Reaction of NO on Ni−Pt Bimetallic Surfaces Investigated with

ARTICLE pubs.acs.org/JPCC

Reaction of NO on Ni
Pt Bimetallic Surfaces Investigated with Theoretical Calculations Shiuan-Yau Wu, Yu-Chieh Lin, and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal University, 88, Section 4, Ting-Chow Rd. Taipei, Taiwan 116 ABSTRACT: We applied periodic density-functional theory to investigate the adsorption and dissociation of NO on bimetallic surfaces, including the xNi@Pt(111), NixPt4
x(111), and (4
x)Pt@Ni(111) surfaces (x = 0
4). For all bimetallic surfaces, NO is preferentially adsorbed on Ni-rich sites, and the adsorption energies increase with an increasing number of top-layer Ni atoms on the surface. When the top-layer compositions are equal (but with varied composition of inner layers), the adsorption energy of NO on these surfaces decreases in the order xNi@Pt(111) > NixPt4
x(111) > (4
x)Pt@Ni(111), whereas the NO dissociation barriers increase in the opposite order; a larger adsorption energy of NO leads to a smaller NO dissociation barrier. We employed the local density of states to investigate the inner-layer effect of the various surfaces; the inner-layer Pt atoms of the 4Ni@Pt(111) surface caused the greatest upshift of the d-band center (of top-layer Ni atoms) toward the Fermi energy, so that the 4Ni@Pt(111) surface exhibited the greatest NO adsorption energy,
2.97 eV, and the smallest NO dissociation barrier, 1.20 eV. In contrast, the inner-layer Ni atoms of the (4
x)Pt@Ni(111) surfaces cause a downshift of the d-band center from the Fermi energy and show much smaller NO adsorption energies and larger NO dissociation barriers. The order of reactivity for dissociation of NO (4Ni@Pt(111) > Ni(111) > NixPt4
x(111) > Pt(111) > 4Pt@Ni(111)) indicates that various combinations of Ni and Pt would produce varied catalytic effects.

1. INTRODUCTION Bimetallic systems have been studied extensively for their novel catalytic properties in the field of heterogeneous catalysis. Because of the interlaced arrangement of the various atoms at the surface that induces either an electronic effect or an ensemble effect, or both, the activity and selectivity of catalysts is modified.1
6 A systematic understanding is desirable of the relations between the compositions and properties of alloys in catalyst design, so that we might predict the reactivity on various surfaces, such as monometallic surfaces, alloy surfaces, and overlayers. In the literature,7 the electron density at the Fermi level is found to correlate well with the interaction between adsorbates and the type of surface; for example, the adsorption of CO on Ni or Cu or Ni/Cu alloys, in which the partially filled d band of nickel has a greater electron density at the Fermi level than that of the Cu surface, results in a much enhanced reactivity of CO toward a Ni surface. According to the Sabatier principle,8 if the reactivity of a pure metal becomes too strong, it might poison the surface, whereas excessive weakness might result in a poor turnover frequency. Accordingly, a bimetallic surface would become attractive for catalysis if we might tune the bimetallic reactivity on adjusting the composition of an alloy, such as Ni/Cu. The Ni/Pt system is an attractive model of a bimetallic surface, which already serves in the reforming of methanol,9 ethanol, and ethanediol according to TPD and HREELS experiments,10 and by density-functional theory (DFT) calculations.11 These bulk metals (Ni and Pt) have similar geometric properties: they both r 2011 American Chemical Society

crystallize in the face-centered-cubic (fcc) system with lattice parameters 3.52 (Ni) and 3.92 Å (Pt). To achieve a distinct surface reactivity, a particular catalyst that possesses the appropriate activity and selectivity for a particular reaction is generally required.12 For example, a 3d transition metal is more active than a 5d metal from the same column of the periodic chart; this difference results in a dissociative adsorption of NO on Ni but molecular adsorption on a Pt surface at a small coverage. On the basis of this selective reactivity, we could design the Ni/Pt system to have a superior selectivity for the NO reaction. Adsorption of NO on monometallic or alloyed surfaces has been widely discussed,13
19 but adsorption of NO on a Ni/Pt(111) alloyed surface appears to have no reported systematic investigation. In this work, we used NO as a probe molecule to test the adsorption and dissociation properties on Ni/Pt (111) bimetallic surfaces. This investigation of NO adsorption and dissociation is technologically relevant because it is an important reaction to control the pollutants released into the atmosphere.19
25 We designed the Ni/Pt (111) bimetallic systems from two aspects: one is a well-mixed arrangement for varied NixPt4
x (x = 0
4) ratios, including Ni4, Ni3Pt, Ni2Pt2, NiPt3, and Pt4(111) surfaces, which represent Ni/Pt alloys in various proportions; the varied ratio NixPt4
x has been studied experimentally and theoretically.26 Alternatively, we substituted the top layer of a Received: December 29, 2010 Revised: March 8, 2011 Published: March 30, 2011 7538

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Figure 1. Schematic structures and locations of all Ni
Pt bimetallic surfaces: (a
e) indicate Ni4, Ni3Pt, Ni2Pt2, NiPt3, and Pt4(111) surfaces, respectively; (f
i) and (j
m) indicate 1
4Ni@Pt(111) and 1
4Pt@Ni(111) surfaces, respectively. The light blue and blue spheres represent Ni and Pt atoms, respectively. T, B, F, and H represent top, bridge, fcc hollow, and hcp hollow sites, respectively; index number “1” denotes the Ni
rich sites and “2”corresponds to Pt
rich sites of all bimetallic surfaces.

Table 1. Calculated Work Function Wf and Bader Charge of Each Top-Layer Atom of NixPt4
x(111) Surfaces Bader charge (|e| of each top atom) surfaces

Wf/eV

Ni
0.03

Pt

Ni4(111)

5.09

Ni3Pt(111)

5.23

þ0.14


0.55

Ni2Pt2(111)

5.39

þ0.25


0.32

NiPt3(111)

5.59

þ0.37


0.18

Pt4(111)

5.83


0.04

Pt (22)-(111) surface with one to four Ni atoms (xNi@Pt(111) surfaces), to model the variable coverage of Ni deposited on Pt surface.9,27 The same arrangement of a Ni (22)-(111) surface with one to four Pt atoms ((4
x)Pt@Ni(111) surfaces) was considered to model the opposite condition, the varied coverage of Pt deposited on a Ni surface. 28 We calculated the adsorption and dissociation energies of NO to compare the catalytic ability of these surfaces and discuss the detailed electronic characteristics of these surfaces, including the local density of states, and the d-band center to understand clearly the bimetallic effect of varied Ni
Pt composition.

2. COMPUTATIONAL METHODS We performed all calculations with the Vienna ab initio simulation package (VASP),29
31 based on DFT and the projector-augmented wave method (PAW).32,33 The Kohn
Sham equations were solved in a self-consistent manner under the generalized-gradient approximation (GGA)34 with the Perdew
Wang (PW91)35 exchange-correlation formulation.36 All structure optimization was based on the conjugate gradient
minimization scheme under a spin-polarized consideration. The Monkhorst
Pack mesh k-points (5  5  5) and (551)36 were used for bulk

and surface calculations respectively, with energy truncated at 400 eV. To establish the well-mixed alloy surfaces, we calculated the lattice parameters for various NixPt4
x bimetallic bulks, including Ni4, Ni3Pt, Ni2Pt2, NiPt3, and Pt4; the calculated values were 3.52, 3.65, 3.72, 3.88, and 3.99 Å, respectively, which values fit Vegard’s law satisfactorily. Among these values, the calculated bulk lattice parameter of pure Ni crystal, 3.52 Å, agrees well with the experimental value, 3.51 Å, whereas that for the pure Pt lattice exceeds the experimental data by 2%, indicating that the employed methods and models function properly in our system. We chose the (111) surface for each NixPt4
x crystal because this surface is the most stable for the fcc system with the smallest surface energy. The p(22) lateral cell of the (111) surface was modeled as periodically repeating slabs comprising six atomic layers. In the well-mixed structures shown in Figure 1a
e, each layer in the NixPt4
x-p(22) cell is composed of x Ni and (4
x) Pt atoms; for instance, the Ni3Pt-p(22) cell has three Ni and one Pt atoms in each layer. The work function (Table 1) of pure Pt (Pt4) is larger than that of pure Ni (Ni4), which produces a negative charge distribution of Pt atoms on all alloyed surfaces (Ni3Pt, Ni2Pt2, and NiPt3), and it also causes a downward shift of the d-band centers on increasing the proportion of Pt in the NixPt4
x systems, an important property to affect the catalytic ability. In addition to the well-mixed bimetallic system, we considered another method to combine two metals. According to experimental observation, the Ni surface has a greater activity for adsorption of NO than Pt metal; we substituted the top-layer atoms of Pt(111)-(22) slab with one to four Ni atoms (large activity) to tune upward the small-activity Pt(111) surface, defined as 1
4Ni@Pt(111) surfaces (Figure 1f
i), in which the 4Ni@Pt(111) surface resembles a thin layer of Ni plated on the Pt surface. The opposite conformations on replacing the top layer of the Ni(111)-(22) slab with one to four Pt atoms, designated as 1
4Pt@Ni(111) surfaces (Figure 1j
m), which 7539

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Table 2. Calculated Adsorption Energies and Vibration Wavenumbers of NO Molecule on the Best Sites of Various NixPt4
x(111) Surfaces (x = 0
4) with the Best Dissociation Path and Its Dissociation Energies, Corresponding to Surfaces in Figure 1a
e best site

Eads/eV

wavenumber/cm
1

Ni4(111)

F


2.48

1523

NO(F) f N(F) þ O(F)

1.49

Ni3Pt(111) Ni2Pt2(111)

F1 F1


2.47
2.21

1515 1525

NO(F1) f N(F1) þ O(F2) NO(F1) f N(F1) þ O(F1)

1.72 1.71

NiPt3(111)

F1


2.03

1517

NO(F1) f N(F1) þ O(F1)

1.86

Pt4(111)

F


1.88

1537

NO(F) f N(F) þ O(F)

2.50

NixPt4
x(111) surfaces

best dissocn path

Edis/eV

may tune downward the too great activity of a pure Ni(111) surface, were tested. For all bimetallic slab models, the bottom three atomic layers were frozen and set to the estimated bulk parameters, whereas the remaining layers were fully optimized. All slabs were separated by a vacuum spacing greater than 15 Å, which ensures no interaction between the slabs. The adsorption energy was calculated according to the following equation Eads ¼ E½slab þ adsorbate
E½slab
E½adsorbate in which E[slab þ adsorbate], E[slab], and E[adsorbate] are the calculated electronic energies of species adsorbed on the surface, clean surface, and free molecule, respectively. Vibrational wavenumbers of adsorbed structures were analyzed on diagonalizing the Hessian matrix of selected atoms within the VASP approach. Under the present computational conditions, the harmonic vibrational wavenumber of free NO was calculated to be 1914 cm
1, 0.5% larger than experimental value 1904 cm
1. The nudged-elastic-band (NEB)37,38 method was applied to locate transition structures, and paths of minimum energy (MEP) were constructed accordingly.

3. RESULTS AND DISCUSSION 3.1. Adsorption and Dissociation of NO on NixPt4
x(111) Surfaces. An electronic characteristic of the NO molecule is that

it has one unpaired electron (2π* orbital, unfilled) residing on the N atom, which might accept a d-electron from the surface metal, resulting in a N-downward orientation preferentially for NO adsorption on the metal surface in most experimental observations39 and theoretical calculations.12,18,19 The monometallic surfaces, Ni(111) and Pt(111), offer four adsorption positions for NO molecule: top (T), bridge (B), and two 3-fold hollow sites, fcc (F), and hcp (H). In our work, the fcc sites are the most stable for NO adsorption on both pure Ni(111) and Pt(111) surfaces; the calculated adsorption energies are
2.48 and
1.88 eV, respectively, slightly greater than for their corresponding hcp sites,
2.47 eV on Ni(111) and
1.72 eV on Pt(111) surfaces. The adsorption sites of bimetallic surfaces, Ni3Pt, Ni2Pt2, and NiPt3(111) are more complicated because of the two distinct metal atoms in the top layers. For instance, in the Ni3Pt (111) surface we distinguish the top sites as TNi (marked T1) and TPt(T2), bridge sites as BNi
Ni(B1) and BNi
Pt(B2), fcc hollow sites as F3Ni(F1) and F2NiPt(F2), and hcp hollow sites as H3Ni(H1) and H2NiPt(H2); index number “1” corresponds to the site coordinated with more Ni atoms, but “2” with more Pt atoms. All these sites are shown in Figure 1b; those for Ni2Pt2 and NiPt3(111) surfaces are shown in Figure 1, c and d, respectively. The calculated adsorption energies are listed in Table 2; the F1 sites are the most stable on all Ni3Pt, Ni2Pt2, and NiPt3(111) surfaces, with adsorption energies
2.47,
2.21, and
2.03 eV,

Figure 2. Potential energy diagram for NO dissociation path (NO f N þ O) on various bimetallic surfaces: the Ni(111) surface is used, for example, to represent an initial state (IS), transition state (TS), and final dissociation state (FS) for NO dissociation; the adsorption energy (Eads) and dissociation barrier (Edis) are also defined.

respectively, indicating that NO molecule prefers to stand at fcc hollow sites coordinated with more Ni atoms. With an increased proportion of Pt in the NixPt4
x surfaces, the adsorption energies decrease gradually from that of pure Ni (Ni4) surface to pure Pt (Pt4). Although various mechanisms of NO reduction are proposed, especially the N
NO and N
N recombination,16 it is agreed that NO dissociation is an important elementary step to form an adsorbed N atom on the surface. In our previous work,40 the energy barriers for dissociation of NO on Ni(111)-(22) and -(33) slabs are similar (about 1.5 eV), and the reaction energy is slightly endothermic for a (22) slab but exothermic 0.67 eV for the (33) counterpart. In addition, the adsorbed atoms, such as N and O, affect mainly the reaction energies, hence the equilibrium.41 We therefore infer that the value of the dissociation barrier depends mainly on its surface characteristic (varied composition or surface indices), but to a lesser extent on the externally enforced factors on the surface, such as the presence of additional N(a) or O(a), or the variation of coverage. As a consequence, in the following sections we focus on the discussion of energy barrier for NO dissociation with respect to the various Ni
Pt compositions. On either a pure Ni(111) or Pt(111) surface, the favored location of the NO is on the fcc site, whereas the final state (N þ O) of NO dissociation occurs at fcc
fcc sites, which are the most stable sites for the adsorbed N and O atoms. The calculated reaction path is thus from NO(F) to N(F) þ O(F), in which the N atom stays still and the dissociated O atom moves toward another fcc site via a bridge location; the transition structures are all near the bridge sites for both Ni(111) and Pt(111) surfaces, with dissociation barriers 1.49 and 2.50 eV, respectively; the reaction coordinates, including various energies and states, are 7540

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Table 3. Calculated Adsorption Energies and Vibration Wavenumbers of NO Molecule on the Best Sites of Various xNi@Pt(111) Surfaces (x = 1
4) with the Best Dissociation Path and Its Dissociation Energies, Corresponding to Surfaces in Figure 1i
l xNi@Pt(111) surfaces

best site

Eads/eV

wavenumber/cm
1

best dissocn path

Edis/eV

1Ni@Pt(111)

F1


2.18

1530

NO(F1) f N(F1) þ O(F1)

1.63

2Ni@Pt(111) 3Ni@Pt(111)

F1 F1


2.47
2.78

1509 1472

NO(F1) f N(F1) þ O(F1) NO(F1) f N(F1) þ O(F2)

1.55 1.25

4Ni@Pt(111)

F


2.97

1475

NO(F1) f N(F1) þ O(F1)

1.20

Table 4. Calculated Adsorption Energies and Vibration Wavenumbers of NO Molecule on the Best Sites of Various xNi@Pt(111) Surfaces (x = 1
4) with the Best Dissociation Path and Its Dissociation Energies, Corresponding to Surfaces in Figure 1j
m best site

Eads/eV

wavenumber/cm
1

best dissocn pathway

Edis/eV

1Pt@Ni(111)

F1


2.11

1536

NO(F1) f N(F1) þ O(F1)

1.89

2Pt@Ni(111)

F1


1.65

1567

NO(F1) f N(F1) þ O(F1)

2.40

3Pt@Ni(111) 4Pt@Ni(111)

F1 T-tilted


1.22
0.93

1702 1762

NO(F1) f N(F1) þ O(F2) NO(T) f N(F) þ O(F)

2.71 3.34

xNi@Pt(111) Surfaces

defined in Figure 2. This large difference of energy barriers agrees with the statement that dissociative adsorption of NO occurs on the Ni surface but molecular adsorption on the Pt surface.39 For the mechanism of NO dissociation on bimetallic surfaces Ni3Pt, Ni2Pt2, and NiPt3(111), we consider the most probable NO(F1) site to be the initial state in each case because of its greatest adsorption energy relative to other adsorption sites on these surfaces. As shown in Figure 1b, the nearest F2 and H1 sites to the NO(F1) site on the Ni3Pt(111) surface are possible locations for O dissociated from the NO molecule, but the transition structure is unstable for direct dissociation of the O atom toward H1 via the T1 site. The most practical path is thus NO(F1) f N(F1) þ O(F2), with energy barrier 1.72 eV. Similarly, two paths on Ni2Pt2(111) are considered: NO(F1) f N(F1) þ O(F1), and NO(F1) f N(F1) þ O(F2). The former exhibits a smaller barrier than the latter (1.71 and 2.38 eV, respectively), because the former dissociated O atom passes its transition structure via the B1 site (Ni-rich, more stable), instead of the B2 site for the latter. There are, likewise, two paths for NO dissociation on the NiPt3(111) surface: NO(F1) f N(F1) þ O(F1) and NO(F1) f N(F1) þ O(F2), with energy barriers 1.86 and 2.47 eV, respectively. The NiPt3(111) surface exhibits a greater NO dissociation barrier than those of other proportions of bimetallic surfaces; furthermore, the barrier of the NO(F1) f N(F1) þ O(F2) path (2.47 eV) even approaches that on the pure Pt(111) surface (2.50 eV). 3.2. Adsorption and Dissociation of NO on 1
4Ni@Pt(111) Surfaces. Here we discuss the adsorption of NO on Ni@Pt(111) surfaces, which possess an effect distinct from that of the well-mixed bimetallic surfaces. As shown in Table 3, NO tends to be adsorbed identically on all F1 sites of 1
3Ni@Pt(111) surfaces but on the F site of the full Ni covering, 4Ni@Pt(111) surface (on which all F sites are indistinguishable, Figure 1i), and the adsorption energy gradually increases with increasing substitution number of the top-layer Pt by Ni atoms. The calculated NO adsorption energy is
2.18 eV on 1Ni@Pt(111) surface, larger than that on the NiPt3(111) counterpart (
2.03 eV) (both 1Ni@Pt(111) and NiPt3(111) surfaces have similar top-layer formations, 1:3 proportion of Ni to Pt). The NO (F1) adsorption energy increases to
2.47 eV on the 2Ni@Pt(111) surface, much larger than on Ni2Pt2(111), even

approaching the value of the pure Ni(111) surface (
2.48 eV). It further increases to
2.78 eV on the 3Ni@Pt(111) surface, already larger than for pure Ni(111) (by 0.30 eV); as expected, when all top-layer atoms are replaced by Ni atoms, 4Ni@Pt(111), the greatest NO-adsorption energy
2.97 eV arises. Because of the same composition of the first-layer surfaces, the dissociation paths of NO on 1
4Ni@Pt(111) and NixPt4
x(111) surfaces are similar: on 1
3Ni@Pt(111) and NixPt4
x(111) (x = 1
3) surfaces, the O atom of NO(F1) dissociates toward the nearest F1 sites (or F2 site for 3Ni@Pt(111) surface) via the B1 position with the N atom unmoved, (N(F1) þ O(F1), or N(F1) þ O(F2)); in contrast, on either 4Ni@Pt(111) or Ni(111), the best path is NO(F) f N(F) þ O(F). Nevertheless, the 1
4Ni@Pt(111) surfaces are expected to be more effective for NO dissociation than their NixPt4
x(111) counterparts because of the larger NO adsorption energies. The calculated NO dissociation barriers gradually decrease from 1 to 4Ni@Pt(111) surfaces (1.63, 1.55, 1.25, and 1.20 eV, respectively). The values of the barrier for 1 and 2Ni@Pt(111) surfaces approach that of the pure Ni(111) surface (1.49 eV), whereas those of 3 and 4Ni@Pt(111) surfaces are even smaller, indicating that a gradual deposition of Ni on a Pt surface would effectively improve the activity of the surface toward NO molecule. 3.3. Adsorption and Dissociation of NO on 1
4Pt@Ni(111) Surfaces. Opposite to the Ni@Pt(111) surfaces, the effect of Pt deposition is expected to decrease the adsorption energy and to increase the dissociation barrier of NO on 1
4Pt@Ni(111) surfaces (Table 4). The calculated adsorption energies decrease markedly,
2.11,
1.65,
1.22, and
0.93 eV from 1 to 4Pt@Ni(111) surfaces, respectively, in agreement with expectation. The NO molecules prefer to locate on all F1 sites of 1
3Pt@Ni(111) surfaces (similar to Ni@Pt(111) counterparts), but locate exceptionally on the tilted top site of the fully covered Pt, 4Pt@Ni(111) surface. (In general, NO prefers to locate on the hollow site of the transition-metal (111) surfaces, but the stable tilted adsorption structure also appears on Ir(111),12 Ag(111), and Au(111)42 surfaces of larger lattice parameters and with slightly larger adsorption energy than that of the hollow site.) In addition, the NO dissociation paths on 1
3Pt@Ni(111) surfaces differ from that of the 4Pt@Ni(111) surface; the former (NO at F1 site) proceeds with the dissociated O atom moving toward another hollow site, whereas the latter 7541

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Figure 4. Local density of states (LDOS) projected on the top-layer atoms of (a) (4
x)Pt@Ni(111) and pure Ni (111) surfaces (x = 0
4); (b) NixPt4
x(111) surfaces (x = 0
4); and (c) xNi@Pt(111) and pure Pt(111) surfaces (x = 0
4).

Figure 3. Comparison of (a) adsorption energy and (b) activation energy for NO dissociation on NixPt4
x(111), xNi@Pt(111), and (4
x)Pt@Ni(111) surfaces; the letters a
m labeling the points correspond to Figure 1.

(tilted NO at top site) with both the dissociated N and O atoms moving toward the opposite F sites. Because of the smaller adsorption energies of NO on Pt@Ni(111) surfaces (relative to Ni@Pt(111) and NixPt4
x(111) surfaces), the slightly weakened N
O bond requires a larger activation energy for NO dissociation. Increasing the number of Pt substitutions increases the activation energies for NO dissociation; the calculated energy barriers are 1.89, 2.40, 2.71, and 3.34 eV for 1 to 4Pt@Ni(111) surfaces, respectively. NO dissociation on the 4Pt@Ni(111) surface entails the largest activation energy, whereas the calculated barrier on 2Pt@Ni(111) surface is nearest that of the pure Pt(111) surface (2.50 eV). 3.4. Comparison of NO Reactivities on NixPt4
x(111), Ni@Pt(111), and Pt@Ni(111) Surfaces. The NO adsorption energies and dissociation barriers on various bimetallic surfaces are drawn in Figure 3, a and b, respectively, from which we observe the following trends. (i) The NO adsorption energies are strongly correlated with the energy barriers for NO dissociation: the larger the adsorption energy is, the smaller is the dissociation barrier. The greater the interaction between the molecule and the surface is, the larger is hence the charge transfer from the surface to the antibonding orbital of NO molecule, which results in a decreased N
O bond dissociation barrier. (ii) When the toplayer compositions are equal, for instance, 3:1 proportion of Ni to

Pt in Ni3Pt(111), 3Ni@Pt(111), and 1Pt@Ni(111) surfaces, corresponding to x = 3 in Figure 3, the adsorption energies have the order 3Ni@Pt(111) > Ni3Pt (111) > 1Pt@Ni(111), whereas the dissociation barriers follow the opposite order (1Pt@Ni(111) > Ni3Pt (111) > 3Ni@Pt(111)), regardless of any proportion of Ni to Pt (x = 0 to 4), indicating that the varied inner-layer composition has a varied effect on NO reactivity. To understand thoroughly the bimetallic effect of varied compositions, we calculated the local density of states (LDOS) projected onto top-layer metal atoms of these pure and bimetallic surfaces.43 A comparison of LDOS projected onto the top layers of pure Ni(111) and 1
4Pt@Ni(111) surfaces is shown in Figure 4a, in which the congregated d-band character of pure Ni(111) displays a strong band intensity about the Fermi energy (EF) (0 to
2 eV). When the substituted number of Pt atoms increases, the d-bandwidth broadens resulting in a gradually decreasing d-band intensity (0 to
2 eV), and the broadened d-bandwidth of 4Pt@Ni(111) even expands to the least energy
8.4 eV. The top-layer-atom projected LDOS for pure Ni(111), NixPt4
x(111) (x = 1
3), and pure Pt(111) are shown in Figure 4b, in which the d-band character of pure Pt(111) presents a greater d-bandwidth and smaller band intensity (0 to
2 eV) than for the pure Ni(111) counterpart, whereas the d-band widths of NixPt4
x(111) (x = 1
3) gradually congregate toward that of pure Ni(111). In contrast, for the system xNi@Pt(111) (x = 1
4) in Figure 4c, gradual substitution of the top-layer Pt by Ni atoms produces a rapidly increasing d-band intensity (0 to
2 eV), resulting in a maximum about
1 eV at the total substitution of the 4Ni@Pt(111) surface. We conclude that increasing the Pt atoms on the top layer of surfaces results in an expanded d-band structure, but increasing Ni atoms aggregate the d-band intensity about EF. The individual DOS of gaseous NO molecule exhibits an obvious 2π* orbital maximum about EF (∼0.12 eV), indicating that the d-band intensity (of top-layer atoms) about EF might be connected to electron transfer from the surface atom to the NO 2π* orbital; the 7542

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Figure 5. Calculated d-band centers of the corresponding d-band state of the top-layer atoms in Figure 4; the letters a
m labeling the points correspond to Figure 1.

increased d-band intensity about EF produces an increased interaction between the metal and the molecule. It is well-known that, if the d-band center approaches EF, the surface presents an increased interaction with the adsorbed molecule.44
46 From our calculated d-band centers corresponding to the d-band state of the top-layer atoms (Figure 5), relative to the corresponding adsorption and dissociation energies of NO molecule (Figure 3), we found that the nearer the EF d-band center is, the larger is the NO adsorption energy and the smaller is the NO dissociation barrier that the surface exhibits, in agreement with the known reports. In Figure 6, we compare the top-layer-atom projected LDOS with that of the same top-layer compositions from other bimetallic systems to explain how the various inner-layer atoms affect the surface activities. As shown in Figure 6a, the absolute Ni top layers in pure Ni(111) and 4Ni@Pt(111) surfaces exhibit similar d-band characters, congregated about the Fermi energy with great intensity, but the latter possesses a maximum d-band intensity (about
1.4 eV), a narrower d-band structure, and a upwardly shifted d-band center (at
1.13 eV relative to
1.47 eV). In contrast, the projected LDOS of the absolute Pt top layers in pure Pt(111) and 4Pt@Ni(111) surfaces (Figure 6b) exhibit broad and weak d-band structures, with the latter of wider and smaller intensity, and a largely downward shift of the d-band center (at
3.58 eV relative to
2.28 eV in pure Pt(111)). This phenomenon appears also for all other Ni
Pt bimetallic surfaces; when the top-layer compositions are equal, the inner Pt layers of xNi@Pt(111) surfaces enhance the top-layer d-band intensity (upwardly shifted d-band center), whereas the inner Ni layers of (4
x)Pt@Ni(111) surfaces cause the opposite effect (broad and weak and downwardly shifted d-band center). As a consequence, the order for d-band intensity about EF (shown in Figure 6c
e), is invariably xNi@Pt(111) > NixPt4
x(111) > (4
x)Pt@Ni(111), consistent with the order of increasing NO adsorption energy and decreasing energy barrier for NO dissociation on these surfaces. We applied a Bader-charge calculation to explain the varied inner-layer effect. The calculated Bader charge of each top-layer atom is
0.027|e| for pure Ni(111), and
0.044|e| for pure Pt(111). Because the work function of Pt is larger than that of Ni atom (in Table 1), the electronic charge would flow from Ni into Pt layers in all these Ni
Pt bimetallic

Figure 6. Comparison of local density of states (LDOS) projected onto the same composition of top-layer atoms. (a) Only Ni atoms on the top layer, (b) only Pt on the top layer, and (c
e) represent the composition of 3:1, 2:2, and 1:3 Ni
Pt ratios on the top layers, respectively.

structures, so that the top Ni layer of 4Ni@Pt(111) surface exhibits a calculated partial positive charge on each Ni atom (þ0.16|e|), and the top Pt layer of 4Pt@Ni(111) surface exhibits a partial negative charge on each Pt atom (
0.14|e|). Accordingly, the flowing partial charge of the surface layer (of 4Ni@Pt(111) surface) into the inner Pt layers causes the upwardly shifted (top-Ni layer) d-band center toward the Fermi energy relative to that of pure Ni(111); it would thus be unstable for all Ni@Pt catalysts and more reactive toward NO. For the 4Pt@Ni(111) surface the charge transfers from inner Ni layers into surface Pt atoms, causing the downwardly shifted d-band center (of the top Pt layer) away from the Fermi energy, and the surface becomes more stable than that of pure Pt, and hence less reactive toward NO. The inner-layer effect for well mixed NixPt4
x(111) surfaces is not obvious because the composition of each layer is equal, so that the electron transfer occurs mostly in each individual layer rather than within layers; the location of the d-band center and NO reactivity of NixPt4
x(111) surfaces are all between that of xNi@Pt(111) and (4
x)Pt@Ni(111) surfaces.

4. CONCLUSION We discuss systematically the adsorption and dissociation of NO on various Ni
Pt bimetallic systems, including xNi@Pt(111), NixPt4
x(111), and (4
x)Pt@Ni(111) surfaces. On either the Ni-rich or Pt-rich surface, the Ni atom is the active site for the NO molecule, and its number on the surface affects the NO adsorption energy and dissociation barrier; the more Ni atoms the surface has, the larger is the NO adsorption energy and the smaller is the NO dissociation barrier, whereas, on the NixPt4
x(111) (x = 1
3) surfaces, the NO adsorption energies and dissociation barriers are between those of pure Ni and Pt(111) counterparts, indicating that well-mixed systems might effectively tune the catalytic ability. 7543

dx.doi.org/10.1021/jp112346u |J. Phys. Chem. C 2011, 115, 7538–7544

The Journal of Physical Chemistry C The partially substituted surface, 2Ni@Pt(111), exhibits a catalytic ability for NO adsorption and dissociation near that of a pure Ni(111) surface (in Figure 3), indicating that a slight deposition of active metal (Ni atom) onto the less active surface (Pt surface) might achieve a catalytic ability similar to that of the pure active surface (Ni surface). According to the LDOS analysis, in the fully top-layer substituted bimetallic systems, 4Ni@Pt(111) and 4Pt@Ni(111) surfaces, the inner-layer atoms enhance the reaction characteristics of the top-layer atoms, so that the 4Ni@Pt(111) surface presents a greater reactivity for NO molecule than that of a pure Ni(111) surface, whereas the 4Pt@Ni(111) surface exhibits even less NO affinity than that of pure Pt(111). In this work, the overall best catalyst has a Ni overlayer on bulk platinum; as nickel is much less expensive (about $29 per kg) than platinum (about $116 per g), the catalyst of choice for this reaction would be pure Ni(111). It seems that theNi
Pt system in this reaction is unrealistic in terms of cost, but the idea of the bimetallic effect is still useful, and we here provide this fact; for instance, quantum-chemical computation and experimental observation prove that RhCu18 and RhAg19 surfaces possess enhanced catalytic activity of NO reaction but cost less than the pure Rh catalyst. Incidentally, the Pt-based (Pt@M, M = Ru, Rh, or Pd) catalysts43,47 invariably exhibit a catalytic ability toward CO oxidation superior to that of pure Pt metal (pure Pt is considered to be a suitable catalyst for CO oxidation), which can be also applied in our Pt
Ni system. In our trial calculation result about the oxidation of CO on pure Pt(111) and 4Pt@Ni(111) surfaces, the energy barriers for CO þ O f CO2 are 0.99 and 0.64 eV, respectively, indicating an enhanced catalytic behavior on 4Pt@Ni(111) for much less cost than pure Pt. Finally, we trust that our work might provide effective information about the bimetallic effect to assist experimentalists to control the catalytic behavior (either to enhance or to diminish) by depositing various coverages of top-layer atom or altering the inner-layer compositions.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ886-2-29309085. Fax: þ886-2-29324249.

’ ACKNOWLEDGMENT National Science Council of Republic of China (NSC 992113-M-003-007-MY3) supported this work; National Center for High-performance Computing provided computer time and facilities. ’ REFERENCES (1) Sachtler, W. H. M. Faraday Discuss. Chem. Soc. 1981, 72, 7. (2) Rordiguez, J. A. Surf. Sci. Rep. 1996, 24, 223. (3) Chen, J. G.; C. A. Menning, C. A.; Zellner, M. B. Surf. Sci. Rep. 2008, 63, 201. (4) Lam, Y. L.; Criado, J.; Boudart, M. Nouv. J. Chim. 1997, 1, 461. (5) Schneider, U.; Busse, H.; Link, R.; Castro, G. R.; Wandelt, K. J. Vac. Sci. Technol. A 1994, 12, 2069. (6) Liu, P.; Nørskov, J. K. Phys. Chem. Chem. Phys. 2001, 3, 3814. (7) Demirci, E.; Carbogno, C.; Gross, A.; Winkler, A. Phys. Rev. B. 2009, 80, 085421. (8) Sabatier, P. Ber. Dtsch. Chem. Ges. 1911, 44, 1984.

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