process in organic chemistry, 1–3 which can also be used as a potential method for hydrogen production from biomasses. 4 The most efficient catalysts for dehydrogenation of alcohols are based on expensive and noble metals 3–11 making a challenge for industrial applications. Therefore large efforts have been devoted to develop effective and cheap catalysts based on abundant elements. 2 ACS Paragon Plus Environment

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Recently it has been demonstrated experimentally that Ni nanoparticles deposited on ΘAl2 O3 support (Ni/Θ-Al2 O3 ) show high activity for catalytic dehydrogenation of secondary alcohols. 2

OH R1

O

Ni/Al2O3 (1 mol%) in o-xylene, 144 oC

R2

R1

R2

+ H2

(1)

(1)

Under the conditions in the eqn. (1), various aliphatic secondary alcohols were dehydrogenated by Ni/Al2 O3 to yield the corresponding ketones with good to high yield (7493%), whereas unsupported Ni metal nanoparticles and Θ-Al2 O3 were inert for the reaction. 2 For dehydrogenation of cyclooctanol under the low catalyst loading conditions at 180 ◦ C, Ni/Al2 O3 shows turnover number of 4050, 2 which is the highest value of the non-noble metal heterogeneous catalysts. Adopting a model dehydrogenation of isopropanol on Ni/Al2 O3 at 60 ◦ C monitored by in situ infrared measurement, a cooperative mechanism shown in eqns. (2)-(4) are proposed, where a basic site on the support oxide abstracts proton from an OH group of alcohol and a metallic Ni center acts as a C-H activation site. 2

(CH3 )2 CH−OH + Al + Al−O

(CH3 )2 CH−OAl + Al−OH

(2)

(CH3 )2 CH−OAl + Ni0

(CH3 )2 C−O + Ni−H + Al

(3)

Ni−H + Al−OH

Ni0 + H2 + Al−O.

(4)

Similar mechanism has been also proposed for Ag/Θ-Al2 O3 10 and Pt/Al2 O3 3 systems, suggesting that the metal-acid-base cooperative catalysis can be a general concept for the design of Al2 O3 -supported metal catalysts. However, in spite of the importance of the processes a systematic theoretical description of the reaction mechanism has been lacking, and hence rational design of noble metal-free catalysts for this reaction is still difficult. 3 ACS Paragon Plus Environment


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In the present work we fill this gap and report results of the theoretical investigation on the catalytic activity of Ni13 cluster supported on Θ-Al2 O3 (010) surface for the oxidant-free catalytic hydrogen elimination from the simplest secondary alcohol - isopropanol, C3 H8 O. The main idea that stands behind our work is that even catalytically inactive materials can be functionalized at the nanoscale via the size, structure, morphology, and support effects. The interface between two inert materials can also act as active site for catalytic reactions. 12 We have demonstrated that dehydrogenation of C3 H8 O on the free Ni13 cluster is a two-step process with the first H elimination from the alcohol hydroxyl group followed by C-H bond cleavage. Our calculations show that in the case of free Ni13 cluster H elimination from the OH group of C3 H8 O is the rate determining step. The successive C-H bond cleavage is considerably promoted by intermediate formation of the C-Ni bonding. We have demonstrated that in the case of Ni13 clusters supported on Θ-Al2 O3 (010) surface the isopropanol molecule adsorbs on the Al surface atom in the close vicinity of the nickel cluster, which results in considerable decrease in barrier for H transfer due to formation of the complementary adsorption sites at metal/support interface. The described mechanism provides fundamental understanding of the process of the oxidant-free catalytic dehydrogenation of alcohols on supported nickel clusters and opens new routes for a rational design of novel type of nanocatalysts based on cheap, abundant noble-metal-free materials.

Methods The calculations are carried out using density-functional theory (DFT) with the gradientcorrected exchange-correlation functional of Perdew-Burke-Ernzerhof (PBE) 13 as implemented in the PWSCF package. 14 The Kohn-Sham equations were solved using the projectoraugmented wave (PAW) technique. 15 The energy cutoffs of 50 Ry for wavefunctions and 300 Ry for charge density are chosen to guarantee convergence of the total energies and forces with the tolerance 10−4 Ry and 10−3 Ry/au, respectively. The selected cutoffs con-


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siderably exceed the minimum suggested values for the all used PAW pseudopotentials. We have confirmed that further increase of cutoffs plays negligible role on adsorption energies. The self-consistency of the density matrix is achieved with a tolerance of 10−5 . All calculations are spin polarized. A climbing image nudged elastic band (CI-NEB) method has been used for the search of the transition states and minimum energy pathways. 16 Periodic boundary conditions are used for all systems, including free molecules and clusters. In the case of finite size systems we have used the cubic unit cell with the lattice parameter a = 25 ˚ A, which is large enough to make intermolecular interactions between replicated species negligibly small (less than 10−4 Ry). In the present work the Θ-Al2 O3 lattice has been optimized using the Monkhorst-Pack 17 10×10×10 k-point mesh for Brillouin zone sampling. The calculated lattice parameters of a monoclinic C2/m structure of Θ-Al2 O3 , a = 11.924 ˚ A, b = 2.938 ˚ A, c = 5.666 ˚ A, and β = 104.0◦ are in a good agreement with the experimental values 18 of a = 11.85 ˚ A, b = 2.904 ˚ A, c = 5.622 ˚ A, and β = 103.8◦ and results of previous theoretical calculations. 19,20 The Θ-Al2 O3 (010) surface is represented by the four-layer slab containing 3×2 unit cells (240 atoms). Here and below, the surface index (010) refers to monoclinic unit cell of Θ-Al2 O3 , which corresponds to the (110) face of the oxygen sublattice. It has been demonstrated that namely this face of Θ-Al2 O3 surface is preferentially exposed. 21–23 In all calculations the bottom two layers in the slab are fixed, and all other atoms are fully relaxed. The periodically replicated slabs are separated by a vacuum region of 25 ˚ A. Only the Γ point is used for sampling the Brillouin zone due to the large size of the supercell. No dipole corrections were applied because the change in energy obtained after including a dipole correction for the model system of Ni13 on Θ-Al2 O3 (010) was found to be negligible (below 0.05 eV). The Θ-Al2 O3 (010) surface can be described as layers of the bended 5-atom Al2 O3 chains. Surface relaxation results in rumpling of 0.45 ˚ A, as shown in Figure 1. The undercoordinated Al atoms on the surface are moving inward by 0.17 - 0.26 ˚ A, while O atoms coordinated with two Al atoms in the first layer are moving outward by 0.19 ˚ A. Oxygen atoms at the ends of



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bended 5-atom Al2 O3 chains, which are coordinated with one surface Al atom and one Al atom from the second layer remain untouched. 0.45Å rumpling

Figure 1: Side and top view of Θ-Al2 O3 (010) 3×2 slab. Relaxation of the Θ-Al2 O3 (010) surface results in a surface rumpling. Aluminum atoms are yellow-green-colored and oxygen atoms are red-colored. In the case of top view only the first layer atoms are colored, while the deeper layers are shown in light gray. In order to understand the basic features of the nickel - alumina interaction we have started our investigation with the search for the most stable adsorption sites of a single Ni atom on Θ-Al2 O3 (010) surface. Twenty starting non-equivalent configurations of Ni on Θ-Al2 O3 (010) have been optimized including relaxations of the first two atomic layers of the surface. The results of such calculations including corresponding binding energies of the first 4 most stable structures are presented in Figure S1 of Supporting Information. We have found that Ni atom prefers to bridge two low coordinated O atoms on Θ-Al2 O3 (010). Therefore we can suggest that nickel nanoparticle deposited on Θ-Al2 O3 (010) would prefer to maximize its interaction with the low coordinated oxygen on the surface. As a model of nickel nanoparticle we consider icosahedral Ni13 cluster with the calculated magnetic moment 6 ACS Paragon Plus Environment

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of 0.62 µB per atom in agreement with the previous studies. 24,25 In order to obtain the most stable geometries of Ni13 supported on Θ-Al2 O3 (010) surface we have generated 15 nonequivalent starting configurations of Ni13 clusters on Θ-Al2 O3 (010) with different orientation and position in respect to the surface. These structures have been fully optimized on the alumina surface, with accounting for the relaxation of all the nickel atoms as well as the top two layers of the four layer slab of alumina. The bottom two layers in the slab were fixed. Thus, we have taken into account deformations of the cluster structure due to the interaction with the support as well as structural relaxations on the Θ-Al2 O3 (010) surface due to its interaction with the supported cluster. Such approach corresponds to the modeling of the soft landing of nickel cluster on the surface. Figure S2 presents five stable structures of Ni13 clusters on Θ-Al2 O3 (010) surface. The most stable configurations of the adsorbed C3 H8 O on free and supported Ni13 cluster were obtained by generating a large number of initial geometries by adding adsorbant in different nonequivalent positions and orientations on Ni13 and Ni13 /Θ-Al2 O3 (010) systems. These structures have been optimized without any geometry constraints. The similar approach has been successfully used in our previous works on adsorption and dissociation of various molecules on free and supported metal clusters and surfaces. 26–36

Results and discussion Hydrogen elimination reaction from isopropanol on free Ni13 cluster Let us consider interaction of isopropanol with the free icosahedral Ni13 cluster. Our DFT calculations demonstrate that isopropanol binds on Ni atom of Ni13 surface via its hydroxyl group forming O-Ni bond of 2.0 ˚ A, as shown in Figure 2a. The adsorption energy of C3 H8 O to Ni13 is calculated to be −0.75 eV. Here, the adsorption energy of C3 H8 O to Ni13 is defined as Ead = Etot (C3 H8 O-Ni13 ) − Etot (C3 H8 O) − Etot (Ni13 ), 7 ACS Paragon Plus Environment

(5)


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where Etot (C3 H8 O-Ni13 ) denotes the total energy of the complex system C3 H8 O-Ni13 , while Etot (C3 H8 O) and Etot (Ni13 ) are the total energies of the non-interacting C3 H8 O and Ni13 species, respectively.

a)

c)

b)

Figure 2: (a) The optimized structure of C3 H8 O on Ni13 and the most stable structures after (b) first and (c) second H transfer from isopropanol to the nickel cluster. Such configuration of the adsorbed isopropanol on the Ni13 surface favors H elimination from the hydroxyl group of the alcohol to the metal. Figure 2b shows the most stable configuration of the system after first H transfer, with H adsorbed at the edge of icosahedron next to the Ni atom where C3 H7 O binds. This is exothermic process which releases 0.38 eV if compared with C3 H8 O-Ni13 configuration. After H elimination from the hydroxyl group to the nickel cluster the O-Ni bond length decreases up to 1.75 ˚ A. The second H transfer occurs via C-H bond cleavage, releasing additional 0.47 eV counted from the energy of the parent C3 H7 O-Ni13 -H structure. The final C3 H6 O-Ni13 -2 H structure formed after transfer of two H atoms from the adsorbed isopropanol molecule to the nickel cluster is shown in Figure 2c. Here one H atom adsorbs at the edge of icosahedron, bridging two Ni atoms, and another H adsorbs at the triangular face, capping three Ni atoms. Figure 3 demonstrates the calculated energy profile along the reaction pathway for hy-


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TS1

TS2 INT1 INT2 INT3

Figure 3: Energy profile for dehydrogenation from isopropanol on free Ni13 cluster. Optimized geometries of the reaction intermediates and transition states geometries are shown in inserts. drogen elimination from isopropanol on the free Ni13 cluster. The considered reaction is a two-step process with the first H transfer from the alcohol hydroxyl group followed by C-H bond cleavage. Our calculations show that H transfer from OH group of C3 H8 O to the free Ni13 cluster is the rate determining step with the barrier of 0.95 eV, while the C-H bond cleavage requires the barrier of 0.41 eV. The corresponding transition state (TS) structures are shown in insert. After the first H transfer from the hydroxyl group of isopropanol to the metal the C3 H7 O intermediate approaches closer to the nickel cluster decreasing the O-Ni bond length from 2.0 ˚ A to 1.75 ˚ A. This is important condition for the subsequent C-H bond cleavage. In order to activate C-H bond and promote the second H transfer C3 H7 O molecule moves close to the Ni13 cluster forming temporary C-Ni bonding which can be clearly seen from the geometry of the transition state TS2, as shown in Figure 3. After C-H bond cleavage takes place the final C3 H6 O product moves backwards from the cluster, breaking temporary C-Ni bond. The described mechanism of C-H bond cleavage promoted by the interaction 9 ACS Paragon Plus Environment


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with the metal is consistent with the known models based on the intermediate formation of a metal-carbon bond. 37

Hydrogen transfer from isopropanol to Ni13 /Θ-Al2 O3 (010) Figure 4 presents the optimized structure of Ni13 cluster on Θ-Al2 O3 (010) surface. Our calculations demonstrate that Ni13 cluster undergoes strong structural rearrangements due to interaction with the support. As a result of such rearrangement one of the Ni vertex atoms on the Ni13 surface is substituted by the Al atom from the surface which anchors Ni13 cluster while the Ni atom is moving aside maximizing interaction with the low coordinated O atoms on Θ-Al2 O3 (010) surface. The Ni12 Al core can be considered as a deformed icosahedron structure, which is clearly seen in insert of Figure 4. Such configuration results in a strong binding of Ni13 to Θ-Al2 O3 (010) with the binding energy of 7.25 eV. It is also interesting to note that as a result of interaction with the surface, the magnetic moment of Ni13 increasing from 8 µB for the free Ni13 to 9.6 µB for the supported cluster.

Figure 4: Top (right) and side (left) view of optimized structure of Ni13 cluster on ΘAl2 O3 (010) surface. In insert all surface atoms are grey colored except of one anchoring Al atom. The optimized structure of Ni13 cluster on Θ-Al2 O3 (010) surface has been used for further investigation of adsorption preferences of isopropanol on Ni13 /Θ-Al2 O3 (010) system. 10 ACS Paragon Plus Environment

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We have generated more than 40 initial configurations, placing C3 H8 O in different positions and orientations in the vicinity of the supported Ni13 cluster and performed geometry optimization accounting for relaxations in isopropanol, nickel cluster and the first two layers of Θ-Al2 O3 (010) surface. We have found that all adsorbed configurations can be classified into three groups according to C3 H8 O adsorption (i) on top of the surface Al atom in the close vicinity of Ni13 /Θ-Al2 O3 (010) interface, as shown in Figures 5a and 5b; (ii) on the side of the supported Ni13 cluster in the vicinity of Ni13 /Θ-Al2 O3 (010) interface, as shown in Figure 5c; (iii) on top of Ni13 far from the support, as shown in Figure 5d.

a)

b)

c)

d)

Figure 5: Optimized structures of C3 H8 O adsorbed on the Θ-Al2 O3 (010) surface in the close vicinity of Ni13 cluster with H atom of the hydroxyl group directed either (a) at the low coordinated surface O atom, Ead =−1.67 eV or (b) at the Ni13 cluster, Ead =−1.07 eV; (c) on the supported Ni13 cluster in the close vicinity of Θ-Al2 O3 (010), Ead =−0.70 eV; (d) on top of Ni13 cluster far from the support, Ead =−0.63 eV. Our calculations demonstrate that in the case of C3 H8 O adsorption on Θ-Al2 O3 (010) 11 ACS Paragon Plus Environment


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surface in the vicinity of Ni13 cluster isopropanol adsorbs on top of the surface Al atom forming O-Al bond, while H atom of the hydroxyl group directed either at the low coordinated surface O atom (Figure 5a) or at the Ni13 cluster (Figure 5b). The corresponding adsorption energies of C3 H8 O on Ni13 /Θ-Al2 O3 (010) for configurations shown in Figures 5a and 5b are −1.67 eV and −1.07 eV, respectively. As one can see from calculations, the interaction of H from the hydroxyl group of alcohol with the surface oxygen considerably stabilizes C3 H8 O molecule on Θ-Al2 O3 (010) surface. In the case of C3 H8 O adsorption on the supported Ni13 cluster the adsorption energies calculated for configurations shown in Figures 5c and 5d are −0.70 eV and −0.63 eV, respectively. Thus, C3 H8 O adsorbs on the supported Ni13 cluster slightly weaker, than on the free one. Hydrogen bonding between isopropanol and surface oxygen atom (Figure 5c), slightly stabilizes C3 H8 O molecule adsorbed on Ni13 in the close vicinity of the Θ-Al2 O3 (010) surface if compared with adsorption on top of Ni13 cluster far from the support. It is clear that interaction of C3 H8 O with the Θ-Al2 O3 (010) surface in the close vicinity of the Ni13 cluster (Ead ∼ −1.7 eV) is much stronger than with the free or supported Ni13 cluster itself (Ead ∼ −0.7 eV), therefore the perimeter interface area between the nickel cluster and the support plays an important role of the active sites for C3 H8 O adsorption. It is important to note that for adequate description of the reaction rates one should analyze the free energy profile. For such an analysis one can use different approaches, for example Blue Moon ensemble method 38 or calculating harmonic frequencies for all involved intermediates and transition states and using conventional statistical mechanics to derive free energy profiles. However, for the large systems such calculations are computationally very time demanding and go far beyong the scope of the present work. On the other hand change in free energy along the reaction pathway can be estimated approximately, by calculating entropy of the free molecules in the ideal gas approximation 39 and putting it to 0 for the molecules adsorbed on the surface. This is a good approximation, because molecules lose the translational and rotational


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degrees of freedom upon adsorption on extended surface. Namely these degrees of freedom make the largest contribution to the total entropy. We have successfully used this approach in our previous works on calculation of free energy profiles for the Oxygen Reduction Reaction (ORR) on various catalytic surfaces. 40–42 As entropy of all species adsorbed on the surface is zero, accounting for free energy contribution will not change the energy profile for the reaction on the surface, but can change the adsorption energy of the initially free molecule. Thus, if the entropy loss is too large, isopropanol can be unstable on the surface and its desorption can be the rate-determining process. The entropy of the free isopropanol calculated at the PBE/cc-pVDZ level with the use of Gaussian 09 43 is 299.4 JK−1 mol−1 . That corresponds to the loss of 0.93 eV in energy upon adsorption on the catalyst (alumina-supported cluster) at T =298.15 K, while calculated electronic energies of adsorption of isopropanol on the surface are -1.67 eV and -1.07 eV for configurations (a) and (b) shown in Figure 5, respectively. Therefore, adsorption energy of isopropanol is large enough to outweigh the entropy loss, making the adsorbed species to be the kinetically relevant states at the room temperature. As was mentioned above, the first H transfer from the hydroxyl group of C3 H8 O to the metal is the rate determining step for hydrogen elimination from isopropanol on the free Ni13 cluster with the barrier of 0.95 eV, while the C-H bond cleavage requires overcoming of the considerably lower barrier, if C can form intermediate bonding with Ni atom. Moreover, the first H transfer is a highly exothermic process, which is the important point for the reaction on the alumina-supported cluster. Therefore in the case of the supported Ni13 cluster we perform detailed investigation on mechanisms of H elimination from C3 H8 O adsorbed in the close vicinity of the cluster/surface interface in order to understand how to make this process more effective. Previously we have demonstrated that for the chemical reactions on metal clusters the most stable structures are not always highly reactive, and an ensemble of isomeric structures must be taken into account for adequate description of the reaction rates. 44–46 Therefore in the present work we consider H transfer not only from the most stable configuration



of isopropanol adsorbed on Ni13 /Θ-Al2 O3 (010), as shown in Figure 5a, but also from the less bound structures presented in Figures 5b-d. For each of the four initial configurations of C3 H8 O on Ni13 /Θ-Al2 O3 (010) we have considered up to 15 different trial configurations with the transferred H from OH group either directly to Ni13 cluster or to the surface and performed full geometry optimization of the system (bottom two layers of Θ-Al2 O3 (010) remain frozen). For investigation of the C-H bond cleavage we selected two typical initial configurations INTa2 and INTb2 obtained after the first H transfer from the OH group of C3 H8 O and performed optimization for up to 22 different trial configurations with H transferred from the adsorbed C3 H7 O. All corresponding transition states have been obtained using NEB-CI method. 16

0.5 0.0 -0.5

Energy, eV

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-1.0 -1.5 -2.0

TS1 +0.95 eV

TS2

INT1 +0.05 eV

TSb1

INTb1 TSa1 +0.24 eV

+0.41 eV

INT2 +1.90 eV

INTa1

TSa2 TSb2

+0.38 eV

INTb2

-2.5 -3.0

INT3 INTa3 INTb3

INTa2

-3.5

Reaction coordinate Figure 6: Energy diagram for H elimination from isopropanol adsorbed at the perimeter interface between the supported Ni13 cluster and Θ-Al2 O3 (010) in configuration INTa1 (blue lines) and INTb1 (red lines). Energy profile for dehydrogenation of isopropanol on free Ni13 cluster (black lines) is shown for comparison. Zero energy corresponds to the noninteracting C3 H8 O molecule and the catalyst (either free or alumina-supported Ni13 cluster). Optimized geometries of the reaction intermediates and products are shown in inserts.


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Figure 6 demonstrates energy diagram calculated for H elimination isopropanol adsorbed at the perimeter interface of the supported Ni13 cluster on Θ-Al2 O3 (010) surface. Optimized geometries of all relative intermediates are presented in Figure S3 of Supporting Information. In the case of the most bounded configuration of C3 H8 O adsorbed on Ni13 /Θ-Al2 O3 (010) with the hydroxyl group directed at the low coordinated O atom on the surface (configuration INTa1 shown in Figure 5a), H transfers to the surface oxygen atom, forming OH group, which further transfers to the edge of the supported Ni13 cluster bridging one Ni atom from the cluster and Al atom on the surface. Such transition involves large rearrangement of the surface structure at the perimeter interface area and formation of the O vacancy. The remaining C3 H7 O intermediate (INTa2) binds to the Θ-Al2 O3 (010) surface nearby the supported cluster bridging 2 Al atoms. The overall barrier for such complex transformation is 0.24 eV, which is considerably lower than the hydrogen transfer barrier from OH group of C3 H8 O adsorbed on the free Ni13 cluster. However the successive C-H bond cleavage requires very large activation barrier TSa2 of 1.9 eV. Moreover the resulting intermediate INTa3 energetically not favorable in comparison with the initial strongly bounded INTa2 configuration. The high activation barrier TSa2 for the C-H bond cleavage can be explained by inability for C3 H7 O which is adsorbed in INTa2 configuration to form C-Ni bonding due to position of C3 H7 O in respect to the supported Ni13 cluster. In the case of configuration INTb1 shown in Figure 5b where OH group directed at the nickel cluster the H transfers to the Ni13 cluster with a negligible barrier of 0.05 eV. The resulting C3 H7 O intermediate shifts closer to the nickel cluster with O atom bridging one surface Al atom and one Ni atom from the cluster. This is the key condition for the successive C-H bond cleavage which is considerably promoted by formation of the C-Ni bonding in transition state TSb2. As the C3 H7 O intermediate binds in the close vicinity of the Ni13 cluster formation of the C-Ni bonding become possible which lower considerably the barrier for C-H bond cleavage to 0.38 eV in a similar way as for the case of the free Ni13 cluster. Figure 7 demonstrates energy diagram calculated for H transfer from the OH group of



1.0

b)

TSd1

0.5 +1.07 eV

TSc1

0.0

Energy, eV

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+0.89 eV

-0.5 -1.0

INTd1 INTc1

INTc2 INTd2

-1.5 -2.0 -2.5

Reaction coordinate Figure 7: Energy diagram for H elimination from the OH group of isopropanol adsorbed on the supported Ni13 cluster in configuration INTc1 (light blue lines) and INTd1 (green lines). Zero energy corresponds to the noninteracting C3 H8 O molecule and the catalyst (aluminasupported Ni13 cluster). Optimized geometries of the reaction intermediates are shown in inserts.


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isopropanol directly adsorbed on the supported Ni13 cluster either close to the Θ-Al2 O3 (010) surface in configuration INTc1 (light blue lines) close to the Θ-Al2 O3 surface or in configuration INTd1 (green lines) on top of the Ni13 cluster. The calculated barriers for H elimination from the OH group of isopropanol adsorbed on the supported nickel cluster in configurations INTc1 and INTd1 are 0.89 eV and 1.07 eV, respectively. Therefore H elimination from the OH group of isopropanol adsorbed on the supported Ni13 cluster occurs similar to the case of free Ni13 and the barrier for the rate determining step does not change. In this case we have not performed further investigation of the C-H cleavage because of trivial reasons. As we have discussed above, in the case of free Ni13 cluster, the transfer of the H atom from OH group is the rate determining step. Considerable decrease in this barrier for the supported Ni13 clusters, when isopropanol adsorbs on Θ-Al2 O3 (010) in the vicinity of Ni13 cluster (cases INTa1 and INTb1) demonstrates that Ni13 supported on Θ-Al2 O3 (010) is considerably more active for hydrogen elimination from isopropanol if compared with the free cluster, with the main reaction sites located at the perimeter interface between the metal cluster and alumina support.

Conclusion We have considered reaction of the catalytic hydrogen elimination from isopropanol adsorbed on free and supported Ni13 cluster. The Θ-Al2 O3 (010) surface has been selected as a support, because in the recent experimental works by Shimizu et al. namely Ni/Θ-Al2 O3 catalyst demonstrates the high catalytic activity for dehydrogenation of secondary alcohols. 2 We have found that dehydrogenation of C3 H8 O on the free Ni13 cluster is a two-step process with the first H elimination from the alcohol hydroxyl group followed by C-H bond cleavage. The first H transfer from OH group of C3 H8 O to free Ni13 is the rate determining step with the barrier of 0.95 eV, while the C-H bond cleavage requires the barrier of 0.41 eV. In the case of Ni13 clusters supported on Θ-Al2 O3 (010) the isopropanol molecule adsorbs on top of



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the surface Al atom in the close vicinity of the nickel cluster, which results in considerable decrease in barrier for H elimination due to formation of the complementary adsorption sites at metal/support interface. Thus the calculated barrier for the first H transfer from the OH group of C3 H8 O becomes negligibly small ∼ 0.05 eV, while the barrier for the C-H bond cleavage is 0.38 eV similar to the case of the free cluster. It is demonstrated that intermediate formation of the C-Ni bonding in transition state considerably promotes C-H bond cleavage. When formation of the carbon-metal bonding is not possible the barrier for C-H cleavage is high. The described mechanism provides fundamental understanding of the process of the oxidant-free catalytic dehydrogenation of alcohols on supported nickel clusters and can serve as a tool for rational design of novel type of nanocatalysts based on abundant noble-metal-free materials.

Acknowledgement This work was supported by the Japan Society for the Promotion of Science (JSPS KAKENHI Grants 15K05387 and 26288001), the FLAGSHIP2020 program supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan within the priority study5 (Development of new fundamental technologies for high-efficiency energy creation, conversion/storage and use), partially supported by the Development of Environmental Technology using Nanotechnology from MEXT and partly performed under the management of the ”Elements Strategy Initiative for Catalysts and Batteries (ESICB)” supported by MEXT program Elements Strategy Initiative to Form Core Research Centerh (since 2012). The computations were performed at the Research Center for Computational Science, Okazaki, Japan.


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Supporting Information Available The following files are available free of charge.

Optimized strictures of a single Ni atom

and Ni13 cluster on Θ-Al2 O3 (010) surface.

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Graphical TOC Entry 0.5

TS1

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-1.0

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Reaction coordinate


Interface Effects in Hydrogen Elimination Reaction from Isopropanol

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