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J. Phys. Chem. B 2006, 110, 18724
COMMENTS Comment on “Electrooxidation of COad Intermediated from Methanol Oxidation on Polycrystalline Pt Electrode” C. Gutie´rrez* Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, C. Serrano, 119, 28006 Madrid, Spain ReceiVed: May 18, 2006 Xu et al.1 have applied normal pulse voltammetry (NPV) to the study of the electrooxidation of carbon monoxide adsorbed on a smooth polycrystalline platinum, using the equations derived by Matsuda2 in 1980 for species under conditions of semiinfinite linear diffusion only. This essential requirement was not fulfilled at all in Xu et al.’s work,1 in which the reacting species, CO, was adsorbed, and therefore, its electrooxidation could not be controlled by its diffusion from the bulk, all the more so because there was no CO in the bulk (adsorbed CO was produced by electrooxidative adsorption of methanol). Xu et al.1 claimed that “...the limiting current was controlled by content of COad produced on the electrode surface and this behavior was similar to semi-infinite”. Three references3-5 were given in support of this surprising assertion, but actually none of them supports it in the least: ref 3 concerns electroactive species confined within a polymer film thicker than the distance of charge transport at the longest sampling time, so that seimiinfinite free diffusion is obtained, in ref 4 the word “diffusion” does not appear at all, and in ref 5 only the diffusion of CO and OH on the surface of Pt and Ru particles is qualitatively discussed. Since the electrooxidation of adsorbed CO cannot occur under conditions of free semiinfinite diffusion, the NPV study of this process by Xu et al.1 is completely invalid. Therefore, perhaps these comments could stop here, but other serious flaws in their work deserve a comment as well. 1. The surprising claim was made1 that “The poisonous intermediate of methanol oxidation on a Pt electrode was validated to be COad by electrochemical method”, namely, by cyclic voltammetry (CV). To begin with, the experimental system was not clean, as clearly evidenced by the high slope of the CV in the region of Pt oxidation, which in clean systems is a current plateau. Furthermore, far more detailed electrochemical studies have already been reported by a host of researchers. And, most importantly, as long ago as 1981, adsorbed CO was unequivocally identified6 as a product of methanol electrooxidative adsorption on Pt by in situ IR spectroscopy. 2. Adsorbed CO was said1 to be produced by holding the potential at -0.2 V vs Ag/AgCl in the presence of methanol, although at this potential (about 0.0 V vs the relative hydrogen electrode, RHE), there is little or no electrooxidative adsorption of methanol on Pt.7 Apparently, the authors did not hold the electrode at 0.0 V vs RHE before introducing it in the cell, and therefore, the electrode was exposed to higher (open-circuit) potentials. * Tel: +34-915619400 ext 1327. Fax: +34-915642431. E-mail:
[email protected] It is worthwhile to point out that in the excellent SEIRAS work8 in which formate was first identified as an active intermediate of methanol oxidation on Pt, and in which both linear and bridge-bonded CO were present from the beginning of the CV at 0.05 V vs RHE, the spectra corresponded to the second and following CVs, and therefore, the adsorbed CO was formed in the preceding negatiVe scan.9 3. It was hypothesized1 that the coverage of adsorbed CO produced by the electrooxidative adsorption of methanol increased with time according to a “Langmuir monolayer model” in which time is substituted for the concentration, namely, θ ) bt/(1 + bt), where θ, b, and t are the coverage, the adsorption rate, and time, respectively. This is a travesty of Langmuir’s equation, given without any justification whatsoever. Furthermore, the coverage is given in units of mol/L, which is simply absurd. Incidentally, a maximum “coverage” of 33 mol/L of CO was reached, this being higher than the molarity of liquid CO, 28.3 mol/L. 4. Although the NPV experiments were carried out in the presence of 1 M methanol in solution,1 the observed currents were attributed only to the electrooxidation of adsorbed CO, although obviously dissolved methanol was also electrooxidized. Actually, the currents of “adsorbed CO” electrooxidation in Figure 71 are even higher than those of “dissolved methanol” electrooxidation in Figure 2 in another work by the same authors,10 with the only difference being the 0.5 V higher initial potential and the 5-10 times higher waiting time in the latter case, in which consequently the amount of adsorbed CO, and therefore the currents, should have been much higher than those in Figure 7,1 eVen if they were exclusiVely due to the electrooxidation of adsorbed CO. The highest “diffusion-limited current” of “dissolved methanol”10 was 2.4 mA cm-2, which is about 40 times lower than the calculated stationary diffusion-limited current. Therefore, this application of NPV was also completely wrong. 5. Anodic peaks were found in the NPV experiments (Figure 71), instead of the plateaus which are the hallmark of diffusionlimited currents. 6. The plots of the anodic peak current densities vs the inverse square root of the sampling time (Figure 81) do not define straight lines, as claimed1, but convex or concave lines. References and Notes (1) Xu, W.; Lu, T.; Liu, C.; Xing, W. J. Phys. Chem. B 2006, 110, 4802. (2) Matsuda, H. Bull. Chem. Soc. Jpn. 1980, 53, 3439. (3) Sato, K.; Yamaguchi, S.; Matsuda, H.; Ohsaka, T.; Oyama, N. Bull. Chem. Soc. Jpn. 1983, 56, 2004. (4) Herrero, E.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. 1995, 99, 10423. (5) Roth, C.; Martz, N.; Hahn, F.; Le´ger, J.-M.; Lamy, C.; Fuess, H. J. Electrochem. Soc. 2002, 149, E433. (6) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J. Electroanal. Chem. 1981, 121, 343. (7) Caram, J. A.; Gutie´rrez, C. J. Electroanal. Chem. 1992, 323, 213. (8) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680. (9) Ye, S. Personal communication. (10) Xu, W.; Lu, T.; Liu, C.; Xing, W. J. Phys. Chem. B 2005, 109, 7872.
10.1021/jp063044o CCC: $33.50 © 2006 American Chemical Society Published on Web 08/31/2006
