Surface Science 548 (2004) 209–219

www.elsevier.com/locate/susc

Electric field effects in the chemisorption of CO on bimetallicRhCu surface models

Silvia Gonz�aalez a,b, Carmen Sousa a, Francesc Illas a,*

a Departament de Qu�ıımica F�ıısica i Centre Especial de Recerca en Qu�ıımica Te�oorica, Universitat de Barcelona i Parc Cient�ııfic

de Barcelona, C/ Mart�ıı i Franqu�ees 1, 08028 Barcelona, Spainb Departamento de Qu�ıımica, Universidad Aut�oonoma Metropolitana-Iztapalapa, P.O. Box 55-534, C.P. 09340 M�eexico D.F., Mexico

Received 13 August 2003; accepted for publication 7 November 2003

Abstract

Cluster models and hybrid density functional theory calculations are used to study the effect of an external electric

field on the properties and bonding mechanism of CO on various RhCu bimetallic surfaces. The presence of an external

electric field induces changes in the geometry and vibrational frequency of the chemisorbed molecule. Several methods

of analysis establish that the electric field effects are not just of electrostatic nature but do affect the bonding mecha-

nisms as well. In any case, the electric field effects observed for various RhCu bimetallic surfaces are not at all different

from what is known for monometallic surfaces. The present study permits to conclude that the effect of an external

electric field on the properties of CO adsorbed on bimetallic RhCu surfaces of different composition follows the same

trend exhibited by the single metal surfaces.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Metallic surfaces; Alloys; Surface electronic phenomena (work function, surface potential, surface states, etc.); Density

functional calculations; Carbon monoxide; Chemisorption

1. Introduction

Promotion is one of the fundamental issues in

heterogeneous catalysis [1,2] because it permits one

to improve the catalytic properties of a given

material with a minimal manipulation. Promotion

may be caused by the introduction of a chemical

additive (chemical promotion) or by the applica-

tion of an external electric potential (electro-

chemical promotion). While chemical promotion

* Corresponding author. Tel.: +34-934021229; fax: +34-

934021231.

E-mail address: francesc.illas@ub.edu (F. Illas).

0039-6028/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2003.11.009

has been long known and used, electrochemical

promotion (or non-faradaic electrochemical mod-ification of catalytic activity; NEMCA effect) was

discovered only twenty years ago [3,4]. The interest

in this phenomenon has been continuously grow-

ing because of its unique way to allow modifying

the activity and selectivity of the catalytic system

in a controlled manner [5–8]. Electrochemical

promoted processes do not only exhibit very large

increases in reaction rate with respect to the un-promoted catalytic processes; significant improve-

ments in the selectivity have also been observed [4].

The easiness to control the electric potential in

electrochemically promoted systems contrasts with

the difficulty to control the rather large number of

ed.

210 S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219

variables that play a role in any electrochemical

process. In fact, electrochemical systems are

characterized by the presence of many simulta-

neous effects such as solvent, impurities, adsorbed

species, competing reactions, etc. Therefore, for a

better understanding of electrochemical promo-tion but also of the complex electrochemical

environment, it is desirable to identify and sepa-

rate contributions arising from each different

effect. To this end, many authors have used theo-

retical models to analyze the role played by an

external electric field in the chemisorption prop-

erties of various adsorbates on metallic surfaces

[9–24]. Bagus et al. [9–11] concluded that thechanges in the vibrational frequencies of simple

molecules like CO and CN� adsorbed on Cu(1 0 0)

induced by the presence of an external electric field

were due to a vibrational Stark effect; this is

without any significant change in the bonding

mechanism of these molecules to the surface. This

conclusion was questioned by Head-Gordon and

Tully [12] who suggested that electrostatic andchemical effects could not be rigorously separated.

Later on, several analysis have shown that, in fact,

the presence of an external electric field induces

noticeable chemical changes in the chemisorption

bond [13,14,20,21,23,24]. In particular, it has been

shown that the electric field provokes changes in

the population 2p� molecular orbital of the ad-

sorbed CO molecule and, hence, in the importanceof the metal to CO backdonation bonding mech-

anism [13,14]. Lambert et al. have used this argu-

ment to explain the changes in the selectivity of

some reactions on chemical promoted metallic

surfaces in terms of changes in the electronic

density in the molecular orbital caused by an

internal electric field [25].

Up to now, almost all studies regarding the ef-fect of an external electric field on the properties

of adsorbed molecules on metals concern mono-

metallic surfaces. However, bimetallic systems are

increasingly used and are preferred as catalysts

in many industrial chemical reactions because,

compared to each separate component, the com-

bination of two metals with different catalytic pro-

perties results in a better catalytic performance[26–30]. Bimetallic RhCu constitutes a paradig-

matic example which has been studied as early as

1975 [31] and experienced renewed interest in the

past few years [32–42]. Recently, chemisorption of

CO on RhCu(1 1 1) surface models has been theo-

retically studied [41]. In particular, it has been

shown that from the study of the geometry and

vibrational frequency of the chemisorbed mole-cule it is not possible to obtain information about

the alloy composition. On the other hand, it has

also been shown that the vibrational frequency

of chemisorbed CO permits to assign the chemi-

sorption site in a rather univocal way.

In the present work, we continue the previous

study about bimetallic RhCu by analyzing the ef-

fect of a uniform external electric field on theproperties of CO chemisorbed on various bime-

tallic surfaces of different Rh–Cu composition.

The motivation to carry out such study comes

from the fact that, upon alloying, large effects on

the electronic structure of the components are

observed by photoemission and X-ray Absorption

Near Edge Structure [43,44]. Changes in the elec-

tronic structure of the alloy have also been ob-served by Rodriguez and Goodman [29] who

found a linear relationship between the core level

shift of the metals and the adsorption energy of

CO. The formation of the heterometallic bond and

the concomitant changes in the electronic structure

may result or not in a different response of the

bimetallic surface to the presence of an external

electric field. Therefore, the main goal of thepresent paper is to investigate the effect of an

external electric field on a bimetallic system and to

see whether the changes induced by such electric

field are similar to those encountered for mono-

metallic surfaces or if, on the contrary, there are

significant differences in the properties of the ad-

sorbed molecule. To this end, cluster models are

employed to study the effect of an external electricfield in the interaction of CO with RhCu and we

make use of density functional theory (DFT) and

of the DFT implementation [45] of the constrained

space orbital variation (CSOV) technique [46–48]

to compute the interaction energy and to dissect it

into its various physically meaningful contribu-

tions. An important conclusion of this work is that

the effect of an external electric field on thechemisorption properties does not depend on the

composition of the alloy since, for each compo-

S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219 211

sition, the electric field modifies chemisorption

properties in a similar way.

2. Surface cluster models and computational details

In this paper different cluster models have been

used to simulate mono (Rh and Cu) and RhCu

bimetallic (1 1 1) surfaces. The employed models

are the same used in previous studies about

molecular hydrogen dissociation [40] and CO

chemisorption on these bimetallic systems [41].

The Rh10, Rh9Cu, Rh4Cu6, RhCu9 and Cu10

cluster models contain seven atoms in the firstlayer and three in the second one in such a way

that the central atom in the upper layer has the

coordination of a surface atom in a (1 1 1) surface.

Rh10 and Cu10 model the corresponding (1 1 1)

surfaces and have been used as reference to com-

pare the activity of the bimetallic systems towards

CO adsorption. Bimetallic Rh9Cu and RhCu9

models represent the Rh- or Cu-rich phases pre-dicted by the RhCu phase diagram [49] whereas

the Rh4Cu6 cluster represents a mixed composition

and has been included mainly for comparison

purposes. The inter-atomic distances in the clusters

considered have been kept fixed. For the mono-

metallic models, the cluster geometries have been

fixed as in Rh or Cu bulk [50] with dRhRh ¼ 2:69 �AAand dCuCu ¼ 2:56 �AA, whereas in the bimetalliccompounds dRhCu ¼ 2:59 �AA is a weighted interpo-

lated value between the two first distances. Notice

that these distances are similar to those experi-

mentally measured by extended X-ray absorption

fine structure (EXAFS) for RhCu clusters of �14�AA (dRhRh ¼ 2:68 �AA, dCuCu ¼ 2:62 �AA and dRhCu ¼2:64 �AA) [51]. Additional details about these cluster

models can be found in previous works [40,41]. Inall cases, the CO molecule adsorbs perpendicular

to the surface with the C atom interacting directly

with the surface metal atom located at the centre

of the cluster model. Hence, for Rh10, Rh4Cu6 and

RhCu9 the CO is adsorbed on top of a Rh atom

while for Cu10 and Rh9Cu the CO molecule

interacts directly with a Cu atom.

To study the effect of an external electric fieldeffect in the CO chemisorption on the above de-

scribed surface cluster models a uniform electric

field has been included in the Hamiltonian on the

system. Calculations including the presence of this

uniform electric field have been carried out using a

first principles density functional theory based

method. In particular, we use the B3LYP hybrid

functional, which combines the gradient correctedBecke�s three parameters hybrid exchange func-

tional [52] with the correlation functional of Lee,

Yang and Parr [53], as implemented in the HON-

DO99 [54] version of HONDO95.3 [55] and in the

Gaussian98 [56] computational packages. The

cluster metal atoms were described by relativistic

small-core effective core potentials and double-fbasis sets reported by Hay and Wadt [57]. Forthe CO molecule we use the Alhrichs TZV basis

augmented with d functions [58]. Calculations

including explicitly a uniform electric field have

been carried out to obtain the equilibrium geo-

metry of the adsorbed molecule, the internal CO

vibrational frequency, the interaction energy with

respect to the CO and M10 separated systems,

and the occupation of the CO(2p�) and CO(5r)molecular orbitals as function of the intensity of a

uniform external electric field. For each electric

field intensity, the interaction energy is decom-

posed into Pauli repulsion, CO or metal cluster

intra-unit polarization and inter-unit donation,

the usual various contributions to chemisorption

bond as defined by Bagus et al. [46–48]. The inter-

unit donation arises almost exclusively from theCO(5r) molecular orbital to the metal surface and

the backdonation from the d atomic orbitals of the

cluster metal atom directly interacting with CO to

the CO(2p�) molecular orbital.

Chemisorption geometry and internal CO

stretching frequency have been obtained by means

of the Gaussian98 computational package; the

interaction energy and CSOV analysis was carriedout using HONDO99; finally, the extent of back-

donation from metal d orbitals to the CO(2p�)

orbital has been calculated by means of the orbital

projection technique introduced by Nelin et al. [59]

using Kohn–Sham orbitals. In all cases, the lowest

singlet closed-shell electronic state has been con-

sidered; this neglects possible low-lying open shell

states which are likely to arise from cluster arti-facts. On the other hand, it has been shown that

low-lying electronic states of small metal clusters

212 S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219

are very close in energy and exhibit a similar

chemistry [60]. Electric field intensities of F ¼�0:01, )0.005, +0.005 and +0.01 a.u. (0.01 a.u.¼5.14 · 10�7 V/cm) have been considered because

these are of the order of the electric fields which

are present in the electrochemical environment.The use of larger values for the intensity of the

electric field does not bring any additional effect in

the stretching frequency of free and adsorbed CO

[19]. The electric field is oriented perpendicular to

the surface in the C–O molecular axis in such a

way that negative values pull electrons out of the

surface towards the vacuum.

3. Results and discussion

The calculation of the vibrational frequency of

a free CO molecule in absence or presence of an

external electric field permits a direct comparison

with experiment and hence provides a check on the

consistency of the present computational model.The computed value of the harmonic vibrational

frequency for free CO is 2219 and 51 cm�1 larger

than the experimental one which is 2170 cm�1 [61].

This difference can be corrected including points in

the anharmonic region of the potential energy

curve which is next fitted to a third degree poly-

nomial to extract the harmonic frequency. The

stretching frequency computed in this way, 2195

Table 1

Calculated values for the equilibrium perpendicular distance of CO to

vibrational frequency (mCO) and the adsorption energy (Eint) correspon

models. The adsorption energy is defined in a way that a positive val

Adsorption on top of Rh

Rh10 Rh4Cu6

dM–C (�AA) 1.841 (1.84a) 1.880

dCO (�AA) 1.147 (1.15a) 1.153

mCO (cm�1) 2059 (2015c) 2009

Eint (kcalmol�1) 28.1 (39e) 42.48

Experimental data, only available for CO on Rh(1 1 1) and on Cu(1 0aRef. [64].bRef. [65].cRef. [66].dRef. [67].eRef. [70].f Ref. [71].

cm�1 is closer to the experimental one and the

anharmonic constant is 11.02 cm�1 is also in good

agreement with the experimental value of 13.46

cm�1 [61]. As it is well known, the presence of an

external electric field induces changes in the

stretching frequency and this change is usuallyreferred to as vibrational Stark effect. It is char-

acterized by the so-called Stark tuning rate value

(STR), the first derivate of the molecular stretch-

ing frequency with respect to the intensity of the

external electric field (dm=dF ). The present calcu-

lated STR is 4.95 · 10�7 cm�1/V/cm, which is in a

rather good agreement with the value for the free

CO molecule reported by Lambert [62] obtainedusing a semiclassical model and some spectro-

scopic data (4.29 · 10�7 cm�1/V/cm) and is also

close to the more direct experimental measured

value reported later also by Lambert which is of

5.27 ± 0.27 · 10�7 cm�1/V/cm [63].

Before starting the discussion about the electric

field effects on CO chemisorption, let us briefly

consider the geometrical parameters, CO vibra-tional frequency and adsorption energy for ad-

sorbed CO without external electric field reported

in Table 1. These are very close to those previously

reported and the only difference comes from the

basis set of CO; Ahlrichs TZV plus polarization

in the present work compared to the standard

6-31G** basis used in Ref. [41]. Therefore, the

discussion corresponding to the values in absence

the surface (dM–C), the C–O inter-atomic distance (dCO); the COding to the interaction of CO with mono and bimetallic surface

ue indicates an exothermic process

Adsorption on top of Cu

RhCu9 Rh9Cu Cu10

1.946 2.037 2.201 (1.9b)

1.148 1.136 1.131 (1.13b)

2032 2117 2154 (2078d)

11.8 3.1 )3.3 (10f )

0), are given in parenthesis.

S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219 213

of electric field will be omitted and we would like

just to point out that geometries and vibrational

frequencies are in good agreement with available

experimental data [64–67] which have been in-

cluded in Table 1 for comparison. Calculated

chemisorption energies for CO on Rh(1 1 1) sur-faces are comparable to the values reported from

temperature programmed desorption (TPD)

[68,69] and time-resolved electron energy loss

spectroscopy (TREELS) [70] although somewhat

smaller. Interestingly enough, TPD measurements

for CO on Cu(1 1 1) suggest that the adsorption

energy is �30 kcalmol�1 smaller than the one

corresponding to the Rh(1 1 1) surface [71]. Thistrend is also properly predicted by the present

cluster B3LYP calculations. Finally, notice that a

discussion concerning the variation of the CO

adsorption energy with respect to the composition

of the bimetallic RhCu alloy has been reported

also in [41]. In particular, the non-monotonic

trend in this quantity has been explained by the

difference on charge transfer mechanisms and onthe Pauli repulsion due to changes in electronic

charge density caused by heterometallic bond.

The changes induced by the external electric

field will be studied by looking at the effect pro-

duced on the C–O distance (dCO), stretching fre-

quency CO (mCO) and occupation of the 2p�

molecular orbital of the adsorbed molecule. The

adsorption energy on each metallic or bimetallicsurface will also be discussed although we can

anticipate that, compared to the properties men-

tioned above; the changes induced in the adsorp-

tion energies are much smaller. Before starting the

discussion it is convenient to remember that a

negative sign of electric field is such that the elec-

tric field pulls electrons out of the surface toward

the vacuum. In case the electric field effects forbimetallic surfaces follow the trend observed for

single metal surfaces, one would expect that in

going from negative to positive electric fields the

inter-atomic distance of CO decreases because the

backdonation contribution will be favored for

negative fields. In the same way, one expects that

the larger the contribution from backdonation the

larger the red shift of the vibrational frequencyand, consequently, the smaller the absolute value

of the vibrational frequency of adsorbed CO.

Finally, the larger backdonation contribution for

negative fields must be accompanied by a higher

occupation of the CO(2p�) molecular orbital and

hence this value must decrease in going from neg-

ative to positive values of the electric field. Fig. 1

reports the variation of dCO, mCO and the occupa-tion of the CO(2p�) molecular orbital with respect

to the electric field. From this figure one does not

see any differential behavior between monometal-

lic and bimetallic model surfaces considered in

this work, or between the different composition of

the RhCu alloys. Moreover, the trend followed

by the three properties is precisely as anticipated in

the previous discussion and the variation of dC–O,mC–O and the occupation of the CO(2p�) molecular

orbital with respect to the electric field intensity is

almost linear. In fact, a linear relationship for dCOhas also been reported by other authors in study-

ing the electric field effects on single metal surfaces

[13,14,21,23]; the linear trend of mCO with respect

to the electric field intensity agrees with reported

results for CO on several metal surfaces [9,10,13,14,21,23,72] and, finally, the occupation of the

CO(2p�) molecular orbital follows the trend pre-

viously found for CO on Cu(1 0 0) [9] and Pt(1 1 1)

[13,14]. Notice that the change in the CO(2p�)

occupation indicates that a external uniform elec-

tric field produces chemical changes. This is in

agreement with the previous work of several au-

thors for CO on single metal surfaces [13,14,21–24] and is in contrast with the original interpre-

tation of Bagus et al. [9] who suggested that

the electric field effect did not affect the bonding

mechanism. Another important point concerns the

slope associated to this linear dependence of dCO,mCO and the occupation of the CO(2p�) molecular

orbital which is similar for all cases implying that

the differences with composition found in absenceof electric field [41] are also preserved, at least for

the electric field intensity range studied in the

present work. The slope of mCO versus the external

electric field intensity merits an additional com-

ment. The values obtained for the Rh10, Rh4Cu6,

RhCu9, Rh9Cu and Cu10 are 15.00, 14.23, 14.62,

15.39 and 15.39 in units of 10�7 cm�1/V/cm. The

close similarity of these values seems to suggestthat the effect of the electric field on the interac-

tion of CO with these metal surfaces is mainly of

Fig. 1. Variation of the structural properties of the CO molecule adsorbed on the different surface clusters models with respect to the

intensity of a uniform external electric field. (a) CO equilibrium distance, (b) CO adsorbed vibrational frequency, (c) occupation of the

2p� molecular orbital as derived from the projection operator technique.

214 S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219

electrostatic origin. However, the analysis of the

population of the 2p� molecular orbital showsthat, in all cases, there is a small but noticeable

effect in the chemical bond. In any case, from the

preceding discussion one must conclude that the

variations in the chemisorption properties with

respect an electric field follow the same trend in

single metal or bimetallic surfaces. This is the main

conclusion of the present work and will be rein-

forced by the discussion of the interaction energywhich is given below.

The interaction (or adsorption) energy, Eint, is

defined as the negative of the value obtained by

subtracting the energy of the fragments from the

energy of the supersystem at the equilibrium

geometry, this is

S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219 215

EintðF Þ ¼ �fEF ðCO–RhmCunÞ� ½EðCOÞ þ EðRhmCunÞ�g; ð1Þ

where EF indicates that the energy of the super-

system has been computed in presence of an

external field of intensity F . The changes in the

total interaction energy, Eint, induced by the elec-

tric field will be given by

DEintðF Þ ¼ EintðF Þ � Eintð0Þ: ð2Þ

These changes are similar for the cluster where

CO interacts directly with Rh or Cu. Accordingly,

a detailed discussion will be given for the interac-

tion directly above Rh only. Fig. 2 (dashed lines)

shows that for these clusters the changes in Eint are

small but significant; DEint varies roughly by 10

kcalmol�1 which for RhCu9 is of the order of Eint

in absence of the external field; notice that the

form of the curves in Fig. 2 is such that Eint is al-

ways enhanced by the presence of the electric field

as expected. Part of the stabilization induced by

the external field arises from purely electrostatic

contributions and another part is due to chemical

changes induced by the electric field. To support

this statement Fig. 2 (solid lines) also reports thevalues of the adsorption energy computed with

respect to the fragments, but in this case the energy

of the fragments is also obtained in presence of the

external electric field. Hence,

Ein

t(k

cal

mol

-1)

15

25

35

45

55

-0.01 -0.005 0

5

15

25

35

-0.01 -0.005 0

Fig. 2. Variation of the interaction energy between CO and the Rh

intensity of a uniform external electric field and referred to the energy

field (dashed lines) and referred to the energy of the fragments compu

system geometry is the equilibrium configuration corresponding to ea

E0intðF Þ ¼ �fEF ðCO–RhmCunÞ

� ½EF ðCOÞ þ EF ðRhmCunÞ�g: ð3Þ

and

DE0intðF Þ ¼ E0

intðF Þ � Eintð0Þ: ð4Þ

The variation of E0int, DE

0intðF Þ defined in Eq. (4), is

considerably smaller than that of DEint, thus indi-

cating that a large part of the changes produced by

the external electric field have an electrostatic

origin. For the cluster models where CO interacts

directly with a surface Rh atom DE0intðF Þ ranges

from 2.2 kcalmol�1 for Rh10 to 2.9 kcalmol�1 for

RhCu9. These values provide a rough measure of

the extent of chemical changes induced by theelectric field. It is important to notice that the

observed effect in the single metal Rh10 metal

cluster is also observed for the two bimetallic,

Rh4Cu6 and RhCu9, cluster models thus reinforc-

ing the conclusion that the effect of an external

electric field in the chemisorption of CO on single

metal or bimetallic RhCu surfaces is essentially the

same. In any case, these effects are quite small andto extract further conclusions it is better to analyze

the different contributions to the interaction en-

ergy in some detail. This is achieved in a rather

straightforward way by making use of the CSOV

analysis.

* Rh10Rh4Cu6RhCu 9

0.005 0.01

F(a.u.)0.005 0.01

-centred models (Rh10, Rh4Cu6, RhCu9) as a function of the

of the fragments computed in the absence of the external electric

ted in the presence of the external electric field (solid lines). The

ch electric field intensity.

216 S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219

The CSOV analysis has been carried out using

the densities of the fragments computed in pre-

sence of the corresponding electric field. In this

way, the final total interaction energy is the one

described in Eq. (3) and the electrostatic contri-

bution to the adsorption energy can be easily ob-tained taking the difference between the values in

Fig. 2. Therefore, this CSOV analysis provides a

direct measure of the chemical changes in each

contribution to the chemical bond which are in-

duced by the external electric field. Fig. 3 sum-

marizes the results of the CSOV analysis for CO

on top of the central Rh atom of the Rh4Cu6

cluster model; the remaining systems exhibit thesame trend and consequently are not further dis-

cussed.

The bars diagram in Fig. 3 reports separately

the contribution of each orbital variation to the

interaction energy; this is simply obtained as the

difference between the energies obtained in two

successive steps of the CSOV analysis. The first

step corresponds to the energy of the CO–Rh4Cu6

obtained by simply superimposing the densities of

the two fragments in presence of the correspond-

ing electric field. The corresponding energy is well

above the energy of the separate systems and,

hence, the interaction is repulsive; this is simply the

result of the Pauli repulsion. The introduction of

an external electric field modifies the Pauli repul-

sion but this is only an electrostatic effect because,at this step, the densities of the two fragments are

-120

-80

-40

0

40

80

120

- 0.01 - 0.005 0 0.0

Est

ep(i

)-E

step

(i-1

) (k

cal m

ol-1

))-

(-

Fig. 3. Variation of the different contributions to the interaction en

function of the intensity of a uniform external electric field as obtain

equilibrium configuration in presence of each electric field and a negati

the vacuum.

frozen. The Pauli repulsion decreases from nega-

tive to positive fields as expected since negative

fields concentrate more density in the surface re-

gion where CO is located; this is the largest single

contribution to the chemisorption bond and the

one where the electric field effect is larger. We havealso computed the values of the Pauli repulsion

obtained using the densities of the fragments ob-

tained in absence of the electric field. However, the

change in the Pauli repulsion is less than 2%. All

other contributions to the chemical bond tend to

compensate the initial Pauli repulsion, the different

contributions are clearly marked in Fig. 3. The

first bonding contribution corresponds to the metalcluster model polarization in response to the

presence of CO. Moreover, the larger the Pauli

repulsion the larger the substrate polarization, the

two effects tend to cancel each other and this is one

of the reasons for the small overall effect in the

total interaction energy (Fig. 3). Here it is worth to

point out that the use of the densities of the frag-

ments obtained in absence of the external electricfield results in substantial changes in the cluster

model polarization because in this way the elec-

trostatic effect on the bare cluster is also included,

this is precisely the reason to carry out the CSOV

analysis as indicated above. The next contribution

in Fig. 3 corresponds to the donation from the

cluster molecular orbitals to the CO virtual ones,

this is essentially p-backdonation and follows thesame trend of the two previous contributions al-

SCF

Donation (from CO)

Polarization of CO

Backdonation (to CO)

Polarization of Rh4Cu6

Pauli repulsion

of

F (a.u.)05 0.01

ergy (kcalmol�1) between CO and Rh4Cu6 surface model as a

ed from the CSOV decomposition. The system geometry is the

ve field is such that electron are pulled out of the surface towards

S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219 217

though to a lesser extent. This is because although

backdonation is expected to be increased when, as

a result of the external field, the electron density is

pulled towards CO, the extent of this effect does

also depend on the capability of the CO 2p�

molecular orbital to accept electronic population.Consequently, backdonation is larger for negative

fields and in going from a field intensity of )0.01to one of +0.01 a.u. varies from 50.1 to 37.2

kcalmol�1, this is a strong indication that chemi-

cal changes are important in agreement with the

results of the occupation of the CO(2p�) orbital

as obtained from the projection operator tech-

nique. It is worth comparing these values withthose corresponding to the interaction of CO

with Rh10 (48.1 kcalmol�1 for F ¼ �0:01 and 34.0

kcalmol�1 for F ¼ þ0:01) and with RhCu9 (42.0

kcalmol�1 for F ¼ �0:01 and 27.9 kcalmol�1 for

F ¼ þ0:01), again providing strong evidence that

single metal and bimetallic surfaces exhibit the

same behavior. We close this discussion of the

CSOV analysis by pointing out that the COpolarization contributions is only little affected by

the presence of the external electric field and that

the CO donation to the metal surface follows the

opposite trend of the p-backdonation with respect

to the electric field intensity as expected from the

reversal sense of this donation.

4. Conclusions

The effect of an external electric field on the

properties and bonding mechanism of CO on

various RhCu bimetallic surfaces has been stud-

ied using cluster models and hybrid density

functional theory calculations. The presence of

the external electric field induces changes in thegeometry and vibrational frequency of the chemi-

sorbed molecule. The analysis of the occupation

of the CO(2p�) antibonding molecular orbital of

the adsorbed molecule permits to firmly establish

that the electric field effects are not just of elec-

trostatic nature but do affect the bonding mecha-

nisms as well. The Constrained Space Orbital

Variations method of analysis has also beenapplied to the metallic and bimetallic systems

studied in the present work. This permits to

distinguish between electrostatic and chemical

changes induced by the electric field. Overall, the

electric field effects observed for the present bi-

metallic surfaces are not at all different from what

has been reported for monometallic surfaces.

Therefore, the main conclusion of the presentwork is that the effect of an external electric field

on the properties of CO adsorbed on bimetal-

lic RhCu surfaces of different composition is the

same as in the single metal surfaces. While the

present conclusions have been obtained for a

given bimetallic alloy it is likely that they will

apply to other bimetallic systems as well.

Acknowledgements

This research has been supported by the Span-

ish DGICYT grant BQU2002-04029-CO2-01 and,

in part, by Generalitat de Catalunya grants

2001SGR-00043 and Distinci�oo de la Generalitat de

Catalunya per a la Promoci�oo de la Recerca Uni-

versit�aaria (F.I.). Computer time was provided by

the Centre de Supercomputaci�oo de Catalunya,

CESCA, and Centre Europeu de Paral.lelisme de

Barcelona, CEPBA, through generous grants from

Universitat de Barcelona, Fundaci�oo Catalana per a

la Recerca. S.G. is grateful to the University of

Barcelona and to CONACyT (M�eexico) for sup-

porting her predoctoral research.

References

[1] L.L. Hagedus, R. Aris, A.T. Bell, M. Boudart, N.Y. Chen,

B.C. Gates, W.O. Haag, G.A. Somorjai, J. Wei, Catalyst

Design: Progress and Perspectives, Wiley, New York, 1987.

[2] G. Ertl, H. Kn€oozinger, J. Weitkamp, in: Hanbook of

Heterogeneous Catalysis, vols. 3–5, Wiley-VCH, Weiheim,

1997.

[3] C.G. Vayenas, S. Bebelis, S. Neophytides, J. Phys. Chem.

92 (1988) 5083.

[4] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625.

[5] J. Pritchard, Nature 343 (1990) 592.

[6] C.G. Vayenas, S. Brosda, C. Pliangos, J. Catal. 203 (2001)

329.

[7] B. Grzybowska-Swierkosz, J. Haber, Annual Reports on

the Progress of Chemistry No. 91, Royal Soc. of Chem.,

Cambridge, 1994.

[8] C.G. Vayenas, M.M. Jaksic, S. Bebelis, S. Neophytides,

in: J.O.M. Bockris, B.E. Conway, R.E. White (Eds.),

218 S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219

The Electrochemical Activation of Catalysis, Kluwer

Academic/Plenum, New York, 2001.

[9] P.S. Bagus, C.J. Nelin, W. M€uuller, M.R. Philpott, H. Seki,

Phys. Rev. Lett. 58 (1987) 559.

[10] P.S. Bagus, C.J. Nelin, K. Hermann, M.R. Philpott, Phys.

Rev. B 36 (1987) 8169.

[11] P.S. Bagus, G. Pacchioni, Surf. Sci. 236 (1990) 233.

[12] M. Head-Gordon, J.C. Tully, Chem. Phys. 175 (1993)

37.

[13] F. Illas, F. Mele, D. Curulla, A. Clotet, J.M. Ricart,

Electrochem. Acta 44 (1998) 1213.

[14] D. Curulla, A. Clotet, J.M. Ricart, F. Illas, Electrochem.

Acta 45 (1999) 639.

[15] G. Pacchioni, R.J. Lomas, F. Illas, J. Mol. Catal. A 119

(1997) 263.

[16] A. Markovits, M. Garc�ııa-Hern�aandez, J.M. Ricart, F. Illas,

J. Phys. Chem. 103 (1999) 509.

[17] M. Garc�ııa-Hern�aandez, A. Markovits, A. Clotet, J.M.

Ricart, F. Illas, Kluwer series on theoretical chemistry and

physics, Kluwer, Progr. Theor. Chem. Phys. B 7 (2001)

211.

[18] M. Garc�ııa–Hern�aandez, U. Birkenheuer, A. Hu, F. Illas,

N. R€oosch, Surf. Sci. 471 (2001) 151.

[19] M. Garc�ııa-Hern�aandez, D. Curulla, A. Clotet, F. Illas, J.

Chem. Phys. 113 (2000) 364.

[20] D. Curulla, A. Clotet, J.M. Ricart, Surf. Sci. 460 (2000)

101.

[21] M.T.M. Koper, R.A. Van Santen, S.A. Wasileski, M.J.

Weaver, J. Chem. Phys. 113 (2000) 4392.

[22] M.T.M. Koper, R.A. Santen, J. Electroanal. Chem. 476

(1999) 64.

[23] S.A. Wasileski, M.T.M. Koper, M.J. Weaver, J. Phys.

Chem. B 105 (2001) 3518.

[24] S.A. Wasileski, M.J. Weaver, Faraday Discuss. 121 (2002)

285.

[25] R.M. Lambert, A. Palermo, F.J. Williams, M.S. Tikhov,

Solid State Ionics 136 (2000) 677.

[26] J.A. Rodriguez, Surf. Sci. Rep. 24 (1996) 223.

[27] V. Ponec, G.C. Bond, Catalysis by Metals and Alloys,

Elsevier, Amsterdam, 1995.

[28] G. Somorjai, Introduction to Surface Chemistry and

Catalysis, John Wiley & Sons Inc., New York, 1994.

[29] J.A. Rodriguez, D.W. Goodman, Science 257 (1992) 897.

[30] J.H. Sinfelt, Bimetallic Catalysts: Discoveries, Concepts

and Applications, Wiley, New York, 1983.

[31] J.K.A. Clarke, A. Peter, J. Chem. Soc. Faraday Trans. 72

(1975) 1817.

[32] B. Coq, R. Dutartre, F. Figueras, A. Rouco, J. Phys.

Chem. 93 (1989) 4904.

[33] J.A. Anderson, C.H. Rochester, Z.J. Wang, J. Mol. Catal.

A 139 (1999) 285.

[34] J. Szanyi, D.W. Goodman, J. Catal. 145 (1994) 508.

[35] R. Khrishnamurthy, S.S.C. Chuang, K. Ghosal, Appl.

Catal. 114 (1994) 109.

[36] F. Solymosi, J. Cser�eenyi, Catal. Lett. 34 (1995) 343.

[37] F.M.T. Mendes, M. Schmal, Appl. Catal. A 151 (1997)

393.

[38] M. Fern�aandez-Garc�ııa, A. Mart�ıınez-Arias, I. Rodriguez-

Ramos, P. Ferreira-Aparicio, A. Guerrero-Ruiz, Langmuir

15 (1999) 5295.

[39] P. Reyes, G. Pecchi, J.L.G. Fierro, Langmuir 17 (2001)

522.

[40] S. Gonzalez, C. Sousa, M. Fern�aandez-Garc�ııa, V. Bertin, F.

Illas, J. Phys. Chem. B 106 (2002) 7839.

[41] S. Gonzalez, C. Sousa, F. Illas, Surf. Sci. 531 (2003)

39.

[42] C. Sousa, V. Bertin, F. Illas, J. Phys. Chem. B 106 (2002)

7839.

[43] M. Fern�aandez-Garc�ııa, J.A. Anderson, G.L. Haller, J.

Phys. Chem. 100 (1996) 16247.

[44] N. Martensson, R. Nyholm, H. Cal�een, J. Hedman, B.

Johansson, Phys. Rev. B 24 (1981) 1725.

[45] A.M. M�aarquez, N. L�oopez, M. Garc�ııa-Hern�aandez, F. Illas,

Surf. Sci. 442 (1999) 463.

[46] P.S. Bagus, K. Hermann, C.W. Bauschlicher Jr., J. Chem.

Phys. 81 (1984) 1966.

[47] P.S. Bagus, K. Hermann, Phys. Rev. B 33 (1986) 2987.

[48] P.S. Bagus, F. Illas, J. Chem. Phys. 96 (1992) 8962.

[49] T.B. Massalsiki, H. Okamoto, P.R. Subramanian, L.

Kacprezak, Binary Alloy Phase Diagrams, second ed.,

ASM, Materials Parks, Ohio, 1992.

[50] See, for instance, Available from <http://www.webele-

ments.com>.

[51] G. Meitzner, G.H. Via, F.W. Lytle, J.H. Sinfelt, J. Chem.

Phys. 78 (1983) 882.

[52] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.

[53] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988)

785.

[54] M. Dupuis, A.M. Marquez, E.R. Davidson, HONDO99,

1999.

[55] M. Dupuis, A.M. Marquez, E.R. Davidson, HONDO

95.3. Quantum Chemistry Program Exchange, QCPE,

Indiana University, Bloomington, IN 47405.

[56] M.J. Frisch et al. Gaussian 98, Revision A.6, Gaussian

Inc., Pittsburgh PA, 1998.

[57] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299.

[58] A. Schafer, H. Horn, R. Ahlrichs, J. Chem. Phys. 97 (1992)

2571.

[59] C.J. Nelin, P.S. Bagus, M.R. Philpott, J. Chem. Phys. 87

(1987) 2170.

[60] J.M. Ricart, J. Rubio, F. Illas, P.S. Bagus, Surf. Sci. 304

(1994) 335.

[61] K.P. Huber, G. Herzberg, in: Molecular Spectra and

Molecular Structure, Constants of Diatomic Molecules,

vol. 4, Van Nostrand-Reinhold, New York, 1979.

[62] D.K. Lambert, Solid State Commun. 51 (1984) 297.

[63] D.K. Lambert, J. Chem. Phys. 89 (1988) 3847.

[64] M. Gierer, A. Barbieri, M.A. Van Hove, G.A. Somorjai,

Surf. Sci. 391 (1997) 176.

[65] S. Andersson, J.B. Pendry, J. Phys. C 13 (1980) 2547.

[66] R. Linke, D. Curulla, M.J.P. Hoptstaken, J.W. Niemans-

verdriet, J. Chem. Phys. 115 (2001) 8209.

[67] C.J. Hirschmugl, Y.J. Chabal, F.M. Hoffman, G.P.J.

Williams, Vac. Sci. Technol. A 12 (1994) 2229.

S. Gonz�aalez et al. / Surface Science 548 (2004) 209–219 219

[68] D.G. Castner, B.A. Sexton, G.A. Somorjai, Surf. Sci. 71

(1978) 519.

[69] T.W. Root, L.D. Schmidt, Surf. Sci. 150 (1985) 173.

[70] D.H.Wei, D.C. Skelton, S.D.Kevan, Surf. Sci. 381 (1997) 49.

[71] W. Kirstein, B. Kr€uuger, F. Thieme, Surf. Sci. 176 (1986)

505.

[72] P.S. Bagus, G. Pacchioni, Electrochim. Acta 36 (1991)

1669.