PtO formation

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

The phenomenon of oscillating reactions is widely recognized in chemistry. In those cases in which a surface-boxmd species or the surface itself is involved in the complex sequence of steps, the EQCM offers the prospect of additional information. An example of this is the role of an oxide layer in the oxidation of formaldehyde at Pt and Rh [174], Similarly, in the oxidation of 2-propanol at Pt electrodes, it was found that the oscillations (in potential and mass, at constant ourent) increased in amphtude until the positive extreme of the potential excursion reached a value consistent with PtOH and/or PtO formation [175]. Oscillations in mass at open-circuit potential were also observed during the dissolution of Cu in sulfate media when the solution concentration of Cu " " was sufficiently high (c > 0.045 mol dm ) [176], although in this case the potential excursions were such that oxide formation/dissolution was ruled out. 

In summary, it can be concluded that platinum dissolution and catalyst particle growth are particularly fast during potential transients because of the delayed PtO formation. Furthermore, accelerated oxidation of the carbon support is observed under cycling conditions. These mechanisms result in cell degradation lowering the durability of the fuel cell significantly. 

Zero-dimensional transient kinetic Darling and Meyers (2003) rate model for Pt dissolution including electrochemical Pt dissolirtion, electrochemical PtO formation, and chemical PtO dissolution (not linked to cell performance)... 

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

The two predominant features in Figure 3.24 are attributable to the 4f orbitals of the Pt electrode. The two peaks were deconvoluted as shown into a main peak and a smaller satellite peak. At potentials > 0.7 V vs. SCE, a peak at 77.1 eV was observed which was attributed to PtO. On the basis of these results, those of Kim et ai (1971), and the coulometric and ellipsometric data discussed above, Augustynski and Balsenc (1979) proposed that the signal attributed to the Pt 4f orbitals shifted via formation of PtO was only observed after the formation of the phase oxide, since it is only after this place exchange that the chemical environment of the Pt atoms is modified... 

In the oxides PdO and PtO, twelve electrons are available for bond formation. This number is sufficient for dsp% hybridization, and from the fact that each Pt atom is surrounded by four oxygen atoms, forming a square group Pt04, it is evident that this hybridization actually occurs. The unit cell of PtO is represented in Figure 40. 

The thermodynamic data for the platinum oxides are not well established. However, a reasonable value for the free energy of formation of the lower oxide, PtO, at 527°C. is —1 kcal./mole (5, 25). This corresponds to an oxygen dissociation pressure at 527°C. of 0.28 atm.—i.e., bulk PtO is unstable toward decomposition to bulk Pt for oxygen pressures below 0.28 atm. Bulk Pt02 is, of course, even less stable. Nevertheless, it has been reported that at this temperature and an oxygen pressure... 

When a typical active material is employed as the anode, a number of additional species generated on the electrode surface must also be considered. They can influence the process performance, causing additional chemical reactions on the electrode surface if the redox couple remains at the surface (i.e., Pt/PtO), or in the bulk solution if the electrogenerated species are dissolved (i.e., A1/A13+). A scheme outlining the processes that need to be considered in the anodic electrochemical zone is shown in Fig. 4.3. The first process to be taken into account is the formation of oxidized species on the electrode surface. These species can either remain on the surface or move toward the bulk zone. In the latter case, mass transfer to the bulk zone and possible chemical reactions in this zone must be considered. 

With several other transition metal monoxides, covalent and special electronic effects that are not compatible with the ideal stmctnre manifest themselves. As examples, PdO and PtO display the characteristic tendencies of heavier d ions for sqnare-planar coordination by adopting different structures while a severe Jahn Teller distortion yields essentially fonrfold planar coordination in CuO. In the cases of TiO, VO, and NbO, formation of weak metal to metal bonds are seen as defect variants of the rock salt stmcture. 

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