Design of Robust and Reactive Nanoparticles with Atomic Precision

Aug 10, 2009 - Al12X (X = Ni, Pd, Pt, Ti, and Zr) Clusters: Promising Low-Cost and High-Activity Catalysts for CO Oxidation. Ling Guo and Xiao Zhang...
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J. Phys. Chem. C 2009, 113, 15559–15564

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Design of Robust and Reactive Nanoparticles with Atomic Precision: 13Ag-Ih and 12Ag-1X (X ) Pd, Pt, Au, Ni, or Cu) Core-Shell Nanoparticles Hyun You Kim, Da Hye Kim, Ji Hoon Ryu, and Hyuck Mo Lee* Department of Materials Science and Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Korea ReceiVed: May 29, 2009; ReVised Manuscript ReceiVed: July 17, 2009

Density functional theory calculations and a modified reaction model confirm that the initial high CO oxidation reactivity of a 13Ag-Ih nanoparticle from an icosahedron (Ih) structure is immediately diminished as the nanoparticle is transformed to an amorphous state by a reaction-driven structural change. The adsorption of O2 and the formation of a four-center intermediate metastable state from coadsorbed CO and O2 positively charge the 13Ag-Ih nanoparticle, and the repulsive force between the Ag atoms causes the reaction-driven structural change of the 13Ag-Ih nanoparticle. When one central Ag atom is substituted with a solute atom, a core-shell type of 12Ag-1X-Ih (X ) Pd, Pt, Au, Ni, or Cu) bimetallic nanoparticle is stabilized. Among them, we propose the 12Ag-1Pd nanoparticle as a robust and reactive Ag-based bimetallic nanoparticle for CO oxidation. The results show that the structural fluxionality accounts for the catalytic activity of small nanoparticles. 1. Introduction Since Haruta’s pioneering findings that small gold nanoparticles can catalyze CO oxidation even at or below room temperature,1,2 studies on the catalytic effect of small nanoparticles have attracted considerable attention.1,3-5 Recent reports have attributed the remarkable catalytic activity of nanoparticles to smaller nanoparticles than where attention has been directed thus far.6-11 Nanoparticles as small as 55 atoms8 or even about 10 atoms7,9,10 have shown considerable catalytic activity in several reactions. The suggested factors that make nanoparticles catalytically active include dynamic structural fluxionality,12-17 the size effect,13,14,18-21 interactions with a support,9,12-14 excess electrons,22,23 and the presence of low-coordinated surface atoms.23-27 Of these, the structural fluxionality of nanoparticles, the importance of low-coordinated surface atoms, and the size effect have been reported in the greatest detail. Several theoretical studies have reported that small nanoparticles adjust their structure during adsorption and transition state formation; they also contend that these structural changes are beneficial.12-14,16 Another recent exceptional study of Falsig et al. has clearly confirmed that the presence of low coordinated surface atoms is essential for excellent CO oxidation activity of gold nanoparticles.25,26 In other studies, Roldan et al. found that the size of Au nanoparticles is critical for an oxygen dissociation reaction, and Yudanov et al.19 and Han et al.20 highlighted the fact that particle size and surface structure have an important effect on the adsorption properties of Pd and Pt nanoparticles. Yet, in spite of these recent fruitful discussions, details on the origin of the excellent catalytic properties of small nanoparticles are still sketchy. Another promising area of research on the catalytic activity of nanoparticles involves bimetallic nanoparticles.28-33 When a solute element is added, tunable catalytic activity can be expected. Bimetallic catalysts based on Pt, Pd, Au, or Ag have been widely studied,28 as has the structural stability of various * To whom correspondence should be addressed. Tel: +82-42-350-3334. Fax: +82-42-350-3310. E-mail: [email protected].

sizes of bimetallic nanoparticles.28-30,34-38 Because of differences in the atomic size and surface energy of two composing elements, a core-shell or three-shell onionlike structure has been reported in several systems.28-30,34,35 Other recent studies on bimetallic nanoparticles have reported that the solute element chemically interacts with the host element and modifies the catalytic properties.31,33 However, the exact quantitative role of the solute element is still under debate. Here we examine how the structural fluxionality and the presence of the solute element affect the catalytic activity of nanoparticles. Using density functional theory (DFT) calculations and a microkinetic reaction model, we show how the CO oxidation reactivity of an unsupported 13-atom Ag-icosahedron (Ih) nanoparticle diminishes with structural change; we also show that the addition of a Pd atom prevents the diminishment of reactivity. By selecting a Ag nanoparticle, which is generally regarded as a less active catalyst for CO oxidation than an Au nanoparticle,25,26 we attempt to clarify the effect of the solute element (Pd, Pt, Au, Ni, and Cu). Recent reports have experimentally shown that unsupported nanoparticles39,40 or nanoparticles supported on chemically inert supporting materials8 are catalytically active. In the light of those studies, we investigate unsupported nanoparticles. As such, we exclude undesired electronic and morphological interactions between the nanoparticle and the support. Initially, we considered two types of nanoparticles: 13Ag-Ih and 13Ag-Cubo-Oh (cubo-octahedron) (see Figure S1a,b, Supporting Information). Molecular simulation studies on the structural stability of small nanoparticles based on many-body force fields generally confirm that a highly symmetric Ih or Cubo-Oh is the most stable structure.41,42 A first-principle study has also predicted that the amorphous state of a 13-atom nanoparticle is more stable than symmetrical nanoparticles.41 For our study, we adopted a general approach and accordingly used the symmetrical structures of Ih and Cubo-Oh as the initial structures of the nanoparticles. Because Ih and Cubo-Oh nanoparticles have been generally reported in various nanoparticles, irrespective of their size, we expect the current results to be highly applicable to studies that investigate how the size,

10.1021/jp905047h CCC: $40.75  2009 American Chemical Society Published on Web 08/10/2009

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coordination number, and presence of solute elements affect the catalytic properties of nanoparticles. We used a CO oxidation reaction, which is the simplest and most representative reaction, to analyze the reactivity of nanocatalysts. For a quantitative comparison of the CO oxidation reactivity of the nanoparticles, we used a microkinetic model; we also calculated the Sabatier activity (SA) from the DFTderived thermodynamic parameters (the details of which are given in the Supporting Information).25,43 2. Computational Methods We performed GGA-level spin-polarized Kohn-Sham DFT calculations with the atomic orbital based DMol3 code44,45 and the revised-PBE functional.46 The Kohn-Sham equation was expanded in a double-numeric quality basis set with polarization functions (DNP). The orbital cutoff range was 5.0 Å. The DFT semicore pseudopotential47 was used to treat the core electrons of heavy Ag and Pd atoms. We used a Fermi smearing method with a window size of 0.002 hartree (1 hartree ) 27.2114 eV). The energy, force, and displacement convergence criterion were set to 10-5 hartree, 0.002 hartree/Å, and 0.005 Å, respectively. The transition state calculations were performed using synchronous transit methods, the linear synchronous transit, and quadratic synchronous transit,48,49 in combination with the conjugate gradient minimization algorithm50 for subsequent refinement. 3. Results and Discussion 3.1. Modified Microkinetic Reaction Model: CO Oxidation by Small Nanoparticles. Liu et al. reported that an association mechanism that follows the Langmuir-Hinshelwood mechanism is essential for CO oxidation catalyzed by the step edge of an Au surface.51 Nørskov and co-workers also used an association mechanism to show that the low-coordinated surface atoms are responsible for the high catalytic activity of small Au nanoparticles.25 Whenever an association mechanism is used in CO oxidation, coadsorbed CO and O2 molecules evolve into a four-center intermediate metastable (MS) state (O-O-CO, the geometry of which is shown in stage 2 of Figure 1 and Figure S2c, Supporting Information), and the MS state is dissociated into a gas phase CO2 molecule and an adsorbed O atom. In contrast, a dissociation mechanism, a general CO oxidation path on a metal surface,25,51 requires a dissociation of adsorbed O2. Table 1 shows the adsorption energy of the CO and O2 molecules on the 13Ag-Ih and 13Ag-Cubo-Oh nanoparticles. The free energy of the gas phase O2 molecule at 300 K is 0.44 eV, while that of the CO molecule is 0.39 eV. On account of these results, we only considered cases in which the adsorbates more strongly interact with nanoparticles than their gas-phase free energy. Moreover, because of the low adsorption energy of the CO and O2 molecules, we excluded the 13Ag-Cubo-Oh nanoparticle from further considerations. We also ignored the dissociation mechanism of CO oxidation because the calculated activation energies of the O2 dissociation on the 13Ag-Ih nanoparticles were 3-5 times higher than the energy barriers of the association mechanism (as was the case with the 12Ag-1X bimetallic nanoparticles). Additionally, a dissociation reaction path requires more adsorption sites than an association mechanism. We postulate that the association mechanism accelerates the reaction and produces a higher volume of CO2. On the basis of an association mechanism of CO oxidation, we tested all possible CO-O2 coadsorption geometries and

Figure 1. CO oxidation on the 13Ag-Ih nanoparticle, with the most reactive reaction pathway (in red) and the corresponding geometry of each stage. After two CO molecules are oxidized, the structure of the nanoparticle becomes amorphous (Am). The energy of the 13Ag-Ih nanoparticle plus two CO molecules and one O2 molecule is taken to be zero. Ex is the energy of the xth stage relative to the initial state. Because of the reaction-driven structural change, the released total reaction energy (E4 ) -7.11 eV) is equal to the sum of the energy of oxidation of two CO molecules (-6.22 eV) and the energy of the Ih to Am transition (-0.89 eV). TS represents the location of the transition state.

TABLE 1: Relative Structural Stability and CO Oxidation Reactivity of Studied Nanoparticlesa 13Ag-Ih 13Ag-Cubo-Oh 13Ag-Am 12Ag-1Pd-Ih 12Ag-1Pt-Ih 12Ag-1Ni-Ih 12Ag-1Au-Ih 12Ag-1Cu-Ih

∆Ebind (eV)

O2 Ead (eV)

CO Ead (eV)

SA

-0.65 -0.61 -050 -0.70 -0.79 -0.62 -1.21 -0.64

-0.88 -0.10 -0.26 -0.57 -0.41 -0.50

-0.23

-0.45 -0.89 -1.75 -3.00 -3.93 -0.13 -1.08

-0.65 -1.05

-0.20

a

∆Ebind is the differences in the total binding energy of nanoparticles relative to the 13Ag-Ih particle, which scales the structural stability of nanoparticles relative to the 13Ag-Ih nanoCO particle. EOad2 is the energy of adsorption of O2, and Ead is the energy of adsorption of CO on nanoparticles. SA is the Sabatier activity.

Figure 2. The highest occupied molecular orbital (HOMO) of nanoparticles with molecularly adsorbed oxygen: (a) a 13Ag-Ih nanoparticle and (b) a 12Ag-1Pd-Ih nanoparticle. This plot is for the surface where the probability of finding an electron is equal to 0.02 e/Å3.

constructed corresponding CO oxidation pathways. The oxidation process of the first two CO molecules by the 13Ag-Ih particle is presented in Figure 1. Interestingly, with adsorbates on the surface, the 13Ag-Ih nanoparticle underwent a structural change. The coadsorbed CO and O2 emerged into an MS state. The energy barriers of the MS intermediate formation varied

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in a reasonable range (0.17-0.44 eV; these low energy barriers do not terminate the reaction). The morphology of the MS state was consistent, irrespective of the initial location of the reactants. Because the structure of the 13Ag-Ih particle changes in relation to the initial relative location of CO and O2 molecules, all possible coadsorption geometries create their own reaction pathway (see Figure 1). Later, the MS state was dissociated into a CO2 molecule and a residual O atom. Because of the high exothermicity of CO2 desorption, the activation energy would be very low. We found that another CO molecule was directly adsorbed on the residual O atom and oxidized to CO2 without any energy barrier. The oxidation of a second CO molecule by a residual O atom follows the Eley-Rideal mechanism.43 Thus, with the considerable CO partial pressure, the life span of the residual O atom is very short. On the basis of these results, we propose the following simple sequential reactions for CO oxidation by the 13Ag-Ih nanoparticle:

CO(g) + * S CO*

(R1)

O2(g) + * S O2*

(R2)

CO* + O2* + CO(g) S 2CO2(g) + 2*

(R3)

The first two reactions, R1 and R2, are unactivated; hence, they are fast and in equilibrium. The oxidation of two CO molecules is described in the third reaction, R3, because the residual O atom is directly oxidized by the second CO molecule without an energy barrier. The coverage of the residual O atom, therefore, is very low and has no influence on the overall reaction rate. In our modified reaction model, the Sabatier rate of forming CO2, rs, is equal to the maximum rate of reaction R3. 3.2. CO Oxidation by the 13Ag-Ih Nanoparticle: Reaction Driven Structural Change. Using a two-layered, 12-atom model nanoparticle, Nørskov and co-workers reported that the SA for the CO2 formation of their Ag nanoparticle is around -1.0 [273 K, p(CO) ) 0.01 bar, and p(O2) ) 0.21 bar].25 However, the SA of the 13Ag-Ih nanoparticle, as calculated with our modified reaction model and the DFT-derived parameters, was -0.23 under the same conditions, and this SA result is similar to the SA of Nørskov’s Au model nanoparticle.25 Nørskov noted that their model particle is aimed at somewhat larger nanoparticles, in the range from 2 to 5 nm.25,26 Recently, in a natural extension of Nørskov’s findings, Roldan et al. studied O2 dissociation on several Au nanoparticles of various sizes (Au25, Au38, Au55, and Au79).21 They found that a specific Au nanoparticle with 38 atoms, Au38, can facilitate O2 dissociation. A smaller Au nanoparticle, Au25, just partially dissociates an O2 molecule, but slightly larger particles, Au55 and Au79, cannot dissociate an O2 molecule. Their results show that even though the presence of low-coordinated surface atoms is a minimum requirement for the superior catalytic activity of nanoparticles, the catalytic activity of nanoparticles is very sensitive to particle size. In this context, we postulate the following: when the size and coordination number of nanoparticles are carefully controlled, a nanoparticle composed of a relatively less active species that has previously been excluded from the design of new catalysts can exhibit a high level of activity. The high CO oxidation reactivity of the 13Ag-Ih nanoparticle is a typical example. Because of the high SA (-0.23), the 13Ag-Ih particle is potentially a good catalyst for CO oxidation. However, before

the catalytic activity of the 13Ag-Ih nanoparticle can be validated, a more precise study of the reaction-driven structural change is necessary. After two CO molecules are oxidized to CO2, the structure of the 13Ag-Ih particle is completely changed into an amorphous state (as shown in the geometry of stage 4 and the inset of Figure 1 and Figure S1c, Supporting Information) and closely resembles the morphology of an amorphous particle; note that Oviedo and Palmer have suggested that this type of structure is the most stable structure of the 13Ag nanoparticle.41 We found that the stability of the amorphous 13Ag (13Ag-Am) particle exceeds that of the 13Ag-Ih particle by as much as 0.89 eV. As shown in Figure 1, the energy released from the structural change also contributes to the total reaction energy. Note, however, that the energy of oxidation of two CO molecules is -6.22 eV; hence, if the 13Ag-Ih nanoparticle is a catalyst, the total reaction energy should be equal to -6.22 eV. Dynamic morphological changes have also been reported to occur in gold nanoparticles,12-14,16 core-shell bimetallic nanoparticles,52 nanowires,53 and on a gold surface.15 The studies of Lopez and Nørskov,16 Hakkinen et al.,14 and Hrbek et al.13 all claim that this type of change is beneficial to reactions because the dynamic structural change facilitates the reaction by holding the adsorbates more strongly and lowering the reaction energy barrier. On the other hand, Hrbek et al. showed that adsorptiondriven morphological change decreases the CO adsorption property of a gold surface.15 Although the structure of the 13Ag nanoparticle was transformed from Ih to Am after the oxidation of two CO molecules, if the 13Ag-Am particle shows moderate catalytic activity for the CO oxidation as well, then more CO molecules can be oxidized. As such, the 13Ag nanoparticle would be a good CO oxidation catalyst, irrespective of its structure. In addition, if the amorphous particle could be easily transformed back to the initial Ih particle, such dynamic structural change would underlie the CO oxidation catalyzed by the 13Ag-Ih nanoparticle. However, these conditions were not fulfilled. Because the energy difference between the Ih and Am particles is as high as 0.89 eV, two things are verly likely, namely, the amorphous particle cannot spontaneously change back to the Ih particle at low temperature (which is an industrially important requirement for CO oxidation) and the structural transition from Ih to Am is irreversible. Moreover, because CO does not strongly bind to the 13Ag-Am nanoparticle, it is not a CO oxidation catalyst (see Table 1). Consequently, the high initial CO oxidation activity of the 13Ag-Ih nanoparticle declines as the structure of the 13Ag nanoparticle changes to amorphous. Sun et al. experimentally and theoretically showed that the amorphous Pt nanoparticle does not catalyze a hydrogen oxidation reaction, though the crystalline nanoparticle does.54 Our quantitative analyses confirm that a collapse in catalytic activity during the structural change from a crystalline to amorphous state would be a general tendency in small nanoparticles. We stated that the high energy difference (0.89 eV) between the Ih and Am reaction pair leads to a decline in the initially high catalytic activity of the 13Ag-Ih nanoparticle. The energy and morphology of isomers of very small nanoparticles are highly dependent on particle size.55,56 Therefore, if a certain Ag nanoparticle composed of slightly more or less atoms than our 13Ag nanoparticles can generate an initial-final reaction pair with a relatively small energy difference, the final structure can be transformed back to the initial structure at room temperature. As such, the nanoparticle is a good catalyst, and the reaction-

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TABLE 2: Mulliken Charge and Spin Density of the 13Ag-Ih Nanoparticle, with and without Molecularly Adsorbed Oxygena Mulliken charge (spin density) 13Ag-Ih + O2

13Ag-Ih Ag shell

Ag core

Ag1

Ag2

Ag core

-0.021 (0.394)

0.257 (0.068)

0.156 (-0.002)

0.183 (-0.023)

0.282 (-0.002)

a

Refer to Figure 2 for geometries.

Figure 3. Adsorbate-induced charge redistribution of the 13Ag-Ih nanoparticle: (a) O2 adsorption, (b) O2 and CO coadsorption, and (c) MS intermediate formation. The O2 adsorption occurs through a charge transfer from the nanoparticle to an O2 molecule. The CO molecule donates a charge to the nanoparticle. The formation of the MS intermediate state depletes the electrons on the nanoparticle.

driven structural change does not deactivate the nanoparticle but contributes to the reactivity. Recall that the current study is focused on unsupported nanoparticles. Because the reducible metal oxide supports donate some electrons to the nanoparticles,18,57-62 an 13Ag nanoparticle supported on such reducible oxides may have different catalytic properties. 3.3. Origin of the Reaction Driven Structural Change. The adsorption of an O2 molecule, which is a prior step for CO oxidation, requires excess electrons; thus, charge transfer occurs readily during molecular adsorption of O2.22 Figure 2 shows that the π* antibonding orbital of the O2 molecule takes electrons from the nanoparticle. The Mulliken charge and spin density profiles presented in Table 2 confirm that an adsorbed O2 molecule preferentially depletes the unpaired electrons of the nanoparticles. The charge transfer from the nanoparticle to an O2 molecule is accompanied by the transfer of unpaired electrons. Figure 3a shows that the O2 adsorption on the 13Ag-Ih nanoparticle positively charges the nearby Ag atoms (Ag1 and Ag2 in Figure 2 and Table 2). Such electron depletion sheds light on the origin of the reaction-driven structural change of the 13Ag-Ih nanoparticle. The structure of the 13Ag-Ih nanoparticle with O2 on the bridging position shows that the local symmetry of the Ih nanoparticle is broken by the O2 adsorption (See Figures 2a and 3a). The repulsive force between positively charged Ag1 and Ag2 atoms and the Ag core atom is the likely cause of this behavior. We also found that the positively charged 13Ag-Ih nanoparticle is energetically unstable. Because the surface of the nanoparticles is covered with low coordinated atoms, the positive charging of the nanoparticle naturally makes it unstable. Accordingly, as the adsorbates take electrons, the 13Ag-Ih nanoparticle is subject to a strong driving force, which spontaneously changes the nanoparticle to an energetically more stable structure. The adsorption of O2 positively charges the 13Ag-Ih nanoparticle and breaks the symmetry, thereby causing the spontane-

ous structural change of the nanoparticle. The CO molecule is known as an electron donor.31,63 However, additional adsorption of the CO molecule in the presence of the preadsorbed O2 molecule does not return the 13Ag-Ih nanoparticle to the initial Ih structure (Figure 3b). Moreover, Figure 3c shows that during the formation of the MS state from the coadsorbed CO and O2 molecules, the 13Ag-Ih nanoparticle loses more electrons and the severe depletion of electrons leads to the reaction-driven structural change. 3.4. Design of Robust and Reactive 12Ag-1X-Ih (X ) Pd, Pt, Au, Ni, or Cu) Core-Shell Nanoparticles. Bimetallic nanoparticles have mainly been considered as a system in which the electronic properties of each component are combined. A mixed bimetallic surface, otherwise known as a near-surface alloy, is a representative example.64 When a solute element is added to a host nanoparticle, the preferential surface segregation of an element with low surface energy generally occurs. Ruban et al. reported that Ag atoms selectively segregate to the surface layer of several Ag-based bimetallic alloys.65 Our previous MD simulation results showed that Ag atoms segregate to the surface of Ag-Pd29,30,66,67 and Ag-Pt68 nanoparticles. Baletto et al. reported the core-shell structures with Ag shell layer as a stable structure of Ag-Cu69 and Ag-Ni70 bimetallic nanoparticles. In our previous studies on the structural evolution of the 135Ag-16Pd bimetallic nanoparticle, we reported the structural stability of the nanoparticle whose surface is covered with many Ih-Pd units.29,30,71 Rossi et al. also showed that the magic polyicosahedral Ag-Ni and Ag-Cu nanoparticles are structurally stable.72 Recently, Chen and Johnston showed that the stable basic Ih units such as Ih13 or Ih55 can be a structural motif of larger Ag-Au bimetallic nanoparticles.73 Because the reactiondriven structural change leads to the decline in the high initial CO oxidation reactivity of the 13Ag-Ih nanoparticle, given the reported robustness of the core-shell and Ih structures in several Ag-based bimetallic nanoparticles, we introduced a Pd, Pt, Au, Ni, or Cu core atom into the 13Ag-Ih nanoparticle as a structural stabilizer to prevent any reaction-induced structural change. The stabilization of Au nanoparticles by a central impurity atom was reported in the seminal paper of Pyykko and Runeberg.32 They showed that the endohedral 12Au-1W-Ih nanoparticle has a special stability with a high HOMO-LUMO gap at around 3 eV. Subsequently, Wang et al. experimentally observed 12Au-1W-Ih and 12Au-1Mo-Ih nanoparticles.74 The 18-electron rule was suggested as a propensity rule for the structural stability of cagelike structure.32,74,75 Our 12Ag-1X-Ih nanoparticles, however, do not obey the 18-electron rule. For example, as a result, the 12Ag-1Pd-Ih nanoparticle has a rather narrow HOMO-LUMO gap, 0.34 eV; this behavior is similar to that of the 12Au-1Pd-Ih nanoparticle as reported by Arratia-Perez and Hernandez-Acevedo.76 Surprisingly, however, we found that core atoms stabilize the Ih nanoparticle (Table 1). For example, the total binding energy of the 12Ag-1Pd-Ih nanoparticle is increased by as much as 1.75 and 0.84 eV relative to the 13Ag-Ih and 13Ag-Am with a Pd core atom. Table 1 shows that all heterogeneous solute core atoms except for Au strengthen the Ih structure and that, as a result, our strategy is promising. Interestingly, the structure of the 12Ag-1Au-Ih nanoparticle was very flexible, as the binding energy predicts. The structure of Ag-Au bimetallic nanoparticles has been widely studied.68,77,78 On the basis of DFT calculated mixing enthalpy, Ryu et al. reported the Ag-shelled alloy structure in 147Ag-13Au bimetallic nanoparticle.68 Wang et al. found that chemically synthe-

13Ag-Ih and 12Ag-1X Core-Shell Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15563 TABLE 3: Mulliken Charge and Spin Density of the 12Ag-1Pd-Ih Core-Shell Nanoparticle, with and without Molecularly Adsorbed Oxygena Mulliken charge (spin density) 12Ag-1Pd-Ih + O2

12Ag-1Pd-Ih Ag shell

Pd core

Ag1

Ag2

Pd core

-0.008 (0.300)

0.091 (0.318)

0.152 (0.034)

0.157 (0.037)

0.076 (0.095)

a

Refer to Figure 2 for geometries.

Ag2 atoms and the Pd core atom are weak. As such, the Pd core atom prevents the reaction-driven structural change and works as a structural stabilizer. Figure 4. CO oxidation catalyzed by the 12Ag-1Pd-Ih bimetallic nanoparticle. This figure has a similar organization to Figure 1. Because there is no reaction-driven structural change, the total reaction energy released (E4 ) -6.22 eV) is equal to the energy of oxidation of two CO molecules (-6.22 eV).

sized Ag-core-Au-shell nanoparticle spontaneously transforms to alloy nanoparticle through the elemental diffusion at 373 K.77 Chen and Johnston reported that Ag-core/Au-shell segregation is energetically favorable for 13-atom Ag-Au nanoparticle and pointed out the change transfer from Ag to Au as a likely reason.78 We postulate that our result is analogous to these previous findings. The 12Ag-1Au-Ih nanoparticle was severely disordered just with adsorbed O2 molecule and we were not able to bind a CO nearby a O2 molecule. Naturally, the 12Ag-1Au-Ih does not catalyze the CO oxidation as well as the 13Ag-Ih nanoparticle. Although Pd, Pt, Ni, and Cu core atoms sufficiently stabilize the Ih structure, because a CO molecule does not strongly interact with the 12Ag-1Pt-Ih and 12Ag-1Cu-Ih nanoparticles, we omitted these two nanoparticles. Moreover, the calculated SA values of the 12Ag-1Pd-Ih and 12Ag-1Ni-Ih nanoparticles confirm that the 12Ag-1Pd-Ih nanoparticle is the best catalyst for CO oxidation (Table 1). Figure 4 shows the complete oxidation cycle of the first two CO molecules catalyzed by the 12Ag-1Pd-Ih nanoparticle. In accordance with our strategy, with the Pd atom at the core site the nanoparticle did not appear to undergo any reaction-driven structural change (structural change during CO oxidation is marginal) during the reaction. Because the Ih structure is conserved during the CO oxidation process, all the coadsorption geometries converged to a single energy level at stage 2 (Figure 4) and the Ih structure was completely maintained after the oxidation of the two CO molecules. Clearly, the 12Ag-1Pd-Ih nanoparticle is a catalyst, because the initial state of the 12Ag-1Pd-Ih was completely preserved and the released total reaction energy (-6.22 eV) is exactly double the CO oxidation energy (-3.11 eV). The calculated SA (-0.65) gives further credence to the finding that the 12Ag-1Pd-Ih nanoparticle is a moderate catalyst. The introduction of the Pd core atom lowers the surface electron density of the nanoparticle (see Table 3). Because the outer electron configuration of a Pd atom is 4d,10 the total amount of valence electrons is lowered if we substitute the core of the 13Ag-Ih nanoparticle with a Pd atom. On the other hand, the 12Ag-1Pd-Ih particle maintains perfect symmetry because the Pd core atom is weakly charged, and thus, the repulsive forces between the Ag1 and

4. Concluding Remarks On the basis of computational results we propose the following: i The 13Ag-Ih nanoparticle is the most reactive one for CO oxidation out of three isomers of the 13Ag nanoparticle (Ih, Cubo-Oh, and Am). ii The reaction-driven structural change deactivates an unsupported 13Ag-Ih nanoparticle that is initially highly reactive for CO oxidation. iii The repulsive force between Ag atoms that lose their electrons as a result of the O2 adsorption and MS state formation breaks the local symmetry of the 13Ag-Ih nanoparticle, and the subsequent reconstruction of the nanoparticle leads to a reaction-driven structural change. iv Introduction of heterogeneous core atoms to the 13Ag-Ih nanoparticle provides structural robustness to the Ih structure. The 12Ag-1Pd-Ih core-shell nanoparticle is the most reactive system. v The extra stabilization of the Ih structure by the Pd core atom prevents the reaction-driven structural change and yields moderate CO oxidation activity. The Pd core atom lowers the repulsive force between atoms and consequently reduces the driving force for the reaction-driven structural change. This work is one of the early attempts to quantitatively determine how the morphology of nanoparticles affects the catalytic activity of nanoparticles. Quantitative analyses on the CO oxidation reactivity of nanoparticles confirm that the morphology of nanoparticles rules the catalytic activity of nanoparticles. We found that a heterogeneous core atom can stabilize a flexible nanoparticle and thus prevent the decline in the catalytic activity accompanied with the structural evolution. Moreover, besides its usefulness as a catalyst, this type of rigid core-shell nanoparticle can be used as a building block for nanostructures. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (the Ministry of Education, Science and Technology, MEST) (No. 2009-0059348) and the Nano R&D program through the KOSEF funded by the MEST (No. 20090082472). Supporting Information Available: Details on the microkinetic model, complete structural data of the 13Ag-Ih, 13AgAm, and the 12Ag-1Pd-Ih nanoparticles (Figure S1) and figures on the reaction intermediates of CO oxidation by the 13Ag-Ih

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