Understanding the Stability and Electronic and AdsorptionProperties of Subnanometer Group XI Monometallic and BimetallicCatalystsNatalie Austin and Giannis Mpourmpakis*

Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15621, United States

ABSTRACT: Density functional theory (DFT) was used to calculate thephysicochemical and (CO and O2) adsorption properties of six-atom Ag, Au, andCu clusters and their bimetallic combinations. The physicochemical propertiesinclude the cohesive energy (binding energy per atom), electron affinity (EA),ionization potential (IP), d-band center (dc), charge transfer, and frontier orbitallocalization on the clusters. Our results demonstrate that the cohesive energy ofAu−Cu bimetallic catalysts decreases with increasing Au composition, whereas forAu−Ag bimetallic clusters, there was no variation with composition. Regarding theadsorption properties, we found that CO, acting as an electron donor molecule,preferentially interacts with low coordinated sites of the monometallic clusters thatlocalize the LUMO orbitals. The CO adsorption on the bimetallic clusters isaffected by the charge that is transferred between the different metals. O2, acting asan electron acceptor molecule, preferentially interacts with clusters that exhibit dcvalues closer to the LUMO level of O2. We revealed a linear trend between theaverage O2 binding energy on the different sites of the clusters and the dc of the clusters. This trend explains the high oxophilicityof Cu compared to the other clusters. Additionally, we identified a cooperative adsorption of CO and O2, with preadsorbed COenhancing the adsorption of O2. This effect becomes significant in the case of the Cu6 cluster, where O2 adsorption in thepresence of CO results to the formation of a partially oxidized cluster. The oxidation of the cluster is supported by charge analysisshowing electron density loss from the Cu6 cluster to O2. Overall, this work highlights that unravelling the adsorption behavioron bimetallic catalysts becomes very challenging at the subnanometer scale.

■ INTRODUCTION

The control of metal−adsorbate interactions is key to thedesign of catalysts with optimal performance.1−4 Nanosizedcatalysts designed for the CO oxidation reaction are beneficialfor controlling emissions in the environment as well as forpurifying hydrogen in the steam re-forming of hydrocarbons. Inaddition, the fundamental understanding of simple oxidationreactions, such as the CO to CO2 reaction, can serve as a probeto understand mechanisms in more complex oxidationprocesses on nanomaterials.Gold (Au) catalysts have been proven to be chemically inert

in bulk; however, they become exceptionally active at thenanoscale,5 thus highlighting the importance of nanoparticlesize on catalytic activity. Haruta was the first to show that Aunanoparticles 2−5 nm in diameter were exceptionally reactive,6

especially in low-temperature oxidation.7,8 The CO oxidationactivity of small Au nanoparticles is primarily attributed to theirlow-coordinated sites (which are mainly located at the edge andcorner sites of the nanoparticles).9−11 Theoretical studies haveshown that the binding energy (BE) of the adsorbates increaseswith decreasing surface coordination number (CN) of Au.12,13

As a result, the emerging consensus is that the CO oxidationactivity on Au increases with decreasing nanoparticle size.13

However, understanding the catalytic behavior of Au at thesubnanometer scale becomes even more challenging, since

support, electronic, and stability effects on the clusters cancontribute to a “magic number” activity, with specific clustersizes being active and others inert.14,15 As an example of thiscomplexity, it has been proposed that the reactivity of small Auclusters is controlled by the shape of the HOMO (highestoccupied molecular orbital)−LUMO (lowest unoccupiedmolecular orbital) orbitals16 as well as by their directionality.12

Bimetallic catalysts often exhibit better catalytic activity thantheir monometallic counterparts.3,17−21 Particularly, there is anincreasing interest in Au-based nanoparticles such as AuAg andAuCu which have shown high activity for CO oxidationcompared to monometallic gold.22−24 Currently, a fundamentalunderstanding of the catalytic behavior of these bimetallics islacking.18 Understanding the bimetallic properties at thesubnanoscale increases in complexity compared to themonometallic because, in addition to the determination ofproperties as a function of nanoparticle size and shape,composition will also be an important factor. The growinginterest in the study of bimetallics can potentially reveal uniquephysicochemical characteristics that can be fine-tuned withmetal composition to produce catalysts with optimal activity.18

Received: April 24, 2014Revised: July 19, 2014Published: July 21, 2014

Article

pubs.acs.org/JPCC

© 2014 American Chemical Society 18521 dx.doi.org/10.1021/jp504015a | J. Phys. Chem. C 2014, 118, 18521−18528

Understanding the chemisorption behavior of CO and O2 onAg, Au, and Cu clusters is of high importance to control theircatalytic properties. Theoretical studies have shown that CO,acting as an electron donor molecule, would preferentially bindthe site of the clusters where the LUMO orbitals are localized.16

Previous research has shown that CO preferentially binds to thecorners than the edges of Au6 clusters due to the lowercoordination of the corners (CN = 2) compared to the edges(CN = 4) and the localization of the LUMO orbital on thecorners of the cluster.11 In addition, CO binds much strongerthan O2 on neutral and positively charged Au6 clusters, which asa result leads to poisoning by CO and deactivation of thecatalyst. However, O2 strongly adsorbs on negatively chargedAu clusters, and transient activity is observed.15 Experimentalresearch has shown that on small Au clusters CO and O2adsorb cooperatively.25,26 Even though a lot of research efforthas been focused on the CO and O2 adsorption on Au, theadsorption behavior on Au-based bimetallic clusters is still notwell understood.In this work we used density functional theory (DFT)

calculations to investigate the stability of M6 (M = Ag, Au, Cu)clusters and their bimetallic combinations. In addition, weinvestigated the CO and O2 adsorption behavior as a functionof the catalyst’s structural11 and electronic16 characteristics. Atthis subnanometer scale, research has revealed that the catalyticactivity is cluster size specific (e.g., Au8 is more active clusterthan Au6).

15 As a result, understanding the physicochemicalproperties of small size clusters can provide guidelines for theirpotential use as effective catalysts.11,27

■ COMPUTATIONAL METHODS

We used the B3LYP hybrid functional combined with theLANL2DZ basis set as implemented in the Gaussian 09program package28 to investigate the structural, electronic, andadsorption properties of six-atom metal clusters. These M6clusters, with M = Ag, Au, Cu (<1 nm diameter) and theirbimetallic combinations (39 clusters in total under inves-tigation) have a planar, D3h geometry. This planar geometry isthe most stable for the six-atom monometallic clusters of Cu,Ag, and Au.27,29−34 As a result of this highly stable geometry ofthe monometallics, we selected the planar structure toinvestigate the effect of metal composition variation on thephysicochemical and adsorption properties of the clusters. Thecombination of the method and basis set has been successfullyapplied to investigate adsorption on bimetallic group XI

clusters3 as well as the CO oxidation reaction on small Auclusters.15,35 All the clusters and the adsorbates were fullyrelaxed without any symmetry constraints, and the obtainedstructures were further verified as minima with frequencycalculations. Different spin states have been considered in ourcalculations. Equation 1 was used to calculate the cohesiveenergy (binding energy (BE) per atom) of the clusters:

= − −E xE yEBE/6 ( (A B ) (A) (B))/6x y (1)

where x + y = 6 and E is the total electronic energy of theclusters with composition A, B (this can be any combination ofAu, Ag, and Cu). The cohesive energy is used as a descriptor ofthe average metal bond strength on the clusters. This propertyis used to understand the relative stability between the clustersof different compositions. We accounted for singlet, triplet, andquintet spin states of the neutral clusters, and we found that thesinglet was always the energetically most preferred state.Equations 2 and 3 were used to calculate the electron affinity(EA) and the ionization potential (IP) of the clusters,respectively:

= −E EEA (negative cluster) (neutral cluster) (2)

= −E EIP (positive cluster) (neutral cluster) (3)

The neutral clusters are of singlet spin state, whereas thenegative and positive clusters of doublet. Equation 4 was usedto calculate the binding energies of the adsorbates on theclusters, where the adsorbates are the CO and O2 molecules:

= − −−E E EBE cluster adsorbate cluster adsorbate (4)

All different systems had singlet spin states except for the onesthat O2 was present. In the presence of O2 the spin of thesystem was always triplet (O2 adsorption and coadsorption onthe clusters and gas phase molecular O2).All the clusters have two equivalent adsorption sites (see

Figure 1) exhibiting coordination numbers of two and four:corners (CN = 2) and edges (CN = 4). Natural bond orbital(NBO) analysis was used to calculate the charge distribution onthe clusters. The molecular orbital analysis was performed withthe use of the Avogadro 2.0 program package.36

■ RESULTS AND DISCUSSIONFigure 1 illustrates the cohesive energy (BE/n) of the clustersas a function of Au composition. The Cu6 cluster was found tobe the most stable (most negative cohesive energy value of

Figure 1. Cohesive energy of M6 clusters (M = Cu, Ag, Au) as a function of Au composition (AuCu: blue; AuAg: red; dashed lines serve as a guideto the eye).

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−84.35 kcal/mol) while the Au6 and Ag6 clusters were the leaststable (−34.68 and −27.64 kcal/mol, respectively). As can beobserved by Figure 1, the BE/n of the bimetallic clusters is alinear combination of the BE/n of the monometallic clusters.For instance, the BE/n of the AuCu clusters becomes lessnegative as the Au composition of the cluster increases. Inaddition, the BE/n for the AuAg clusters did not varysignificantly with Au composition. These trends are the resultof the large difference in the BE/n of Au6 and Cu6monometallic clusters and the comparable BE/n between Au6and Ag6 clusters (Au: −34.68 kcal/mol; Ag: −27.64 kcal/molcompared to −84.35 kcal/mol for Cu). The BE/n of Cu6cluster is almost 3 times larger than that of Ag6 and Au6.Additionally, the overall cohesive energy trends of themonometallic clusters (|BE/n|: Cu6 > Au6 > Ag6) do notfollow the experimental cohesive energies (CE) of the bulkmetals (CEAu = 87.9, CEAg = 68.0, CECu = 80.5 kcal/mol).37 Atthis subnanoscale cluster size, the cohesive energy of Cu issignificantly enhanced compared to that of Ag and Au clusters.Figure 2 shows the EA of both the monometallic and

bimetallic clusters as a function of Au composition. It was

determined that Cu3Au3 (Cu on the corners and Au on theedges as shown in Figure 1) had the highest (most negative)EA (−79.91 kcal/mol). The reason we observed this highelectron affinity was that the negatively charged Cu3Au3 clusterrestructured to the more energetically stable Au3Cu3 structure(Au atoms on the corners). In the latter structure, the negativecharge is located at the Au (corner) atoms (Au moreelectronegative than Cu) reducing the overall electrostaticrepulsions on the cluster. This is the only case we observedrestructuring on the negatively charged clusters. According toFigure 2, unlike with the cluster BE/n, EA is not a linearcombination of the EA of the monometallic clusters. The EA ofAuCu is larger (more negative) than that of the AuAgbimetallic clusters in compositions of Au ≥ 50%, and itbecomes larger for AuAg bimetallic clusters of Au compositionssmaller than 50%. EA is an important property of the clustersince it defines its ability to accept electrons from the metaloxide supports.15,17 The charging of the clusters can in turnaffect the adsorption energies of the reactants and the catalyticbehavior of the cluster.15,35 We have also calculated the IP ofthe clusters, as presented in Figure 3, as a function of Aucomposition. Similar to the EA, the IP is an essential clusterproperty because it determines the cluster’s ability to donateelectrons from interactions with materials (e.g., oxides) that areused as supports in heterogeneous catalysis. The Ag6 (154.79kcal/mol) and Cu6 (157.61 kcal/mol) clusters have a lower IPthan the Au6 (194.99 kcal/mol) cluster. This trend occursbecause Au is the most electronegative metal in group XI of the

periodic table, and it does not tend to lose electrons easily. Thisis also depicted in Figure 2 where Au has lower (more negative)electron affinity values than Cu and Ag. The IPs shown inFigure 3 did not appear to exhibit any trend associated with Aucomposition. It should be noted that we observed a transitionfrom planar to three-dimensional structures in the positivelycharged clusters.Since Figure 1 shows that mixing Au with Cu increases the

stability of the cluster (resulting to a more negative cohesiveenergy), we used this trend to analyze CO adsorption on themonometallic clusters and on AuCu bimetallics with 50% Aucomposition. At this specific composition, eight differentstructures exist. We selected the clusters with the highest andlowest cohesive energies: AuCu bimetallic clusters with BE/nvalues of −65.90 and −59.86 kcal/mol associated with theclusters Au3Cu3 and Cu3Au3, respectively (total energydifference between the clusters is ∼36 kcal/mol). TheAu3Cu3 cluster has the Au atoms on the corners and the Cuatoms on the edges, whereas the Cu3Au3 cluster has the Cuatoms on the corners and Au on the edges.Table 1 shows our calculated BEs for CO adsorption on the

corner and edges of the clusters. These CO adsorption energies

are weaker than the ones calculated for CO adsorption on Pt6(BE = −62.95 kcal/mol).38 Additionally, the binding energyvalues of Table 1 are significantly stronger than that for COadsorption on the top site of a 10-atom cluster which modeledthe (111) surface (Cu: −2.66; Ag: −2.28; Au: 1.35 kcal/mol).3

The CO adsorption behavior was analyzed as a function of thecoordination number of the adsorption site and the LUMOlocalization on the clusters. As shown in Table 1, the BE wasstronger on the corners than the edges for the monometallicclusters. This trend was expected since we have shown that thelower coordinated sites exhibit stronger CO bindingenergies.11,12 However, this trend was not observed in thebimetallic clusters. CO binds stronger to the edges of Au3Cu3than to its corners. When CO was adsorbed on the edge of theCu3Au3 cluster, it restructured to Au3Cu3 with CO adsorbed onthe corner. In order to further understand the CO adsorption

Figure 2. EA of clusters as a function of Au composition (color codeas in Figure 1).

Figure 3. IP as a function of Au composition of the clusters (colorcode as in Figure 1).

Table 1. BE of CO on the Corners and Edges of the Clusters(Values in kcal/mol)

clusters BE CO corner BE CO edge

Ag6 −6.67 −4.51Au6 −17.27 −9.66Cu6 −15.94 −14.78Au3Cu3 −10.63 −15.56Cu3Au3 −20.54 restructured to Au3Cu3 CO corner

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behavior, we analyzed the LUMO orbital localization on theclusters as shown in Figure 4. Figure 4 clearly shows that the

LUMO orbitals are localized at the corners of the clusters. Thisis important because CO as an electron donor wouldpreferentially bind to the sites of the clusters where theLUMO is localized.16 This observation further supports thepreferential binding of CO to the corners of the monometallicclusters.Figure 5 presents the HOMO−LUMO gaps of the clusters

with respect to the Cu6 LUMO orbital. Research has shown

that large HOMO−LUMO gaps represent structures of higherstability.11,30 In Figure 5, Au6 and Au3Cu3 exhibit the largestgaps and their HOMO and LUMO energies are positioned atcomparable levels. This is because according to Figure 4, thesetwo clusters localize their HOMO and LUMO orbitals on thesame (Au) atoms and the orbitals exhibit the similar symmetry.Cu6 and Cu3Au3 can be grouped together since they show theclosely related orbital localization/symmetry and HOMO−LUMO gap behavior. However, just as with the coordinationnumber, the LUMO orbital analysis cannot explain the higherBE of CO at the edges of Au3Cu3.

Figure 6 illustrates NBO charge distribution analysis on theclusters. The positive charge is located at the corners of themonometallic clusters and of the Cu3Au3 cluster. However, inthe case of Au3Cu3 the positive charge is located at the edges.The negatively charged corners of Au3Cu3 (electron local-ization) is the result of the higher electronegativity of Aucompared to Cu. CO, which acts as an electron donor,preferentially binds the sites of the cluster that are positivelycharged or “electron poor”. As a result, this charge distributionanalysis can explain the stronger binding of CO on the edges ofAu3Cu3. One aspect of Table 1 that we have not discussed isthe restructuring of Cu3Au3 with CO adsorbed on the edge toAu3Cu3 with CO adsorbed on the corner. The reason weobserved this restructuring is because Au3Cu3 is more stablethan Cu3Au3 as shown in Figure 1. This is not surprising, sincewe have very recently shown that adsorption of CO oxidationspecies (CO and O2) on the Au6

− cluster can change thecluster’s structural characteristics (cluster breathing duringreactions).15

In addition, we investigated the BE of CO on the bimetallicclusters as a function of CO coverage. Zhai et al. have shownthat CO binding to the apex sites of the Au6 (the three corners)does not significantly disturb the structure of the bare cluster.39

The lack of disturbance to bare cluster is a trend also observedin the Au3Cu3 cluster with placement of CO on the corner sites.As it can be observed from Figure 7 the BE of CO on the

corners of Au3Cu3 is weakly affected by CO coverage, with anaverage BE per CO molecule approximately at −10 kcal/mol(BE per CO = (Ecluster−COn − E − nECO)/n). In addition to therestructuring that occurred when CO was adsorbed on the edgeof Cu3Au3, we also observed restructuring when three COatoms were adsorbed on the corners of Cu3Au3. This, again, is aresult of the higher stability (more negative cohesive energy) ofthe Au3Cu3 cluster compared to Cu3Au3. As a final note on theCO coverage effects, we observe a symmetry transition from

Figure 4. HOMO and LUMO Orbitals of Ag6, Au6, Cu6, Au3Cu3, andCu3Au3 clusters.

Figure 5. HOMO and LUMO gaps of Ag6, Au6, Cu6, Au3Cu3, andCu3Au3 clusters with respect to the energy level of the Cu6 LUMOorbital.

Figure 6. NBO charge distribution on the clusters. The numbers in red show the total charge transferred within the cluster (Δq).

Figure 7. CO coverage effects on Au3Cu3 and Cu3Au3 clusters. Thenumbers in red represent the binding energy per CO molecule.

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D3h to C2v on both Au3Cu3 and Cu3Au3 clusters when two COmolecules bind the edge atoms. This is because the presence oftwo CO molecules disturbs the symmetry of the cluster in away to facilitate transition to C2v symmetry.Table 2 describes the BE behavior of O2 on the corners and

edges of the metal clusters as well as the average BE on the

cluster. The calculated O2 adsorption energies on Ag6 fallwithin the range of reported literature values of O2 BEs on Agnclusters (n = 1−7; BE: −3.16 to −25.18 kcal/mol).40

Additionally, O2 binding on Cu6 is stronger than BEs previouslyreported on Cu2 (−4.7 kcal/mol) but weaker than on Cun (n =3−5; BE: −14.9 to −39.1 kcal/mol).41 It has been shown byNørskov’s group that the adsorbate binding energy can becorrelated with the d-band center (dc) of the metal.42,43 As aresult, we calculated the density of states (DOS) of the dorbitals and the dc of the clusters as presented in reference.44

Figure 8 illustrates the DOS, dc, and HOMO−LUMO levels forthe clusters with respect to the O2 LUMO orbital. The reasonwe chose the LUMO of O2 as energy reference in Figure 8 isbecause O2 acts as an electron acceptor molecule, and theenergy level of its LUMO plays an important role in bonding.According to Figure 8, the overall dc trend is Cu6 > Cu3Au3>Au3Cu3 > Au6 > Ag6. In Figure 9 we plotted the average BE ofO2 on the cluster as a function of the cluster dc. The average BEof O2 is linearly related to the dc of the clusters showing thatthe higher the dc (closer to the O2 LUMO), the stronger the O2

binding. This trend, although developed on subnanometermetal clusters, follows Nørskov’s d-band model for adsorptionon periodic metals surfaces.42 It is worth noticing that anattempt to relate the average and site specific CO bindingenergy on the clusters (Table 1) with the dc did not give clearlinear trend as in the case of O2 adsorption.As shown with CO adsorption on the monometallic clusters

(Table 1), the BE of O2 on the corners is stronger than on theedges (Table 2). The weaker adsorption of O2 compared toCO has been also observed on neutral and positively chargedAu6 clusters.

15 Adsorption behavior on the various sites of thecluster was examined using HOMO orbitals and dc of theclusters. O2 acting as an electron acceptor molecule wouldprefer to bind to sites of the clusters that the HOMO islocalized.16 Figure 4 shows that the HOMO is localizedprimarily on the corner sites of the clusters. Orbital analysissupports higher binding on the corners than the edges of the

Table 2. BE of O2 on the Corners and Edges and the AverageBE of the Clusters (Values in kcal/mol)

clusters BE O2 corner BE O2 edge av BE

Ag6 −4.95 −3.36 −4.15Au6 −3.58 −2.41 −3.00Cu6 −11.75 −7.77 −9.76Au3Cu3 −3.16 −4.86 −4.01Cu3Au3 −12.09 −2.48 −7.28

Figure 8. d-orbital density of states (DOS), d-band center (dc), and HOMO−LUMO orbitals of the clusters with respect to the energy level of O2LUMO orbital. Within each color code (representing the different clusters), the lines below the DOS lines represent from left to right the dc,HOMO, and LUMO orbitals of the clusters.

Figure 9. O2 average BE on the different clusters vs dc of the clusters(black dashed line serves as a guide to the eye).

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monometallics and the Cu3Au3 cluster. However, this is notsufficient to explain the O2 adsorption behavior of Au3Cu3. Tounderstand the slightly stronger binding of O2 to the edge ofAu3Cu3 rather than the corner, we turn back to the O2adsorption behavior on the monometallic clusters Au6 andCu6. As shown in Table 2, we found that Cu6 had a higheraffinity (−9.76 kcal/mol) to bind O2 than Au6 (−3.00 kcal/mol) due to the oxophilic nature of Cu compared to Au.22 Thisis supported by our dc cluster analysis (Figure 8) where wefound that the dC of Cu6 (purple line) lies closer to the LUMOlevel of O2 than the dC of Au (navy blue line). Consequently,the presence of Cu on the edges of the bimetallic Au3Cu3cluster increases the O2 adsorption energy on this specific site(compared to Au6 cluster). On the other hand, adsorption onthe Au atoms of the Au3Cu3 (corner) and Cu3Au3 (edge)bimetallic clusters is significantly weaker compared to the caseof Cu6. If we compare the O2 adsorption on the same sites ofthe clusters that exhibit the same metal composition, weobserve that O2 adsorption does not change significantly frommonometallic to bimetallic clusters (compare O2 on corners ofAu6 vs Au3Cu3, on corner of Cu6 vs corner of Cu3Au3 and onedge of Au6 vs edge of Cu3Au3). Exception from this behavior isthe adsorption of O2 on the edge of Cu6, which is −7.77 kcal/mol and changes to −4.86 kcal/mol on the edge of Au3Cu3(approximately 40% adsorption energy drop). The reason forthis adsorption change is that both the HOMO and dc ofAu3Cu3 (orange line) are energetically located further awayfrom the LUMO of O2 than the corresponding levels of Cu6 asshown in Figure 8. As a final note, we observed that O2adsorption on any site of the clusters did not result to anyrestructuring, and the clusters retained the D3h symmetry. Thisis due to the weaker adsorption of O2 on the clusters (BE range= −2.4 to −12.0 kcal/mol) compared to CO (BE range = −4.5to −20.5 kcal/mol).Finally, to analyze the BE behavior of coadsorbed species, we

performed calculations with CO adsorbed on the corner and O2adsorbed on the edge of the metal clusters. According to Figure10, the addition of O2 to the clusters with preadsorbed CO

results to an increase in the BE of O2. The values in parenthesesin Figure 10 represent the adsorption of O2 when CO ispreadsorbed on the clusters. All these adsorption energies arestronger than the corresponding ones presented in Table 2(edge adsorption). Our theoretical observation is supported bymass spectrometry experiments on small clusters which showed

that CO and O2 adsorb cooperatively instead of compet-itively.25,26 A profound example in our observations is the caseof Cu6, where the BE of O2 on its edge atom is −7.77 kcal/moland the adsorption is strengthened to −21.27 kcal/mol whenCO is preadsorbed on the corner. The reason for thissignificant energy change is that the Cu6 cluster is oxidized asstructurally shown in the inset of Figure 8 and further verifiedby NBO charge analysis (Cu6 loses one electron). This resulthighlights the significance of preadsorbed CO on the clusterespecially since this oxidation behavior was not observed whenO2 was singularly adsorbed to the edge of the Cu6 cluster.

■ CONCLUSIONSIn this study we investigated the cohesive energy (BE/n),electronic and adsorption properties of six-atom Ag, Au, andCu, and bimetallic combinations of AuAg and AuCu metalclusters. Our results demonstrated that the cohesive energy ofthe bimetallic clusters is a linear combination of the BE/n of themonometallic clusters. As a result, the BE/n of small Au clusterscan be significantly enhanced when mixed with Cu (formationof bimetallic clusters). We also calculated the electron affinities(EA) and ionization potential (IP) of the clusters. We foundthat the AuCu bimetallic clusters have higher tendency toreceive electrons than the AuAg clusters in compositions of Au≥ 50%. This trend is reversed in Au compositions smaller than50%. We did not observe any trend with IP calculations. Inaddition, we determined that the preferential binding of CO tothe corners compared to the edges of monometallic clusterscould be explained by the CN and the LUMO localization onthe clusters. However, these descriptors are not sufficient toexplain the CO adsorption on the bimetallics, and one has toaccount for the charge distribution among the atoms of thecluster. NBO charge distribution analysis revealed that theedges of Au3Cu3 (Au atoms occupy the corners) are positivelycharged, consequently resulting in the stronger binding of COon the edges (CN = 4) than on the corners (CN = 2) of thecluster. The restructuring of Cu3Au3 to Au3Cu3 with COadsorption highlighted the importance of the cohesive energyof the cluster (Au3Cu3 is more stable than Cu3Au3). Whenmore than one CO molecules adsorbed on the corner atoms ofthe Au3Cu3 cluster, the BE of CO on the cluster did not varysignificantly, thus supporting structural integrity against COcoverage. In addition to CO adsorption, we investigated O2adsorption and coadsorption (with CO) on the clusters. Wefound that the average O2 BE on the cluster can be linearlyrelated to the dc of the cluster. Additionally, we determinedthat the preferential binding of O2 to the corners instead of theedges on the monometallic clusters and bimetallic Cu3Au3 canbe explained by the CN and HOMO localization on theclusters. The higher oxophilicity of Cu6 compared to Au6 wasexplained in terms of dc of the clusters. Our study also revealedthat the adsorption of O2 on the corners of Au3Cu3 issignificantly weaker than that at the corners of Cu6 (O2 binds inboth cases Cu atoms) due to the fact that the HOMO orbital ofAu3Cu3 has been energetically shifted away from the LUMOorbital of molecular O2. The coadsorption of CO and O2showed that the BE of O2 increases with preadsorbed CO,which consequently supports a cooperative interaction betweenthe molecules. Additionally, we discovered that a significantenhancement in the BE of O2 on Cu6 due to preadsorbed COleads to the formation of a partially oxidized cluster. Chargeanalysis supported the oxidation of this cluster by illustrating itsloss of electron density to O2. Overall, this study demonstrates

Figure 10. Coadsorption of CO and O2 on Ag6, Au6, Cu6, Au3Cu3, andCu3Au3 clusters. CO is adsorbed on the corner and O2 on the edge ofthe clusters.

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a high degree of complexity in the adsorption behavior ofsubnanoscale bimetallic catalysts.

■ AUTHOR INFORMATIONCorresponding Author*E-mail gmpourmp@pitt.edu; phone 412-624-7034 (G.M.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the Center for Simulation and Modeling(SAM) at the University of Pittsburgh for computationalsupport. This research has been supported by start-up fundsfrom the University of Pittsburgh.

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