5074
J . Phys. Chem. 1994,98, 5074-5083
Electrochemistry of Methanol at Low Index Crystal Planes of Platinum: An Integrated Voltammetric and Chronoamperometric Study E. Herrero,+K. Franaszczuk, and A. Wieckowski' Department of Chemistry, University of Illinois, Urbana, Illinois 61801 Received: October 25, 1993; In Final Form: March 10, 1994"
W e have studied a catalytic decomposition of methanol on low Miller index platinum surfaces, Pt( 1 1 l ) , Pt( 1 lo), and Pt( 100) in perchloric, sulfuric, and phosphoric acids a t room temperature. The instantaneous methanol oxidation current is unaffected by the methanolic CO formation (surface poisoning) and depends on platinum surface structure and composition of supporting electrolyte with respect to the anions. The highest oxidation current, 156 mA.cm-*, is observed with the Pt( 110) electrode in perchloric acid solution a t 0.200 V vs Ag/AgCl reference. In terms of turnover, this current translates to 163 molecules.(Pt site)-'-s-l, a high rate exceeding previous expectations in methanol electrode kinetics. Overall, the oxidation current changes by 3 orders of magnitude between the extreme cases examined in this study. Breaking up the total effect into individual components shows that the surface geometry and anionic effects are roughly comparable. Therefore, we have a n evidence that anion-platinum interactions are as important in determining the methanol oxidation rate as is the surface geometry of the P t catalyst. Being encouraged by the magnitude of the oxidation current, especially with the Pt( 110) electrode, and by the control of the oxidation process through the structural and electrochemical variables of this research, we also report that the rate of methanolic CO formation follows the same pattern as does the oxidation current. Namely, the CO poisoning is the highest for the P t ( l l 0 ) electrode in perchloric acid and the slowest with the Pt( 11 1) electrode in phosphoric acid. We conclude that optimizing the structure of clean platinum, and solution composition, is not a sufficient remedy for platinum deactivation and that the CO poisoning process must be addressed with new force in basic research on platinum fuel cell catalysis.
Introduction Methanol is catalytically oxidized on platinum electrodes producing CO1 and six electrons per CH3OH molecule, making this organic compound a promising fuel in the direct oxidation fuel cell.1-3 Unfortunately, if not prevented by some physical or chemical conditions, a catalytic poison, namely methanolic CO, develops, thus impeding the oxidation. Basic research aimed at optimizing the methanol oxidation rates via surface structrual adjustment using, for instance, well-defined, single-crystal electrodes+s is both timely and challenging. The connection between electrochemistry of well-defined platinum electrodes and real-life electrocatalysis arises from the fact that fuel cell reactions occur on small platinum particles that are essentially single crystals and have distinct microstructures and surface order .9 The sites on crystallographic terraces (facets) have a relatively low reactivity compared with those in edges and defects (ref 10 and references therein). Equally important, the unevenly distributed defects and terraces induce steric and electronic differences among the catalytic sites" that may affect reactivity. Both types of the site arrangement, facets and defects, can be modeled using macroscopic single-crystal planes. For instance, flat Pt(ll1) and Pt(100) planes may imitate the facets on the metal particle while the corrugated Pt( 110) face may simulate the defects, at least to some extent. Previous structural research in the gas-phase heterogeneous catalysis with singlecrystal surfacesl~I4has brought conclusive information on the significance of surface structure in activation of selected bonds, like C-0 and C-H (refs 11 and 14 and references therein). The structural approach practiced with the catalytic solid/gas interface is likewise appropriate in electrochemistry as demonstrated, for instance, by an extreme sensitivity of methanol electrooxidation to the surface crystallographic atomic order of platinum.4s f
Permanent address: University of Alicante, Spain.
* To whom correspondence should be addressed.
@
Abstract published in Aduunce ACS Absfracrs, April 15, 1994.
In addition toour search for an understanding of how platinum surface structure and the composition of supporting electrolyte affect methanol oxidation rates, we want to know whether providing the right catalytic surface morphology will reduce the CO poisoning. We also ask if a low level of poisoning automatically guarantees a good catalytic efficiency. It is a prerequisite in catalysis-related research to learn about the rate/poison relationship from the surface crystallographic perspective since this approach may provide significant practical conclusions. In this respect, it is also important to discuss what methanol oxidation mechanism is suggested by the new rate data and to check if the new mechanism favorably compares to some of the earlier proposals. Previous studies of methanol electrooxidation rates with welldefined electrodes has been carried out using ~oltammetry.4.5.~ In contrast, our studies reported in ref 6, which we continue here, have employed a fast chronoamperometric technique. This potential step, rather than potential sweep approach, gave us rates of the elementary processes involved in methanol decomposition without the complications of CO poisoning. In this report, we have expanded the number of surfaces and solutions studied and (i) have quantified not only the effects of platinum surface structure but also the solution composition on methanol oxidation rates, (ii) have measured the Tafel slopes and kinetic isotope effects involved in methanol oxidation, (iii) have used the rate data to demonstrate how the methanol oxidation mechanism changes with surface geometry, and (iv) have proposed an integrated chronoamperometric and cyclic voltammetric interpretation of the observed rate events (structure/function). We have found, in accordance with Adzic et al.,4J that the Pt( 1 11) electrode is the most poison resistant. However, this electrode gives a small initial (instantaneous) current that degrades slowly to an even lower level. On the other hand, the Pt(l10) electrode is the most efficient but deactivates rapidly. We have found that the surface geometry and anionic influence on methanol oxidation rates are roughly comparable. Evidently, anion-platinum in-
0022-365419412098-5074%04.50/0 @ 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 5075
Electrochemistry of Methanol Uuuer ootential
5
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'
,
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.
,
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Figure 1. Potential step program used in the chronoamperometric experiments.
teractions are as important in determining the rates of catalytic oxidation of methanol as is the proper surface geometry of the Pt catalyst.
0 5
'b 3
Experimental Section 1. Electrode Preparation, Chemicals, and Instruments. Spherical platinum single crystals, 0.2 cm in diameter, were cut and polished to produce three surfaces of (loo), (1 lo), and (1 11) crystallographic orientations, within 5' a c c ~ r a c y . ~ J 5The J ~ real surface area was determined using hydrogen adsorption-desorption charges.6 Each of the surfaces was chemically and electrochemically cleaned and ordered by the hydrogen-air, flameannealing method.15J6 Immediately before the measurements, the hot crystal was cooled in the nitrogen-hydrogen gas to ca. 200 OC, quenched in ultrapure water, and transferred to an electrochemical cell. The following chemicals used were: Millipore water (18 Mn-cm), 99.9% heavy water (Sigma Chemical Co.), reagent grade sulfuric, perchloric, and phosphoric acids (Fischer), reagent grade methanol-h (Fischer), and 99% methanol-d (Aldrich). The twocompartment electrochemical cell was equipped with a Luggin capillary. In HCIO, solution, the iR drop was measured and compensated for by using a positive feedback option of the PAR 273 potentiostat. (In H2SO4 and H3P04, the iR drop did not affect the oxidation currents.) The experiments were carried out at room temperature of 20 f 1 OC. All of the voltammograms were recorded a t 50 m V d . The electrodes were used in a meniscus configuration to avoid exposure of other then desired planes of the crystal to the electrolyte. All potentials were measured against the Ag/AgCl reference in 1 M NaCl solution. 2. Chronoamperometric Rate Data Acquisitions. A sequence of activating potential steps was applied to an electrode via PAR 237 potentiostat that wasinterfaced toan IBM PC/ATcomputer. The sequence was followed by measurements of the methanol oxidation current at a constant potential, as shown in Figure 1.6 The steps were applied between the predetermined, lower and upper potential biases. The upper potential was selected so that complete oxidative removal of surface CO was achieved, but no surfacedisorder occurred. The lower potential was always at the onset of hydrogen evolution. As shown by voltammetry, the activating pulsing procedure induced some modifications to the electrode surfaces that resulted in poor reproducibility of the methanol oxidation data. In perchloric acid, especially with the Pt(ll1) and Pt(100) electrodes, such effects were particularly acute, most likely due to accumulation of small amounts of surface chloride, present in HC104 as impurity: or due to a slow perchlorate reduction process (to chloride anions).l7J* In other electrolytes, while surface contamination effects were not found, surface defect formation become noticeable, thus affecting the reproducibility. To minimize such adverse effects, only one activating potential step was applied. For the Pt( 110) electrode,
O
L
0
20
40
60
80
100
t/ms Figure 2. Potential step, current-time plots for the Pt(l11) electrode in 0.1 M HC104 at E = 0.200 V: (A) the total current vs time plot obtained with 0.2 M CHsOH in solution, (B) the blankcurrent (without methanol in solution), and (C) the total current - blank current (circles) and the fitting least-squares curve (solid line).
the changes in the voltammetric properties and the catalytic activity upon the potential steps were much less pronounced, and two steps could be used. This was beneficial to the properties of this particular surface since it is very active and easily oxidizable. Hence, improved reproducibility was observed with two activating cycles. The waiting time a t the low and high bias was 1 s. Before the current transients were recorded, the electrodes were potentiostated for 10 ms at a potential that was 60 mV more positive than the lower bias (Figure 1). The delay of 10 ms was sufficient to reduce remnants of platinum oxides (water discharge products) formed at the upper potential but short enough to avoid a noticeable methanolic CO formation. From voltammetry, after the pulse sequence was completed, we found that there were no significant suppression of hydrogen adsorption states. Therefore, no methanolic CO poison was on the surface to interfere with the chronoamperometric data acquisition. A chronoamperometric sample of data used for the determination of the instantaneous currents, i,=o,6 and rate constants for adsorption of methanolic CO, kad (eq l), consisted of a set of 300 current-time points. The sampling rate was 1 ms for the Pt( 111) and Pt( 1 10) electrodes in HC104 and H2S04solutions, 5 ms for these two electrodes in H3P04 solution, and 5 ms for the Pt( 100) electrode in all of the solutions. (The Pt( 100) electrode displayed a capacitance of approximately 3 times higher than the Pt( 1 11) surface.) The blank current transients due to oxidation of adsorbed hydrogen and the double-layer charging were subtracted from the total oxidation currents measured in methanolcontaining solutions. These blank transients were negligibleafter 3 ms in HClO, and H2SO4, after 15 ms in H3P04, or after 15 ms with the Pt(100) electrode in all solutions studied. Figure 2 shows typical chronoamperometric curves representing (i) methanol oxidation current without the blank current correction, (ii) the blank transient, and (iii) methanol oxidation current after correcting for the blank. The current decays with time until it approaches a steady-state value that is usually very small.
5076 The Journal of Physical Chemistry, Vol. 98,No. 19,1994
Herrero et al.
TABLE 1: Voltammetric Peak Current Densities (the "icy"Row), Instantaneous Current Densities, and Tafel Slopes for Methanol Oxidation Process on the Platinum Single-Crystal Electrodes Studied in This Work
0.74 0.30 32.4 24.5 icv, mA-cm-2 1.80 0.57 0.24 156 6.5 iE.0.2 V,mA.cm-2 2.80 1.37 nmb 17.0 97 4.8 i ~ 4 . V, 4 mA.cm-2 122 121 123 119 Tafel slope, mV/decade 123 The instantaneous currents were obtained at 0.200 and 0.400 V. nm = not measured.
As a result of the high blank current at short times, the instantaneous oxidation current is not directly measurable. Therefore, the square-root extrapolation6 was performed (Figure 2C, solid line). In view of surface stability of methanolic CO in the electrode potential range i n v e ~ t i g a t e d , l - ~the , ~ J drop ~ in the oxidation current is accounted for by the C O formation, and the potentiostatic current-time relationship has the following form+
i = it,,
- ema:kadt
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0 -50 -100
ema:kadf)z
+ emaxkadt + emaxkadt where kad is the rate constant for methanolic C O adsorption/site blocking process, 4 is the number of electrons produced in methanolic CO formation, e is the elementary charge, Npt is the total number of surface Pt sites, Omax is the maximum coverage of the CO, and m is the number of surface sites occupied by one methanolic C molecule. Four quantities shown in eq 1, i,,o, kad, Omax,and m, have been treated as adjustable parameters and determined from the experimental i-t data for using the leastsquares method (a Simplex algorithm6).
100
50
0 -50 -100
B
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Results 0
1. Cyclic Voltammetry. Cyclic voltammograms for the -50 surfaces investigated in the three solutions, with and without methanol present, are shown in Figures 3-8. The voltammetric -100 C peak current densities obtained in the methanol-containing solutions are given in Table 1 (see the icv row in Table 1). The -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 data for the individual electrodes will be treated separately in the next three subsections of this section. 1 .I, The Pt(l11) Electrode. Figure 3 shows cyclic voltamE/V ( A g / A g W mograms of the Pt(ll1) electrode2&z4 in the studied solutions Figure 3. Cyclic voltammograms for the Pt( 1 1 1) electrode in (A) 0.1 M without methanol. As expected, s ~ l f a t e ~and ~ - ~p*h ~ s p h a t e ~ ~ - ~ l HClOI, (B) 0.1 M HzS.04, and (C) 0.1 M HjPO4. (All of the adsorption shift the unusual splitting segment of the voltammovoltammogramsin this study weremeasured at thescan rateof 50 mV.s-I.) gram to lower potentials and prevent surface oxidation. The quality of the Pt( 1 1 1) single-crystal surfaces can be assessed by positive-going sweep is taken, the second peak (a2) increases, the absence of the peak at -0.19 V, the appearance of wellindicating that this is due to the C O oxidation process. When developed spikes at 0.42 and 0.19 V in HC104 and HzSO4, electrodes with more intrinsic surface defects, or electrodes with respectively, and the flatness of the region between 0.60 and 0.70 defects induced by prolonged cycling, were used, an increase in V in HC104.20*23324 the a3 peak current was systematically observed. Therefore, we Cyclic voltammetric curves for methanol oxidation at the Ptassign the peak a3 to methanol oxidation on the defect sites. In (1 11) electrode in solutions containing 0.2 M methanol are shown parallel with the increase in the height of the peak current a3, the in Figure 4. The current is the lowest in phosphoric acid and the contribution of the peak a l to the voltammogram decreases. We highest in perchloric acid (Table 1). As discussed in the conclude that this latter peak originates from methanol oxidation Introduction (ref 5 and references therein, as well as more recent on the ideal Pt( 111) plane. studies692,33), the (1 11) plane of platinum is the least susceptible The current decrease past the maximaS is not caused by the to poisoning by methanol chemisorption products. Therefore, methanolic CO formation since the surface CO in this potential some clearly measurable currents are recorded at low potentials range is easily oxidizable and cannot block platinum sites. In in the positive-going sweep. Also, the hydrogen part of the perchloric acid, the decrease in the oxidation current coincides voltammogram does not show a noticeable site-blocking effect. with the terminating spike of the unusual splitting of the Nevertheless, some surface poisoning takes place as shown by the voltammogram, that is, between 0.05 and 0.20 V2&24(cf. Figures fact that currents measured on the negative-going scan were 9 and 3A). Since the spike is due to the initial stage of platinum always higher than those measured on the positive-going scan. oxidation,24-27.34which may indicate a completion of the full OH The current recorded on the positive-going scan shows small adlayer,Z1 the current decay is most probably caused by the but distinctive voltammetric features. In H3P04,because of the oxidation. However, insulfuricand phosphoric media, thecurrent low rate of methanolic C O formation, these features can be easily falloff above 0.35 V cannot be related to platinum oxidation since identified and assigned (Figure 4C). If the electrode is held in this takes place only at much higher potentials20324-29(Figure the hydrogen region, thus allowing the CO to form, and the
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 5077
Electrochemistry of Methanol -0.4 2.0
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E/V (Ag/AgC1) Figure 4. Cyclic voltammograms for the Pt( 111) electrode in (A) 0.1 M HCIO4 + 0.2 M CH30H, (B) 0.1 M H2S04+ 0.2 M CH3OH, and (C) 0.1 M Hap04 + 0.2 M CHsOH. 3B,C). We will return to this point in the Discussion section (see also the chronoamperometric results presented below). 1.2. The P t ( l l 0 )Electrode. Voltammetric curves for the Pt(1 10) electrode in the clean supporting electrolytes are shown in Figure 5 . The specific adsorption of sulfate and phosphate, vis&vis an apparent absence of a correspondingly strong perchlorate adsorption, causes a shift in the onset of hydrogen adsorption to more negative potentials. There is also a small, but noticeable, difference between the hydrogen adsorption-desorption profiles in phosphoric and sulfuric ,media, indicating that phosphate is adsorbed more strongly than sulfate (see Discussion). This is confirmed by the observation that the platinum oxidation current is more suppressed in phosphoric acid than in sulfuric acid (Figure 5). Voltammograms obtained in solutions containing methanol are shown in Figure 6 . (The peak currents are given in Table 1.) Clearly, the (1 10) crystallographic plane of platinum generates the highest voltammetric current. There is no significant current measured on the negative-going sweep reflecting a more pronounced Pt( 110) oxidation as compared to that of Pt(ll1). The beginning of the peak potential for the (1 10) electrode is shifted 200 mV in the positive direction vs that measured with the Pt(1 11) electrode. This shows that the electrooxidation of methanolic CO-needed for the catalytic site release-was shifted to more positive potentials. A sharp decrease in the oxidation current at 0.60 Vcoincides with theonset of the Pt( 110) surfaceoxidation (Figure 5 ) . for the z'3' The Pt(zOO)E'ectrode' The voltammetric Pt(100) electrode in the clean solutions are shown in Figure 7. As with other platinum surfaces studied in this work, anionspecific adsorption shifts the adsorption states from 0.21 V in HC104 to more negativevalues. In solutionscontaining methanol, the oxidation peak currents are higher than with Pt( 11 1) but lower than with Pt( 110) (Figure 8 and Table 1). As in the case of the Pt( 1 10) electrode in H2S04and H3P04solutions, the decaying branch of the oxidation current past the maxima can be ascribed to the initial phase of platinum surface oxidation.
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E/V (Ag/AgCl)
Figure 5. Cyclic voltammograms for the Pt(ll0) electrodein (A) 0.1 M HC104, (B) 0.1 M H2SO4, and (C) 0.1 M H3PO4. -0.4
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E/V (Ag/AgCl) Figure 6. Cyclic voltammograms for the Pt(ll0) electrode in (A) 0.1 M HC1O, + o.2 cH30H,(B)o.l H 2 ~+0o.2 4 CH~OH,and (c) 0.1 M H3PO4 + 0.2 M CH3OH. (See the increase in the voltammetric current in Figure 7 at 0.45 V.) Notably, in phosphoric acid electrolyte, instead of the welldevelopedvoltammetricpeakon thenegative-goingscan, acurrent plateau is found. We interpret this observation as resulting from the lower rate of CO poisoning due to phosphate adsorption. If this is so, thevoltammetricdata indicate that the rateof adsorption
5078 The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 0.4
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Herrero et al.
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E/V (Ag/AgCl) Figure 9. Semilogarithmic log(if,o) vs electrode potential plots for methanol oxidation on the Pt( 111) electrode obtained under potential step conditions in 0.1 M HCIO, (triangles),0.1 M HzSO4 (circles), and 0.1 M H3P04 (squares). Methanol concentration was 0.2 M. The i,.o current is the instantaneouscurrent; see text. Solid lines are calculated Tafel lines. Broken lines are extrapolated Tafel lines to higher potentials.
-100 100
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E/V (Ag/AgCU Figure 7. Cyclic voltammograms for the Pt( 100) electrodein (A) 0.1 M HC104, (B) 0.1 M HzSO4, and (C) 0.1 M H3P04.
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E/V (Ag/AgCU Figure 10. Semilogarithmic log(if,o) vs electrode potential plots for methanol oxidationon the Pt( 1 10) electrodein 0.1 M HClO, (triangles), 0.1 M HzS04 (circles), and 0.1 M H3PO4 (squares). Methanol concentration was 0.2 M. See Figure 9 caption for explanation of the solid and broken lines.
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Figure 8. Cyclic voltammograms for the Pt( 100) electrode in (A) 0.1 M HC104 + 0.2 M CHIOH, (B) 0.1 M HzS04 + 0.2 M CHsOH, and (C) 0.1 M H3P04 + 0.2 M CHIOH.
Y V (Ag/AW) Figure 11. Semilogarithmic log(i,,o) vs electrode potential plots for methanol oxidation on the Pt( 100)electrodein 0.1 M HCIO, (triangles), 0.1 M (circles), and 0.1 M HIPOI (squares). Methanol concentration was 0.2 M. See Figure 9 caption for explanation of the solid and broken lines.
of methanolic CO on the Pt( 100) electrode is lower than on the Pt(ll0) electrode, where such an effect is not found. This is confirmed in the potential step studies reported below. 2. Chronoamperometry (Potential Step Studies). The type of current-potential plot that we have processed is illustrated in Figure 2C. All of the instantaneous methanol oxidation currents,
if=o,6 at two potentials, 0.200 and 0.400 V, are given in Table 1. They are also plotted vs the electrode potential (as logarithms) in Figures 9-1 2. Tafel slopes obtained from the plots are also given in Table 1. A brief inspection of the slopes shows that the main mechanistic difference occurs between the Pt( 11 1) and Pt(1 10) electrodes on one hand, and the Pt(100) electrode on the
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 5079
Electrochemistry of Methanol
TABLE 3: Rate Constants k.a (ms-l) Involved in Methanolic CO Adsorption (Site Blocking) Process: See E4 1 E = 0.200V E = 0.400 V Pt(ll1) Pt(ll0) Pt(100) Pt(ll1) Pt(ll0) Pt(100) 0.1 M HC104 0.26 2.9 0.30 1.3 nm' 0.051 0.1 M HzS04 0.070 0.16 0.037 0.18 0.25 0.056 0.1 MHjP04 0.0057 0.019 0.041 0.022 0.033 0.013 nm = not measured. V, presumably due to increasingly stronger anion effects, and some contribution of surface CO oxidation, to the methanol oxidation rates. Consequently, if deuterated water is used, and hydrogen atoms of the methanolic 0-H groups are exchanged with deuterium atoms from the solvent, the currents are only 1.5-2 times lower vs those measured in light water (not shown). Such effects can be explained through solvent effect on chemical reaction rates.6935J6 Twovertical lines have been added to Figure 12 to discriminate against three potential ranges of the Pt( 11 1) electrode in which the oxidation of methanol responds to the increases in electrode potential in a dissimilar way: the linear part of the Butler-Volmer plot (range I), the deviations from the Butler-Volmer linearity, range I1 (i.e., where the rateofcurrent increase with the electrode potential decelerates) and the current decrease with potential, range I11 (that does not apparently relate to clean surface oxidation). The events accounting for these distinctive currentpotential segments of Figure 12 will beconsidered in the Discussion section. 2.2. The Pt(100) Electrode. The i,,o data obtained with the Pt( 100) electrode (Figure 11 and Table 1) show that theoxidation rates are intermediate between those with Pt( 11 1) and Pt( 110) surfaces (see above, subsection 1.3). The Tafel slope range is very short but gives formally the slope of 60 mV-decade-1, confirming earlier data obtained by voltammetry.37 Evidently, a different rate-determining step is involved in methanol oxidation on the Pt( 100) electrode than on the (1 11) and (1 10) electrodes. There is also a flat maximum in current at 0.20 V, where the electrolyte is HC104, and current plateaus in H2S04and H3P04 above 0.10 V. A comparison of the rate data obatined with light and heavy methanol gave very high kinetic isotope values, averaging 7 (Table 2). This isotope effect is much higher than those observed with other platinum single-crystal planes used in this study (see Discussion). 2.3. Methanolic CO Formation. The rate constants for the methanolic CO formation (surface poisoning) obtained from eq 1 are given in Table 3. The data show that the more catalytic the surface, the higher the surface poisoning rate; Le., the poisoning rate changes with platinum surface geometry in the order of Pt( 110) > Pt( 100) > Pt( 11 1). The data also show that not only the instantaneous current but also the rate of the current decay due to the methanolic CO formation depends on the composition of the supporting electrolyte. That is, the catalytic oxidation currents and the rate of poisoning follow the same order: HC104 > H2S04 > H3P04 (Table 1). ~
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0.3
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E/V (Ag/AgCU Figure 12. Semilogarithmic log(i,,O) vs electrode potential plots for methanol oxidation on the Pt( 111) electrodein 0.1 M HC104(triangles) and in 0.1 M H2SO4 (circles). Indicated are three distinctive potential ranges that are referred to in text. Methanol concentration was 0.2 M.
See Figure 9 caption for explanation of the solid and broken lines.
TABLE 2: Kinetic Isotope Effects (the Ratios of Instantaneous Currents with CH3OH and CD3OH) Involved in Methanol Oxidation E = 0.400 V E = 0.200 V Pt(ll1) Pt(ll0) Pt(100) Pt(ll1) Pt(ll0) Pt(100) 0.1MHC104 4.1 4.8 5.8 4.1 4.6 2.12 0.1 M H2SO4 3.2 3.0 8.1 3.1 1.23 3.2 3.4 9.5 0.1 M H3P04 4.3 4.2 7.6 3.0 other. We will present the data obtained with the (111) and Pt( 110) surfaces first, followed by those obtained with the Pt(100) electrode. 2.1. The P t ( l l 1 ) and P t ( l l 0 ) Electrodes. The Pt(ll1) electrode exhibits the lowest oxidation current of the three low index planes (Table l ) , in agreement with the earlier voltammetrid-5 and chronoamperometric6 results. In perchloric acid the current measurements yield a linear range in the ButlerVolmer kinetics from 0.10 to 0.40 V (Figure 9, triangles). As already reported in ref 6 for sulfuric acid electrolyte, the plots gave the Tafel slope (the reciprocal of the slopes in Figures 9-1 2) of 120 mv-decade-l and, correspondingly, n,a = 0.5. (The same Tafel slope was previously found in methanol adsorption on polycrystalline ~1atinum.I~)Notably, in H2S04 and H3P04 media, a deviation from the Tafel linearity was found starting at 0.20 V (Figures 9 and lo), that is, at a potential 200 mV more negative than in perchloric acid medium (Figure 12). While the drop in the chronoamperometric current at 0.40 V for the Pt(1 1 1) electrode in perchloric acid may be attributed to the initial stage of platinum oxidation, such an interpretation is not plausible for the sulfuric and phosphoric electrolytes since the oxide formation process begins at more positive potentials (see above, subsection 1.1). With the platinum (1 10) electrode and HC104 electrolyte, the oxidation rates are very high, and our method is not applicable for rate measurements above 0.20 V. At 0.200 V, the current is 156 mA.cm-2 (Table l), which corresponds to a turnover of 163 molecules.(Pt site)-'-s-'. This is the highest current (and turnover) measured in this study. The Butler-Volmer plot in an appropriate potential range (Figure 1 1 ) gives again the Tafel slope of 120 mV-decade-'. The instantaneous currents with heavy methanol (CD30H) with the Pt( 11 1) and Pt( 110) surfaces are 3-4 times lower that with light methanol, CH30H. The ratios of the instantaneous currents for the light and heavy methanol, Le., the kinetic isotope effects, are given in Table 2. The obtained values are in the range that is typical of a primary isotopic effect, implying that a C-H bond scission is involved in the rate-determining step.6 At 0.400 V, the isotope effects are lower than that taken at 0.200
~~~~
Discussion 1. Potential Step Measurements. While cyclic voltammetry is undoubtedly a useful technique, the potential step method offers a more straightforward means to extract detailed mechanistic information, and quantitative rate constants, for the methanol oxidation process. The advantages of analyzing current decays include the ability to measure oxidation current immediately following the potential stepping (the instantaneous currents, turnovers), a separation of the elementary reactions from methanolic CO formation, the identification of the relationship between the rates of oxidation and the rates of poisoning, and the determination of isotope effects at fixed potentials to which the electrode is stepped. Furthermore, the potential step experiments have allowed us to determine the functional relationships between
Herrero et al.
5080 The Journal of Physical Chemistry, Vol. 98, No. 19, I99'4 TABLE 4 Solution Components, the pH, Ionic Strength, and Adsorotion Strength, Effective in Methanol Oxidation Rates ionic anion adsorption oxidation solution strengthb strengthC pHb rates" weak 1.5 0.034 0.1 M HC104 highest 1.0 0.12 strong medium 0.1 M 1.6 0.024 very strong lowest 0.1 M H3P04 0 The qualitative summary of the quantitative rate data in Table 1. Calculated from equilibria data in CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1991. From data in refs 28 and 31; see text. the potentiostatic rates and potential. This is in contrast to cyclic voltammetry where the functional form must be assumed a priori. In view of practically unlimited diffusion delivery of methanol to the surface and the relatively small current measured (Table l ) , the drop in methanol oxidation current under potential step conditions is mainly accounted for by surface poison formation, most likely chemisorbed C O
CH30H
+
co,d, + 4H+ + 4e-
(2)
which completes with the oxidation of methanol to C02:196938
CH30H + H,O
-
CO,
+ 6H+ + 6e-
(3)
To obtain this instantaneous methanol oxidation current information, the extrapolation of the current-time response to zero time has to be made on the milliseconds time scale6 (Figure 2). Overall, a significant change in the oxidation rates approaching 3 orders of magnitude between the extreme cases has been observed (Table 1; see the row ~E.;o.~v, the third and fourth columns). These limiting cases determine the extent of tunability of the methanol oxidation kinetics on the low index planes of platinum at room temperaturein the commonly used electrolytes. Breaking up the total rate effect into the individual substrate and environment contributions shows that the surface geometry and solution composition effects are roughly comparable (Table 1; see the figures in the row i ~ ~ o . 2 Therefore ~). we report, for the first time, that these solution composition factors are as important in determining the rates of methanol oxidation as is the proper surface geometry of the Pt catalyst. 2. Solution Components Effective in Oxidation Rates. The first column of Table 4 gives a qualitative ranking of methanol oxidation rates obtained in the three studied solutions (The qualitative summary of the quantitative rate data in column i ~ ~V, 0in Table . ~ 1, obtained with the three surfaces studied). The solution pHs and ionic strength are given in the two next columns. Inspection of the data shows that the rates correlate neither with a change in solution pH nor with a change in the ionic strength. The next column contains information on the order of the anion adsorption strength obtained for polycrystalline39 and singlecry~tal2~93' platinum electrodes: H3P0, > H2SO4 > HC104 (see below). This adsorption trend correlates very well with the increase in the methanol oxidation current. As shown in the seminal paper by Clavilier,20anion adsorption on platinum is sensitive to the surface structure. In addition to the surface structure dependency, anions are engaged in competitive chemisorption with water decomposition products-mainly surface OHZI-and with adsorbed hydrogen.2&29,34,40 If the least strongly adsorbed anion, perchlorate, is specifically adsorbed at all, it is adsorbed in the most positive potential range of this study. It competes with the O H formation process but does not interfere with hydrogen adsorption in the conventional hydrogen potential range. Therefore, the methanol oxidation current in perchlorate media is free of any major effects by specific adsorption of anions. In sulfuric and phosphoric acid solutions, it is well recognized that the anion desorption from the Pt( 111) electrode coincides with the beginning of the unusual splitting of volta-
mmogram measured upon the negative-going electrode scan, 0.20 V (Figure 3B,C). (Adsorption begins at the most negative potential of the splitting, 0.00 V.)20,25-29 With the other two crystal faces the anion desorption is triggered by hydrogen adsorption, that is, at 0.23 V for Pt( 100) and 0.00 V for Pt(ll0) (Figures 5A and 7A). That is, sulfate and phosphate adsorption precedes, or occurs in parallel with, methanol oxidation in the potential range examined. Consequently, the anion adsorption affects in sulfate and phosphate media on methanol oxidation are very strong. Recent radiochemical studies of adsorption of sulfate and phosphate on low the Miller index planes of platinum have helped to discriminate between the phosphate, sulfate, and perchlorate adsorption.28.31 In this investigation 0.1 M solutions of HC104 containing 1 mM sulfuric and phosphoric acid were used. Since both sulfateand phosphate were adsorbed from moreconcentrated perchloric acid solution to high coverage, they are capable of displacing surface perchlorate easily. Therefore, the notion that perchlorate is not meaningfully adsorbed is conformed using single-crystal28-31rather than polycrystallineelectrodes.39Further, using the Pt(100) electrode, we have found that the surface concentration of phosphate is 3 times that of sulfate. With the Pt(l11) and Pt(ll0) electrodes, much broader surface concentration vs electrode potential maxima were obtained with phosphate than with sulfate, also pointing a t the stronger phosphate that sulfate adsorption. Since the order of adsorption strength, H3P04 > H2SO4 > HC104, correlates with the trend of decreasing methanol reactivity (Table 4), we conclude that, for a given platinum crystal face, anion adsorption is the main factor accounting for the range of methanol oxidation rates reported in this paper. 3. The Linear Range of the Butler-Volmer Plot. The linear range of the Butler-Volmer plot (the Tafel range) for methanol oxidation a t the Pt( 111) electrode in perchloric acid solution, with the Tafel slope of ca. 120 mv-decade-', is sufficiently large to conclude that the elementary (rds) step is a C-H bond splitting and a concerted electron charge-transfer process: rds
CH,OH
(CH,OH),d,
+ H+ + e-
(4)
This amounts to confirmation of one of the main mechanistic statements of Bagotsky et al. onmethanoldecompositionprocesses at polycrystalline p l a t i n ~ m . ~The I step 4 is in contrast to the elementary step in methanol decomposition on 'dry" platinum, where the rate-determining step is the methanolic 0-H split (ref 6 and references therein.)
CH30H- (CH30),ds +
(5)
With other electrodes and solutions, the linear range is shorter, but the Tafel slope for the Pt(ll1) and Pt(ll0) electrodes is the same as that for Pt( 11 1) in perchloric acid solution. We propose (albeit with less confidence, due to the shorter Tafel range) that reaction 4 is again rate determining. With thePt( 100) electrode, the H/D isotope labeling data of Table 2 indicate that, as with other two surfaces, a C-H split is rate determining. However, the Tafel slope is 60 mV-decade-1, the same value as found in ref 37. Since these two independent investigations give the 60 mV-decade-' slope, we attach a mechanistic significance to the Tafel slope value and conclude that the rate-determining step involves a C-H bond scission preceded by a reversible electrochemical step involving one-electron charge-transfer process and the first C-H bond breakassuming no D / H exchange before step 7):42
CH30H
-
+ H+ + e-
(6)
( C H O H ) & + H+
(7)
(CH,OH),d, rds
(CH2OH)a,s-
Electrochemistry of Methanol loo 10
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 5081
/I
A
'
:
+
+++
A
++++
e
+
e
t
1 .
e
e
e
0.1
0.0
e
0.1
0.2
0.3
0.4
E/V (Ag/AgCU
Figure 13. Semilogarithmiclog(i,=o) vs electrode potential plots for the Pt(ll1) electrode in 0.1 M HzSO4 containing 0.2 M CHlOH (filled circles), 0.2 M HCOOH (crosses), and 0.2 M HCHO (triangles).
Since, in reactions 6 and 7, the equilibrium and kinetic isotope effects are combined, the net isotopic effect is higher than the purely kinetic effect for reaction 2 (Table 2). We also notice that the transition state formed between the surface species CHzOH and CHOH, reaction 7-the CHlOH species activated in the adsorbed state-may be more loose than the transition state formed between the associatively (weakly) adsorbed methanol and the CHzOHads (reaction 4). The kinetic the0ry3~predicts higher kinetic isotope effects for relaxed rather than for rigid activated complexes, the situation that may be applicable to our case.36 The Tafel lines for methanol oxidation are downshifted along the current axis from perchloric to phosphoric acid (Figures 9 and 10). At 0.10 V, where our current measurements begin, the anion coverage is already at saturation on the Pt( 110) electrode and at an approximately one-third coverage on the Pt( 11 1). For the first surface (Figure lo), it suffices to conclude that a monolayer of surface sulfate is decreasing the catalytic activity of platinum, as discussed in the previous subsection of this paper. On the P t ( l l l ) , quite surprisingly, the increase in the anion adsorption from one-third to 1 ML coverage is not changing the slopes of the Tafel lines (Figure 9). Close examination of the voltammetric data shows, however, that the Pt(ll1) surface is not uniform; it has two groups of surface sites of different energy (Figure 3). If this is so, the more reactive sites, e.g., surface defects, are affected by the anion adsorption at less extreme potentials than the less reactive sites on the (1 11) terraces. While the defect surface concentration is not higher than 2% (Figure 3), the defects are very active, especially vis-&vis a very inactive Pt( 111) plane. In view of the high defect activity, the suppression of their catalytic properties by adsorbed anions is evidently reducing the total oxidation current in a stepwise manner, as shown in Figure 9. We emphasize that the shift between the parallel linesalong thecurrent axis in Figures lOand 11 constitutes the main solution composition effect on the methanol oxidation rates identified in this study. 4. Deviations from the Tafel Kinetics (Range I1 in Figure 12). Above 0.20 V, a clear deviation from the Tafel kinetics is observed, Figures 9 and 10 for the Pt(ll1) and Pt(ll0) electrodes, respectively (see range I1 in Figure 12 for the (1 11) orientation). In the first approximation, the catalytic site blocking by the specifically adsorbed anions may account for the rate deceleration and ultimately for the current decay above 0.40 V; see below (range 111, Figure 12). However, auxiliary experiments with formic acid and formaldehyde suggest this explanation not to be applicable (Figure 13). Namely, the data indicate that in the potential range I1 (from 0.20 to 0.40 V) the catalytic oxidation of formic acid and formaldehyde gives currents that are 3 and 10 times higher, respectively, than those observed for methanol. Regarding the anion effects, this observation confirms earlier evidence obtained by radiochemistry that adsorption of sulfate2'
and phosphatejl is reversible and that the exchange of the surface and bulk anions is fast. Because of the fast exchange, the frequency of collisions of methanol molecules with the bare platinum surface may not be much affected by the anion adsorption. Instead, on the surface covered by the specifically adsorbed anions, less methanol collisions may be reactive to generate proportionally less oxidation current. In other words, this conclusion is not consistent with the assumption that the site accessibility may explain the deviation from the semilogarithmic plots of Figures 9 and 10. Because of the failure of the simple site-blocking model, we need to look for another explanation to account for the rate data. We notice, for instance, that the second harmonic generation (SHG) results indicate that sulfate adsorption induces modifications of platinum surface nonlinear ~usceptibility.~3 This provides conclusive evidence that the surface electrons participate in the anion chemi~orption.~~ On the other hand, the SHG data give a strong basis for an assumption that the surface anions, when present during methanol electrolysis, affect the electrons engaged in the methanol transition-state formation, reactions 4,6,and 7. This may profoundly affect the way methanol decomposes to more stable surface and/or solution products. Therefore, one possibility is that when the platinum-anion interaction strength increases with electrode potential, the electronic effects caused by the anions also increase and ultimately affect the Tafel kinetics. We also notice that the methanol reaction sites in sulfuric and phosphoric solutions are surrounded by the specifically coadsorbed anions," as redox centers are surrounded by ligands in chemical or electrochemical redox reactions. Therefore, one may consider a modified S a ~ e a n and t ~ ~Chidsey46 model to account for the reduction in the rate increase (with the electrode potential). The means for discrimination between these two models, or a possibility to combine them, are now being examined. In fact, a unified model for the Pt( 111) and Pt( 110) surfaces may not be found since the saturation coverage of the anions on the Pt( 110) electrode are obtained at much less positive potentials than on Pt( 111).3' Inspection of the data obtained with the Pt(100) electrode (Figure 1 1) indicates that beyond the short Tafel range, methanol oxidation current instead of increasing slightly decreases with the increase in the electrode potential. Several factors may contribute to this development, such as (i) excessively strong interaction of the Pt(100) surface with water, including a possibility of an early water discharge,28 (ii) the reconstruction of the Pt( 100) plane upon the methanol reaction events, and (iii) a different character of the anion interaction with the Pt(100) surface than that with the other surfaces. At this point, we may only state that, for the reasons that are not fully understood, the behavior of thePt( 100) electrodeas a methanol oxidationcatalyst is very different from that of other surfaces studied in this work. 5. Current-Potential Decays in Voltammetric and Potential Step Measurements. The voltammetric oxidation current passes through a maximum and decays to a small level at potentials more positive than those of the peak values (Figures 4,6,and 8). Likewise, the instantaneous currents obtained under potential step conditions decay at potentials not much more positive than are the voltammetric peak potential values, e.g., Figure 12. Undoubtedly, these events observed under voltammetric and chronoamperometric conditions are closely interconnected. In some of the instances, the decays may be accounted for by platinum oxidation, which is always detrimental for methanol electrolysis (see Results). However, for the Pt(ll1) surface in sulfuric and phosphoric acid, there is no evidence that such a surface oxidation is occurring below 1.0 V on the potential scale in use.20 Markovic and Ross, confronted with a very similar problem, have proposed an anion site-blocking process as an origin for the voltammetric current decay.' In particular, they postulated that the current drop is coincidental with the small, but clearly noticeable, voltammetric feature at 0.45 V (Figure 3B) that was
SO82 The Journal of Physical Chemistry, Vol. 98, No. 19, 1994
believed to be related to some additional uptake of However, while plausible for sulfuric acid solution, this explanation is not adequate for phosphoric acid where the current-potential line in a broad vicinity of the 0.40 V value is featureless (Figure 3C). The lack of the coincidence notwithstanding, a change in surface anion properties must account for the rate decrease, since no other potentially effective factors have become apparent through this or the earlier investigation^.^ A possibility to consider is that, at potentials as positive as 0.40 V, there may be an increase in two-dimensional order within the anion adsorbates47and their hydration water molecules, as in thecaseof coadsorption of sulfate and water (hydronium ion) on the Au( 1 11) electrode~.~sThis may result in a two-dimensional water-anion cross-linking that inhibits access of methanol molecules to the bare catalytic sites. While there is very little information on such a cross-linking processes on platinum, a long-range order character of the sulfate adsorbateon the Pt( 1 11) electrode has already been demonstrated in UHV research.22328 Therefore, the reduction of the site accessibility through the lateral sulfate-water interactions is proposed to account for thevoltammetric and chronoamperometric current decays at the electrode potential of ca. 0.4 V. 6. Oxygen-like Species Involved in Methanol Oxidation. Dehydrogenation of methanol to surface CO, reaction 2, does not require a participation of an oxygen-containing species to go to completion. On the contrary, a CO2-producing process via CH2OH, reaction 3, and methanolic CO oxidation to COz need one extra oxygen to fulfill the reaction stoichiometries. While the decomposition modes of the CHzOH intermediate are unknown, it is most probably highly reactive and may intercept oxygen from any source available, including interfacial or near surface bulk water molecules49 (or may competitively degrade to methanolic CO, reaction 2, if the methanol C O formation process goes via CHzOH intermediate). However, the process that ultimately dominates the methanol electroactivity at potentials lower that 0.30 V is the methanolic C O formation, as recently reaffirmed by Gasteiger et al. (ref 50 and references therein). The oxidative removal of the CO releases catalytic sites that are used in bulk methanol oxidation; e.g., see Figures 4,6, and 8. If efficient and sustained, such a site removal process may ultimately lead to a meaningful, steady-state methanol oxidation. Therefore, a source of oxygen for the site release process is of considerable mechanistic and practical interests. The voltammetric part of this single-crystal study shows that on the negative-going scan the methanol oxidation current is not explicitly correlated with water discharge process observed in solutions without methanol. Consider the oxidation on Pt( 100) electrode (Figures 7 and 8). The water discharge to a high coverage H206+,ds species28 @)
(9) or to a low coverage Pt-OH,d,
is clearly dependent on solution composition. That is, the discharge current maximum is at 0.20 V in HC104 and at 0.35 V in H2SO4 and overlaps with Pt surface oxidation in H3P04 (Figure 7). This shows that the increase in the adsorption strength shifts the discharge in the positive direction. However, the voltammetric peak current for methanolic CO and bulk methanol oxidation is practically unchanged (Figure 8). On the contrary, in sulfuric and phosphoric acid, water discharge on the Pt( 110) appears at -0.10 and -0.15 V, respectively, Le., is shifted slightly in the negative direction, but the position of the onset of methanol oxidation is again invariant with the solution composition, Figure 6. (The overpotential could be a nucleation overpotential.51)
Herrero et al. Further, whereas the replacement of perchloric by phosphoric acid makes the Pt( 11 1) electrode surface inactive for the water discharge at potentials of interest (Figure 3A,C), this surface passivation leaves the position of the methanol oxidation peak practically unaffected. In all the instances discussed above, methanolic CO and surface water coexist on the surface catalytic sites of platinum, including those experiments where the electrode potential is undergoing a voltammetric change to more positive values (Figures 4, 6, and 8). At the voltammetric peak beginning, the electrode potential becomes sufficiently positive to make the adsorbed CO and water molecules more reactive. As a result, the oxygen transfer from water to methanolic CO may take place without water decomposing first to a distinctive and catalytically active, OH,& phase. This is an essence of the methanolic CO oxidation mechanism proposed by BieglerS2and W i e c k ~ w s k and i ~ ~ this mechanism is indirectly confirmed in this study.
Conclusions Using electrodes made of low index crystal planes of platinum and light and heavy methanol, we have presented evidence that methanol mechanism changes with surface geometry. With the Pt(ll1) and Pt(ll0) surfaces, a C-H bond cleavage and a concerted electron-transfer reaction are involved in the ratedetermining step. With the Pt( 100) electrode, the ratedetermining step is a second C-H bond cleavage, preceded by an electrochemical equilibrium reaction involving the first C-H bond break. The instantaneous currents for methanol-free of methanol self-inhibition effects-show that the methanol oxidation rates depends significantly on the platinumsurface geometry. Specifically, the Pt(ll0) electrode is the most active and the Pt( 11 1) is the least active. Moreover, the rates change when the solution composition in which methanol oxidation occurs is altered, and this rate-reducing, environmental effect follows the relative adsorption strength of the anions: H2PO4- > HS04- > C104-. Overall, at a constant electrode potential, the methanol oxidation rate may change as much as by 3 orders of magnitudes. This shows that one may achieve a considerable control over the methanol oxidation rate using the structural and solution composition variables. We believe that our findings are related to the practice of fuel cell catalysis by providing more evidence on surface structure effects in methanol oxidation rates and by showing quantitative information on anion effects in controlling the rates. We reemphasize that our data show that the anionic effects may be as strong as the structural ones. Since unexplored synthetic options remain for producing electrolytes that are simultaneously surface noninteracting and have properties compatible with fuel cells, our observations may provide a basis for pursuing such new syntheses. One may also notice that anionic surface properties mimic, at least to some extent, those of the functional groups in conducting polymers used in fuel cell technology. Synthesizing polymers that will have functional groups exhibiting the least surface interactions also appears to be worth considering. Finally, with all of the single-crystal planes, the currents measured when surface CO is present are very small, and the rate of the C O formation followsthe same rate pattern as does methanol oxidation to C02. The quantitative information obtained in this study clearly shows that, while platinum of an optimized surface crystallography can generate large instantaneous oxidation current, this favorable catalytic property cannot be utilized in practice because of the rapid CO poisoning.
Acknowledgment. Helpful comments by Dr. Radoslav Adzic of the Brookhaven National Laboratory are highly appreciated. This work was supported by the National Science Foundation, Grant DMR 89-21538, administered by the Materials Research Laboratory of the University of Illinois, and by the Air Force
Electrochemistry of Methanol
The Journal of Physical Chemistry, Vol. 98, No. 19, 1994 5083
Office of Scientific Research (AFOSR-89-0368). A.W. acknowledges a continued collaboration with Professor R. I. Masel, Department Of Engineering* uluc, and E-H. acknowledges support by Generalitat Valenciana as an award of a FPI grant.
References and Notes R.;Van der Noot, T. J .
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