Hydrogen Spillover to Copper Clusters on Hydroxylated Î³-Al2O3 - The

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Hydrogen Spillover to Copper Clusters on Hydroxylated γ‑Al2O3 Gang Feng,†,‡,§ Maria Verónica Ganduglia-Pirovano,†,∥ Chun-Fang Huo,‡,⊥ and Joachim Sauer*,† †

Department of Chemistry, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, P. R. China

‡

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S Supporting Information *

ABSTRACT: Density functional theory is used to examine the binding of Cun (n = 1−5) clusters onto γ-Al2O3(110) surfaces. Weak binding of Cu clusters onto a clean surface leads to three-dimensional growth. On a hydroxylated surface, hydroxyl H atoms spillover to the deposited Cu clusters, yielding oxidized Cu clusters that bind more strongly to the surface. For a single Cu atom and the Cu dimer, the energy barriers for H-spillover are 0.81 and 0.64 eV, respectively. Release of H2 from the hydroxylated surface is endothermic for dimers and tetramers, exothermic for single Cu atoms and trimers, and thermoneutral for pentamers. The barrier for this reaction, calculated for a single Cu atom, is 0.80 eV.

1. INTRODUCTION Surface-rich transition metal particles, a few nanometers in diameter, supported on metal oxides represent the largest class of solid catalysts used in industrial processes.1,2 Growth of active particles into larger, less active, aggregates is regarded as a major, inevitable problem. Ostwald ripening can be influenced by the strength of the metal−support interaction, and it is known that small inorganic ligands, e.g., Cl− and OH− in particular, can facilitate high dispersion degrees.3,4 Recent progress in atomistic understanding of metal−support interactions has been made due to surface science investigations on model surfaces, see, e.g., refs 5, 6 and advances in aberrationcorrected scanning transmission electron microscopy (STM).7,8 For Pt nanoparticles on γ-Al2O3, STM studies revealed significant differences between the particle sizes of the oxidized and the reduced forms of the catalyst. Moreover, it has been shown that the conditions at which reduction by H2 is achieved do have an influence.7 The Pt/γ-Al2O3 system has also been studied computationally using density functional theory (DFT), and the results obtained for the Pt13 cluster under reforming conditions have been summarized in ref 9. DFT has also been applied to small Rhn clusters (n = 1−5) on hydrated and nonhydrated γ-Al2O3 surfaces.10 Here, we report on a DFT study of small copper particles (Cun, n = 1−5) on a hydroxylated γ-Al2O3 surface compared with a clean, nonhydroxylated one. We have found that on the hydroxylated γ-Al2O3 surface hydrogen atoms can migrate from the hydroxyl group of the support onto the metal. We further show that this reverse H-spillover and the subsequent H2 desorption play a crucial role in the Cu cluster growth. We adopt the γ-Al2O3 surface model of ref 11 used before in studies of metal atoms and clusters on this surface,9,12 although it © XXXX American Chemical Society

is presently not completely clear how this surface looks like under different conditions.7,13 The interaction of Cun (n = 1−4) with the γ-Al2O3 surface14 was studied before by DFT, as also was the interaction of Au,15 Pd,12,16 Pt,17 and Ni.18 For single Cu atoms19 and also for single (Pt, Pd, Au, and Ag)20 and (Ni, Pd, and Pt)21 atoms, reverse Hspillover21,22 has been found. Reverse H-spillover has also been reported before for transition metal clusters, e.g., Ir4 and Ir6 clusters,22 and M6 clusters (M = Fe, Pd, Pt, Ag, Rh, Au) in acidic zeolites.23−25

2. METHODS AND MODELS Density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP)26,27 is applied. Spinpolarized calculations are carried out with the PW91 generalized-gradient exchange and correlation functional.28 A plane-wave basis set with a kinetic energy cutoff of 350 eV is used. Core electrons are described by the projected augmented wave method29,30 with core radii of 1.10, 1.90, 1.90, and 2.30 Bohr for H, O, Al, and Cu atoms, respectively. For Cu, only the 3d and 4s electrons are in the valence space. The Brillouin zone is sampled with a 3 × 3 × 1 k-point mesh for the p(1 × 1), p(2 × 1), and p(1 × 2) surface unit cells, generated by the Monkhorst− Pack algorithm. The convergence criteria are 1.0 × 10−4 eV for the self-consistent field (SCF) energy, 1 × 10−3 eV for the total energy, and 0.05 eV/Å for the atomic forces for structure optimizations. Bader charges31 are calculated to determine a possible charge separation between atoms. Transition structures Received: April 21, 2018 Revised: July 6, 2018

A

DOI: 10.1021/acs.jpcc.8b03764 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

single Cu atom and added one, two, three, and four Cu atoms, which was followed by MD simulations at 700 K for 4 ps (2 fs time step and H mass equal to 2) and subsequent cooling from 700 to 100 K during 6 ps. Finally, structures were further relaxed at 0 K. We are aware of the fact that there is no guarantee that our approach always yields the global minimum structure, but this is also true for other techniques such as genetic algorithms and replica exchange MD. For comparison, neutral CunHm (n = 1−5, m = 0−3) gasphase clusters were calculated using an a = b = c = 15 Å cell. Charged systems cannot be easily treated using VASP. Therefore, the charged [CunHm]m+ gas-phase clusters were calculated using Turbomole33 employing the RI34-PBE functional35 and the TZVP basis set.36 To check the compatibility of these two approaches, the structures and dehydrogenation energies of the neutral CunHm (n = 1−5, m = 1−3) clusters were calculated with both PW91/plane wave (VASP) and RI-PBE/ TZVP (Turbomole). As shown in Table 1 and in the Supporting Information (Figure S1 and Table S2), the PW91 and RI-PBE/ TZVP results for the dehydrogenation energy per 1/2(H2) differ only by 0.01−0.08 eV, with 12% as the largest deviation, which is within the accuracy range of the PW91 functional. We considered both the clean and hydroxylated γ-Al2O3(110) surfaces using the models of Digne et al.11 Our calculated lattice parameters of bulk γ-Al2O3, a = 5.585, b = 8.406, c = 8.079 Å, and α = γ = 90°, β = 90.555°, are in good agreement with those of Digne et al.11 The (110) surface accounts for more than 70% of the exposed surface of γ-Al2O3 under real catalytic conditions.11,38 It is fully dehydroxylated at 1150 K and has 8.9 OH/nm2 at around 600 K,11 which is the level of hydroxylation considered in our study. A p(1 × 1) eight-layer slab model was adopted for Cun/γAl2O3 (n = 1−4), which contains 16 Al2O3 units with a 15 Å vacuum region in the z direction. Tests showed that the adsorption energies of Cu4 calculated for p(1 × 1) and p(1 × 2) slabs differ by less than 0.12 eV, 3.0%. For Cu5/γ-Al2O3, p(1 × 2) and p(2 × 1) eight-layer slabs were chosen to reduce the interaction with Cu5 clusters in neighboring cells. The four topmost layers (each layer has Al4O6 composition, top views of Figure 1 show the two topmost layers) of the slabs along with the adsorbed metal atoms were fully relaxed, and the four bottom layers were fixed to their bulk positions. The vibrational frequencies are used to calculate enthalpies and Gibbs free energies in the standard harmonic oscillator, rigid rotor, ideal gas approximation. For the solid components, only vibrational contributions are considered, whereas for gas-phase

(TSs) for the migration of Cu atoms, H-spillover to deposited Cu atoms, and H2 desorption have been localized using the nudged elastic band method32 with eight equally spaced images along the reaction pathway. Vibrational frequencies and normal modes are calculated by diagonalization of the mass-weighted force constant matrix, obtained by numerical differentiation of analytically calculated forces as implemented in VASP. The atoms of the outmost Al4O6 layer (p(1 × 1) slab, Figure 1; note

Figure 1. Side and top views of the p(1 × 1) cell of the clean (left) and hydroxylated (right) γ-Al2O3(110) surfaces. H, O, and Al atoms are shown in white, red, and gray, respectively. Coordination numbers of the surface Al and O atoms are given as subscripts.

that the top view figure also shows the sublayer atoms, so it appears like Al8O12) and the adsorbate atoms are displaced by 0.02 Å in both directions for each Cartesian coordinate. All reported transition structures are saddle points on the potential energy surface with only one imaginary frequency, and all reported minimum-energy structures have only real frequencies. To find the lowest-energy Cun/γ-Al2O3 structures, several structural optimizations were started from reasonable guesses at various adsorption positions as well as from structures generated by short molecular dynamics (MD) simulations. To generate the latter, we started from the most stable adsorption structure for a

Table 1. Aggregation Energies per Atom and Growth Energies, Eagg/n and Egrowth (eV), Respectively; Average Coordination Number of Cu Atoms by Cu Atoms, NCu; and Average Cu−Cu Bond Distances, R (pm) for Neutral Cun Clusters (n = 1−5) in the Gas Phase and Supported on the Clean γ-Al2O3 Surfacea gas phaseb n 1 2 3 4 5

NCu

Eagg/n

Egrowth

1 2 2.5 2.8

0.00 1.12 (1.12) 1.26 (1.23) 1.63 (1.56) 1.77 (1.72)

0.00 2.23 (2.25) 1.55 (1.44) 2.74 (2.65) 2.35 (2.25)

Cun/surf R

c

225 (222) 238 (233) 238 (235) 238 (236)

NCu

Nav

Eagg/n

Egrowth

Eads/n

Cu−O

Cu−Al

1 2 2 2.4

4 4 4.7 4 4.2

0.00 1.07 0.94 1.10 1.10

0.00 2.14 0.68 1.57 1.11

1.47 1.42 1.18 0.98 0.85

200 201 205 208 207

258 252 252 245 255

R

q

229 241 238 248

0.19 0.06 0.36 0.24 0.30

a For the supported clusters, adsorption energies per atom, Eads/n (eV); average total coordination numbers, Nav; sum of Bader charges, q (e); and average Cu−O and Cu−Al bond lengths (pm) are given. bThe results for the gas-phase clusters correspond to Turbomole (and VASP) calculations. cFor bulk Cu (coordination number 12), the calculated binding energy is 3.45 eV per Cu atom, the calculated Cu−Cu bond distance is 258 pm, and the experimental value is 259 pm.37

B


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Figure 2. Structures, bond distances (pm), and relative energies (eV, in parentheses) for neutral (left) and charged (right) CunHmm+ (n = 1−5, m = 0− 3) as obtained with Turbomole. (H and Cu atoms are shown in white and blue, respectively.)

species, rotational and translational contributions are also included. The clean and hydroxylated γ-Al2O3(110) surfaces have been described in detail before.11 Figure 1 shows side and top views of the p(1 × 1) γ-Al2O3(110) slab, where the Al and O atoms of the surface layer are numbered and their coordination numbers are given as subscripts. The relaxed clean γ-Al2O3(110) surface (Figure 1, left panel) is corrugated. Due to a symmetry plane, the O(2)3c and O(3)3c, the O(5)2c and O(6)2c, and the Al(1)4c and Al(2)4c sites are equivalent. Three water molecules were added to the p(1 × 1) surface cell to get the hydroxylated surface with 8.9 OH/nm2 (Figure 1, right panel). One of them, H2O(8), is attached to Al(3). Two are dissociated with one hydroxyl group, O(9)H, bonded to Al(4) and the other one, O(7)H, forming a bridge between the

Al(1) and Al(2) sites. The two protons form OH groups with the O(3) and O(6) surface atoms. As reported by Digne et al., the surface Al atoms behave as Lewis acid sites and thus are active sites for water adsorption.11 Water adsorption is stronger at the Al(4) site because this is the site with highest acidity, followed by the Al(1,2,3) sites. Since the Al(1) and Al(2) sites are close, the OH group resulting from the dissociation of water can form a bridge between them.

3. RESULTS 3.1. Definitions. For gas-phase clusters, the total aggregation energy is defined as the energy of formation of Cun from n Cu atoms Eagg (n) = −[Egas(n) − nEgas(1)] C

(1) DOI: 10.1021/acs.jpcc.8b03764 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. Structures for Cun (n = 1−5) on γ-Al2O3: (a) clean surface; (b) hydroxylated surface without H-spillover; and (c) hydroxylated surface with H-spillover. (H, O, Al, and Cu atoms are shown in white, red, gray, and blue, respectively; selected bond distances are in pm; coordination numbers of Cu atoms are given as subscripts.)

Egas(n) is the total energy of the n-atom cluster. The negative aggregation energy (a positive number) is also known as binding energy. The growth energy is defined as the energy of formation of the n-atom cluster from the (n − 1)-atom cluster and one Cu atom

For Cu clusters on the surface, the aggregation energy is defined as Eagg (n) = −[Esurf (n) + (n − 1)Esurf (0) − nEsurf (1)]

(3)

Egrowth(n/n − 1) = −[Egas(n) − Egas(n − 1) − Egas(1)]

and the growth energy as

(2) D


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Figure 4. Growth energies, Egrowth(n/n − 1), and aggregation energies per Cu atom, Eagg(n)/n, for Cun clusters (n = 1−5) in the gas phase, on the clean and the hydroxylated γ-Al2O3 surfaces with and without H-spillover.

A single Cu atom coordinates to the O(4)2c, O(5)2c, Al(3)4c, and Al(4)3c surface sites. In the Cu2 surface cluster, the second Cu atom coordinates to the first Cu atom and to the Al(4)3c and O(2)3c ions. The Cu−Cu bond distance is 4 pm longer (229 pm) than that of the gas-phase dimer. The experimental40 and computed (Turbomole) values are 225 pm. As in the gas phase,39 the dimer has the shortest Cu−Cu bond distance among all deposited Cu clusters. Adsorbed Cu3 forms a triangle with Cu−Cu bond distances of 237, 242, and 245 pm (Figure 3a). It is hypothetically obtained from the adsorbed dimer by replacing the Cu3c atom by a Cu4c− Cu4c dimer. One of the Cu4c atoms connects to a next layer oxygen atom and to Al(4)2c, which moves 144 pm upward and gives up a coordination to the next layer oxygen atom. A cave with an interatomic distance of 379 pm (see Figure S2) forms under Al(4)2c and the deposited Cu3. Such caves may host subsurface hydrogen atoms.41,42 On the γ-Al2O3 surface, Cu4 does not assume the rhombic planar structure that is most stable in the gas phase but forms a triangle with one exocyclic Cu atom, which in the gas phase is 0.48 eV less stable than the rhombic planar one (cf. Figure 2) but 0.31 eV more stable than the next gas phase square Cu4 isomer shown in Figure 3 of ref 43 (not shown here). The exocyclic Cu atom coordinates to Al(4)2c and O(3)3c. Looking at the bond distances, the structure can also be understood as one 235 pm dimer attached side-on to a second 242 pm dimer. The most stable Cu5 isomer in the gas phase is a planar, trapezium structure that contains three triangles (Figure 2). On the surface, the O(4) atom inserts into the Cu−Cu bond of the shorter parallel side of the trapezium. The resulting structure consists of two Cu3 triangles sharing one Cu atom that lies on the same symmetry plane as the Al(3) and O(4) surface atoms. The two symmetry-equivalent Cu atoms connected to O(5)2c and O(6)2c, respectively, are fivefold-coordinated, whereas the two Cu atoms connected to O(2)3c and O(3)3c are threefoldcoordinated. 3.3.2. Stabilities. Table 1 shows the aggregation energies per atom, the growth energies, and the Bader charges for Cu clusters adsorbed on the clean γ-Al2O3 surface compared with gas-phase clusters. In the gas phase, the aggregation energy increases monotonically with the cluster size as found before,43 whereas on the surface, it is nearly constant from n = 2 on (Figure 4). The growth energy shows the well-known even−odd alternation,39,43,44 which is due to the unpaired electron on a single

Egrowth(n/n − 1) = −[Esurf (n) + Esurf (0)] − [Esurf (n − 1) + Esurf (1)]

(4)

Here, Esurf(n) and Esurf(0) are the total energies of a γ-Al2O3 surface cell (either clean or hydroxylated) with a Cun cluster and of the Cu-free surface cell, respectively. The energy of adsorption of a Cun cluster on the γ-Al2O3 surface is defined as Eads(n) = −[Esurf (n) − Egas(n) − Esurf (0)]

(5)

This definition is also used for the H-spillover structures. The energy of hydrogen desorption per 1/2(H2) molecule from a CunHm species on the surface (or in the gas phase) is defined as 1 i1 y Edesjjj H 2zzz = E(Cu nHm − 1) + Egas(H 2) − E(Cu nHm) 2 k2 {

(6)

3.2. CunHm Gas-Phase Clusters. As mentioned above for the neutral CunHm clusters (n = 1−5, m = 0−3), calculations are performed with the Perdew−Burke−Ernzerhof (PBE) functional35 and a TZVP basis set36 using Turbomole 6.1,33 as well as with the PW91 functional and a 350 eV plane-wave cutoff employing VASP. Both the structures and binding energies agree well, see Table 1 and Figure S1 (Supporting Information), and previous results.39,40 Figure 2 shows the PBE/TZVP structures for the neutral and charged CunHm(m+) clusters (n = 1−5, m = 0−3) as obtained with Turbomole. The calculated energies of these clusters are given in Table 1; see also the Supporting Information (Table S1). The H atoms in the neutral CunHm clusters are negatively charged (−0.15 to −0.30 e). 3.3. Cun on Clean γ-Al2O3. Although also on the γ-Al2O3 surfaces there are several isomers for each cluster size, in this and the following sections, we present the lowest-energy structures only because these are the relevant ones for the questions we are interested in, namely, the difference between clean and hydroxylated surfaces including H-spillover. 3.3.1. Structures. For Cun on the clean γ-Al2O3 surface (n = 1−5), both optimization and MD simulations yield the same most stable structures, which are shown in Figure 3. More details are given in the Supporting Information (Figures S2−S6). E


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Table 2. Adsorption and Aggregation Energies per Atom, Eads/n and Eagg/n (eV); Growth Energies, Egrowth (eV); H-Spillover Energies, Espill (eV), and Sum of Bader Charges, q (e), for Cun Clusters (n = 1−5) on the Hydroxylated γ-Al2O3 Surface before and after H-Spillover, Cun/H6O3-Surf and CunHm/H6−mO3-Surf, Respectivelya Cun/H6O3-surf n

Eads/n

Eagg/n

Egrowth

1 2 3 4 5

0.94 0.81 0.72 0.55 0.42

0.00 0.99 1.01 1.19 1.19

0.00 1.99 1.04 1.74 1.20

CunHm/H6−mO3-surf

R

Cu−O

q

m

Espillb

224 234 235 243

198 194 199 198 203

0.14 0.02 0.21 0.08 0.15

1 1 2 2 3

0.50 1.36 0.96 1.62 1.50

R

Cu−O

Cu−H

q

232 249 244 252

203 189 203 199 203

153 158 161 164 167

0.81 0.88 1.68 1.72 2.50

a

Average Cu−Cu bond distances, R (pm), and Cu−O and Cu−Al bond lengths (pm) are given. bEnergy of the H-spillover reaction, eq 7.

the shortest Cu−O and Cu−Cu distances (194 and 224 pm, respectively) among the Cun surface clusters investigated. The Cu−Cu distance is virtually the same as that of the gas-phase dimer (Table 1). There is another dimer structure that is only 4.5 meV less stable and has both Cu atoms coordinated to surface oxygen atoms with a Cu−Cu distance of 226 pm (see Figure S3). The Cu trimer attaches with one Cu atom to O(1)3c and with another one to O(4)2c. This creates two threefold-coordinated Cu atoms (Figure 3), whereas the third Cu atom remains twofold-coordinated. The Cu tetramer retains its rhombic structure, which is the most stable in gas phase on the surface, whereas the Cu pentamer forms a trigonal−bipyramidal structure, which is not the most stable isomer of Cu5 in the gas phase (cf. Figure 2). These most stable structures of the Cun clusters with n = 2−5 can be generated by successively adding one Cu atom to the structure with n − 1 Cu atoms. 3.4.2. Stabilities. Table 2 shows the adsorption and aggregation energies per atom, growth energies, Bader charges (q), and average bond distances (pm) for Cun (n = 1−5) on the hydroxylated γ-Al2O3 surface. The total adsorption energies increase from 0.94 (n = 1) to 2.21 eV (n = 4). The average adsorption energy per Cu atom decreases monotonically from 0.94 (n = 1) to 0.42 eV (n = 5). As for the nonhydroxylated surface, the growth energy shows the even−odd alternation. Except for the dimer, aggregation and growth energies are larger on the hydroxylated than on the clean γ-Al2O3 surface. 3.5. H-Spillover for Cun on Hydroxylated γ-Al2O3. On the hydroxylated surface, we find that H-spillover structures, CunHm/H6−mO3-surf, in which protons have migrated from surface hydroxyl groups onto the Cun clusters

Cu atom (Figure 4), for both the gas-phase and the adsorbed clusters. It also shows up in the average Bader charges for adsorbed Cun species. The total adsorption energy for Cu clusters on the clean γ-Al2O3 surface (Table 1) increases with the cluster size, but the increment per atom becomes smaller, 1.37, 0.70, 0.39, and 0.33 eV from a single Cu atom to Cu2, from Cu2 to Cu3, and so on, respectively, resulting in a monotonous decrease of the adsorption energy per atom from Cu to Cu5 (Table 1). To obtain further insight into the growth of Cu clusters on the clean γ-Al2O3 surface, we performed MD simulations for 10 Cu atoms in a p(1 × 2) cell to keep the coverage of 5 Cu atoms in one p(1 × 1) cell. We find that 10 Cu atoms in the p(1 × 2) cell aggregate into one large cluster, which is 0.71 eV more stable than two Cu5 clusters in neighboring cells. In addition, the Cu5 cluster in a p(1 × 2) cell is by 0.53 and 1.11 eV more stable than Cu2/Cu3 and single Cu/Cu4 in neighboring cells, respectively. The latter value is identical with the growth energy for n = 5 in Table 1, which confirms that the results are not affected by the cell size, p(1 × 1) or p(1 × 2). On the clean γ-Al2O3 surface, the energy barriers for migration of a single Cu atom and a Cu2 dimer are 0.68 and 1.02 eV, respectively (see Figure S6 in the Supporting Information). For well-defined model catalysts that are usually prepared by metal evaporation on the support, e.g., in ref 45, it will be very likely that these barriers are overcome and migration occurs, whereas with (industrial) powder catalysts that are usually prepared by impregnation of the support with active metal compounds, it will depend on the calcination temperature if this migration occurs. Moreover, with sufficiently high operation temperatures, migration may be observed during the catalytic process. 3.4. Cun on Hydroxylated γ-Al2O3. 3.4.1. Structures. Figure 3, middle, shows the structures of Cun clusters (n = 1−5) on the hydroxylated γ-Al2O3 surface; see Table 2 for average bond distances and Supporting Information (Figures S2−S6) for more details. The low-coordinated Al sites on the clean γAl2O3 surface, Al(4)3c and Al(3)4c, have increased their coordination by one upon hydroxylation and became Al(4)4c and Al(3)5c. They are no longer the preferred adsorption sites for Cu atoms. The Cun clusters adsorb preferentially in the region enclosed by the −O(5)−Al(2)−O(2)−O(1)−Al(1)− O(6)−O(4) atoms, which is not fully hydroxylated and therefore the coordination numbers of the Al and O ions in that region are lower. A single Cu atom attaches to one of the low-coordinated surface O atoms, O(1)3c. The Cu dimer attaches with one of its Cu atoms to the same O(1)3c site. The binding to the surface O atom is accompanied by polarization of Cu2 (the Bader charges on the terminal and surface-connected Cu atoms are −0.21 and +0.22 e, respectively), and the strong binding to O(1)3c leads to

Cu n/H6O3‐surf → Cu nHm/H6 − mO3‐surf

(7)

are always more stable than the H-free Cun clusters, Cun/H6O3surf. Figure 5 shows possible surface reactions and their relative energies. Moreover, hydrogen may desorb from the H-spillover structures according to Cu nH m/H6 − mO3‐surf → k /2H 2 + Cu nHm − k /H6 − mO3‐surf

(8)

3.5.1. Structures. Figure 6a,b (bottom panels) shows the CunHm/H6−mO3-surf H-spillover structures for n = 1 and 2 and m = 1 (see Figure 3, Tables 2 and 3 as well as Figures S4 and S5 (Supporting Information), for additional details). In the CuH+/ [H5O3]−-surf H-spillover structure, the CuH+ species attaches with its Cu atom to the O(3)4c, O(7)3c, and O(9)2c surface sites. With 153 pm (Figure 6a), the Cu−H bond is 2 pm longer than in the gas-phase CuH+ species (Figure 2). We note here that after F


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Table 3. Adsorption and Aggregation Energies per Atom, Eads/n and Eagg/n, Growth Energies, Egrowth (All in Electronvolt), for Cun Clusters on the Hydroxylated γ-Al2O3 Surface after H-Spillover and Desorption of 1/2(H2), CunHmCunHm−1/H6−mO3-Surfa n

m

Eads/n

Eagg/n

Egrowth

1 2 3 4 5

1 1 2 2 3

2.05 0.99 1.16 0.66 0.72

0.00 0.56 0.34 0.49 0.39

0.00 1.12 −0.11 0.94 −0.03

R

Cu−O

230 244 241 252

194 196 201 197 201

Cu−H

q

163 163 167 165

0.63 1.26 1.42 1.43 2.27

a

Average distances (pm) and sum of Bader charges for all Cu atoms, q (e), are also given.

Figure 5. Energies (eV) for Cun clusters (n = 1−5) on the hydroxylated γ-Al2O3 surface relative to the total energies of the isolated Cun clusters and the hydroxylated γ-Al2O3 surface. (Cun/H6O3-surf: Cun adsorbed on the hydroxylated surface; TS: transition structure for H-spillover; CunHm/H6−mO3-surf: H-spillover structures with H atoms on top of Cun; k/2(H2)+CunHm−k/H6−mO3-surf: after k/2(H2) desorption from the surface) (for n = 1, 2, 3, 4, 5, respectively, m = 1, 1, 2, 2, 3 and k = 1, 1, 1, 2, 1).

Figure 7. Transition structures and relative energies (in electronvolt) for H2 desorption from the CuH/H5O3-surf system (H, O, Al, and Cu atoms are shown in white, red, gray, and blue, respectively; bond distances are in picometer).

shorter than in the gas-phase Cu2H+ species (Figure 2). It is the shortest Cu−Cu distance among the five H-spillover structures. The [Cu3H2]2+ surface species coordinates with its three Cu atoms to the O(3)3c, O(4)2c, O(6)3c, O(8)2c, O(9)2c, and Al(3)5c surface sites (Figure 3c). Its two H atoms are bridging two Cu atoms each, as in the Cu2H+ species. After desorption of one hydrogen atom (Figure S5), the Cu atoms of [Cu3H]+ form a triangle with one bridging H atom and fivefold coordination of the three Cu atoms. [Cu4H2]2+ forms a distorted diamond structure with two bridging H atoms on its top (Figure 3c), which is different from the gas-phase structure (Figure 2). Three Cu atoms of Cu4 are fivefold-coordinated, and another is fourfold-coordinated. [Cu5H3]3+ (Figure 3c), with one threefold-coordinated H atom and two bridging H atoms, forms a distorted trapezoid structure, which is obtained by adding one Cu atom to the diamond Cu4 structure. Two Cu atoms are fivefold-coordinated, two are sixfold-coordinated, and one is threefold-coordinated. 3.5.2. Stabilities. Tables 2 and 3 show adsorption and aggregation energies per atom as well as growth energies for the H-spillover structures on the hydroxylated γ-Al2O3 surface. The sum of Bader charges on all Cu atoms (q) and average bond distances (pm) are also given. Hydrogen desorption energies (with respect to 1/2(H2)) for the H-spillover structures in comparison to the corresponding gas-phase clusters are given in Table 4. As for the adsorption of H-free Cun clusters, the total adsorption energies of the CunHm clusters in the H-spillover structures increase for n = 1−4 from 1.44 to 3.83 eV (calculated from Table 2 as Eads + Espill) and decrease for n = 5 to 3.61 eV. The average adsorption energies per Cu atom decrease monotonically. The Bader analysis shows that in the H-spillover structures the Cu cluster is partially oxidized (Table 2). The total charge on the Cu atoms is about 0.8−0.9 e per H atom, which suggests the notation [CunHm]m+/[H6−mO3]m−-surf.

Figure 6. Transition structures and relative energies (eV) for Hspillover reactions Cun/H6O3-surf → [CunH]+/[H5O3]−-surf, for (a) n = 1 and (b) n = 2. (H, O, Al, and Cu atoms are shown in white, red, gray, and blue; selected bond distances are given in picometer.)

hydrogen desorption the Cu+ ion tightly binds to O(7)3c and O(9)2c with distances of 191 and 197 pm, respectively (see Figure 7). In crystalline Cu2O, the Cu−O distance is 184 pm.46 In the Cu2H+/[H5O3]−-surf structure (Figures 3 and 6b), the two Cu atoms of the Cu2H+ unit bridge the O(8)2c and O(9)2c sites. Both Cu atoms are threefold-coordinated. The Cu−Cu bond distance is 232 pm (Figures 3, right panel, and 6b), 7 pm G


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Table 4. Reaction Energy, ΔEdeh, Zero-Point Vibrational Energy, ΔEZPV, Enthalpy, ΔHT, and Gibbs Free Energy, ΔGT, for the [CunHm]m+[H6−mO3]m−-Surf → [CunHm−k]m+[H6−mO3]m−-Surf + k/2(H2) (Gas) Dehydrogenation Reaction on the Hydroxylated γ-Al2O3(110) Surfacesa [CunHm−k]m+

100 kPab

[CunHm]m+[H6−mO3]m−-surf

10 kPac

n, m

k

ΔEdeh

ΔEdeh

ΔEZPV

ΔH100

ΔH300

ΔH600

ΔG300

ΔG100

ΔG300

ΔG600

1, 1 2, 1 3, 2 3, 2 4, 2 4, 2 5, 3 5, 3 H2

1 1 1 2 1 2 1 2

−0.90 1.26 −0.39 −d 1.25 0.53 0.21 −d 0.00

−0.61 0.99 −0.36 1.02 1.19 0.95 0.01 1.09 0.27

−0.03 0.18 −0.06 −0.15 −0.07 −0.12 −0.02 −0.10 0.29

−0.63 1.21 −0.40 0.90 1.13 0.86 0.01 1.02 0.33

−0.61 1.32 −0.39 0.95 1.15 0.90 0.03 1.08 0.40

−0.61 1.48 −0.39 0.97 1.16 0.91 0.05 1.12 −0.07

−0.79 0.87 −0.55 0.54 0.97 0.51 −0.18 0.65 0.17

−0.69 1.11 −0.46 0.78 1.07 0.73 −0.06 0.89 −0.13

−0.82 0.84 −0.58 0.48 0.94 0.45 −0.21 0.59 −0.65

−1.04 0.29 −0.77 0.01 0.72 −0.01 −0.46 0.09

a

The values for H2 in the gas phase are also given, all in electronvolt. bStandard pressure (1 bar). cPartial pressure of 0.1 bar. dNo SCF convergence.

4. DISCUSSION 4.1. Growth and Aggregation of Cun on the Clean γAl2O3 Surface. The aggregation energy per atom and growth energy for Cun on the on γ-Al2O3 surface are plotted in Figure 4 and compared with the gas-phase values. In the gas phase, the aggregation energy per atom increases with the cluster size. This is also true for the growth energy if even and odd cluster sizes are considered separately. The gain in aggregation energy, i.e., the growth energy, is larger when a cluster with closed electronic shells (even number of valence electrons) is formed. Whereas for the Cu dimer the aggregation (∼1.1 eV/atom) and growth energies (∼2.2 eV/atom) are similar for the gas-phase clusters and on the clean surface, for the larger clusters, the growth energies on the clean surface are always smaller than in the gas phase and the aggregation energy per atom is almost constant with the cluster size. The energies in Table 1 suggest unlimited growth of Cu clusters on the clean γ-Al2O3 surface. A limiting value of the growth energy for very large clusters (1.44 eV) is obtained by comparing the adsorption energy for a single Cu atom on the Cu(100) surface, 2.91 eV, with its adsorption energy on the clean γ-Al2O3 surface (1.47 eV). The calculated energy barriers for diffusion of a single Cu atom and a Cu dimer on the clean γAl2O3 surface (see Figure S6 in Supporting Information) are 0.68 and 1.02 eV, respectively, which are smaller than the corresponding adsorption energies. This indicates that on the clean γ-Al2O3 surface cluster growth is also kinetically possible. This result is particularly relevant for model catalysts that are typically created by metal evaporation on the support under ultrahigh vacuum conditions. 4.2. Comparison of Cun on γ-Al2O3 and MgO. The binding of Cun clusters on the clean γ-Al2O3 surface is strong compared to the binding on the clean and perfect MgO(100) surface. For a single Cu atom, the adsorption energy is 1.47 eV (Table 1), which is 0.53 eV more than the 0.94 eV we calculate for the MgO(100) surface. The adsorption energy of 0.98 eV/ atom for Cu4 on γ-Al2O3 (Table 1) is also 0.50 eV larger than the value of 0.48 eV reported for MgO(100).47 The stronger binding of Cu on the γ-Al2O3 surface compared to that on the MgO(100) surface is also reflected by the 3 + 1 → 4 cluster growth energy, which is 1.17 eV smaller on the γ-Al2O3 surface than in the gas phase (1.57 compared to 2.74 eV, Table 1), whereas on the MgO(100) surface, it is only 0.86 eV smaller than in the gas phase (1.69 compared to 2.55 eV).47 The energy

For a single Cu atom and for Cu2, we have calculated the energy barriers for the H-spillover reaction, which are 0.81 and 0.64 eV, respectively (Figure 6), and the reaction is in both cases exoenergetic by 0.5 and 1.36 eV, respectively (Figures 5 and 6); for TSs and final states, see Figure 6. Table 4 shows reaction energies, enthalpies, and Gibbs free energies for the dehydrogenation reaction (eq 8). The energies show an even−odd alternation with the cluster size, for both the gas-phase [CunHm]m+ clusters and the surface-supported clusters. This is due to the larger stability of the closed shell systems with even electron numbers, namely, Cu+ (a d10 system), Cu3H2+, and Cu5H23+, compared to Cu2+ and Cu4H2+, which have an unpaired electron (see also Figure 5). We conclude that a single Cu atom and Cu3 could reduce one surface hydroxyl proton to 1/2(H2) in an exoenergetic reaction. For Cu5H33+, H2 formation is slightly endoenergetic but dehydrogenation will be driven by the entropy gain connected with releasing H2 into the gas phase as the Gibbs free energies show. Table 4 shows values for T = 300 K and two H2 pressures, standard (1 bar) and 0.1 bar. H2 release from Cu5H33+ surface species is favored even for very low temperatures (100 K), but releasing two hydrogen atoms (as H2) from Cu3H22+ and Cu5H33+ requires more than 1 eV because the species formed, Cu32+ and Cu5H3+, respectively, have an odd number of valence electrons. However, at high temperature and low H2 partial pressure, this may become possible as the very small positive values of the Gibbs free energy for 600 K and 10 kPa indicate. At such conditions, releasing two hydrogen atoms form Cu4H22+ also becomes possible because it requires less energy than releasing one hydrogen atom. The dehydrogenation energies of the charged [CunHm]m+ (n = 1−5, m = 1−3) gas-phase clusters (Table 4) are in accordance with their surface counterparts [CunHm]m+[H6−mO3]m−-surf but always more pronounced, i.e., in the gas phase, exoenergetic reactions are more exoenergetic and endoenergetic reactions more endoenergetic than on the surface. To check if H2 desorption from the hydroxylated γ-Al2O3 surfaces with H-spillover is also kinetically feasible, we calculated the barrier for H2 desorption for the example of the CuH/H5O3surf structure to produce H2 (gas) and Cu/H4O3-surf (Figure 7). The barrier of 0.80 eV is smaller than the 1.44 eV obtained upon formation of the H-spillover structure, i.e., Cu(g) + HOsurf → HCu−O-surf, and thus H2 desorption is a viable process. H


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results in a surface chemical reaction, 2OH− + Co0 → 2O2− + H2 + Co2+. Jensen et al.53 also reported that hydroxylation influences the equilibrium morphology of Cu nanoclusters and pre-exposing α-Al2O3 to water leads to increased Cu wetting due to the Al−O−Cu bond formation. Experiments by Niu et al.54 demonstrated that hydroxyl surface coverage critically affects the ability of Cu to “wet” α-Al2O3(0001) at 300 K.

of formation of a tetramer from two dimers on the γ-Al2O3 surface is 1.36 eV smaller than in the gas phase (0.28 compared to 1.64 eV), but on the MgO surface, it is only 0.72 eV smaller than in the gas phase (1.06 compared to 1.78 eV).47 The different gas-phase values of this study and ref 48 are due to different functionals (PW91 compared to BP86) and basis sets (plane wave vs Gaussian). On the clean γ-Al2O3 surface, the Cu atoms bind to both the surface Al and O atoms and the interaction is strong enough to change the structure of the Cun cluster compared to that in the gas phase. For example, Cu4 changes to a “Y”-shaped structure, and in Cu5, one surface O atom inserts into a Cu−Cu bond. In contrast, on the MgO(001) surface, Cu clusters largely retain their gas-phase structures and adsorb perpendicular to the surface with their Cu atoms binding to surface O sites only.48−50 This is due to the fact that the interaction is dominated by polarization and that the polarizability of the flat Cun clusters (n = 1−5) is larger within the plane of the Cu atoms than perpendicular to the plane of Cu atoms. 4.3. Reverse H-Spillover on the Hydroxylated γ-Al2O3 Surface. Figure 4 and Tables 2 and 3 show that energies of aggregation and growth on the hydroxylated and the clean surface are very similar but that reverse H-spillover changes the growth behavior significantly. Whereas the adsorption energies for Cun on the hydroxylated surface, −2.05, −2.98, −3.49, −3.83, and −3.61 eV, for n = 1−5, respectively, are still of the same order as on the clean surface (−1.47, −2.84, −3.54, −3.93, and −4.26 eV, respectively), the aggregation energies are only about one-half of their values on the clean surface, 0.5 compared to 1.1 eV. After H-spillover, the growth energy of Cu3 and Cu5 becomes even negative, which indicates that Cu2 and Cu4 have no ability to get one more Cu atom. It means that the growth of Cun on the hydroxylated surface is not favored due to the Hspillover reaction and that Cu2 and Cu4 can be the most favored species on the hydroxylated surface. Sanz and Hernández19 studied the reaction of Cu atoms with the hydroxylated α-Al2O3(0001) surface. Two types of hydroxyl groups are formed on water addition to the clean α-Al2O3(0001) surface, bridging hydroxyl groups when a proton attaches to the surface O atoms and terminal hydroxyl groups attached to surface Al atoms, which become fourfold-coordinated. This surface interacts more strongly with Cu atoms than the γ-Al2O3 surface studied here. The adsorption energy for a single Cu atom that binds to the twofold-coordinated oxygen site of the terminal AlIV-OH group is 1.25 eV compared to 0.94 eV (Table 2) for the binding to a threefold-coordinated surface O atom on the γAl2O3 surface. The total surface reaction, H-spillover and H2 desorption, Cu/HO-surf → Cu−O-surf + 1/2(H2) (gas), is 1.35 eV exoenergetic for the hydroxylated α-Al2O3 surface19 and 1.11 eV for the γ-Al2O3 surface. This yields for the total reaction, HOsurf + Cu(gas) → Cu+/O−-surf + 1/2(H2), −2.6 eV for α-Al2O3 and −2.05 eV for γ-Al2O3. In the final surface structures, Cu+ is coordinated to one surface O atom of the α-Al2O3 surface, whereas on γ-Al2O3, it is bridging two O surface atoms. These findings are consistent with previous experiments showing that surface hydroxyls play a critical role in the Cu growth.51 Experimental evidence for reverse H-spillover on alumina has been produced by Heemeier et al.,45 who found that Rh cluster growth on an hydroxylated thin alumina film supported on NiAl(110) is associated with a strong chemical interaction in which surface hydroxyl groups are consumed. In a combined experimental and computational study, Chambers et al.52 found that Co deposition on fully hydroxylated α-Al2O3

5. CONCLUSIONS The adsorption energies of small Cun clusters on the γAl2O3(110) surface show even−odd alternation. Stepwise addition of the first, third, and fifth adsorbed Cu atoms results in smaller adsorption energy increments than those from the addition of the second and fourth Cu atoms. Odd-atom Cu clusters, single Cu atoms, and Cu3 clusters, in particular, show a stronger ability to reduce surface hydroxyls, yielding molecular H2 . On the clean γ-Al2O3(110) surface, Cu clusters bind weakly to the surface, resulting in a three-dimensional growth mode. In contrast, on the hydroxylated surface, reverse H-spillover from surface hydroxyls to the deposited Cun (n = 1−5) clusters and subsequent H2 desorption lead to oxidized Cu clusters that bind more strongly to the alumina surface. These surface reactions favor small even-atom clusters and prevent the aggregation of deposited Cu clusters.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b03764. Total energies and zero-point vibrational energies, dehydrogenation energies for CunHm clusters, and additional figures with structure information (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maria Verónica Ganduglia-Pirovano: 0000-0003-2408-8898 Joachim Sauer: 0000-0001-6798-6212 Present Addresses §

Institute of Applied Chemistry, College of Chemistry, Nanchang University, No. 999 Xuefu Road, Nanchang 330031, P. R. China (G.F.). ∥ Institute of Catalysis and Petrochemistry, CSIC, C/Marie Curie 2, 28049 Madrid, Spain (M.V.G.-P.). ⊥ National Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing 101400, P. R. China (C.F.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the German Research Foundation (DFG) within the Cluster of Excellence “Unifying Concepts in Catalysis” (EXC314) and by the National Natural Science Foundation of China (Grants Nos. 20703052, 21673270, and 21763018). The calculations were partially carried out at the facilities of the North-German Supercomputing Alliance HLRN. G.F. thanks the International Max Planck Research School “Complex Surfaces in Materials I


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(18) Liu, Z.; Wang, Y.; Li, J.; Zhang, R. The Effect of γ-Al2O3 Surface Hydroxylation on the Stability and Nucleation of Ni in Ni/γ-Al2O3 Catalyst: a Theoretical Study. RSC Adv. 2014, 4, 13280−13292. (19) Sanz, J. F.; Hernández, N. C. Mechanism of Cu Deposition on the α-Al2O3 (0001) Surface. Phys. Rev. Lett. 2005, 94, No. 016104. (20) Xiao, L.; Schneider, W. F. Surface Termination Effects on Metal Atom Adsorption on α-Alumina. Surf. Sci. 2008, 602, 3445−3453. (21) Briquet, L. G. V.; Catlow, C. R. A.; French, S. A. Platinum Group Metal Adsorption on Clean and Hydroxylated Corundum Surfaces. J. Phys. Chem. C 2009, 113, 16747−16756. (22) Uzun, A.; Dixon, D. A.; Gates, B. C. Prototype Supported Metal Cluster Catalysts: Ir4 and Ir6. ChemCatChem 2011, 3, 95−107. (23) Vayssilov, G. N.; Gates, B. C.; Rösch, N. Oxidation of Supported Rhodium Clusters by Support Hydroxy Groups. Angew. Chem., Int. Ed. 2003, 42, 1391−1394. (24) Vayssilov, G. N.; Rösch, N. Reverse Hydrogen Spillover in Supported Subnanosize Clusters of the Metals of Groups 8 to 11. A Computational Model Study. Phys. Chem. Chem. Phys. 2005, 7, 4019− 4026. (25) Shor, E. A. I.; Nasluzov, V. A.; Shor, A. M.; Vayssilov, G. N.; Rösch, N. Reverse Hydrogen Spillover onto Zeolite-supported Metal Clusters: An Embedded Cluster Density Functional Study of Models M6 (M = Rh, Ir, or Au). J. Phys. Chem. C 2007, 111, 12340−12351. (26) Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (27) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (28) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces - Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671−6687. (29) Blöchl, P. E.; Först, C. J.; Schimpl, J. Projector Augmented Wave Method: Ab Initio Molecular Dynamics with Full Wave Functions. Bull. Mater. Sci. 2003, 26, 33−41. (30) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, 17953−17979. (31) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (32) Jónsson, H.; Mills, G.; Jacobsen, K. W. In Classical and Quantum Dynamics in Condensed Phase Simulations; Berne, B. J., Ciccotti, G., Coker, D. F., Eds.; World Scientific: Singapore, 1998; p 385. (33) Eichkorn, K.; Treutler, O.; Ö hm, H.; Häser, M.; Ahlrichs, R. Auxiliary basis sets to approximate Coulomb potentials (Chem. Phys. Letters 240 (1995) 283-290). Chem. Phys. Lett. 1995, 242, 652−660. (34) Treutler, O.; Ahlrichs, R. Efficient Molecular NumericalIntegration Schemes. J. Chem. Phys. 1995, 102, 346−354. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (36) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian-Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (37) Straumanis, M. E.; Yu, L. S. Lattice Parameters, Densities, Expansion Coefficients and Perfection of Structure of Cu and of Cu-In α Phase. Acta Crystallogr., Sect. A 1969, 25, 676−682. (38) Tsyganenko, A. A.; Mardilovich, P. P. Structure of Alumina Surfaces. J. Chem. Soc., Faraday Trans. 1996, 92, 4843−4852. (39) Guvelioglu, G. H.; Ma, P. P.; He, X. Y.; Forrey, R. C.; Cheng, H. S. Evolution of Small Copper Clusters and Dissociative Chemisorption of Hydrogen. Phys. Rev. Lett. 2005, 94, No. 026103. (40) Rohlfing, E. A.; Valentini, J. J. UV Laser-excited Fluorescence Spectroscopy of the Jet-cooled Dimer. J. Chem. Phys. 1986, 84, 6560− 6566. (41) Sohlberg, K.; Pennycook, S. J.; Pantelides, S. T. Hydrogen and the Structure of the Transition Aluminas. J. Am. Chem. Soc. 1999, 121, 7493−7499.

Science” for a fellowship within the Joint Doctoral Program of the Chinese Academy of Sciences and the Max Plank Society for support. He also thanks Dr. Long Huang (Beijing Research Institute of Chemical Industry SINOPEC) and Dr. Haijun Jiao for helpful discussions.

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REFERENCES

(1) Gates, B. C. Supported Metal Clusters: Synthesis, Structure, and Catalysis. Chem. Rev. 1995, 95, 511−522. (2) Ertl, G.; Knözinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; VCH: Weinheim, 1997. (3) Brown, M. A.; Fujimori, Y.; Ringleb, F.; Shao, X.; Stavale, F.; Nilius, N.; Sterrer, M.; Freund, H.-J. Oxidation of Au by Surface OH: Nucleation and Electronic Structure of Gold on Hydroxylated MgO(001). J. Am. Chem. Soc. 2011, 133, 10668−10676. (4) Blaser, H.-U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M. Supported Palladium Catalysts for Fine Chemicals Synthesis. J. Mol. Catal. A: Chem. 2001, 173, 3−18. (5) Freund, H.-J. The Surface Science of Catalysis and More, Using Ultrathin Oxide Films as Templates: A Perspective. J. Am. Chem. Soc. 2016, 138, 8985−8996. (6) Lykhach, Y.; Kozlov, S. M.; Skála, T.; Tovt, A.; Stetsovych, V.; Tsud, N.; Dvořaḱ , F.; Johánek, V.; Neitzel, A.; Mysliveček, J.; Fabris, S.; Matolín, V.; Neyman, K. M.; Libuda, J. Counting Electrons on Supported Nanoparticles. Nat. Mater. 2016, 15, 284−288. (7) Sinkler, W.; Sanchez, S. I.; Bradley, S. A.; Wen, J.; Mishra, B.; Kelly, S. D.; Bare, S. R. Aberration-Corrected Transmission Electron Microscopy and In Situ XAFS Structural Characterization of Pt/γAl2O3 Nanoparticles. ChemCatChem 2015, 7, 3779−3787. (8) Bradley, S. A.; Sinkler, W.; Blom, D. A.; Bigelow, W.; Voyles, P. M.; Allard, L. F. Behavior of Pt Atoms on Oxide Supports During Reduction Treatments at Elevated Temperatures, Characterized by Aberration Corrected Stem Imaging. Catal. Lett. 2012, 142, 176−182. (9) Raybaud, P.; Chizallet, C.; Mager-Maury, C.; Digne, M.; Toulhoat, H.; Sautet, P. From γ-Alumina to Supported Platinum Nanoclusters in Reforming Conditions: 10 Years of DFT Modeling and Beyond. J. Catal. 2013, 308, 328−340. (10) Shi, X.-R.; Sholl, D. S. Nucleation of Rhn (n = 1−5) Clusters on γAl2O3 Surfaces: A Density Functional Theory Study. J. Phys. Chem. C 2012, 116, 10623−10631. (11) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. Use of DFT to Achieve a Rational Understanding of Acid-basic Properties of γ-Alumina Surfaces. J. Catal. 2004, 226, 54−68. (12) Valero, M. C.; Raybaud, P.; Sautet, P. Influence of the Hydroxylation of γ-Al2O3 Surfaces on the Stability and Diffusion of Single Pd Atoms: A DFT Study. J. Phys. Chem. B 2006, 110, 1759− 1767. (13) Wischert, R.; Florian, P.; Copéret, C.; Massiot, D.; Sautet, P. Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View. J. Phys. Chem. C 2014, 118, 15292− 15299. (14) Li, J. R.; Zhang, R. G.; Wang, B. J. Influence of the Hydroxylation of gamma-Al2O3 Surfaces on the Stability and Growth of Cu for Cu/ gamma-Al2O3 Catalyst: A DFT Study. Appl. Surf. Sci. 2013, 270, 728− 736. (15) Nasluzov, V. A.; Shulimovich, T. V.; Shor, A. M.; Bukhtiyarov, V. I.; Rö sch, N. Small Gold Species Supported on Alumina. A Computational Study of α-Al2O3(0001) and γ-Al2O3(001) Using an Embedded-cluster Approach. Phys. Status Solidi B 2010, 247, 1023− 1031. (16) Valero, M. C.; Raybaud, P.; Sautet, P. Nucleation of Pdn (n = 1-5) Clusters and Wetting of Pd Particles on γ-Al2O3 Surfaces: A Density Functional Theory Study. Phys. Rev. B 2007, 75, No. 045427. (17) Liu, Y.; Cen, W.; Feng, G.; Chu, Y.; Kong, D.; Yin, H. First Principles Study on the Adsorption of Ptn (n = 1−4) on γ-Al2O3 (110) Surface. Appl. Surf. Sci. 2014, 313, 424−431. J


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The Journal of Physical Chemistry C (42) Jennison, D. R.; Schultz, P. A.; Sullivan, J. P. Evidence for Interstitial Hydrogen as the Dominant Electronic Defect in Nanometer Alumina Films. Phys. Rev. B 2004, 69, No. 041405. (43) Guvelioglu, G. H.; Ma, P. P.; He, X. Y.; Forrey, R. C.; Cheng, H. S. First Principles Studies on the Growth of Small Cu Clusters and the Dissociative Chemisorption of H2. Phys. Rev. B 2006, 73, No. 155436. (44) Ingólfsson, O.; Busolt, U.; Sugawara, K.-i. Energy-resolved Collision-induced Dissociation of Cun+ (n = 2−9): Stability and Fragmentation Pathways. J. Chem. Phys. 2000, 112, 4613−4620. (45) Heemeier, M.; Frank, M.; Libuda, J.; Wolter, K.; Kuhlenbeck, H.; Bäumer, M.; Freund, H. J. The Influence of OH Groups on the Growth of Rhodium on Alumina: A Model Study. Catal. Lett. 2000, 68, 19−24. (46) Ghijsen, J.; Tjeng, L. H.; Vanelp, J.; Eskes, H.; Westerink, J.; Sawatzky, G. A.; Czyzyk, M. T. Electronic-structure of Cu2O and CuO. Phys. Rev. B 1988, 38, 11322−11330. (47) Inntam, C.; Moskaleva, L. V.; Yudanov, I. V.; Neyman, K. M.; Rösch, N. Adsorption of Cu4, Ag4 and Au4 Particles on the Regular MgO(001) Surface: A Density Functional Study Using Embedded Cluster Models. Chem. Phys. Lett. 2006, 417, 515−520. (48) Inntam, C.; Moskaleva, L. V.; Neyman, K. M.; Nasluzov, V. A.; Rösch, N. Adsorption of Dimers and Trimers of Cu, Ag, and Au on Regular Sites and Oxygen Vacancies of the MgO(001) Surface: a Density Functional Study Using Embedded Cluster Models. Appl. Phys. A: Mater. Sci. Process. 2006, 82, 181−189. (49) Musolino, V.; Selloni, A.; Car, R. Structure and Dynamics of Small Metallic Clusters on an Insulating Metal-oxide Surface: Copper on MgO(100). Phys. Rev. Lett. 1999, 83, 3242−3245. (50) Barcaro, G.; Fortunelli, A. The Interaction of Coinage Metal Clusters with the MgO(100) Surface. J. Chem. Theory Comput. 2005, 1, 972−985. (51) Fu, Q.; Wagner, T.; Rühle, M. Hydroxylated α-Al2O3 (0001) Surfaces and Metal/α-Al2O3 (0001) Interfaces. Surf. Sci. 2006, 600, 4870−4877. (52) Chambers, S. A.; Droubay, T.; Jennison, D. R.; Mattsson, T. R. Laminar Growth of Ultrathin Metal Films on Metal Oxides: Co on Hydroxylated α-Al2O3(0001). Science 2002, 297, 827−831. (53) Jensen, M. C. R.; Venkataramani, K.; Helveg, S.; Clausen, B. S.; Reichling, M.; Besenbacher, F.; Lauritsen, J. V. Morphology, Dispersion, and Stability of Cu Nanoclusters on Clean and Hydroxylated α-Al2O3(0001) Substrates. J. Phys. Chem. C 2008, 112, 16953−16960. (54) Niu, C.; Shepherd, K.; Martini, D.; Tong, J.; Kelber, J. A.; Jennison, D. R.; Bogicevic, A. Cu Interactions with α-Al2O3(0001): Effects of Surface Hydroxyl Groups versus Dehydroxylation by Ar-ion Sputtering. Surf. Sci. 2000, 465, 163−176.

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