A first principles analysis of CO oxidation over Pt and Pt66.7%Ru33.3%

(111) surfaces

Sanket Desai, Matthew Neurock *

Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22904-4741, USA

Received 2 February 2003; received in revised form 27 May 2003; accepted 29 May 2003

Electrochimica Acta 48 (2003) 3759�/3773

www.elsevier.com/locate/electacta

Abstract

Nonlocal gradient corrected DFT slab calculations were carried out to determine the overall reaction energies along with the

barriers for the activation of water and the oxidation of CO over well-defined Pt(111) and PtRu(111) surfaces. We attempt to

elucidate the features that control the bifunctional mechanism proposed for the oxidation of CO in solution. The addition of Ru to

Pt along with the presence of solution helps to enhance the elementary steps that comprise the bifunctional mechanism. The

activation of water over Pt(111) in the vapor phase is energetically unfavorable with an activation barrier of �/142 kJ/mol and

overall heat of reaction of �/53 kJ/mol. Water will desorb before it will react on Pt(111). The presence of solution reduces the barrier

to about 75 kJ/mol. The addition of Ru lowers the barrier for water activation in the vapor phase. Alloying Pt with Ru lowers the

barriers for the homolytic as well as the heterolytic activation of water in solution. The presence of solution, however markedly

favors the heterolytic activation which leads to the formation of an adsorbed OH* surface intermediate, a proton which migrates

into solution and an electron. The energetics for this path over the Pt66.7%Ru33.3%(111) surface were calculated to be quite favorable

with an activation barrier of �/27 kJ/mol and an overall energy of reaction of �/26 kJ/mol. Ab initio molecular dynamics results

indicate that water can be activated over PtRu at 300 K. The resulting surface Ru-OH group that forms induces the adsorption and

subsequent activation of water at a neighboring Pt site. The surface hydroxyl intermediate continues to diffuse across the surface via

a sequence of proton transfer steps. The barrier for the subsequent oxidation of adsorbed CO by surface OH groups over Pt(111) in

the vapor phase is 86 kJ/mol with an overall reaction energy of 21 kJ/mol. The barrier is reduced to 71 kJ/mol when carried out over

the Pt66.7%Ru33.3% alloy in the vapor phase. The barrier for the homolytic disproportionation of CO and OH in solution (over

Pt66.7%Ru33.3%) is 90 kJ/mol with an overall reaction energy of �/24 kJ/mol. The heterolytic path which involves the oxidation of CO

by OH to form CO2�/H�(aq)�/e�/ is more favorable than the homolytic path. The activation barrier for the heterolytic path over

Pt66.7%Ru33.3% is �/60 kJ/mol with an overall reaction of �/6 kJ/mol (exothermic). These results indicate that at potentials which are

less than or equal to the potential of zero total charge, water activation over Pt(111) may be difficult. CO oxidation, however is more

likely limiting over the Pt66.7%Ru33.3% alloy at these potentials. These results only hold for the ideal conditions studied herein

including ideal single crystal surfaces, an ideal liquid water solution phase and the absence of an applied potential.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: First principles analysis; CO oxidation; DFT slab calculations; Bifunctional mechanism; Solvent effects; PtRu

1. Introduction

Site blocking or poisoning of Pt electrodes by CO at

concentrations of less than 1% presents a critical

obstacle for meeting the desired catalytic activity or

overpotentials at the anode of the direct methanol and

the reformate PEM fuel cells [1�/3]. For direct methanol,

it is difficult to avoid CO, since it is produced either as a

direct intermediate or as a side reaction product [1,4].

Surface poisoning significantly degrades fuel cell per-

formance and is a primary contributor to low power

densities. The oxidative stripping of CO from Pt occurs

at high overpotentials which range from 0.7 to 0.8 V

[5,6]. It is well established that alloying Pt with Ru [2,7�/

9], Mo [7,8,10], Sn [7,8,10,11], Fe [12] and other non-

precious metals including Co, Fe, and Ni [13�/17] can

significantly enhance the tolerance of the anode to CO

poisoning. The addition of Ru lowers the overpotential

* Corresponding author. Tel.: �/1-434-924-6248; fax: �/1-434-982-

2658.

E-mail address: mn4n@virginia.edu (M. Neurock).

0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0013-4686(03)00509-7

by anywhere between 0.1�/0.3 V [2,3,5] depending on the

composition and atomic structure of the alloy that

forms. The optimal PtRu alloy is thought to be between

Pt50%Ru50% and Pt80%Ru20% [2�/4,7,8,18] depending onhow the catalyst is synthesized. Pt tends to segregate to

the surface, thus obscuring surface characterization

results. Smotkin recently reported that Ru can phase

segregate out leaving the alloy enriched in Pt at the

surface [19]. They estimate that for a high surface area

catalyst, Pt50%Ru50% may really be closer to a

Pt80%Ru20% alloy at the surface [19]. Two mechanisms

have been proposed to explain the significant shift in theoverpotential as Ru is alloyed into Pt. The first is known

as the ‘‘ligand’’ effect. Ru induces changes in the

electronic structure of Pt which ultimately weakens the

Pt�/CO bond [20�/22]. The second is referred to as the

‘‘bifunctional’’ mechanism [2,3,7,8,23,24]. In the bifunc-

tional mechanism, Ru activates water to form the active

oxidant, i.e. a surface hydroxyl intermediate (Ru�/OH),

which subsequently oxidizes CO bound to a neighboringPt site. The bifunctional mechanism is commonly

written as:

H2O�Ru 0 OH�Ru�H�(aq)�e� (1)

CO�Pt�OH�Ru 0 CO2�H�(aq)�e��Pt�Ru

(2)

These steps are considered overall reaction paths. Thetrue identity of the elementary steps and the active

oxidant are still debated. For example, the active

oxidant may be OH, water or some other form of

adsorbed oxygen. In addition, the oxidation step shown

in Eq. (2) could also occur via the formation of the

COOH* intermediate which would then react to form

CO2, H�/(aq) and e�/. For simplicity, we focus herein

primarily on steps 1 and 2 above.There is theoretical as well as experimental evidence

that indicates that both the ligand and bifunctional

mechanisms can occur under different conditions.

Which of these mechanisms dominates however is still

actively debated. Wieckowski and Masel [5] used 13C-

NMR, temperature programmed desorption spectro-

scopy and cyclic voltammetry to begin to quantify the

relative magnitudes of the ligand and bifunctionaleffects. Their results indicate that the addition of Ru

leads to a total reduction in the overpotential by 170�/

260 mV [5]. Only 40 mV out of this total, however, was

attributed to an electronic effect induced by alloying.

The remaining 130�/220 mV was the thought to be the

result of bifunctional mechanism [5]. Similar ideas have

also been proposed by Iwasita et al. [25].

The addition of Ru lowers the overpotential for COoxidation by 170�/260 mV. The effect of Ru is still

debated in the literature. A number of earlier studies

suggest that water activation over Pt may be rate

limiting especially at lower potentials since water itself

is not activated until 0.7 V. Recent results by Koper et

al. [26], however, indicate that CO oxidation is more

likely the rate limiting step on Pt(111). The results arebased on the analysis of Tafel slope measurements

which were found to be significantly lower than would

be expected if water activation was rate limiting. The

primary role of Ru then is to help drive the equilibrium

from water to its dissociated state. Gasteiger et al. [2]

and others [27,28] have proposed that over PtRu alloys,

the direct oxidation of CO(Pt) by OH(Ru) (Eq. (2)) is the

rate limiting [25]. This suggests that the contact betweenCO(Pt) and OH(Ru) is critical. Koper et al. [27] indeed

showed by kinetic Monte Carlo simulations that the

resulting current densities were markedly increased as

the rate constants for diffusion were increased. There is

still some debate, however, as to whether all of the

reactivity occurs at the boundary [5,27,29] or whether

some of what is seen is due to OH spillover onto Pt [23]

or CO diffusion onto Ru.Advances in surface science, in-situ spectroscopy and

electrochemical have been used to provide an under-

standing of nature of the active surface and its role in

dictating the electrocatalytic kinetics [3,5]. Ab initio

quantum mechanical methods offer a complementary

tool to help elucidate a more complete understanding of

the how the atomic as well as the electronic structure can

dictate the reactivity of metal catalyzed reactions.Modeling electrocatalytic systems, however, presents a

number of challenges for ab initio quantum mechanical

methods. These challenges go beyond the traditional

difficulties for vapor phase metal catalysis which include

the effect of the size and shape of the particle, the

influence of the support, the effect of alloying, and the

interaction between surface species. Modeling electro-

catalytic systems requires the ability to model the effectsof solution, applied potential, and electrolytes. These are

currently active areas of research for a number of

theoreticians but are quite challenging due to their

complexity. Anderson et al. have performed some of

the first theoretical studies using the semiempirical

ASED-MO method [30,31] and, more recently, ab initio

methods [32,33] in order to model electrochemical

surface reactions. In particular, they examined boththe oxidation and reduction of water as well as the

oxidation of CO over Pt and developed an approach by

which to incorporate electric field effects. These results

are quite promising but may be somewhat limited by the

small size of the models used for both the metal

substrate as well as the solution phase.

Herein we focus on the oxidation of CO over Pt and

PtRu alloys. We explicitly probe the effects of carryingthese reactions out in the vapor phase as well as in the

solution phase. We do not, however, attempt to analyze

the effects of an applied potential, electric fields or the

presence of electrolyte. These features can be very

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/37733760

important in capturing the true kinetics for fuel cell

systems and ultimately need to be explored in detail. In

addition, we only examine the Pt(111) and the

Pt66.7%Ru33.3% alloy surfaces as models. Based onexperimental findings discussed above concerning the

optimal alloy surface composition, the Pt66.7%Ru33.3%

was chosen as an active surface model. Because of the

very idealized reaction environment used herein, the

results can only loosely related to what happens under

actual fuel cell operating conditions. Despite the differ-

ences, we believe that the trends derived from this study

are informative.

2. Calculation details

Self-consistent field (SCF) electronic energy calcula-

tions were performed using plane wave density func-

tional theory as implemented in the Vienna Ab-initio

Simulation Program [34�/36]. A cut off kinetic energy of

25 Rydberg was used to maintain a finite plane wavebasis set. This cut off was found to be sufficient to attain

convergence in both the adsorption and reaction en-

ergies. Electronic states were computed using a 4�/4�/1

k-point grid in the first Brillouin zone. We have verified

that the calculated energetics converge at the chosen k-

point grid density. For calculations performed in the

vapor phase, there is less than a 2 kJ/mol change in the

energy when we increase the k point sampling to 7�/7�/

1. The accuracy is further improved when we move to

the liquid phase calculations. The Perdew Wang (PW91)

functional [37] was used within the Generalized Gradi-

ent Approximation (GGA) to describe non-local ex-

change and correlation effects. Non-local gradient

corrections were found to be important since the local

density approximation (LDA) [38,39] is known to

underestimate the relatively weak interactions such ashydrogen bonds that form between water molecules

[40,41]. Frozen core, scalar relativistic, Vanderbilt ultra-

soft pseudopotentials were used to describe the core

electrons [42].

In order to begin to understand the effects of alloying

we examine the simple closepacked Pt(111) surface and

the Pt66.7%Ru33.3% alloy (66.7% Pt, 33.3% atomically

mixed Ru) as model surfaces. The alloy favors a facecentered cubic structure with an optimal lattice constant

of 2.78 A. Three parallel layers, each with a Pt:Ru ratio

of 2:1, were chosen to describe the thickness of the metal

slab. The top two layers were allowed to relax during

geometry optimization. The metal atoms in the bottom

layer are not directly involved in the reaction and were

therefore held fixed at their bulk positions. The calcula-

tions were performed within a 3�/3-unit cell, with aspacing of 10 A to separate the metal slab and its

periodic images in the direction normal to the surface

plane. In order to model the solution phase, the unit cell

was packed with 23 water molecules, corresponding to a

solution density of �/1 gm/cc. The inter- and the intra-

molecular geometries of the water molecules were

relaxed during the calculation to allow for the formationof hydrogen bonds. Optimization led to the formation

of a hydrogen bonding network.

Adsorption energies (DEADS) were all calculated using

the following equation:

DEADS�Eadsorbate=metal�Eadsorbate�Emetal slab (3)

where, Eadsorbate/metal, Eadsorbate and Emetal slab are the

electronic energies of the adsorbate bound to the metal

surface, the isolated adsorbate, and the bare metal slab

respectively.

Reaction energies (DER�N) were calculated similarly

as the difference in the electronic energies of theproducts and the reactants:

DER�N�Eproducts�Ereactants (4)

where, Eproducts and Ereactants are the electronic energy ofthe products and the reactants respectively.

The convention used here is that a positive value of

DER�N indicates an endothermic reaction, whereas a

negative value refers to an exothermic and hence

thermodynamically favorable reaction.

The reaction paths are a function of the optimized

reactant and product states. The reactants and products

chosen were those which correspond to the lowestenergy states. Despite this assumption, the optimization

scheme can sometimes overcome local energy minima to

find lower energy states.

Transition states were all first isolated using the

nudged elastic band approach developed by Jonsson et

al. [43]. The nudged elastic band method interpolates a

series of images between the reactant and product states.

Each image is then optimized in parallel to map theminimum energy reaction path. For each image and

every iteration, the forces acting on the nuclei are

decomposed into components parallel and perpendicu-

lar to the reaction path. In order to prevent the images

from rolling over to either the reactant or the product

states (local minima on the potential energy surface)

during optimization, the component of the force parallel

to the reaction path is set to zero. The nuclear positionsare, however, fully optimized along the normal to the

reaction path. This approach helps converge the images

to the minimum energy path. A spring interaction

between the images is added to maintain the spacing

between adjacent images to a constant. The highest

point along the minimum energy path provides an initial

transition state structure for the chosen reaction path.

Quasi-Newton optimization was subsequently per-formed on this initial state to help converge and isolate

the transition state. The system size is too prohibitive to

carry out full numerical second derivative calculations

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/3773 3761

to establish the frequencies on the isolated state to help

confirm their identity. Instead, the structures were

slightly perturbed from their transition states. They

fall back to either the reactant or product statesdepending on the initial perturbation.

As discussed earlier, our model of the reaction

environment is overly simplistic whereby we analyze

the effects of solution and a single well-dispersed alloy

structure. We ignore, for the time being the effects on an

applied potential. Our results would be most compar-

able to those found at the potential of total zero charge

in an electrochemical cell. In the following sections wediscuss the results for both vapor phase as well as

solution phase activation of water and CO oxidation.

3. Results and discussion

3.1. Dissociation of water

3.1.1. Vapor phase

3.1.1.1. Pt(111). In the vapor phase, water adsorbs

atop a single Pt atom through its oxygen atom and is

tilted off the axis normal to the surface with an energy of

30 kJ/mol. At higher coverages the adsorption energy

can increase up to 40�/50 kJ/mol due to hydrogen

bonding. Water dissociates over Pt(111) homolytically

leading to formation of metal hydride and metalhydroxyl intermediates. The reactant and product states

were fully optimized. Atomic hydrogen shows a very

slight preference for the three-fold fcc site. The differ-

ence between the atop, bridge and three fold sites,

however, is less than a 1 kcal/mol. This is consistent with

other studies on the adsorption of hydrogen on Pt

[44,45]. The path is essentially the same as that for water

activation over the Pt33.3%Ru66.7% (111) surface which isshown in Fig. 1 and discussed in the next section. The

reaction involves a direct metal atom insertion into the

O�/H bond, thus resulting in a three center (H�/Pt�/O)

transition state. The transition state is quite late along

the reaction channel whereby there is a considerable

stretch of the O�/H bond. The hydride intermediate that

forms adsorbs at the three-fold fcc hollow site whereas

the hydroxyl intermediate adsorbs atop of a Pt atomsite. The barrier for this reaction over Pt(111) is quite

high at �/142 kJ/mol. Under UHV conditions, water

would preferentially desorb before it would ever un-

dergo dissociation. This is consistent with known

experimental results [46,47].

3.1.1.2. PtRu alloy. By alloying the Pt(111) surface with

progressively more Ru we find that the barrier toactivate water in the gas phase is successively decreased.

The barrier over the pure Ru(0001) surface was found to

be �/92 kJ/mol which is 50 kJ/mol lower than pure

Pt(111). It is interesting to note that even if only one out

of every three metal atoms is Ru (Pt66.7%Ru33.3%), the

barrier is still reduced by 37 kJ/mol (�/105 kJ/mol). The

reaction coordinate which is depicted in Fig. 2 is

essentially identical to that described for water activa-

tion over Pt(111) as discussed above. The overall energy

for reaction is endothermic by �/53 kJ/mol. The

Fig. 1. Reaction coordinate for the activation of water over the

Pt66.7%Ru33.3%(111) surface. (A) Adsorbed reactant state (DEads�/�/34

kJ/mol), (B) transition state (Eact�/�/105 kJ/mol), and (C) product

state (DEP�R�/�/53 kJ/mol). The Ru atom inserts into the O�/H bond

of water, forming a three-center complex.

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/37733762

energetics and mechanism of the reaction are affected by

the presence of a solution environment.

3.1.2. Solvent phase dissociation of water

3.1.2.1. Pt(111). In solution, water remains bound atopa Pt atom through a weak van der Waals interaction and

thus remains stable on the surface. Hydrogen bonding,

however, now begins to play a more important role in

solvating the surface layer. The water molecules ad-

sorbed at the surface are now nearly planar with the

surface. In this configuration, they can begin to max-

imize hydrogen bonding between the hydrogen atoms on

the adsorbed water molecules and the oxygen atomsfrom the water molecules in solution. Water bound to

the metal begins to take on a bilayer like structure.

Water is bound flat to the surface through its oxygen

whereas neighboring solution phase water molecules

hydrogen bond to these species but also interact with the

surface through one of their hydrogen atoms which is

directed toward the surface. This forms a water over-

layer which subsequently forms hydrogen bonds withadditional water molecules in solution as shown in Fig.

2. The structural arrangement of water close to the

surface is similar to that proposed by Doering and

Madey for multilayer adsorption of water over

Ru(0001) [48]. The network of hydrogen bonds between

the water molecules extends within the unit cell as wellas across adjacent unit cells, thereby simulating the

effect of an aqueous environment.

In the vapor phase, water was found to dissociate

homolytically thus forming surface hydride and hydrox-

yl intermediates (Eq. (5)). The presence of solution,

however, opens up a second possible reaction pathway

which involves the heterolytic activation of water to

form a proton (which migrates into the solution phase)an OH surface intermediate and an electron (Eq. (6)).

H2O�0 OH��H� (5)

H2O�0 OH��H� (aq)�e� (6)

The heterolytic activation of water is possible because of

the stabilization of protons by water.

The heterolytic activation of water over Pt(111) was

found to be the more favorable of the two paths. The

activation baffler for the heterolytic path is �/75 kJ/mol.

The overall reaction energy was also found to be �/75

kJ/mol thus reflecting that the potential energy surfacein the region of the transition and product states is quite

flat. Charge transfer appears to occur at the transition

state. Once the charge transfers the proton can readily

Fig. 2. Optimized structure of water adsorbed on the Pt66.7%Ru33.3%(111) surface in the presence of an aqueous environment. The arrows represent

the path along which the adsorbed water prefers to dissociate. For the sake of clarity, only the water molecules involved in the chemistry are

highlighted. Hydrogen bonds are shown by dotted lines.

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/3773 3763

move from the transition state structure into solution.

The potential energy surface therefore increases up to

the transition state but then remains relatively flat

whereby the proton is stabilized by the surroundingsolution. This is consistent with the literature on proton

migration through solution where the energy barriers

are only on the order of 2.7 kcal/mol. Ab initio

molecular dynamics studies show the formation and

breakup of various different water clusters. This is

attributed to the relatively shallow potential energy

surface that forms due to the hydrogen bonding net-

work. This is similar to recent findings by Marx et al.[49,50] who indicate that proton transfer in pure water

can be quite rapid. There is a low activation barrier for

hydrogen transfer as the result of the shallow potential

due to the local hydrogen bonding.

While the presence of solution helps to markedly

lower the activation barrier and the overall endothermi-

city, this step is still thought to be rate-limiting under the

conditions examined here. This is qualitatively consis-tent with experimental results which suggest that at

much lower potentials the activation of water is rate

limiting. In addition, these results agree with those

recently reported by Anderson [33] for the activation

of water over a 2 atom Pt cluster.

3.1.2.2. PtRu alloy. Both reactions (5) and (6) were

found to be thermodynamically more favorable having

lower activation barriers than the dissociation of waterin the vapor phase. The reaction leading to the forma-

tion of adsorbed surface hydride and hydroxyl inter-

mediate was found to be endothermic by �/43 kJ/mol,

while that leading to the formation of protons in the

aqueous phase and an adsorbed OH- intermediate

(which subsequently transfers its charge to the metal)

was significantly less endothermic at �/26 kJ/mol.

Transition state calculations indicate that the activationbarrier for the homolytic pathway involving the forma-

tion of a surface hydride (Eq. (5)) is �/90 kJ/mol while

that for the heterolytic path involving the formation of

an aqueous proton is only �/27 kJ/mol. In the presence

of solution, water is more likely to dissociate to form an

OH- surface intermediate and a proton rather than

surface hydroxyl intermediate and hydrogen adatom

thus favoring the heterolytic path. The results describedhere are for the potential of zero total charge. Qualita-

tively they can be extended to more positive potentials.

At potentials which are more negative, the hydride form

becomes more favorable.

The hydroxyl intermediate that forms sits atop of a

Ru atom site. It is partly stabilized by the solvent and

partly stabilized by the metal surface. The electron that

is produced upon the heterolytic dissociation is deloca-lized over the oxygen as well as the metal surface. The

proton, on the other hand, desorbs from the surface into

solution whereby it can be stabilized by a shell of solvent

molecules. Static calculation indicate that the proton is

most stable as the H5O2� Zundel intermediate in

solution which resides near the metal surface but is

clearly stabilized by hydrogen bonding with solution(Fig. 3). Only one water molecule separates the proton

from the surface hydroxyl group. Subsequent dynamics

simulations, however, show that proton is quite mobile

and can take on the H5O2� and the H4O9

� forms as well

as additional hydrogen bonding states. This is consistent

with recent results reported by Marks et al. [50] who

indicate that the proton defect is delocalized over

different states in the hydrogen bonding network andfluctuates between H5O2

�, H4O9� as well as other

intermediates.

The transition state structure for the dissociation of

water in the presence of solution is shown in Fig. 4. In

the reactant state, water adsorbs atop of Ru and forms

hydrogen bonds with neighboring water molecules. The

reaction coordinate involves the stretch of the OH bond.

The presence of solution here helps to stabilize theheterolytic splitting water. The aqueous solution pro-

vides a much better medium than the metal surface to

stabilize the formation of an Hd� intermediate. The

proton therefore preferentially transfers directly into

solution. At the transition state, the proton that is

formed as the result of the activation of water is

transferred to a neighboring water molecule in solution.

The solution molecule in turn transfers one of itshydrogen atoms to the next adjacent water molecule.

The dissociation of water into hydroxyl intermediates

and protons thus involves a concerted migration of

protons from the adsorbed water molecule into solution

along the hydrogen-bonded water network. The water

molecules which make up the hydrogen bonding net-

work provide a direct route for proton conduction away

from the adsorbed water. This is analogous to theGrotthus mechanism for proton transfer through solu-

tion and has also been suggested by Koper [51] for the

activation of adsorbed water over Rh.

In the vapor phase, the dissociation of water was

found to be mediated by the Ru surface, whereby

ruthenium activates the O�/H bond. Both, the hydroxyl

and hydrogen intermediates that form are stabilized by

the surface. In the presence of the solution, theanalogous metal-mediated route, which leads to the

formation of surface bound hydroxyl and hydride

species (Eq. (5)), was found to be enhanced by the

presence of water. Solution molecules interact with each

other only via hydrogen bonds. The solution structure

can readily rearrange during the course of the reaction

to help stabilize the hydrogen as it dissociates from

adsorbed water to the Ru surface. The solution plays anindirect role in that it helps to stabilize the partial

charges on the transition state complex. This effect

however is rather weak as the barrier is lowered by only

15 kJ/mol from that in the vapor phase. The preferred

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/37733764

route in solution is the heterolytic path, which involves

the direct participation of the solution molecules and

leads to the formation of surface hydroxyl group aproton and an electron (Eq. (6)). The metal is involved

in the initial activation (weakening) of the O�/H bond of

water. Dissociation, however, involves the cooperative

coordination between the metal and water solution

whereby an electron is transferred to the metal along

with the OH surface intermediate, whereas the proton

stabilizes into the water solution.

3.1.3. Dynamics of water on PtRu

In order to capture the influence of the fluctuating

nature of the solution at temperatures greater than 0 K

and associated entropic effects, we carried out ab initiomolecular dynamics simulations for liquid water over

the Pt66.7%Ru33.3% alloy. The dynamics were performed

within the canonical NVT ensemble for up to 2 ps in

time with a time step of 1 fs. The temperature was

maintained near constant at 300 K using a Nose-Hoover

thermostat [52,53]. The other computational parameters

were described earlier in Section 2.

The dynamic simulations were started using theoptimized structures found from the static water/metal

interface calculations. Snapshots of the system at

various time intervals in the simulation are shown in

Fig. 5. Water preferential adsorbs only at the Ru sites.

At about 0.8 ps of the dynamics run, one of the

adsorbed water molecules begins to heterolytically

dissociate, transferring one of its hydrogen atoms

directly into solution. The dissociation leads to the

formation of a solvated proton, and a surface hydroxyl

intermediate centered at the initial Ru adsorption site.

The negative charge is delocalized over the top layer of

the metal surface, particularly near to the Ru adsorption

site, and the adsorbed hydroxyl intermediate. The

reaction products formed and the reaction pathway

are in agreement with the predictions from static

calculations.

The dissociation of water at the Ru site acts to induce

coadsorption of water at a neighboring Pt site. The

OH� intermediate at the Ru site abstracts a proton

from the water that has adsorbed on the Pt site. As a

result, the Ru�/hydroxyl intermediate is converted into

Ru�/water, whereas the Pt�/water molecule is converted

into Pt�/hydroxyl. The hydroxyl species thus diffuses

from Ru to Pt. The hydroxyl intermediate subsequently

diffuses to another neighboring Ru site during the

course of the dynamics via a similar proton transfer

path.

This suggests that the diffusion of the surface

hydroxyl intermediate is not due to Brownian motion

Fig. 3. DFT optimized structure of the reaction products formed from the heterolytic dissociation of water. The hydroxyl intermediate is bound atop

Ru whereas the proton is solvated in water (DEP�R�/26 kJ/mol).

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/3773 3765

along the surface, but instead the result of proton

transfer between adsorbed water molecules and the

hydroxyl intermediates as is shown schematically in

Fig. 6. This may be important in describing the kinetics

for CO oxidation. The results suggest that surface

hydroxyl groups can readily migrate from Ru to Pt

provided that there is a free path along the Pt sites.

Water initially adsorbs and activates on Ru. It can

continue to transfer along Pt as long as there are vacant

sites nearby where water can adsorb and provide

additional protons for abstraction. Our calculations

indicate the overall reaction energy for proton transfer

from water adsorbed on an adjacent Pt atom to the Ru�/

OH intermediate (Pt�/OH2�/Ru�/OH0/Pt�/OH�/Ru�/

OH2) is nearly thermoneutral at 5 kJ/mol.

This path of proton transfer as a potential mechanism

of surface diffusion may be of direct relevance to the fuel

cell chemistry where CO oxidation is thought to occur at

the Pt�/Ru boundaries [18,54]; CO forms islands on Pt

whereas water is adsorbed and activated on Ru. As CO

at the periphery is oxidized to CO2, more CO begins to

diffuse from the interior to the Pt�/Ru boundaries.

Recent evidence by Davies et al. [23] suggests that in

addition to the reaction at the Pt�/Ru boundary, there

may be spill-over of the oxidant onto Pt. Our results

indicate that OH groups can migrate onto Pt but they

require vacant Pt sites to provide a diffusion path. In

addition, diffusion must be significantly faster than

adsorption and desorption. Both of these are significant

assumptions and need to be further explored before fullyaccessing the relevance of these dynamic simulations to

fuel cell catalysis.

3.2. CO�/OH disproportionation over Pt�/Ru

The second step of the bifunctional mechanism

involves the disproportionation of CO with the hydroxylintermediates. Again, we examine the chemistry over

Pt(111) and the well-dispersed Pt66.7Ru33.3(111) alloy in

the vapor phase as well as in the solution phase in order

to probe the effects of alloying and solvation.

3.2.1. Vapor phase surface reaction

3.2.1.1. Pt(111). The most favorable adsorption sites

for coadsorbed CO and OH on Pt(111) are on adjacent

bridge and atop sites, respectively. These sites are

directly across from one another and form a 3-fold

surface site. It is well known that DFT tends to predict

that CO will adsorb at the higher fold coordination sites

whereas experiments show that CO binds atop [58]. Thecalculated energy differences between these sites, how-

ever, are actually quite small. CO and OH can subse-

quently react to form CO2 and surface hydrogen. The

Fig. 4. Transition state structure for the heterolytic dissociation of water into adsorbed hydroxyl intermediates and protons (Eact�/27 kJ/mol).

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/37733766

reaction therefore involves both the formation of an

OC�/OH bond as well as the dissociation of the O�/H

bond. If the reaction only involved the formation of the

OC�/OH bond then one might suspect that Pt (and other

metals to the right in the periodic table) would be more

active than Ru since the weaker metal-adsorbate bonds

should tend to favor coupling reaction paths. Ru,

however, aids the scission of the OH bond just as it

does in the activation of water. The CO�/OH reaction

proceeds by the simultaneous displacement of the

adsorbed CO and OH intermediates toward one another

along with the metal atom insertion into the OH bond.

The barrier for this reaction in the vapor phase over

Pt(111) was found to be �/86 kJ/mol whereas the overall

reaction energy was endothermic by �/21 kJ/mol. This is

in very good agreement with a value of 85 kJ/mol

estimated from the experimental TPD results by Weibel

et al. [55]. They found a peak in the desorption spectra

at 340 K which they attributed to CO2 formation from

the reaction of CO and OD on Pt(111). The barrier for

CO�/OH (or OD) in the vapor phase is significantly

lower than that calculated for the activation of water

over Pt(111).

The CO�/OH disproportionation over Pt(111) in

solution proceeds by a heterolytic path thus forming

CO2*�/H*(aq) and e(�/) as products. The barrier for

the heterolytic path was found to be �/79 kJ/mol. The

overall reaction energy was calculated to be �/22 kJ/mol

Fig. 5. Snapshots taken from an ab-initio molecular dynamics simulation performed at 300 K. (a) shows the initial structure wherein water is

adsorbed over the Ru sites on the surface. (b) shows the Ru-bound hydroxyl intermediate and the solvated proton that form on the dissociation of

water. (c)�/(f) indicate the migration of the hydroxyl intermediates over the surface from Ru (shown as darker atoms on the surface) to Pt (shown as

the lighter atoms) to Ru sites via proton transfer.

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/3773 3767

endothermic. CO, however, can also react with an

adsorbed hydroxyl to form the COOH surface inter-

mediate. This reaction, shown in Eq. (7), is only 10 kJ/

mol less favorable than the reaction to form CO2�/

H��/1e�.

CO��OH�0 COOH� DER�N��4 kJ=mol (7)

With such a small change in the energy difference, it is

difficult to distinguish which of these paths (Eq. (2) or

Eq. (7)) would be more prevalent under electrocatalytic

conditions.

3.2.1.2. PtRu. On the clean Pt66.7%Ru33.3% alloy sur-

face, OH preferentially adsorbs atop Ru while COprefers the Pt bridge site (Fig. 7a). In the vapor phase,

the O�/H bond is activated by the direct participation of

the metal. The reaction coordinate here also involves the

simultaneous formation of an OC�/OH bond along with

the simultaneous scission of the O�/H bond as is shown

in Fig. 7B. Ru inserts into the O�/H bond, thus

elongating the O�/H bond from 0.99 A in its reactant

state to 1.40 A at the transition state. The products ofthe reaction in the vapor phase are CO2 and a surface

bound hydrogen atom. CO2 weakly interacts with the

surface in a di-s adsorption mode through its carbon

and oxygen atoms. The resulting atomic hydrogen binds

at the three-fold hollow site on the surface (Fig. 7c). The

overall homolytic reaction was found to be endothermic

by�/11 kJ/mol and has an activation barrier of �/71 kJ/

mol. In the vapor phase, the energetics are only slightlymore favorable on the alloy than on pure Pt. The

energetics and the mechanism, however, may change in

the presence of the solution.

3.2.2. Solution phase surface reaction

The favored adsorption sites for the CO and OH

intermediates in solution are found to be the same as

those in the vapor phase*/CO prefers the Pt�/Pt bridge

site whereas OH binds atop Ru (Fig. 8). An important

point to note is that the surface hydroxyl intermediate

can form hydrogen bonds with the water solution thus

stabilizing the reactant state.

The disproportionation between the coadsorbed CO

and OH surface intermediates in solution can either

occur homolytically*/leading to the formation of CO2

Fig. 6. Schematic representation of pathways for the dissociation of

water and the subsequent migration of hydroxyl intermediates over the

surface. The arrows represent the paths for proton transfer.

Fig. 7. CO oxidation over Pt66.7%Ru33.3%(111) in vapor phase (a) CO

and OH coadsorbed on Pt66.7%Ru33.3%(111) surface (b) Transition

state for CO oxidation (Eact�/�/71 kJ/mol) (c) Products CO2 and H

(DER�N�/�/11 kJ/mol).

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/37733768

and surface hydrogen (Eq. (8)) or heterolytically*/

leading to the formation of CO2, a proton and an

electron (Eq. (9)).

CO��OH�0 CO2��H� (8)

CO��OH�0 CO2��H��e� (9)

The former is analogous to the reaction that occurs inthe vapor phase; the latter is possible in the solution

phase because of the energy gain in the stabilization of

protons in water.

We have examined the energetics associated with both

of these reactions in solution. The homolytic reaction

that leads to the formation of CO2 and a surface hydride

(Eq. (8)) was calculated to be endothermic by �/24 kJ/

mol and has an activation barrier of �/90 kJ/mol. Thisreaction is 13 kJ/mol less favorable than the vapor phase

reaction, likely because there is a loss in the solvation of

the polar hydroxyl group in the reactant state that is not

compensated for by the formation of the surface bound

products when carried out in solution.

In the heterolytic reaction path (Eq. (9)), CO and OH

react at the metal�/solution interface to form weakly

adsorbed CO2, an electron which is delocalized over themetal (in an actual electrocatalytic system, the electron

would instead be drawn out of the metal as current)

whereas the proton would migrate into solution (Eq.

(9)). This heterolytic solution phase coupling reaction is

slightly exothermic at �/6 kJ/mol thus making it more

favorable than the homolytic vapor phase reaction (�/11

kJ/mol) and the homolytic solution phase surface

reaction (�/24 kJ/mol). This is primarily due to the

greater degree of solvation of the protons as compared

to that of chemisorbed hydrogen. The activation barrier

for the heterolytic reaction is �/60 kJ/mol, which is 30

kJ/mol lower than that for the homolytic solution phase

reaction and 11 kJ/mol lower than that for the vapor

phase path. The disproportionation of CO and OH at

the solution�/metal interface is therefore more likely to

form CO2 and a proton rather than CO2 and a surface

hydrogen. The CO2 that forms adsorbs di-s at the

surface through the carbon and oxygen atoms. If we

allow the proton to remain in the local vicinity of the

CO2, it will react with the CO2d� surface intermediate to

form the COOH* surface species. This path is only

favorable when the electron is not pulled out of the

metal fast enough. The proton resides in solution as an

H5O2� where it is solvated by two water molecules (Fig.

9). There is experimental evidence as well as very recent

theoretical results over Pt [56] which indicate that

COOH and formic acid are possible intermediates or

products [18]. Anderson [56] suggests that the COOH

intermediate can subsequently react with a second

Fig. 8. Reactants CO and OH co-adsorbed over well-dispersed Pt�/Ru alloy. CO prefers to adsorb at the Pt�/Pt bridge site. OH adsorbs atop Ru.

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/3773 3769

surface hydroxyl intermediate to form CO2 and H�/

(aq). This, however, presumes that there are two

adjacent hydroxyl groups next to the bound CO

intermediate or that OH migration is rapid.

The reaction at the solution�/metal interface is similar

to that at the vapor�/metal interface in that it involves

the simultaneous coupling of the carbon of CO with the

oxygen of the OH intermediate, and the activation of the

O�/H bond of the hydroxyl intermediate. In the presence

of the solution, the latter was found to occur by the

direct involvement of the solution phase water mole-

cules. During the reaction, the O�/H bond of the

hydroxyl group begins to stretch. Here, however, it is

aligned toward the solution where it can form hydrogen

bonds rather than toward the surface. The transition

state associated with the reaction is shown in Fig. 10.

The OH group dissociates heterolytically. The oxygen is

transferred to the adsorbed CO to form a surface CO2

intermediate. The proton, on the other hand, transfers

to a neighboring water molecule in solution, which in

turn transfers one of its hydrogen atoms to the next

adjacent water molecule in the form of a proton. The

disproportionation in solution thus occurs by the

concerted movement of protons from the hydroxyl

group into the solution along the hydrogen bonds of

water. The heterolytic reaction appears to be morefavorable than the homolytic reaction by 30 kJ/mol.

This reaction is less affected by the presence of solution

than the water activation step.

3.3. Comparing reaction paths

A comparison of the of the overall reaction energies

for water activation and CO�/OH disproportionation

over the pure Pt(111) in solution indicates that water

activation is 48 kJ/mol more endothermic. The activa-

tion barriers, however, are very similar. The barrier for

water activation is �/75 kJ/mol whereas the CO�/OH

disproportionation step is �/79 kJ/mol. At the potentialswhich are lower than or equal to the potential of zero

total charge, water activation is likely rate controlling.

We can not make any statements, however, with respect

to higher potentials.

In moving to the Pt66.7%Ru33.3% alloy, the activation

of water in solution drops from �/75 kJ/mol over

Pt(111) to �/27 kJ/mol over the Pt66.7%Ru33.3% alloy

[57]. The CO�/OH disproportionation reaction, on theother hand, decreased by only 19 kJ/mol in moving from

Pt(111) (79 kJ/mol) to the Pt66.7%Ru33.3% (111) alloy (60

kJ/mol). Over the Pt66.7%Ru33.3% alloy, the dispropor-

Fig. 9. The products that result from the CO�/OH disproportionation over the Pt�/Ru alloy in solution include the formation of CO2 and proton

and an electron. (EP�R�/�/6 kJ/mol). CO2 adsorbs di-sigma. Proton resides in solution as H5O2�.

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/37733770

tionation step now becomes the more limiting of the two

steps involved in the bifunctional mechanism. This is

consistent with the results reported by Gastegier et al. [2]and others [27]. The results here agree well with those

reported by Wieckowski and Masel [5] who indicate that

addition of Ru primarily enhances the bifunctional

mechanism. The electronic effects of alloying Pt with

Ru which also promote this reaction were found to be

less important. These comparisons, however, are still

preliminary since the relative energetics for the two

reactions may be altered when electric field effects areincluded.

4. Summary/conclusions

Alloying Pt with Ru significantly lowers the activation

barriers as well as the overall reaction energies for both

steps (water activation and CO oxidation) in the

bifunctional mechanism. The analysis of these reactions

in the vapor phase is useful for setting ideas. The

presence of solution, however, is critical since it not

only influences the overall energetics but also changesthe actual mechanism. The activation of water over

Pt(111) is energetically not very favorable when carried

out in the vapor phase. The activation barrier for water

over Pt(111) is �/142 kJ/mol and endothermic by �/53

kJ/mol which indicates that water would preferentially

desorb before dissociating over Pt under UHV condi-

tions. While the presence of solution significantly lowers

the barrier to dissociate water down to 75 kJ/mol, it is

still considered to be a rate limiting step at potentials less

than or equal to the potential of zero total charge.

Without having explored higher potentials or even the

effect of potential, we can not explicitly comment on

what will happen at higher potentials.Alloying Pt with Ru at a ratio of Pt66.7%Ru33.3%,

significantly lowers the barrier for the dissociation of

water in both the vapor and liquid phases. The barrier

for the heterolytic activation of water over the

Pt66.7%Ru33.3%(111) surface in the presence of solution

decreased to �/27 kJ/mol. The significant drop in the

activation barrier is the result of both alloying as well as

solution effects. Water activation over the

Pt66.7%Ru33.3%(111) surface no longer appears to be

rate limiting. In addition, the preliminary results from

ab initio MD simulations suggest that the presence of

surface hydroxyl groups can induce water to adsorb and

activate at neighboring Pt sites. The migration of surface

hydroxyl intermediates appears to occur via a low

energy proton transfer path. The diffusion of surface

hydroxyl intermediates onto Pt requires a path of vacant

Fig. 10. Transition state structure for the CO�/OH disproportionation over well dispersed Pt�/Ru alloy in solution (Eact�/�/60 kJ/mol).

S. Desai, M. Neurock / Electrochimica Acta 48 (2003) 3759�/3773 3771

Pt sites which would allow water to activate at Ru and

diffuse over Pt ensembles.

The second step in the bifunctional mechanism, the

oxidation of CO, occurs via the disproportionation of

CO and OH. This reaction proceeds homolytically in the

vapor phase over the pure Pt(111) with an activation

barrier of �/86 kJ/mol and an overall reaction energy of

�/21 kJ/mol. The barrier is reduced to 71 kJ/mol when

carried out over the Pt66.7%Ru33.3%(111) surface. Pt aids

in the coupling of CO and OH whereas Ru helps to

activate the O�/H bond. The barrier for this same

homolytic activation path is increased to 90 kJ/mol

when carried out in presence of solution. The reactant

state for the homolytic disproportionation of CO and

OH is stabilized more so than the transition state thus

leading to the increased barrier. In solution, the hetero-

lytic path which results in the formation of CO2�/

H�(aq)�/e(�/) is much more favorable than the homo-

lytic path. The barrier is reduced to 60 kJ/mol. The

transition state involves the formation of a much more

charged separated state (Pt�/CO2d��/Hd�) which is

stabilized by solution much more so than the reactant

state. If the negative charge that forms is allowed to

remain localized near the CO2 it will react with the

resulting proton to form COOH. If, however, the charge

is transferred through the metal the resulting

products are CO2(g), an aqueous phase proton and an

electron. Overall the CO oxidation path is less affected

by the presence of solution than the water activation

path.

The end result is that both alloying and the

presence of solution enhance water activation

as well as CO disproportionation steps. These

effects, however, impact the water activation much

more than the CO�/OH disproportionation step.

The barrier for water activation is lower than that

for CO oxidation over the Pt66.7%Ru33.3%(111)

surface when the reaction is carried out in the presence

of solution. This suggests that for the ideal conditions

examined here, that there may be a change in the rate

limiting step from water activation to CO�/OH dis-

proportionation as we begin to alloy the Pt(111) surface

with Ru.

Acknowledgements

We would like to acknowledge the DuPont Chemical

Company for partial support of this work and would

also like to thank the NCSA Supercomputing Center at

the University of Illinois for the computing resources.

M.N. would like to thank Professor Marc Koper for

invaluable discussions.

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