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On the reaction pathway for methanol and carbon monoxide electrooxidation on Pt-Sn alloy versus Pt-Ru alloy surfaces
Electrochimico Acta.Vol.41. No. 16, pp. 2587-2593, 1996 Copyripht0 1996 ElsevierSciena Ltd. Printedin &eat Britain.Allrightsreserved @x3-4686/96 $15.0 + 0.00
Pergamon
ON THE REACTION PATHWAY FOR METHANOL AND CARBON MONOXIDE ELECTROOXIDATION ON Pt-Sn ALLOY VERSUS Pt-Ru ALLOY SURFACES K. WANG, H. A. GASTEIGER, N. M. MARKOVIC and P. N. Ross, JR* Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA (Acceptedfor
publication
1 November 1995)
Abstract-It has been observed in this work and previous studies that Pt,Sn alloy surfaces are very effective catalysts for CO electrooxidation, but not for methanol electrooxidation. Since CO,, is postulated to be an intermediate in methanol electrooxidation on Pt alloy surfaces, the relative inactivity of Pt,Sn for methanol oxidation appears paradoxical. We present an explanation for this apparent contradiction in terms of a unique state of CO,,, on this surface, which is not the same state of CO,,, as occurs on either Pt-Ru or pure Pt surfaces. It is also not a state of CO,,, which is produced by methanol dehydrogenation. The state is unique in the sense that a significant fraction of CO,,, is oxidized at a much lower (-c 400mV) potential than the rest of the CO,,, , a phenomenon that does not occur on any other Pt and Pt-alloy surfaces examined in the same way. This CO state is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by CO,,, at low coverage. Copyright 0 1996 Elsevier Science Ltd Key words: Electrocatalysis, Pt-Sn alloys, Pt-Ru alloys, carbon monoxide, methanol.
1. INTRODUCTION
kc0
CO,,, + OH,,, -CO*
The electrocatalytic properties of Pt-Ru and Pt-Sn alloys have been widely studied in the last three decades due to their promising activity for the electrooxidation of carbon monoxide, methanol and other C, compounds that are potential fuels for fuel cells[l]. There have been more consistent reports of enhanced activity (relative to pure Pt) for Pt-Ru alloys than for Pt-Sn alloys. For example, for Pt-Ru the activity is almost always higher than that of pure Pt regardless of the method of preparation of the catalysts or of the method of testing the activity, eg[2-51. On the basis of recent systematic studies of electrooxidation of CO and methanol on wellcharacterized Pt-Ru alloys in our laboratory[6-81, an oxidation pathway for methanol oxidation has been proposed to explain the effect of Ru on enhancing the activity of Pt. Several reaction steps are involved in this model, represented by the following equations: kd CH,0H,,e*CH30Hoda
-
COad,+4H+
+4e-
(1) H20e*OH,d,
+ H+ + e-
(2)
l Author to whom correspondence should be addressed: Mail Stop 2-100, 1 Cyclotron Road, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA.
+ H+ + e-
(3)
where COad, represents one of possible products of the dehydrogenation of methanol, k, is the rate constant of methanol dehydrogenation and k,, the rate constant of CO oxidation into CO,. It is proposed that step 1 occurs primarily at ensembles of Pt sites while step 2 occurs primarily at Ru sites, and step 3 occurs at Pt-Ru pair sites. This model, in conjunction with the adsorption properties of Pt and Ru for CO, OH and methanol, explains our experimental finding that a Pt-Ru alloy surface with a ca. 10% Ru concentration has the maximum activity for methanol oxidation: the maximum concentration of three-fold Pt sites, which are expected to be most active ensembles for methanol adsorption, coordinated to a Ru atom occurs at about 10 atomic% Ru[6-71. However, the literature on the effect of Sn as a promoter of catalytic activity of Pt toward methanol oxidation turns out to be much less consistent. Some studies report Sn enhances the activity of Pt[9-151, while others report either an inhibition of the methanol oxidation reaction or an activity comparable to that of pure Pt[2, 16-171. Even in cases where there are reports of enhanced activity, the enhancements are much smaller than those reported for Pt-Ru alloys, es rarely are the enhancements more than a factor of 5. One consistent feature of the reports of enhanced activity is that the states of the two metals in the catalyst are not well known, ie they may not be alloyed, whereas the reports of little or no
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K. WANG et al.
enhancement by Sn have come from studies using well characterized bulk Pt-Sn alloy electrodes, specifically Pt,Sn. One of the reports of little or no enhancement by a Pt,Sn alloy surface was from this laboratory[16]. In an attempt to clarify the contradictions and to understand the role of Sn as a promoter in the methanol oxidation, Haner and Ross[l6] conducted a study using a combination of electrochemical and UHV surface analytical methods with the low index single-crystal faces of Pt,Sn alloy and Pt, with the latter being modified by having Sn(I1) in solution. They observed that none of the alloy surfaces were more effective catalysts than any of the pure platinum surfaces under the measurement method used (slow potential sweep) and there was a significant enhancement (at least a factor of 6) on Pt(l11) and Pt(100) due to Sn(I1) in the electrolyte at very low concentration, eg 1 PM. A hybrid homogeneous-heterogeneous mechanism involving a Sn(II)/Sn(IV) redox “shuttle” sequence was proposed to account for the enhancement. The results of Haner and Ross would appear to reconcile much of the disparities in the literature concerning Pt-Sn catalysts. If Sn is not alloyed with Pt in the catalyst, it is unstable and dissolves in strong acid upon immersion in the cell. As a consequence, one could observe an enhancement due to the presence of Sn(II)/Sn(IV) in solution, the magnitude depending on the concentration of Sn ions in solution. The largest enhancement found by Haner and Ross was about a factor of 6, comparable to the largest enhancements reported for Pt-Sn catalysts in the literature. It was still not clear why those Pt-Sn alloy surfaces were not active for methanol oxidation. In terms of the reaction pathway, the bifunctional mechanism discussed above for the Pt-Ru catalyst would appear to apply as well to Pt-Sn, since Sn in the Pt-Sn catalyst, like Ru in the Pt-Ru catalyst, is suggested to facilitate the adsorption of oxygencontaining species such as OH,,,[18-193. But molecular orbital calculations by Anderson et a/.[201 predict that Sn in a Pt-Sn alloy would not dissociate water preferentially, and that Pt-Sn alloy would not be an effective catalyst for either CO or methanol electrooxidation. These calculations, which used the same computational methodology that correctly predicted the activity of Pt-Ru[21], appeared to provide the answer. However, the mystery deepened when we examined the kinetics of CO oxidation on one of the Pt,Sn alloy surfaces used previously (the 110 crystal). As we reported recently[22], using the same methodology as we used to study the oxidation of CO on Pt-Ru alloys, we found that Pt,Sn was even more active than any Pt-Ru alloy for CO oxidation. As we report elsewhere[23], the clean annealed Pt,Sn( 111) surface is even more active for CO oxidation, with the potential for the onset of oxidation shifted negatively with respect Pt,Sn(l 10) alloy by 250mV. These new results produce the following (apparent) paradox: Pt,Sn is not effective at oxidizing CO ,,d8produced by methanol dehydrogenation, but is very effective at oxidizing CO dissolved in solution. This paradox would appear to require us to invoke a new reaction pathway for the oxidation of dissolved CO on Pt-Sn alloys.
In this paper, we have re-examined the activity of the Pt,Sn(llO) single crystal surface toward methanol oxidation in acid solution attempting to confirm the previous results and develop a more detailed understanding of the interactions of methanol, CO and water with this surface. One of the experimental effects we wanted to re-examine was the time effect. Previously, we had measured the methanol oxidation kinetics using slow, eg 1 mV/s, sweeping from a negative potential of immersion. In our studies of Pt-Ru, we found that even at 1 mV/s one does not achieve an equilibrated condition on the surface at each potential, and that one needs to use potential step measurements to approach a true steady-state condition that would emulate what one might see in a fuel cell polarization measurement. As we report here, even in potential step measurements, the Pt,Sn (20% Sn) surface is found to be much less active than the Pt-Ru (10% Ru) surface for methanol oxidation, and only slightly more active than pure Pt. We present an explanation of the paradoxically high activity of Pt,Sn for the oxidation of dissolved CO without invoking a new reaction pathway, ie the same reaction pathway but with uniquely different states of adsorbed CO on Pt,Sn. 2. EXPERIMENTAL A typical experiment consisted of UHV preparation of the Pt,Sn(llO) sample surface followed by electrochemical characterization of CO or methanol oxidation. The Pt,Sn(llO) alloy sample used in this study was the same crystal as used in previous UHV and electrochemical studies[ 161. The initial UHV cleaning of the electrode surface was achieved by Ar + sputtering/annealing and the cleanliness of the surface was examined with AES. Details of LEIS and AES analysis procedures are given in our earlier papers[22-25). The UHV-prepared surface was then transferred into an insertable rotating disk electrode assembly as described earlier[22], and then placed into the electrochemical cell for electrochemical characterization. The sample was immersed into the electrolyte under potential control at O.OV. A saturated calomel electrode was used as the reference electrode and was isolated from the cell by a bridge. All potentials, however, are reported versus the reversible hydrogen electrode in the same electrolyte (the). Sulfuric acid (Baker Ultrex Grade, 0.5 M) was used as the supporting electrolyte. Methanol (0.5 M) was injected into the electrochemical cell using a gastight syringe. It was thoroughly deaerated by purging with Ar gas prior to injection. For CO experiments, gaseous carbon monoxide (Matheson, Research Grade), was bubbled through the electrolyte and either purged from the cell with Ar for anodic stripping measurements, or passed over the top of the electrolyte for rde kinetic measurements. 3. RESULTS
AND DISCUSSION
3.1. Surface composition Previous experiments have established that the elemental composition of an annealed Pt,Sn( 110)
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On the reaction pathway for methanol and carbon monoxide electrooxidation surface is Pt : Sn = 1 : 1, which is the thermodynamically preferred termination of the bulk crystal in the (110) orientation[24]. The dashed lines in Fig. l(a) represent an AES spectrum and in l(b) a LEIS (low energy ion scattering) spectrum acquired on a annealed clean Pt,Sn(llO) surface. A surface with 20% Sn concentration, which we expected to be a more favorable surface than the 50% Sn surface for methanol oxidation, was used for all the electrochemical measurements in this study, and was created by a mild sputter-etching with 0.5 keV Ar + ions. The LEIS and AES spectra characterizing the 20% surface were shown as the solid lines in Fig. 1.
I
-
Augerelectronenergy [eV] 1. I.,
I.
-0.5
keVAr+ _._......9~ K/m"
3.2. Carbon monoxide oxidation The polarization curves for CO electrooxidation on Pt, Pt-Ru and Pt-Sn alloys, when the solution is saturated with pure CO gas, at ambient temperature and 1600 rpm are shown in Fig. 2. This figure clearly illustrates the dramatically higher rate of oxidation of CO on Pt,Sn surface than on Pt, Ru, or any Pt-Ru alloy surface. The latter samples are polycrystalline disks sputter-etched in UHV in the same manner as the Pt,Sn sample. The onset potential for CO oxidation is shifted negatively by more than 0.4 V with respect to pure Pt and 0.3 V with respect to Pt-50% Ru, the latter being the most active surface for the oxidation of adsorbed CO[7]. These shifts represent rate constants that are lo5 and lo3 higher, respectively. As we reported previously[26], the onset potential for the oxidation of solution phase CO on Pt and Pt-Ru alloys is shifted positively by 0.2 to 0.3 V relative to the potential for the oxidation of adsorbed CO. This shift is the consequence of the competition between CO and H,O (to
0.55
0.65
o.75
E,/E,
[eV]
Fig. 1. AES (a) and LEIS (b) spectra obtained before (solid lines) and after (dashed lines) an annealed Pt,Sn(llO) surface was mildly sputtered with SOOeV Ar + ions. The Sn concentration on the surfaces were estimated by means of LEIS to be 50% and 20%, respectively.
pure CO (25'C;O.5 M H,SOJ
0.2
0.0
0.6 0.4 E/V
0.8
Pt,Sn ------ pure Ru 50% Ru .___ pure Pt
-
0.0
0.2
0.4
0.6
0.8
1.0
E/V [RHE] Fig. 2. Potentiodynamic (20mV/s) CO oxidation current on the sputtered PtsSn(ll0) (co 20% Sn) compared with those measured on sputter-cleaned Pt, Ru and Pt-Ru (ca. 50% Ru). All the measurements were performed with a rotating disk electrode in 0.5 M H,SO, saturated with CO at 25°C at a rotation rate of 1600rpm. The insert provides a magnification of the low current density region, showing the positive-going sweeps for the four surfaces. The potential was held for 30s at the negative potential limit before each sweep.
K. WANGet al.
2590
form OH) for the Ru sites, ie the presence of CO in solution makes it more difftcult to nucleate OH on the Ru sites. An important difference between Pt-Sn alloys and Pt-Ru alloys is that CO has no affinity for the Sn sites[27], so that solution phase CO will not interfere with the OH nucleation on Sn sites, if indeed that is the functionality of the Sn. An important distinction, which we present below, is the relation between the potential for the onset of oxidation of solution phase CO and the potential for the oxidation of adsorbed CO on Pt,Sn. 3.3. Methanol oxidation Methanol oxidation currents were measured using the same potential step method employed in our earlier study of Pt-Ru alloys. In these experiments, the sputter-cleaned surface was immersed and held at 25 mV. Then the potential was stepped to SO0mV, which is the most positive potential one can use and not dissolve Sn from the surface[22]. Figure 3 shows the results of these experiments. For the direct comparison, the results of stepping experiments conducted under the same conditions for the pure Pt and two other Pt-Ru alloy electrode surfaces are displayed in the same figure. At very short-times, eg < 1 s (not shown), the Pt,Sn(llO) electrode exhibited a comparable activity to the Pt and the Pt-Ru electrodes, suggesting that the affinity of Pt toward methanol adsorption was not significantly reduced by Sn at this concentration in the surface. In the first few seconds (see the insert of Fig. 3) the current measured for the Pt,Sn(llO) electrode dropped even faster than that for the pure Pt, indicating that the
OSM C%OH @ OSV --FT,Sn (Xs”s= 0.2) ------hRu (xR.‘p0.3) -----PtRu
&U’S = 0.1)
_ - -purePf
’
------.-----..___.____________
.‘........... .............. .............__..... ,..... .,......, _,...._
lo2
x35
.
f
1000 0
10
5
15
electrode surface was poisoned with CO rapidly. However, after about 20 s, the current decrease for the Pt,Sn(llO) electrode became much slower, whereas the current measured for the Pt electrode continuously decreased, eventually becoming lower than the current for the Pt,Sn(llO) electrode. In contrast, the activity for both the Pt-Ru(lO%) and the Pt-Ru(30%) surfaces did not decrease significantly with time after a few seconds. Over long periods of time, all the four surfaces showed very steady activities. The steady-state activity of the Pt,Sn(llO) surface showed a factor of 3 enhancement (which is small compared with that of Pt-Ru alloys) for methanol oxidation over the pure Pt, but for times smaller than ca. 10 s. the Pt,Sn(llO) surface was actually less active than Pt, consistent with what we had reported previously with the potentiodynamic measurements[16] The other low index surfaces of Pt,Sn actually show no enhancement over Pt even at long times. From the nature of the transient currents, and our previous knowledge of CO accumulation on Pt and Pt-Ru alloy surfaces from FTIR measurements[KJ, it appears that methanol dehydrogenation on Pt,Sn surfaces is rapidly poisoned by the accumulation of CO, and that these alloys are not capable of oxidizing CO,,, from the surface at low potential. The potentiodynamic data for these surfaces in methanol provide some further insight into the relative rates of accumulation of CO. We compared the coverage by CO,,, formed from methanol dehydrogenation on three surfaces, Pt, Pt-10% Ru, and Pt,Sn, using cyclic voltammetry and measuring (qualitatively) the amount of CO on the surface from the loss of hydrogen adsorption pseudocapacitance. The results are shown in Fig. 4. The potential range was limited to OSV, since Sn dissolves from the surface above this potential. As the potential step data in Fig. 3 indicated, the accumulation of CO,,, to its equilibrium coverage is a relatively slow process, requiring nearly a minute in the potential step experiment and even longer under sweeping conditions, eg ten sweeps with an elapsed time of 500s. In the potentiodynamic curves in Fig. 4, the accumulation of CO,,, on the surface is seen as the decrease in the hydrogen adsorption/desorption pseudocapacitance in the potential region between 0.0550.35V. Note that on the first sweep, the rate of dehydrogenation on Pt,Sn is about the same as on pure Pt, but on the tenth sweep the rate of methanol oxidation is somewhat higher on Pt,Sn, in qualitative agreement with the potential step measurements. Note also the enormous difference in methanol oxidation current on the Pt-10% Ru alloy surface on the tenth sweep relative to the other two surfaces. On Pt and the Pt-10% Ru alloy surfaces, there is a loss of ca. 70-80% of the original charge (the base voltammetry) in this region; on Pt,Sn there is only a 50% reduction in capacitance. There is not a 1 : 1 correspondence between the loss of adsorbed H and the coverage by CO,,, on Pt surfaces[28], ie
time [mill]
Fia. 3. Potentiostatic methanol oxidation current densities measured on the sputtered Pt,Sn(llO), pure Pt, Pt-Ru(lO% Ru) and Pt-Ru(30% Ru) electrode surfaces at 500mV in 0.5 M methanol t 0.5 M H,SO,.
(1 - 0,) f &o (4) since 1 CO molecule can block more than one H-site, eg bridge-bonded CO. It is also known that from direct adsorption of CO on Pt the hydrogen
On the reaction pathway for methanol and carbon monoxide electrooxidation
0.0
0.2
0.4 E/V
0.6
[RHE]
Fig. 4. Voltammograms with and without the presence of methanol in 0.5 M H,SO, acquired on (a) pure Pt, (b) PtRu(lO% Ru) and (c) the sputtered Pt,Sn(llO) electrodes. Sweep rate was 20mV/s.
adsorption/desorption pseudocapacitance is reduced to zero, ie hydrogen adsorption is completely blocked, at CO coverage less than saturation . We have not, therefore, attempted to quantify the COad, coverages from methanol dehydrogenation from the results in Fig. 4, but it is sufficient for our purposes in this study to note that qualitatively from the blocking of hydrogen adsorption the Pt,Sn surface may actually have less CO,,+, at steady-state than either the pure Pt or the 10% Ru alloy surfaces; it is most certainly not higher. It appears then from the potentiodynamic data that the accumulation of COad, on the Pt,Sn surface blocks the methanol dehydrogenation step equation (1) more strongly than it does on either pure Pt or the Pt-Ru alloy surfaces, ie the Pt,Sn is actually “less poisoned” than the pure Pt surface yet the overall oxidation rate is only marginally higher. 3.4 States of adsorbed CO All of the above results indicate that the key to resolving the paradoxical activity of Pt,Sn is to be found in understanding the states of CO,, on the Pt,Sn surface and how the nature of these states on this surface depend on the source, ie whether the CO,,, comes from direct adsorption of CO from solution, or from the dehydrogenation of methanol. We have previously reported[8] coverages of CO,,,, formed from methanol dehydrogenation as measured by FTIR on both polycrystalline Pt and Pt-Ru alloys. On both poly Pt and Pt-10% Ru, the adsorption isotherm has a classical “bell-shaped” potential dependence, with the maximum coverage at OSV of ca. 0.55 of the saturation coverage. The saturation
2591
coverage is that from the direct adsorption of CO from solution, which has been measured many times on polycrystalline Pt (see[29] for a review) using anodic stripping coulometry, and is typically 0.85 0.9 CO/Pt atom. An interesting discussion of the saturation coverage of CO on the low index single crystal surfaces of Pt was presented recently by Weaver and co-workers[28]; their values were 0.67 for (111) and 1.0 for (110) and (100). The saturation coverage of CO by direct adsorption is independent of potential in the potential region of interest here. In contrast to these Pt-rich surfaces, the maximum coverage of CO on Pt-50% Ru alloy surface from methanol dehydrogenation was less than 5% of the saturation coverage, one tenth of that on pure Pt, consistent with the previous observation that this surface is the most active for the oxidation of CO,,,[7]. Yet even though the Pt-50% Ru alloy surface is nearly “unpoisoned”, it is only slightly more active for methanol oxidation at steady-state than the pure Pt surface, since the Ru atoms (50%) block sites for methanol dehydrogenation. We reiterate these considerations here to emphasize the delicate balance in activity between steps (1) and (3) the ideal catalyst surface must have for methanol oxidation. For pure Pt and Pt-Ru alloy surfaces, we have shown[7] that the oxidation of dissolved CO involves a different balance, a balance between CO,,, and OH,,, in reaction (3). A detailed discussion of this point is given in [7], and for our purposes here we will use a simplied mode1 below in comparing Pt,Sn with Pt-Ru alloy. Both CO and OH are adsorbed on both Pt and Ru atoms of the Pt-Ru alloy surface, which is to say that there is a competition between these molecules for the same site, whether this site is a Pt or a Ru atom. Theoretically, we can see the balance between the coverage by COad, and OH,,, needed to maximize the rate of oxidation by writing a simple expression for step (3) as the rds, I = 2Fk, eco eoH
(5)
and if tIco + eoH = 1, then it is easily seen that dIJd&, = 0 when 0co = l/2, ie when the surface is half covered by each species. Experimentally, we can see this balance in action by examining the anodic stripping of COad, pre-adsorbed from CO in solution. These curves are shown in Fig. 5 for all three surfaces of interest here, Pt, Pt-Ru (30% Ru), and Pt,Sn. In these experiments, the electrode was held at 0.1 V for 10 min in a solution saturated with CO at 1 atm., then the cell was purged with argon to remove CO from solution, followed by the potential sweep shown. Looking only at the curves for Pt and Pt-Ru for the moment, when there is no CO in solution to re-adsorb, as each COad, molecule is oxidized, an empty site is available for OH adsorption without competition from CO in solution, the peak current (maximum rate) for both Pt and Pt-Ru does in fact occur when about 50% of the CO has been removed, as predicted from our simple model. The cathodic shift of the peak current maximum on Pt-Ru relative to Pt is a manifestation of the greater affinity of the Ru sites for OH, and thus when a CO,,, molecule is oxidized from a Ru site it is more
2592
K.
.... baseCV
.----- CO
, -after
0.0
0.2
0.4
0.6
1
stripping CO stripping
(20 mV/s)
.------’
WANG et
0.8
1.0
E/V [RHE]
Fig. 5. CO stripping voltammograms in 0.5 M H,SO, on (a) pure Pt, (b) Pt-Ru(30%) and (c) the sputtered Pt,Sn(llO) electrodes. CO was pre-adsorbed by holding potential at 0.1 V for 1Omin on all these surfaces.
rapidly replaced by an OH than when it is oxidized from a Pt site. Accounting for all the detailed differences in the stripping peaks on Pt and Pt-Ru alloy surfaces does, of course, require a more sophisticated model[7] than the one just described. We can also see experimentally when the balance between COad, and OH,,, needed to maximize the rate of oxidation is difftcult to obtain. When there is CO in solution, and there is rapid mass transport of the CO to the surface, as with the rde configurations in Fig. 2, there is a competition between CO and Hz0 (to form OH) for the Ru sites, and the presence of CO in solution makes it more diflicult to nucleate OH. At low potential (< 0.6 V), there is an imbalance in the afftnity of the Pt-Ru surface for CO vs. OH, and when a CO,,, molecule is oxidized it is much more likely to be replaced by another COadsr ie the surface is “swamped” with CO. As we pointed out in section 3.2, this causes an large anodic shift in the onset potential for the oxidation of solution phase CO relative to the onset for the stripping of adsorbed CO. An important difference between Pt,Sn alloy and Pt-Ru alloys is that CO has no affinity for the Sn sites[27], so that solution phase CO will not interfere with OH nucleation on Sn sites, if we apply the same model to Pt,Sn. The oxidative stripping of CO on the Pt,Sn surface reveals some surprising and significantly different behavior. The stripping begins at even lower potential than on any Pt-Ru alloy surface, as low as 0.25V, but only a small fraction of the total CO,,, present, as inferred from the complete extinction of the hydrogen adsorption/desorption pseudocapacitance, is stripped at potentials near this onset poten-
al.
tial. There are in fact two anodic waves for CO,,, stripping on Pt,Sn, a second (not shown) starting at about 0.65 V with a peak at about 0.75V, ie very Pt-like, that is convoluted with anodic current from Sn dissolution. We discussed the deconvolution of these charges in detail in another paper[30]. For our purposes here, we note only that the total coverage of CO,,, on Pt,Sn calculated from the charge under both waves is greater than 0.9 CO/Pt (actually quite close to 1.0) and as high or higher than the CO/Pt ratio calculated from the anodic stripping charges for the Pt or Pt-Ru alloy surface. From the charge under the first anodic peak (ea. 90 &/cm2), we estimate the coverage of the low potential state to be ca. 30% of the total CO present at saturation. The state of COad, on Pt,Sn which is oxidized in the first wave appears to be an extremely important state in the reaction pathway for the oxidation of CO. There are several pieces of evidence that support this observation. First, after oxidizing this state of CO, when the potential sweep was reversed, the removal of the state leads to a reappearance of hydrogen adsorption/desorption pseudocapacitance; but holding the potential or cycling the potential in the hydrogen region does not repopulate the state from the other CO still on the surface. The state can only be repopulated by exposing the surface to CO dissolved in solution. There is also no evidence for this state in the voltammetry in the presence of methanol (Fig. 4) so it is not a state which can be formed from methanol dehydrogenation. The onset potential for the oxidation of CO dissolved in solution, shown in Fig. 2, is exactly the same as the onset potential for the oxidation of this state of COad, in Fig. 5. As we discussed previously[26], on Pt, Ru and any Pt-Ru alloy, the onset potential for oxidation of dissolved CO is shifted positively with respect to the onset potential for the oxidation of CO,,,; this difference between the two types of surfaces is fundamental and is a consequence of the absence of competition between CO and water for the same sites on the Pt-Sn alloy surface. From the properties described above, we have termed this catalytically active intermediate state of CO,,, the “high coverage” state. There appears to be a similar state of CO,,, on the alloy surface when CO is adsorbed in the absence of electrolyte, ie adsorption in UHV[27]. A direct comparison was made in [27] of the adsorption of CO on Pt,Sn(hkf) to the pure Pt(hkl) surface of the same orientation using the same methodology. On the pure Pt surfaces there are no states of CO that desorb in the temperature range of 250-3OOK, but on the Pt,Sn surfaces cu. lo-30% of the CO desorbs in this temperature range, with the (111) surface having the largest fraction in this state. This low temperature state is populated only after the high temperature states are filled and required relatively high doses of CO, eg 1OOL (1 L = 1 torr.sec). The heat of adsorption of the low temperature state was estimated to be 70 kJ/mol, or about 20 kJ/mol lower than the heat of adsorption at saturation on the comparable Pt surface, and is an adsorption energy that would be “weak chemisorption”. Classical considered models[3 l] for weakly chemisorbed molecules predict high surface mobility, and hence high react-
On the reaction pathway for methanol and carbon monoxide electrooxidation
ivity for bimolecular surface reactions like step (3), other factors being the same. Although we have no direct evidence that this state exists on the Pt,Sn surface at 3OOK, using the standard expressions[32] for isotherms for CO on Pt surfaces, we can calculate that the state would be populated at 300K at CO pressures above cu. 0.1 Pa. Since CO adsorption on the Pt(ll1) surface is characterized by strong repulsive interaction, eg at >0.5 CO/Pt the heat of adsorption decreases dramatically[33], it was concluded in [27] that strong repulsive interaction between CO molecules adsorbed on Pt sites on the Pt,Sn(lll) surface was the reason why the fraction of the low temperature state of CO,,, was highest on this surface. We therefore suggested in [23] that the repulsive interaction from the “crowding” of CO on the Pt sites on the surface is the reason why Pt,Sn(lll) is even more active for the oxidation of dissolved CO than the (110) surface. This high coverage state on Pt,Sn surfaces, eg >0.9 CO/Pt[23], cannot be created by methanol dehydrogenation on any Pt surface, since the dehydrogenation reaction requires a critical ensemble of free Pt sites[6], and is thus strongly attenuated by a relatively low coverage of COnds, eg <0.6 CO/Pt[S].
4. CONCLUSIONS A small enhancement of methanol oxidation on a sputtered Pt,Sn( 110) surface was observed in longertime potential step measurements. The enhancement is, however, an order of magnitude smaller than those observed on Pt-Ru alloys, eg Pt-10% or 30% Ru, using the same methodology. On the other hand, Pt,Sn is very active for the oxidation of dissolved carbon monoxide (CO), with an onset potential approximately 300mV lower than the most active Pt-Ru alloy surface, and more than SOOmV lower than on polycrystalline Pt. The apparently paradoxical results can be explained in terms of a unique state of CO,,, on this surface, which is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by CO,,, at low coverage. Acknowledgements-Special thanks are due to Lee Johnson for preparing the Pt,Sn rotating disk electrode and Frank Zucca for his invaluable help in keeping the rotator rotating. This work was sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Oflice of Transportation Technologies, Electric and Hybrid Propulsion Division of the US Department of Energy, under Contract DE-AC03-76SFOOO98.
REFERENCES 1. R. Parsons and T. VanderNoot, J. Electroana!. Chem. 257,9 (1988).
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B. Beden, F. Kadirgan, C. Lamy and M. M. Leger, J. E[ectroanal. Chem. 127,75 (1981). M. Watanabe, M. Uchida and S. Motoo, J. Electroana[. Gem. 229,395 (1987). J. B. Goodenough, A. Hamnett, B. J. Kennedy, R. Manoharan and S. A. Weeks, J. Elecrroanal. Chem. 240, 133 (1988). M. M. P. Janssen and J. Moolhuysen, Electrochimica Acta. 21, 869 (1976). H. A. Gasteiger, N. M. Markovic, P. N. Ross. Jr. and E. J. Cairns, J. Phys. Chem. 97, 12020 (1993). H. A. Gasteiger, N. M. Markovic, P. N. Ross, Jr. and E. J. Cairns, f. Phys. Chem. 98,617 (1994). N. M. Markovic, H. A. Gasteiger, P. N. Ross Jr., X. Jiang, I. Villegas and M. J. Weaver, Electrochimica Acta. 40,91 (1995). 9. K. J. Cathro, J. Electrochem. Sot. 116, 1608 (1969). 10. M. R. Andrew, J. S. Drury, B. D. Mchicol. C. Pinnington and R. T. Short, J. Appl. Electrochem. 6, 99 (1976). 11. M. M. P. Janssen and J. Moolhuysen: (a) J. Catalysis 46,280(1977);(b) Electrochimica Acta 21, 861 (1976). 12. A. A. Mikhailova, N. V. Ostrova and Y. V. Vasil’ev, EIektrokhimiya 13, 518 (1977). 13. M. Watanabe, Y. Furuuchi and S. Motoo, J. Electroanal. Chem. 191,367 (1985). 14. B. Bittins-Cattaneo and T. Iwasita, J. Electroanab Chem. 238,151(1987). 15. C. T. Hable and M. S. Wrighton, Langmuir 7, 1305 (1991). 16. A. N. Haner and P. N. Ross, J. Phys. Chem. 95, 3740 11991). 17. S. A. Campbell and R. Parsons, .I. Chem. Sot. Faraday Trans.88(6). . ,, 833 (19921 18. Y. B. Vassiliev, \;. S. i)agotzky, N. V. Osetrova and A. A. Mikhailova, J. Electroanal. Chem. 97,63 (1979). 19. M. Shibata and S. Motoo, .I. Electroanal. Chem. 209, 151 (1986). 20. A. B. Anderson, E. Grantscharova and P. Shiller, J. Electrochem. Sot. 142, 1880 (1995). 21. A. B. Anderson and E. Grantscharova, J. Phys. Chem. 99,9149 (1995). 22. H. A. Gasteiger, N. M. Markovic and P. N. Ross, Jr., J. Phvs. Chem. 99.8945 (19951. 23. H.-A. Gasteigei, N. h. Mirkovic and P. N. Ross, Jr., Catalysis Letters 36, 1 (1996). 24. A. N. Haner, P. N. Ross, Jr. and U. Bardi, Catalysis Letters 8, 1 (1991). 25. H. A. Gasteiger, P. N. Ross, Jr. and E. J. Cairns, Surj: Sci. 293.67 (1993). 26. H. A. Gasteiger, N. M. Markovic and P. N. Ross, Jr., J. Phys. Chem. 99.8290 (1995). 27. A. N. Haner, P. N. Ross, Jr., U. Bardi and A. Atrei, J. Vat. Sci. Technol. A lo,2718 (1992). 28. M. J. Weaver, S. C. Chang, L. W. Leung, X. Jiang, M. Rubel, M. Szklarczyk, D. i?urawski and A. Wieckowski, .I. Electroanol. Chem. 327,247 (1992). 29. K. Kinoshita and P. Stonehart, in Modern Aspxs of Electrochemistry (Edited by J. O’M. Bockris and B. E. Conway). Vol. 12, D. 183. Wiley Interscience. New York _ and Ldrdon, (19jf). 30. H. A. Gasteiger, N. M. Markovic and P. N. Ross, Jr., (in press). 31. A. Clark, Theory of Adsorption and Catalysis, p.17-21. Academic Press,.Ndw York and London, (i970): 32. H. Ibach, W. Erley and H. Wagner, Surf. Sci. 92, 29 (1980). 33. H. Steininger, S. Lehwald and H. Ibach, Sur& Sci. 123, 264 (1982).
Pergamon
ON THE REACTION PATHWAY FOR METHANOL AND CARBON MONOXIDE ELECTROOXIDATION ON Pt-Sn ALLOY VERSUS Pt-Ru ALLOY SURFACES K. WANG, H. A. GASTEIGER, N. M. MARKOVIC and P. N. Ross, JR* Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA (Acceptedfor
publication
1 November 1995)
Abstract-It has been observed in this work and previous studies that Pt,Sn alloy surfaces are very effective catalysts for CO electrooxidation, but not for methanol electrooxidation. Since CO,, is postulated to be an intermediate in methanol electrooxidation on Pt alloy surfaces, the relative inactivity of Pt,Sn for methanol oxidation appears paradoxical. We present an explanation for this apparent contradiction in terms of a unique state of CO,,, on this surface, which is not the same state of CO,,, as occurs on either Pt-Ru or pure Pt surfaces. It is also not a state of CO,,, which is produced by methanol dehydrogenation. The state is unique in the sense that a significant fraction of CO,,, is oxidized at a much lower (-c 400mV) potential than the rest of the CO,,, , a phenomenon that does not occur on any other Pt and Pt-alloy surfaces examined in the same way. This CO state is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by CO,,, at low coverage. Copyright 0 1996 Elsevier Science Ltd Key words: Electrocatalysis, Pt-Sn alloys, Pt-Ru alloys, carbon monoxide, methanol.
1. INTRODUCTION
kc0
CO,,, + OH,,, -CO*
The electrocatalytic properties of Pt-Ru and Pt-Sn alloys have been widely studied in the last three decades due to their promising activity for the electrooxidation of carbon monoxide, methanol and other C, compounds that are potential fuels for fuel cells[l]. There have been more consistent reports of enhanced activity (relative to pure Pt) for Pt-Ru alloys than for Pt-Sn alloys. For example, for Pt-Ru the activity is almost always higher than that of pure Pt regardless of the method of preparation of the catalysts or of the method of testing the activity, eg[2-51. On the basis of recent systematic studies of electrooxidation of CO and methanol on wellcharacterized Pt-Ru alloys in our laboratory[6-81, an oxidation pathway for methanol oxidation has been proposed to explain the effect of Ru on enhancing the activity of Pt. Several reaction steps are involved in this model, represented by the following equations: kd CH,0H,,e*CH30Hoda
-
COad,+4H+
+4e-
(1) H20e*OH,d,
+ H+ + e-
(2)
l Author to whom correspondence should be addressed: Mail Stop 2-100, 1 Cyclotron Road, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA.
+ H+ + e-
(3)
where COad, represents one of possible products of the dehydrogenation of methanol, k, is the rate constant of methanol dehydrogenation and k,, the rate constant of CO oxidation into CO,. It is proposed that step 1 occurs primarily at ensembles of Pt sites while step 2 occurs primarily at Ru sites, and step 3 occurs at Pt-Ru pair sites. This model, in conjunction with the adsorption properties of Pt and Ru for CO, OH and methanol, explains our experimental finding that a Pt-Ru alloy surface with a ca. 10% Ru concentration has the maximum activity for methanol oxidation: the maximum concentration of three-fold Pt sites, which are expected to be most active ensembles for methanol adsorption, coordinated to a Ru atom occurs at about 10 atomic% Ru[6-71. However, the literature on the effect of Sn as a promoter of catalytic activity of Pt toward methanol oxidation turns out to be much less consistent. Some studies report Sn enhances the activity of Pt[9-151, while others report either an inhibition of the methanol oxidation reaction or an activity comparable to that of pure Pt[2, 16-171. Even in cases where there are reports of enhanced activity, the enhancements are much smaller than those reported for Pt-Ru alloys, es rarely are the enhancements more than a factor of 5. One consistent feature of the reports of enhanced activity is that the states of the two metals in the catalyst are not well known, ie they may not be alloyed, whereas the reports of little or no
2588
K. WANG et al.
enhancement by Sn have come from studies using well characterized bulk Pt-Sn alloy electrodes, specifically Pt,Sn. One of the reports of little or no enhancement by a Pt,Sn alloy surface was from this laboratory[16]. In an attempt to clarify the contradictions and to understand the role of Sn as a promoter in the methanol oxidation, Haner and Ross[l6] conducted a study using a combination of electrochemical and UHV surface analytical methods with the low index single-crystal faces of Pt,Sn alloy and Pt, with the latter being modified by having Sn(I1) in solution. They observed that none of the alloy surfaces were more effective catalysts than any of the pure platinum surfaces under the measurement method used (slow potential sweep) and there was a significant enhancement (at least a factor of 6) on Pt(l11) and Pt(100) due to Sn(I1) in the electrolyte at very low concentration, eg 1 PM. A hybrid homogeneous-heterogeneous mechanism involving a Sn(II)/Sn(IV) redox “shuttle” sequence was proposed to account for the enhancement. The results of Haner and Ross would appear to reconcile much of the disparities in the literature concerning Pt-Sn catalysts. If Sn is not alloyed with Pt in the catalyst, it is unstable and dissolves in strong acid upon immersion in the cell. As a consequence, one could observe an enhancement due to the presence of Sn(II)/Sn(IV) in solution, the magnitude depending on the concentration of Sn ions in solution. The largest enhancement found by Haner and Ross was about a factor of 6, comparable to the largest enhancements reported for Pt-Sn catalysts in the literature. It was still not clear why those Pt-Sn alloy surfaces were not active for methanol oxidation. In terms of the reaction pathway, the bifunctional mechanism discussed above for the Pt-Ru catalyst would appear to apply as well to Pt-Sn, since Sn in the Pt-Sn catalyst, like Ru in the Pt-Ru catalyst, is suggested to facilitate the adsorption of oxygencontaining species such as OH,,,[18-193. But molecular orbital calculations by Anderson et a/.[201 predict that Sn in a Pt-Sn alloy would not dissociate water preferentially, and that Pt-Sn alloy would not be an effective catalyst for either CO or methanol electrooxidation. These calculations, which used the same computational methodology that correctly predicted the activity of Pt-Ru[21], appeared to provide the answer. However, the mystery deepened when we examined the kinetics of CO oxidation on one of the Pt,Sn alloy surfaces used previously (the 110 crystal). As we reported recently[22], using the same methodology as we used to study the oxidation of CO on Pt-Ru alloys, we found that Pt,Sn was even more active than any Pt-Ru alloy for CO oxidation. As we report elsewhere[23], the clean annealed Pt,Sn( 111) surface is even more active for CO oxidation, with the potential for the onset of oxidation shifted negatively with respect Pt,Sn(l 10) alloy by 250mV. These new results produce the following (apparent) paradox: Pt,Sn is not effective at oxidizing CO ,,d8produced by methanol dehydrogenation, but is very effective at oxidizing CO dissolved in solution. This paradox would appear to require us to invoke a new reaction pathway for the oxidation of dissolved CO on Pt-Sn alloys.
In this paper, we have re-examined the activity of the Pt,Sn(llO) single crystal surface toward methanol oxidation in acid solution attempting to confirm the previous results and develop a more detailed understanding of the interactions of methanol, CO and water with this surface. One of the experimental effects we wanted to re-examine was the time effect. Previously, we had measured the methanol oxidation kinetics using slow, eg 1 mV/s, sweeping from a negative potential of immersion. In our studies of Pt-Ru, we found that even at 1 mV/s one does not achieve an equilibrated condition on the surface at each potential, and that one needs to use potential step measurements to approach a true steady-state condition that would emulate what one might see in a fuel cell polarization measurement. As we report here, even in potential step measurements, the Pt,Sn (20% Sn) surface is found to be much less active than the Pt-Ru (10% Ru) surface for methanol oxidation, and only slightly more active than pure Pt. We present an explanation of the paradoxically high activity of Pt,Sn for the oxidation of dissolved CO without invoking a new reaction pathway, ie the same reaction pathway but with uniquely different states of adsorbed CO on Pt,Sn. 2. EXPERIMENTAL A typical experiment consisted of UHV preparation of the Pt,Sn(llO) sample surface followed by electrochemical characterization of CO or methanol oxidation. The Pt,Sn(llO) alloy sample used in this study was the same crystal as used in previous UHV and electrochemical studies[ 161. The initial UHV cleaning of the electrode surface was achieved by Ar + sputtering/annealing and the cleanliness of the surface was examined with AES. Details of LEIS and AES analysis procedures are given in our earlier papers[22-25). The UHV-prepared surface was then transferred into an insertable rotating disk electrode assembly as described earlier[22], and then placed into the electrochemical cell for electrochemical characterization. The sample was immersed into the electrolyte under potential control at O.OV. A saturated calomel electrode was used as the reference electrode and was isolated from the cell by a bridge. All potentials, however, are reported versus the reversible hydrogen electrode in the same electrolyte (the). Sulfuric acid (Baker Ultrex Grade, 0.5 M) was used as the supporting electrolyte. Methanol (0.5 M) was injected into the electrochemical cell using a gastight syringe. It was thoroughly deaerated by purging with Ar gas prior to injection. For CO experiments, gaseous carbon monoxide (Matheson, Research Grade), was bubbled through the electrolyte and either purged from the cell with Ar for anodic stripping measurements, or passed over the top of the electrolyte for rde kinetic measurements. 3. RESULTS
AND DISCUSSION
3.1. Surface composition Previous experiments have established that the elemental composition of an annealed Pt,Sn( 110)
2589
On the reaction pathway for methanol and carbon monoxide electrooxidation surface is Pt : Sn = 1 : 1, which is the thermodynamically preferred termination of the bulk crystal in the (110) orientation[24]. The dashed lines in Fig. l(a) represent an AES spectrum and in l(b) a LEIS (low energy ion scattering) spectrum acquired on a annealed clean Pt,Sn(llO) surface. A surface with 20% Sn concentration, which we expected to be a more favorable surface than the 50% Sn surface for methanol oxidation, was used for all the electrochemical measurements in this study, and was created by a mild sputter-etching with 0.5 keV Ar + ions. The LEIS and AES spectra characterizing the 20% surface were shown as the solid lines in Fig. 1.
I
-
Augerelectronenergy [eV] 1. I.,
I.
-0.5
keVAr+ _._......9~ K/m"
3.2. Carbon monoxide oxidation The polarization curves for CO electrooxidation on Pt, Pt-Ru and Pt-Sn alloys, when the solution is saturated with pure CO gas, at ambient temperature and 1600 rpm are shown in Fig. 2. This figure clearly illustrates the dramatically higher rate of oxidation of CO on Pt,Sn surface than on Pt, Ru, or any Pt-Ru alloy surface. The latter samples are polycrystalline disks sputter-etched in UHV in the same manner as the Pt,Sn sample. The onset potential for CO oxidation is shifted negatively by more than 0.4 V with respect to pure Pt and 0.3 V with respect to Pt-50% Ru, the latter being the most active surface for the oxidation of adsorbed CO[7]. These shifts represent rate constants that are lo5 and lo3 higher, respectively. As we reported previously[26], the onset potential for the oxidation of solution phase CO on Pt and Pt-Ru alloys is shifted positively by 0.2 to 0.3 V relative to the potential for the oxidation of adsorbed CO. This shift is the consequence of the competition between CO and H,O (to
0.55
0.65
o.75
E,/E,
[eV]
Fig. 1. AES (a) and LEIS (b) spectra obtained before (solid lines) and after (dashed lines) an annealed Pt,Sn(llO) surface was mildly sputtered with SOOeV Ar + ions. The Sn concentration on the surfaces were estimated by means of LEIS to be 50% and 20%, respectively.
pure CO (25'C;O.5 M H,SOJ
0.2
0.0
0.6 0.4 E/V
0.8
Pt,Sn ------ pure Ru 50% Ru .___ pure Pt
-
0.0
0.2
0.4
0.6
0.8
1.0
E/V [RHE] Fig. 2. Potentiodynamic (20mV/s) CO oxidation current on the sputtered PtsSn(ll0) (co 20% Sn) compared with those measured on sputter-cleaned Pt, Ru and Pt-Ru (ca. 50% Ru). All the measurements were performed with a rotating disk electrode in 0.5 M H,SO, saturated with CO at 25°C at a rotation rate of 1600rpm. The insert provides a magnification of the low current density region, showing the positive-going sweeps for the four surfaces. The potential was held for 30s at the negative potential limit before each sweep.
K. WANGet al.
2590
form OH) for the Ru sites, ie the presence of CO in solution makes it more difftcult to nucleate OH on the Ru sites. An important difference between Pt-Sn alloys and Pt-Ru alloys is that CO has no affinity for the Sn sites[27], so that solution phase CO will not interfere with the OH nucleation on Sn sites, if indeed that is the functionality of the Sn. An important distinction, which we present below, is the relation between the potential for the onset of oxidation of solution phase CO and the potential for the oxidation of adsorbed CO on Pt,Sn. 3.3. Methanol oxidation Methanol oxidation currents were measured using the same potential step method employed in our earlier study of Pt-Ru alloys. In these experiments, the sputter-cleaned surface was immersed and held at 25 mV. Then the potential was stepped to SO0mV, which is the most positive potential one can use and not dissolve Sn from the surface[22]. Figure 3 shows the results of these experiments. For the direct comparison, the results of stepping experiments conducted under the same conditions for the pure Pt and two other Pt-Ru alloy electrode surfaces are displayed in the same figure. At very short-times, eg < 1 s (not shown), the Pt,Sn(llO) electrode exhibited a comparable activity to the Pt and the Pt-Ru electrodes, suggesting that the affinity of Pt toward methanol adsorption was not significantly reduced by Sn at this concentration in the surface. In the first few seconds (see the insert of Fig. 3) the current measured for the Pt,Sn(llO) electrode dropped even faster than that for the pure Pt, indicating that the
OSM C%OH @ OSV --FT,Sn (Xs”s= 0.2) ------hRu (xR.‘p0.3) -----PtRu
&U’S = 0.1)
_ - -purePf
’
------.-----..___.____________
.‘........... .............. .............__..... ,..... .,......, _,...._
lo2
x35
.
f
1000 0
10
5
15
electrode surface was poisoned with CO rapidly. However, after about 20 s, the current decrease for the Pt,Sn(llO) electrode became much slower, whereas the current measured for the Pt electrode continuously decreased, eventually becoming lower than the current for the Pt,Sn(llO) electrode. In contrast, the activity for both the Pt-Ru(lO%) and the Pt-Ru(30%) surfaces did not decrease significantly with time after a few seconds. Over long periods of time, all the four surfaces showed very steady activities. The steady-state activity of the Pt,Sn(llO) surface showed a factor of 3 enhancement (which is small compared with that of Pt-Ru alloys) for methanol oxidation over the pure Pt, but for times smaller than ca. 10 s. the Pt,Sn(llO) surface was actually less active than Pt, consistent with what we had reported previously with the potentiodynamic measurements[16] The other low index surfaces of Pt,Sn actually show no enhancement over Pt even at long times. From the nature of the transient currents, and our previous knowledge of CO accumulation on Pt and Pt-Ru alloy surfaces from FTIR measurements[KJ, it appears that methanol dehydrogenation on Pt,Sn surfaces is rapidly poisoned by the accumulation of CO, and that these alloys are not capable of oxidizing CO,,, from the surface at low potential. The potentiodynamic data for these surfaces in methanol provide some further insight into the relative rates of accumulation of CO. We compared the coverage by CO,,, formed from methanol dehydrogenation on three surfaces, Pt, Pt-10% Ru, and Pt,Sn, using cyclic voltammetry and measuring (qualitatively) the amount of CO on the surface from the loss of hydrogen adsorption pseudocapacitance. The results are shown in Fig. 4. The potential range was limited to OSV, since Sn dissolves from the surface above this potential. As the potential step data in Fig. 3 indicated, the accumulation of CO,,, to its equilibrium coverage is a relatively slow process, requiring nearly a minute in the potential step experiment and even longer under sweeping conditions, eg ten sweeps with an elapsed time of 500s. In the potentiodynamic curves in Fig. 4, the accumulation of CO,,, on the surface is seen as the decrease in the hydrogen adsorption/desorption pseudocapacitance in the potential region between 0.0550.35V. Note that on the first sweep, the rate of dehydrogenation on Pt,Sn is about the same as on pure Pt, but on the tenth sweep the rate of methanol oxidation is somewhat higher on Pt,Sn, in qualitative agreement with the potential step measurements. Note also the enormous difference in methanol oxidation current on the Pt-10% Ru alloy surface on the tenth sweep relative to the other two surfaces. On Pt and the Pt-10% Ru alloy surfaces, there is a loss of ca. 70-80% of the original charge (the base voltammetry) in this region; on Pt,Sn there is only a 50% reduction in capacitance. There is not a 1 : 1 correspondence between the loss of adsorbed H and the coverage by CO,,, on Pt surfaces[28], ie
time [mill]
Fia. 3. Potentiostatic methanol oxidation current densities measured on the sputtered Pt,Sn(llO), pure Pt, Pt-Ru(lO% Ru) and Pt-Ru(30% Ru) electrode surfaces at 500mV in 0.5 M methanol t 0.5 M H,SO,.
(1 - 0,) f &o (4) since 1 CO molecule can block more than one H-site, eg bridge-bonded CO. It is also known that from direct adsorption of CO on Pt the hydrogen
On the reaction pathway for methanol and carbon monoxide electrooxidation
0.0
0.2
0.4 E/V
0.6
[RHE]
Fig. 4. Voltammograms with and without the presence of methanol in 0.5 M H,SO, acquired on (a) pure Pt, (b) PtRu(lO% Ru) and (c) the sputtered Pt,Sn(llO) electrodes. Sweep rate was 20mV/s.
adsorption/desorption pseudocapacitance is reduced to zero, ie hydrogen adsorption is completely blocked, at CO coverage less than saturation . We have not, therefore, attempted to quantify the COad, coverages from methanol dehydrogenation from the results in Fig. 4, but it is sufficient for our purposes in this study to note that qualitatively from the blocking of hydrogen adsorption the Pt,Sn surface may actually have less CO,,+, at steady-state than either the pure Pt or the 10% Ru alloy surfaces; it is most certainly not higher. It appears then from the potentiodynamic data that the accumulation of COad, on the Pt,Sn surface blocks the methanol dehydrogenation step equation (1) more strongly than it does on either pure Pt or the Pt-Ru alloy surfaces, ie the Pt,Sn is actually “less poisoned” than the pure Pt surface yet the overall oxidation rate is only marginally higher. 3.4 States of adsorbed CO All of the above results indicate that the key to resolving the paradoxical activity of Pt,Sn is to be found in understanding the states of CO,, on the Pt,Sn surface and how the nature of these states on this surface depend on the source, ie whether the CO,,, comes from direct adsorption of CO from solution, or from the dehydrogenation of methanol. We have previously reported[8] coverages of CO,,,, formed from methanol dehydrogenation as measured by FTIR on both polycrystalline Pt and Pt-Ru alloys. On both poly Pt and Pt-10% Ru, the adsorption isotherm has a classical “bell-shaped” potential dependence, with the maximum coverage at OSV of ca. 0.55 of the saturation coverage. The saturation
2591
coverage is that from the direct adsorption of CO from solution, which has been measured many times on polycrystalline Pt (see[29] for a review) using anodic stripping coulometry, and is typically 0.85 0.9 CO/Pt atom. An interesting discussion of the saturation coverage of CO on the low index single crystal surfaces of Pt was presented recently by Weaver and co-workers[28]; their values were 0.67 for (111) and 1.0 for (110) and (100). The saturation coverage of CO by direct adsorption is independent of potential in the potential region of interest here. In contrast to these Pt-rich surfaces, the maximum coverage of CO on Pt-50% Ru alloy surface from methanol dehydrogenation was less than 5% of the saturation coverage, one tenth of that on pure Pt, consistent with the previous observation that this surface is the most active for the oxidation of CO,,,[7]. Yet even though the Pt-50% Ru alloy surface is nearly “unpoisoned”, it is only slightly more active for methanol oxidation at steady-state than the pure Pt surface, since the Ru atoms (50%) block sites for methanol dehydrogenation. We reiterate these considerations here to emphasize the delicate balance in activity between steps (1) and (3) the ideal catalyst surface must have for methanol oxidation. For pure Pt and Pt-Ru alloy surfaces, we have shown[7] that the oxidation of dissolved CO involves a different balance, a balance between CO,,, and OH,,, in reaction (3). A detailed discussion of this point is given in [7], and for our purposes here we will use a simplied mode1 below in comparing Pt,Sn with Pt-Ru alloy. Both CO and OH are adsorbed on both Pt and Ru atoms of the Pt-Ru alloy surface, which is to say that there is a competition between these molecules for the same site, whether this site is a Pt or a Ru atom. Theoretically, we can see the balance between the coverage by COad, and OH,,, needed to maximize the rate of oxidation by writing a simple expression for step (3) as the rds, I = 2Fk, eco eoH
(5)
and if tIco + eoH = 1, then it is easily seen that dIJd&, = 0 when 0co = l/2, ie when the surface is half covered by each species. Experimentally, we can see this balance in action by examining the anodic stripping of COad, pre-adsorbed from CO in solution. These curves are shown in Fig. 5 for all three surfaces of interest here, Pt, Pt-Ru (30% Ru), and Pt,Sn. In these experiments, the electrode was held at 0.1 V for 10 min in a solution saturated with CO at 1 atm., then the cell was purged with argon to remove CO from solution, followed by the potential sweep shown. Looking only at the curves for Pt and Pt-Ru for the moment, when there is no CO in solution to re-adsorb, as each COad, molecule is oxidized, an empty site is available for OH adsorption without competition from CO in solution, the peak current (maximum rate) for both Pt and Pt-Ru does in fact occur when about 50% of the CO has been removed, as predicted from our simple model. The cathodic shift of the peak current maximum on Pt-Ru relative to Pt is a manifestation of the greater affinity of the Ru sites for OH, and thus when a CO,,, molecule is oxidized from a Ru site it is more
2592
K.
.... baseCV
.----- CO
, -after
0.0
0.2
0.4
0.6
1
stripping CO stripping
(20 mV/s)
.------’
WANG et
0.8
1.0
E/V [RHE]
Fig. 5. CO stripping voltammograms in 0.5 M H,SO, on (a) pure Pt, (b) Pt-Ru(30%) and (c) the sputtered Pt,Sn(llO) electrodes. CO was pre-adsorbed by holding potential at 0.1 V for 1Omin on all these surfaces.
rapidly replaced by an OH than when it is oxidized from a Pt site. Accounting for all the detailed differences in the stripping peaks on Pt and Pt-Ru alloy surfaces does, of course, require a more sophisticated model[7] than the one just described. We can also see experimentally when the balance between COad, and OH,,, needed to maximize the rate of oxidation is difftcult to obtain. When there is CO in solution, and there is rapid mass transport of the CO to the surface, as with the rde configurations in Fig. 2, there is a competition between CO and Hz0 (to form OH) for the Ru sites, and the presence of CO in solution makes it more diflicult to nucleate OH. At low potential (< 0.6 V), there is an imbalance in the afftnity of the Pt-Ru surface for CO vs. OH, and when a CO,,, molecule is oxidized it is much more likely to be replaced by another COadsr ie the surface is “swamped” with CO. As we pointed out in section 3.2, this causes an large anodic shift in the onset potential for the oxidation of solution phase CO relative to the onset for the stripping of adsorbed CO. An important difference between Pt,Sn alloy and Pt-Ru alloys is that CO has no affinity for the Sn sites[27], so that solution phase CO will not interfere with OH nucleation on Sn sites, if we apply the same model to Pt,Sn. The oxidative stripping of CO on the Pt,Sn surface reveals some surprising and significantly different behavior. The stripping begins at even lower potential than on any Pt-Ru alloy surface, as low as 0.25V, but only a small fraction of the total CO,,, present, as inferred from the complete extinction of the hydrogen adsorption/desorption pseudocapacitance, is stripped at potentials near this onset poten-
al.
tial. There are in fact two anodic waves for CO,,, stripping on Pt,Sn, a second (not shown) starting at about 0.65 V with a peak at about 0.75V, ie very Pt-like, that is convoluted with anodic current from Sn dissolution. We discussed the deconvolution of these charges in detail in another paper[30]. For our purposes here, we note only that the total coverage of CO,,, on Pt,Sn calculated from the charge under both waves is greater than 0.9 CO/Pt (actually quite close to 1.0) and as high or higher than the CO/Pt ratio calculated from the anodic stripping charges for the Pt or Pt-Ru alloy surface. From the charge under the first anodic peak (ea. 90 &/cm2), we estimate the coverage of the low potential state to be ca. 30% of the total CO present at saturation. The state of COad, on Pt,Sn which is oxidized in the first wave appears to be an extremely important state in the reaction pathway for the oxidation of CO. There are several pieces of evidence that support this observation. First, after oxidizing this state of CO, when the potential sweep was reversed, the removal of the state leads to a reappearance of hydrogen adsorption/desorption pseudocapacitance; but holding the potential or cycling the potential in the hydrogen region does not repopulate the state from the other CO still on the surface. The state can only be repopulated by exposing the surface to CO dissolved in solution. There is also no evidence for this state in the voltammetry in the presence of methanol (Fig. 4) so it is not a state which can be formed from methanol dehydrogenation. The onset potential for the oxidation of CO dissolved in solution, shown in Fig. 2, is exactly the same as the onset potential for the oxidation of this state of COad, in Fig. 5. As we discussed previously[26], on Pt, Ru and any Pt-Ru alloy, the onset potential for oxidation of dissolved CO is shifted positively with respect to the onset potential for the oxidation of CO,,,; this difference between the two types of surfaces is fundamental and is a consequence of the absence of competition between CO and water for the same sites on the Pt-Sn alloy surface. From the properties described above, we have termed this catalytically active intermediate state of CO,,, the “high coverage” state. There appears to be a similar state of CO,,, on the alloy surface when CO is adsorbed in the absence of electrolyte, ie adsorption in UHV[27]. A direct comparison was made in [27] of the adsorption of CO on Pt,Sn(hkf) to the pure Pt(hkl) surface of the same orientation using the same methodology. On the pure Pt surfaces there are no states of CO that desorb in the temperature range of 250-3OOK, but on the Pt,Sn surfaces cu. lo-30% of the CO desorbs in this temperature range, with the (111) surface having the largest fraction in this state. This low temperature state is populated only after the high temperature states are filled and required relatively high doses of CO, eg 1OOL (1 L = 1 torr.sec). The heat of adsorption of the low temperature state was estimated to be 70 kJ/mol, or about 20 kJ/mol lower than the heat of adsorption at saturation on the comparable Pt surface, and is an adsorption energy that would be “weak chemisorption”. Classical considered models[3 l] for weakly chemisorbed molecules predict high surface mobility, and hence high react-
On the reaction pathway for methanol and carbon monoxide electrooxidation
ivity for bimolecular surface reactions like step (3), other factors being the same. Although we have no direct evidence that this state exists on the Pt,Sn surface at 3OOK, using the standard expressions[32] for isotherms for CO on Pt surfaces, we can calculate that the state would be populated at 300K at CO pressures above cu. 0.1 Pa. Since CO adsorption on the Pt(ll1) surface is characterized by strong repulsive interaction, eg at >0.5 CO/Pt the heat of adsorption decreases dramatically[33], it was concluded in [27] that strong repulsive interaction between CO molecules adsorbed on Pt sites on the Pt,Sn(lll) surface was the reason why the fraction of the low temperature state of CO,,, was highest on this surface. We therefore suggested in [23] that the repulsive interaction from the “crowding” of CO on the Pt sites on the surface is the reason why Pt,Sn(lll) is even more active for the oxidation of dissolved CO than the (110) surface. This high coverage state on Pt,Sn surfaces, eg >0.9 CO/Pt[23], cannot be created by methanol dehydrogenation on any Pt surface, since the dehydrogenation reaction requires a critical ensemble of free Pt sites[6], and is thus strongly attenuated by a relatively low coverage of COnds, eg <0.6 CO/Pt[S].
4. CONCLUSIONS A small enhancement of methanol oxidation on a sputtered Pt,Sn( 110) surface was observed in longertime potential step measurements. The enhancement is, however, an order of magnitude smaller than those observed on Pt-Ru alloys, eg Pt-10% or 30% Ru, using the same methodology. On the other hand, Pt,Sn is very active for the oxidation of dissolved carbon monoxide (CO), with an onset potential approximately 300mV lower than the most active Pt-Ru alloy surface, and more than SOOmV lower than on polycrystalline Pt. The apparently paradoxical results can be explained in terms of a unique state of CO,,, on this surface, which is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by CO,,, at low coverage. Acknowledgements-Special thanks are due to Lee Johnson for preparing the Pt,Sn rotating disk electrode and Frank Zucca for his invaluable help in keeping the rotator rotating. This work was sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Oflice of Transportation Technologies, Electric and Hybrid Propulsion Division of the US Department of Energy, under Contract DE-AC03-76SFOOO98.
REFERENCES 1. R. Parsons and T. VanderNoot, J. Electroana!. Chem. 257,9 (1988).
2593
B. Beden, F. Kadirgan, C. Lamy and M. M. Leger, J. E[ectroanal. Chem. 127,75 (1981). M. Watanabe, M. Uchida and S. Motoo, J. Electroana[. Gem. 229,395 (1987). J. B. Goodenough, A. Hamnett, B. J. Kennedy, R. Manoharan and S. A. Weeks, J. Elecrroanal. Chem. 240, 133 (1988). M. M. P. Janssen and J. Moolhuysen, Electrochimica Acta. 21, 869 (1976). H. A. Gasteiger, N. M. Markovic, P. N. Ross. Jr. and E. J. Cairns, J. Phys. Chem. 97, 12020 (1993). H. A. Gasteiger, N. M. Markovic, P. N. Ross, Jr. and E. J. Cairns, f. Phys. Chem. 98,617 (1994). N. M. Markovic, H. A. Gasteiger, P. N. Ross Jr., X. Jiang, I. Villegas and M. J. Weaver, Electrochimica Acta. 40,91 (1995). 9. K. J. Cathro, J. Electrochem. Sot. 116, 1608 (1969). 10. M. R. Andrew, J. S. Drury, B. D. Mchicol. C. Pinnington and R. T. Short, J. Appl. Electrochem. 6, 99 (1976). 11. M. M. P. Janssen and J. Moolhuysen: (a) J. Catalysis 46,280(1977);(b) Electrochimica Acta 21, 861 (1976). 12. A. A. Mikhailova, N. V. Ostrova and Y. V. Vasil’ev, EIektrokhimiya 13, 518 (1977). 13. M. Watanabe, Y. Furuuchi and S. Motoo, J. Electroanal. Chem. 191,367 (1985). 14. B. Bittins-Cattaneo and T. Iwasita, J. Electroanab Chem. 238,151(1987). 15. C. T. Hable and M. S. Wrighton, Langmuir 7, 1305 (1991). 16. A. N. Haner and P. N. Ross, J. Phys. Chem. 95, 3740 11991). 17. S. A. Campbell and R. Parsons, .I. Chem. Sot. Faraday Trans.88(6). . ,, 833 (19921 18. Y. B. Vassiliev, \;. S. i)agotzky, N. V. Osetrova and A. A. Mikhailova, J. Electroanal. Chem. 97,63 (1979). 19. M. Shibata and S. Motoo, .I. Electroanal. Chem. 209, 151 (1986). 20. A. B. Anderson, E. Grantscharova and P. Shiller, J. Electrochem. Sot. 142, 1880 (1995). 21. A. B. Anderson and E. Grantscharova, J. Phys. Chem. 99,9149 (1995). 22. H. A. Gasteiger, N. M. Markovic and P. N. Ross, Jr., J. Phvs. Chem. 99.8945 (19951. 23. H.-A. Gasteigei, N. h. Mirkovic and P. N. Ross, Jr., Catalysis Letters 36, 1 (1996). 24. A. N. Haner, P. N. Ross, Jr. and U. Bardi, Catalysis Letters 8, 1 (1991). 25. H. A. Gasteiger, P. N. Ross, Jr. and E. J. Cairns, Surj: Sci. 293.67 (1993). 26. H. A. Gasteiger, N. M. Markovic and P. N. Ross, Jr., J. Phys. Chem. 99.8290 (1995). 27. A. N. Haner, P. N. Ross, Jr., U. Bardi and A. Atrei, J. Vat. Sci. Technol. A lo,2718 (1992). 28. M. J. Weaver, S. C. Chang, L. W. Leung, X. Jiang, M. Rubel, M. Szklarczyk, D. i?urawski and A. Wieckowski, .I. Electroanol. Chem. 327,247 (1992). 29. K. Kinoshita and P. Stonehart, in Modern Aspxs of Electrochemistry (Edited by J. O’M. Bockris and B. E. Conway). Vol. 12, D. 183. Wiley Interscience. New York _ and Ldrdon, (19jf). 30. H. A. Gasteiger, N. M. Markovic and P. N. Ross, Jr., (in press). 31. A. Clark, Theory of Adsorption and Catalysis, p.17-21. Academic Press,.Ndw York and London, (i970): 32. H. Ibach, W. Erley and H. Wagner, Surf. Sci. 92, 29 (1980). 33. H. Steininger, S. Lehwald and H. Ibach, Sur& Sci. 123, 264 (1982).