Oxidation of platinum

Halogen oxidation of platinum(II) complexes generally occurs trans, as does reaction with H202 to afford dihydroxy complexes (Figure 3.106). 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

During the anodic polarization of platinum to potentials of about 3.0 V (RHE), one or several layers (but no more than three) of chemisorbed oxygen are formed, which sometimes are called the a-oxide of platinum. The limiting thickness of these layers is about 1.3 nm. They can be studied both by electrochemical methods and by ellipsometry. At more positive potentials phase-oxide surface layers, the p-oxides are formed. The quantitative composition and structure of these layers and the exact limits of potential for their formation depend on many factors composition of the electrolyte solution, time of polarization, surface history, and often remain unknown. 

It is interesting to note that according to photoelectrochemical measurements, the a-oxide of platinum exhibits a photoeflect that is typical for n-type semiconductors, while for the P-type oxide the sign of the photoeflect is opposite, and typical for a p-type semiconductor. This is an indication for important dilferences in the properties of these two types of oxide layer. 

Parmigiani F, Kay E, Bagus PS. 1990. Anomalous oxidation of platinum clusters studied by X-ray photoelectron-spectroscopy. J Electron Spectrosc Relat Phenom 50 39-46. 

Hayden BE, Fletcher D, Suchsland J-F, Williams LJ. 2009. The influence of Ft particle size on the surface oxidation of platinum—Fart 1 Reduced titania support. Fhys Chem Chem Fhys. DOI 10.1039/b817553e. 

The first reported porphyrin complexes of platinum(IV) date from 1980 and were obtained by hydrogen peroxide oxidation of platinum(II) porphyrin complexes in an acidic medium (HC1).479 Since then oxidation of platinum(II) complexes of other porphyrins has been achieved by the same method,480 and by chlorine,481 or bromine482 oxidation. Reaction with iodine did not lead to oxidation and treatment of platinum(IV) porphyrin complexes with iodide resulted in reduction to platinum(II). 

R. J. Puddephatt No. Nobody has prepared such complexes and the synthesis is not trivial. Substitution of halide ligands in octahedral platinum(IV) derivatives is typically very slow, and a better route (suggested by J. K. Kochi) might involve oxidation of platinum(II) metallacyclobutanes with peroxides. It would certainly be worthwhile to attempt this synthesis in view of the promise of enhanced reactivity. 

A mechanism for CO oxidation, proposed by Gilman and still referred to, involves the reaction of linearly adsorbed CO with an adjacent water molecule as the oxygen donor. He proposed that the oxidation of COad must be completed before the oxidation of idatinum can take place. McCallum etal. studied the oxidation of COad by stepping the potential into the region where oxidation of platinum occurs simultaneously with COad oxidation. They concluded that the oxidation of COad occvirs at the perimeter of the growing islands of Pt-OH, which acts as the oxygen donor. 

Figure 2—17 shows the voltammogram for PtClOO). At the hydrogen region, there are two peaks, one at 250 mV, the other at 350 mV. The oxidation of platinum starts at 850 mV. The peak at 350 mV disappeared after the electrode experienced the formation and reduction of Pt-OH. 

The cyclic voitammogram for Pt (111) in 5 M sulfuric acid is shown in Fig. 2-21. Compared with that in 0.5 M sulfuric acid (Fig. 2-15), the anodic part of the two split hydrogen adsorption-desorption areas was compressed in the cathodic direction and became two sharp peaks while the cathodic part did not change its shape very much. The asymmetric peak at 700 mV shifted cathodicly and became more symmetric and sharp. The oxidation of platinum shifted about 100 mV in the anodic direction. All these changes could be attributed to the increase in specific adsorption of anions or the decrease of the activity of water as well as the pH change. 

Fig. 2-21 Cyclic voltammogram of Pt(lll) in 5 M H2SO4 at 50mV/s. (---) Jirst sweep (---) after experiencing oxidation of platinum... 

Sometimes, during oxidative voltammetry, we obtain a current peak caused by the oxidation of platinum to form Pt02- In order to avoid the formation of such a peak, we can deliberately form a substantial... 

According to reaction (559) these reactions can occur by an oxidation reaction of platinum(II). With one-electron oxidants, the intermediate formation of platinum(III) complexes occurs. The best early example of this type of reaction is in the oxidation of PtCi - and Pt(en)2+ by IrIVCl - in the presence of free chloride ion. The presence of platinum(III) intermediates has been inferred from the rate law.2036 For the oxidation of platinum(II) by gold(III) the kinetic data are consistent with a mechanism requiring a complementary two-electron transfer,2037 with a rate independent of chloride ion. For the substituted pyridine derivatives PtCkLj (L = substituted pyridine) however, the third-order rate law is found with first-order dependencies on PtCl I, Aum and Cl-.2038 Comparisons have been made with the amine complexes PtC L (L = NH2R).2039... 

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