Isomerization in Gold Clusters upon O2 Adsorption - The Journal of

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Isomerization in Gold Clusters upon O2 Adsorption Published as part of The Journal of Physical Chemistry virtual special issue “Mark S. Gordon Festschrift”. Min Gao,†,‡ Daisuke Horita,† Yuriko Ono,† Andrey Lyalin,*,§ Satoshi Maeda,*,†,‡ and Tetsuya Taketsugu*,†,‡,§ †

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8245, Japan § Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Material Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ‡

ABSTRACT: A systematic investigation is performed on structural transformations in small neutral gold clusters (Au3−Au12) induced by O2 adsorption, with the use of the fully automated reaction path search techniques, i.e., anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods, implemented in the global reaction route mapping (GRRM) program. For each size of Au cluster, the most stable structure, low-energy isomers, and a network of isomerization pathways are determined. The located Aun− O2 adsorption forms can be classified into two groups: η1-AunO2, where only one oxygen atom is adsorbed on Aun, and η2-AunO2, where both oxygen atoms are adsorbed on Aun in a bridged manner. These two adsorption forms can be transformed to each other with a low barrier. The isomerization pathways of gold clusters upon O2 adsorption are compared with those obtained for the pure gold clusters without O2. It is demonstrated that O2 adsorption promotes structural transformations in gold clusters considerably by lowering the isomerization barriers. The presence of the unpaired electron is a necessary condition for O2 adsorption in the η1-AunO2 form, as well as the subsequent cluster isomerization and the O−O bond activation. These conclusions are consistent with the recent experimental results by Fielicke et al.

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INTRODUCTION Catalysis by nanoscaled particles is significant research topics in modern nanoscience. Gold is one of the most studied metals in nanocatalysis because of its unique catalytic properties emerging at nanoscale.1−3 Moreover, gold nanoparticles demonstrate extraordinary catalytic activity and selectivity even at room temperature, which is very attractive for industrial and chemical applications. The enormous interest in gold nanoparticles is stipulated by the fact that the catalytic activity of gold nanoparticles can be controlled and tuned by their size, structure, morphology, charge state, support materials, etc., see, e.g., refs 4 and 5 and references therein. Small gold clusters can possess various structures in a range of the one-dimensional nanowires, the two-dimensional nonosurfaces, the threedimensional nanostructures, and even the cagelike structures.6−9 Theoretical investigations on structure of gold clusters have been performed using various global optimization techniques and local geometry optimization methods.10−12 A lot of works have been devoted to catalytic processes involving gold clusters.13−16 It is noted that clusters of the same size but with different geometrical structure can possess different reactivity.16−18 The most explored type of catalytic reactions with the gold clusters is the oxidation reaction by molecular oxygen. In order © XXXX American Chemical Society

to understand the origin of the catalytic activity of gold clusters, several possible mechanisms have been proposed for oxidation reaction. The first one is a preliminary dissociation of O2 adsorbed on a gold cluster followed by the consequential oxidation of the reactant molecule by the atomic oxygen. The second is a direct oxidation reaction between the activated molecular oxygen adsorbed on the cluster surface and the reactant.19−25 No matter which route oxidation reaction follows, the adsorption and activation of O2 on gold clusters is always the key initial step. It has been found that O2 is adsorbed on gold clusters in two different forms: η1-AunO2 form with only one O atom bonded to the gold cluster surface,26,27 and η2-AunO2 form with O2 molecule bridging two gold atoms on cluster surface. Recently, Fielicke et al.26 observed these two adsorption forms of O2 on small oddnumber sized gold cluster on the basis of vibrational spectra of the O−O stretching mode. The presence of two vibrational fundamentals for Au7O2 suggests that there are one or more isomers of the cluster, either in the parent gold structure or in the adsorption structure of O2. Existence of such isomers has Received: September 30, 2016 Revised: January 12, 2017 Published: January 17, 2017 A

DOI: 10.1021/acs.jpcc.6b09919 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C also been observed previously for O2 adsorbed on anionic gold clusters.27 It is well-known that gold clusters may possess structural isomers which can be easily transformed to each other due to thermal effects or interaction with the adsorbates.28 Such effect is known as “dynamical fluxionality”,29 and isomer-transformations can occur at room temperature on a time scale that is typical for chemical reactions.28,30−36 Even for a very small gold cluster, a number of local minimum structures exist on the potential energy surface. As a result, the network of rearrangement reactions for gold clusters is extremely complicated and only fully automated methods for a systematic location of the rearrangement pathways among various isomers can be useful to unravel such networks. Despite of intensive theoretical and experimental studies, clear understanding of the process of structural rearrangements in metal clusters during chemical reactions is still lacking. The present work fills this gap by examining structural transformations induced in small gold clusters by O2 adsorption. Such calculations have recently become possible by the anharmonic downward distortion following (ADDF) method37−39 and the artificial force induced reaction (AFIR) method40,41 implemented in the GRRM program.42 In the present study, we explored the isomerization pathways between different isomers, including pathways between η1AunO2 and η2-AunO2, to reveal mechanism of catalytic reactions by gold-cluster. Recently, we demonstrated that ADDF and AFIR methods can be used as a promising tool for prediction of the reactivity of small gold clusters in the regime of structural fluxionally and effective search for the most favorable paths for single bond activation reactions.17 In the present work, we have extended applications of ADDF and AFIR methods to investigations of structural transformations in gold clusters (Au3−Au12) induced by adsorption of oxygen molecule. An exhaustive search for the possible structural transformations is performed between the most stable cluster structures and lowenergy isomers. The present strategy can identify the lowest transition state for isomerization pathways of gold clusters with the systematic procedure. It is demonstrated that O2 is adsorbed on gold clusters either in the η1-AunO2 or in the η2AunO2 form. These two adsorption forms can be transformed to each other with simultaneous isomerization of gold clusters with a low barrier.

Gaussian09.50 It is noted that the LC-DFT scheme gives reliable orbital energies for molecular systems.51 The energetics are discussed based on the free energy evaluated with T = 173 K, which was used in the experiment by Fielicke et al.26 The free-energy correction was estimated assuming ideal gas, rigidrotor, and harmonic oscillator models for translational, rotational, and vibrational motions, respectively, at each stationary point. The search for pathways of O2 adsorption on the sizeselected gold clusters was performed in the following steps: (1) Search for low-lying isomers of Aun clusters, and low-barrier pathways of their interconversion by the ADDF method; (2) Search for low-lying O2 adsorption structures on gold clusters, by pushing O2 to the most stable isomer of gold cluster obtained in step 1 by the multicomponent (MC)-AFIR method;17 (3) Search for all low-barrier pathways between the global minimum and the local minima of AunO2 structures by single-component (SC)-AFIR method,40 with all the geometries obtained in step 2 as initial structure and with an artificial force parameter, γ = 100 kJ/mol.

COMPUTATIONAL DETAILS Stable structures and their generation pathways for AunO2 were searched systematically by ADDF and AFIR methods implemented in the GRRM program with energy gradients calculated by the SIESTA program.43−45 The energy gradients were computed on the basis of density function theory (DFT) with PBEPBE functional.46 The double-ζ plus polarization basis sets were used to treat 2s22p4 and 6s15d10 valence electrons of O and Au atoms, respectively.47 The core electrons for Au atoms are represented by the Troullier−Martins normconserving pseudopotentials.48 An energy cutoff of 100 Ry was chosen to guarantee the convergence of energies and gradients. A common energy shift of 50 meV was employed. All the transition states were verified to have only one imaginary frequency, and the connectivity with reactant and product has been confirmed by intrinsic reaction coordinate calculations,49 using GRRM. The orbital correlation diagram for Aun + O2 → AunO2 was analyzed by the spin-restricted open-shell Kohn− Sham method with long-range correction (LC)-wPBE functional for the geometry calculated at the level of PBEPBE by

Figure 1. Most stable geometries for gold clusters Aun (n = 3−12).

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RESULTS AND DISCUSSION Following the step (1) described in a previous section, an ensemble of isomers of small gold clusters Aun (n = 3−12) and the isomerization pathways between different isomers have been obtained by the ADDF method. We previously reported structures of gold-cluster isomers in the low-energy region obtained by the same computational scheme.17 It is well-known that gold clusters possess many isomer structures, and the number of isomers grows with increase in a cluster size. The structural and chemical properties of small gold clusters have been studied extensively.52−61 Figure 1 shows the most stable

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structure for each-sized Aun. The most stable isomer has a planar geometry in accordance with the previous theoretical6,62,63 and experimental studies.52 The differences in electronic energy between the most stable structure and the second (third) low-lying isomers for Au8 is 16.4 (17.1) kJ/mol that is consistent with B3LYP results7,61 and very recent CCSD(T) results (19.2 (18.4) kJ/mol).58−60,64 The present PBE calculations well reproduce the structures obtained in both experimental 52 and high-level ab initio computational works.60,63 A spin multiplicity was verified to be doublet for odd-number sized gold clusters, and singlet for even-number sized ones. Geometrical structures of the obtained isomers for Aun were reported in our previous paper.17,18 An oxygen molecule is expected to be adsorbed on various sites of gold clusters. The number of possible adsorption sites for O2 should increase with increasing in cluster size, n. Following step 2, the most stable gold-cluster isomers obtained in step 1 are used to a systematic search of structures of O2 B


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The Journal of Physical Chemistry C

Figure 2. Most stable O2-adsorption geometries in η1-AunO2 form and η2-AunO2 form for (a) odd-number sized and (b) even-number sized gold clusters. Below each geometry are shown the O2-adsorption energy, calculated from a difference of the free energy at 173 K (EAun−O2 − EAun − EO2). The numbers in parentheses are the relative free energies of Aun in η2-AunO2 form at 173 K, compared to the most stable isomer (this number is omitted when Aun structure in η2-AunO2 form is the most stable one). The free energy is given in kJ/mol.

Figure 3. O−O bond length, RO−O, in O2 adsorbed in η1-AunO2 (black line) and η2-AunO2 (red line) forms on gold clusters with odd (left) and even (right) number of Au atoms.

adsorption on Aun by the MC-AFIR method.18 For each goldcluster isomer, O2 was randomly placed in the vicinity of clusters in a number of nonequivalent positions and orientations, and then AunO2 adsorption structures were searched by using SC-AFIR method. During the search by SC-AFIR method, not only pathways for changes in the adsorption site of O2 but also those involving rearrangements in the Aun part were explored systematically. With this, the best path leading to the most stable Aun−O2 complex starting from the initial complex in which O2 is adsorbed to the most stable Aun, was identified. It should be noted that pathways leading to structures in which O−O bond is cleaved are excluded in the present search with the small γ value. The most stable structures for O2 adsorption on Aun in η1AunO2 form and η2-AunO2 form are shown in Figure 2. The number below each geometry indicates the O2-adsorption energy which is defined as EAun−O2 − EAun − EO2 where EA

denotes the gas-phase free energy of molecule A at 173 K. As shown here, O2 prefers to be adsorbed on the edges of 2D cluster’s structures, while the terrace of 2D is usually inactive.65 Our calculations demonstrate that O2 can be adsorbed on gold clusters either in η1-AunO2 or η2-AunO2 form. In η1-AunO2 adsorption, O2 prefers to be adsorbed on the most stable cluster structure, while in η2-AunO2 adsorption, O2 prefers to be adsorbed on the energetically higher isomers of the cluster in some cases (n = 3, 7, 8, 9, 11, 12). In even-number sized gold clusters, the O2-adsorption energy is small in most cases, and only the η2-Au4O2 complex shows a relatively large adsorption energy (−21.6 kJ/mol), which is consistent with the experimental observation of Au4O2. Taking into account this result, we hereafter focus on gold clusters with odd number, Au3, Au5, Au7, Au9, and Au11, if not specified otherwise. In Aun−O2, the interaction between Aun and O2 should influence the bonding of O2. Figure 3 shows variations of the C


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The Journal of Physical Chemistry C O−O interatomic distance (RO−O) in η1-AunO2 and η2-AunO2 complexes. In η1-AunO2 form, RO−O of the odd-number sized cluster was enlarged to 1.28−1.29 Å (1.22 Å for free O2), and the O−O stretching frequency decreased to 1260−1303 cm−1 (1509 cm−1 for free O2) in accordance with the experimental data.26 The red-shift in O−O stretching frequency in η1-AunO2 complex indicates the weakening of O−O bond due to the interaction with gold cluster. The adsorption energy of O2 and gold clusters was ranged from −27.7 to −34.0 kJ/mol at the temperature 173 K that was used in the experiments by Fielicke et al.26 In η2-AunO2 form, O2 is adsorbed on less stable goldcluster isomers except for the case of Au5. Thus, O2 adsorption on neutral gold clusters results in a considerable rearrangement of their structures. The O−O bond length was calculated to be 1.32−1.36 Å, which is similar to the one of the superoxide state (1.38 Å for free O2−). The ability of small odd-number sized gold clusters to adsorb and activate molecular oxygen is well-known and has been explained by the transfer of unpaired valence electron in the highest occupied molecular orbital (HOMO) of gold cluster to antibonding 2π* orbital of O2 which weakens the O−O bond and activates oxygen molecule in the following catalytic reactions, as described in previous works.27,66 We calculated Bader charges localized on O2 in the η1-AunO2 and η2-AunO2 complexes. The charge of O2 in η2-AunO2 complexes is about 0.4 e, which is approximately twice as large as the one in η1AuO2. The gold cluster structures in η2-AunO2 complexes are similar to structures of the corresponding free cationic gold clusters.18,67 This result indicates that the structure of gold clusters highly depends on their charge state, which can change during reactions. The O2-adsorption energy on Aun in η2-AuO2 is ranged from −43.6 to −75.0 kJ/mol, which is approximately twice of those in η1-AunO2 complexes. The O−O stretching frequency in η2-AuO2 complexes was calculated to be 1043− 1156 cm−1. These results indicate that the adsorbed O2 molecule in η2-AunO2 complexes is activated stronger than in η1-AunO2 complexes. Here we discuss the difference of Aun−O 2 binding mechanism in η1-AunO2 and η2-AunO2 on the basis of the molecular orbital correlation diagram. The diagram for n = 3 case is shown in Figure 4. In the case of η1-Au3O2, the singly occupied molecule orbital (SOMO) of gold cluster makes the bonding orbital with 2π* (in-plane) orbital of O2. The resultant bonding orbital (HOMO−1) is dominated by 2π* (in-plane) of O2, indicating a slight electron transfer from Aun to O2. The SOMO of η1-AunO2 comes from 2π* (out-of-plane) orbital of O2. On the other hand, in the case of η2-Au3O2, the situation is largely different. As shown in Figure 4, there is no interaction between SOMO of Au3 and 2π* of O2. The orbital interaction between Au3 and O2 occurs mainly between HOMO−8 of Au3 and 2π* (in-plane) of O2, resulting in the bonding orbital (HOMO−12) and antibonding orbital (HOMO−1). Three electrons from Au3 and one electron from O2 occupy these two orbitals, while 2π* (out-of-plane) orbital of O2 remains SOMO in the η2-Au3O2 complex, indicating one electron transfer from Au3 cluster to O2. This electron transfer activates the O−O bond more strongly than in the case of η1-Au3O2. This orbitalcorrelation picture is general for the interaction of odd-number sized gold cluster and O2, which provides us insights to the understanding of O2-activation mechanism by gold cluster. Isomerization of AunO2. Gold clusters possess a number of the energetically close-lying isomers, and some catalytically active isomers become thermodynamically available. At the

Figure 4. Molecular orbital correlation diagram for (a) η1-Au3O2 and (b) η2-Au3O2 calculated by the LC-wPBE method for the geometry determined by PBEPBE.

finite temperature, O2 can be adsorbed either on the most stable gold cluster or the other cluster isomers, and promote consequent isomerization reaction of gold clusters. Following the step (3) described in Computational Details, the SC-AFIR method was applied to all the isomers of AunO2 obtained in step 2, to get the isomerization pathways for AunO2. In order to understand the structural rearrangement pathways of gold clusters induced by O2 adsorption, we compare the isomerization pathways of gold cluster with and without O2. Figure 5 shows the most favorable isomerization pathways of Aun and AunO2 (n = 3, 5, 7, 9). There are two processes that can lead to a formation of η2-AunO2 structures. The first one is the initial isomerization of the gold cluster followed by O2 adsorption in η2-AunO2 form, while the second one is barrierless adsorption of O2 in η1-AunO2 form on the most stable clusters, followed by isomerization to the η2-AunO2 form. In the case of Au3 (Figure 5a) the isomerization of bare gold cluster occurs with an energy barrier of 16.6 kJ/mol. On the other hand, with the existence of O2 molecule, gold cluster can be highly stabilized. The geometry of Au3 in η1-Au3O2 first changes to the more compact triangle structure via one transition state, TS0−1, with almost no barrier, followed by a structural transformation to the more stable bridged-form η2Au3O2 through the second TS1−2. Only in the case of Au5 (Figure 5b), gold cluster does not change its structure in η1 and η2 forms upon O2 adsorption. Both η1-Au5O2 and η1-Au7O2 structures transform to the corresponding η2 forms via one TS. The only difference is that TS of Au7O2 includes both an isomerization of Au7 and the change in adsorption-form of O2 (Figure 5c). The energy barrier for structural transformation of gold cluster is almost the same for the bare cluster of Au7 and the O2 adsorption cluster, Au7O2. For Au9 and Au11 (Figure 5d,e), the isomerization process becomes more complicated. In both cases, the energy barrier for gold cluster structural transformations becomes low by O2 adsorption. The isomerization of gold cluster and the change in O2 adsorption-form occur separately along the reaction pathways from η1-AunO2 to η2-AunO2. In the case of η2-Au11O2, the parent gold cluster D


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Figure 5. Free energy profiles along the most favorable isomerization pathways leading to the most stable Aun−O2 complex starting from the initial complex in which O2 is adsorbed to the most stable Aun in the η1-form (solid line) and the related pathways of bare gold clusters (dashed line): (a) n = 3, (b) 5, (c) 7, (d) 9, and (e) 11.

clusters rather than the charge state of clusters. This conclusion is in accordance with the recent experimental findings.67 O−O Bond Cleavage via Collision of Gold Clusters. In oxidation reactions by molecular oxygen, the rate-determining step usually corresponds to the O−O bond cleavage of the activated O2 due to interaction with the reactant molecule. Such O−O bond cleavage does not occur on the small free neutral even-sized gold clusters. Moreover, O2 adsorption on the even-number sized ones is not so stable. Since O2 can be adsorbed on the odd-number sized gold cluster and becomes activated, it is interesting to examine whether two odd-number sized gold clusters can promote O2 bond cleavage or not. One can assume that if clusters in the beam can collide with each other, they can fuse and form larger species. Such an event can rarely occur under the experimental conditions described in ref 67; the probability for such collision and fusion in the experiment may be not that high in the gas phase due to the small concentration of gold clusters. However, with changing the experimental condition, for example, with the existence of additional surface, the probability of collision with the gold clusters will increase. Such effect of coalescence is well-known for the supported clusters, when clusters diffuse on the surface and are merged. We have already discussed that O2 is adsorbed strongly on odd-number sized Aun cluster forming η2-AunO2 complex. The direct O−O bond cleavage on Aun is not favorable energetically, but it can be promoted by coadsorption of the reactant molecule such as ethylene36 and water.72 To examine the O−O bond cleavage process by a collision of Au clusters, we employed the MC-AFIR method for the collision of the stable η2-Au5O2 complex and Aun (n = 3, 5, 7). The most favorable energy profiles for the corresponding reaction pathways leading to O−O bond cleavage on Au8, Au10, and Au12 clusters are shown in Figure 7. In these figures, the energy profiles are given relative to a sum of energies of free Au5, O2, and Aun (n = 3, 5, 7). The barriers for O2 bond cleavage in fusion of Au5 + Au3, Au5 + Au5, and Au5 + Au7 clusters were calculated to be 123.8, 30.0, and 26.7 kJ/mol, respectively. These values are much lower than the corresponding barriers of O2 dissociation on single Au8, Au10,

possesses considerable structural rearrangement as a result of O2 adsorption. Such mechanism of oxygen binding highlights the significance of the structural flexibility of gold clusters for oxygen activation. It is usually accepted that in order to activate O2 molecule, negatively charged gold cluster is more appropriate. For example, Au8 adsorbed on the MgO surface with oxygen vacancy demonstrates a high catalytic activity for the lowtemperature oxidation of CO, partly due to the electron transfer from the defective MgO surface.68 However, it has been demonstrated that positively charge gold clusters can also be active for CO oxidation reaction.7,19,69,70 In order to clarify the origin of the catalytic activity of odd-number sized gold clusters, we investigate adsorption and activation of O2 on the cationic and anionic Au8. The geometry of charged gold cluster has been reported in our previous work.18 Several isomers of cationic Au8 exist with very small energy difference. The most stable structure of cationic Au8 is the one with Cs symmetry at room temperature, which is consistent with the experimental result.71 For the case of anionic Au8, the most stable isomer is similar to the neutral one of D4h symmetry18 which is also consistent with the experimental result.71 Interestingly, O2 can be activated on both negatively and positively charged Au8 as shown in Figure 6. It is demonstrated that O2 can be adsorbed on Au8+ and Au8− in the η2-Au8O2 form with the O−O bond enlarged up to 1.35 Å. Geometry of Au8+ is slightly changed after adsorption of O2. However, in the case of Au8−, the cluster changes back to the one similar to the neutral cluster due to the electron loss by O2. Therefore, O2 adsorption and activation is influenced by the presence of the unpaired electron in gold

Figure 6. Most stable geometries for O2 molecule on cationic and anionic Au8. The bond length for O−O is given in Å. E


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adsorption promotes structural transformations in gold clusters considerably by lowering isomerization barriers. The presence of an unpaired electron rather than the charge state of the cluster is a necessary condition for O2 adsorption in the η1AunO2 form, subsequent cluster isomerization, and activation of the O−O bond in accordance with the recent experimental findings.67 Finally, we demonstrated that O2 bond cleavage on small neutral gold clusters can occur as a result of cluster collision and fusion processes, resulting atomic oxygen on the even-number sized gold clusters.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.L.). *E-mail: [email protected] (S.M.). *E-mail: [email protected] (T.T.). ORCID

Andrey Lyalin: 0000-0001-6589-0006 Tetsuya Taketsugu: 0000-0002-1337-6694 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partly supported by a grant from JSPS KAKENHI with Grant Numbers JP15K05387 and JP26288001, a grant from the FLAGSHIP2020 program supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, within the priority study 5 (Development of New Fundamental Technologies for High-Efficiency Energy Creation, Conversion/Storage and Use), and partly performed under the management of the Elements Strategy Initiative for Catalysts and Batteries (ESICB), supported by the MEXT program Elements Strategy Initiative to Form Core Research Center (since 2012). The present work is also partly supported by the MEXT program Development of Environmental Technology using Nanotechnology. The computations were partly performed at the Research Center for Computational Science, Okazaki, Japan.

Figure 7. Energy profiles along the reaction pathways for a bond cleavage of O2 on even-number sized Aun in red line and on the two odd-number sized clusters Au5 and Aun+5 in black line ((a) n = 8, (b) 10, and (c) 12).

and Au12 clusters, i.e., 300.2, 244.9, and 209.0 kJ/mol, respectively. The energy difference between dissociated O2 adsorption on gold clusters and the formation energy of gold cluster are also compared. Although the energy of dissociated O2 is not so stable, the intermediate of dissociated O2 can reach the bottom of a local funnel of the O−Aun−O type with very small barrier which is more stable than Aun. The present results demonstrate that O−O bond cleavage can occur during collision and fusion of two odd-number sized gold clusters, resulting in atomic oxygen adsorption on the small neutral even-number sized gold clusters. It provides principally novel mechanism of O2 dissociation on gold clusters, leading to a new interesting effect of O2 dissociation induced by collision.

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CONCLUSIONS In this paper, we performed a systematic investigation on isomerization in neutral gold clusters (Au3−Au12) induced by O2 adsorption. A full network of structural transformation pathways between the large numbers of low-lying energy isomers is obtained by ADDF and SC-AFIR methods in GRRM program. It is demonstrated that the order of energies of isomers in gold clusters can be easily changed by interaction with adsorbate, and the most stable form of gold cluster with adsorbed O2 does not always correspond to the most stable structure of the parent gold cluster. It is shown that O2 is adsorbed on the gold clusters either in η1-AunO2 or η2-AunO2 form. These two adsorption forms can be transformed to each other with a very low barrier. It is demonstrated that O2 F


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Isomerization in Gold Clusters upon O2 Adsorption - The Journal of

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