10421
J. Phys. Chem. 1995,99, 10421-10422
Comments on "Electrochemistry of Methanol at Low Index Crystal Planes of Platinum: An Integrated Voltammetric and Chronoamperometric Study" W. Vielstich" and X. H. Xia Institut f i r Physikalische Chemie, Universitat Bonn, Wegelerstrasse 12, 53115 Bonn, Germany Received: January 12, 1995
In a very interesting paper in this Journal by Herrero, Franaszczuk, and Wieckowski which we hereafter refer to as ref 1, the authors have used cyclic voltammograms showing the methanol oxidation current from an acidic 0.2 M methanol solution at the 111, 110, and 100 single crystal surfaces of platinum. In addition, current-time transients have been taken, produced by potential steps from f 6 0 to f 4 2 0 mV (RHE) and to +620 mV, respectively, followed up to 100 ms. From the data obtained rate constants for the oxidation of methanol to C02 were calculated, evaluated via Tafel plots for the different surfaces. The influence of varying the anions in the electrolytic solution was also investigated using 0.1 M solutions of HC104, H2S04, and H3P04. Mechanism of Methanol Oxidation at Platinum. The evaluation of the experimental data by the authors has been made assuming a parallel pathway for the reaction as it is known from the oxidation of formic acid:
+ 4Hf + 4eCH,OH 4-H 2 0 - CO, + 6H' + 6eCH,OH
+
CO
................
...................:. ................. ....
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'
1
"
"
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'
0,2 0,3 0.4 potential (V vs. RHE)
'
0,5
0,6
Figure 1. First potential scan at a porous polycrystalline platinum layer 0.1 M HClOI, starting at 50 mV with 10 mV/s: in 0.2 M CH30H (a) current as function of potential; (b) simultaneously recorded mass signal for CO1 ( d e = 44). The lower curves are from a blank experiment without methanol in solution.
+
(1)
(2)
Competing with the CO formation according to reaction 1, one has an altemative pathway which leads directly to the final product C02. However, the existence of such a direct parallel pathway has never been proved. Conversely, up to f 4 5 0 mV no C02 formation can be observed as will be demonstrated below. As a parallel pathway to reaction 1, it has been observed via on-line mass spectroscopy2the formation of methyl formate? but occurring at a rate smaller than the rate of reaction 1 by 2 orders of magnitude. Therefore, with an efficiency of almost loo%, we have to consider reaction 1 as the pathway for methanol oxidation. The formation of C02 is practically proceeding via the intermediate CO only. Therefore, the current-time transients from potential steps to f 4 2 0 mV cannot be caused by reaction 2 as assumed by the authors. COz Formation Starts at Polycrystalline Platinum above +420 mV. Experimental investigations in our 1ab0ratorqP.~ have shown that the formation of C02 at polycrystalline Pt does not start before the oxidation of CO is taking place, Le., not before +420 mV. This investigation had been done in an on-line MS experiment using a porous Pt layer as electrode, sputtered on a porous polymer membrane at the entrance to the mass spectrometer.2 Figure 1 shows with curve a the current of a first potential scan taken at the porous Pt layer in a 0.2 M CH3OH 0.1 M HC104 solution, starting at 60 mV with 10 mVls, and with curve b the simultaneously recorded mass signal for C02 ( d e = 44). At first, it can be clearly seen that COz formation from methanol oxidation takes place only above f 4 2 0 mV. Second, a pronounced current peak starts already near +200 mV versus RHE. This current results from the dehydrogenation of adsorbed methanol and formation of adsorbed CO according
+
0022-365419512099- 10421$09.0010
0,6t
,
,
0
20
,
,
,
40
,
60
,
,
80
1 0
time (s)
Figure 2. (a) Current-time transient of a potential step from +60 to +420 mV (RHE) at the surface of sputtered polycrystalline platinum in 0.2 M CHsOH 0.1 M HC101. (b) Simultaneonsly recorded mass signal for COz ( d e = 44). These are netto-curves, i.e., the blank curves (without methanol in solution) are subtracted from the total current vs time plot.
+
to reaction 1. The delivery of charges below the oxidation potential of CO is limited by the coverage of the Pt surface with CO. Current-Time Transients below +420 mV Are Due to Dissociative Adsorption. The formation of CO according to reaction 1 below the CO oxidation potential is due to a 0 1995 American Chemical Society
Comments
10422 J. Phys. Chem., Vol. 99, No. 25, 1995
The transient in our Figure 2 is to prove that only charges are delivered but not CO;?. The respective on-line MS experiment is done on a porous polycrystalline platinum layer. As in the paper of Herrero et al., the potential step is made from 60 to 420 mV. The current-time transient obtained is of comparable form as the one in ref 1, but only very low C02 formation is observed. Finaly, we like to show that the current peak due to dehydrogenation during the first scan at polycrystalline platinum6 is to be seen at least on one single crystal surface also. An according voltammogram taken at a Pt(110) surface is given in Figure 3.
0,06
0,04
g
0,02
u
’
-0,Ol 0,o
I
0.2
0,4
0,6
0.8
I
potential (V vs. RHE)
Figure 3. First potential scan as in Figure 1, but using Pt(100) single crystal surface and without using on-line mass spectrometry. Dotted lines represent the blank without methanol in solution.
dissociative adsorption of methanol molecules, resulting in a complete dehydrogenation. The electric charges delivered during this process are responsible for the current-time transients observed in the study of Herrero et al.’ In our laboratory we have taken current-time transients under respective conditions in order to prove this statement.
References and Notes (1) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (2) Bittins-Cattaneo, B.; Cattaneo, E.; Konigshoven, P.; Vielstich, W. In Bard, A. J., Ed. Electroanalytical Chemistry; Marcel Dekker: New York,
1991; Vol. 17, p 181. (3) Iwasita, T.; Vielstich, W. J. Electroanal. Chem. 1986,201, 403. (4) Krausa, M.; Vielstich, W. DECHEMA Monogr. 1993,128, 161. (5) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994,379, 307. (6) Iwasita, T.; Nart, F. C.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1990,94, 1030. JP950147Q
