The dopant vanadium enhances CO adsorption on gold clusters

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The CO adsorption on vanadium-doped gold clusters AunV with n = 1–14 is studied by ... cage, the low-coordinated Au atoms become the preferred sites for CO ...
Theoretical study of AunV-CO, n = 1–14: The dopant vanadium enhances CO adsorption on gold clusters Pham Vu Nhat, Truong Ba Tai, and Minh Tho Nguyen Citation: J. Chem. Phys. 137, 164312 (2012); doi: 10.1063/1.4761892 View online: http://dx.doi.org/10.1063/1.4761892 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v137/i16 Published by the American Institute of Physics.

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THE JOURNAL OF CHEMICAL PHYSICS 137, 164312 (2012)

Theoretical study of Aun V-CO, n = 1–14: The dopant vanadium enhances CO adsorption on gold clusters Pham Vu Nhat,1,2 Truong Ba Tai,1 and Minh Tho Nguyen1,3,a) 1

Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium Department of Chemistry, Can Tho University, Can Tho, Vietnam 3 Institute for Computational Science and Technology at HoChiMinh City (ICST), Quang Trung Software Park, HoChiMinh City, Vietnam 2

(Received 20 June 2012; accepted 7 October 2012; published online 29 October 2012) The CO adsorption on vanadium-doped gold clusters Aun V with n = 1–14 is studied by density functional theory computations, using the BB95 and B3LYP functionals along with the cc-pVDZPP basis for metals and cc-pVTZ for non-metals. When both Au and V sites are exposed, CO adsorption on V is thermodynamically favorable because with partially filling d orbitals vanadium is more willing to interact with CO empty or filled orbitals. When vanadium is confined inside a gold cage, the low-coordinated Au atoms become the preferred sites for CO attachment. The presence of V tends to reinforce CO adsorption as compared with the bare gold clusters. The diatomic AuV is predicted to have the largest CO adsorption affinity as it has a typical π -back donation bond. Aun V–CO complexes typically have the larger CO binding energies and larger CO frequency shift than the isoatomic gold-carbonyl Aun+1 -CO counterparts. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4761892] I. INTRODUCTION

Contrary to the peculiar inertness of its bulk, gold exhibits either in the form of nano-scale or finely dispersed on metal oxide surfaces an enhanced catalytic activity for many gas-phase reactions such as CO oxidation, propylene epoxidation, NO reduction, water-gas shift, and methanol synthesis,1–4 . . . In the CO combustion catalyzed by gold, the CO adsorption on Aun clusters constitutes a fundamental step determining the catalysis mechanism. There have therefore been an increasing number of experimental and theoretical studies of Aun CO complexes in both neutral and charged states.5–12 Prior investigations revealed that irrespective of the charged states of the gold host, CO is likely to attack to lowcoordinated gold atoms, and due to the predominance of a charge transfer from CO to gold over a π -back donation, the adsorption strength obeys the ordering as cation > neutral > anion. The chemical bonding in carbonyl complexes is normally understood by the Blyholder model,13 which describes the metal-CO bonding as donor–acceptor interactions. Owing to its lone pair of electrons, carbon monoxide tends to react with metal clusters via facile association reactions, in which either molecular or dissociative chemisorptions are involved, rather than breaking molecular bonds and then making stronger new bonds between the resulting fragments and the metal surface.14 The reactivity of gold clusters dispersed on metal oxides toward CO has also received much attention in recent times since such supported gold catalysts exhibit surprisingly high catalytic activities.15 Furthermore, this activity depends not only on the cluster size but also on the nature of the solid a) E-mail: [email protected].

0021-9606/2012/137(16)/164312/12/$30.00

substrate. For example, the best catalytic activity for the CO → CO2 reaction is observed at the size Au13 , and system sizes containing 8 and 20 gold atoms when dispersed on Mg(OH)2 and MgO surfaces, respectively.3, 16, 17 In contrast, gold particles on TiO2 substrate become the most effective in CO oxidation at the sizes corresponding to a few hundred atoms.2 Previous studies on the doped gold clusters leave no doubt that the presence of impurity not only induces geometric and electronic changes, but also strongly influences the physical and chemical properties of host gold systems.18–22 Consequently, the reactivity of Au-based catalysts can be tuned, and it is necessary to extend investigations of CO adsorption on binary clusters. A large amount of research work has thus been performed on the catalytic mechanism of CO combustion by bimetallic clusters including Aun Agm −/+ ,23–27 Aun Cum + ,28 Aun Xm (X = Cu, Ag, Pd, Pt),28–31 Au12 X (X = V− , W),32–34 Aun X (X = H, Li, Na; n = 1–7),35 Aun Y (n = 1–9),36 MAun + (M = Ti, Fe, Au; n = 1, 6, 7)37 and MAun Om + (M = Ti, Fe; n = 1, 4–7; m = 1–2).38 For Aun Ptm , it was found that CO adsorption on the Pt site is much stronger than that on the Au site, and if only Au sites are available for CO adsorption, the strongest adsorption occurs at ∼25% Pt composition.31 Similarly, a replacement of Au in pure gold clusters by Pd atoms significantly improves their CO attraction.29 In contrast, the presence of Cu, Ag tends to reduce such activity of gold clusters, and a systematic decrease of CO adsorption affinity with increasing silver or copper content was observed.25, 29 Recently, Lin et al. also found a drop in the CO reactivity toward gold clusters after one gold atom is replaced by one yttrium for most of the sizes considered.36 Furthermore, the net charges of CO in Aun YCO complexes are consistently negative, in such a way that CO basically plays the role of a global electron acceptor.36 For Aun Cum + CO complexes,

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Zhao et al.28 however pointed out an electron density flow from CO to metals and the CO binding energies to clusters decrease with respect to the increasing number of copper atoms as the amount of electron transfer decreases. It is thus expected that the greater the charge involved the larger the binding energy is. Even though the Au12 V− anion was suggested as an improved catalyst for CO oxidation,32 very little information on CO interaction with Aun V clusters has been reported. Albeit some kinetic parameters of CO adsorption on Aun V clusters in both neutral and cationic states were experimentally determined by Le et al.,39 our knowledge about adsorption and combustion on these bimetallic clusters is still very limited. In this context and in relation to our recent theoretical investigation on the Aun V clusters,22 we report here a detailed examination on the adsorption of a single CO molecule on a series of vanadium-doped gold clusters Aun V with n ranging from 1 to 14. The main purpose is to probe the CO adsorption behavior on these binary metallic clusters, including the binding sites, energies, and molecular mechanism. In addition, a comparison of the calculated results obtained for both doped and pure gold systems allows us to evaluate the inherent effects of the V dopant. The chemical bonding is also examined to probe further into the ways CO and Au clusters interact with each other. II. COMPUTATIONAL METHODS

Density functional theory (DFT) computations are carried out to identify the lowest-energy structures of the combined Aun VCO complexes and thereby to evaluate the CO binding energies. All calculations are performed using the GAUSSIAN 09 suite of program.40 It is rather hard to find a current functional that is good for both transition metals and main group elements. For example, the description of bonding in transition metal compounds typically requires functionals with lower amount of Hartree–Fock exchange, while functionals with high percentage of Hartree–Fock exchange could be more appropriate for main-group compounds. It is also well known that no functional is suitable for all chemical properties. One functional can be good for a specific property but much less good for another one. It is therefore usual to select the functionals according to their performance for the properties considered. In this study the meta-GGA BB9541, 42 functional is employed in conjunction with the correlation consistent ccpVDZ-PP and cc-pVTZ basis sets for geometry optimization and energetic calculations as well. The basis set cc-pVDZPP43 with an effective core potential (ECP) is applied for both Au and V metals, while the all electrons cc-pVTZ basis set is used for C and O. The former already includes relativistic effects that are crucial in the treatment of heavy elements such as gold. Harmonic vibrational frequencies are invariably calculated to confirm the character of optimized geometries as local minima on the potential energy surface. The rationale for the selection of the BB95 functional is given below. The main reason for choosing the pseudopotential-based correlation consistent basis sets cc-pVaZ-PP (a = D, T, Q, 5) is that they are obviously suitable for describing the chemical

J. Chem. Phys. 137, 164312 (2012)

bonding of the coinage metals in yielding some of the most accurate carried out to date. As for a necessary calibration of the performance of the functionals employed, we first carry out some benchmark computations for the dimers Au2 and AuV, along with the adsorption energy of CO on Au+ cation, using various types of functionals, namely four pure GGAs BP86,41, 44 BPW91, PW91,45 PBE;46 six hybrid-GGAs B3P86, B3PW91, B3LYP,47–49 B2PLYP,50 PBE0,51, 52 B98;53 two meta-GGAs TPSS,54 BB95; and three meta hybrid-GGAs TPSSh,55 M06,56 M06–2X,56 and the CCSD(T) method.57, 58 As stated above, the basis sets employed are the cc-pVTZ-PP for metals and the cc-pVTZ for non-metals. The computed results are summarized in Table I, which also includes the available experimental data for the purpose of comparison. For the homo-diatomic Au2 , all functionals tested tend to overestimate the bond length. The deviations from the experiment vary from 0.04 Å (TPSSh) to 0.10 Å (M06). Consistent with the tendency for overestimation of bond length, the De (Au2 ) values obtained from hybrid functionals are too low as compared to the experimental value of 2.29 ± 0.02 eV.59 Generally, hybrid and hybrid-meta GGA functionals are not quantitatively accurate for predicting bond length and bond energy between gold atoms as they are likely to underbind them. On the contrary, pure and meta GGA functionals perform better for such parameters. The computed De (Au2 ) values using the BB95, PBE and TPSS functionals are around 2.28–2.29 eV, and thus in excellent agreement with experiment. Similarly, functionals containing an amount of HartreeFock exchange consistently yield the longer equilibrium distance (re ) for AuV than those with zero or lower Hartree–Fock exchange. Unfortunately, the experimental information on the bond length of the hetero-diatomic species is not available for a benchmark. Concerning the dissociation energy of AuV, BPW91 calculations provide more reliable result than any other functional considered. The value of De (AuV) = 2.50 eV by BPW91 is compared well with the experimental value of 2.51 ± 0.09 eV.59 Several functionals that also yield De (AuV) value lying within the error bar (±0.09 eV) include the B3LYP, B3P86, BB95, and TPSSh. Based on the agreement between the measured data and our computed results for both Au2 and AuV, a non-hybrid functional, namely the meta-GGA BB95, appears to be an advantageous choice for the systems containing these elements. However, this functional tends to overestimate the adsorption energy of CO on Au+ ,60 where the B3LYP describes it better (see Table I). Both functionals also reproduce well the ionization energy and electron affinity of the diatomic species,61, 62 though like other functionals considered, they tend to underestimate the experimental vibration ωe (Au2 ).59 Therefore, we select the two BB95 and B3LYP functionals for most calculations carried out in this study. As for a further calibration of the basis set employed, some molecular properties of the dimers Au2 and AuV are also computed using the basis sets with diffuse functions. The results are found to be marginally modified by the basis sets. For example, at BB95/aug-cc-pVTZ-PP level, the IE and EA values of Au2 correspond to 9.34 and 1.82 eV, respectively,

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TABLE I. Theoretical and experimental results of bond length R (Å), dissociation energy De (eV), ionization energy IE (eV), electron affinity EA (eV), and vibrational frequency ω (cm−1 ) for Au2 and AuV, along with the CO adsorption energy Eads on cation Au+ (eV). Au2 Method BP86 PW91 BPW91 PBE B98 B3PW91 B3LYP B3P86 B2PLYP PBE0 TPSS BB95 TPSSh M06 M062X CCSD(T) Exptl

AuV

Re

De

IE

EA

ωe

Re

De

IE

EA

ωe

Eads

2.520 2.519 2.523 2.521 2.534 2.520 2.547 2.513 2.512 2.516 2.510 2.521 2.509 2.575 2.550 2.497 2.472a

2.27 2.31 2.18 2.29 2.12 2.00 1.96 2.11 1.48 2.03 2.28 2.29 2.18 2.17 1.54 2.15 2.29a (±0.02)

9.53 9.44 9.34 9.37 9.18 9.21 9.28 9.83 8.44 9.10 9.23 9.33 9.14 9.10 8.62 9.18 9.20b (±0.21)

2.08 1.97 1.91 1.91 1.71 1.82 1.89 2.39 1.41 1.73 1.79 1.80 1.72 1.77 1.61 1.78 1.92c

173 173 172 172 165 174 166 177 180 176 178 172 179 160 164 187 191a

2.465 2.451 2.476 2.470 2.504 2.498 2.512 2.485 2.479 2.500 2.469 2.470 2.482 2.500 2.550 2.477

2.71 2.66 2.50 2.65 2.64 2.28 2.48 2.45 1.98 2.29 2.70 2.57 2.55 2.39 2.29 2.76 2.51a (±0.09)

7.18 7.08 7.06 7.06 6.58 7.03 8.26 7.58 7.55 7.32 6.89 6.97 6.85 7.05 6.94 8.04

1.17 1.13 1.07 1.06 1.31 1.01 1.10 1.53 0.82 0.94 0.90 1.02 0.85 1.23 0.98 1.16

231 232 227 228 219 223 219 227 236 223 232 228 228 211 210 143

2.60 2.66 2.50 2.64 2.10 2.19 2.08 2.28 1.47 2.21 2.45 2.51 2.29 1.92 1.63 2.04 2.08c (±0.15)

a

Taken from Ref 59. Taken from Ref 61. c Taken from Ref 62. d Taken from Ref 60. b

as compared to the values of 9.33 and 1.80 eV at BB95/ccpVTZ-PP level. Similarly, the adsorption energies of CO on Au+ cation computed using the aug-cc-pVTZ-PP basis set in conjunction with either the BB95 or B3LYP functional are equal to those obtained using the cc-pVTZ-PP. In this paper, we report on CO adsorption energies, which are defined as a measure of the strength of CO adsorption to a specific Aun V cluster, as follows (Eq. (1)): Eads = E(CO) + E(Aun V) − E(Aun VCO),

(1)

where E(CO), E(Aun V), and E(Aun VCO) are the total energies of CO, Aun V, and Aun VCO, respectively. Hence, as for a convention, a positive value of Eads corresponds to a favorable adsorption. In addition, this parameter can be used to evaluate the relative stability of a specific carbonyl complex. The natural bond orbital (NBO) charges of atoms are computed by using the NBO5.G software.63 We use NBO charges for electronic population analysis instead of Mulliken charges because the former are likely to be more reliable.29 The densities of states, which are used to assign the contributions of atomic orbitals to a particular molecular orbital,64 are plotted using the PyMolyze-2.0 program.65

III. RESULTS AND DISCUSSION

To simplify the presentation of results, the shape of molecular orbitals of a few structures and the Cartesian coordinates of all equilibrium structures discussed hereafter are given in the supplementary material.66

A. Equilibrium structures of Aun VCO adducts

The optimal structures of the Aun V clusters with n = 1– 14 were systematically determined in a recent investigation,22 and the corresponding results are taken in our present study of CO adsorption. For the sake of consistency, their structures are now reoptimized using the BB95 functional. Structures of the most stable forms of Aun VCO adducts are presented in Figures 1 and 2, along with their CO adsorption energies computed at both BB95 and B3LYP methods. In general, for clusters smaller than Au11 V when both Au and V sites are exposed, the latter is consistently more favorable to adsorb CO than the former (see Figure 1). In Sec. III B, we discuss the shapes of these species in some detail. n = 1. For the dimer AuV, CO prefers to anchor on the V site, forming the complex 1n-I (Figure 1) having Cs symmetry and a high spin 5 A ground state. In AuVCO 1n-I, the V–C and C–O bond lengths are around 2.00 and 1.16 Å, respectively. The V–CO linkage is thus somewhat longer than a typical M–CO distance in metal carbonyls (typically < 1.80 Å), but it is about equal to a metal-alkyl bond. CO can also bind to the Au atom of AuV, although the resulting complex is much less stable than 1n-I (∼1.0 eV). It has been pointed out that due to the relativistic effect, an AuX bound is substantially destabilized if X is more electronegative than gold.67, 68 Here, attachment of CO on V is more energetically favored than on Au as the relativistic effect of V is much smaller than that of Au.69 n = 2. The lowest-energy structure of Au2 VCO 2n-I has Cs symmetry in which CO directly attaches to V of AuVAu. The V–CO distance in Au2 VCO is ∼1.99 Å,

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FIG. 2. Lowest-energy structures considered for Aun VCO complexes (n = 10–14) and their CO adsorption energies. The values in square brackets are computed using the B3LYP functional.

shapes and are formed by CO attacking the dopant V. The former has a V–C distance of around 1.99 Å and a CO binding energy of 1.28 eV, while the values of the latter amount to ∼1.98 Å and 1.36 eV, respectively. n = 6 and 7. By adding one CO molecule to the lowestenergy Au6 V through the dopant site, we obtain the most stable form Au6 VCO 6n-I. Other isomers 6n-II and 6n-III, which are built upon the 3D structure of Au6 V, stay at around 0.32–0.42 eV above 6n-I. FIG. 1. Lowest-energy structures considered for Aun VCO complexes (1–9) and their CO adsorption energies. The values in square brackets are computed using the B3LYP functional.

compared to 2.00 Å of AuV–CO. However, its CO binding energy of 1.26 eV is smaller than that of AuVCO (1.50 eV, BB95). n = 3. The global minimum found for Au3 VCO, i.e., 3nI in Figure 1, is a triangular pyramid (Cs ) with a V–C bond length of 1.97 Å. Such geometry is slightly distorted from C3v symmetry and it is the first 3D structure of mono-carbonyl complexes located in this work. n = 4 and 5. Optimal structures of both Au4 VCO 4nI and Au5 VCO 5n-I adducts (Figure 1) have also 3D

The optimal structure 7n-I of the larger system Au7 VCO can be viewed as arising from attachment of one more gold atom to 6n-I, while the next low-lying conformation 7nII with the 3D backbone Au7 V is located around 0.24 eV (BB95) above. n = 8 and 9. These systems appear to be extremely sensitive with the functionals employed. Indeed, at the BB95 level, the form 8n-I, which is built upon a part of an icosahedron, is computed to be 0.44 eV lower in energy than 8n-II, which comes from a planar configuration of Au8 V. At the same level, the most stable form 9nI of Au9 VCO is obtained by adding one gold atom to 8n-I and possesses a three-fold axis. A slight distortion of 9n-I leads to 9n-II, whose energies are separated by

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TABLE II. Ground and lower-lying states of Aun VCO complexes, with n = 1–14, and relative energies (RE) computed using the BB95 and B3LYP functionals. RE (eV) n

Isomers

1

1n-I (Cs ) 1n-II (Cs ) 2n-I (Cs ) 3n-I (Cs ) 4n-I (Cs ) 5n-I (Cs ) 6n-I (C2 ) 6n-II (Cs ) 6n-III (Cs ) 7n-I (Cs ) 7n-II (C1 ) 7n-III (Cs ) 8n-I (C2v ) 8n-II (Cs ) 8n-III (Cs ) 9n-I (C3 ) 9n-II (Cs )

2 3 4 5 6

7

8

9

State 5 A 5 A 4 A 3 A 4 A 3 A 4A 4 A 4 A 3 A 3A 3 A 4A 2 4 A 4 A 3A 3 A

RE (eV)

BB95

B3LYP

n

Isomers

0.00 1.07 0.00 0.00 0.00 0.00 0.00 0.32 0.42 0.00 0.24 0.40 0.00 0.44 0.44 0.00 0.01

0.00 0.95 0.00 0.00 0.00 0.00 0.00 0.10 0.29 0.00 0.66 0.19 0.75 0.00 0.75 0.70 0.21

9

9n-IV (Cs ) 9n-V (Cs ) 10n-I (C1 ) 10n-II (Cs ) 10n-III (Cs ) 10n-IV (C1 ) 11n-I (Cs ) 11n-II (C2v ) 11n-III (Cs ) 12n-I (C1 ) 12n-II (C2v ) 13n-I (Cs ) 13n-II (Cs ) 13n-III (Cs ) 14n-I (Cs ) 14n-II (Cs ) 14n-III (Cs )

only 0.01 eV. The isomer 9n-V coming from a 2D Au9 V is 0.81 eV less stable than 9n-I. However, B3LYP computations reveal a much different energy landscape. Accordingly, two forms 8n-II and 9n-V become the global minima. The 8n-I, is now a local minimum, being 0.75 eV above 8n-II, while 9n-I is characterized at this level to be a transition state with an imaginary frequency of 25i cm−1 . The hybrid functional thus significantly favors the 2D conformations of the Aun V moiety. Further information on their relative energies at both levels of calculation is given in Table II. n = 10. The global minimum 10n-I of Au10 VCO can be viewed as a capped derivative of 9n-II. The second isomer 10n-II is reached by adding the CO molecule to an on-top Au atom of a defect icosahedron. This is also the last cluster of the series in which the impurity is the most favorable site for binding carbonyl group. Though the isomers 10n-I and 10n-II have comparable energy content, being separated by only 0.09 eV (BB95 value), their CO adsorption energies are somewhat different from each other, namely 1.33 and 0.99 eV, respectively. Besides, it is interesting to note that B3LYP calculations confirm 10n-II to be a first-order saddle-point as it has an imaginary frequency of 20i cm−1 and is 0.75 eV above 10n-I. n = 11. At the BB95 level, the lowest-energy structure 11n-I is formed by adding CO to the top-side gold atom of an incomplete icosahedron Au11 V. The lower-lying isomer 11n-II, which is built upon an incomplete cuboctahedron, is found to be a transition state even though it has a tiny imaginary frequency of 10i cm−1 and is ∼0.2 eV less stable than 11n-I. On the contrary, B3LYP predicts 11n-II as the lowest-energy form of Au11 VCO, while 11n-I is 0.55 eV above and

10

11

12 13

14

State 3 A 3 A 4A 4 A 4 A 4A 3 A 3B

2 3 A 2A 2A 1 1 A 1 A 1 A 2 A 2 A 2 A

BB95

B3LYP

0.46 0.81 0.00 0.09 0.12 0.28 0.00 0.18 0.43 0.00 0.37 0.00 0.07 0.08 0.00 0.11 0.26

0.21 0.00 0.00 0.75 0.78 0.75 0.00 0.56 0.00 0.00 0.05 0.07 0.00 0.17 0.30

has an imaginary frequency of 27i cm−1 . Nonetheless, it is clearly seen that CO preferentially adsorbs on a vertex gold atom rather than on the V atom as the V–CO complex 11n-III is ∼0.43 eV (BB95 value) energetically above the gold-carbonyl complex 11n-I. As a result, the CO absorption energy abruptly drops in going from Au10 V (1.33 eV) to Au11 V (0.95 eV). n = 12. From Au12 V, CO clearly has no other choice than attaching on Au atoms as the dopant V is now located completely inside the gold cage. We find two forms competing for the ground state of Au12 VCO, i.e., 12nI and 12n-II, with CO molecule capping on one apex gold atom of an icosahedron and of a cuboctahedron, respectively. The latter is located at 0.37 eV above the former and has an imaginary frequency of 15i cm−1 at BB95 level. However, B3LYP calculations reverse the energy ordering, in that the 12n-I is now not even a local minimum and lies 0.6 eV above 12n-II. These observations can be understood as cuboctahedra and icosahedra are very closely related and they can easily interconvert to each other.70 Graciani et al.32 examined a multiple CO adsorption on the anion Au12 V− using the plane-wave pseudo-potential DFT calculations. These authors reported an average binding energy of about 1.0 eV. However, this value cannot directly be compared to the present values for the neutral Au12 V-CO, due in addition to the use of a different method that we cannot calibrate. n = 13 and 14. Contrary to Au12 VCO, several genuine minima with small energy separations are located for Au13 VCO and Au14 VCO using both BB95 and B3LYP functionals. For instance, with BB95 functional, two forms 13n-II and 13n-III are predicted to be 0.07 and 0.08 eV, respectively, above the most stable form 13n-I. In this case, all these conformations can be considered

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as the ground state of the complex Au13 VCO. Similarly, some nearly iso-energetic structures of Au14 VCO are also detected. Three isomers 14n-II, 14n-III, and 14n-III are calculated to be marginally, 0.11–0.26 eV, above the most stable form 14n-I. One noticeable and common point here is that the most stable form of these complexes is based on the core of an icosahedral cluster Au12 V and all exhibit the lowest possible spin state as their ground state.

B. Lowest-energy structures of Aun CO adducts

In order to probe the effects of impurity on the ability to attract CO of the host clusters, we now explore the optimal structures of the Aun CO complexes resulting from addition of CO to the lowest-energy pure gold clusters. There have been several theoretical studies on structures of small pure gold clusters.71, 72 We here select the results from a recent report by Assadollahzadeh et al.,73 in which the predicted structures of Aun are consistent with a combined experimental and theoretical vibrational investigation.74 Optimized geometries of the Aun CO species, and then CO adsorption energies, are computed using the BB95 and B3LYP functionals in conjunction with the same cc-pVDZ-PP basis set for Au and the cc-pVTZ basis set for C and O. Previously, several groups6, 10, 12, 36 also used DFT calculations to predict the CO adsorption on both neutral and charged Aun clusters containing up to ten gold atoms. CO adsorption on the larger clusters, i.e., Au11 , Au12 , and Au13 , was also evaluated,8 but their structures are much different from the lowest-energy isomers recently reported.73 In this work, for the purpose of comparison, we extend the investigations up to the size of Au15 . In general agreement with the previous work, our results point out that CO prefers to be anchored at the on-top sites of pure gold clusters. For the purpose of comparison Figure 3 gives the shapes and binding energies of the optimal Aun CO species. The CO binding energies are positive for all clusters, implying that the adsorption reactions are exothermic. The BB95 values agree well with previous predictions,6, 9, 10, 32, 36 while those obtained from the B3LYP are consistently lower. Although the measured CO adsorption energies were reported in the literature for some small anions Aun − (n = 1–3),24 and a series of cations Aun + (n = 1–65),60 no experiment is available for the neutral systems. Initial structures for Aun CO geometry optimization are constructed by attaching CO to the optimal structures of pure gold clusters through low-coordinated gold atoms. Attachment of CO causes rather negligible structural changes for Aun species. In fact, the Au–Au bond length of Au2 CO is ∼2.51 Å as compared to ∼2.52 Å in free Au2 . For larger systems, free gold clusters and gold frameworks in Aun CO almost bear the similar shape and geometrical parameters. Among free gold clusters considered, Au14 has a particular low CO affinity (0.50 eV), while Au3 induces the highest value (1.34 eV). Another striking observation is that the CO adsorption energies of the smallest clusters sizes, i.e., Au2 , Au3 , and Au4 , turn out to be much higher than those of the larger sizes.

FIG. 3. Lowest-energy structures considered for the Aun CO complexes (n = 2–15) and their CO adsorption energies. The values in square brackets are computed using the B3LYP functional.

C. Effects of vanadium on the CO adsorption

1. Binding energies and charge populations

In recent papers, Lin and co-workers36 confirmed that the CO affinity of pure gold clusters in general decreases upon replacement of one gold atom by one yttrium (Y). As previously pointed out,75 the presence of Y draws an increase of electron density on gold atoms due to the charge transfer from Y to Au. Such increase leads to a shift of the lowest unoccupied molecular orbitals towards higher energies and thereby weakens the σ -donor bond of CO and gold atoms.25 Similarly, the adsorption energies of CO on the pure gold clusters in cationic states are also greater than those on the neutral and anionic states because the CO → metal electron donation is typically more important than the metal → CO back donation.6 In other words, in these cases the CO molecule tends to play its classical role as a nucleophile and thus its attack to positively charged sites is favored. Figure 4 illustrates the variations of CO adsorption energies on both V-doped Aun V and pure Aun systems. As stated above, though the BB95 functional appears to be more reliable than the B3LYP in predicting geometries and energetics of pure and V-doped gold clusters, it tends to overestimate the CO absorption energy, where the latter appears to describe

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FIG. 4. The CO adsorption energies on Aun V and Aun clusters as a function of cluster size. The results are obtained at B3LYP/cc-pVaZ–(PP) level.

somewhat better. Hence, we here report the B3LYP values for this quantity. For the pure clusters, the smallest species, i.e., Au2 , Au3 , and Au4 , have much higher affinity with CO than the larger sizes. Their CO binding energies are calculated to be around 1.2 eV as compared to the highest value of 0.8 eV obtained for the remaining sizes. Among the pure clusters considered, Au14 is characterized by the lowest CO absorption energy of 0.5 eV, while the second lowest value of 0.6 eV is estimated for Au13 . Concerning bimetallic Aun V clusters, the largest binding energy is predicted for the dimer AuV (1.14 eV), then a significant drop occurs in going to larger sizes. From Au3 V to Au10 V, the Eads values exhibit even-odd oscillations with distinct maxima at n = 4, 6 and n = 8. The remaining species with only Au sites exposed, namely the Au11 V–Au14 V cages, have a much smaller CO absorption energy, in which the highest value (0.71 eV) is found for Au14 V. As seen in Figure 4, introduction of V leads to a significant change in trend of CO absorption on gold clusters. Particular differences are observed for clusters in the range of n = 1–10 as CO directly attaches to V atom and become less remarkable for larger clusters. Except for small sizes of n = 1–3, such a presence is accompanied with an increase of CO adsorption affinity. After replacing one Au by V, the CO binding energies of Au3 and Au4 are reduced from 1.34 (Au3 ) and 1.28 eV (Au4 ) to 0.85 (Au2 V) and 0.83 eV (Au3 V), respectively. On the contrary, the larger clusters generally have higher Eads values than their iso-atomic pure counterparts Aun . The Au11 V cluster is an exceptional case, as the BB95 method predicts for it a higher Eads (0.95 eV) than for Au12 (0.90 eV), whereas B3LYP yields a reverse ordering. Interactions between metals and CO moiety in carbonyl complexes are basically characterized by electron donation and acceptance, in such a way that charge transfer is a key element in their mechanism. The natural charges distributed on the adsorbed CO in Aun VCO complexes are summarized

J. Chem. Phys. 137, 164312 (2012)

in Table III. On the one hand, from Au5 VCO to Au10 VCO, the net charge of adsorbed CO in complexes where CO directly anchors on the V atom is consistently found to be positively charged and tends to increase monotonously with respect to the cluster size. For example, the natural charge of CO in 5n-I, 8n-I, and 10n-I are computed to be +0.04, +0.08, and +0.13 electron, respectively (BP95 values). This clearly reflects the existence of a charge transfer from CO to metals upon complexation, and such trend becomes more effective in larger systems. On the other hand, the forming of Au–CO bonding is accompanied by a charge transfer from clusters to CO as the adsorbed CO is now negatively charged. AuVCO is a special case as it possesses a typical π -back bonding between V and CO (see Figure 6). Indeed, the natural charge of CO in AuVCO complex is quite negative (–0.18 electron), indicating that an amount of electron has been transferred back from metal to CO. There also exists a backward charge transfer in Au11 VCO and Au12 VCO species, but it is not significant as in AuVCO. The charge of absorbed CO in these adducts and that of free CO molecule are comparable (Table III). Results for systems with n from 2 to 4 are somewhat sensitive with the functionals employed. While B3LYP predicts a forward electron transfer from CO to metals, the BB95 suggests an opposite process. Similar phenomena are also observed for the Au13 VCO and Au14 VCO adducts. It is not straightforward to make a meaningful comparison between our calculated results with the experiment recently reported by Le et al.39 because they involve two different stories. Le et al. performed a kinetic study, and most of their conclusions were based on the rate ratios kf /kD (kf being the bimolecular rate coefficient for the formation of the cluster–CO complex and kD being the unimolecular dissociation rate coefficient of the complex), and not on the CO adsorption energy. While the latter energetic parameter was not derived in Ref. 39, we actually compute it instead of the rate ratios. 2. Bonding lengths and vibrational frequencies

In Table III, we list the computed results for CO adsorption process on binary Aun V clusters, including bond lengths, CO and M–CO stretching frequencies, NBO charges of absorbed C in complexes, and CO binding energies. As a consequence of the stronger relativistic effects, the Au– CO bond lengths in gold-carbonyl complexes are consistently shorter than those of the V–CO counterparts in the vanadium-carbonyl complexes (Table III). However, the latter is characterized by a much larger Eads value than the former. Especially, the smallest vanadium-carbonyl complex AuVCO has the longest V–CO and C–O bond distances. In this case, an amount of electron has effectively been transferred from metal to CO (NBO charge of C in AuVCO being more negative than that of C in free CO). Thus the existence of a π back-donation enhances the metal–CO bond in weakening the C–O bond. The interaction between AuV and CO is the strongest which is consistent with the fact that the V–CO bond has a partial triple character. Therefore, in spite of having a longer M–CO distance, AuVCO has the largest CO binding energy. Nevertheless, this kind of bond

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TABLE III. Calculated results for CO adsorption on Aun V clusters. Values are obtained by BB95/cc-pVDZ-(PP) computations. NBO charges of C and O given in square brackets are computed using the B3LYP functional. For free CO, BB95 values are 1.138 Å, 2117 cm−1 and +0.45 electron [+0.49 at B3LYP]. Bond distance (Å) Species

V–CO

1n-I 1n-II 2n-I 3n-I 4n-I 5n-I 6n-I 6n-II 6n-III 7n-I 7n-II 7n-III 8n-I 8n-II 8n-III 9n-I 9n-II 9n-III 9n-IV 9n-V 10n-I 10n-II 10n-III 10n-IV 11n-I 11n-II 12n-I 12n-II 13n-I 13n-II 13n-III 14n-I 14n-II 14n-III

2.005

NBO charge of C and O in Aun VCO (au) C–O

ν CO (cm−1 )

ν M–CO (cm−1 )

1.962

1.159 1.153 1.152 1.151 1.151 1.150 1.149 1.154 1.148 1.148 1.151 1.145 1.151 1.148 1.147 1.150 1.149 1.147 1.149 1.147 1.150 1.147 1.150 1.152 1.147

1961 1988 1997 1996 1997 2006 2011 1975 2044 2014 1987 2064 1991 2015 2052 1997 2004 2052 2010 2024 1999 2049 1999 1983 2049

418 331 411 427 410 417 407 423 384 408 405 382 419 406 377 414 410 375 401 412 408 384 402 391 372

1.946

1.147

2049

389

1.943 1.968 1.968 1.946 1.982 1.977

1.147 1.147 1.147 1.147 1.146 1.146

2047 2047 2048 2048 2049 2050

391 369 369 387 353 358

Au–CO

2.044 1.985 1.959 1.984 1.973 1.985 2.000 1.949 1.982 2.004 1.951 1.998 1.985 1.956 1.999 2.000 1.958 2.010 1.976 2.008 1.950 2.007 2.014

significantly weakens the C–O bond since the anti-bonding orbital (LUMO of CO) does not accommodate electron occupancy. This is clearly reflected in the CO vibrational frequency, as shown in Table III. The AuVCO 1n-I is computed to have the lowest-energy C–O stretching mode among complexes considered. The CO stretching frequency in AuVCO is ∼1961 cm−1 (BB95 value), while in other complexes such vibrations occur at generally above 2000 cm−1 . The presence of V as dopant results in a larger C–O frequency red-shift. The evolution of CO stretching frequencies of Aun CO and Aun VCO as a function of cluster size is illustrated in Figure 5. In agreement with previous reports,6, 10, 24, 36 the CO frequencies in Aun CO adducts exhibit a clear odd– even oscillation up to Au10 . Following replacement of one Au atom by one V, such a pattern is destroyed. Instead, the CO frequencies of Aun VCO complexes are predicted to increase monotonically with respect to cluster size up to Au7 VCO. Then an abrupt drop takes place at Au8 VCO and then a sudden increase at Au11 VCO. In general, we find that CO stretching frequencies are shifted to a much distinguishably lower

C 0.22 [0.21] 0.27 [0.33] 0.32 [0.40] 0.37 [0.45] 0.37 [0.44] 0.42 [0.49] 0.44 [0.51] 0.38 [0.47] 0.30 [0.38] 0.45 [0.51] 0.43 [0.50] 0.34 [0.39] 0.45 [0.51] 0.44 [0.50] 0.32 0.48 0.49 [0.56] 0.32 0.47 [0.54] 0.46 [0.53] 0.49 [0.53] 0.33 0.47 0.46 0.32 [0.39] 0.333 [0.39] 0.46 [0.39] 0.53 [0.39] 0.35 [0.40] 0.48 [0.37] 0.54 [0.38] 0.37

O − 0.40 [−0.45] − 0.41 [−0.45] − 0.38 [−0.40] − 0.38 [−0.40] − 0.38 [−0.40] − 0.38 [−0.40] − 0.38 [−0.40] − 0.38 [−0.40] − 0.38 [−0.41] − 0.38 [−0.39] − 0.38 [−0.40] − 0.36 [−0.39] − 0.37 [−0.39] − 0.37 [−0.39] − 0.37 − 0.37 − 0.37 [−0.39] − 0.38 − 0.38 [−0.40] − 0.37 [−0.38] − 0.36 [−0.39] − 0.37 − 0.38 − 0.39 − 0.38 [ − 0.41] − 0.37 [ − 0.41] − 0.35 [−0.40] − 0.36 [−0.42] − 0.38 [−0.42] − 0.36 [−0.41] − 0.36 [−0.42] − 0.38

Eads (eV) 1.50 0.43 1.26 1.33 1.28 1.36 1.42 1.06 0.96 1.36 1.29 0.97 1.41 1.46 0.97 1.40 1.41 0.96 1.25 1.35 1.33 0.99 1.30 0.81 0.95 1.07 1.02 0.95 0.94 0.93 0.82 0.86

frequency range in species that CO molecule directly attaches to V atom. For instance, CO frequencies in 1n-I and 2n-I amount to 1961 and 1997 cm−1 as compared to 2049 cm−1 of 11n-I and 12n-I, respectively. However, no linear correlation can be established between CO vibrational frequency (vCO ) and adsorption energy (Eads ). For example, while the Eads values follow the ordering 6n-I > 5n-I ≈ 7n-I, the CO frequency sequence is 7n-I > 6n-I > 5n-I. Nonetheless, an interesting point is that the gold-carbonyl complexes, i.e., systems from 11n-I to 14n-I, consistently have higher vCO values and lower binding energies than corresponding vanadiumcarbonyl complexes. Such a correlation between vCO and Eads will be discussed in Sec. III D. D. Chemical bonding in Aun VCO complexes

As stated above, the interaction between CO and transition metals in carbonyl complexes is classically characterized by both σ and π electron components. An overlap of the nonbonding C electron pair with the partially filled d-orbitals

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FIG. 5. CO stretching frequencies in the optimal Aun CO and Aun VCO complexes as a function of cluster size.

gives rise to the σ bond. On the other hand, the π bonds arise from an overlap of filled d-orbitals on the metal with a pair of π 2p * antibonding orbitals of the CO. Gold has no empty d orbitals (the d shell is fulfilled) and thus an electron donation from the σ 2p nonbonding CO orbital to gold atoms is rather difficult. On the contrary, vanadium with partially filling d orbitals is willing to donate as well to accept electrons. Hence, interaction between CO and V turns out to be stronger than that of CO and Au. This is in line with our calculated results predicting that for small Aun V clusters, the dopant V binds more favorably to CO than the Au core. For larger clusters, Au atoms become the favored sites simply due to a steric effect, as V is now completely confined inside a gold cage. Information from frontier orbitals further provides us with some deeper insights into the bonding mechanism between CO and clusters. The most important combinations are related to the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals of CO and frontier orbitals of clusters. Depending on their symmetry and energy gap, the forward or backward donation bond will be prevailing. The problem becomes more complicated in open-shell systems because the singly occupied molecular orbitals (SOMO) can equally donate or accept electrons. Nevertheless, the orbital symmetry and energy are also the key factors deciding the bonding formation. Let us take a look at AuVCO as an example. The ground state of AuV is a quintet state with the valence orbital configuration 5 + : . . . (2σ )2 (3δ)1 (2π )1 (2π )1 (3σ )1 . Shapes and symmetries of these orbitals are given in Figure S1 of the supplementary information.66 In this case, the energy difference between the 3σ orbital (AuV) and the CO σ 2p -nonbonding (HOMO(CO)) is much larger than that between 2π orbitals (AuV) and the CO π 2p *-antibonding (LUMO(CO)). Hence, the charge transfer from 2π orbitals of AuV to LUMO(CO) is more effective than the reversed process, which is in agreement with the NBO analysis above. In addition, the LUMO(CO) can interact with the orbital 3δ of AuV, form-

FIG. 6. Shapes and symmetries of frontier MOs for AuV (upper) and Au6 V (lower) before and after interactions with CO.

ing further π -type bonding orbital. Consequently, the backdonation plays a much more important role than the forwardtransfer, and in AuVCO a charge amount is actually transferred from AuV to CO. The combination between these orbitals is illustrated in Figure 6. It is also stressed that the singly occupied orbitals of AuV have large contributions from d(V) AOs, as a consequence, CO prefers to bind to V than to Au site. The resulting adduct thus prefers a bent structure over a linear form as a result of the 3δ(V)–LUMO(CO) interaction. A similar interaction disappears in molecules with C∞v geometry because of the dδ (V) orbitals and LUMO(CO) do not overlap with each other. To get stabilization, the AuVCO structure needs to adopt a bent geometry. Figure 6 in addition shows the shapes and symmetries of orbitals generated from combinations between frontier MOs of CO and Au6 V. In its ground state, Au6 V has an orbital configuration of 4 B: . . . (1a2 1b1 2b1 3b1 2a), which are given in Figure S2 of the supplementary information.66 The occupancy, with three unpaired electrons on three b orbitals, gives rise to an electronic state 4 B. Due to the difference in symmetry, neither the LUMO nor the HOMO of CO does combine with the SOMO 3b. In contrast, while combinations between lower-energy SOMOs 1b and 2b with LUMO(CO) yield the π -bonding orbitals, the LUMO 2a can interact with HOMO(CO), giving rise to the σ -bonding orbital. These bonding orbitals are fully occupied. This demonstrates the existence of both forward and backward donations but the former (charge transfer from CO to metal) is somewhat dominant over the latter in the 6n-I adduct, as mentioned above, based on NBO charges. A direct correlation between the CO frequency and CO adsorption energy is not clearly established because both parameters depend on two different bonding environments.

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FIG. 7. Total and partial density of states for two Au8 VCO isomers.

While the former directly depends on the C and O bonding, the latter is the consequence of the C to metal attachment. However, in the whole system, one type of bonding may to some extent affect the other, and we find that goldcarbonyl complexes with smaller binding energies constantly possess higher CO frequencies than vanadium-carbonyl complexes. This observation can be understood on the basis of an analysis of density of states (DOS) for the atoms in a complex. Figure 7 displays the DOS of two isomers of Au8 VCO, namely 8n-I and 8n-III. Both isomers have the same Au9 V backbone but the site bound to CO and the binding energy differ from each other. In the stronger adsorption complex 8n-I, CO is attached to V whereas in the weaker adsorption complex 8n-III, CO absorbs on a low-coordinated Au atom. In 8n-III, two distinct bonding peaks between the C

and O atoms are visibly observed in the DOS graphic. One peak is located at around –12.0 eV and the other is above –2.5 eV. However, the peak corresponding to the CO bonding at around –2.5 eV does not appear on the DOS of 8n-I. Therefore, the CO bond in 8n-I is getting weaker compared to that in 8n-III, which is manifested in the CO frequency shift. In 8n-III, the CO frequency is actually shifted from 2117 cm−1 of free CO to 2052 cm−1 , as compared to 1991 cm−1 in 8n-I. In general, vanadium-carbonyl complexes have a weaker VCO bond and a larger CO frequency shift than gold-carbonyl complexes (see Table III) as a result of a shrink of the bonding peak between C and O at around –2.5 eV. The DOS plots also emphasize that the interaction between V and CO in 8n-I is more effective than that involving Au and CO in 8n-III. The resulting binding energy of 8n-I (1.41 eV) is thus much larger than that of 8n-III (0.97 eV).

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IV. CONCLUDING REMARKS

In this contribution, we perform a detailed theoretical study of the CO adsorption on vanadium-doped gold clusters Aun V in the range of n = 1–14, using the BB95 and B3LYP functionals in conjunction with the pseudo-potential cc-pVDZ-PP basis set for metals and the full-electron ccpVTZ basis set for non-metals. For the purpose of comparison, the CO adsorption on pure gold clusters with the sizes up to Au15 is also investigated to probe the effects of dopant V atom. In terms of geometrical viewpoint, when both sites Au and V are exposed, CO adsorption is energetically favorable on V atom because with partially filling d orbitals vanadium is more willing to undergo an interaction with either empty or fulfilled orbitals of CO. However, for larger systems, when vanadium is now completely doped inside the gold cage, lowcoordinated Au atoms become, as in the pure clusters, the preferred sites. The binding energies of Aun VCO adducts are also computed and compared with those of Aun+1 CO species to evaluate the effects V on the CO adsorption affinity. In the smallest systems such as Au2 V, Au3 V and Au4 V, the CO interaction in V-doped clusters is weaker than those of the pure hosts. For the larger systems, the presence of V invariably results in a reinforcement of CO adsorption. Among the doped clusters considered, AuV has the highest CO adsorption affinity as it can form a typical π -back donation bond between V and CO. Results obtained on the geometries, net charges and densities of states concur with the view that vanadium-carbonyl complexes have larger CO binding energies and larger CO frequency shift than those in their isoatomic gold-carbonyl counterparts. The calculated properties of the systems with n = 8– 12 appear to be extremely sensitive with the functionals employed. The hybrid functional B3LYP is significantly biased towards 2D conformations for metal frameworks. Moreover, while the meta-GGA BB95 predicts an icosahedral growth pattern in these species, the former yields a cuboctahedral evolution. In addition, several lower-lying isomers with very small energy gaps (