J. Phys. Chem. 1995, 99, 9143-9148
9143
Potential Dependence of CO(ads) Oxidation by OH(ads) on Platinum Anodes. Molecular Orbital Theory Alfred B. Anderson* and E. Grantscharova5 Chemistry Department, Case Westem Reserve University, Cleveland, Ohio 441 06-7078 Received: September 29, 1994; In Final Form: March 24, 199.5@
An atom superposition and electron delocalization molecular orbital (ASED-MO) study has been made of the electrochemical oxidation of the CO poison on the Pt anodes that is generated during methanol fuel cell operation. The interaction of CO(ads) with the oxidant OH(ads) is shown to be productive only at high surface coverages, as high coverages activate CO(ads) by weakening its bond to the surface. The potential dependence of this reaction, modeled by shifting the metal valence band parametrically, has the wrong behavior for it to be the rate-limiting step. A rate-limiting step which has good qualitative agreement with measured apparent activation energies at different potentials is the generation of OH(ads) from HzO(ads), for which the activation energy barrier is calculated to decrease as the potential increases.
Introduction The mechanism for the oxidative removal of the CO poison from platinum anodes has been a topic of intense investigation for the past 30 years. This strongly adsorbed species blocks the surface, quickly damping currents produced by fuel cells running on methanol or other organic fuels. The overall reaction for removing CO is CO(ads)
+ H,O - CO, + 2H' + 2e-
(1)
but the mechanistic details have been elusive. Gilman suggested in 1964 a mechanism where an adsorbed water molecule attacks an adsorbed carbon monoxide molecule with subsequent and separate deprotonation and electron transfer steps.' A recent theoretical study by Shiller and Anderson2was not supportive of this particular mechanism. In it, molecular orbital calculations showed that as the potential increased, as modeled by shifting the platinum valence band down on the energy scale, the activation energy barrier for the reaction in eq 1 increased. The measured overpotential for CO oxidation on platinum is about 0.6 V and is a kinetic phenomenon. The thermodynamic potential for this reaction lies close to 0 V versus the reversible hydrogen electrode. Thus, a viable mechanism must have the proper potential dependence for its slow step; that is, its activation energy must decrease as the applied potential is increased. The above step was calculated to have the opposite behavior. A factor acting to make the adsorbed water molecule inactive toward adsorbed CO was the increase in its lone-pair donation bond strength as the potential increased, making the platinum surface a better Lewis acceptor. Since the water molecule was calculated to rise about 1 8, higher from the surface on going to the transition state, while the CO(ads) rose less than 0.1 A, the increased potential, which caused the H?O(ads) to be held more tightly on the surface, served to increase the activation energy needed to reach the transition state. As might be expected from this, further theoretical investigation3 showed that a nonadsorbed water molecule could react with CO without the activation barrier increasing with potential.? In fact, a H2O-CO intermediate was calculated for the form with a low nearly potential-independent barrier. This 8 On leave from the Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria 1 1 13. Abstract published in Advance ACS Abstracts, May 1, 1995. @
0022-3654/95/2099-9143$09.00/0
species, which is bent and bound through C to a surface Pt atom, desorbed in a reductive elimination sense, reducing the metal surface by two electrons while, it was postulated, the two protons would simultaneously transfer to water molecules, forming hydronium ions. This process was activated by increased potential because the occupied C-Pt bonding orbital rises as this bond breaks, promoting two electrons to this Fermi level. This process takes less energy as the Fermi level becomes more stable. Experimental evidence in support of the mechanism wherein there is a direct attack of CO(ads) by H20 is sketchy. Wieckowski observed a weak isotope effect when D20 was used as the solvent, with only small entropy changes for the activation step over platinized Pt electrode^.^ The entropy result means that water decomposition to OH(ads) by the mechanism
Pt
+ H,O
OH(ads)
+ Hf + e-
(2)
must not be occurring because if it did the solvation of the proton would substantially decrease the entropy of the system. Thus, it was suggested that OH(ads) did not play the role of oxidant others had proposed it did.4 The absence of a large isotope effect also led Wieckowski to conclude that the activated complex of H20 and CO(ads) is reactant-like and that its deprotonation is not rate limiting. This is all consistent with the quantum chemical modeling in ref 2. However, there is a continuing belief that OH(ads) is in some circumstances the oxidant of CO(ads). Gasteiger et al.5 have presented circumstantial evidence that OH(ads) is an oxidant by showing that apparently isolated Ru atoms, alloyed in a polycrystalline pt surface, shifted the CO electrooxidation current by -0.25 V. This was interpreted as the Ru atoms in the surface providing nucleation sites for OH(ads) formation. This OH(ads) was presumed to oxidize CO adsorbed on adjacent Pt in the rate-determining step CO(ads)
+ OH(ads) - CO, + H+ + e-
(3)
It was subsequently found that during methanol oxidation on polycrystalline Pt surfaces, with about 1 in 10 atoms replaced by Ru atoms, the activation barrier was about 0.3 eV, while on surfaces with about 1 in 3 and 1 in 2 atoms replaced by Ru atoms the activation barrier doubled to about 0.6 eV. The 0.3 eV barrier was assigned to the activation energy for CO diffusion 0 1995 American Chemical Society
9144 J. Phys. Chem., Vol. 99, No. 22, 1995
Anderson and Grantscharova
over Pt to OH bound to Ru. The 0.6 eV barrier was assigned to the early stages of CH30H oxidative dehydrogenation having higher activation energy when, due to the surface alloying, surface Pt ensembles were no longer present. The Ru atoms in the Pt surface were presumed to generate active OH in the platinum double-layer region, which is the range -0.3 to -0.7 V and is bordered at the low potential by the desorption of hydrogen and at the high potential by OH(ads) formation. Thus, the CO(ads) stripping oxidation peak moves from -0.7 V on Pt to as low as -0.45 V for 46% Ru surface composition. Other evidence for OH(ads) playing a role in the oxidation of CO adsorbed on Pt comes from the hanging meniscus rotating disk measurements of Lim6 The sharply peaked CO oxidation voltammograms observed were attributed to OH(ads) formation on the polycrystalline platinum electrodes. Raicheva and co-workers found by measurements in the temperature range 15-60 "C that activation energies are about the same for oxidizing various alcohols over Pt.' Furthermore, they decreased as the potential increased. Gasteiger et al. interpreted this to mean that there is likely to be a negative potential shift for the onset of the nucleation of OH(ads) as the temperature is increased.8 Given the circumstantial evidence for OH(ads) acting as an oxidant for the removal of the CO(ads) poison on platinum electrodes as C02, we have undertaken an atom superposition and electron delocalization molecular orbital (ASED-MO) study of the reaction in eq 3. This is done in the same spirit as the above-mentioned ASED-MO study2 of the oxidation of CO(ads) by H20 given in eq 1. That is, two-layer-thick cluster models are used to represent the Pt( 111) surface modeled in these works. All Pt atoms touching the adsorbed species are coordinated by all nearest neighbors in our models. The cluster valence band shifting technique is used to represent the effects of changing the potential applied to the electrode. We study 0-1 eV stabilization of the Fermi level of the clusters, corresponding approximately to going from 0 to 1 V on the normal hydrogen electrode scale. For this type of cluster modeling of the platinum surface, our standard parameters, corresponding to the 0 V band position, have given an adequate description of a variety of chemisorption and reaction phenomena as measured at the vacuum interface. As ref 15 shows, the work function of the Pt(ll1) surface in vacuum is a bit over 1 eV greater than the projected value of the work function of the normal hydrogen electrode at 0 V on the NHE scale. However, CO adsorption increases the work function, and coadsorption of H20 and CO will lead to values between the vacuum result and the 0 V electrode result. Consequently, we have decided to pick our standard Pt parameters to represent 0 V as a model for 0 V on the NHE scale. Our emphasis is necessarily on the topic of change, that is, on how properties of adsorbed H20 and CO and their reactions change as the Fermi level of the electrode surface is stabilized. We cannot in this model predict quantitatively the potential at which an activation energy will assume a certain value, but we believe we can predict whether it increases or decreases as the potential increases. Therefore, our focus is on establishing or corroborating chemical trends and then explaining them with concepts of perturbationmolecular orbital theory for chemical b ~ n d i n g .This ~ technique has been used to model the potential dependencies of adsorbed CO and the generation of 0 2 from H20, among other potentialdependent phenomena: these studies have been r e ~ i e w e d . ~ Two reactions are examined in this work. First, we analyze the formation of OH(ads) from adsorbed HzO by the mechanism H,O(ads)
-
OH(ads)
+ H(ads)
(4)
1-folb'sites Figure 1. Bulk superimposable P t 1 8 cluster used to model the (1 11) surface in the calculations. Nearest neighbor distance is 2.77 A. Arrows indicate adsorption and reaction sites.
and the potential dependence of the transition state energy. We note that the entropy change for this will be small, overcoming the problem of eq 2 as stated by Wieck~wski.~ This is followed by a study of the reaction of OH(ads) with CO(ads) up to a transition state: OH(ads)
+ CO(ads) - HO.
CO(ads)
(5)
The potential and coverage dependence of the activation energy for this step are modeled. HzO Adsorption and Generation of OH(ads) An earlier ASED-MO studylo of H20 molecular adsorption and dissociation on Pt( 111) and on Pt( 111) with coadsorbed 0 atoms yielded low activation energies for the generation of OH(ads) H(ads) according to eq 4 and also for the proton transfer reaction
+
H,O(ads)
+ O(ads) - 2 OH(ads)
In that study, employing a one-layer cluster model of the (111) surface, the H20 molecule was found to bind most stably on a 1-fold atop site. In this work we use the larger P t l 8 cluster shown in Figure 1 and also study the potential dependence of the activation energy for the OH-forming reaction in eq 5. The parameters we use, given in Table 1, are essentially those in ref 10 and are based on the The Ptl8 band structure can be roughly partitioned as shown in the right-hand column of Figure 2. At the bottom is a region of Pt 6s states, above are nearest neighbor Pt-Pt d bonding orbital energy levels and then Pt d nonbonding orbital levels, and then nearest neighbor d-d antibonding orbitals dominate the electronic structure. The 6s and 6p orbitals are mixed throughout, and above the antibonding region of the d set they are empty. Binding of the H20 molecule is characterized as lone-pair donation to the platinum surface. As the correlation diagram in Figure 2 (which is for the 0 V potential in our model potential scale) illustrates, the antibonding counterparts of the H20 3a1 lone-pair donation are practically all empty and for the lbl lone-pair ~t donation they are not highly occupied. However, despite the lack of occupation of major antibonding counterparts to the water lone-pair plus surface bonding orbitals, water adsorption is still relatively weak. This is because there is a closed-shell repulsion between the water lone-pair orbitals and the low-lying Pt 6s, 5d bonding, nonbonding, and most of the 5d antibonding surface orbitals. There is a net lone-pair charge donation to the surface due to the lack of occupation of many antibonding adsorbate-substrate orbitals, and this contributes to the observed work function decrease.I5
CO(ads) Oxidation by OH(ads) on Platinum Anodes
J. Phys. Chem., Vol. 99, No. 22, 1995 9145
TABLE 1: Parameters Used in the Calculations (See Refs 10-14): Principal Quantum Numbers, n, Diagonal Hamiltonian Matrix Elements, H (eV), Orbital Exponents, (au), and Linear Coefficients, c, for Double.< d Orbitals P
S
d
atom
n
H
t
n
H
5
n
H
CI
51
c2
52
Pt
6 2 2 1
-10.5 -15.09 -26.98 -12.1
2.554 1.6583 2.1460 1.2
6 2 2
-6.46 -9.76 -12.12
2.25 1.6180 2.1270
5
-11.1
0.65581
6.013
0.57150
2.396
C
0 H
TABLE 2: H20 and CO Adsorption Energy, E (eV), Height, h (A), and Mulliken Charge on H20, q (-e) on the 1-Fold Atop Site of the Ptls Cluster (Figure 1) as a Function of Relative Potential (V) Modeled by Shifting the Cluster Valence Banda potential
E(H20)
E(C0)
h(H2O)
h(C0)
q(H20)
0 112 1
1.81 2.29 2.86
2.46 2.86 3.29
1.79 1.75 1.73
2.02 2.03 2.07
0.76 1.oo 1.27
The HOH angle is 120’ in each case.
0.41
Figure 2. Calculated correlation diagram for H2O binding to a central 1-fold site of the Pt18 cluster in Figure 1. The hatched region at the top of the doubly filled band region indicates singly occupied orbital energy levels at the top of the d band: there are eight unpaired electrons. The bracketed 6s, bonding (b), nonbonding (n.b.), and antibonding (a.b.) regions are as discussed in the text.
The calculated water molecule adsorption energy increases as the potential is increased as given in Table 2. The valence band shifts corresponding to the potential shifts are achieved by decreasing the Pt valence s, p, and d H values in Table 1 by amounts of 1/2 and 1 eV. The Fermi level is calculated to decrease by practically the same amounts. Note from the adsorption energies for H20 and CO, also given in Table 2, that CO systematically is predicted to bind more strongly than H20. The calculated increase with potential of the H20 binding energy and the accompanying decrease in water molecule height above the surface as well as an increased H20 charge with increasing potential are expected from Figure 2: as the Pt surface band moves down, the lone-pair orbitals donate more strongly to the partially empty antibonding part of the d band and there is less closed-shell repulsion with the doubly occupied part. By the time the potential has reached 1/2 V in Table 2, the charge on the water molecule has reached +l. If a previously used rule applies, deprotonation to the double layer might be expected in alkaline electrolytesI6 according to eq 2, but for acidic solutions used in fuel cell studies this reaction will require high potentials and the generation of OH(ads) by the surface reaction of eq 4. Transition states for water OH bond cleavage on the cluster surface as a function of otential were found by stretching one of the OH bonds in 0.1 increments, optimizing the complete structure, and plotting the energies. The resulting reaction energy profiles are in Figure 3. The activation energies are calculated to decrease by 65% as the potential increases from 0 to 1 V, as shown in Table 3. For each potential the H20 molecule bonds are “V”-shaped, throu h 0 to the 1-fold site, when the OH bond is stretched by 0.1 to 1.1 A, and when it is stretched by 0.2 A to 1.2 A, the structure changes to “L”shaped, with the OH bond bridging two Pt atoms, as shown in Figure 4. Table 4 gives structure parameters and molecular
R
1
6 b = w
pov
0’31lRZV
0.2 0.1
1
0.0
+v
1.1 1.2 1.3 0 - H Distance (A)
Figure 3. Potential dependence of the reaction energy profile for OH bond scission in water bound to the Pt(ll1) cluster model.
TABLE 3: Calculated Activation Energies, E , (eV), for OH Bond Scission According to the Reaction in Eq 4 at Three Different Potentials (V) E,
potential 0 112 1
0.44 0.33 0.17
charges for the three near transition state structures with 0.2 A OH structures. The increase in H20 charge with potential in Table 4 and the decrease in transition state OH bond stretch with potential that is visible in Figure 3 indicate the dominance of OH o donation. This becomes stronger as the potential increases. As the OH bond is stretched, the o* orbital mixes more and more, gaining large occupation after the transition state is passed. For further discussion of the orbital interactions in the transition state, ref 10 may be consulted. CO(ads) Oxidation by OH(ads): Low Coverage
The reaction of CO(ads) with OH(ads) to form C02 according to eq 3 on a metal surface at low coverage ought to have a reaction energy profile similar to that for the oxidation of CO(ads) by O(ads) at low coverage, but with a somewhat lower activation energy. Single-crystal ultrahigh-vacuum studiesI7 found an activation barrier of 1.7 eV for the reaction CO(ads)
+ O(ads)
-
C02
(7)
This decreased to 0.7 eV at high coverage. ASED-MO calculations yielded 1.6 eV for the low-coverage barrier.Is
Anderson and Grantscharova
9146 J. Phys. Chem., Vol. 99, No. 22, 1995
\
Pt
Pt
Figure 4. Orientation of H20 at the transition states for OH bond
scission. TABLE 4: Potential (V) Dependencies of Calculated H 0 Properties in Near Transition State tructures with 0.2 OH Stretches: Oxygen Height, ho ( ), Oxygen Displacement from Atop Site toward Second Pt Atom, do (A), Hydrogen Height, h~ (A), and HtO Mulliken Charge, q(-eP
A
1
potential
ho 1.56 1.54 1.52
0 1I2 1 a
do
hH 1.40 1.38 1.36
0.57 0.57 0.56
4 0.85 1.16 1.48
The HOH angle is 103" in each case. See Figure 3.
1
0.61 OV
0.41 0.2
1.4 1.6 1.8
OC-OH Distance (i) Figure 5. Potential dependence of energies of the CO(ads) (ads) system as functions of the HO-CO distance.
+ OH-
TABLE 5: Calculated Activation Energies E, (eV) for Combining CO and OH According to Eq 5 at Three Different Potentials (V) potential
E,
0 112 1
0.98 1.13 1.40
+ 02-+ 2h'
-
CO,
(8)
where h+ stands for missing electrons or holes at the metal Fermi level in the vicinity of the 02-(ads). Thus, in eq 8 two electrons are promoted from the low-lying 0 2p orbitals to the Fermi level, and this promotion is likely to contribute to the activation barrier. Rewriting eq 3 in the form CO(ads)
+ OH-(ads) + h+ - CO, + H+ + e-
\Pt
Figure 6. Transition state structure for CO(ads) oxidation by OH(ads).
the Fermi level so that an electron flows into the anode and through the circuit. Whether the activation barrier is higher or lower than that for oxidizing CO(ads) by O(ads) remains to be seen. Our calculated reaction energy profiles for combining reacting CO(ads) and OH(ads) beginning with them adsorbed on adjacent sites to just beyond the transition state for CO bond formation, eq 5, are in Figure 5 , and activation energies are listed in Table 5. The adsorption energy for CO and OH together on adjacent sites is calculated to be 0.20 eV less than the sum of individual adsorption energies on the Ptl8 cluster, indicating some repulsion. We note that OH is calculated to be about as stable in the central 3-fold site of the Ptlg cluster as on the 1-fold site. Preferences for the 1-fold site are 0.04, 0.12, and 0.23 eV at 0, 1/2, and 1 V potentials, respectively. The general transition state structures for OH beginning in the 1-fold site are as in Figure 6; parameters are in Table 6. The calculated value for 0 V potential, 1.O eV, is significantly less than 1.6 eV previously calculated for the O(ads) oxidant. However, 1.O eV is not only too high, given that methanol oxidation has an activation barrier of -0.4 eV,' but the calculated E, at positive potentials are higher, which is the wrong potential dependence for this to be the rate-limiting step. For OH beginning in the central 3-fold site the calculated E, decrease slightly with increased potential, but the values are far higher, at about 2.1,2.0, and 1.9 eV. This means that either an entirely different pathway is followed for C02 production, one with a lower activation energy, such as H20 reacting with CO(ads) as studied in ref 2, or something influences the present mechanism to lower the barrier. The latter possibility is considered next. CO(ads) Oxidation by OH(ads): High Coverage
Equation 7 is a reductive elimination from the surface. To bring this out, it is rewritten CO(ads)
I Pt
(9)
it is evident that only one hole is reduced in this case compared to two in eq 8.19 However, a second electron is promoted to
If Gasteiger and co-workers8 are right, OH(ads) reacts as fast as it is formed with CO(ads). High coverages of CO are expected to form, ultimately poisoning the surface. We have modeled the effects of -1/2 monolayer CO and -112 monolayer OH on the oxidation reaction by means of the model shown in Figure 7. An extended Huckel study has also been made of coadsorbed CO and OH.*O As may be seen in Figure 8, our calculated reaction energy profiles and the activation energies are significantly stabilized at 0 and 1 V for -1/2 monolayer CO coverage and also for -1/2 monolayer OH coverage at 0 V. The transition state structures are only slightly changed compared with low coverage, the maximum change being 11O for the OCO angle and f 0 . 1 6 8, for the Pt-OH distance at high CO coverage; most changes are far smaller. We attribute this high-coverage activation to weakened CO and OH adsorption energies. The CO adsorption energy is well-known to decrease markedly at high coverage, and there is evidence that
+
CO(ads) Oxidation by OH(ads) on Platinum Anodes
J. Phys. Chem., Vol. 99, No. 22, 1995 9147
TABLE 6: Initial and Transition State Structures at Three Potentials (V) for CO(ads) Oxidation by OH(adsy potential
state initial transition initial transition initial transition
0 1/2 1
dco
0.0 0.65
0.0 0.73 0.0 0.85
RR-co 2.025 2.071 2.035 2.100 2.044 2.153
Rc-o 1.162 1.186 1.157 1.190 1.153 1.181
qco 0.52 0.39 0.64 0.52 0.76 0.79
&-OH
1.611 1.705 1.592 1.697 1.580 1.597
ROH 1.006 1.007 1.008 1.010 1.011 1.014
qOH 0.51 0.35 0.83 0.67 1.07 1.16
eoco
@HoC
110.6
105.0
120.0
110.3
124.4
106.8
Initially, CO and OH are perpendicular to the surfaces. CO displacement from atop site, dco (A),Pt-CO intemuclear distnace, R~-,-o (A), C - 0 intemuclear distance, Rc-0 (A), CO Mulliken charge, qco ( - e ) , Pt-OH intemuclear distance, RR-OH(A), 0-H internuclear distance, RO-H (A), OH Mulliken charge, qOH ( - e ) , 0 - C - 0 angle @wo(deg), and H-0-C angle, @Hoc(deg). See Figure 6.
Discussion and Conclusions
CO or OH
\
OH
Figure 7.
high CO and high OH coverage models for studying the effects of high coverage on the CO(ads) OH(ads) reaction. Circles are all either CO or OH. Squares are the adsorbed reactants. For surrounding CO on the 1-fold site, the intemuclear distance and height, 1.16 and 2.02 A, respectively, are based on optimization of the cluster in Figure 1, as are the respective values of 1.18 and 1.65 8, for the 2-fold bridging site. The corresponding values for OH are 1.00, 1.61, 1.00, and 1.12 A. Pt18
+
s Y
g )r
C
w
C
B .-9
-
Y
1.4 -
1.21.0-
??
Pt,(111)
pJov
0.8- d
0.6-
1v
1v
0.40.2-
1.4 1.6 1.8 2.2
OC-OH Distance (A) Figure 8. Reaction energy profiles and their potential dependencies for low and high (Figure 7) coverages of CO at 0 and 1 V and for high coverage of OH at 0 V.
the same holds for OH.21 On the Pt18 model of Figure 1 the calculated CO adsorption energy is 2.46 eV at 0 V. When OH is on an adjacent site this is 2.26 eV. When 5CO OH are surrounding the CO adsorption site, as in Figure 7, the value is 1.27 eV. The weakening of the CO adsorption bond at high coverage might be expected to have a through-space closedshell or van der Waals repulsion component, since some COCO distances are -2.5 A, and a through-bond component, since extant surface ligands form bonding interactions with surface metal orbitals which interfere with bonding with additional ligands. We have crudely separated these components by calculating the interaction of CO with 5CO OH arrayed as in this high-coverage model, but without the metal atoms included in the calculations. A repulsion of 0.18 eV is obtained, which is a small portion of the calculated 1.19 eV decrease in CO adsorption energy. Thus, we conclude that at roughly halfmonolayer coverage through-bond repulsions are strong compared to through-space repulsions.
+
+
This study illustrates the importance of coverage effects and electrochemical potential on surface chemistry. For the mechanism whereby CO(ads) is oxidized on Pt by OH(ads), the activation banier is too high to correspond to experiment at low coverage. When high coverage is assumed, CO(ads) and OH(ads) are activated for reaction with each other. Since we modeled -1/2 monolayer coverage and found a large lowering of the activation energy for this step, to less than the value of the measured apparent activation energy from ref 7, and since even higher coverage is probable, we suggest that this reaction is not the slow step. Another argument in favor of this conclusion is that the calculated activation energy for this step increases as the potential increases, whereas measured apparent activation energies decrease with increased potential. We note that the measured values’ are larger than found for methanol oxidation on 1 in 10 Ru in Pt alloy surfaces at 0.4 V (0.4 eV), for which the slow step was attributed to CO diffusion to OH bound to Ru in the surface. Measured values for various alcohols on polycrystalline Pt are about 0.5 eV at about 0.9 V and about 0.3 eV at about 1.35 V.7 Although the absolute values of calculated OH(ads) formation activation energies from adsorbed H20 are lower, there is a decrease in these energies as potential increases, at the rate of 0.32 e V N for 1/2 to 1 V compared to 0.44 e V N around 1 V from e~periment.~ On this basis we conclude that it is possible that the slow step in the removal of the CO(ads) poison from platinum anodes in methanol fuel cells is the generation of the OH(ads) oxidant. That Ru alloyed into the anode is able to generate OH(ads) at a lower potential than surface Pt atoms will be demonstrated in the next paper,22providing support for the interpretations of Gastiger et al.598 Acknowledgment. This work was funded by ARPA through ONR Contract No. N00014-92-J-1848. References and Notes (1) Gilman, S . J. Phys. Chem. 1964, 68, 70. (2) Shiller, P.; Anderson, A. B. J . Electroanal. Chem. 1992,339, 201. (3) Wieckowski, A. J. Electroanal. Chem. 1977, 78, 229. (4) Petry, 0. A,; Podlovchenko, B. I.; Frumkin, A. N.; Lal, H. J . Electroanal. Chem. 1965, I O , 253. ( 5 ) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J . Phys. Chem. 1994, 98, 617. (6) Lin, S. A. Ph.D. Thesis, Case Westem Reserve University, 1991. (7) Raicheva, S. N.; Christov, M. V.; Sokolova, E. I. Electrochim. Acta 1981, 26, 1669. ( 8 ) Gasteiger, H. A,; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Electrochem. SOC. 1994, 141, 1795. (9) Anderson, A. B. J . Electroanal. Chem. 1990, 280, 37. (10) Anderson, A. B. Surf. Sci. 1981, 105, 159. (1 1) Ionization potentials are taken from the following: Lotz, W. J. Opt. SOC.Am. 1970, 60, 206. (12) Empty 6p orbital ionization potentials are deduced using the lowest 6p 6s optical excitation energy taken from the following: Moore, C., Ed. Atomic Energy Levels, Vol. 111; Circular of the National Bureau of Standards 467; 1958; p 181. (13) Clementi, E.; Raimondi, D. L. J . Chem. Phys. 1963, 38, 2686. 8
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Anderson and Grantscharova
9148 J. Phys. Chem., Vol. 99, No. 22, 1995 (14) Basch, H.; Gray, H. B. Theoret. Chim. Acta 1966, 4 , 367. (15) Wagner, F. T., Moylan, T. E. Proc.-Electrochem. Soc. 1992, 9211, 25. (16) Anderson, A. B. Proc.-Electrochem. Soc. 1992, 92-11, 434. Mehandru, S. P.; Anderson, A. B. J . Electrochem. Soc. 1989, 136, 158. (17) Gland, J. L.; Kollin, E. B. J . Chem. Phys. 1983, 78, 963. (18) Ray, N. K.; Anderson, A. B. Surf. Sci. 1982, 119, 35. (19) A reviewer pointed out that our calculated OH(ads) charges are positive even at 0 V (Table 6). whereas its charge is around -0.7 when bound to cluster models of Cu( 11l), according to self-consistentcalculations. See: Hermann, K.; Witko, M.; Pettersson, L. G. M.; Siegbahn, P. J . Chem. Phys. 1993, 99, 610. Using standard parameters, a two-layer-thick Cu3l cluster, and heights calculated by Hermann et al., we obtained OH(ads)
charges of -0.7 and -0.5 for the 1-fold and 3-fold fcc sites, respectively. This suggests the possibility that, due to 0 2p back-donation when in OH(ads) on Pt due to the higher Pt work function (ref 15) and larger Pt d orbitals, there is much less electron transfer to OH on Pt. Whether or not OH(ads) might actually bear a positive charge on Pt cannot be said for certain in the absence of self-consistent calculations. (20) Estiu, G.; Maluendes, S.; Castro, E. A. Arvia, A. J. J . Electroanal. Chem. 1990, 283, 303. (21) Mooney, C. E.; Anderson, L. C . ; Lunsford, J. H. J . Phys. Chem. 1993, 97, 2505. (22) Anderson, A. B.; Grantscharova, E. J . Phys. Chem. 1995,99,9149. JP9426350
