Chemical Physics Letters 498 (2010) 120–124

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Chemical Physics Letters

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Theoretical study of CO oxidation on small gold cluster anions:Role of the carbonate adducts

Ling Lin a, Minh Tho Nguyen a,b,⇑a Department of Chemistry, Katholieke Universiteit Leuven, B-3001 Leuven, Belgiumb Institute for Computational Science and Technology of HoChiMinh City, Thu Duc, HoChiMinh City, Vietnam

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 May 2010In final form 17 August 2010Available online 20 August 2010

0009-2614/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.cplett.2010.08.030

⇑ Corresponding author at: Department of ChemLeuven, B-3001 Leuven, Belgium. Fax: +32 16 32 79 9

E-mail address: minh.nguyen@chem.kuleuven.be (

CO oxidation on Au�2n (n = 1 – 4) cluster anions are investigated using DFT methods. O2 prefers to bind on-top to a gold atom. Binding energies of O2 on Au�2n are in the range of 0.8–1.2 eV. CO molecule can theninsert into the O–O bond of Au2nO�2 to form carbonate species, that are the most stable Au2nCO�3 isomers,and thus expected to play an important role in CO oxidation. The negative charges of the gold carbonatesare mainly distributed on the CO3 moieties. The gold carbonate anions can yield CO2 by direct dissocia-tion or react with another CO to produce 2(CO2).

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Small gold nanoparticles and clusters have been found to beable to efficiently catalyze the carbon monoxide oxidation reaction,and this finding has gained much interest from both experimentaland theoretical chemists alike [1,2]. Two classical mechanismshave been proposed to interpret the catalytic effect of the goldmedium. The first which is known as the Langmuir–Hinshelwoodmechanism, involves an initial co-adsorption of CO and O2 on goldand a subsequent dissociation of O2. The second is the Eley–Ridealmechanism in which two units take part in the process: one CO orO2 unit is adsorbed on the cluster, whereas the other unit reacts di-rectly with the pre-adsorbed molecules in the gas phase [3–7].

The co-adsorption of CO and O2 on the gold anions Au�n (n = 2–20) has also been investigated. While it has been found that O2 ad-sorbs molecularly on gold, there was no evidence for a single O-atom adsorption. Perhaps more interestingly, O2 was found to beable to react only with gold anions with even number of goldatoms (with an exception for Au�16) [8,9]. This behavior was sug-gested to be linked with the ability of the Au�n anion to transferan electron to the O2 molecule. In some theoretical studies[10,11] on the co-adsorption of CO and O2 on small Au�n anions,the binding energies of O2 on even-number Au�n were found tobe much larger than those of the odd-number Au�n , which thusagrees with experiment. The reactivity of CO with gold anions isalso size-dependent, but there is no odd–even oscillation as inthe case of O2 [8].

CO oxidation on Au�2 has extensively been studied by bothexperiment and theory [12–16], and the Eley–Rideal mechanism

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istry, Katholieke Universiteit2.M.T. Nguyen).

was proposed for this reaction. Au-O2 complexes were detectedin different experiments [12,13,15,16] indicating that CO oxidationmechanism on small gold anions involves di-oxygen species ratherthan atomic oxygen. Some indications for formation of an interme-diate Au2CO�3 were derived from experiment, and two alternativestructures were predicted for this intermediate. Both structureswere characterized theoretically by low activation barriers fordecomposition yielding CO2 [13]. CO oxidation mediated by the tri-atomic species Auþ3 , Au3 and Au�3 has also been studied theoreti-cally [17]. Three pathways were explored, and it was predictedthat all triatomic species, regardless of the charge state, are ableto facilitate the CO oxidation but with different mechanisms. Thehighly stable Au-carbonate species were however suggested notto be the necessary intermediates [17]. A theoretical investigationon the CO oxidation on Au�6 [18] predicted that the reaction pro-ceeds through the Langmuir–Hinshelwood mechanism. Two struc-tures were proposed for the initial reactants, and both of them arepredicted to produce the first CO2 through a four-centered inter-mediate structure (CO–OO), and this was followed by an elimina-tion of the remaining O on the cluster by another CO. It wasconcluded that the CO oxidation was more facile when O2 wasbonded to the apex and CO to the nearest lateral site of Au�6 [18].Let us note that CO combustion on Au�6 was also previously pro-posed to follow a carbonate-like intermediate [8].

Studies on CO oxidation on the neutral and positively chargedAu9 and Au13 showed that CO have higher binding energy thanO2, irrespective of the charge and spin state of the gold clusters.Different pathways involving CO–OO four-centered and carbon-ate-like intermediates were considered, and the Eley–Rideal mech-anism was proposed [19–21].

Although a large number of studies have been reported on theCO oxidation on gold clusters, the mechanisms responsible forthe catalytic activities are not well understood yet. In this context,

L. Lin, M.T. Nguyen / Chemical Physics Letters 498 (2010) 120–124 121

we set out to carry out a theoretical study of the CO oxidation on aseries of small Au�n anions with even number of gold atoms. Themain purpose is to identify the relevant reaction pathways, andthereby to probe the possible formation and the role of the carbon-ate units along the catalytic processes.

Figure 1. Shape of the most stable Au2nO�2 (n = 1–4) complexes with the relativeenergies (eV) obtained with BP86/G (G: cc-pVDZ for C and O, cc-pVDZ-PP for Au).

Table 1Binding energies of O2 at the most favorable site of Au�2n (n = 1–4) (Eb), the distance ofO–O [d(O–O)], and the natural charge distributed on the adsorbed O2 [q(O2)] in theoptimal Au2nO�2 complexes.

Structures Eb d(O–O) q(O2)eV Å electron

Au2O�2 1.24 1.311 �0.54Au4O�2 1.07 1.302 �0.48Au6O�2 (A) 1.09 1.336 �0.57Au6O�2 (B) 1.06 1.307 �0.50Au8O�2 0.85 1.292 �0.41

2. Computational methods

All electronic structure calculations are carried out with the aidof the GAUSSIAN 03 program packages [22]. Geometry optimizationsand subsequent harmonic vibrational frequency calculations areperformed using density functional theory (DFT) with the pureBP86 functional [23,24]. The correlation-consistent cc-pVDZ-PP ba-sis set is employed for gold atom [25,26] (PP stands for an effectivecore potential replacing the core electrons), while for carbon andoxygen, the all electron cc-pVDZ basis set is used.

It is well known that the results obtained for transition metalclusters are strongly dependent on the functional employed. Thereason for our choice of the BP86 functional arises from our ownexperience with the assignment of the far-infrared spectra of theyttrium-doped gold clusters AunY (n = 1–9) by comparing the cal-culated vibrational spectra with experiment [27]. This studyshowed that the BP86 functional is appropriate for gold clusters.In the previous study, coupled-cluster CCSD(T) calculations werealso used for single-point energy calculations, and the energy dif-ferences vary but they are rather small. More importantly, the en-ergy ordering of the different isomers considered is mostly similarbetween the two sets of BP86 and CCSD(T) results, in particular forthe lower-lying isomers. This implies that the relative energies pre-dicted by BP86 for stationary points on a potential energy surfaceare acceptable.

The natural bond orbital (NBO) analysis is used for evaluatingthe charges transfer in the interactions, and they are computedby using NBO 5.G code [28] integrated into the GAUSSIAN 03 programsuite. To analyze the electronic distribution, we consider the elec-tron localization function (ELF) [29–32], and the isosurfaces areplotted with the graphical program gOpenMol [33,34].

Figure 2. Shape of the most stable Au2nCO�3 (n = 1–4) complexes.

3. Results and discussion

The lowest-lying structures of the small gold anions consideredAu�2n (n = 1–4) are taken from Ref. [35] and reoptimized using theBP86/G method (the basis set G includes the set cc-pVDZ for Cand O, and the set cc-pVDZ-PP for Au). We first consider the inter-action of gold anions with molecular oxygen. Different bindingsites are tested for O2 adsorption on the most stable structures ofAu�2n (n = 1–4), and the lower-lying Au2nO�2 complexes are shownin Figure 1 along with their relative energies.

Au2nO�2 (n = 1–4) complexes have been observed in the reactionof O2 with gold anions from experiments [8,13,14]. Our calcula-tions show that all of the Au2nO�2 (n = 1–4) adducts tend to preferlow spin state (doublet), and the corresponding quartet state arehigher in energy by �1.0 eV. For Au�2 , Au�4 , and Au�8 , O2 prefers toon-top binds to one gold atom, and the corresponding bindingenergies amount to 1.24, 1.07 and 0.85 eV, respectively (Table 1),which are much larger than the values obtained when O2 bindsto other sites. For Au�6 , the structure A formed by O2 bridgingtwo gold atoms is similarly favorable as the isomer B, and the cor-responding binding energies amount to 1.09 and 1.06 eV,respectively.

Our results obtained for the O2 binding energies agree well withthose reported in a previous theoretical study using PW91 func-tional [10]. Among the Au�2n (n = 1–4), O2 interacts most stronglywith the diatomic entity Au�2 whose binding energy amounts to1.24 eV (cf. Table 1). The experimental binding energy for O2

adsorption on Au�2 has been evaluated to be 1.01 ± 0.14 eV [36].The O–O stretching frequencies of Au2O�2 and Au4O�2 are predictedto be 1145 cm�1 (142 meV) and 1162 cm�1 (144 meV), respec-tively, and the latter agrees well with the experimental value of152 ± 10 meV [14].

In the most stable Au2nO�2 adducts, the O2 molecule departsfrom the Au plane resulting in non-symmetrical structures, exceptfor the structures Au2O�2 and Au6O�2 (A), which are planar. The O–Odistances and the net charges distributed on O2 in the Au2nO�2 com-plex are listed in Table 1. The O–O bond lengths are calculated inthe range of 1.292–1.336 Å, which are elongated as compared tothat of the isolated molecular oxygen in the gas phase (1.24 Å inthe ground triplet state). NBO analysis shows that the net chargesof O2 in the optimal Au2nO�2 complexes are in the range of �0.41 to�0.57 electron, implying that the Au�2n anion behaves as an elec-tron donor in their interaction with O2. The electrons are mainlytransferred to the p* orbital of O2 molecule, leading to the super-oxo-like species with an elongated O–O bond.

Figure 3. ELF isosurfaces of Au2nCO�3 (n = 1–4) (n = 0.70).

Figure 4. Potential energy surfaces of CO oxidation

122 L. Lin, M.T. Nguyen / Chemical Physics Letters 498 (2010) 120–124

Having examined the pre-adsorption of O2 on the Au�2n clusters,the adsorption of CO on different sites of the Au2nO�2 complex arenow studied. It is found that in all the sizes studied, the carbonatespecies (containing a CO3 unit) are consistently the lowest-energyisomer for Au2nCO�3 , and this concurs with experimental findings.As a matter of fact, the co-adsorption of O2 and CO on gold anionshave been observed in the experiment, and it has been found thatthe presence of adsorbed O2 may enhance the subsequent adsorp-tion of CO on the cluster, especially for Au�2 and Au�4 [8,13]. Let ustake Au2CO�3 as an example. Eight possible isomers, whose shapeare given in Figure S1 of the Supplementary Information (SI) are lo-cated, and the carbonate 2a formed by a CO insertion into the O–Obond of the most stable Au2O�2 turns out to be the lowest isomer inenergy. The isomer 2c with CO binding to the other gold atom ofthe same Au2O�2 adduct is found to be much higher in energy by2.74 eV. This result agrees well with the results previously re-ported [13] using a different computational method. The structuresof the lowest-lying carbonate structures Au2nCO�3 (n = 1–4) areshown in Figure 2, and several additional lower-lying isomers lo-cated are shown in Figure S1 (Supplementary Information). Sincethe carbonate adducts are thermodynamically more stable thanthe other Au2nCO�3 isomers, it is expected that they play a certainrole in the CO oxidation on small gold anions.

via a carbonate species on Au�2 (a) and Au�4 (b).

L. Lin, M.T. Nguyen / Chemical Physics Letters 498 (2010) 120–124 123

It should be noted that although an observation of anAu2CO3CO� adduct was not reported in a previous experimentalstudy [13], it does not mean such adduct does not exist. It mightbe omitted in the previous experimental assignment.

The following discussion will mainly focus on the gold carbon-ates. ELF and NBO analyses are now performed to probe their elec-tronic properties. The corresponding ELF isosurfaces displayed inFigure 3 show that for the Au2nCO�3 carbonates, there are localiza-tion domains around the nuclei of gold representing the cores. Forthe CO3 unit, except for the core basins, the valence basins betweenC and O atoms are responsible for the bonds. In addition, the va-lence basins around the O, which represents the lone pairs, appearto be involved in the interactions between the gold and carbonateunits. The net charges on the CO3 units of Au2nCO�3 are calculated tobe �0.97, �0.95, �0.92, and �0.91 electron, for n = 1, 2, 3, and 4,respectively, and this suggests that following insertion of CO intothe Au2nO�2 (n = 1–4) complexes, a significant amount of electronsare actually transferred from the gold cluster to the CO3 unit. Afterthe interaction, the gold units only carry slightly negative charges,which seem to increase with the increasing size of the clusters. Allof the negative charges on the CO3 units are distributed on the oxy-gen atoms (� �0.6 electron), while the carbon atoms are positivelycharged (� +0.97 electron). To probe further the reaction mecha-nism, portions of the potential energy surfaces describing the

Figure 5. Potential energy surfaces of CO oxidation via a carbonate species on Au�6(a) and Au�8 (b).

decomposition of Au2nCO�3 to produce CO2 are constructed, andthe results are shown in Figures 4 and 5.

As mentioned in the introduction, Au2CO�3 was detected in theCO oxidation process on the dimer Au�2 from experiment, and itwas confirmed to be a reaction intermediate [13], that is consistentwith our calculated results. For Au2CO�3 , two reaction channels arepredicted (Fig. 4a), namely (i) it can release one CO2 by direct dis-sociation and produce a highly reactive Au2O� (endothermic by1.27 eV), which will react readily with another CO molecule andthen produce CO2, or (ii) the carbonate can react with CO and pro-duce a Au2CO3ðCOÞ� complex, subsequently it will produce twoCO2 molecule via a transition structure with a small energy barrierof 0.1 eV. The latter barrier is much lower than the value predictedin a previous study [13] using a BO–LSD–MD method. It is worthnoting that the species Au2O�, Au2CO�2 , and Au2CO3ðCOÞ� werenot observed in the experimental studies [13].

Similar reaction channels can also be predicted for Au4CO�3 toproduce CO2. Corresponding geometries and energies of the rele-vant structures are shown in Figure 4b. Direct CO2 dissociationfrom Au4CO�3 is predicted to be endothermic by 1.28 eV, and theproduced Au4O� anion readily reacts with CO to release anotherCO2 molecule. The Au4CO�3 carbonate can also react with anotherCO molecule yielding a Au4CO3ðCOÞ� complex, which is expectedto produce two CO2 molecules via a transition structure, but theassociated energy barrier of only 0.03 eV is not meaningful. Noexperimental details about the intermediate in CO oxidation onAu�4 has been reported, except for the detection of Au4O�2 [14]and co-adsorption of O2 and CO on Au�4 [8].

Our calculations predict that both Au6CO�3 (Fig. 5a) and Au8CO�3(Fig. 5b) can produce CO2 by direct bond cleavage, and the reac-tions are endothermic by 1.19 and 1.05 eV, respectively. The pro-duced Au6O� and Au8O� fragments are highly reactive andreadily react with an additional CO releasing CO2. The gold carbon-ates Au6CO�3 and Au8CO�3 can also react with another CO producingtwo CO2 molecules. We have attempted to optimize the structuresof the complexes formed by Au6CO�3 and Au8CO�3 with CO, but theoptimizations invariably led to the gold cluster plus two CO2, andthis suggests that such complexes are not stable. Let us note thatthe species Au6CO�3 and Au6O� have been detected in experiments,and the anion Au�6 is found to have extremely high activity towardsCO oxidation [8], which is in line with our calculated results.

Overall, a novelty of this study is that we find very stableAu2nCO�3 (n = 1–4) carbonate species. In Ref. [17], it was concludedthat Au-carbonate species were not necessary as intermediates inCO oxidation based on a molecular dynamics simulation for Au�3 .In the present study, different sizes of gold anions [Au�2n (n = 1–4)] are considered, and therefore our results may differ from thoseobtained for other sizes. Let us note that in some previous studies[16], the Au-carbonate species were predicted to be important,which agree well with our results. In other words, the correspond-ing Eley–Rideal mechanism appears to be more likely.

4. Concluding remarks

In the present study, quantum chemical calculations using den-sity functional theory are carried out to study the CO oxidation onAu�2n (n = 1–4) clusters. The binding energies of O2 on the pure Au�2n

(n = 1–4) clusters are predicted to be in the range of 0.85 to1.24 eV, and it prefers to bind on-top to a gold atom, except forAu�6 , for which the O2 can bridge two gold atoms with similar bind-ing energy. Following the O2 pre-adsorption, the CO molecule caninsert into the O–O bond and form the gold carbonate species,which are found to be the most stable structures among all thepossible Au2nCO�3 isomers. In this context, they are expected toplay a role in the CO oxidation on Au�2n following thus an Eley–

124 L. Lin, M.T. Nguyen / Chemical Physics Letters 498 (2010) 120–124

Rideal mechanism. The gold carbonates can produce CO2 by eitherdirect bond cleavage or reaction with another CO producing twoCO2 molecules. During the oxidation process, a charge transfer oc-curs from the gold unit to the CO3 unit, and then back to gold whenCO2 is produced.

Acknowledgements

The authors are indebted to the Flemish Fund for Scientific Re-search (FWO-Vlaanderen), and the KULeuven Research Council(GOA and IDO research programs) for support. LL thanks INPACfor a postdoctoral fellowship. MTN thanks the ICST for supportinghis stays in Vietnam.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cplett.2010.08.030.

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