Hydrogen-platinum bond, polarization

At the platinum electrode the individual steps of the four-electron reaction cannot be studied separately. Slope b has its usual value of about 0.12 V, but in contrast to what is seen at the mercury electrode, the polarization is practically independent of solution pH (i.e., the potential at a given current density shifts by 0.06 V in the negative direction when the pH is raised by a unit). It follows that the reaction rate depends on hydrogen ion concentration. The step in which an electron and a proton are transferred while the 0-0 bond is broken is probably the ratedetermining step. 

The works of Maurel and Tellier 82), Rhzicka and Cerveny 83, 84), Litvin, Freidlin, and Tilyaev (55) and Brown and Ahuja 86), who have used extensive series of alkenes, confirmed the Lebedev s rule. With 1-alkenes C -Cn) on palladium, platinum, and rhodium catalysts, the initial reaction rate decreased with the length of the chain, and with Pd and Pt a hnear dependence on the number of carbon atoms was obtained 83) (series 53). An example of the influence of the number of substituents on the carbon atoms of the double bond is shown in Fig. 4. It is evident that the mere presence of the substituent is more important than its nature. However, this secondary factor has been accounted for by using the sums of the Taft polar (7 or steric constants for all substituents on C=C. Cerveny and Rfizicka 84) have found excellent linear relationships between the initial hydrogenation rate of 15 alkenes on 3 different Pt catalysts and 2 (series 54), and... 

Catalytic hydrogenation of fluoro- and difluOTO-m- and tran -butenedioic acids and their esters frequently affords products only of hydrogenolysis, especially when platinum oxide (Adams catalyst) is used. Hydrogenolysis is favored in polar solvents, and is found to be more effective when rhodium or nickel rather than palladium are used as the catalysts (Table 2). Hydrogenolysis takes place prior to the saturation of the double bonds fluoro- and 2,3-difluoro-succinic acids do not suffer any loss of fluorine under the same reaction conditions. ... 

In many papers in which the reaction rates or adsorption coefficients of a series of substrates were obtained the effect of structure is discussed only qualitatively. Rilzicka and Cerveny (5) found that the rate of hydrogenation of 1-olefins C6-C17 in ethanol under usual conditions on platinum, palladium and rhodium catalysts decreased monotonically with the chain length. Since in this olefinic series the chain length did not affect (68) the relative adsorptivity of olefins, as demonstrated in Table II, the observed phenomenon may be explained only by the effect exerted by the chain length on the rate constants. Since, moreover, the polar effect of substituents in this series is virtually the same, an explanation can be sought in the steric hindrance of the adsorbed double bond by the free end of the olefin molecule. The trans isomers were adsorbed much more weakly than the respective cis... 

The aim of this work was to develop and present a new method for the synthes selective platinum catalysts for the hydrogenation of unsaturated aldehydes to unsatui alcohols. The method of preparation presented above makes it possible to obtain cata possessing polar platinum surface, due to the presence of active centers near the intei metal - partially reduced transition metal oxide. This transition metal oxide adsorb carbonyl oxygen atom whereas adjacent platinum atom interacts with carbonyl ca atom. In such a situation, the C=C bond is quite far from the surface so its adsorption i favored. There are also some additional parameters which should be taken consideration. The first is the size of platinum crystallites located on the catalyst sur Large platinum crystallites cause planar adsorption of aldehyde molecule in such adsorption of both double bonds is highly probable. It is known that a more sur... 

The effect of chemisorbed sulfur on the HER has been studied using a sulfur radiotracer method in combination with polarization measurements [23]. In the case of single-crystal (110) platinum in acid, the chemisorbed sulfur causes a large decrease in the HER rate, apparently due to a blocking effect of sulfur on the sites of adsorption of the weakly bonded hydrogen. The HER, however, is not completely poisoned by sulfur, even at the sulfur-saturation coverage [6 =0.8). 

Scheme 6-2. Schematic representation of selective catalytic hydrogenation of isomeric undecenoic adds in polymerized micelle encapsulated colloidal platinum catalysts. The position of the double bond in the substrate determines the necessary orientation for 3ccess to the hydrogenation site, and this is mediated by the ordered polar micelle walls. (Adapted from ref. [261].)...

Silica-supported platinum catalysts with a thin film of [C4dmim][OTf] (l-butyl-2,3-dimethylimidazolium trifluoromethanesulfonate) were investigated by the Lercher group, with a focus on the interactions between ionic liquid, support and platinum particles [83]. The interaction of silica and the ionic liquid occurs via hydrogen bonds. Platinum clusters modify the electron density of the ionic liquid, which changes the polarity of the ionic liquid within certain levels. The ionic liquid protects the platinum cluster from oxidation, as the platinum particles in ionic liquid-coated systems are in the zero oxidation state. SCILL-catalysed ethene hydrogenation showed that the interaction of the ionic liquid with the platinum surface is weaker than the interaction of ethene and the platinum surface. 

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