Theoretical Investigation of Adsorption of Molecular Oxygen on Small

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Theoretical Investigation of Adsorption of Molecular Oxygen on Small Copper Clusters Xiuxiang Yuan, Liuxia Liu, Xin Wang, and Mingli Yang* Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China

Koblar Alan Jackson* Department of Physics, Central Michigan University, Mt. Pleasant, Michigan 48859, United States

Julius Jellinek* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

bS Supporting Information ABSTRACT: Adsorption of molecular oxygen on CuN (N = 2
10) clusters is investigated using density functional theory under the generalized gradient approximation of PerdewBurke-Ernzerhof. An extensive structure search is performed to identify low-energy conformations of CuNO2 complexes. Optimal adsorption sites are assigned for low-energy isomers of the clusters. Among these are some new arrangements unidentified heretofore. Distinct size dependences are noted for the ground state CuNO2 complexes in stability, adsorption energy, Cu
O2 bond strength, and other characteristic quantities. CuNO2 with odd-N tend to have larger adsorption energies than their even-N neighbors, with the exception of Cu6O2, which has a relatively large adsorption energy resulting from the adsorption-induced 2D-to-3D structural transition in Cu6. The energetically preferred spin-multiplicity of all the odd-N CuNO2 complexes is doublet; it is triplet for N = 2 and 4 and singlet for N = 6, 8, and 10.

1. INTRODUCTION Understanding the interactions of metal clusters with small molecules is of importance from both fundamental and applied points of view. For example, growth patterns of atomic clusters can be explored by inspecting their interactions with small molecules.1
8 Many studies have pointed out that metal clusters can serve as nanocatalysts with unique properties, even though the understanding of their catalytic mechanisms is far from complete.9
15 Explorations of interactions between molecules and metal clusters have been carried out for various adsorbate-cluster systems.16
36 Here we report results of a computational study of molecular adsorption of O2 on small CuN, N = 2
10, clusters. Copper clusters have been the subject of many experimental37
47 and theoretical48
68 investigations. Studies52
59 based on density functional theory (DFT) calculations revealed that copper clusters adopt planar structures for N e 6, three-dimensional layered morphologies for N e 16, and icosahedron-based compact structures for larger sizes in their lowest energy conformations. Adsorption of small molecules on copper clusters has been studied by several authors.69
82 The adsorbates include H2, CO, O2, H2O, H2S, and others. Experimentally, the interaction between oxygen and copper clusters was first addressed by Winter et al.1 in time-of-flight mass spectrometry based studies. r 2011 American Chemical Society

CuN with closed shells, for example, N = 2, 8, 18, ..., were found to be nonreactive with O2. On the theoretical side, Padilla-Campos78 investigated adsorption of atomic and molecular oxygen on CuN with N e 8 using DFT/B3LYP calculations. Significant size dependence and even
odd alternation effects were noted in the reactivity of the clusters. Florez et al.79 used DFT/BLYP calculations to characterize the interactions of O2 with CuN, N = 1
7, and showed that the condensed Fukui function could be utilized as a predictor of the optimal adsorption sites. The various studies indicated that the interaction of O2 with copper clusters is size and site specific and that the adsorption of the molecule, when it happens, is strong. The goal of this study is to determine the energetically preferred configurations of CuN-O2, N = 2
10, complexes, their adsorption patterns and energetics, their geometrical and electronic properties, and the evolution of these with the size of the clusters. As has been pointed out in earlier investigations, in considering adsorbate addition one has to consider not only the most stable structures of the clusters, but also their other lowReceived: January 5, 2011 Revised: May 20, 2011 Published: July 06, 2011 8705

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Figure 1. Patterns of O2 (smaller spheres) adsorption on CuN (larger spheres) clusters investigated in this work.

energy isomers, as the adsorption energetics of the latter may be competitive with or even exceed that of the most stable conformations. Earlier, DFT-based calculations56
68 identified a number of stable isomeric forms of copper clusters. In a series of recent studies59,67,83 we continued to explore the structures and electronic properties of small and intermediate size neutral and anionic CuN. Here we consider O2 adsorption on neutral CuN in the size range N = 2
10. The outcome is a number of new, heretofore unidentified energetically preferred CuN-O2 conformations. Inclusion of these in the analysis leads to a new understanding of the size-evolution of O2 adsorption on CuN. The paper is organized as follows. In section 2 we outline the methodological framework used. The results and their discussion are presented in section 3. A brief summary is given in section 4.

2. METHODOLOGICAL FRAMEWORK The structures of the low energy isomers of CuN, N = 2
10, we use in this study are taken from our earlier work.59 These were obtained from a large pool of candidate conformations resulting from a combined tight-binding/DFT search. The calculated ionization potentials of the most stable structures are in good agreement with the measured data.37,38 Three to six lowest energy isomers, depending on their energy separations, are considered for O2 addition for each cluster size. The various adsorption sites and arrangements considered for each isomer are shown in Figure 1. Two patterns, a single-bonded and a double-bonded, were explored for adsorption over Cu atoms. Three and four patterns were considered for O2 addition over Cu
Cu bonds and Cu
Cu
Cu triangular faces, respectively. In the case of bonds, the O2 was placed both parallel and perpendicular to the bond in a plane parallel to it and also perpendicularly over the bond center in a plane containing it. In the case of triangular faces, the O2 was placed parallel to the triangle along its three medians. A fourth placement, perpendicular to the plane of the triangle above its center, was also considered. Four-fold faces can be viewed as two (pairs of) triangles, and all the different initial parallel orientations of O2 with respect to these triangles were considered. A vertical placement over the center of each 4-fold face was explored as well. Following the same pattern, different initial “parallel” and “perpendicular” arrangements of O2 were used for 5-fold faces. The computations were performed within the framework of the generalized gradient approximation to DFT with the PerdewBurke-Ernzerhof (PBE) exchange-correlation functional.84 An

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extensive all-electron basis set of 20 primitive Gaussians contracted to [7s5p4d] for Cu and [5s4p3d] for O, as implemented in the NRLMOL code,85 was utilized. The quality of the basis set for copper has been verified in our earlier studies.59,67,83 The basis set for oxygen produces an O
O bond length of 1.22 Å and a stretching frequency of 1544 cm
1, which are in good agreement with the measured values of 1.21 Å86 and 1549 cm
1,87 respectively. Because the ground electronic state of O2 is a triplet, both singlet and triplet states for even-N clusters and doublet and quartet states for odd-N clusters were examined for the CuN-O2 complexes. Spin-polarized calculations were performed for openshell systems. For comparison (see below), we carried out singlepoint calculations within DFT/B3LYP88,89 and restricted open shell MP2 (ROMP2)90 using the LANL2DZ pseudopotential/ basis set91 for Cu and the 6-31G(d,p) basis set for O. The structures of the CuN-O2 complexes were optimized using the Broyden-Fletcher-Goldfarb-Shanno algorithm92 with no symmetry constraints imposed. Normal mode analysis was performed for the three lowest energy conformations of the CuNO2 complexes for each N to ensure that these conformations correspond to true minima on the respective potential energy surfaces.

3. RESULTS AND DISCUSSION 3.1. Optimized CuN-O2, N = 2
10, Structures. For each N, we obtained a number of low energy structures of the CuN-O2 complexes. Figure 2 displays the three most stable structures, which are labeled as A, B, and C, in increasing order of their total energies or, equivalently, decreasing order of their relative stabilities. For example, 3A is the lowest energy structure of Cu3-O2, followed by 3B and 3C. The three lowest energy structures for a given N are not necessarily obtained from the same isomer of the parent CuN cluster. For example, conformations A and B for N = 4 and N = 5 are generated from different isomers of Cu4 and Cu5, respectively. This underscores the need for considering a sufficient number of low energy isomers of CuN in a search for low energy conformations of CuN-O2. As a rule, the most stable CuN-O2 complexes are obtained by adding O2 to the most stable isomer of CuN. The case of N = 6 is an exception. The Cu6 parent of all three conformations 6A, 6B and 6C of Cu6O2 is a 3-D isomer (C2v) of Cu6, rather than the planar (C3v), most stable form of this cluster.52,56,58 The relative energies of the complexes and their spin-multiplicities are given in Table 1. Their energy separations vary with the number N of the Cu atoms in the complex. For N = 2, all three structures are triplets. The singly bonded 2A complex is more stable than the doubly bonded 2B by 0.14 eV, whereas in the 2C conformation the two O atoms bond to different Cu atoms, forming a four-membered ring structure whose energy is 0.45 eV higher than that of the 2A structure. For N = 3, the singly bonded 3C conformation is less stable than either the multiply bonded 3A or the doubly bonded 3B. The energy separation between the complexes is significant, 0.34 eV between 3A and 3B, and 0.28 eV between 3B and 3C. The multiplicity of all three conformations is doublet. For N = 4, the planar 4A and the 3-D 4B adsorption structures are nearly degenerate, whereas the planar 4C conformation lies 0.18 eV higher in energy than the 4A conformation. The spin state of the 4A structure is triplet, and it is singlet for the 4B and 4C arrangements. In the case of N = 5, the two O atoms bridge to two Cu atoms in all three conformations. The 5A and 5C structures are formed from the same planar isomer (C2v) 8706

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Figure 2. Three lowest-energy conformations of CuN-O2 complexes for N = 2
10.

of Cu5, an isosceles trapezoid that is the most stable form of this cluster, but their energies differ by 0.58 eV. The energy of the 3-D 5B conformation is close to that of the 5C conformation. The spin-multiplicity for all three cases is doublet. The most stable form of the bare Cu6 is a planar triangle with C3v symmetry, but all the three lowest energy isomers of Cu6O2 are 3-D structures. This is an example of the case when the energetically most favorable adsorption of the molecule occurs on cluster isomers other than the most stable one. Moreover, the singlet states are much more stable than the corresponding triplet states. For N g 7, the three lowest energy adsorption complexes are 3-D and they are generated by the same respective most stable isomers of CuN. Their spin multiplicities are either 1 or 3 for the cases of even N, and 2 for the cases of odd N. For example, the 8A and 10A conformations are singlets, but the 8C and 10B structures are triplets. Below we discuss the implications of the different electronic configurations for the adsorption properties of O2 on CuN. Single-point B3LYP calculations produce a similar energy ordering of the adsorption conformations with a few exceptions (cf., Table 1). The same lowest energy structures are obtained for all N but N = 6, for which the 6B structure emerges as slightly more stable than the 6A structure. Also, there is a change in the energy ordering of the second and third adsorption conformations for N = 5, 7, and 10. The number of possible adsorption structures increases dramatically with the size of CuN. DFT studies of O2 adsorption by Florez et al.79 and Padilla-Campos78 covered the size ranges of up to N = 7 and 8, respectively. We found a number of new low

energy adsorption conformations in these ranges. We have also identified lowest energy structures of CuN-O2 complexes that are different from those of Florez et al.79 for N = 2, 4, 6, and 7, and of Padilla-Campos78 for N = 2
4 and 6
8. In ref 79, the most stable structure for N = 2 is our 2C conformation, whereas the lowest energy structures for N = 4, 6, and 7 are not among the three shown for each of these sizes in our Figure 2. In ref 78, the most stable structures for N = 2, 3, and 7 are our conformations 2B, 3B, and 7B, respectively. The lowest energy conformations for N = 4, 6, and 8 are not among the three shown for each of these cases in our Figure 2. Only for N = 5 is the most stable structure in ref 78, the same as our 5A. Our results for N = 9 and 10 appear to be the first predictions for these sizes. As our results differ from those of the earlier DFT studies, which also are at odds with each other, regarding the different low energy conformations of CuNO2, it is important to try to validate them all. To this end, we performed ROMP290 single point calculations of all those conformations that were predicted as the most stable structures of CuNO2, N = 2
10, by all the studies carried out so far, including the present one. We employed ROMP2, rather than unrestricted MP2, to remove spin contamination that was found to be significant in some cases. With a single exception, the ROMP2 energies of the newly identified most stable conformations shown in Figure 2 are lower than those of the most stable structures obtained in earlier studies (cf. Table 1). The exception is for the case of N = 2: Conformation 2A is slightly less stable than conformation 2B (by 0.03 eV) at the ROMP2 level. 8707

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Table 1. Spin Multiplicities (Spin), Relative Total Energies (RE), Binding Energies per Atom (BE), and Adsorption Energies (AE) of CuNO2 Complexes, a PBE b

B3LYP c

ROMP2

spin

RE

BE

AE

RE

RE

2A

3

0.00

1.41

0.73

0.00

0.03

2B (2P)d

3

0.14

1.34

0.59

0.13

0.00

complex

Table 2. Averaged Cu
Cu, Cu
O, and O
O Bond Lengths (in Å) Calculated for the Most Stable CuN and CuNO2 Complexesa

2C (2F)

3

0.45

1.19

0.28

0.40

0.08

3A (3F)

2

0.00

1.78

1.94

0.00

0.00

3B (3P) 3C

2 2

0.34 0.62

1.67 1.58

1.60 1.32

0.39 0.53

0.50

4A

3

0.00

1.77

1.21

0.00

0.00

4B

1

0.02

1.76

1.19

0.21

4C

1

0.18

1.72

1.03

0.54

4F

3

0.57

1.62

0.64

0.60

1.82

Cu
Cu

Cu
O

CuN

O
O

nCu
O

CuNO2

Cu2O2 Cu3O2

2.254 2.310

2.271 2.412

1.886 1.944

1.271 (1277) 1.359 (1080)

2 2

Cu4O2

2.411

2.481

1.917

1.360 (976)

2

Cu5O2

2.422

2.651

1.899

1.377 (975)

2

Cu6O2

2.470

2.563

1.942

1.497 (705)

3

Cu7O2

2.460

2.516

1.968

1.359 (1007)

2

Cu8O2

2.453

2.562

1.993

1.539 (670)

4

Cu9O2

2.482

2.618

1.950

1.358 (1006)

2

Cu10O2

2.454

2.557

1.951

1.627 (501)

4

a

The numbers in parentheses are the vibrational frequencies of the O
O bond in cm
1. The last column indicates the number of Cu
O bonds in each complex.

4P

3

0.28

1.70

0.94

0.21

5A (5F, 5P)

2

0.00

1.97

2.00

0.00

5B 5C

2 2

0.57 0.58

1.85 1.85

1.56 1.41

0.80 0.55

6A

1

0.00

2.01

1.66

0.06

6B

1

0.08

2.00

1.58

0.00

6C

1

0.21

1.98

1.45

0.58

6F

3

1.12

1.83

0.33

0.73

2.84

6P

3

0.85

1.87

0.60

0.50

2.65

7A

2

0.00

2.06

1.21

0.00

0.00

7B (7P) 7C

2 2

0.01 0.12

2.06 2.05

1.20 1.09

0.24 0.20

7F

2

0.26

2.03

0.95

0.35

0.31

8A

1

0.00

2.11

0.88

0.00

0.00

8B

1

0.00

2.11

0.87

0.02

Figure 3. O2 adsorption energy (in eV) for the most stable CuNO2 complexes.

8C

3

0.02

2.11

0.86

0.00

8P

3

0.19

2.08

0.68

0.11

0.41

9A

2

0.00

2.19

1.79

0.00

9B 9C

2 2

0.06 0.21

2.18 2.17

1.73 1.58

0.05 0.38

10A

1

0.00

2.22

1.58

0.00

10B

3

0.44

2.18

1.14

0.36

10C

1

0.47

2.18

1.11

0.53

which leads to the formation of four Cu
O bonds as, for example, in the cases of 8A and 10A. One notices that more Cu
O bonds are favored in larger clusters. Table 2 lists the lengths of the bonds between Cu
Cu, Cu
O, and O
O atoms near the adsorption sites in the most stable CuNO2 conformations. The Cu
Cu distances shown are averages over the Cu
Cu bonds formed by the Cu atoms(s) that are directly involved in O2 adsorption. The corresponding averaged Cu
Cu distances in the bare CuN clusters are also listed for comparison. The Cu
O distances are averages over the Cu
O bonds. Adsorption of O2 increases the lengths of Cu
Cu bonds adjacent to the adsorption site. For most clusters, this increase is the range 0.9 - 0.13 Å. For Cu2O2 the increase is very small, 0.02 Å and in Cu5O2, the increase is relatively large, 0.23 Å. These results correlate well with the adsorption energies, which are smallest (0.02 eV) in the former case, and largest (3.13 eV) in the latter. The Cu
O bond lengths do not appear to be correlated to adsorption energies. For example, the Cu
O distances are nearly the same (1.886 and 1.899 Å) for Cu2O2 and Cu5O2, respectively. As seen in Table 2, the Cu
O distances tend to be longer in the larger clusters in which the Cu atoms have higher coordination than in smaller clusters. Cu3O2 is an exception to this trend, however. The shortest O
O bond, just 0.05 Å longer than that in an isolated O2, is found in the conformation 2A. With the exception

0.00

a

All in units of eV. The labeling of the complexes corresponds to that in Figure 2. RE values are shown for PBE and B3LYP, as well as spinrestricted MP2 (ROMP2) calculations. b BE = {N  E(Cu) + E(O2)
E(CuNO2)}/N. c See text. d P and F denote the lowest energy conformations obtained in ref 78 and ref 79 respectively.

One can identify five distinct patterns of O2 adsorption on low energy isomers of CuN, N = 2
10 (cf., Figure 2). The first is attachment of one O atom to one Cu atom, as in the cases of 2A and 3C. The second is attachment of two O atoms to the same Cu atom that leads to formation of a Cu
O
O triangle, as in the cases of 2B and 3B. In the third pattern, the two O atoms are attached to different neighboring Cu atoms and form a Cu
Cu
O
O four-membered ring as, for example, in cases of 2C, 3A, and others. In the fourth pattern, one of the O atoms bonds with one Cu atom, whereas the other bonds with two different Cu atoms; examples are the cases of 4B, 6A, and others. In this type of O2 adsorption, three Cu
O bonds are formed. Finally, in the fifth pattern, each O atom attaches to two different Cu atoms,

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Figure 4. Amount of charge transfer (in units of electron charge) from CuN to O2 in the most stable CuNO2 complexes.

of the 5A case, the O
O bond lengths in the odd-N complexes have a fairly constant value of about 1.36 Å. In the even-N complexes they vary from 1.271 Å to 1.627 Å. One also notices an even
odd alternation in the O
O bond lengths for the cases of 5A to 10A, with the longer ones characteristic of even-N complexes and the shorter ones characteristic of odd-N complexes. As discussed further below, this even
odd alternation is opposite from that found in earlier studies. Florez et al.79 for N = 1
7 and Padilla-Campos78 for N = 1
8, found the O
O bond lengths to be longer for odd-N cases and shorter for even-N cases. Figure 3 shows the O2 adsorption energy for the most stable configurations of CuNO2 as a function of N. The adsorption energy is defined as Ead ¼ EðO2 Þ þ EðCuN Þ
EðCuN O2 Þ

Figure 5. Net electron spin density of the most stable open shell CuN-O2 complexes (cf., Figure 2, isovalue = 0.008 au).

ð1Þ

where E(X) is the total energy of system X. For N = 2 and N = 8, the adsorption energy is small, which is in agreement with the experimental finding1 that closed shell Cu clusters are less reactive. With the exception of Cu7, the energy of O2 adsorption on even-N CuN is smaller than that on neighboring odd-N clusters. Florez et al.79 found a similar even
odd alternation in O2 adsorption energy, but for the size range N = 2
5. The break in the trend at N = 7 indicated by our results may be a consequence of the fact that the energetically preferred structure of CuN changes from planar at N = 6 to 3D at N = 7. This suggests that changes in trends of adsorption energy versus cluster size may be an indicator of structural/shape changes. Charge transfer occurs when O2 adsorbs on copper clusters. Figure 4 shows the computed charge transfer in the lowest energy CuNO2, N = 2
10, complexes evaluated using the natural population analysis (NPA).93 Charge transfer occurs for all N from CuN to O2. As seen in the figure, it initially increases with N, and then shows an odd
even alternation in the size range N = 5
10. The amount of transferred charge is around 0.8 e for all the odd-N complexes, but varies for the even-N complexes. In the two triplet cases, N = 2 and N = 4, relatively less charge is transferred, about 0.5 and 0.8 e, respectively, while in the singlet cases relatively more charge is transferred, about 1.2 e for N = 6, and 1.4 e for N = 8 and 10. An increase in the amount of charge transfer correlates with an increasing length and decreasing vibrational frequency of the O
O bond. As shown in Table 2, the O
O bond lengths in the singlet structures 6A, 8A and 10A are 0.29, 0.33, and 0.42 Å longer than in the free O2; these values are to be compared with elongations of only 0.06 and 0.15 Å in the triplet 2A and 4A structures, and of about 0.15 Å in the odd-N complexes. Structures 6A, 8A, and 10A have O
O frequencies that lie between 500 and 700 cm
1, while the triplet 2A and 4A conformations, as well as all the odd-N complexes, have O
O

Figure 6. Energy profiles corresponding to reactants, reaction intermediates, transition states, and products of O2 adsorption on Cu4 and Cu7. The attachment of O2 by a single bond occurs with no barrier.

frequencies in the range 975
1275 cm
1 (Table 2). The calculated stretching frequency of a free O2 is 1544 cm
1. The suggestion of earlier studies78,79 that even-N CuN clusters should be less reactive with molecular oxygen is not supported by our results. Instead, we have found that both the odd-N and the even-N clusters, with the exception of Cu2O2 and Cu4O2, form very stable complexes with O2. The main reason for the difference between our results and the findings of earlier studies is the set of new lowest energy CuNO2 complexes, especially for the cases of even-N = 6, 8, and 10, identified in this study. Earlier treatments identified the triplet state as the preferred one for all the even-N CuNO2 complexes. Similar to the cases of 2A and 4A presented here, the binding in these triplets was found to be relatively weak and to involve relatively little charge transfer to O2.78 In contrast, our results predict the singlet as the energetically most preferred state for the complexes with N = 6, 8, and 10 copper atoms. These singlet structures are characterized by 8709

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Figure 7. Structures of Cu7 and Cu8 clusters with multiple adsorbed O2 molecules.

Figure 8. Total adsorption energy as a function of the number of adsorbed O2 molecules on the Cu7 and Cu8 clusters. The total adsorption energy is evaluated as Ead = E[CuN] + xE[O2]
E[CuN(O2)x].

multiple (n > 2) Cu
O bonds and a larger degree of charge transfer from CuN to O2. Both the odd-N complexes and the triplet 2A and 4A structures have the O2 molecule attached over a Cu
Cu bond of the cluster, which results in the formation of a four-membered Cu
Cu
O
O ring. The formation of multiple bonds between CuN and O2 apparently becomes possible for clusters that are large enough to provide more than two neighboring Cu binding sites in appropriate positions. Three Cu
O bonds first appear in the 4B complex of Cu4O2. This complex is a singlet, and its energy is only 0.02 eV higher than that of 4A. It is interesting that one does not find multibonding arrangements in the lowest energy conformations of odd-N complexes, although structure 7B, which possesses three Cu
O bonds, is nearly degenerate with 7A. An inspection of Figures 3 and 4 indicates that there is no simple connection between the trends in the charge transfer and the adsorption energy in the most stable conformations of CuNO2 complexes. Relatively greater charge transfer does not imply relatively stronger binding. For example, the greatest charge transfer occurs in Cu8O2 where the adsorption energy is relatively small. The adsorption energies are defined not only by the degree of stability of the CuNO2 complexes, but also the stability of the parent CuN clusters. The lower adsorption energies in even-N CuNO2 complexes result primarily from the higher stability of the corresponding parent CuN clusters, which is particularly pronounced in closed shell cases such as N = 2 and 8. The position of the HOMO and the HOMO
LUMO gaps of the CuN clusters appear to have little effect on O2 adsorption. The HOMO and LUMO levels, and the HOMO
LUMO gaps,

of CuN clusters are presented in Table 2S and Figure 1S. The position of the HOMO level is relatively constant across the CuN clusters. The LUMO shows slightly more variation, but the variation in the HOMO
LUMO gap shows no clear correspondence to the adsorption energies shown in Figure 3. The HOMO orbitals and their s- and d-partial contributions are shown in Table 3S. For most clusters, the HOMOs are mainly d-like and delocalize among all Cu atoms. The net electron spin densities of the open-shell CuNO2 complexes with N = 2, 3, 4, 5, 7, and 9 are shown in Figure 5. In all these cases, the spin density is localized mostly around the two oxygen atoms with only a small fraction distributed around the copper atoms that are closest to O2. In the context of cluster-based catalysis, energy barriers to adsorption can be as important as adsorption energies. To get a sense of the nature of energetic barriers to O2 adsorption on CuN, we have explored the formation of the complexes 4A and 7A, starting from their corresponding bare clusters and molecular O2. Because the NRLMOL code is not equipped to allow searches for transition states, we used the Turbomole program94 with PBE/ def2-TZVP95 for this purpose. In Figure 6 are displayed the energy profiles of the reactants, reaction intermediates, transitionstate structures, and products. A two-step formation mechanism is found energetically favorable for both complexes. First, O2 approaches a single atom of CuN, forming an intermediate with one Cu
O bond. No energy barrier is observed in this step. In the second step, the other O atom approaches an adjacent Cu atom and forms a transition state structure. A small energy barrier has to be overcome in both cases before the final adsorbed structure is obtained, with the barrier of 4A somewhat lower than that of 7A (0.02 and 0.08 eV, respectively). This is consistent with the fact that in 4A O2 remains in the triplet spin state and has more molecular character than in 7A. For both 4A and 7A, the barriers are much smaller than the energy difference between the reactants and the intermediates. As a result, the barriers could be easily overcome given the exothermicity of the first steps. Saturation coverage is another concept that is important in the context of cluster-based catalysis. The lowest-energy structures of Cu7 and Cu8 were selected to study the coverage dependence of O2 adsorption. For a given number of adsorbate molecules, x, a large number of candidate CuN-(O2)x (x = 2
7 or 8) structures were built by hand and then relaxed within PBE/def2-TZVP. Figure 7 presents the lowest-energy structures found for Cu7-(O2)x and Cu8-(O2)x. The structures of Cu7O2 and Cu8O2 are given in Figure 2. Two basic trends can be seen in the figures. First, for small values of x, the adsorbate molecules 8710

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The Journal of Physical Chemistry A tend to make multiple bonds to the cluster, while for larger x, they tend to make only a single bond to the cluster. Second, the adsorbate molecules tend to spread out as much as possible over the cluster surface. We note that molecular adsorption is found in all cases, even for large values of x. Figure 8 presents the total adsorption energy of CuN-(O2)x complexes versus x, the number of adsorbed O2 molecules. Ead rises approximately linearly up to x = 3 for Cu7 and up to x = 5 for Cu8 before leveling off in the case of Cu7 and decreasing for Cu8. The adsorption energy per adsorbate thus decreases after x = 3 and 5 for Cu7 and Cu8, respectively.

4. SUMMARY In this study we reexamined the energetically preferred structural arrangements and electronic states of CuNO2, N = 2
10, complexes. A number of new such arrangements, states, and interaction patterns is identified and characterized. These are obtained by considering the addition of O2 to different sites of not only the most stable configuration of CuN for each N, but also those of other low energy isomers of the clusters. Patterns of O2 adsorption on CuN as a function of cluster size are discussed. Trends in the O2 adsorption energy, charge transfer from CuN to O2, O
O bond lengths, and the CuN-O2 binding energy, all considered as a function of N, are analyzed. It is pointed out that adsorption characteristics of a cluster for small molecules (e.g., adsorption energy) may serve as indicators of a possible sizeinduced structural/shape transition. New singlet structures with multiple (n > 2) bonding are identified as the most stable conformations of CuNO2 for N = 6, 8, and 10. These conformations are characterized by a relatively large transfer of charge from CuN to the O2, which results in lengthening of the O
O bond and reduction of its vibrational frequency. Calculations on representative CuN-O2 complexes suggest that O2 adsorption on CuN is barrierless. Finally, studies of the dependence of the adsorption energy on O2 coverage were conducted for Cu7 and Cu8, yielding the result that the adsorbate energy per O2 molecule is roughly constant up to three adsorbates for Cu7 and five adsorbates for Cu8, after which it decreases. ’ ASSOCIATED CONTENT

bS

Supporting Information. Cartesian coordinates of the most stable conformations, and details of the HOMO, LUMO, and HOMO
LUMO gaps of CuN clusters. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (M.Y.); [email protected] (K.A.J.); [email protected] (J.J.).

’ ACKNOWLEDGMENT M.Y. was supported by NSFC (Grant No. 20873088), Project sponsored by SRF for ROCS (Grant No 20091341-11-10), and the Major State Basic Research Development Program of China (Grant No. 2011CB606200). A part of the computational work was carried out on the High-Performance Computers of Physics at Sichuan University. J.J. was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, U.S. Department of Energy, under Contract No.

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DE-AC02-06CH11357, and by the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. K.A.J. was supported by the U.S. Department of Energy under Award Number: DESC0001330.

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