Shell Bimetallic Surfaces

ARTICLE pubs.acs.org/JPCC

Reactivity of C
C Scission on Ni-Based Core/Shell Bimetallic Surfaces Investigated with Quantum-Chemical Calculations Yu-Chieh Lin and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal University, 88, Section 4, Ting-Chow Road, Taipei, Taiwan 116

bS Supporting Information ABSTRACT:

With quantum-chemical calculations, we investigated the cleavage of the C
C bond in molecules of type R
G, with R = CH3 and G = CO, CN, on the (111) surface of pure Ni and core/shell Cu/Ni and Pt/Ni surfaces. We chose here the three most typical and commonly encountered functional groups to investigate the scission of the catalytic C
C bond on these surfaces. To molecules of two kinds, CH3CO and CH3CN, we added CH3CH2 as a reference similar to an alkane itself, to perform the scission of the C
C bond on the specified metal surfaces. All three molecules exhibit the greatest adsorption energies on the core/shell Pt/Ni surface, according to the order Pt/Ni > Cu/Ni > Ni. The order of adsorption energy is generally CH3CO > CH3CH2 > CH3CN on all surfaces, but CH3CN adsorbed more strongly than CH3CH2 on the Pt/Ni surface. CH3CO has the least barrier on all surfaces; the order of its activation energy is Pt/Ni > Ni > Cu/Ni, whereas the order of activation energy for the other two molecules is Ni > Cu/Ni > Pt/Ni, with CH3CN smaller than CH3CH2. The barriers for cleavage of the C
C bond of CH3CO, CH3CN, and CH3CH2 on the (most active) core/shell Pt/Ni surface are 1.30, 1.47, and 1.84 eV, respectively. The local density of states (LDOS) is projected on the top layer of the pure metal Ni, Cu, and Pt, and the core/shell Cu/Ni and Pt/Ni to rationalize the calculated outcomes.

1. INTRODUCTION Bimetallic systems, which have been widely investigated by either experimental1
5 or computational means,6
13 are among the most promising catalysts.10
19 Metallic surfaces with a core
shell architecture have shown a great potential for catalytic activity in their superior electrical,20,21 optical,22
24 and magnetic25
27 properties. This innovative structure allows us to expose the desired metal (shell), and we can vary at will the core metal to modify the electronic and geometric properties of the shell for enhanced activity of the proposed reactions. In brief, by means of this method, we could minimize the quantity of the shell metal but attain its highest quality. Because nickel is known for its strong reactivity in various catalytic processes such as decomposition of methanol28 or C2Hy species,29 conversion of methane into syngas,30 reverse Fischer
Tropsch reaction,31 and catalyzed formation of carbon nanotubes,32 we applied it as the shell metal to have direct contact with the adsorbates and chose Cu (which is in the same period with Ni) and Pt (in the same VIIIB group with Ni) as the core metals. With a r 2011 American Chemical Society

similar size to Ni, Cu enabled us to compare the electronic property, whereas with Pt of similar electronic property, we studied the size effect with respect to Ni, toward the catalytic behavior for the scission of the C
C bond of molecules of type R-G, with R = CH3 and G = CO, CN. Because C
C bonds exist of several kinds, we chose here the three most typical and commonly encountered functional groups to study the catalytic scission of the C
C bond on these surfaces. Hence we have molecules of two kinds, CH3CO and CH3CN, to which we added CH3CH2 as a reference similar to alkane itself, to perform the scission of the C
C bond on the specified metallic surfaces.

2. COMPUTATIONAL METHODS We performed all calculations with the Vienna ab initio simulation package (VASP)33
35 based on density functional Received: June 15, 2011 Revised: August 25, 2011 Published: August 29, 2011 19231

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The Journal of Physical Chemistry C theory (DFT) and the projector-augmented wave method (PAW).36,37 The Kohn
Sham equations were solved in a selfconsistent manner under the generalized-gradient approximation (GGA)38 with the Perdew
Wang (PW91)39 exchangecorrelation formulation. All structure optimization was based on the conjugate gradient
minimization scheme under a spinpolarized consideration. The Brillouin zone is sampled with a Monkhorst-Pack grid.40 The calculations were performed using (4  4  4) and (4  4  1) Monkhorst-Pack mesh k-points for bulk and surface, respectively, with energy truncated at 400 eV. We chose the (111) surface for the monometallic or core/shell crystal because this surface is the most stable for the facecentered-cubic (fcc) system with the smallest surface energy. The p(3  3) lateral cell of the (111) surface was modeled as periodically repeating slabs comprising six atomic layers. The top view of the slab is shown in Figure 1. To establish the core/shell model, we consulted various experimental accounts;41
47 there are many systematic methods to produce these core/shell metallic surfaces. For the purpose of constructing a model that can simulate the real core/shell surfaces, we arranged the top layer to be the shell metal, nickel, among the six atomic layers of the surface slab; the other five layers of the same atoms, copper or platinum, act as the core. These core/shell models including pure nickel are shown in Figure 2. For all monometallic or bimetallic slab models, the bottom four atomic layers were frozen and set to the estimated bulk

Figure 1. Top view of schematic depiction of Ni(111) surface used in the present work to show the varied coverage.

Figure 2. Top view of schematic structures of a p(3  3) supercell of (a) Ni(111) and core/shell bimetals of (b) Cu/Ni (111) and (c) Pt/Ni (111) surfaces.

ARTICLE

parameters, whereas the remaining layers were fully optimized. We tested the three-layer-frozen model for all surfaces for comparison with the above four-layer frozen model: the energy difference between each surface model is 15 Å, which ensures no interaction between the slabs. The adsorption energy was calculated according to Eads ¼ E½slab þ adsorbate-E½slab-E½adsorbate in which E[slab + 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. The nudged-elastic-band (NEB) method48,49 was applied to locate transition structures; paths of minimum energy (MEP) were constructed accordingly. Frequency calculations were also applied to these transition structures, and only one imaginary frequency was obtained to confirm the transition states.

3. RESULTS AND DISCUSSION To elucidate the interaction between a metal surface and its adsorbates, it is pivotal to discover the binding properties and energies in these systems. Hence we provide all results calculated for the adsorbates, including adsorption energies, geometries, charge analysis, and possible barriers for the C
C scission; we present also the local density of states (LDOS) projected on the top layer of the monometallic or bimetallic surfaces to enlighten about the variation of the d band of a core/shell composition. In Figure 3, we provide the calculated charge dispersion of R and G groups in the three adsorbates in the gaseous phase and their C
C bond lengths; we found that CH3CO has the largest dispersed charge (0.66 |e|), whereas CH3CH2 has the least (0.09 |e|); the C
C bond length follows the order: CH3CH2 > CH3CN > CH3CO, which directly correlates with the charge dispersion. 3.1. Adsorption Properties of CH3CO, CH3CN, and CH3CH2 on Ni, Cu/Ni, and Pt/Ni Surfaces. Three geometries for CH3CO

are favorable for adsorption on each surface, all prone to bind with the metal atom through their CO group, with the methyl group rising away from the surface, shown in Figure 4a
c. The structure of CH3CO that binds with the surface via its central C atom, forming only one single bond with the surface, (Top-η1-C) (Figure 4a), exhibits the least stability; whereas the one in Figure 4b, with C and O atoms each bound to one top-site metal atom in a η1η1 (C,O) configuration holds a greater adsorption stability. In Figure 4c, the C atom favors binding at the bridge site, whereas the O atom favors binding at the top site in a η2η1(C,O) fashion; there both atoms are saturated, four and two bonds,

Figure 3. Calculated C
C bond length and charge dispersion in the gaseous phase of CH3CO, CH3CN, and CH3CH2, respectively. 19232

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respectively, exhibiting the greatest adsorption energies,
3.29,
3.52, and
4.27 eV of CH3CO on pure Ni and core/shell

Figure 4. Top and side views of all possible adsorption geometry structures of (a
c) CH3CO, (d
f) CH3CN, and (g) CH3CH2 on pure Ni, Cu/Ni, and Pt/Ni surfaces, respectively.

Table 1. Calculated Adsorption Energies of (a
c) CH3CO, (d
f) CH3CN, and (g) CH3CH2 on Pure Ni, Cu/Ni, and Pt/Ni Surfaces with All Possible Adsorption Structures, Drawn in Figure 4a
g surface

Ni

site CH3CO

CH3CN

CH3CH2

Cu/Ni

Pt/Ni

binding energies/eV

(a) top-η1(C)


2.98


3.11


3.39

(b) η1η1(C,O)


3.27


3.37


4.08

(c) η2η1(C,O)


3.29


3.52


4.27

(d) η1η1(C,N)


0.68


0.92


1.75

(e) η2η1(C,N)


0.92


1.49


1.99

(f) η1η3(C,N)


1.15


1.57


2.33

(h) top-η1(C)


1.82


1.92


2.18

Cu/Ni and Pt/Ni, respectively. The detailed adsorption energies of CH3CO, CH3CN, and CH3CH2 on these surfaces appear in Table 1. CH3CN also has three preferred adsorption geometries on each surface, in common with CH3CO, such that all tend to show binding with the metal atom via their functional group (CN), as the methyl group tilts away from the surface. Unlike CH3CO, the CN group in CH3CN must interact with the surface through both C and N atoms in η1η1(C,N), η2η1(C,N), and η1η3(C,N) configurations in Figure 4d
f, respectively. The adsorption stability in these three fashions is Figure 4f > 4e > 4d, consistent with the greater coordination numbers with the surface metal (four, three, and two of CH3CN to the surface, respectively) yielding the more stable adsorption geometry. CH3CN with Figure 4f adsorption geometry would consequently have the best adsorption energy (Table 1),
1.15,
1.57, and
2.33 eV, on pure Ni, Cu/Ni, and Pt/Ni surfaces, respectively. There is only one preferred adsorption structure of CH3CH2 on each surface (Figure 4g) in which the C atom of CH2 is the only atom binding to the surface; the adsorption energies are
1.82,
1.92, and
2.18 eV on pure Ni, Cu/Ni, and Pt/Ni, respectively (Table 1). In comparison with the other two molecules, CH3CH2 is slightly more stable than CH3CN except on the Pt/Ni surface, whereas CH3CO is still the most stable and has the largest adsorption energies (within
3.3 to
4.3 eV) on all three surfaces. This comparison is also drawn in Figure 5a in which the line curve of the adsorption energies of CH3CO (pink line) is far beneath the others. We clearly observe the comparison between adsorption energies not only for the same molecule on varied surfaces but also for the three molecules on the same surface; the slopes of the three lines indicate the degree of adsorption difference between the monometallic Ni and the core/shell Cu/Ni and Pt/Ni surfaces. The results show that the Pt/Ni surface adsorbs CH3CO and CH3CN much better than pure Ni and Cu/Ni, whereas the mild slope of the gray line (for CH3CH2) implies a similar interaction between CH3CH2 and the three surfaces. 3.2. C
C Scission of CH3CO, CH3CN, and CH3CH2 on Ni, Cu/Ni, and Pt/Ni Surfaces. In Figure 6a
c, we describe the potential-energy profiles for cleavage of the C
C bond in CH3CO, CH3CN, and CH3CH2, respectively (from the most stable adsorption configurations), on the three surfaces; the detailed results of the activation and reaction energies appear

Figure 5. Comparison of (a) adsorption energy and (b) activation energy of CH3CO, CH3CN, and CH3CH2 on pure Ni, core/shell Cu/Ni, and Pt/Ni, respectively. 19233

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from its originally upright position (away from the surface) to the top site of a metal atom. In the transition structure, the C
C bond is greatly stretched, and the CO fragment alters its position from lying down to upright with C bonded to a three-fold site and O away from the surface. The final product contains two fragments similar to the transition structure in which CO locates on a three-fold site (fcc) and CH3 on the nearest top site along the C
C bond scission coordinate. Our calculated energy barriers for the C
C cleavage on pure Ni, Cu/Ni, and Pt/Ni surfaces are 1.23, 1.07, and 1.30 eV energies, respectively. The path for C
C rupture of CH3CN is similar to that of CH3CO, described in Figure 6b; it begins from the most stable state that coordinates with the surface of four bonds. During the stretching of the C
C bond, the CH3 group moves gradually toward the final position in which CH3 is on the top site via the C atom binding with the surface and the N atom of CN gradually moves upward away from the hollow site of the surface. In the transition structure, the length of the C
C bond is ∼2.1 Å; the location of the CH3 group is unstable such that it finally moves toward another top site of the metal atom to be completely detached from the CN group with the N atom standing perpendicularly on top of the C atom sitting on the original three-fold site of the metal surface. The calculated activation energies for the detachment of this C
C bond are 1.75, 1.71, and 1.47 eV on Ni, Cu/Ni, and Pt/Ni, respectively. The major difference of the C
C bond splitting for adsorbed CH3CH2 (Figure 6c) from the other two adsorbates is that the final fragment CH2 travels to a bridge site to obtain a tetrahedral-like structure instead of the hollow site like CO and CN, whereas CH3 still preferred to move toward the top site. The barriers for scission of the C
C bond in adsorbed CH3CH2 on Ni, Cu/Ni, and Pt/Ni are 2.03, 1.98, and 1.84 eV, respectively. The scission of the C
C bond of CH3CO is the only exothermic reaction on all three surfaces (ΔH =
0.22 to
0.48 eV) relative to the similar dissociation of the other two adsorbates, all endothermic on these surfaces (CH3CN ≈ 0.94 to 1.1 eV and CH3CH2 ≈ 0.06 to 0.47 eV, listed in Table 2). The three C
C bond cleavage reactions in the gaseous phase are all endothermic; the equations and our calculated heats of reaction are as follows CH3 COðgÞ f CH3ðgÞ þ COðgÞ ΔH 1 ¼ 0:43eV

ð3-2-1Þ

CH3 CNðgÞ f CH3ðgÞ þ CNðgÞ ΔH 2 ¼ 6:47eV

ð3-2-2Þ

CH3 CH2ðgÞ f CH3ðgÞ þ CH2ðgÞ ΔH 3 ¼ 6:22eV

Figure 6. Energy diagram of C
C bond scission for (a) CH3CO, (b) CH3CN, and (c) CH3CH2 on pure Ni and core
shell Cu/Ni and Pt/Ni surfaces, respectively. The numbers inserted represent for the activation energies of each step.

in Table 2. During the C
C cleavage of CH3CO, the methyl group (CH3) gradually moves downward toward the surface

ð3-2-3Þ

These equations show that the heat of reaction for C
C scission of CH3CO in the gaseous phase is slightly endothermic, whereas the other two are strongly endothermic, CH3CO , CH3CH2 < CH3CN. According to our calculated data of adsorption energies of the reactants and products (in Table S1 of the Supporting Information), ΔH of these three reactions on the specified surfaces are plausible for the C
C scission of CH3CO being the only exothermic reaction; whereas the other two are endothermic. For instance, the adsorption energies of CH3, CO, and CH3CO on the Ni surface are
2.17,
1.91, and
3.29 eV, respectively, whereas ΔH for C
C bond scission of CH3CO in the gaseous 19234

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Table 2. Calculated Greatest Adsorption Energies (Eads), Bader Charge Analysis (chg.), Integrated Overlap Area Percent (A%), C
C Bond Length (dC
C), and C
C Bond Elongation Difference (Δd) of CH3CO, CH3CN, and CH3CH2 on pure Ni, Cu/Ni, and Pt/Ni Surfaces with the Best C
C Cleavage Paths and Its Activation and Reaction Energies Corresponding to Figure 6a
c adsorbate CH3CO

CH3CN

surface

site

A%b

dC
C/Å

Δd/Åc

C
C bond scission

ΔH/eV


3.29


0.32

2.81

1.517

0.091

1.23


0.34


0.49

3.14

1.520

0.094

1.07


0.48

Pt/Ni


4.27


0.56

3.89

1.516

0.090

1.30


0.22


1.15


0.82

3.46

1.506

0.057

1.75

0.94


1.57
2.33


0.89
0.91

3.51 4.02

1.510 1.514

0.061 0.065

1.71 1.47

1.1 1.08

Ni

η1η3(C,N)

CH3CO f CH3 + CO

Ea/eV


3.52

2 1

Cu/Ni Pt/Ni CH3CH2

chg. |e|a

Cu/Ni

Ni

η η (C,O)

Eads/eV

CH3CN f CH3 + CN


1.82


0.16

2.63

1.517

0.046

2.03

0.55

Cu/Ni


1.92


0.19

2.73

1.519

0.048

1.98

0.47

Pt/Ni


2.18


0.23

2.95

1.520

0.049

1.84

0.06

Ni

top(C)

CH3CH2 f CH3 + CH2

a

Charge transfer from the surface Rto adsorbates. b Integrated overlap area in LDOS between adsorbate (s,p orbitals) and shell Ni (d orbital), which has R been normalized by the formula: ( overlap area)/( shell Ni area)  100%. c C
C bond elongation difference as compared with the gas phase of each adsorbate.

phase is 0.43 eV, causing ΔHsur of C
C bond scission of CH3CO on the Ni surface (ΔHsur = (
2.17) + (
1.91)
(
3.29) + (0.43) =
0.36 eV) to agree with our directly calculated data to be exothermic. The same derivation is applicable to the other two reactions and results in slightly endothermic (∼1.0 eV or less) on all surfaces, which implies that the C
C bond scission of CH3CN and CH3CH2 would be greatly beneficial thermodynamically on the metal surfaces relative to the gaseous phase. In Figure 5b, on comparison of the adsorbates on the same surface, the order of the decreased C
C cleavage barriers is CH3CH2 > CH3CN > CH3CO on the three surfaces, in accord with the increased order of C
C bond elongation difference (Δd in Table 2, the difference of C
C bond length between the gaseous phase and adsorbate on the metallic surface, approximately 0.05, 0.06, and 0.09 Å for CH3CH2, CH3CN, and CH3CO, respectively) so that a greater C
C bond elongation difference correlates with a smaller barrier to cleavage. Comparison of the C
C cleavage barriers of the same adsorbate on three surfaces yields the result in Table 2: the two adsorbates CH3CN and CH3CH2 exhibit an order opposite to their adsorption energies in that the greater adsorption energy correlates with a smaller barrier. In contrast, for the dissociation of CH3CO on the Pt/Ni surface, the adsorption energy is the greatest (
4.27 eV) among the three surfaces but the C
C cleavage barrier is the greatest (1.30 eV). 3.3. LDOS and Charge Analysis of the Adsorption and C
C Bond Activation Energies for CH3CO, CH3CN, and CH3CH2 on Ni, Cu/Ni, and Pt/Ni Surfaces. We employed the Bader charge analysis that showed that the charge transfer from the surfaces to each adsorbate after adsorption (in Table 2) correlated with the surface adsorption energies: the greater charge transfer correlated with the greater adsorption energy. We applied the calculated results of the local density of states (LDOS) to explain those outcomes. The d-band curve projected on the first layer of the three pure metals (in Figure 7a) shows that although pure Cu has the sharpest band it has scarcely any electron density near the Fermi level, whereas the d-band of pure Pt is the broadest and has a similar strength over the whole band; the bandwidth of pure Ni falls between but has a stronger band maximum near the Fermi level, which indicates that pure Ni more readily donates its electrons to the adsorbates. Relative to the d-band of pure Ni (red line in Figure 7b), the d-band of shell

Ni after coating onto Cu (Cu/Ni) or Pt (Pt/Ni) congregates and becomes sharper and stronger and shifted toward the Fermi level (EF). If the d-band center approaches EF, then the surface interacts more strongly with the adsorbed molecule.50
55 From our calculated d-band centers corresponding to the d-band state of the top-layer Ni for three surfaces, the d-band center (εd) of the overlayer Ni shifts from
1.33 (pure Ni) to
0.79 (Cu/Ni) and to
0.68 (Pt/Ni), which thus results in gradually increased adsorption energies of the same adsorbate adsorbed from pure Ni to Cu/Ni and to Pt/Ni surfaces (Table 1). Furthermore, the maximum near the Fermi level in the band of overlayer Ni of Pt/Ni is the sharpest and strongest, which, as a consequence, is the most favorable to donate electrons to the adsorbed molecules; the maximum in the shell Ni of Cu/Ni shows similarly but is slightly weaker and farther from the Fermi level. This smaller interaction with the adsorbate explains well our calculated adsorption energies of the three adsorbates being the greatest on the Pt/Ni and then the Cu/Ni surface. Figure 7c
e displays that among the same adsorption geometry of a molecule on varied surfaces the value of the LDOS integrated overlap area percent (in Table 2) between the adsorbates (s and p orbitals) and the shell Ni (d orbital) has invariably the order Pt/Ni > Cu/Ni > Ni, which is consistent with the outcome of our calculated adsorption energies. In Table 3 appears the integrated overlap area percent for each adsorbate, from the shell Ni d and carbon p orbitals, for the initial and transition states, and the difference of the integrated overlap area percent (ΔA) between the initial and transition states directly correlates to the C
C cleavage activation energy on the three surfaces. The smaller ΔA means that there are smaller stability differences between initial and transition states, thus causing less energy change from initial to transition state as well as the activation energy. For example, the activation energies of CH3CO are 1.23, 1.07, and 1.30 eV on Ni, Cu/Ni, and Pt/Ni with ΔA = 1.1, 0.7, and 1.3, respectively.

4. CONCLUSIONS We discuss the C
C cleavage of three adsorbates, CH3CO, CH3CN and CH3CH2, on pure Ni, core/shell Cu/Ni, and Pt/Ni surfaces. Among these three molecules, CH3CO adsorbate is the most stable on all three surfaces, whereas CH3CN is the least stable, but it appears to be more stable on Pt/Ni than CH3CH2; 19235

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Figure 7. Local density of states (LDOS) projected on the top layer for (a) pure metal Ni (shell metal), Cu, and Pt (core metal), respectively, and (b) shell Ni of pure Ni and core/shell Cu/Ni and Pt/Ni, respectively, LDOS for (c) CH3CO, (d) CH3CN, and (e) CH3CH2, with the greatest adsorption energy on pureR Ni and core/shell R Cu/Ni and Pt/Ni, respectively. The integral overlap percent (A%) is also shown in the Figure. A% is normalized according to ( overlap area)/( shell Ni area)  100%.

for each adsorbate, the stability on the surfaces is Pt/Ni > Cu/Ni > Ni. The adsorbed CH3CO appears to break the C
C bond much more easily than the other two adsorbates; whereas CH3CH2 requires the most energy to run this reaction, this phenomenon is directly correlated with the C
C bond elongation difference between the molecule in the gaseous and adsorbed phases. For each adsorbate at three surfaces, the barriers

for scission of the C
C bond correlate with ΔA between the initial and the transition states: the larger ΔA correlates with a larger barrier. Here we offer a calculated result on the significant catalytic properties of surfaces of type core/shell by means of scission of the C
C bond; the outcome shows that a substrate metal such as Pt and Cu can alter the electronic configuration of the overlayer Ni so as to provide it with enhanced activities. 19236

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Table 3. Integral Summation of Overlap Area Percent (A%) in LDOS for Each Adsorbate Between the Shell Ni (d) and C (p) on Ni, Cu/Ni, and Pt/Ni Surfaces at Initial (IS) and Transition (TS) States and the Difference between IS and TS Overlap Area Percent (ΔA) Ai % (initial state)/arb. unitsa

At% (transition state)/arb. unitsa

ΔAb

Ni

1.23

17.0

15.9

1.1

Cu/Ni

1.07

20.6

19.9

0.7

Pt/Ni

1.30

21.8

20.5

1.3

Ni

1.75

23.1

20.5

2.6

Cu/Ni Pt/Ni

1.71 1.47

24.5 30.8

22.1 29.2

2.4 1.5

Ni

2.03

23.2

21.8

1.4

Cu/Ni

1.98

25.1

23.9

1.3

Pt/Ni

1.84

31.1

29.9

1.2

Surface

CH3CO

CH3CN

CH3CH2

a

Ea/eV

adsorbate

R R Normalized according to ( overlap area)/( shell Ni area)  100%. b ΔA = |At%
Ai%|.

’ ASSOCIATED CONTENT

bS

Supporting Information. Calculated adsorption energies of the reactants (CH3CO, CH3CN, and CH3CH2) and the products (CH3, CO, CN, and CH2) on Ni, Cu/Ni, and Pt/Ni surfaces, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +886-2-29309085. Fax: +886-229324249.

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