Platinum-induced reactions

There is clear evidence that adsorbate and alloyed metal atoms on platinum surface promote CO electro-oxidation. The reduced overpotential is primarily a result of the promotion of the activation of water. The subsequent kinetics are determined by the details of a Langmuir-Hinshelwood reaction between the adsorbed oxidant (OH) and adsorbed CO. Evidence is also presented that relates this promotion (or poisoning) of CO electro-oxidation to tolerate CO in hydrogen feeds in the hydrogen electro-oxidation reaction. An alternative mechanism that may operate at low potentials [79,113] may be that the reduction in CO adsorption energy on platinum induced by Ru [86,113,114] results in a higher equilibrium concentration of nonpoisoned sites. The relative importance of these mechanisms is a function... 

The reaction between the PtCh ion and alkanes can be induced by irradiation [26]. When a solution of hexachloroplatinic acid and n-hexane in acetic acid is irradiated by light (A > 300 nm) [26a,c,d] or y-quanta [26b], a 7t-coinplex of hex-1-ene with platinum(II) is formed in addition to isomeric chlorohexanes. This complex has been isolated in the form of the pyridine adduct, (hex-l-cne)PtCl2Py. The yield of the it-complex in the y-induced reaction reaches 17% based on Pt. The photochemical reaction is of first order with respect to hexane. It is interesting that the photochemical reaction with hex-2-ene affords the n-complex ofhex-l-ene (Scheme VII.5). This complex can be also prepared by the reaction ofhex-l-ene with PtCL, " under irradiation. It should be noted that the photoinduced reaction of PtCl6 or PtCU with olefins is a convenient method for the synthesis of various n-o efin complexes of platinum(II) [27]. The photostimulated reaction of PtCU with stilbene does not occur, possibly due to steric restrictions. 

When manganese(III) ions are used to induce reaction between hypo-bromous acid and sugars, oscillations appear in the potential of a platinum electrode in the solution. Nine common mono- and di-sacccharides were studied by such polarography and sustained oscillations of the Belousov-Zhabotinsky type were noted when a nitrogen 38 13... 

Oxidation of the platinum(II) center by sUver(I) salts has occasionally been observed when attempting to prepare adducts containing platinum-silver bonds (for example, see Sect. 4.2.1). One can speculate that these oxidations could proceed via an inner-sphere process involving the transient formation of a Pt -Ag mixed species. Two examples have recently appeared in which the bimetallic species can be observed prior to oxidatively induced reactions of dimethylplati-num(II) and dime thy Ipalladium(II) species. 

In 1818 the French scientist Louis Thenard discovered hydrogen peroxide, and studied its decomposition over several materials. Thenard was also among the first to discover that metals such as iron, copper, and platinum induce the decomposition of ammonia (1913). Thenard s work on H2O2 decomposition is of particular interest because it represents one of the earliest measurements of the rate of a reaction. 

Quantitative estimation of cyclohexane in the presence of benzene and aUphatic hydrocarbons may be accompHshed by a nitration-dehydrogenation method described in Reference 61. The mixture is nitrated with mixed acid and under conditions that induce formation of the soluble mononitroaromatic derivative. The original mixture of hydrocarbons then is dehydrogenated over a platinum catalyst and is nitrated again. The mononitro compounds of the original benzene and the benzene formed by dehydrogenation of the cyclohexane dissolve in the mixed acid. The aUphatic compound remains unattacked and undissolved. This reaction may be carried out on a micro scale. 

There are also examples of induced complex formation, an essential step of which is always an oxidation-reduction reaction. Rich and Taube found that the rate of exchange between PtCl and Cl was considerably increased by addition of cerium(rV). In the presence of this oxidizing agent a labile complex of Pt(III) is formed, the chloride of which is easily exchangeable. Exchange of platinum between PtCl and PtClg is similarly rapid via the intermediate labile PtCIs complex formed by cerium(IV). 

Platinum-loaded Ti02 systems can be considered as a short-circuited photo-electrochemical cell where the Ti02 semiconductor electrode and metal Pt counterelectrode are brought into contact [159]. Light irradiation can induce electron-hole (e -h +) pair formation and surface oxidation and also reduction reactions on each Pt/Ti02 particle (Figure 4.11). These powder-based systems lack the advantage of... 

The final cyclization manifold has been realized with a different ruthenium catalyst (Scheme 22). The cationic [Cp Ru(MeCN)3]PF6 induces exclusive endo-dig cyclization of both homopropargylic and bis-homopropargylic alcohols.29 73 The clean reaction to form a seven-membered ring is noteworthy for several reasons intramolecular exo-dig cyclization with bis-homopropargylic alcohols is not well established, the platinum-catalyzed case has been reported to be problematic,80 and the selectivity for seven-membered ring formation over the exo-dig cyclization to form a six-membered ring is likely not thermodynamic. The endo-dig cyclization manifold was thus significant evidence that a re-examination of alkyne hydrosilylation mechanisms is necessary (see Section 10.17.2). 

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