Platinum stable intermediates

Although platinum(IV) is a stable oxidation state of platinum, the complex PtCl2 will oxidize hydrazinium ion. Hydrogen ion does not affect the rate. The reaction proceeds via a platinum(III) intermediate and a protonated hydrazyl radical which decomposes to N2 and... 

The last example suggests that the platinum-vinylidene intermediate is more stable than the corresponding ferrocenyl-stabilized carbonium ion. 

In a first set of experiments, the voltammetric behaviour of sodium dithionite was investigated in alkaline solution (pH around 12.5), by variation of the rotation rate of the platinum-disc electrode for different concentrations of sodium dithionite. In Fig. 6.1, current-potential curves are shown, obtained at different rotation rates of the electrode in a solution with constant sodium dithionite concentration. Two anodic waves are observed. In principle, sodium dithionite is the only electroactive species in solution, therefore it is supposed that both well-separated waves can be attributed to the oxidation of sodium dithionite with formation of a relatively stable intermediate product in the first wave. 

Harting et al. reported the results of DFT and AIMD studies for the oxidation of methanol on the Pt(l 1 1) face in aqueous solution.122 Their work reveals that the oxidation of methanol is initiated at the moment when a hydrogen bond forms between the OH group of the methanol and a water molecule. The initial step of the reaction is the cleavage of a CH bond with the bond direction points towards the platinum surface. This is followed by a rapid dissociation of the methanol OH bond, which leads to formation of a formaldehyde as a stable intermediate within the timescale of the simulation. Charge delocalization is achieved by the formation of a Zundel ion H502+ in the aqueous phase. 

Several surface oxygen species have been postulated based on kinetic parameters, stable intermediates, and in situ observations during oxygen reduction or evolution. As in the case of hydrogen, platinum is the most well-studied electrocatalyst. Optical measurements (122-126) show that a freshly developed oxygen layer on platinum behaves reversibly up to 0.95 V. However, rapid aging yields an irreversibly bound layer. Chemisorption of OH is assumed to occur in this potential region (123,124,127,128) formed by z. + + H- + e (27)... 

Apart from platinum s intermediate nature on bonding, another point in platinum s favor is availability platinum can be purchased in various suitable forms at a reasonable price some noble metals are difficult to find and purchase. The word noble means here stable and of course that is a first point one wants in an electrocatalyst. It must be a catalyst, not enter into the reaction. It is meant to accelerate the reaction. It must itself be stable, thermally and electrochemically. On the last point, platinum is only fairly good because oxide-free platinum does start itself to dissolve around 1.0 V on the normal hydrogen scale. By using it in anodic reactions in a potential range anodic to 1.0 V, Pt(II) is likely to get into the solution and may be deposited on the cathode. 

On bare platinum the reduction proceeds via catalytic hydrogenation to o-nitroanihne (Scheme 2), which is reduced further to o-phenylenediamine. On the other hand, on UPD-modified platinum surfaces, the reduction occurs at much more positive potentials and follows the electronation mechanism, that is, the direct exchange of electrons between the dinitroso tautomeric form and the modified electrode surface. The first step is now the reduction to o-benzoquinone dioxime that appears as a stable intermediate over a wide potential range (0.60 to 0.45 V) before it is reduced further to the final products. 

The adsorption rate, coverage, and stability of H2S species are strongly affected by temperature [18]. It is known that the adsorbed H2S and SH are highly unstable on Pt, while the adsorbed S and H are the most stable intermediates on Pt [19]. The formation of the sulfide film on the platinum surface not only causes poisoning of the catalyst surface but also makes it impossible for the fuel cell to recover from contamination [17, 20, 21]. 

In contrast to mercury, the I—U curves of the O2 reduction on platinum metals consist only of one wave. The limiting diffusion current corresponds nearly to a four-electron process. Separate studies of the reduction of O2 and H2O2 demonstrate in agreement with this result that the two species are reduced in the same potential range. Reaction 9 may occur as a parallel reaction to the sequence of reactions 12 and 14 in acid electrolytes. A similar statement holds for reaction 4 and the sequence of reactions 13 and 15 in alkaline media. Dissolved hydrogen peroxide is not formed as a stable intermediate in reactions 4 and 9. However, the intermediate production of H202ad or adsorbed peroxide radicals cannot be ruled out. For this reason the following scheme was proposed [12] previously for the O2 reduction on active smooth platinum ... 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

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