Electro-oxidation of adsorbed

Potential-Step Electro-Oxidation of Adsorbed Species... 

Jusys Z, Massong H, Baltruschat H. 1999. A new approach for simultaneous DEMS and EQCM Electro-oxidation of adsorbed CO on Pt and Pt-Ru. J Electrochem Soc 146 1093 1098. 

The paradoxical electrochemical behavior of PtSn based catalysts makes its stndy more controversial. Nevertheless, the catalytic enhancement of the electro-oxidation of adsorbed CO on PtSn catalysts is generally accepted. To explain the difference in activi-... 

Figure 11 (A) Stripping voltammetry (20 m Vs at 55 °C) of CO layers on humidified PEM fuel-cell anodes (1) platinum catalyst (2) platinum/molybdenum catalyst. Voltammetry in the absence of adsorbed CO on the platinum/molybdenum catalyst is shown in (3). Molybdenum-mediated electro-oxidation of adsorbed CO takes place on the alloy catalyst in the peak at 0.45 V and at lower overpotentials [79]. (B) Steady-state polarization curves of PEM fuel-cell anode at 85 °C for platinum (squares) and platinum/molybdenum catalysts in the presence of 100 ppm CO (filled points) and pure H2 (unfilled points). (From Ref 79.)... 

In-situ ATR-FTIR spectroscopic study of electro-oxidation of methanol and adsorbed CO at Pt-Ru alloy. J. Phys. Chem. B, 108, 2654-2659. 

Figure 13.5 Potential-step electro-oxidation of formic acid on a Pt/Vulcan thin-film electrode (7 p,gptcm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH upon stepping the potential from 0.16 to 0.6 V (electrol)Te flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCOOH oxidation to CO2. (b) Solid line, m/z = 44 ion current transients gray line, potential-step oxidation of pre-adsorbed CO derived upon HCOOH adsorption at 0.16 V, in HCOOH-ftee H2SO4 solution.

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

Figure 3.35 shows the potential dependence of the integrated band intensity of the linear CO observed in the experiment described above and the corresponding variation in the methanol oxidation current. The latter was monitored as a function of potential after the chemisorption of methanol under identical conditions to those employed in the IRRAS experiments. As can be seen from the figure the oxidation of the C=Oads layer starts at c. 0.5 V and the platinum surface is free from the CO by c. 0.65 V. The methanol oxidation current shows a corresponding variation with potential, increasingly sharply as soon as the CO is removed strong evidence in support of the hypothesis that the adsorbed CO layer established at 0.4 V acts as a catalytic poison for the electro-oxidation of methanol. 

Genera/. The central goal of fundamental electrochemical kinetics is to find out what electrons, ions, and molecules do during an electrode reaction, hr this research, one is not only concerned with the initial state (Le., the metal and the reactants in the solution next to the electrode surface before the reaction begins) and the final product of the reaction, one also has to know the intermediate species formed along the way. Thus, all practical electrode reactions (say, the electro-oxidation of methanol to C02) consist of several consecutive and/or parallel steps, each involving an intermediate radical, e.g., the adsorbed C-OH radical. I Iowcver, one finds that intermediates can be classed into two types. 

This reorganisation also explains the decrease of the current of reduction peak IVC and the formation of the third oxidation peak (IVa). However, both peaks and also peak IIIC are too large to be explained by reduction or oxidation of adsorbed Co(II)TSPc only. It is assumed that this is the result of an electrocatalytic reaction, such as reaction with Co(II)TSPc in solution. This is confirmed by the fact that these peaks disappear when the Co(II)TSPc-modilied gold electrode is scanned in a pH 12 buffer in the absence of Co(II)TSPc in solution. In addition, the peak currents of peak IIIC in the first scan and peaks IIIC and IVC at the scan of maximum coverage vary linearly with Co(II)TSPc concentration (Fig. 7.5). Note that small peaks are observed at the same potentials where peaks IIIC and IVC occurred with solutions containing Co(II)TSPc. Electrochemical measurements of TSPc without Co show also a reduction wave at these potentials, explaining the ring reduction of CoTSPc in solution. This confirms the fact that Co(II)TSPc is adsorbed at the surface of the electrode and electro-catalyses the oxidation/reduction of Co(II)TSPc transported from solution towards the electrode surface. 

It has been suggested that the first step of reaction (6) may be the formation of a carboxylic species COOHads. Carboxyl radicals have indeed been observed by Zhu et al." for potentials lower than 0.65 V using Fourier Transform infrared Reflectance Absorption Spectroscopy with the Attenuated Total Reflection mode (ATR-FTtR). Moreover Anderson et al." made numerical simulation which indicated that the formation of an adsorbed carboxylic species was energetically more favorable. Here, it has to be noted that the electro-oxidation of CO being a stracture sensitive reaction (sensitive to the superficial stracture symmehy" and to the presence of surface defects) this species can be used to study the activity of a catalyst but also as a molecular probe to characterize the catalytic surface. ... 

It can also be observed from this figure that Sn-containing catalyst is a more effective catalyst for the oxidation of CO than that containing Ru, as a lower onset potential of the oxidation wave is obtained with the former catalyst. It has also to be noted that PtSn catalysts are less active towards methanol electrooxidation than PtRu catalysts (see Section IV. 1). ° However, adsorbed CO species are proposed as reaction intermediates of methanol electro-oxidation, which seems to lead to a paradoxical behavior of PtSn based catalysts. In CO stripping experiments, a negative shift of the onset potential for the oxidation of adsorbed CO on PtSn also occurs. " On the basis of in situ infrared spectroscopy studies coupled with electrochemical measurements, Mo-... 

The electro-oxidation of H2/CO mixtures is a very complicated reaction with many variables, e.g., temperature, CO concentration, and pH. From a practical standpoint, the structure sensitivity is not so interesting, since at any realistic level of CO, e.g., >10 ppm CO, and temperature the pure-Pt surface is highly poisoned by adsorbed CO, and the electrode polarization is impractically large. It is, however, still of fundamental importance to know the structure sensitivity of the reaction on pure Pt in order to understand the properties of Pt-based alloy catalysts that are not so highly poisoned by the CO, i.e., so-called CO-tolerant catalysts. For our purposes here, we discuss only one characteristic measure of the structure sensitivity, shown in Figure 14. For a wider range of results, we refer the interested reader to [54] and references therein. Figure 14 shows the current for H2 oxidation at 50-mV... 

For the electro-oxidation of an adsorbed monolayer of CO, however, synergistic effects between Pt and Ru are obvious from the time dependence of the IR spectra, i.e., there is a coupling of the reaction on the composite surface. Figure 14(a) shows the temporal variation of integrated band densities after a potential step from 90 mV to 450 mV. At 450 mV no CO is oxidized on Pt(lll), and one would expect an uneven concentration of CO on Ru or Pt, reflected in the IR-CO stretching band. 

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