J. Phys. Chem. 1995, 99, 9149-9154
9149
Catalytic Effect of Ruthenium in Ruthenium-Platinum Alloys on the Electrooxidation of Methanol. Molecular Orbital Theory Alfred B. Anderson* and E. Grantscharova9 Chemistry Department, Case Westem Reserve University, Cleveland, Ohio 44106-7078 Received: September 29, 1994; In Final Form: March 24, 1 9 9 9
An atom superposition and electron delocalization molecular orbital (ASED-MO) study has been made of the catalytic effect of Ru alloyed into Pt electrodeson the oxidation of the adsorbed CO poison that is generated during methanol fuel cell operation. Using cluster models of the Pt( 111) surface, the Ru in Pt( 111) alloy surface, and the Ru(OOO1) surface, it is found that the H20 adsorption energy to surface atoms increases in the order Pt in Pt(111) < Ru in Pt( 111) < Ru in Ru(0001). An inverse relationship is found for OH activation OH(ads) H(ads) takes energies: the stronger the adsorption, the more easily the reaction H20(ads) place. The values and potential dependencies of these activation energies are consistent with OH(ads) being the oxidizing species that removes the CO(ads) poison. Both the Pt and the alloy surfaces are calculated to have increasing activation energies with increasing potential for the CO(ads) oxidation step. In both cases high coverage of coadsorbed CO, OH, or other species will activate CO(ads) and OH(ads) by weakening their chemisorption bond strengths.
-
Introduction The significant enhancement in efficiency of platinum electrodesfor methanol electrooxidation caused by alloying with ruthenium was reported 30 years ago by Bockris and coworkers.’ Petry et al.2 found immediately thereafter the highest activity for Ru contents of 5-10% and advanced the hypothesis that alcohol dehydrogenation is rate determining on the bare platinum electrode and that when steady state is reached, the removal of chemisorbed oxidation products determines the rate. They suggested that in acid solution OH radicals oxidize these surface species; as they pointed out, an oxidizing role for OH in the electrooxidationof organic molecules had been suggested as far back as the 1920s3and was assumed by Bockris as well.’ Petry et al. also found methanol to be practically unoxidized on ruthenium electrodes. A mechanism for the catalytic effect of 5- 10% ruthenium alloyed into platinum was not suggested in this paper, but an implication that the ruthenium atoms may be providing OH radicals is easily drawn. In the intervening years between the early work and the present it has been established that the oxidation product that is poisoning the platinum electrode surface and requiring a high (-0.8 V versus the standard hydrogen electrode) overpotential for oxidative removal is carbon m ~ n o x i d e . Although ~ once thought to be a poison, adsorbed formyl, HCO, is now thought to be a short-lived reaction intem~ediate.~ Very recently, Gasteiger and co-workers5have shown a -0.25 V catalytic shift in the electrooxidationpotential at fixed current for carbon monoxide adsorbed on polycrystalline rutheniumplatinum alloy surfaces with about 50% of each atom present. They attributed this to the ability of the Ru atoms to generate OH(ads) at lower potentials than occurs on platinum atoms in Pt or Pt alloy surfaces. As they pointed out, Ru decomposes water at a much lower potential than Pt, that is, around 0.2 V, compared to around 0.8 V for Pt. The formation of OH adsorbed on Ru atoms or on Ru-Pt pair sites on the alloy from H20 decomposition at low potential is inferred in cyclic On leave from the Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria 1113. Abstract published in Advance ACS Abstracts, May 1, 1995. @
0022-365419512099-9149$09.00/0
+
~ ~ l t a m m o g r a mThis ~ . ~ ,OH(ads) ~ was presumed to oxidize CO(ads): CO(ads)
+ OH(ads) -CO, + H+ + e-
(1)
where CO might be on either a Pt or a Ru atom adjacent to an OH which is presumably bonded to Ru. In their study, wherein temperature variations to determine apparent activation energies were not made, the measured overpotential for CO removal over pure Ru was shifted by 0.15 V compared to pure Pt. This indicated less activity on pure Ru, which adsorbs CO as strongly as pure Pt but was argued to adsorb OH more strongly than Pt. The line of reasoning being used was that the more strongly the reactants are adsorbed, the higher the overpotential (or activation energy) for reaction 1 to occur. Given that the overpotential shift was greatest, -0.25 V, for the 50% alloy surface, the authors postulated a synergistic effect based on weakened OH adsorption on Pt-Ru “pair sites”. It is important to consider the implication of the fact that Gasteiger et al. made their studies at high CO coverages, by oxidative stripping of adsorbed monolayers. New variables are introduced when the CO poison is introduced on an electrode surface by methanol oxidation. These have been addressed in other recent publications by Gasteiger et a1.6$8They found that methanol neither adsorbs nor undergoes oxidation below 0.75 V over a Ru electrode and that on a Pt electrode currents due to methanol oxidation appear at -0.2 V.6 Increasing substitution of surface Pt atoms by Ru atoms quickly attenuates this methanol reaction, which led them to believe that ensembles of about three Pt atoms are needed for oxidative electrodecomposition of methanol to CO(ads); Ru atoms, presumably blocked by H20 and its decomposition products, are ineffective. Unless annealed, the Ru atoms in the. Pt alloy surfaces are believed to be randomly dispersed, so that when more Ru atoms are present, the connected Pt ensembles will be, on average, smaller in size.5 The Ru sites serve, it was believed, the role of providing OH(ads) at low potential for oxidizing the CO(ads). Gasteiger et al.’s temperature-dependent studies of methanol electrooxidation8 have provided interesting information that they 0 1995 American Chemical Society
9150 J. Phys. Chem., Vol. 99, No. 22, 1995
Anderson and Grantscharova
TABLE 1: Parameters Used in the Calculations (see ref 9): Principal Quantum Numbers, n, Diagonal Hamiltonian Matrix Elements, H (eV), Orbital Exponents, 6 (au), and Linear Coefficients, c, for Double-c d Orbitals P
S
d
atom
n
H
4
n
H
P
n
H
CI
51
c2
52
Pt Ru C 0 H
6 5 2 2 1
-10.5 -9.37 -15.09 -26.98 -12.1
2.554 2.078 1.6583 2.1460 1.2
6 5 2 2
-6.46 -6.11 -9.76 -12.12
2.25 1.778 1.6180 2.1670
5 4
-11.1 -10.5
0.655 81 0.533 99
6.013 5.378
0.571 50 0.63679
2.396 2.303
relate to the mechanism of the oxidation of the CO(ads) poison. On pure Pt the onset of methanol oxidation shifted negatively by -50 mV at 60 "C compared to 25 "C, which was interpreted to mean that the activation energy for OH(ads) formation was overcome at lower potential when the temperature was higher, a suggestion which we supported theoretically in our previous paper.9 For the Pt-Ru alloy electrodes two activation energies were determined at 0.4 V, 0.31 eV for -7 atom % Ru surfaces and 0.62 eV for -33 and -46 atom % Ru surfaces. The lowcoverage activation energy was attributed to the process of CO motion to the OH(ads) associated with Ru, the subsequent oxidation being fast. The activation energy at the high coverage was attributed to more difficult methanol dehydrogenation steps on surfaces lacking the needed Pt ensembles of the right size. With the above recent experimental results and interpretations in mind, we have set out to make an atom superposition and electron delocalization molecular orbital (ASED-MO) investigation of (i) OH(ads) formation on Ru atoms in the Pt-Ru alloy surface and on a pure Ru surface, (ii) the oxidation of CO adsorbed on an adjacent Pt atom in the alloy or on an adjacent Ru atom in the Ru case, and (iii) the CO adsorption energy at different sites on the Pt, Ru, and alloy surfaces. Potential dependencies of all of these phenomena are studied by using the surface valence band shifting technique. Comparison with our companion study for Pt9 will complete an overall mechanistic picture of CO electrooxidation which is in comfortable agreement with the observations of Gasteiger et aL5s6s8and supportive of the interpretations which they give. Methods and Models All atomic parameters are listed in Table 1. The Pt, C, 0, and H parameters are the same as in our companion Pt study,9 and the added Ru parameters come from sources referenced there. Our standard parameters have Pt diagonal Hamiltonian matrix elements that are 1.5 eV greater than the negative valence state IPS, and for C, H, and 0 they are 1.5 eV less: such shifts emulate self-constancy by reducing orbital polarizations (charge transfer) for diatomic fragments and metal-ligand bonds into a reasonable range. For Ru the matrix elements are shifted 2 eV instead of 1.5 eV in order to reduce electron transfer to the Pt cluster. Exponent shifts from the literature values, when made, improve the calculated diatomic bond lengths. To model different metal and alloy surface potentials, all valence diagonal Hamiltonian matrix elements for each metal atom of the cluster model of the surface are shifted the same amount. The shift in the Fermi level is nearly the same as the parameter shift, and no fine adjustment is felt to be necessary for the purpose of making the Fermi level shifts exactly 1/2 and 1 V for this qualitative trend-seeking study. Cluster models of fcc Pt( 111) and hcp Ru(OOO1) surfaces look the same because they are two layers thick. The Ml8 clusters are in Figure 1, along with our RuPt18model of the alloy surface. In this work the chemistry studied takes place on 1-, 2-, and 3-fold sites involving the three atoms of the central triangle. We do not model the effects of high coverages of CO or OH
Y-\J CO
OHorH20
Figure 1. Cluster models of the surfaces as used in the calculations. Nearest neighbor distances are 2.77 A for the Pt( 111) model and 2.68 8, for the Ru(001) model; for the alloy model, the Ru atom is at the optimized height of -0.4 A. For these three cases the indicated atoms are the ones upon which adsorption and reactions are studied. for the Ru-Pt alloy because the companion paper dealing with the Pt surface made the point conceming the role of high coverage in reducing the activation barrier for CO(ads) oxidation by OH(ads). As will be shown below, the important catalytic effect of Ru in the Ru-Pt alloy is activating the formation of OH(ads) at lower potentials; the energetics of the oxidation step are similar to what was calculated for the Pt surface and so will be similarly influenced by high coverage. As a final note on the model, we have modeled the (111) surfaces, whereas the experimental work we discuss used polycrystalline electrodes with surfaces containing several crystallographic facets, edges, and comers that come with this morphology. The chemistry explored here should certainly be expected to show variations in rates and potentials, but our focus is on the broader conceptual aspects of the electrocatalytic oxidation of the CO poison, at least for the first step. We also want to remind the reader of the widely known powerful blocking effect of specifically adsorbed anions from the electrolyte. This would, if included, add another dimension to the modeling, but is not considered in this or the companion study.9 For a very recent experimental discussion, see Herrero et a1.I0
HzO Adsorption on Ru Sites Our calculations yield at all three potentials, 0, 1/2, and 1 V, stronger H20 adsorption on the Ru site on the alloy cluster model (Figure 1) than on the Pt site of the platinum surface model. These adsorption energies increase and the H20 heights decrease as the potential increases. Results are given in Table 2, along with the same calculated properties for H20 bound to the Ru(0001) surface cluster model. It is evident that at each potential this H20 lone-pair donation bond strength is strongest for the Ru surface, weakest for the Pt surface, and almost midway between for the isolated Ru site in the Ru-Pt alloy. The different Lewis accepting abilities of the three surfaces can be understood by analysis of electronic structures given in Figure 2. Examining the cluster electronic structures and orbitals, one finds systematic changes in nearest neighbor metal
Catalytic Effect of Ru in Ru-Pt Alloys
J. Phys. Chem., Vol. 99, No. 22, 1995 9151
TABLE 2: Calculated Potential Dependencies of the Hz0 Adsorption Energies, E (eV), Heights, h (A), and HzO Mulliken Charges, q (-e) for the Pt(ll1) Surface, the Ru-Pt Alloy, and the Ru(0001) Surface As Modeled in Figure 1" Pt(ll1) potential 0 1/2 1 a
E
h
Ru in Pt(l11) q
E
q
h
H I H-0-O
Ru(000 1) E
h
q J 0
1.81 1.79 0.76 2.46 1.68 0.63 2.85 1.64 0.74 2.29 1.75 1.00 2.75 1.66 0.84 3.26 1.61 0.97 2.86 1.73 1.27 3.17 1.63 1.08 3.79 1.59 1.22
\M'
J
M
Pt( 111) results are from ref 9.
Figure 3. Orientation of H20 on surface clusters during OH bond scission.
'/*V
IV
1v
0.4
- 0.3 >
g
0.2
C
w
0.1
0.0
Figure 2. Calculated correlation diagrams for HzO bonding to central 1-fold Pt (in Ptle), Ru (in RuPtl7), and Ru (in R u , ~ atoms ) in cluster models of the surfaces in Figure 1. The hatched regions at the tops of P t 1 8 and RuPtl, bands indicate singly occupied orbital energy levels at the tops of the d bands with eight unpaired electrons. The bracketed bonding (b), nonbonding (n.b.), and antibonding (a.b.) regions are discussed in the text.
atom average bonding characteristics going up from the bottoms of the valence bands. At the very bottom in each case, orbitals are largely bonding between nearest neighbors and the valence s coefficients are large (valence s, p, and d atomic contributions are found throughout this valence band). Above this region is a region of largely d-d bonding orbitals, and on top of this is a largely d-d nonbonding region, meaning many orbitals have nodes at nearest neighbor positions. Finally, the top of the valence d band is completed with a region of orbitals that has many nearest neighbor antibonding components. Now as the figure shows, the mainly d part of the Ptl8 band is occupied with electrons almost to the top, so lone-pair orbital interaction with this band has a large closed-shell repulsive energy component: the antibonding counterparts of the 3a1 stabilizations move up to the predominantly s p empty valence band region, so that, effectively, some electrons are promoted from the heart of the d band to the Fermi level, weakening the effect of the donation stabilizations shown in Figure 2. In contrast to this, the Ru cluster, with two fewer electrons per atom, has the d band filled only up to, it tums out, about the top of the nonbonding region. This leaves empty Ru surface orbitals, on average antibonding between nearest neighbors, to participate in stronger donation bonding with the H20 orbitals. When placed in the Pt surface, the Ru d orbitals undergo bonding stabilizations to contribute to the bottom of the Pt d band and the antibonding counterparts are found just above the Fermi level. These orbitals are quite well localized on Ru. The lower bonding Ru-Pt orbitals provide some closed-shell repulsion when H20 bonds to Ru, but the interaction with the Ru-centered empty surface orbitals is strong. The H20 lbl and 3al orbitals donate into these without the need to promote electrons to the Fermi level in antibonding counterparts. It is noted that the H20 charges in Table 2 must be viewed with a realization of the arbitrary aspects of the Mulliken partitioning used in calculating them. Thus, even though H20 donation bonding is stronger on Ru than on Pt, the Mulliken
+
Flu in Pt
J-cj --Ru
1.1 1.2 1.3
1.1 1.2 1.3
1.1 1.2 1.3
0 - H Distance (A)
Figure 4. Potential dependencies of reaction energy profiles for OH bond scission in water bound to Ru(0001), Ru in Pt(l1 l), and Pt(ll1) 1-fold sites. See Figure 1.
TABLE 3: Calculated Activation Energies, E, (eV), for OH Bond Scission According to the Reaction in Eq 4 at Three Different Potentials (V) potential
Pt
Ru in Pt
Ru
0 112 1
0.44 0.33 0.17
0.19 0.13 0.09
0.12 0.08 0.03
charge is less in the Ru case. It is inappropriate to compare these charges, but the calculated variations in H20 charge when adsorbed to a given atom should be meaningful. With this in mind, we deduce from Table 2 that as the potential is increased for each surface system, Pt( 11l), Ru in Pt(l1 l), and Ru(OOOl), the increase H20 donation bond energy correlates with increased H20 charge. The bond energy increases are in response to lowering the Fermi level so that the antibonding counterparts do not have so far to promote electrons and are less occupied. This also results in greater charge donation to the surface because, from perturbation theory, the H20 lone-pair and metal bonding orbitals are more polarized to the metal. H20 heights on all surfaces decrease as the potential increases. For H20 adsorption bond strengths on Pt( 111) and Ru(0001) the d electron count is the dominant variable. This is shown by adding two additional electrons per Ru to the Ru18 valence band. Doing this and using the original H20 structure on the R U Icluster ~ results in an H20 absorption energy of 1.83 eV, which is essentially the same as for the Pt surface clusters, as given in Table 2. By the same token, we find that when the Ptl8 cluster orbital occupation is reduced by 36 electrons to that of R u I ~the , H20 adsorption energy increases to 2.94 eV, which is essentially the same as for the R U Icluster. ~ Thus, for these two metals, the d electron count, Le., the degree of d band filling, determines the H20 adsorption bond strengths. Generation of OH(ads) on Ru Sites We calculate significant decreases in activation energies for generating OH(ads) from adsorbed water on Ru sites com-
Anderson and Grantscharova
9152 J. Phys. Chem., Vol. 99, No. 22, 1995
TABLE 4: Calculated Properties of H20 near OH Scission Transition State Structures, with 0.2 Stretch on Pt, Ru in Pt,and Ru Surface Cluster Models at Three Potentifs (V), Oxygen Height, ho (A), Oxygen Displacement toward Metal Atom Accepting H, do (A), Hydrogen Height, h~ ( ), and Mulliken Charge, q (-e) (See Figure 3)
hi
potential
Pt
ho Ruin Pt
Ru
Pt
RuinPt
Ru
Pt
RuinPt
Ru
Pt
4 RuinPt
Ru
0 112 1
1.56 1.54 1.52
1.17 1.16 1.14
1.46 1.46 1.46
0.57 0.57 0.56
0.21 0.21 0.20
0.54 0.54 0.54
1.40 1.38 1.36
1.09 1.09 1.07
1.33 1.34 1.34
0.85 1.16 1.48
0.61 0.91 1.22
0.77 1.03 1.35
5 v
do
1
0.5
>r
,
0.4
.- I
/n-
c
ul
c
0 .c
0.3
m
2 0.2 c
Y
g
6
Ru in Pt L
W
ov
0.8
Ru in Pt OV
0.6
0.1 1.0 2.0 3.0 4.0 H20 Adsorption Energy (eV)
1.4
Figure 5. Calculated OH bond scission activation energies as functions
1.6
1.8
2.2
of calculated H20 adsorption energies for Pt( 11l), Ru(0001), and Ru in Pt( 111).
OC-OH Distance (A) Figure 6. Calculated reaction energy profiles for CO(ads) bonding to
pared to Pt sites by the mechanism
OH(ads) on Pt, Ru in Pt, and Ru surface cluster models at selected potentials.
H20(ads)
-
OH(ads)
+ H(ads)
(2)
with the transition state shown schematically in Figure 3. Calculated energy profiles for Ru in Pt( 111) and Ru(OOO1) are compared with those for Pt( 111) at 0, 1/2, and 1 V in Figure 4, and graphically estimated activation energies are in Table 3. The very low activation energy for OH(ads) H(ads) formation calculated for the Ru surface cluster model even at low potentials is compatible with the presence of H and 0 species that form at low potentials and the absence of a double-layer potential region. The Ru atom in the Pt( 111) cluster surface has -0.15 eV higher calculated activation energy at the three potentials, but they are still much lower than those calculated for the Pt(111) surface model. Overall, the OH(ads)-forming reaction we have modeled is one of oxidative addition to the surface:
TABLE 5: Calculated Activation Energies E, (eV) for Combining CO and OH According to Eq 5 at Three Different Potentials (V)
+
H,O(ads)
-
OH-(ads)
+ H-(ads) + 2h'
(3)
where h+ represents the formal oxidation of the surface by one electron. On a single metal atom this would correspond to (H-)M*+(OH-), but when on a metal surface, it is convenient to use the h+ (hole) notation. Such reactions at some point have the OH u* orbital becoming occupied as the OH bond stretches, but these transition states are reached before much OH u* mixes in, that is, before enough OH stretching has occurred to bring it down near the Fermi level. Consequently, the strong OH u donation bonding to the surface atoms seems to be responsible for the activation. The greater this interaction, as measured by H20 adsorption energies, the lower is the activation energy for OH bond scission. Figure 5 illustrates this relationship for the Pt( 11l), Ru in Pt(l1 l), and Ru(0001) surfaces. Table 4 gives transition state structures and H20 Mulliken charges for each point in Figure 5. Comparing the charges with those of the initial adsorbed H20 structures in Table 2, it may be seen that in all but one case the H20 charge is greater at the transition state. This is consistent with having
a
potential
Pt
Ru in Pt
Ru
0 1I2 1
0.98 1.13 1.40
1.03 1.10 1.18
1.23 U
Not calculated.
significant donation bonding between the water molecule and the surface in the transition state. Unlike what was found for the H20 adsorption energies, the d electron count has little effect on the OH bond scission activation energies. That is, upon giving the Pt cluster the Ru electron count, the calculated activation energy at 0 V is 0.32 eV, not much changed from the Pt result of 0.48 eV, and when the Ru cluster is given the Pt electron count, the calculated activation energy is 0.09 eV, which is close to the Ru result of 0.12 eV. This means that the antibonding counterpart orbitals to the OH u donation are pushed up into the empty conduction band region. CO(ads) Oxidation by OH(ads) It was found in our earlier studyg that in the low-coverage model, meaning CO and OH alone adsorbed, on Pt( 111) the calculated transition state energy for HO-CO bond formation increased as the potential increased. However, packing CO(ads) or OH(ads) around the reacting species to model -1/2 monolayer coverage activated them through mostly throughbond but also some steric repulsions. Calculated activation energies at 0 V dropped to about 0.3 eV and increased as the potential increased, just as at low coverage. Several reaction energy profiles for bond formation between OH(ads) and CO(ads) are presented in Figure 6 , and the activation energies from these curves are in Table 5. Deprotonation, not modeled here, is assumed to occur after the C - 0 bond has formed in the species -COOH(ads). Again, the
J. Phys. Chem., Vol. 99, No. 22, 1995 9153
Catalytic Effect of Ru in Ru-Pt Alloys
TABLE 6: Initial and Transition State Structures for CO(ads) Oxidation by OH(ads) at 0 and 1 V Potentials on Pt, Ru in Pt, and Ru Surface Cluster Model@ dco
potential
Pt
Ru/Pt
Ru
Pt
0 initial transition 1 initial transition
0 0.65 0 0.85
0 1.07 0 1.21
0 0.75 d
2.02 2.07 2.04 2.15
RM-co Ru/Ptb 2.02 2.08 2.04 2.13
Ro-H
Rco
qco
RM-OH
Ru
Pt
Ru/Pt
Ru
Pt
Ru/Pt
Ru
Pt
Ru/Ptc
Ru
1.92 2.02
1.16 1.19 1.15 1.18
1.16 1.19 1.15 1.18
1.16 1.21
0.52 0.39 0.76 0.79
0.51 0.43 0.76 0.75
0.44 0.31
1.61 1.70 1.58 1.60
1.54 1.56 1.50 1.56
1.51 1.58
eOCo
qOH
@HOC
potential
Pt
Rum
Ru
Pt
RuPt
Ru
Pt
Rum
Ru
Pt
Rum
Ru
0 initial transition 1 initial transition
1.01 1.01 1.01 1.01
1.01 1.01 1.01 1.01
1.01 1.01
0.51 0.35 1.07 1.16
0.28 0.42 0.88 0.91
0.70 0.57
110
117
127
105
90
108
124
122
107
105
In the alloy case CO is initially on Pt and OH is on Ru, and they are perpendicular to the surface. CO displacement from atop site, dco (A), Pt-CO intemuclear distance, RR-CO(A),CO intemuclear distance, RCO(A),Mulliken charge, qco ( - e ) , Pt-OH intemuclear distance, RR-OH,OH intemuclear distance, Ro-H (A), OH Mulliken charge, qOH ( - e ) , 0-c-0 angle, (deg), and H-0-C angle, (deg). See Figure 7. CO bound to Pt. OH bound to Ru. Not calculated.
coco
TABLE 7: Calculated OH Adsorption Energies, E (eV), for OH on Various Sites and Their Potential (V) Dependencies
0
E Pt18
IM
\M’
Figure 7. Transition state structure for CO(ads) oxidation by OH(ads).
activation energy is calculated to increase with potential for the case of OH bound to Ru in Pt( 111). A few partially completed calculations indicate that the same is true for Ru(0001) as well. Initial and transition state structures may be compared in Table 6, and the general structure of the transition states is given in Figure 7. At ‘12 V the activation energies for Pt( 111) and Ru in Pt(ll1) are about the same, and at high coverages, all activation energies should become very low. CO Adsorption on Pt and Ru Surface Sites The motion of CO(ads) across the surface to OH(ads)supporting Ru sites was raised by Gasteiger et al.8 as the probable slow step when the surface concentration of Ru is about 7%. The apparent activation energy for methanol oxidation and, they proposed, CO(ads) motion was 0.31 eV. To explore this suggestion, we have calculated the energy difference between the more stable 1-fold and less stable 2-fold bridging site on the Pt( 111) surface cluster model. Results are 0.31 eV at 0 V, 0.40 eV at 112 V, and 0.45 eV at 1 V. Thus, these numbers are the order of magnitude that can be taken to be consistant with the hypothesis of Gasteiger et al. Previous calculations” using a pt40 cluster yielded 0.16 eV for this difference when parameters corresponding to 0 V were used and also indicated that 2-fold and 3-fold adsorption energies are close to one another. The structural and surface composition details for the motion of CO(ads) toward OH(ads) for reaction on the actual polycrystalline electrode surface, combined with the qualitative nature of the theoretical model, mean that these results support but do not prove that CO motion across the electrode surfaces plays a role in the measured apparent activation energy for methanol oxidation.
potential
1-fold
3-fold
Ru in Ptl2 1-fold
0 1I2 1
3.92 4.41 5.02
3.88 4.29 4.79
4.61 4.85 5.26
Ruis 1-fold 3-fold 5.32 6.16 6.62
5.84 a
Not calculated.
As discussed in ref 5, CO adsorbs on Pt and Ru clean surfaces with nearly the same bond strengths. Although in the present context Ru surface sites are likely to be blocked by waterderived species, calculations have been made. For the 0 V atom parameters, calculated adsorption energies on 1-fold sites are 2.46 eV on Pt18, 2.89 eV on Ruts, and 2.59 eV on Ru in Pt17. This indicates that the alloying of Ru into a Pt surface does not change its bond strength to CO very much. Whereas H20 forms quite a bit stronger donation bonds to Ru than to Pt, as discussed above, CO binding is a more complicated combination of u donation and metal back-donation to its x* orbitals.
OH Adsorption on Pt and Ru Surface Sites Gasteiger and co-workers found a higher overpotential for CO oxidation on pure Ru than on pure Pt and attributed this to the stronger adsorption of OH on Ru than on F t 5 It is of interest to see if our model offers support for this idea. Potentialdependent trends in calculated OH adsorption energies are in Table 7. The OH adsorption energies are indeed stronger on Ru, particularly on the 3-fold site, although our focus has been on OH and CO beginning on adjacent 1-fold sites. In our Pt(111) study we found the activation barrier was doubled for OH restricted to the 3-fold site.9 Thus, this simple model supports the conjecture in ref 5. For Ru in Pt the activation energy (Table 5) is slightly greater for the alloy with OH on Ru at 0 V compared to Pt(ll1) and less at 1/2 and 1 V. This is suggestive of a synergistic effect of the alloy surface, as proposed in ref 5. Concluding Comments We have found that for Pt and Ru in Pt alloy cluster models the CO oxidation step given in eq 1 has a high activation barrier which does not decrease as the potential increases. However, as shown in ref 9, when there is a high coverage of coadsorbed species, the barrier becomes very low. Hence, the slow and
9154 J. Phys. Chem., Vol. 99, No. 22, 1995
Anderson and Grantscharova
potential-dependent step corresponds to an earlier stage of the References and Notes process. We find that H20 activation to form OH(ads) (1) Bockris, J. O’M.; Wroblowa, H. J. Electroanal. Chem. 1964, 7, according to eq 2 has the necessary properties to explain the 428. Dahms,I+; Bockris, J. O’M. J . Electrochem. Soc. 1964, 111, 728. (2) Petry, D. A,; Podlovchenko, B. I.; Frumkin, A. N.; Lal, H. J. key experimental observations in the literature: (i) the activation Electroanal. Chem. 1965, 10, 253. energies on all surfaces decrease as the potential increases and (3) Muller, E. 2.Elektrochem. 1923, 29, 264. Muller, E.; Takegami (ii) the activation energy is lower on the Ru in Pt alloy than on Ibid. 1928, 34, 704. Tanaka, S. Ibid. 1929, 3.5, 38. Pt. The greater activation on Ru has been linked to stronger (4) Leger, J.-M.; Lamy, C. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1021. donation bonding, a consequence of the relative electronic (5) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J . structure. Once OH(ads) forms, there is evidence that the rate Phys. Chem. 1994, 98, 617. of its reaction with CO(ads) may also play some role in the (6) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. rate of CO(ads) removal at a given overpotential. Overall, our (7) Ticanelli, E.; Beery, J. G.; Paffett, M. T.; Gottesfeld, S . J. J. results are closely supportive of the interpretation concerning Electroanal. Chem. 1989, 258, 61. the activities of those systems by Gasteiger and c o - w o r k e r ~ . ~ - ~ ~ ~ (8) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J . It is noted that other oxophilic alloying metal atoms might be Electrochem. SOC.1994, 141, 1795. (9) Anderson, A. B.; Grantsharova, E. J . Phys. Chem. 1995,99,9143. expected to behave similarly to Ru when in Pt. We will report (10) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, our results in a later publication. 98, 5074. (11) Mehandru, S.P.; Anderson, A. B.; Ross, P. N. 1.Cural. 1986,100, 210. Acknowledgment. This work was funded by ARPA through ONR Contract No. N00014-92-J-1848. JP942636G
