Structures of Bimetallic CopperâRuthenium Nanoparticles: Incoherent

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On the Structures of Bimetallic Copper-Ruthenium Nanoparticles: Incoherent Interface and Surface Active Sites for Catalytic Nitric Oxide Dissociation Ryoichi Fukuda, Nozomi Takagi, Shigeyoshi Sakaki, and Masahiro Ehara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09280 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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The Journal of Physical Chemistry

On the Structures of Bimetallic Copper-Ruthenium Nanoparticles: Incoherent Interface and Surface Active Sites for Catalytic Nitric Oxide Dissociation Ryoichi Fukuda,*†‡ Nozomi Takagi,† Shigeyoshi Sakaki,§† and Masahiro Eharaǁ┴† †

Center for the Promotion of Interdisciplinary Education and Research, Elements Strategy

Initiative for Catalysts and Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ‡

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,

Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan §

Fukui Institute for Fundamental Chemistry (FIFC), Kyoto University, Takano-Nishihiraki-

cho 34-4, Sakyou-ku, Kyoto 606-8103, Japan ǁ

Research Center for Computational Science, 38 Nishigo-naka, Myodaiji, Okazaki, 444-8585,

Japan ┴

Department of Theoretical and Computational Molecular Science, Institute for Molecular

Science (IMS), 38 Nishigo-naka, Myodaiji, Okazaki, 444-8585, Japan

*

[email protected], Tel: +81-75-383-3036

ACS Paragon Plus Environment

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ABSTRACT: Bimetallic alloy nanoparticles are promising candidates for replacing platinum group metals utilized in the catalytic removal of nitrogen oxides in exhaust gas. In this study, we investigated the electronic, interfacial, and surface structures of copper/ruthenium alloy nanoparticles by quantum chemical computations using 135-atomic cluster models. We employed Ru-core/Cu-shell models in which the Ru-core takes both fcc (face-centered cubic) and hcp (hexagonal closed-packed) structures. The fcc-core model has a coherent Cu/Ru interface while the hcp-core model involves an incoherent interface. This incoherence results discontinuity in the lattice structure and the valence electronic structure, and generates steplike structures on the surface of the particle. Such a step-like site enhances the catalytic activities for nitric oxide dissociation. The orbital energies suggest that the alloying can control the oxidation tendency of clusters. Charge-transfer occurs between the Cu shell and Ru core; the surface layer of the clusters has a positive charge although the surface atoms are not directly binding to the core Ru atoms. The interfacial structure of core-shell interphase is a crucial factor to be considered in designing the properties of alloy nanoparticles.


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1. INTRODUCTION Catalytic activities of alloy nanoparticles, which are composed of more than two metal elements, are receiving increasing attention because they may replace expensive platinum group elements that are employed in many catalytic processes.1,2 For example, rhodium nanoparticles are utilized as a component of automotive catalysts in order to eliminate nitrogen oxides (NOx) in exhaust gas of gasoline engines by reductive decomposition reactions.3 Because rhodium is a rare element and its resources are unevenly distributed, replacing the rhodium in automotive catalysts by ubiquitous elements is highly demanded; however, we hardly expect a NOx decomposition ability of single component nanoparticles as high as that of rhodium. Alloying can improve the freedom in design and material selection of catalysts. The progress in the theories for modeling alloys is desired for proposing a design guideline for alloy nanoparticles.4,5 The atomistic structures of alloy surfaces have been studied little although it is well recognized that the local structure of a surface adsorption-site significantly affects the activities of catalysts.6–8 However, geometrical structures of alloys have been discussed only qualitatively such as the preference for core-shell structures to solid-solution structures in terms of their thermodynamics and phase diagrams.5,9,10 When we prepare an alloy from metals whose lattice parameters are greatly different from each other, discontinuity appears in the interface termed as incoherent interface.10,11 Such discontinuity affects the surface structure of an alloy particle; therefore, that can be responsible for the catalytic activities. In this study, we investigated a copper-based alloy that contains a small amount of ruthenium because fairly high abilities of copper for nitric oxide (NO) reduction have been reported.12 We are interested in the effect of a small amount of ruthenium on the properties of copper nanoparticles. Since the copper and ruthenium are mutually immiscible,13 copper-


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ruthenium nanoparticles favor a core-shell structure14,15 in which hcp (hexagonal closedpacked) ruthenium exists in core13 because the hcp is the most stable structure for ruthenium. The structure of copper shell may depend on its thickness. If we assume that the copper shell takes the fcc (face-centered cubic) structures same as bulk copper, discontinuity appears in the copper/ruthenium incoherent interface. The discontinuity in the interfaces has been noted in some previous studies. The molecular dynamics simulation showed that the copperruthenium interface takes disordered amorphous-like structure whose thickness is estimated to be 2.91 nm.16 Another study pointed the possibility that a small hcp embryos of Ru atoms may be formed in an fcc copper phase.17 Such a disordered interface would have a significance in nanoscale. Additionally, fcc and hcp phases coexist even in a single particle of the solid-solution alloy of palladium (fcc) and ruthenium (hcp).1 In the case that the amount of ruthenium is very small, it is difficult to characterize the alloy nanoparticles by experimental techniques such as the X-ray diffraction or transmission electron microscope although experimental progress is remarkable for controlling the compositions of alloy nanoparticles.1,2,18,19 The knowledge from computational studies is, therefore, valuable. In this study, we investigated such discontinuity between fcc and hcp incoherent interface of alloy nanoparticles in terms of the electronic structures, surface structures, and catalytic activities for NO decomposition. 2. MODELS AND METHODS Computational Details. For cluster models, molecular geometry optimizations were performed with the DMol3 program in the Material Studio.20,21 The PW91 GGA functional was used for the exchange-correlation term. We used the DND (double-numeric quality with d-polarization functions) basis sets with effective core potentials (ECP); 19 and 16 valence electrons were explicitly considered for Cu and Ru, respectively. Transition states were


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calculated with the linear and quadratic synchronous transit (LST/QST) search method.22 The basis set superposition error correction was not considered for evaluating adsorption energies. The molecular orbital energies of the clusters were calculated using the B3LYP/CRENBL ECP with the contracted basis sets in a double-zeta quality23,24 by the Gaussian 09.25 The atomic charges were calculated by the Hirshfeld’s method26 and NBO (natural bond orbital) method27 using the B3LYP/CRENBL ECP by the Gaussian 09. The NAO (natural atomic orbital) bond-order analyses were also performed. Pure and Ru containing Cu surfaces were simulated with a periodic slab model using Vienna Ab-initio Simulation Package (VASP).28 The PW91 GGA functional and the projector augmented wave (PAW) method were adopted. The kinetic cutoff energy for the plane wave basis sets was fixed at 550 eV. The convergence was tested by increasing the cutoff to 600 eV. The Gamma centered mesh with 2 × 2 × 2 was used for sampling the first Brillouin zone. The linear tetrahedron method (LTM) with Blöchl corrections was used for the Brillouin zone integrations.29 A vacuum spacing of 15 Å was added in the z-direction. The calculation of isolated NO molecule was performed in a (10 Å ×10 Å ×10 Å) unit cell with the 1 × 1 × 1 Gamma centered mesh. Bader charges were obtained with using the program developed by Henkelman group.30 Structure Models. Our models of nanoparticles were constructed in the following manners. The cuboctahedron Cu135 cluster was cut from fcc bulk Cu. Starting from this cuboctahedron cluster, we performed a geometry optimization for the doublet spin state; this is the lowest spin state. As the result of the geometry relaxation, we obtained an icosahedral (Ih) structure. This Ih structure is the same as that obtained by the molecular dynamics simulation in which molten Cu135 cluster was cooled to 500 K.31 We, therefore, believe this is the global minimum structure of Cu135 system. Replacing the 13 core atoms of Cu135 (Ih) by


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Ru atoms yields an fcc packed Cu/Ru core-shell model. This Cu122Ru13 fcc-core model keeps the Ih structure after the geometry relaxation for the singlet spin state (the lowest spin state). We also constructed Cu122Ru13 having an hcp-Ru core by embedding the hcp Ru13 cluster into Cu122 shell that was obtained by eliminating the 13 core atoms from the Cu135 (Ih) cluster. Since there are several possibilities of the orientation between the core and shell components, we examined several orientations as the initial geometry of optimization. Based on several trials, we found that the total energy of the cluster significantly depends on the number of CuRu bonds; therefore, we were able to narrow down the trial orientations to those having the maximum number of Cu-Ru bonds. After the geometry optimization for the singlet spin state (the lowest spin state), we obtained one of the most stable structures of the Cu122Ru13 hcpcore model that has a C2v symmetry. The total energy of this hcp-core model is 115.3 kcal·mol−1 higher than that of the fcc-core model. The relative stability depends on the size of models even if the ratio of Ru to Cu is fixed. The instability is attributable to the interfacial energy that is proportional to the surface area of Ru-core. The energy difference between fcc and hcp clusters is proportional to their volume because energy is size-extensive. Consequently, the stabilization by forming a hcp-core becomes larger than the interfacial destabilization with increasing the number of atoms. We believe this hcp-core model is more realistic for our purpose. Although we did not perform the global search of the energy minimum structure of Cu/Ru clusters, it is likely that the present model structures are reasonably consistent with the findings by the molecular dynamics simulations for the formation of hcp-Ru embryo17 and the crystallization Cu shells.31 We believe our structure models are suitable for our purpose. To confirm that the results are not specific to the 135atomic systems, we additionally employed periodic models. Our model of Cu(111) surface was constructed with 3 ×3 supercell consists of six atomic layers. The supercell contains 54 atoms (Figure 1). The initial structures of Cu(111) surface


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models containing Ru cluster were obtained as follows: A cluster of eight Cu atoms in the third and fourth layers was replaced by Ru atoms. Simply replacing Cu atoms by Ru atoms yields the initial structure of Cu(111) containing an fcc packed Ru cluster (hereafter denoted as fcc-Ru@Cu(111)). The initial structure of Cu(111) containing an hcp packed Ru cluster (hereafter denoted as hcp-Ru@Cu(111)) was obtained by arranging the positions of the Ru atoms to correspond to those of six-layered hcp Ru(111) model, where the bottom layer of the hcp Ru(111) were made to be identical to those of the fcc Cu(111) (see Figure 1). From those initial structures, both the ionic positions and cell shapes were relaxed, except for the atomic positions of the bottom layer. The bottom layer was fixed in the optimized position of bulk cupper. The cell shapes were fixed for the calculations of NO adsorption and decomposition. Low-spin states were obtained after performing the spin-polarized calculations.

Figure 1. The models of the fcc Cu(111) and hcp Ru(111) surface. The encircled atoms were exchanged to generate the initial structure of hcp-Ru@Cu(111) model. 3. RESULTS AND DISCUSSION Figure 2 shows the optimized structures of the clusters. The sizes of icosahedral Cu135 and Cu122Ru13 fcc-core clusters are 12.99 Å and 13.14 Å in diameter (the distance between the farthest Cu atoms), respectively. The shape of Cu122Ru13 hcp-core cluster is approximately


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ellipsoid; its diameters in the three axis are 12.98 Å, 13.13 Å, and 14.16 Å. The Cu/Ru alloy clusters are larger than the pure Cu cluster.

Figure 2. Molecular structures of Cu135 (Ih) and Cu122Ru13 hcp-core clusters. The number of the surface atoms and the coordination number of them are shown. The Cu122Ru13 hcp-core cluster has larger surface area than that of Cu135 and has lowcoordinated surface atoms as shown in Figure 2. Regarding the clusters as a sphere or ellipsoid, the surface areas of Cu135, Cu122Ru13 fcc-core, and hcp-core are 530.1 Å2, 542.4 Å2, and 565.9 Å2, respectively. In the Cu135 and Cu122Ru13 fcc-core clusters, the number of the surface atoms is 80; they contain octacoordinated or nonacoordinated atoms. In the Cu122Ru13 hcp-core cluster, 84 Cu atoms appear on the surface. The coordination numbers of them are five to ten. There are low-coordinated atoms on the surface, such as pentacoordinated and hexacoordinated sites. Such low-coordination sites generate edges, kinks, and steps, which play an important role for catalytic reactivities.6–8 These low-coordinated surface-atoms result from the incoherent interface between the hcp-core Ru and fcc-shell Cu. The doped small Ru cluster affects the lattice structure of copper, the principal component of the alloy. The doping of an fcc-packed Ru cluster does not distort the lattice structure of


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Cu but enlarges the lattice constant. We examined the structural regularity of the clusters in terms of the distribution of Ru-Cu and Cu-Cu distances. The histograms for the frequencies of Ru-Cu and Cu-Cu distances in the clusters are given in Figure 3; this graph corresponds to the radial distribution function but the values in the vertical axis are not normalized.

Figure 3. The histograms for the frequencies of Cu-Cu (upper panel) and Ru-Cu (lower panel) distances appear in the Cu and Cu/Ru clusters. In the Cu/Ru fcc-core cluster, we find a high regularity of the distribution of the Ru-Cu distances. In the bonding region (< 3 Å), there are 72 Cu-Ru bonds that are divided into two classes: Ru-Cu = 2.577 and 2.590 Å. We also find a high regularity of the distribution of the Cu-Cu distances in this cluster. The distribution pattern of Cu-Cu distance in the fcc-core cluster is similar to that in the Cu135 cluster. The distributions around 4.5 Å are slightly


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shifted to the long-distance region. Namely, the lattice structure of Cu shell in the fcc-core cluster is the same as that of pure Cu cluster but is expanded by the Ru core. On the other hand the distributions of Ru-Cu and Cu-Cu distances are spread and disordered in the Cu/Ru hcp-core cluster. This means that the doping of the hcp Ru cluster distorts the lattice structure not only in the Ru-Cu interface region but also in the Cu shell component. We find 78 Cu-Ru bonds between the bond lengths of 2.523 Å and 2.965 Å. This spread width of 0.442 Å is remarkably larger than that of the fcc-core cluster (0.013 Å). The Cu shell takes disordered amorphous-like structure. These findings are consistent with the results of molecular dynamics simulation for Cu-Ru multilayers.16 The disorder in Cu shell finally appears on the surface of the cluster. The coherent/incoherent interface between the core and shell components is relevant to the valence orbital structures of these clusters. The orbital-energy diagram is shown in Figure 4, and the isosurface plots of the valence orbitals are given in Figure S1 in Supporting Information. The energy levels of HOMO (highest occupied molecular orbital) for the Cu135, Cu/Ru fcc-core, and Cu/Ru hcp-core clusters are −4.89, −4.45 and −4.61 eV, respectively. Destabilization of the HOMO in the Cu/Ru clusters suggests that the Cu/Ru alloy particles are easily oxidized in comparison with the pure Cu particle. The oxidation tendency of particles is an important factor of catalytic activity for NO reduction.12 We can expect that the Cu/Ru alloy particles could be a better catalyst for NO reduction than pure Cu particles because the redox cycle between Cu(0) and Cu(II) would occur easily; it has been suggested that this redox cycle relate the NO reduction capability.12


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Figure 4. Valence orbital energy diagrams of the Cu135 and Cu/Ru clusters. The orbital degeneracy is not considered in these diagrams. For the Cu135, the HOMO and SOMO (singly occupied molecular orbital) have large amplitudes in the core region, and the large amplitudes of the LUMO (lowest unoccupied molecular orbital) are located on the surface atoms. The LUMO+1 orbital has a character of the anti-bonding between the core and shell Cu atoms. The HOMO of Cu/Ru fcc-core cluster is destabilized in comparison with that of Cu135 because the HOMO of the fcc-core cluster has a character of the Ru d-orbitals and anti-bonding between core Ru and shell Cu atoms. The anti-bonding character destabilizes the HOMO. The HOMO−1 has large amplitudes on the surface atoms. The large amplitudes of LUMO are located on the hollow sites of the surface (the center of pentagon faces of Ih). The HOMO of the Cu/Ru hcp-core cluster is stabilized in comparison with that of the fcc-core cluster. In contrast to the fcc-core cluster, the HOMO of the hcp-core cluster has a non-bonding character of the Ru orbitals. The nonbonding orbital in the valence region results from the incoherent interface between the core and shell components. The HOMO−1 and HOMO−2 orbitals have the similar non-bonding


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character. The LUMO and LUMO+1 orbitals have anti-bonding characters of Ru-Cu and RuRu. Based on the calculated atomic charges, a charge-transfer (CT) is observed between the Cu shell and Ru core in the Cu/Ru alloy clusters. Figure 5 shows the atomic charges with respect to the distance from the central atom of the clusters. Rough trends of both atomic charges are similar to each other; however, the values of charges are remarkably different. The NBO charges are sometimes one digit larger than the Hirshfeld charges. In the Cu135 cluster, all atoms are almost neutral but core atoms have a small amount of negative charge. The surface atoms are neutral (Hirshfeld) or slightly positive (NBO). In the Cu/Ru cluster, the Ru core has a negative charge. The trends in the atomic charges with respect to the distance coincide between the fcc-core and hcp-core clusters. According to the Hirshfeld charge, approximately two-electron-transfer occurs between the Cu shell and Ru core. Adopting the NBO charge, on the other hand, around 18 to 19 electrons move from Cu shell to Ru core. Each Ru atoms except for the central one accept 1.5 electrons. The Cu atoms in the interfacial layer have a relatively large positive charge around +0.3 (NBO). In the fcc-core cluster, the number of Cu atoms in the interfacial layer is 30, and the total of the atomic charges in this layer is +9.732. Approximately 10 electrons is removed from this interfacial layer. The surface Cu atoms have a small amount of positive charge around +0.1 (NBO). For the fcc-core cluster, the total atomic charge of the surface layer that includes 80 atoms is +7.448 (NBO); the total atomic charge of the surface layer is smaller than that of the interfacial layer.


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Figure 5. Variation of atomic charges (Hirshfeld: closed circle and NBO: open circle) with respect to the distance from the central atom of the clusters. According to both the Hirshfeld and NBO pictures, Cu atoms in the shell are approximately neutral. Positive charges in each Cu atoms are apparently small because the number of Cu atoms are about ten times as many as that of Ru; therefore the positive charges are attenuated in individual Cu atoms. The positive charges are not localized at a particular site or layer; the charges are well delocalized. Based on the total atomic charge of surface layer, the electronic structure of the surface is affected by the Ru core even though the surface atoms are not directly binding to the Ru atoms. Note that these findings suggest the possibility that the doping of Ru enhances the CT between the cluster and adsorbed molecules. Such an effect must be one of controlling factors of catalytic activities, but we do not discuss them in this study because very careful argument is necessary for a quantitative estimation of the CT ability by the density functional theory.32 The calculated natural electron configuration of Cu is 3d104s1, and that of Ru is 4d8(5sp)1.5; the sp shells of Ru are occupied by 1.5 electrons. These differences in the electron


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configuration are reflected in the bonding between metals. The values of the NAO (natural atomic orbital) bond-order for Cu-Cu, Ru-Ru, and Ru-Cu are 0.1–0.2, 0.6–0.7, and 0.2–0.3, respectively in the calculated clusters. The total NAO bond-order by atoms are 6–7 for Ru and 0.7–1.5 for Cu (Supporting Information). Atoms in Ru-core are strongly bonding. The bond-order of Cu-Cu and Cu-Ru are much lower than that of Ru-Ru. From the point of view of bond formation, a discontinuity is found between the core and shell components. Molecular orbitals localized in the core component are found in the Cu/Ru alloy clusters. To confirm that the above findings are not specific to the 135-atomic cluster systems, we performed calculations of periodic slab models .We modeled the surface of bulk copper that contains a small Ru cluster. We adopted the Cu(111) surface because it is the most stable surface and similar to the surface of our cluster models. Due to the limitation of computer resources, we limited the size of Ru core to eight atoms. The lattice structure of the hcp-Ru@Cu(111) is disordered by doping of the hcp-Ru core. The optimized structures of Ru@Cu(111) are shown in Figure 6. The fcc-Ru@Cu(111) exhibits almost ideal hexagonal lattice structures (a = 7.797 Å, b = 7.799 Å, α = 89.5°, β = 90.5°, γ = 120.7°). On the other hand, the hcp-Ru@Cu(111) exhibits significantly distorted lattice structures (a = 7.727 Å, b = 7.782 Å, α = 91.5°, β = 87.4°, γ = 120.5°). The energy difference is 14.7 kcal·mol−1; the fcc-Ru@Cu(111) is more stable. The Ru-Cu bond distances in the fcc-Ru@Cu(111) models are distributed in 2.559–2.706 Å. The Ru-Cu bond distances in the hcp-Ru@Cu(111) are 2.490–3.153 Å. This large distribution width of 0.662 Å is comparable to that of the cluster model. Those results demonstrate that the doping of hcp-Ru disorders fcc lattice of Cu. These findings are consistent with the results from the cluster models.


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Figure 6. The optimal structures of the fcc-Ru@Cu(111) and hcp-Ru@Cu(111) models. The surface of the hcp-Ru@Cu(111) is disordered in comparison with that of the fccRu@Cu(111). The surface of fcc-Ru@Cu(111) is flat. Specifically, the z-coordinates of the surface atoms are almost constant: the variation is 0.056 Å only. On the other hand, the variation in the z-coordinates reaches 0.268 Å for the surface atoms of the hcp-Ru@Cu(111). Namely, the hcp-Ru@Cu(111) surface exhibits significant irregularity. In comparison with the surface of the hcp-core Cu122Ru13 cluster, geometry relaxation in the hcp-Ru@Cu(111) seems to be small. Even though the extent of the geometry relaxation depends on the model systems, we consistently found that the doping of a small hcp-core generates disorder in the surface of fcc-shell. High density of states (DOS) that originate from Cu 3d orbitals are obtained in the energy region of −1.5 eV to −4.0 eV (relative energy from the Fermi level) for the pure Cu(111) and Ru@Cu(111) models (see Figure S2 in Supporting Information). The DOS distribution that originates from Ru 4d appears near the Fermi level (−1.5 eV to 0.0 eV). This DOS structure


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is consistent with the valence orbital structures of the cluster models, where the orbitals that is localized in the Ru-core are found around the HOMO–LUMO region. Bader charge analyses show a small amount of electron-transfer occurs from Cu to Ru. The atomic charges in Ru@Cu(111) are relatively small; all Cu atoms are almost neutral. The gross charges in Ru8 core are −0.128 and −0.206 for the fcc-Ru@Cu(111) and hcpRu@Cu(111), respectively. These trends qualitatively agree with the results of the cluster models although the values of the Bader charge in the surface model are much smaller than the NBO charges in the cluster models. Next, we examined the NO adsorption to the clusters and the N–O dissociation on the clusters. For discussions, we need to investigate all adsorption sites and possible reaction paths; in this study, however, we examined representative adsorption sites and typical adsorption structures only for discussing the effect of the disordered surface on NO dissociation qualitatively. We examined an end-on on-top adsorption (a typical monodentate binding) and a side-on three-fold adsorption (a kind of multiple-site binding), where two Cu atoms are attached to N. The adsorption sites we considered are an edge of the pentagon faces (Ih clusters) and a step-like structure generated by hexacoordinated surface Cu atoms (hcpcore cluster). The adsorption structures and adsorption energies in the low-spin state (singlet or doublet) are summarized in Table 1. The N-O bond is elongated by the adsorption. The effect is significant for the side-on adsorption. In comparison with the Cu135, the adsorption energies for the Cu/Ru fcc-core cluster are small and those for the hcp-core cluster are large. A precise cause of those differences was not found; however, we guess that the extent of the geometry relaxation of the surface and the valence orbital distribution on surface atoms are relevant to the adsorption


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energies. To confirm this assumption, we need a systematic investigation of all possible adsorption sites; however, such an extensive investigation is beyond the scope of this paper. Table 1. Adsorption structures (angstrom and degree) and adsorption energies (∆Eads in kcal·mol−1) of NO on the Cu and Cu/Ru clusters. Cu(111)

Cu135

Cu122Ru13 fcc-core

Cu122Ru13 hcp-core

Cu-N

1.801

1.826

1.894

1.847

N-Oa)

1.183

1.185

1.180

1.194

Cu-N-O

179.7

150.9

136.5

149.4

∆Eads

16.5

23.5

15.1

24.2

Cu-N

1.983, 1.984

1.950, 1.997

1.970, 2.047

1.963, 1.978

N-O

1.226

1.260

1.263

1.265

Cu-O

2.902b)

2.104

2.054

2.099

∆Eads

27.4b)

20.0

16.2

28.3

End-on

Side-on

a)

Calculated bond length of isolated NO is 1.163 Å (Dmol3).

b)

Side-on three-fold structure was not found on Cu(111). End-on three-fold structure is shown.

The NO adsorption structure on the Cu(111) surface model differs from those of the cluster model. Table 1 also shows the calculated adsorption structures and adsorption energies on the Cu(111) model surface. Our computation reproduced the previous results by the PAW PW91 computations for NO on Cu(111) surface.33,34 We could not find a side-on three-fold adsorption structure on the Cu(111): the Cu-O bond was elongated, and an end-on three-fold (hcp-hollow site) structure was finally obtained. The end-on on-top adsorption on the Cu(111) exhibits almost linear adsorption structure, while the end-on on-top adsorption on


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the cluster model shows bend structure. The electronic structures of NO on Cu(111) surface and Cu135 cluster would be different. The NO on Cu(111) has more negative charge: the Bader charge of NO on Cu(111) is −0.31, and the Hirshfeld charge of NO on Cu135 is −0.13 by Dmol3 calculation. Finally, we investigated the NO dissociative adsorption. The dissociation is initiated from the side-on adsorption. Figure 7 shows the energy diagram of the NO dissociation on the Cu and Cu/Ru clusters in the low-spin state. The calculated barrier height is still very high in comparison with rhodium clusters: the reported values are about 40 kcal·mol−1 for Rh7+ cluster.35 The calculated barrier by our model is higher than the previous PAW PW91 studies of NO dissociation on the Cu(111) fcc hollow site: the reported values are 37.1 and 43.4 kcal·mol−1.33,34 Previous studies showed that NO dissociation on the Cu(111) is endothermic. The dissociated product has 7.6 or 13.8 kcal·mol−1 higher energy than the initial adsorbed species.33,34 On the other hand, our cluster model exhibits ectothermic reaction probably due to the stabilization of the dissociated products by surface relaxation. The side-on adsorption energy of the Cu/Ru hcp-core cluster is significantly larger than the adsorption energy on the pure Cu cluster, and the dissociation product on the Cu/Ru hcp-core cluster is remarkably stable in comparison with the product on other clusters. Although we examined a limited number of examples only, we may see a Brønsted−Evans−Polanyi type relation36 between the activation energies and the adsorption energies of the reactant and products for the series of clusters: the activation energies are proportional to the negative values of thee adsorption energy. An interesting feature for the Cu/Ru hcp-core cluster is that the side-on adsorption at a low-coordination site is more stable than the end-on on-top adsorption. The side-on adsorption is the initial state of NO dissociation; therefore, we expect an improved catalytic activity for the Cu/Ru hcp-core cluster.


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Figure 7. Energy diagram of NO decomposition reaction on the Cu and Cu/Ru clusters, where the activation energies (∆Ea) and the relative energies of the reactant and product with respect to the noninteracting species are given. The inserted figures are the structures for the reaction on the Cu122Ru13 hcp-core cluster. 4. CONCLUSION An incoherent interphase interface in core-shell alloy nanoparticles may have disordered surface structures including steps, edges, and kinks that are potential active sites of catalysts, even though the number of core atoms are much smaller than that of shell atoms. The Cu/Ru alloy particle with an incoherent interface between an fcc-packed shell and an hcp core showed an improved catalytic activities for NO dissociation in comparison with the pure Cu cluster and Cu/Ru alloy with a coherent interface. Such structural discontinuity also affected the valence orbital structure through bonding/non-bonding interactions between core and shell components. The HOMO level of the Cu/Ru alloy clusters are higher than that of pure Cu cluster because the HOMO of alloys has the character of an anti-bonding or non-bonding between the core and shell components. This suggests that the Cu/Ru alloy particles are


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easily oxidized and could be a good catalyst for NO reduction. The atomic charge analyses show that CT occurs between the core and shell component; the core Ru atoms accept excess electrons. The positive chargers are distributed over all the Cu atoms. The surface layer shows positive charges even though their Cu atoms are not directly binding to Ru atoms. The periodic model calculations support the impact of the Cu/Ru incoherent interface on the lattice constants, electronic structures, and surface structures. Although further studies including larger periodic slab models are needed to discuss the adsorption and decomposition of NO in detail, these findings show that the structure of the interphase interface can be a crucial factor to be considered in for designing the properties of alloy nanoparticles.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:XXXXX. The isosurface plots of the valence orbitals, density of states plot of Cu(111) and Ru@Cu(111), and Cartesian coordinates of models with the values of atomic charges and bond indices.

AUTHOR INFORMATION Corresponding Author *[email protected], Tel: +81-75-383-3036 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by a MEXT (Ministry of Education Culture, Sports, Science and Technology in Japan) program "Elements Strategy Initiative to Form Core Research Center". The computations were partially performed at the Research Center for Computational Science, Okazaki, Japan.


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TOC GRAPHICS


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Figure 1. The models of the fcc Cu(111) and hcp Ru(111) surface. The encircled atoms were exchanged to generate the initial structure of hcp-Ru@Cu(111) model. Figure 1. 78x54mm (300 x 300 DPI)



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Figure 2. Molecular structures of Cu135 (Ih) and Cu122Ru13 hcp-core clusters. The number of the surface atoms and the coordination number of them are shown. Figure 2 81x69mm (300 x 300 DPI)


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Figure 3. The histograms for the frequencies of Cu-Cu (upper panel) and Ru-Cu (lower panel) distances appear in the Cu and Cu/Ru clusters. Figure 3 112x156mm (300 x 300 DPI)



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Figure 4. Valence orbital energy diagrams of the Cu135 and Cu/Ru clusters. The orbital degeneracy is not considered in these diagrams. Figure 4 88x95mm (300 x 300 DPI)


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Figure 5. Variation of atomic charges (Hirshfeld: closed circle and NBO: open circle) with respect to the distance from the central atom of the clusters. Figure 5 81x85mm (300 x 300 DPI)



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Figure 6. The optimal structures of the fcc-Ru@Cu(111) and hcp-Ru@Cu(111) models. Figure 6. 163x83mm (300 x 300 DPI)


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Figure 7. Energy diagram of NO decomposition reaction on the Cu and Cu/Ru clusters, where the activation energies (∆Ea) and the relative energies of the reactant and product with respect to the noninteracting species are given. The inserted figures are the structures for the reaction on the Cu122Ru13 hcp-core cluster. Figure 7 82x79mm (300 x 300 DPI)



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TOC GRAPHICS TOC GRAPHICS 50x49mm (300 x 300 DPI)


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