Theoretical Calculations on the Oxidation of CO on Au55, Ag13Au42

Article pubs.acs.org/JPCC

Theoretical Calculations on the Oxidation of CO on Au55, Ag13Au42, Au13Ag42, and Ag55 Clusters of Nanometer Size Han-Jung Li and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal University No. 88, Section 4, Tingchow Road, Taipei 116, Taiwan S Supporting Information *

ABSTRACT: Using density-functional theory (DFT), we investigated the oxidation of CO on Au55, Ag13Au42, Au13Ag42, and Ag55 metal clusters of nm size. The structures of oxidation intermediates and at the transition states on the potential-energy surfaces were derived with the nudged-elastic-band (NEB) method. According to our results, the coupling of CO and O2 molecules to form intermediate OCOO has the least energy barrier (0.13 eV) on the Ag13Au42 core−shell nanocluster, whereas the dissociation of the O− O bond of OCOO to form CO2 and O on the Au13Ag42 core−shell nanocluster is the easiest process with a 0.15 eV barrier height. To understand the electronic property of these nanocluster catalysts and their interactions with the adsorbates, we calculated the electron localization functions, Bader charges, and local densities of states; the results were consistent and explicable.

1. INTRODUCTION Among metallic clusters of nanometer size that attract considerable interest because of their significant optical, electronic, thermal, magnetic, and catalytic properties, gold nanoparticles have been studied theoretically1−7 and experimentally8−12 because their catalytic capabilities are distinct from those of bulk gold metal. For instance, with DFT calculation, Hvolbæk et al.7 reported that, despite bulk Au being considered inert, Au particles of size less than 3−5 nm exhibited outstanding reactivity for several chemical reactions at, or even below, 300 K. Experimental work by Turner et al.12 demonstrated that supported 55-atom gold particles (size ∼1.4 nm) are powerful catalysts for the selective oxidation with O2 molecule. Bimetallic nanoparticles composed of two metallic elements have received more attention than monometallic nanoparticles; in particular, recent theoretical13 and experimental14−20 works demonstrated that the core−shell structures show physical and chemical properties different from those of the separate metals. On mixing with a second metal (Ag, Pd, or Cu), Au-based bimetallic catalysts have shown theoretically to improve catalytic activity.21−24 Au/Ag alloy and core−shell structures can be prepared with various methods because of their similar crystal structures and lattice parameters.25−28 Either to control emission from motor vehicles or to remove poisonous CO in a fuel cell, the catalytic oxidation of CO is an important process. Its catalytic mechanism in a heterogeneous system has been widely studied and discussed theoretically.29−31 From the experimental and theoretically investigation of reactions to oxidize CO on supported and unsupported Au nanoparticles, Au nanoparticles were found to be active.32−35 Here we report the potential-energy surfaces © 2012 American Chemical Society

for the oxidation of CO via a Langmuir−Hinshelwood (LH) mechanism on pure (Au55 and Ag55) and core−shell (Ag13Au42 and Au13Ag42) nanoclusters using DFT calculations and address the electronic structures to explain the interaction between adsorbates and clusters of nm size.

2. COMPUTATIONAL METHODS All calculations were performed with the spin-polarized DFT plane-wave method implemented in the Vienna ab initio simulation package (VASP).36−39 The projector-augmentedwave method (PAW)40,41 in conjunction with the PW9142 density functional was employed. For a 30 × 30 × 30 Å3 cubic supercell, the energy was truncated at 300 eV, which was adequate for the required convergence, together with the Γ point for the summation in the Brillouin zone. We calculated adsorption energies according to this equation, Eads = E[adsorbate + nanocluster] − (E[adsorbate] + E[nanocluster])

in which E[adsorbate + nanocluster], E[adsorbate], and E[nanocluster] are electronic energies of adsorbed species calculated for the nanocluster, a free molecule, and a clean nanocluster, respectively. Frequency calculations were applied to verify the adsorbed intermediates and the transition states. The nudged-elastic-band (NEB) method43−45 was applied to Received: March 26, 2012 Revised: May 18, 2012 Published: May 29, 2012 13196

dx.doi.org/10.1021/jp302855n | J. Phys. Chem. C 2012, 116, 13196−13201

The Journal of Physical Chemistry C

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locate transition structures, and paths of minimum energy (MEPs) were constructed accordingly.

Table 1. Calculated d-Band Center, Bader Charges of Core and Shell, Maximum (Zmax) of the Electron Localization Function (ELF) for Au55, Ag13Au42, Au13Ag42, and Ag55 Nanoclusters, with the Adsorption Energy, Geometric Parameters, and Integrated Overlap Area of the LDOS for CO and O2 on Each Nanocluster

3. RESULTS AND DISCUSSION Au55 and Ag55 nanoclusters with icosahedral (ICO) structures tend to be more stable than low-symmetry counterparts.46,47 The four nanoclustersAu55, Ag13Au42, Au13Ag42, and Ag55 in this work were accordingly modeled as icosahedral structures with 13 core atoms and 42 shell atoms to form twenty equivalent triangular faces, shown in Figure 1a. Of six stable

d-band center/eV charge (core)/|e| charge (shell)/|e| Zmax (shell)a Eads(CO)/eV dC−O/Å dAu(Ag)‑C/Å overlap area (A)b Eads(O2)/eV dO−O /Å dAu(Ag)‑O /Å overlap area (A)c

Au55

Ag13Au42

Au13Ag42

Ag55

−2.88 1.233 −1.233 0.203 −0.91 1.15 1.98 5.93 −0.39 1.34 2.09 7.84

−2.82 2.393 −2.393 0.210 −1.12 1.15 2.01 6.65 −0.29 1.34 2.21 7.08

−3.97 −0.850 0.850 0.130 −0.47 1.15 2.08 5.59 −0.97 1.36 2.27 8.18

−3.82 0.784 −0.784 0.145 −0.51 1.14 2.12 5.66 −0.75 1.36 2.29 7.94

a

Maximum of electron localization function (ELF) for shell atoms. Integrated overlap area in LDOS (Figure 4 (a)) between CO molecule (p orbital) and nanocluster (d orbital). cIntegrated overlap area in LDOS (Figure 4 (b)) between O2 molecule (p orbital) and nanocluster (d orbital). b

Fermi energy; the d-band center of Au13Ag42 is left-shifted, at −3.97 eV relative to −3.82 eV. The increased d-band intensity about the Fermi level produces an increased interaction between the metal and the adsorbed molecule;48 the order of CO adsorption energy is accordingly Ag13Au42 > Au55 > Ag55 > Au13Ag42, which correlates directly with the calculated positions of the d-band centers: the more that the d-band center is leftshifted (away from the Fermi-level), the smaller is the adsorption energy. The calculated energy for CO adsorbed on a Au55 nanocluster (−0.91 eV) is near that on Au32 (−1.10 eV),49 Au29 (−1.01 eV),50 and Au10 (−0.95 eV)51 nanoclusters, indicating that this energy depends little on the size of the cluster, whereas the adsorption energy, −0.91 eV, of CO on Au55 is much larger than that, −0.17 eV,52 on Au(111), implying much greater interaction of the adsorbate toward a nanocluster than a metal surface; similar phenomena appeared for the silver counterparts, Ag55 (−0.51 eV) and Ag(111) (−0.29 eV).53 The O2 molecule is adsorbed on each nanocluster via its two oxygen atoms atop the two edge atoms of the cluster in a η1η1 mode (Figure 1(d)), with adsorption energies −0.29, −0.39, −0.75, and −0.97 eV on Ag13Au42, Au55, Ag55, and Au13Ag42, respectively. Figure 3 shows ELF54 diagrams for each nanocluster; the Zmax values of the shell atoms and the Bader charges55−59 are listed in Table 1. The work function60 of pure gold metal (5.10−5.47 eV) is larger than that of pure silver metal (4.52−4.74 eV), which produces a negative charge distribution of Au atoms on Ag13Au42 and Au13Ag42 core−shell clusters. The maximum ELF (Zmax) for shell atoms decreases in a sequence: Ag13Au42 (0.210) > Au55 (0.203) > Ag55 (0.145) > Au13Ag42 (0.130), whereas the adsorption energy of O2 follows the opposite order (Au13Ag42 > Ag55 > Au55 > Ag13Au42). This phenomenon is rationalized in that, on a core−shell Au13Ag42 nanocluster, most electrons withdraw to the core (Au13) atoms, resulting in the smallest Zmax for shell atoms and causing the least repulsion between the lone pair electrons of O2 adsorbate and the electrons of shell atoms; the adsorption energy of O2

Figure 1. Schematic presentation of (a) Au55 nanocluster, (b) adsorption sites of high-symmetry on a triangular face of the Au55 nanocluster (detailed presentation are shown in Figure S1), and optimized structures of (c) CO and (d) O2 adsorbed on the Au55 nanocluster. The Au, C, and O atoms are represented by yellow, gray, and red spheres, respectively.

adsorption sites on a triangular face considered for the adsorbate, two top sitesT1 and T2are atop the vertex and edge atoms respectively of the triangular face, two bridge sitesB1 and B2bridge between two edge atoms and between vertex and edge atoms respectively of the triangular face, and two hollow sitesH1 and H2fcc and hcp respectively are on a triangular face, shown in Figure 1b. For CO adsorption, the molecule prefers the “end-on” configuration on the top site (T1) with a C atom binding to the vertex atom (Figure 1c). The adsorption energies of CO molecule and the corresponding geometric parameters for each nanocluster are listed in Table 1. A CO molecule adsorbed on Au 55 , Ag 13Au 42 , Au 13 Ag42 , and Ag55 nanoclusters with adsorption energies −0.91, −1.12, −0.47, and −0.51 eV, respectively, indicating CO is preferably adsorbed on the Ag13Au42 core−shell nanocluster. To understand thoroughly the bimetallic effect, we calculated the local density of states (LDOS) of pure and core−shell nanoclusters, shown in Figure 2, and the calculated d-band centers, listed in Table1. Our results demonstrate that Au55 and Ag13Au42 nanoclusters exhibit similar d-band character, congregated near the Fermi energy, but the latter possesses a right-shifted d-band center, at −2.82 eV relative to −2.88 eV. In contrast, the d-band characters of Ag55 and Au13Ag42 nanoclusters congregate away from the 13197



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Figure 2. Local density of states (LDOS) of Au55, Ag13Au42, Au13Ag42, and Ag55 (d orbital) nanoclusters.

molecule is consequently largest on the Au13Ag42 nanocluster. In contrast, most electrons withdraw to the shell atoms (Au42) of the core−shell Ag13Au42 nanocluster, thus yielding the largest Zmax on shell atoms and the largest repulsion between shell atoms and the O2 adsorbate; the adsorption energy is thus the smallest. Although the maximum ELF (Zmax) for shell atoms decreases in a sequence order: Ag13Au42 (0.210) > Au55 (0.203) > Ag55 (0.145) > Au13Ag42 (0.130), the sum of charge transfer from cluster to the adsorbed O2 molecule increases in a same order: Ag13Au42 (−0.547) < Au55 (−0.580) < Ag55 (−0.623) < Au13Ag42 (−0.729), which correlates positively with the calculated adsorption energies. Incidentally, we have also presented the adsorption energies of O2 molecule adsorbed atop the T1 and T2 sites with an “end-on” (vertical adsorption) configuration on Ag13Au42, Au55, Ag55, and Au13Ag42 nanoclusters in Table S1. The adsorption energies of O2 molecule adsorbed on T1 and T2 sites with the end-on configuration are

Figure 3. Electron localization function (ELF) diagrams sliced (a) (200) plane in the bounding box of (b) Au55, (c) Ag13Au42, (d) Au13Ag42, and (e) Ag55 nanoclusters.

Figure 4. Local density of states (LDOS) projected on the (a) CO and (b) O2 (p orbital) molecules adsorbed on Au55, Ag13Au42, Au13Ag42, and Ag55 (d orbital) nanoclusters. 13198



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Figure 5. Profile of the potential-energy surface for the oxidation of CO. The inset figures show the side view of optimized structures of CO oxidation on a Au55 nanocluster (the top views of optimized structures are shown in Figure S2); similar results were found for the other three nanoclusters. The corresponding coadsorption energy, reaction barrier, and reaction energy on each nanocluster are listed in Table 2.

CO coadsorbed with a horizontal η1η1 mode O2 (Figure S2(a)) or with a vertical O2 (Figure S2(c)). The oxidation of CO begins with coadsorption of CO and horizontal O2 of which the coadsorption energies (Ecoads‑h) are −1.39, −1.18, −1.43, and −1.17 eV respectively on Au55, Ag13Au42, Au13Ag42, and Ag55 nanoclusters. This coadsorbate might proceed with the O2 molecule associating with the C atom of CO on crossing the transition state (TS1) with reaction barriers 0.24, 0.13, 0.30, and 0.27 eV (Ea1) respectively on Au55, Ag13Au42, Au13Ag42, and Ag55 nanoclusters, forming intermediate OCOO. Alternatively, the oxidation of CO might start with coadsorption of CO and vertical O2, of which the coadsorption energies (Ecoads‑v) being −1.97, −0.96, −0.89, and −0.58 eV, together with the calculated reaction barriers 0.40, 0.42, 0.24, and 0.21 eV (Ea2) in forming intermediate OCOO respectively on Au55, Ag13Au42, Au13Ag42, and Ag55 nanoclusters. However, the vertical coadsorption energies (Ecoads‑v) are smaller than that of horizontal O2 (Ecoads‑h) on Ag13Au42, Au13Ag42, and Ag55 nanoclusters, but larger on Au55. Therefore we predict that the oxidation of CO begins from the adsorbed horizontal O2 would be more preferable on Ag13Au42, Au13Ag42, and Ag55 clusters. For intermediate OCOO to produce CO2 and an O atom, the calculated O−O bond dissociation barriers are 0.27, 0.25, 0.15, and 0.22 eV (Ea3) respectively on Au55, Ag13Au42, Au13Ag42, and Ag55 nanoclusters. This barrier directly correlate with the interaction of the O atom sticking onto the cluster (Figure S2(e)); the stronger is the interaction of this O atom with the cluster, the more easily breaks the O−O bond of intermediate OCOO, such that the barrier for dissociation of the O−O bond is the least (0.15 eV) on Au13Ag42, relative to Ag55, Ag13Au42, and Au55. This interaction is explicable via the following analysis. The left and right panels of Figure 6 show respectively the LDOS projected on the O atom of OCOO directly connecting onto the cluster and the ELF diagrams for OCOO adsorbed on Au55, Ag13Au42, Au13Ag42, and Ag55 nanoclusters. The values of the LDOS integrated overlap area (A) between the O atom and the nanocluster have a trend similar to that of Zmax (maximum of ELF for O−Au(Ag) bond),

smaller than that via its two oxygen atoms atop the two edge atoms of the cluster in a η1η1 mode (horizontal adsorption), indicating O2 is preferably adsorbed in a η1η1 mode on each nanocluster. The detailed analyses of the local density of states (LDOS) projected on CO and O2 adsorbed on each nanocluster are shown in Figure 4, panels a and b, respectively. In Figure 4a, the values of the LDOS integrated overlap area (A) between the CO adsorbate and the nanocluster follow the order Ag13Au42 (6.65) > Au55 (5.93) > Ag55 (5.66) > Au13Ag42 (5.59), consistent with the calculated adsorption energies of CO and indicating that a larger overlap area between CO and the cluster correlates with a greater adsorption energy; for the integrated overlap area between O2 and the nanocluster, the calculated adsorption energies of O2 follow the overlap area order Au13Ag42 (8.18) > Ag55 (7.94) > Au55 (7.84) > Ag13Au42 (7.08). The potential-energy surface for oxidation of CO is shown in Figure 5; the corresponding barriers and reaction energies on each nanocluster are listed in Table 2. The preferred mechanism for the reaction CO + O2 proceeds as CO(gas) + O2(gas) → CO(ads) + O2(ads) → OCOO(ads) → O(ads) + CO2(gas). There are two possible coadsorption structures of CO and O2: Table 2. Calculated Coadsorption Energies of CO and O2 (Ecoads‑h and Ecoads‑v), Reaction Barriers (Ea1, Ea2, and Ea3), and Reaction Energies (ΔE1, ΔE2, and ΔE3) for the Oxidation of CO on Au55, Ag13Au42, Au13Ag42, and Ag55 Nanoclusters

Ecoads‑h/eV Ecoads‑v/eV Ea1/eV ΔE1/eV Ea2/eV ΔE2/eV Ea3/eV ΔE3/eV

Au55

Ag13Au42

Au13Ag42

Ag55

−1.39 −1.97 0.24 −1.33 0.40 −0.75 0.27 −1.96

−1.18 −0.96 0.13 −0.79 0.42 −1.01 0.25 −2.05

−1.43 −0.89 0.30 −0.59 0.24 −1.13 0.15 −2.43

−1.17 −0.58 0.27 −0.73 0.21 −1.32 0.22 −2.47 13199



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Figure 6. Local density of states (LDOS) (left panel) projected on the O atom of intermeduiate OCOO (p orbital) and electron localization function (ELF) diagram (right panel; the sliced plane is shown in Figure S3) for intermediate OCOO adsorbed on (a) Au55, (b) Ag13Au42, (c) Au13Ag42, and (d) Ag55 nanoclusters.

4. CONCLUSIONS For the oxidation of CO on nanosized Au55, Ag13Au42, Au13Ag42, and Ag55 metal clusters, our calculations demonstrate that, on the Ag13Au42 nanocluster, most electrons withdraw to the shell atoms (Au42) thus causing O2 molecule to have the smallest adsorption energy but CO to have the largest; the association to form intermediate OCOO is thus easiest, consistent with the least reaction barrier, Ea1 = 0.13 eV, on a Ag13Au42 nanocluster, relative to the Au55, Ag55, and Ag13Au42 analogues. Dissociation of the O−O bond of intermediate OCOO to form CO2 molecule and an O atom is favored (Ea3 = 0.15 eV) on Au13Ag42 through the strongest interaction between the O atom of intermediate OCOO and the Au13Ag42 nanocluster, resulting in the weakest O−O bond of intermediate OCOO. Our results indicate that a core−shell system might alter the electronic properties of nanoclusters and efficiently tune the catalytic capabilities.

which follows the order Au13Ag42 (A = 3.111, Zmax = 0.760) > Ag55 (A = 3.074, Zmax = 0.751) > Ag13Au42 (A = 3.053, Zmax = 0.745) > Au55 (A = 3.021, Zmax= 0.736). The magnitude of the overlap area is related to the value of Zmax; a larger overlap area (A) indicates a stronger interaction between the O atom of OCOO and the cluster, forming a stronger O−Au(Ag) bond and resulting in a larger value of Zmax and a smaller Ea3 (O−O dissociation barrier); Ea3 accordingly follows the order Au13Ag42 < Ag55 < Ag13Au42 < Au55. Comparing the reaction barriers of Ea1, Ea2, and Ea3 for the oxidation of CO on these four nanoclusters, we discern that this reaction is most facile to occur on core−shell Ag13Au42 relative to its Au55, Ag55, and Au13Ag42 counterparts. Despite that the oxidation of CO might proceed alternatively via O2 dissociation first, then combination with CO to form CO2, our calculated barriers for the dissociation of O2 are about 1 eV, consistent with values greater than 0.9 eV on Au with stepped, flat or particle size,52 which is much larger than our Ea1 and Ea3; accordingly, we deduce that this alternative process would be unlikely. Our calculated result for the oxidation of CO on the core−shell Ag13Au42 nanocluster is predicted to have the least energy barriers relative to the following theoretical work with the DFT method regarding the oxidation of CO on various Au catalysts: Liu et al.52 suggested that the barriers on the Au(221) surface are 0.59 (CO(ads) + O2(ads) → OCOO(ads)) and 0.43 eV (OCOO(ads) → O(ads) + CO2(gas)); Chen et al.50 reported the barriers on a Au38 nanocluster to be 0.69 and 0.40 eV; Gao et al.61 reported barriers 0.35 and 0.55 eV on a Cu13Au42 core− shell nanocluster.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 shows calculated adsorption energies of O2 at T1 and T2 sites on four nanoclusters. Figure S1 shows the Au55 nanocluster, adsorption sites of high symmetry on a triangular face of the Au55 nanocluster, and top and bottom views of a Au55 nanocluster. Figure S2 shows top and side views of optimized geometries for CO oxidation on an Au55 nanocluster; similar results were found on the other three nanoclusters. Figure S3 shows a sliced plane of an electron localization function (ELF) diagram for intermediate OCOO adsorbed on Au55. This material is available free of charge via the Internet at http://pubs.acs.org. 13200



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (886)-2-77346224. Fax: (886)-2-29324249. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS National Science Council of Republic of China (NSC 99-2113M-003-006-MY3) supported this work; National Center for High-performance Computing provided computer time.

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