RSC Advances

PAPER

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article OnlineView Journal | View Issue

DFT study of CO

aKey Laboratory of Automobile Materials (

School of Materials Science and Engineeri

China. E-mail: yfzhu@jlu.edu.cn; jiangq@jlbState Key Laboratory of Meta-stable Mat

University, Qinhuangdao 066004, China

Cite this: RSC Adv., 2015, 5, 1587

Received 21st September 2014Accepted 18th November 2014

DOI: 10.1039/c4ra10881g

www.rsc.org/advances

This journal is © The Royal Society of C

oxidation on Cu2O–Au interfacesat Au–Cu alloy surfaces

D. Liu,ab Y. F. Zhu*a and Q. Jiang*a

Although Au–Cu alloy nanoparticles on inert substrates show high activity for catalyzing CO oxidation, the

corresponding catalytic mechanism is not clear. To clarify the mechanism of this alloy catalysis method, CO

oxidation reactions on Au–Cu alloy surfaces with different surface oxidation states are studied via density

functional theory simulations. The simulation results indicate that on AuCu(111) and Cu2O/Au3Cu(111),

CO and O2 cannot move together for reactions since they are adsorbed on separate Cu sites. On

Cu2O–Au/Au3Cu(111), O2 prefers to be located at the Cu hollow sites near Cu2O–Au interfaces. When

CO diffuses to its neighboring Au sites, they can easily combine to generate CO2, for which the reaction

barriers are no more than 0.42 eV. The Au and Cu synergetic effect for catalyzing CO oxidation can be

realized on Cu2O–Au interfaces at Au–Cu nanoparticle surfaces.

Introduction

Gold was regarded as an inert metal until Haruta et al. foundthat Au nanoparticles dispersed on oxide substrates, such asTiO2 and Fe2O3, show high catalytic activity for low temperatureCO oxidation.1 On bulk gold surfaces, O2 adsorption is weak andcorresponding oxygen dissociation barriers are higher than 1eV,2,3 which makes further CO oxidation impossible. When COand O2 react on Au/TiO2 catalysts, O2 molecules are stronglyadsorbed on the Au nanoparticle–TiO2 substrate circumferen-tial interfaces.4,5 Via this interfacial adsorption mode, activatedO2 molecules can easily react with the CO molecules adsorbedon neighboring Au sites.4,5 However, this synergetic effect incatalysis relies on the type of substrates. The catalytic activity forCO oxidation of the Au/SiO2 catalyst is signicantly reducedbecause inert SiO2 substrates cannot offer the strong adsorptionfor O2 molecules.6

Alloying with a second metal is an effective way to obtainactive Au-based binary nanoparticles on inert supports.Recently, Au, Cu and Au–Cu binary nanoparticles (dimensionalsizes of 2–5 nm) on inert SBA-15 mesoporous silica substrateshave been synthesized for catalyzing CO oxidation.7 Cu/SBA-15is inactive below 100 �C and Au/SBA-15 only possessesmoderate activity at room temperature. However, CO conver-sion remains at 100% from room temperature (RT) to 300 �C onAu–Cu/SBA-15. In addition, the turnover frequencies of COoxidation on Au–Cu/SBA-15 are three times larger than those onAu/MCM-41 mesoporous silica.8 When a large amount of H2 is

Jilin University), Ministry of Education,

ng, Jilin University, Changchun 130022,

u.edu.cn; Fax: +86-431-5095876

erials Science and Technology, Yanshan

hemistry 2015

introduced, the CO conversion rates on Au–Cu/SBA-15 reach60–80% at RT. As the reaction temperature increases to 80 �C(operational temperature for H2 fuel cells), the CO conversionrates are still 50–70%, whereas the corresponding values onlyare 20% and 0% for Au/SBA-15 and Cu/SBA-15.

Structural changes in Au–Cu nanoparticles (Au/Cu atomicratio 1/1) during the catalytic process of CO oxidation areobserved via in situ techniques.9 The Au–Cu binary particlesobtained by calcination have a gold core and a CuxO shell. Toobtain active Au–Cu particles, a subsequent reduction treat-ment is carried out in H2 at 550 �C. Aer this reduction treat-ment, Cu cations are reduced to alloy with the interior Au atomsand the structure of particles reverts to the Au3Cu intermetallicphase. When the reduced Au–Cu nanoparticles are used tocatalyze CO oxidation in the temperature range of RT to 300 �C,their Au3Cu structure hardly changes. In situ XANES resultsindicate that some Cu atoms are oxidized from Cu0 to Cu+ andCu2+, whereas Cu2O and CuO structures are absent in XRDmeasurements.

These Cu cations may come from surface oxidation duringthe catalytic process. The reaction temperature for CO oxidationis much lower than that for nanoparticle calcination.7 At thismoderate temperature range (RT to 300 �C), Au–Cu nano-particles are not totally covered by CuxO oxide layers and thereare still Au sites on their surfaces. Infrared spectroscopymeasurements indicate that aer O2 introduction, CO adsorp-tion on Au sites of Au–Cu nanoparticles is decreased dramati-cally, whereas CO adsorption on Cu sites of these particlesremains unchanged.9 However, the presence of gold improvesthe oxidation resistance of copper. Extended X-ray absorptionne structure (EXAFS) measurements show that there are oneand a half oxygen neighbors for each Cu cation on average onthe surface of the Au–Cu nanoparticles,10 which proves that

RSC Adv., 2015, 5, 1587–1597 | 1587

RSC Advances Paper

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

Cu2O constitutes the majority of the surface oxides. Cu2O is anincomplete oxide, on which O2 can be adsorbed.

It has been reported that Cu2O layers can grow epitaxially onthe surface of Au–Cu particles with small lattice mismatch.11 Inaddition, Cu2O–Au in-plane coherent interfaces can also beformed along the close-packed crystal directions.11,12 Aer thesurface oxidation of Au–Cu nanoparticles, coherent linearinterfaces are generated between the Cu2O and Au surfacelayers. The CO oxidation reactions may just occur on theseCu2O–Au interfaces.13,14

In this work, CO + O2 reactions on Cu2O–Au interfaces atsurfaces of the Au3Cu alloy are simulated by the density func-tional theory (DFT) method. In addition, the bond length,charge transfer (CT) and partial density of states (PDOS) of someadsorption and reaction states during the catalytic process arealso analyzed for understanding the essence of these chemicalreactions. For comparison, the adsorption status and furtherpossible reaction of CO + O2 on AuCu and Cu2O/Au3Cu alloysurfaces are investigated to explain why unoxidized and totallyoxidized Au–Cu nanoparticles are inert for CO oxidation.

Simulation details

In the rst-principles DFT15,16 simulations, the DMol3module17,18 was used for the geometric optimization, reactionprocess imitation and analysis of CT and PDOS. We employedthe generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof functional (PBE)19 to describe exchange andcorrelation effects. During the geometric optimization process,the energy convergence, maximum force and maximumdistance were 5.0 � 10�4 eV, 0.1 eV A�1 and 5.0 � 10�3 A,respectively. In the electronic settings, to reduce the computa-tional cost, the DFT semi-core pseudo-potentials (DSPP)method20 was used to replace core electrons by a single effectivepotential and introduce some degree of relativistic correctioninto the core. The self-consistent eld (SCF) tolerance value was3.0� 10�4 eV and the double numerical plus d-functions (DND)were chosen as the basis set.17 To speed up convergence, athermal smearing of 0.1 eV was applied to the orbital occupa-tion. The calculations were spin unrestricted. In the simulationof searching for transition states (TS), the calculation initiallyperformed a linear synchronous transit (LST)21 maximum,which was followed by energy minimization in directionsconjugated to the reaction pathway. TS approximation obtainedvia LST/optimization was then used to perform a quadraticsynchronous transit (QST) maximization to determine thetransitional states more accurately. The convergence toleranceof the root mean square (RMS) force was 0.15 eV A�1 and themaximum number for a QST step was set as 5. For the CTproperties, atom charges would be calculated via the Hirshfeldpopulation analysis.22 For PDOS calculations, the empty bandswere chosen as 12.

The Au–Cu nanoparticles with dimensional sizes of 2–5 nmusually have polyhedral constructions, such as cubo-octahedrons, icosahedrons and decahedrons.23,24 For thesesmall particles, close-packed facets still occupy the majority ofsurfaces.25,26 The atom arrangement of these facets is the same

1588 | RSC Adv., 2015, 5, 1587–1597

as that of bulk (111) surfaces. Therefore, four layers of (111)slabs with periodicity of (4 � 6) were built for representing thesurfaces of nanoparticles [the in-plane lattice directions areh110i]. When the slabs were used for the geometric optimiza-tion and modeling reaction processes, the underneath twoatomic layers were xed to simulate the interior of Au–Cu alloys.A vacuum of 18 A was added along the normal directions of theslabs. K-points were chosen as 3 � 2 � 1, for which the actualspacing is 0.03, 0.03 and 0.04 A�1 in the three lattice directions.First, two extreme cases are considered. When Au–Cu nano-particles (Au/Cu atomic ratio 1/1) undergo no Cu oxidation andAu surface enrichment, the atom packing of their surface facetsshould be similar to AuCu(111), which is shown in Fig. 1(a).However, when all Cu atoms in Au–Cu nanoparticles areoxidized and enriched at surfaces via calcination at hightemperature, the atom packing of their surface facets should besimilar to CuO(111) and Cu2O(111). In contrast to inert CuOsurfaces, Cu2O surfaces possess active sites for adsorbingreactants.27 The geometric structure of Cu2O(111) is shown inFig. 1(b). Differentiating the oxidization states, there are two Cucation sites at Cu2O(111) surfaces: the oxygen coordinativelysaturated (CuCSA) and unsaturated (CuCUS) surfaces.28 Under theCu surface atom layer, there are two oxygen atom sub-surfacelayers; one layer binds with CuCSA sites and the other bindswith CuCUS sites.

As mentioned above, AuCu nanoparticles maintain theAu3Cu structure when they are only partially oxidized. In thiscase, part of their surface is a single layer of Cu2O oxide growingon the Au3Cu core. Cu2O/Au3Cu(111) represents thisepitaxial system, which is shown in Fig. 1(c). In Cu2O/Au3Cu(111), the oxygen atom sub-surface layer connecting withCuCUS sites is absent since Au–O bonding is forbidden.9,10

However, the oxygen atom sub-surface layer connected withCuCSA sites can be thermodynamically stabilized via bindingwith Cu atoms in the Au3Cu alloy. Because of this oxygen de-ciency on Cu2O/Au3Cu interfaces, the surface CuCUS sites wouldchange to oxygen uncoordinated ones, which are dened asCuUNC. This surface oxide structure is similar to the construc-tion of an Ag2O oxide layer on Ag surfaces.29 Although thegeometric structures of surface oxides are similar to those ofcorresponding bulk oxides, their electronic properties may bequite different because the surface oxides seem like periodic on-surface and sub-surface oxygen adsorption on metal surfaces.The oxygen adsorption and Cu2O surface oxide formation onCu(111) and their electronic properties have been studied indetail via DFT-GGA simulations.30,31 Therefore, it is appropriateto investigate the Cu2O surface oxide on Au3Cu(111) by the samemethod.

There are four types of coherent interfaces between Cu2Oand Au on Au3Cu(111), which differentiate the boundary Cu andO sites (Fig. 2). For all of them, the in-plane linear interfaces aredesigned to lie along the close-packed h110i direction accordingto experimental results.11,12 CuCSA sites can always connect withAu sites on interfaces because they occupy three quarters ofthe whole Cu surface atom layer. There are no Au–O bonds atCu2O–Au interfaces, which means that interfacial CuCSA siteslose one coordinated oxygen atom and are converted to new

This journal is © The Royal Society of Chemistry 2015

Fig. 1 Geometricconfigurationsof (a)AuCu(111) surfaces, (b)Cu2O(111) surfaces, (c)Cu2O/Au3Cu(111) surfacesand (d)Cu2O–Au/Au3Cu(111) surfaces.

Paper RSC Advances

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

oxygen coordinatively unsaturated sites (CuNCUS). In Fig. 2(a andb), CuUNC and CuNCUS sites are both on Cu2O–Au interfaces. Thedifference between them is that the sub-surface oxygen islocated near interfaces for the former and the on-surface oxygenis located near interfaces for the latter. In Fig. 2(c and d), onlyCuNCUS sites are on the Cu2O–Au interfaces and the differencebetween them is as the same as that in Fig. 2(a and b). The total

Fig. 2 The four possible geometric configurations of Cu2O–Au/Au3Cu(1interfaces; (b) CuUNC and CuNCUS sites on interfaces and Oon-surface siteinterfaces; (d) CuNCUS site on interfaces and Oon-surface site near interfac

This journal is © The Royal Society of Chemistry 2015

cohesive energy difference between these slabs and that ofCu2O/Au3Cu(111) and Au/Au3Cu(111), DEslab, derives from thegeneration of Cu2O–Au interfaces. It is impossible to describethe interfacial energy of linear Cu2O–Au interfaces in units ofJoules per square meter. Therefore, the interfacial energy, gin, iscalculated in electron volts per atom, and is calculated via the

formula gin ¼ DEslab

Nin, where Nin is the total number of atoms at

11). (a) CuUNC and CuNCUS sites on interfaces and Osub-surface site nearnear interfaces; (c) CuNCUS site on interfaces and Osub-surface site neares.

RSC Adv., 2015, 5, 1587–1597 | 1589

Fig. 3 Possible adsorption positions of reactants O2, CO and O on (a)AuCu(111) and (b) Cu2O/Au3Cu(111) surfaces.

RSC Advances Paper

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

the Cu2O–Au interfaces in a slab. gin z 0.65 eV per atom forCu2O–Au interfaces in Fig. 2(a and b) and gin z 0.70 eV peratom for Cu2O–Au interfaces in Fig. 2(c and d). The gin valuesare similar to the surface energy values of Au(111).32 The highgin values of Cu2O–Au interfaces may be due to thelattice oxygen deciency of the CuNCUS sites. Nevertheless, if theCu2O–Au interfaces do not form, the edges of Au and Cu2O areexposed to surfaces where the surface energy values areextremely large. To catalyze CO oxidation at Cu2O–Au interfaces,active CuUNC sites are necessary. Because of the higher gin

values and absence of CuUNC sites, the Cu2O–Au interfaces inFig. 2(c and d) are excluded. In Fig. 2(b), on-surface oxygenatoms are adjacent to interfaces, which may hinder theadsorption of reactants. Therefore, the Cu2O–Au/Au3Cu(111)slab with both CuUNC and CuNCUS sites on Cu2O–Au interfacesand sub-surface oxygen atoms adjacent to Cu2O–Au interfaces[Fig. 2(a)] are chosen as the catalytic substrate for the COoxidation reaction, which is also shown in Fig. 1(d).

Results and discussionThe adsorption status and reaction of CO oxidation on AuCuand Cu2O/Au3Cu(111)

The stable and meta-stable adsorption positions of reactantsCO, O2 and O on AuCu and Cu2O/Au3Cu(111) are shown in Fig. 3and the corresponding adsorption energy values, Ead, are listedin Table 1. In general, CO is vertically adsorbed on the top sitesof metal surface atoms, whereas O is adsorbed in the hollowsites of metal surfaces. O2 is adsorbed on metal surfaces inthree states: top-bridge-top (tbt), top-hollow-bridge (thob), andbridge-hollow-bridge (bhob).33 For fcc and hcp hollows, thethob state can be sub-divided into the top-fcc-bridge (t) andtop-hcp-bridge (thb) states. Similarly, the bhob state can be sub-divided into the bridge-fcc-bridge (b) and bridge-hcp-bridge(bhb) states. In addition, the Au and Cu sites should bedistinguished for Au–Cu alloy surfaces.

In Fig. 3(a), CO can be adsorbed on the Au site of AuCu(111)(CO-II) with Ead ¼ �0.32 eV. In comparison, CO binds stronglyto the Cu site (CO-I) due to its low adsorption energy of Ead ¼�0.83 eV. The most stable O2 adsorption position is the Cu2bridge (O2-I). In this adsorption state, although the two oxygenatoms bind separately with two Cu sites, the O–O bond is notparallel to the Cu2 bridge (the angle of interaction is about 40�),which differs from the tbt state. Therefore, this new adsorptionstate is the tilted tbt state. For this stable O2 adsorption posi-tion, Ead ¼ �0.39 eV. When O2 is placed on the AuCu2 or Au2Cuhollows, it will spontaneously move to the Cu2 bridge. The onlyexceptions are O2 adsorption on AuCu2 hollows in the t-Au topand thb-Au top states (O2-II and O2-III). For these two meta-stable adsorption states, the Ead values are �0.19 and �0.13 eV,respectively. In addition, O2 adsorption on the Au2 bridge isendothermic. The O atom adsorption is much stronger thanthat of O2 molecules because the atomic oxygen has lost thestrong O–O bond. The most stable adsorption position foroxygen atoms is the AuCu2 fcc hollow (O-I) with Ead ¼ �4.40 eV.Othermeta-stable O adsorption positions are AuCu2 hcp, Au2Cufcc and hcp hollows (O-II, O-III and O-IV), for which Ead values

1590 | RSC Adv., 2015, 5, 1587–1597

are�4.27,�3.84 and�3.73 eV, respectively. In Fig. 3(b), CO canonly be adsorbed on the CuUNC site of Cu2O/Au3Cu(111) (CO-I)with Ead ¼ �1.12 eV. The most stable O2 adsorption positionis the Cu3 fcc hollow site (O2-I). In the t-CuUNC top state, Eadreaches �0.49 eV. At the same adsorption position, the othertwo meta-stable adsorption states are t-CuCSA top and bhb-CuUNC top (O2-II and O2-III), for which the Ead values are �0.29and �0.24 eV, respectively. Similar to the CO adsorption, O canonly be adsorbed on the Cu3 fcc hollow (O-I) with Ead ¼ �4.04eV.

The possible reaction mechanism of CO oxidation onunoxidized AuCu alloy surfaces is shown in Fig. 4. According tothe reactant adsorption results, O2 can be meta-stably adsorbedon the Cu2 bridge of AuCu(111) in the tilt tbt state. Even if O2

molecules are adsorbed at AuCu2 hollows in the t-Au top andthb-Au top states, they can easily diffuse to the Cu2 bridge with asmall diffusion barrier of Edb ¼ 0.15 eV. In Fig. 4(a–c), when theO2 molecule on the Cu2 bridge is dissociated to form two Oatoms, it needs to overcome the reaction barrier of Erb ¼ 0.74eV. The direct O2 dissociation on AuCu(111) is not easy.Considering the direct reaction of O2 with CO, they must be co-adsorbed on the neighboring surface sites. At the ordered AuCualloy surfaces, the Au and Cu sites are next to each other.

This journal is © The Royal Society of Chemistry 2015

Table 1 Ead values of reactants CO, O2 and O on AuCu(111), Cu2O/Au3Cu(111) and Cu2O–Au/Au3Cu(111)

AuCu(111) Cu2O–Au/Au3Cu(111)

Reactants Positions States Ead (eV) Reactants Positions States Ead (eV)

CO-I Cu Top �0.83 CO-I Au Top �0.28CO-II Au Top �0.32 CO-II CuNCUS Top �1.03O2-I Cu2 Tilted tbt �0.39 CO-III CuUNC Top �0.75O2-II AuCu2 t-Au top �0.19 O2-I Cu3 t-CuUNC top �0.44O2-III AuCu2 thb-Au top �0.13 O2-II Cu3 t-CuNCUS top �0.73O-I AuCu2 fcc hollow �4.40 O2-III Cu3 t-CuCSA top �0.49O-II AuCu2 hcp hollow �4.27 O2-IV Cu3 bhb-CuUNC top �0.40O-III Au2Cu fcc hollow �3.84 O2-V Au2Cu t-CuUNC top �0.05O-IV Au2Cu hcp hollow �3.73 O2-VI AuCu2 thb-Au top �0.21Cu2O/Au3Cu(111) O-I Cu3 fcc hollow �4.25CO-I CuUNC Top �1.12 O-II AuCu2 hcp hollow �4.26O2-I Cu3 t-CuUNC top �0.49 O-III Au2Cu (CuUNC) fcc hollow �3.97O2-II Cu3 t-CuCSA top �0.29O2-III Cu3 bhb-CuUNC top �0.24 O-IV Au2Cu (CuNCUS) fcc hollow �4.00O-I Cu3 fcc hollow �4.04

Paper RSC Advances

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

However, CO prefers to bind with the Cu site of AuCu(111). Toreact with O2 on the Cu2 bridge, CO must move to the Au sitebetween them, as shown in Fig. 4(d–f). A reaction barrier of Erb¼ 0.81 eV is calculated for this movement, whichmeans that COundergoes desorption and re-adsorption during this process.When CO is adsorbed on the Au site with O2 on its neighboringCu2 bridge, their co-adsorption energy increases to Ead ¼ �0.88eV as shown in Fig. 4(f) [Ead ¼ �1.22 eV for the initial state (IS)].

Fig. 4 Illustration of (a)–(c) O2 dissociation reaction, (d)–(f) CO + O2 re

This journal is © The Royal Society of Chemistry 2015

Owing to thismeta-stable adsorption state, the reaction tends tobe reversible and the corresponding Erb is only 0.44 eV. There-fore, on AuCu(111) surfaces, it is difficult for CO on the Cu siteto move close to O2 on the Cu2 bridge for further reactions. Inaddition, O2 molecules can still dissociate into O atoms onAuCu(111) because the corresponding Erb value is similar to thatof O2 dissociation on Pt(111).33 According to Fig. 4(c), aer O2

dissociation, the two atoms are located at the AuCu2 and Au2Cu

action and (g)–(i) CO + O reaction on AuCu(111) surfaces.

RSC Adv., 2015, 5, 1587–1597 | 1591

RSC Advances Paper

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

hollows. The AuCu2 fcc hollow is the most stable position for Oadsorption. O atoms adsorbed on the other three sites can easilydiffuse to the AuCu2 fcc hollow with small Edb values of 0.02–0.25 eV. In Fig. 4(g–i), to react with the O atom on the AuCu2 fcchollow, CO on the Cu site must move to the Au site betweenthem. The Erb for this reaction is as high as 0.81 eV, whereas Erbof the reverse reaction is only 0.35 eV. Therefore, even if O2 isdissociated to O atoms on AuCu(111), they still hardly react withCO molecules.

Because the surface energy of Au is smaller than that of Cu,32

gold atoms are usually enriched at the unoxidized Au–Cu alloysurfaces.34 The Au surface enrichment has been neglected inthis AuCu(111) slab. If the Au enrichment is considered, somelocal parts of the AuCu alloy surface may show an orderedAu3Cu atom arrangement, for which the neighbors of Cu sitesare only Au sites. On this Au3Cu surface layer, Cu2 bridges andAuCu2 hollows are absent. O2 adsorption on the Au2Cu hollowsand AuCu bridges is very weak because the corresponding Eadvalues are higher than �0.10 eV. The gold surface enrichmentleads to weaker O2 adsorption, which cannot change the factthat CO oxidation is impossible on unoxidized Au–Cu alloysurfaces. In addition, the simulation of CO oxidation on Cu/Au(100) surfaces has been performed by the DFT-GGAmethod.35 The Erb of the CO + O reaction on Cu/Au(100) rea-ches 0.6 eV,35 which is a relatively high value. Therefore, thecatalysis of CO oxidation using Au–Cu alloys is still difficult evenif Cu is totally segregated to their unoxidized surfaces.

The possible reaction mechanism of CO oxidation on theCu2O surface oxide of Au–Cu alloys is shown in Fig. 5. One of themost common CO oxidation reaction mechanisms on reducibleoxides or metal-support interfaces is that the oxides providetheir lattice oxygen to react with CO.36 Based on this consider-ation, the possibility of the CO reaction with the on-surfaceoxygen of Cu2O/Au3Cu(111) is investigated rst. In Fig. 5(a–c),the transitional state of this CO + Oon-surface reaction was notobserved via our simulations. The difficulty for CO reactionswith lattice oxygen may be due to the strong bonding betweenthe on-surface oxygen anions and the Cu cations underneath. Ifan on-surface oxygen anion is removed from Cu2O/Au3Cu(111),there is an energy cost of 5.71 eV. For comparison, oxygen atomadsorption on Cu(111) has been also calculated, for which Ead¼�4.91 eV. The lattice Cu–O bonds in Cu2O surfaces are strongerthan the adsorption bonds of oxygen atoms on Cu surfaces.Although oxygen adsorption bonds are weaker, the Erb value ofthe CO + O reaction on Cu(111) is calculated to be as high as0.80 eV. Based on this result, the Erb value of the CO + Oon-surface

reaction on Cu2O/Au3Cu(111) should be higher than 0.80 eV. Inaddition, considering the Eley–Rideal mechanism, the possi-bility of the gas phase CO molecule reaction with the on-surfaceoxygen of Cu2O/Au3Cu(111) is studied. Similarly, no transitionalstate is observed. Therefore, the direct reaction of CO with thelattice oxygen of the Cu2O surface oxide is impossible.

According to the reactant adsorption results, O2 can beadsorbed meta-stably on the Cu3 fcc hollow of Cu2O/Au3Cu(111)in the t-CuUNC top state. Even if O2 molecules are meta-stablyadsorbed on the Cu3 fcc hollow in the t-CuCSA top state andthe Cu3 hcp hollow in the bhb-CuUNC top state, they can easily

1592 | RSC Adv., 2015, 5, 1587–1597

change to the most stable adsorption state with small diffusionbarriers of Edb ¼ 0.18 and 0.03 eV, respectively. In Fig. 5(c–e),when O2 on the Cu3 fcc hollow is dissociated to form two Oatoms, it needs to overcome the reaction barrier Erb ¼ 0.90 eV.The direct dissociation of O2 on Cu2O/Au3Cu(111) is notpossible. When CO and O2 are co-adsorbed on the Cu2O oxide,they cannot be neighbors, as shown in Fig. 5(f). To react with O2

on the Cu3 fcc hollow, CO on the CuUNC site must move to theCuCSA site between them. However, when CO is placed on theinert CuCSA site, it spontaneously leaves Cu2O/Au3Cu(111). Inaddition, the movement of O2 to the neighboring sites of CO isimpossible because O2 cannot undergo exothermic adsorptiononce it has le the CuUNC site. CO and O2 have no opportunity tomeet directly and react on Cu2O/Au3Cu(111). As mentionedabove, Cu2O/Au(111) represents the surfaces of the calcinedAu–Cu particles. Moreover, the surface structures of Cu2O/Au(111) and Cu2O/Au3Cu(111) should be similar because Auand Au3Cu have the same crystal structure and similar latticeconstants. These simulation results can explain why thecalcined Au–Cu nanoparticles are inert for CO oxidation.

Moreover, the O2 dissociation and CO oxidation onCu2O(111) have also been studied using the same DFT-GGAmethod.27,37 The results show that reaction barriers for thedirect O2 dissociation, the CO reaction with the lattice oxygen,and the direct CO reaction with O2, are all higher than 1 eV.27,37

Therefore, CO oxidation reactions cannot be carried out on boththe surface Cu2O oxides of Au–Cu alloys and bulk Cu2O oxides.

Adsorption status and reactions of CO oxidation on Cu2O–Au/Au3Cu(111)

The stable and meta-stable adsorption positions of reactants onCu2O–Au/Au3Cu(111) are shown in Fig. 6 and the correspondingadsorption Ead values are listed in Table 1. In Fig. 6(a), thepossible O2 adsorption positions near or on Cu2O–Au interfacesare given. As mentioned above, the most stable O2 adsorptionposition on Cu2O/Au3Cu(111) is the Cu3 fcc hollow site, at whichEad is �0.49 eV in t-CuUNC top state [O2-I in Fig. 3(b)]. OnCu2O–Au/Au3Cu(111), O2 also prefers to be located at the Cu3 fcchollow site near Cu2O–Au interfaces. When O2 is adsorbed inthe t-CuUNC top state (O2-I), its Ead value is only �0.44 eV.However, in the t-CuNCUS top state (O2-II), Ead for O2 adsorp-tion can reach �0.73 eV. On Cu2O/Au3Cu(111), CuUNC is theonly active Cu site, which offers strong adsorption for O2. OnCu2O–Au/Au3Cu(111), CuUNC and CuNCUS are both active Cusites. When O2 is located at the Cu3 fcc hollow site nearCu2O–Au interfaces in the t-CuNCUS top state, CuUNC andCuNCUS may simultaneously offer strong adsorption for O2,which induces a signicant decrease in Ead. O2 can also bemeta-stably adsorbed on the Cu3 fcc hollow site in the t-CuCSA topstate (O2-III), for which Ead ¼ �0.48 eV. On the Cu3 hcp hollow,O2 is meta-stably adsorbed in the bhb-CuUNC top state with Ead¼ �0.40 eV (O2-IV). In addition, O2 can also be located on onlythe Cu2O–Au interfaces. On the AuCu2 hcp hollow, O2 is meta-stably adsorbed in the thb-Au top state (O2-VI), for which Ead ¼�0.21 eV. On the Au2Cu fcc hollow, where the Cu belongs to theCuUNC sites (O2-V), O2 is weakly adsorbed in the t-CuUNC top

This journal is © The Royal Society of Chemistry 2015

Fig. 5 Illustration of (a)–(b) CO + Oon-surface reaction, (c)–(e) O2 dissociation reaction and (f) CO + O2 reaction on Cu2O/Au3Cu(111) surfaces.

Fig. 6 Illustration of possible adsorption positions of reactants (a) O2

and (b) CO and O on Cu2O–Au/Au3Cu(111) surfaces.

Paper RSC Advances

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

state because the corresponding Ead value is only �0.05 eV. Theadsorption positions of CO and O on Cu2O–Au/Au3Cu(111) areshown in Fig. 6(b). CO can be meta-stably adsorbed on the Au

This journal is © The Royal Society of Chemistry 2015

site (CO-I) with Ead ¼ �0.28 eV. On the CuNCUS and CuUNC sites(CO-II and CO-III), the CO adsorption is stronger due to the lowEad values of �1.03 and �0.75 eV, respectively. The most stableO adsorption position is the AuCu2 hcp hollow (O-II), on whichEad ¼�4.26 eV is determined. On the Cu3 fcc hollow (O-I), the Oadsorption is slightly weaker with Ead¼�4.25 eV. In addition, Oatoms can be adsorbed meta-stably on the Au2Cu fcc hollows atCuNCUS and CuUNC sites (O-IV and O-III) and the correspondingEad values are �4.00 and �3.97 eV, respectively.

Diffuse reectance infrared Fourier transform (DRIFT) andFT-IR results indicate that although CO adsorption on Cu sitesis stronger than that on Au sites of Au–Cu nanoparticles, thecorresponding CO adsorption amount on the former is far lessthan that on the latter.9 This may be because surface Cu sites aredecient before the reactions are carried out. Au enrichment onunoxidized Au–Cu alloy surfaces occurs34 because the surfaceenergy of Au is smaller than that of Cu.32 Before the introduc-tion of CO and O2, Au sites occupy the majority of the surface ofthe reduced Au–Cu nanoparticles. Therefore, at the beginningof catalytic reactions, CO can bind more readily with Au sites. Inthe presence of oxygen, Au–Cu nanoparticles are partiallyoxidized and Cu2O–Au interfaces are formed between localCu2O and Au surface layers. According to this model of reactantadsorption, O2 prefers to be located at the Cu hollow sites nearthe Cu2O–Au interfaces. The diffusion of CO on Au surfaces isvery easy. When CO diffuses to the Au sites near the Cu2O–Auinterfaces and encounters O2 there, they can combine togenerate CO2.

RSC Adv., 2015, 5, 1587–1597 | 1593

RSC Advances Paper

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

The CO oxidation reaction on Cu2O–Au/Au3Cu(111) canrepresent this interface catalytic process, which is shown inFig. 7. First, O2 is stably adsorbed on the Cu3 fcc hollow near theinterfaces in the t-CuNCUS top state. When O2 molecules are inother meta-stable adsorption states, they can easily change tothe most stable state with small diffusion barriers of 0.03–0.15eV. Although the O2 adsorption on Cu2O–Au/Au3Cu(111) isstrong enough, it still cannot be directly dissociated because thecorresponding dissociation barrier is as high as 1.04 eV. WhenCO diffuses to the Au site next to the Cu2O–Au interfaces, it stillhas no electronic interaction with O2. This co-adsorption statecan be recognized as the IS of CO oxidation reactions, which isshown in Fig. 7(a). Subsequently, CO moves to the neighboringAu site of O2, which only requires a small reaction barrier of Erb¼ 0.35 eV to be overcome. In Fig. 7(c), when CO binds with theAu site near interfaces, it leans towards to the O2 molecule. Inthis meta-stable intermediate state (MS), the co-adsorptionenergy of CO and O2 reaches �1.17 eV, which is lower thanthat of IS of Ead ¼ �1.02 eV. Therefore, there is already weakinteraction between CO and O2 in the MS. CO and O2 then movetogether to form themeta-stable intermediate product O–O–CO,through which the O–O bond can be activated and easilydissociated.2 The reaction barrier of this combination process isonly Erb ¼ 0.35 eV. In Fig. 7(e), Ead has reduced to�1.73 eV aerthe formation of O–O–CO, which means that a strong bond isgenerated between CO and O2. In this intermediate state, theO–O bond has been activated, which is benecial for furtherreactions. In Fig. 7(e and f), the transformation of O–O–CO toCO2 and O is easy with Erb ¼ 0.07 eV. Aer the CO2 productmolecule desorbs, the remaining O atom is adsorbed on thestable AuCu2 hcp hollow. When another CO molecule diffuses

Fig. 7 Illustration of (a)–(g) CO + O2 reaction and (h)–(j) CO + O reacti

1594 | RSC Adv., 2015, 5, 1587–1597

to a neighboring Au site, the molecules can combine to generateCO2 with a moderate reaction barrier of Erb ¼ 0.42 eV, as shownin Fig. 7(h–j). In summary, CO oxidation reactions can be easilycarried out on Cu2O–Au interfaces at Au3Cu surfaces.

Cu2O–Au/Au3Cu(111) can only represent Cu2O–Au interfaceslocated on facets of Au–Cu nanoparticles. In addition to facets,there are still some surface atoms on the edges and vertices ofAu–Cu binary particles24 and Cu2O–Au interfaces may passthrough them. It has been demonstrated that these under-coordinated nanoparticle sites can accelerate heterogeneouscatalysis.38 On Au–Cu binary nanoparticles, the CO and O2

molecules adsorbed on edges and vertices should have lowerEad values than those adsorbed on facets. Via this enhancedreactant adsorption, O2 activation and further CO oxidationmay become easier. In future work, we intend to consider COoxidation on Au–Cu clusters with different surface oxidationstates, which can represent the surfaces of these binary nano-particles more realistically.

CT of O2 adsorption and PDOS analysis of CO oxidation onCu2O–Au/Au3Cu(111)

An important premise for CO oxidation on Cu2O/Au3Cu(111) isthat O2 prefers to be located at the Cu sites near Cu2O–Auinterfaces rather than those inside the Cu2O layers. To explainthis preference for reactant adsorption, the CT and bond lengthof O2 adsorption at the most stable positions of Cu2O/Au3Cu(111) and Cu2O–Au/Au3Cu(111) are analyzed, which isshown in Fig. 8. When adsorption bonds are generated betweenO2 molecules and the underlying Cu2O surfaces, charges aretransferred from the substrates to the reactants and theCoulomb forces between them would determine the bond

on on Cu2O–Au/Au3Cu(111) surfaces.

This journal is © The Royal Society of Chemistry 2015

Paper RSC Advances

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

strength.39 The inter-atomic Coulomb forces are proportional tothe absolute value of the product of the positive and negativecharges of the atoms and inversely proportional to the bondlength. In Fig. 8(a and b), the Cu sites of the Cu2O surfaces areall positively charged without reactant adsorption. At the inte-rior of the Cu2O surfaces, the CT values of the CuUNC and CuCSAsites are 0.06 and 0.18, respectively. For the correspondinginterfacial CuUNC and CuCSA sites, the CT values are almostunchanged. The CT value of the CuNCUS sites decreases to 0.09compared with that of CuCSA sites, which is caused by thecoordinate oxygen unsaturation of the CuNCUS sites. In Fig. 8(cand d), aer O2 is adsorbed on both Cu2O/Au3Cu(111) andCu2O–Au/Au3Cu(111), the average CT values of reactants are�0.16 and �0.18, respectively. O2 can obtain more charge fromthe Cu sites near Cu2O–Au interfaces. Nevertheless, the sum ofthe charge product of the three Cu–O adsorption bonds issimilar for O2 adsorption on Cu2O/Au3Cu(111) and Cu2O–Au/Au3Cu(111). In contrast, the sum of the bond length of the threeCu–O adsorption bonds calculated is 6.24 A for O2 adsorptionon Cu2O/Au3Cu(111) and 6.17 A for O2 adsorption on Cu2O–Au/Au3Cu(111). The difference comes from the CuNCUS–O bond,which has a bond length of only 1.96 A. When O2 is adsorbed onthe interfacial Cu3 fcc hollow in the t-CuUNC and t-CuCSAstates, the bond lengths of the CuNCUS–O bond are 2.06 and 2.09A, respectively. This difference in bond length proves ourprevious assumption that the interfacial CuNCUS sites couldoffer enhanced adsorption for O2 in the t-CuNCUS state.Therefore, Cu–O adsorption bonds near the Cu2O–Au interfaceshave similar absolute values of the charge product but shorteradsorption bond lengths compared with those at the interior ofCu2O surfaces, which causes their large Coulomb forces and lowadsorption energy values.

Fig. 8 CT of Cu sites on (a) Cu2O/Au3Cu(111) surfaces and (b) Cu2O–Au/Abond length of Cu–O adsorption bonds when O2 is stably adsorbed on

This journal is © The Royal Society of Chemistry 2015

The increase in charge transfer can only ensure the prefer-ence of O2 adsorption near Cu2O–Au interfaces. The O2 activa-tion and eventual dissociation requires the supplementary roleof CO. The O–O bond of O2 is generated via extensive orbitalhybridization of the 2p electrons. When CO reacts with O2, theorbital hybridization of the O–O bond may decrease and even-tually disappear. To clarify the effect of CO assistance on the O2

activation and dissociation, the PDOS changes in the 2p elec-trons of the reactants during the CO oxidation on Cu2O–Au/Au3Cu(111) are analyzed, which is shown in Fig. 9. First, 2pelectrons of C and O, 3d electrons of Cu, and 5d electrons of Auin all slabs have no spin polarization. In Fig. 9(a), the 2phybridization peak of CO lies at a lower energy level than that ofO2 in IS because the C–O bond is stronger than the O–O bond.In the rst transitional state (TS1), CO has been desorbed fromthe Au site, which induces the movement of its hybridizationpeak to a higher energy level. In Fig. 9(c), the hybridization peakof CO is substantially broadened and weakened in the rstmeta-stable intermediate state (MS1). At the lower energy end ofthe valence bands, PDOS of CO and O2 are both slightlyincreased, which proves the weak interaction of the reactants.Under the inuence of CO, the hybridization peak of O2 alsodecreases. In the second transitional state (TS2), the originalhybridization peak of CO almost disappears. However, thesimultaneous increase in PDOS of CO and O2 becomes morepronounced. In Fig. 9(e), large hybridization peaks areproduced between the 2p electrons of CO and O2 because of theformation of O–O–CO. Before the reaction reaches the meta-stable intermediate state (MS2), the PDOS shapes of the two Oatoms belonging to the O2 molecule are similar. In MS2, PDOSof the O atom connecting the Cu2O substrates is signicantlyreduced at the main hybridization peak (energy level of�6.70 eV).

u3Cu(111) surfaces. CT of O2molecules and its connecting Cu sites and(c) Cu2O/Au3Cu(111) surfaces and (d) Cu2O–Au/Au3Cu(111) surfaces.

RSC Adv., 2015, 5, 1587–1597 | 1595

Fig. 9 PDOS of CO and O2 during their reaction on Cu2O–Au/Au3Cu(111) surfaces. (a)–(g) correspond to the reaction states shown inFig. 7(a–g). The black solid and red short dashed lines represent the Cand O atoms in CO, respectively. The blue short dashed and dottedline represents the O atom in O2, which would form the C–O bondwith CO. The green and dashed dotted line represents the O atom inO2, which would remain on substrates after the production of CO2.

RSC Advances Paper

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

In the meta-stable intermediate product, O–O–CO, the elec-tronic hybridization of this O atom only occurs for the O–Obond. Therefore, the reduced participation of its 2p electrons inhybridization indicates that the O–O bond in O–O–CO is weakercompared with that of the original O2 molecule. In the thirdtransitional state (TS3), PDOS of this O atom is decreasedfurther at the main hybridization peak, which indicates that theO–O bond is almost broken. In Fig. 9(g), the extensive hybrid-ization of the 2p electrons only happens among the other threeC and O atoms, which conrms the complete dissociation of O2

and formation of CO2 in the nal state (FS).

Discussion of the reaction mechanisms of CO oxidation onAu–Cu nanoparticles

The reaction mechanism of CO oxidation on Au–Cu nano-particles can be explained based on our simulation results. Incontrast to the Cu alloy, O2 adsorption and CO oxidation on Aunanoparticles only occurs at vertex sites.40 However, these

1596 | RSC Adv., 2015, 5, 1587–1597

under-coordinated sites occupy a very small proportion of thenanoparticle surface,25,26 which reduces the probability of O2

adsorption and further reactions with CO. Oxidation resistanceexperiments indicate that at 110 �C, the transformation of Cunanoparticles to Cu2O and CuO aggregates only requires severaltens of minutes.40 Inert CuO surfaces cannot provide effectiveadsorption for reactants. CO oxidation cannot be carried out onCu2O surfaces because their active CuCUS sites are far apart. Theoxidation rates of Au–Cu alloy nanoparticles are muchlower than those of pure Cu ones.41 To catalyze CO oxidation onAu–Cu nanoparticles, CO molecules are rst adsorbed on theirsurface Au sites. Owing to the introduction of O2, some Cuatoms of the particles are oxidized to form numerous local Cu2Osurface oxide layers. Simultaneously, coherent interfaces aregenerated between Cu2O and the unoxidized Au layers. O2

activation and CO oxidation can occur on these Cu2O–Auinterfaces. For traditional nanogold catalysts, interfacesbetween Au and oxide substrates are used for O2 activation andCO oxidation. However, for these Au–Cu binary nanoparticles,CO and O2 can directly react on surfaces, and this does notdepend on the substrates. Considering the catalyst design forCO oxidation reactions, O2 cannot be activated and dissociatedon pure Au and Pt nanoparticles. In contrast, Cu, Rh, Ni, Ru,and Co nanoparticles are easily oxidized and lose their surfaceactive sites for O2 adsorption. Via alloying techniques, binarynanoparticles can avoid being completely oxidized and offeractive sites for O2 adsorption and further CO oxidation.

Conclusions

OnAuCu(111), CO is stably adsorbed on the top of the Cu site andO2 prefers to be located at the Cu2 edge in the tilted-tbt state. TheO2 dissociation barrier on the unoxidized AuCu alloy surface is ashigh as 0.74 eV. Because CO and O2 are adsorbed on the separateCu sites, they cannot readily combine to generate CO2 onAuCu(111). On Cu2O/Au3Cu(111), CO is strongly adsorbed on thetop of the CuUNC site and O2 prefers to be located at the Cu3 fcchollow in the t-CuUNC top state. CO cannot react with the latticeoxygen of the Cu2O surface oxides. The direct O2 dissociation onoxidized Au3Cu alloy surfaces is difficult because of the highdissociation barrier value of 0.90 eV. The reaction of CO + O2 onCu2O/Au3Cu(111) is impossible as the reactants cannot movetogether in adsorption states. On Cu2O–Au/Au3Cu(111), O2

prefers to be located near Cu2O–Au interfaces rather than insidethe Cu2O surfaces. CO can be adsorbed on Au surfaces, moder-ately. When CO diffuses to the Au site near Cu2O–Au interfaces, itcan easily react with O2 due to the small reaction barriers of 0.07–0.35 eV. Aer this reaction, the remaining O atom on the AuCu2hcp hollow can also react with CO on the neighboring Au site andthe corresponding reaction barrier is 0.42 eV. TheHirshfeld population and bond length analysis indicate that O2

near Cu2O–Au interfaces can obtain more charge, transferredfrom the substrates and shorter Cu–O adsorption bonds, whichcauses its lower adsorption energy value. In addition, PDOSresults show that the electronic hybridization of the O–O bond inO2 weakens and eventually disappears aer it combines with COto form O–O–CO and CO2 + O.

This journal is © The Royal Society of Chemistry 2015

Paper RSC Advances

Publ

ishe

d on

19

Nov

embe

r 20

14. D

ownl

oade

d by

Jili

n U

nive

rsity

on

03/1

2/20

14 1

3:35

:48.

View Article Online

Acknowledgements

We acknowledge support by the National Key Basic Researchand Development Program (Grant no. 2010CB631001), and theHigh Performance Computing Center (HPCC) of Jilin Universityfor supercomputer time.

Notes and references

1 M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem.Lett., 1987, 2, 405–408.

2 Z. P. Liu, P. Hu and A. Alavi, J. Am. Chem. Soc., 2002, 124,14770–14779.

3 Y. Xu and M. Mavrikakis, J. Phys. Chem. B, 2003, 107, 9298–9307.

4 I. X. Green, W. Tang, M. Neurock and J. T. Yates Jr, Science,2011, 333, 736–739.

5 D. Widmann and R. J. Behm, Angew. Chem., Int. Ed., 2011, 50,10241–10245.

6 S. D. Lin, M. Bollinger and M. A. Vannice, Catal. Lett., 1993,17, 245–262.

7 X. Liu, A. Wang, X. Wang, C. Y. Mou and T. Zhang, Chem.Commun., 2008, 27, 3187–3189.

8 M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura,M. Azuma and M. Hartua, Catal. Lett., 1998, 51, 53–58.

9 X. Liu, A. Wang, L. Li, T. Zhang, C. Y. Mou and J. F. Lee, J.Catal., 2011, 278, 288–296.

10 J. C. Bauer, D. Mullins, M. Li, Z. Wu, E. A. Payzant,S. H. Overbury and S. Dai, Phys. Chem. Chem. Phys., 2011,13, 2571–2581.

11 K. Koga and D. Zubia, J. Phys. Chem. C, 2008, 112, 2079–2085.12 C. H. Kuo, T. E. Hua and M. H. Huang, J. Am. Chem. Soc.,

2009, 131, 17871–17878.13 J. Yin, S. Shan, L. Yang, D. Mott, O. Malis, V. Petkov, F. Cai,

M. S. Ng, J. Luo, B. H. Chen, M. Engelhard and C. J. Zhong,Chem. Mater., 2012, 24, 4662–4674.

14 A. Wang, X. Y. Liu, C. Y. Mou and T. Zhang, J. Catal., 2013,308, 258–271.

15 P. Hohenberg and W. Kohn, Phys. Rev., 1964, 136, B864–B871.

16 W. Kohn and L. J. Sham, Phys. Rev., 1965, 140, A1133–A1138.17 B. Delley, J. Chem. Phys., 1990, 92, 508–517.18 B. Delley, J. Chem. Phys., 2000, 113, 7756–7764.

This journal is © The Royal Society of Chemistry 2015

19 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,1996, 77, 3865–3868.

20 B. Delley, Phys. Rev. B: Condens. Matter Mater. Phys., 2002, 66,155125.

21 T. A. Halgren and W. N. Lipscomb, Chem. Phys. Lett., 1977,49, 225–232.

22 F. L. Hirshfeld, Theor. Chim. Acta, 1977, 44, 129–138.23 B. Pauwels, G. Van Tendeloo, E. Zhurkin, M. Hou,

G. Verschoren, L. Theil Kuhn, W. Bouwen and P. Lievens,Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 63, 165406.

24 T. J. Toai, G. Rossi and R. Ferrando, Faraday Discuss., 2008,138, 49–58.

25 L. Deng, W. Hu, H. Deng, S. Xiao and J. Tang, J. Phys. Chem.C, 2011, 115, 11355–11363.

26 F. Baletto and R. Ferrando, Rev. Mod. Phys., 2005, 77, 371–417.

27 R. Zhang, H. Liu, H. Zheng, L. Ling, Z. Li and B. Wang, Appl.Surf. Sci., 2011, 257, 4787–4794.

28 A. Onsten, M. Gothelid and U. O. Karlsson, Surf. Sci., 2009,603, 257–264.

29 C. I. Carlisle, D. A. King, M. L. Bocquet, J. Cerda andP. Sautet, Phys. Rev. Lett., 2000, 84, 3899–3902.

30 Y. Xu and M. Mavrikakis, Surf. Sci., 2001, 494, 131–144.31 A. Soon, M. Todorova, B. Delley and C. Stamp, Phys. Rev. B:

Condens. Matter Mater. Phys., 2006, 73, 165424.32 Q. Jiang, H. M. Lu and M. Zhao, J. Phys.: Condens. Matter,

2004, 16, 521–530.33 Y. Xu and M. Mavrikakis, J. Am. Chem. Soc., 2004, 126, 4717–

4725.34 H. J. Kang, R. Shimizu and T. Okutani, Surf. Sci., 1982, 116,

L173–L178.35 A. Hussain, J. Phys. Chem. C, 2013, 117, 5084–5094.36 H. Y. Kim, H. M. Lee and G. Henkelman, J. Am. Chem. Soc.,

2011, 134, 1560–1570.37 B. Z. Sun, W. K. Chen and Y. J. Xu, J. Chem. Phys., 2010, 133,

154502.38 I. V. Yudanov, A. V. Matveev, K. M. Neyman and N. Rosch, J.

Am. Chem. Soc., 2008, 130, 9342–9352.39 B. Hammer and J. K. Norskov, Adv. Catal., 2000, 45, 71–129.40 Y. Gao, N. Shao, Y. Pei, Z. Chen and X. C. Zeng, ACS Nano,

2011, 5, 7818–7829.41 Z. Xu, E. Lai, Y. Shao-Horn and K. Hamad-Schifferli, Chem.

Commun., 2012, 48, 5626–5628.

RSC Adv., 2015, 5, 1587–1597 | 1597