VOLUME 101, NUMBER 19, MAY 8, 1997
© Copyright 1997 by the American Chemical Society
LETTERS Potential Dependence of the Yield of Carbon Dioxide from Electrocatalytic Oxidation of Methanol on Platinum(100)† T. D. Jarvi, S. Sriramulu, and E. M. Stuve* UniVersity of Washington, Department of Chemical Engineering, Box 351750, Seattle, Washington 98195-1750 ReceiVed: September 23, 1996; In Final Form: February 21, 1997X
The yield of complete oxidation product (CO2) from electrocatalysis of methanol on a Pt(100) electrode in perchloric acid electrolyte at room temperature was determined by combined measurements of chronoamperometry and linear sweep voltammetry. The fractional yield of CO2 was zero over a potential range of 0.350.45 VRHE and increased monotonically to unity over the potential range of 0.5-0.65 V. The results may be explained by a simple parallel mechanism, in which methanol oxidizes directly to CO2, or by a complex serial mechanism, in which the overpotential for CO oxidation is reduced by solution phase methanol.
Introduction
to be CO, forms as a serial reaction intermediate, reaction 1
Development of a low-temperature, direct methanol fuel cell requires substantial improvements in the kinetics of methanol electrooxidation. One major problem with this reaction is poisoning of the surface by accumulation of CO1-6 and other carbonaceous species such as HCO or COH.7-10 On typical catalysts like carbon-supported platinum or bimetallic platinum/ ruthenium, oxidative removal of adsorbed carbon monoxide requires potentials much greater than the desired potential of a technical anode. The higher activity of Pt/Ru electrocatalysts in comparison with pure Pt is due partly to reduced poisoning levels brought about by ruthenium-enhanced oxidation of carbon monoxide.11-13 Yet even for the most active bimetallic catalysts, anode poisoning remains a problem. Despite considerable research on methanol electrooxidation, a definitive description of the reaction mechanism remains elusive. The main issue is whether the poison, here assumed † Presented at the 212th ACS National Meeting, Orlando, FL, 1996; Paper COLL-51. * Address correspondence to this author. E-mail: Stuve@ u.washington.edu. Fax: 206-543-3778. X Abstract published in AdVance ACS Abstracts, April 15, 1997.
S1089-5647(96)02924-0 CCC: $14.00
CH3OH
–4H+ –4e– r1s
CO
+H2O, –2H+ –2e– r2s
CO2
(1)
or as a side product in a parallel reaction mechanism, eq 2. –4H+ –4e–
r1p
CO
CH3OH
(2) r2p +H2O,
–6H+
–6e–
CO2
Equivalently, one may ask whether methanol can oxidize completely to CO2 at potentials where CO does not oxidize. From a technical perspective, the nature of further research in this area depends upon the answer to this question. If reaction proceeds by the serial route, then further research (if any) must attempt to destabilize adsorbed CO. If, instead, the parallel path is operative, then further research should focus on improving the kinetics and selectivity to CO2, reaction r2p. © 1997 American Chemical Society
3650 J. Phys. Chem. B, Vol. 101, No. 19, 1997 No clear consensus exists as to which mechanism holds. Early results1,14 were interpreted in favor of the parallel mechanism, yet the surfaces used in those studies were not sufficiently well characterized in terms of cleanliness, number of active sites, surface structure, or accumulation of poison. More recent work with single-crystal substrates and time dependent reaction measurements (chronocoulometry, chronoamperometry) does address these concerns. Through comparative studies of carbon monoxide and methanol oxidation on Pt/ Ru electrodes, Gasteiger et al.11,12 found in favor of the serial path, whereby they attributed the Ru-induced enhancement for methanol reaction to increased CO oxidation kinetics. Vielstich and Xia15 similarly found in favor of the serial path as their DEMS (differential electrochemical mass spectrometry) measurements on platinum electrodes showed methanol reacting first to CO (or other carbonaceous species) without production of CO2 for potentials below 0.45 V. Conversely, Herrero et al.16,17 found in favor of the parallel path. Their argument rests on turnover numbers that exceed unity on Pt(100) at potentials where adsorbed carbon monoxide is stable. The conclusion here is that some methanol must react completely to CO2 (r2p) without formation of CO (r1p); otherwise, the surface would saturate with CO, and the turnover number could not exceed unity. In this Letter we present electrochemical measurements of the fraction of methanol that oxidizes completely to CO2 as a function of the potential of a Pt(100) electrode. Reaction studies were performed at constant potential for a fixed period of time (60 s). Accumulation of CO during reaction was measured by linear stripping voltammetry performed immediately after the reaction period. A simple charge balance allows for calculation of the CO2 yield from measured charges, and the resulting yield data are discussed in terms of their mechanistic significance. Experimental Details The experiments were performed in a dual electrochemical cell apparatus operating in an anaerobic environment. The two electrochemical cells were identical, and each consisted of a Teflon electrolyte cup of 0.25 cm3 volume, platinum foil counter electrode of 0.4 cm2 surface area, and palladium R-hydride reference electrode. The electrochemical cells were similar to those previously described,18 and complete details will appear in a later publication. Single-crystal platinum beads were prepared by melting a 0.5 mm diameter platinum wire (ESPI, 99.99%) in a hydrogenoxygen flame to form a bead approximately 1.5 mm in diameter.19 The beads were etched in aqua regia at 60 °C for 30 min to reveal a faceting characteristic of the (100) and (111) crystal faces. The (100) face was aligned by visual inspection and polished with diamond paste to 1 µm smoothness. After polishing, the samples were annealed at 1100 °C in a hydrogenair flame for 30 min.20 Before each electrochemical measurement, the electrode was heated to about 1000 °C for several seconds and quenched in argon-saturated, ultrapure water. The sample was protected by a droplet of ultrapure water during the subsequent transfer to the electrochemical cell. A cyclic voltammogram for samples prepared in this fashion is shown in Figure 1, from which we conclude that the surface is free of contamination and ordered along the (100) plane. Figure 1 compares well with previously reported voltammograms for flame-annealed Pt(100).21,22 The relative heights of the peaks at 0.285 and 0.35 V depend on the exact cooling procedure.21,22 In particular, the lower potential peak increases in height at the expense of the higher potential peak if sample transfer from the flame to the quenching water takes too long.
Letters
Figure 1. Cyclic voltammogram (sweep rate of 50 mV/s) of flameannealed Pt(100), cooled in Ar.
Figure 2. Schematic representation of the multielectrolyte experiment. The upper traces show the current measured for the potential wave forms shown in the lower traces. The arrow denotes electrode transfer from cell A to cell B. Respective concentrations (M) of HClO4 and CH3OH were (a) 0.1, 0.1 and (b) 0.1, 0.
The CO2 yield measurements were performed as follows. Cell B of the two-cell system contained blank electrolyte (0.1 M HClO4), and cell A contained methanol-bearing electrolyte (0.1 M CH3OH in 0.1 M HClO4) or blank electrolyte for rinsing the electrode. After a quick anneal the clean electrode was immersed into cell B for recording of its cyclic voltammogram. The electrode was emersed at 0.075 V and immersed into cell A at the same potential. After 2 s the potential was stepped to the desired reaction potential Er for 60 s while the resulting current transient was recorded as depicted in Figure 2a. The charge passed during the current transient is the reaction charge qr. After the reaction period the electrode potential was returned to 0.075 V. Traces of methanol-bearing electrolyte were removed by rinsing the electrode, still in cell A, with blank electrolyte under potential control. The electrode was then transferred to cell B for measurements of adsorbed, partial oxidation products by linear sweep voltammetry as depicted in Figure 2b. The charge measured under the CO oxidation peak at 0.72 V is the stripping charge qs. The yield of CO2 was determined from qr and qs as described below.
Letters
Figure 3. Reaction charge (a) qr (0, 9) and the stripping charge (b) qs (O, b) as a function of reaction potential Er. Open symbols represent data for cumulative turnovers of less than one and solid symbols for cumulative turnovers greater than one. The size of data points reflects (1 standard deviation in the qr measurement. The lines are guides to the eye.
Electrolyte solutions were prepared from perchloric acid (GFS, doubly distilled), methanol (Fisher, spectrophotometric grade), and ultrapure water (Barnstead Nanopure, 18 MΩ cm resistivity). Both reference electrodes were prepared identically by charging at -150 µA for 15 min daily. Potential control and voltammetry were performed with an EG&G 263 potentiostat. Current transients were recorded by a Tektronix TDS 520 digital oscilloscope. Measured reaction transient currents were at least 2 orders of magnitude less than diffusion-limited currents estimated by the Cottrell equation.23 For all electrochemical measurements only the polished face of the electrode contacted the meniscus of the electrolyte. A drop of blank electrolyte was maintained on the electrode during cell transfers to protect the surface from contamination. All measurements were performed at room temperature, and electrode potentials are reported with reference to a reversible hydrogen electrode in 0.1 M HClO4. Results and Discussion The reaction charge qr and the stripping charge qs are shown as a function of the reaction potential Er in Figure 3. The reaction charge is approximately constant below 0.45 V but increases at 0.5 V and beyond. The qs values scatter about a line at approximately 235 µC/cm2 below 0.65 V, while at higher potential they drop off. The drop in the stripping charge above 0.65 V is attributed to an increase in the rate of CO oxidation (r2s) during the reaction period τ at this potential (compare with Figure 2b). Data corresponding to cumulative turnovers less than or greater than unity are depicted with open and filled symbols, respectively; the surface supports complete turnover for Er g 0.5 V. Cumulative turnovers were calculated on the basis of a six-electron transfer process, a reaction site of one surface atom, and the density of topmost atoms of an ideal Pt(100) surface (1.28 × 1015 cm-2); a reaction charge of 1.23 mC cm-2 corresponds to a cumulative turnover of one. A simple analysis leads from the data reported in Figure 3 to an expression for the fractional yield yc of complete oxidation product (CO2). Here fractional yields of complete yc or partial yp oxidation products are defined as the moles of product formed per mole of methanol reacted during the reaction period τ. To develop the necessary equation, we assume a parallel mechanism
J. Phys. Chem. B, Vol. 101, No. 19, 1997 3651
Figure 4. Fractional yield yc of CO2 as a function of reaction potential. The line is a guide to the eye. Details of the yield calculation are given in the text. Typical standard deviations are shown for some data points.
as shown in eq 2, though the final form of the equation does not depend on the choice of mechanism.24 The number of moles of methanol that react during the reaction period is denoted Nr, the Faraday constant F, and the electrode surface area A. The complete oxidation product is CO2 and we assume that the partial oxidation product is CO, so that six electrons are required for complete oxidation and four electrons for partial oxidation. The reaction charge qr can be expressed as
qr )
F N (6y + 4yp) A r c
(3)
If the CO formed by r1p in eq 2 is oxidized according to r2s in eq 1, the stripping charge required is
qs ) 2
F Ny A r p
(4)
Since the yields of complete and partial oxidation products are related by
yc + yp ) 1
(5)
we may rearrange eqs 3-5 to obtain an expression for the fractional yield of complete oxidation product
yc )
qr - 2qs qr + qs
(6)
The fractional yield as determined by eq 6 is shown in Figure 4 as a function of reaction potential. The results show three distinct potential regions corresponding to different product distributions. At low potential (below 0.50 V), partial oxidation products form exclusively, in agreement with DEMS experiments.15 Reaction at intermediate potentials (0.5-0.65 V) produces both CO and CO2. Finally, beyond the apparent onset of CO oxidation (0.65 V), CO2 is the dominant reaction product. Figure 4 indicates CO2 production occurs at potentials of 0.50.65 V, while CO is predicted to be surface stable at these potentials by either Figure 2b or Figure 3. Furthermore, Figure 4 predicts CO2 as the majority product over this entire potential range. These results may be interpreted according to either the parallel or serial mechanism. The parallel mechanism provides the most direct explanation as the increase in yc above 0.5 V
3652 J. Phys. Chem. B, Vol. 101, No. 19, 1997
Letters
can be interpreted as an increase in r2p relative to r1p. Alternatively, the serial mechanism also explains the data if one assumes that CO oxidation through r2s occurs at appreciable rates over the potential range of 0.5-0.65 V. Such an assumption appears to contradict the voltammetric measurement in Figure 2b, which shows CO to be surface stable below 0.65 V. However, the reaction charge was measured in a methanolcontaining electrolyte, whereas the stripping charge was measured in the absence of methanol. Solution phase methanol could provide the chemical potential necessary to reduce the overpotential of reaction r2s relative to that in methanol free electrolyte. To examine the serial mechanism hypothesis further, note that the tailing edge of the reaction transient in Figure 2a implies the existence of a near steady state. The serial mechanism predicts a steady state when r1s ) r2s. At the end of the reaction period t ) τ, the rate of CO oxidation r2s(τ) must therefore be finite according to the serial mechanism and measured turnover numbers exceeding unity. To test this, we performed one reaction pulse on the electrode at 0.55 V for 60 s in methanolbearing electrolyte (cell A) and then transferred the electrode to blank electrolyte (cell B) and performed a second reaction pulse, again to 0.55 V for 60 s. If r2s were finite, then the current transient at the beginning of the second pulse should be about one-third that at the end of the first pulse. However, we detected no measurable current transient (other than double-layer charging), and the reaction charge for the second pulse was below the limit of detection. A subsequent stripping voltammogram revealed that the CO coverage was the same as in the case where only a single reaction pulse was performed. This result shows that CO does not oxidize appreciably at 0.55 V so that the only manner in which the serial mechanism could hold in this potential range is in the case that solution phase methanol alters the reaction mechanism in some other fashion than that shown in eq 1. In other words, the mechanism for CO oxidation below 0.65 V (if any) is different than that above 0.65 V. The assumption that the only partial oxidation product formed is CO appears in eqs 3 and 4, where the numbers of electrons required for the formation and oxidation of the partial oxidation product are 4 and 2, respectively. Evidence of other stable reaction products exists, namely HCO or COH species.9 The presence of any finite quantity of HCO or COH species as partial oxidation products leads to a wider potential window over which yc is nonzero. This can be seen by considering the analog of eq 6 for the case of either COH or HCO as the partial oxidation product, for which the formation and oxidation steps both require three electrons
yc )
qr - qs qr + qs
(7)
For the same measured qr and qs values, the calculated yc values are greater with this assumption. Thus, the yc values shown in Figure 4 represent the lower bound to the yield of complete oxidation products. These results indicate that methanol electrooxidation on Pt(100) produces CO2 at potentials where CO appears to be surface stable (0.5-0.65 V). Our inability to detect complete oxidation below 0.5 V is consistent with the DEMS results,15 which were
interpreted in favor of a serial mechanism, while our detection of complete oxidation above 0.5 V along with turnover numbers in excess of unity agrees with the previous results and interpretation in favor of the parallel mechanism.16 Additional preliminary results suggest that the parallel mechanism may operate even at very low potentials.25 The simple parallel mechanism, as represented in eq 2, indeed provides the most straightforward interpretation of our results. A simple serial mechanism, as shown in eq 1, cannot describe our results. Instead, a more complex serial mechanism is required in which solution phase methanol reduces the overpotential for CO oxidation. We are continuing our experiments in this area in order to determine which of the simple parallel or complex serial mechanisms best represents electrooxidation of methanol on platinum. Acknowledgment. This work benefited from stimulating discussions with H. Baltruschat, C. Campbell, T. Engel, N. Markovic, and A. Wieckowski. We gratefully acknowledge financial support from the National Science Foundation (CTS9502971). References and Notes (1) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (2) Beden, B.; Bewick, A.; Lamy, C. J. Electroanal. Chem. 1983, 148, 147. (3) Sun, S. G.; Clavilier, J. J. Electroanal. Chem. 1987, 236, 95. (4) Adzic, R. R.; Tripkovic, A. V.; Markovic, N. M. J. Electroanal. Chem. 1983, 150, 79. (5) Iwasita, T.; Nart, F. C.; Lopez, N.; Vielstich, W. Electrochim. Acta 1992, 37, 2361. (6) Chang, S. C.; Ho, Y.; Weaver, M. J. Surf. Sci. 1992, 265, 81. (7) Willsau, J.; Wolter, O.; Heitbaum, J. J. Electroanal. Chem. 1985, 185, 163. (8) Iwasita, T.; Vielstich, W.; Santos, E. J. Electroanal. Chem. 1987, 229, 367. (9) Wilhelm, S.; Iwasita, T.; Vielstich, W. J. Electroanal. Chem. 1987, 238, 383. (10) Bittins-Cattaneo, B.; Cattaneo, E.; Ko¨nigshoven, Pl.; Vielstich, W. In Electroanalytical Chemistry; Bard, A. J. Ed.; Marcel-Dekker: New York, 1991; Vol. 17, p 181. (11) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (12) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (13) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Electroanal. Chem. 1993, 361, 269. (14) Bagostky, V. S.; Vassilev, Yu. B.; Khazova, O. A. J. Electroanal. Chem. 1977, 81, 229. (15) Vielstich, W.; Xia, X. H. J. Phys. Chem. 1995, 99, 10421. (16) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995, 99, 10423. (17) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (18) Borup, R. L.; Sauer, D. E.; Stuve, E. M. Surf. Sci. 1993, 293, 10. (19) Clavilier, J. J. Electroanal. Chem. 1980, 107, 205. (20) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333. (21) Armand, D.; Clavilier, J. J. Electroanal. Chem. 1989, 270, 331. (22) Rodes, A.; Zamakhchari, M. A.; El Achi, K.; Clavilier, J. J. Electroanal. Chem. 1991, 305, 115. (23) Bard, A. J.; Faulkner, J. R. Electrochemical Methods; Wiley: New York, 1980; p 143. (24) Jarvi, T. D.; Stuve, E. M. In Electrocatalysis, Frontiers in Electrochemistry; Lipkowski, J.; Ross, P. N., Jr., Eds.; VCH Publishers: New York, 1997; Vol. V, Chapter 3. (25) Chrzanowski, W.; Sung, Y-E.; Thomas, S.; Shibata, M.; Wieckowski, A. Presented at the 212th ACS National Meeting, Orlando, FL, 1996; Paper COLL-209.
