Potential-dependent methanol

Case Study Potential-Dependent Methanol Dehydrogenation over Aqueous-Phase Pt(111)... 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Tadjeddine and co-workers have used SFG [Guyot-Sionnest and Tadjeddine, 1990 Eisenthal, 1992 Richmond, 2002 Vidal et al., 2002, 2004, 2005] to study the adsorbed CO produced from a variety of solution species, including methanol [Vidal et al., 2002, 2005]. With BB-SFG, we studied the electrochemical kinetics of methanol chemisorption as surface CO, as shown in Fig. 12.13. We used apolycrystal-line Pt electrode and 0.1 M H2SO4 electrolyte with 0.1 M methanol. Figure 12.13a-d characterize the potential-dependent SFG spectra obtained under the voltammetric... 

Although the detection of COads by in situ IR was accepted as not ruling out the existence of other adsorbed species (particularly since the experiments were not quantitative in terms of coverage and the potential-modulation aspect of the technique could render it blind to adsorbed species that do not exhibit a potential-dependent absorption frequency), it was generally accepted that the EMIRS data had ended the long controversy over the nature of the poison derived from methanol. 

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. 

Figure 3.35 Potential dependence or the ft) integrated band intensity of the linear COad< derived from methanol at 0,4 V vs. RHE in I M CH3OH/0.5M H SO and (2) the methanol electro-oxidation current observed after the adsorption of methanol at 0.4 V. From K. Kunimatsu, Berichte der Bunsen-Ceseiischaft jur Phy-sitcafische Chemie. 1900.94. 1025 1030-...

Although the accuracy of this explanation will be discussed later, it is easily understood that the behavior of the electrode is greatly influenced not only by the instantaneous potential of the electrode (potential dependence ) but also by the history of the electrode (time dependence). As this example shows, the electrochemical oxidation of methanol is a series of reactions in which methanol, water, intermediates and surface adsorbates are interacting with each other in various ways, and are yet to be fully understood. 

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

Thus, the conclusion from the spectroscopic investigations into the steady-state oxidation of methanol is that the surface contains a potential-dependent intermediate, single-bonded CO at less positive potentials, and bridge-bonded CO at the more positive potentials. A possible mechanism fitting the conclusion of the spectroscopy is... 

When the reaction has adsorbed chemical intermediates, the surface concentration of which is potential dependent, the situation is difficult and was first put into a quantitative theory by Conway and Gileadi in 1962 and in more detail by Srinivasan and Gileadi in 1967. However, these pioneer authors dealt with submonolayers of simple entities such as H. How to deal with the potential-dependent intermediates in such a (still fairly simple) reaction such as methanol oxidation is not yet in sight (It can be done in principle, but there is still no knowledge of the kinetics of the reactions of the radical intermediates and how they are connected to the sweep rate.)... 

These are expressed in terms of scalar products between the unit axis system vectors on sites 1 and 2 (on different molecules) and the unit vector 6. from site 1 to 2. The S functions that can have nonzero coefficients in the intermolecular potential depend on the symmetry of the site. This table includes the first few terms that would appear in the expansion of the atom-atom potential for linear molecules. The second set illustrate the types of additional functions that can occur for sites with other than symmetry. These additional terms happen to be those required to describe the anisotropy of the repulsion between the N atom in pyridine (with Zj in the direction of the conventional lone pair on the nitrogen and yj perpendicular to the ring) and the H atom in methanol (with Z2 along the O—H bond and X2 in the COH plane, with C in the direction of positive X2). The important S functions reflect the different symmetries of the two molecules.Note that coefficients of S functions with values of k of opposite sign are always related so that purely real combinations of S functions appear in the intermolecular potential. 

Similar calculations that examine the effect of solution on the chemistry at the anode for both the hydrogen and the direct methanol fuel cells are currently begin carried out. While detailed studies on the effect of the potential dependence and solution effects have been studied, no one has begun to couple the two studies. It is clear that this will be very important for future efforts. 

Ba B., Fotouhi B.B., Gabouze N., Gorochov O. and Cachet H. (1993), Dependence of the flatband potential of n-type GaAs on the redox potential in methanol and acetonitrile , J. Electroanal. Chem. 334, 263-277. 

After increasing the electrode potential, the methanol adsorbates oxidize as per the following reactions, depending on the nature of the catalyst and the experimental conditions [125] ... 

Comparing the results of EC-NMR and IR investigations, we find that the potential dependence of C NMR shift and the vibrational frequency of adsorbed CO are primarily electronic in nature, and originate from changes in the f-LDOS. C NMR results show that CO adsorbed on Pt, either directly from CO gas or from methanol oxidation, have the same electronic properties. That is, the chemisorbed product (surface CO) from CO solutions and from methanol decomposition is the same. The electrode potential dependence of the C NMR spectra of CO adsorbed on Pt and Pd nanoparticles provide direct evidence for electric field induced alterations in the E/ -LDOS. In relation to fuel cell catalysis, EC-NMR investigations of Pt nanoparticles decorated with Ru show that there exist two different kinds of CO populations having markedly different electronic properties. COs... 

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