Hydrogen oxidative-addition

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

The main methods for the synthesis of hydride complexes are sufficiently well discussed, their procedure features are interaction of complex compounds with hydrogen, oxidative addition of hydrogen-containing molecules, formation of hydride complexes resulting from cleavage of the C — H bond, and protonation of the central atom of certain complexes [18a,28], Various physical and chemical meth-... 

On the basis of the above results, Krische and co-workers proposed the mechanism depicted in (Scheme 12) (32). The catalytic cycle begins with oxidative addition of the Rh complex to enyne 25, forming the rhodacyclopentene intermediate B-I. Homoljdic hydrogen activation via either (a) hydrogen oxidative addition or (b) a-bond metathesis to form vinyl rhodium species B-II is expected. 

Several mechanisms have been proposed for the platinum-catalyzed homogeneous hydrosilylation reaction. The most commonly invoked mechanism, proposed by Chalk and Harrod in 1965, consists of elementary steps similar to homogeneous hydrogenation, oxidative addition, migratory insertion, and reductive elimination (226). However, this mechanism fails to describe the indnction period or the presence of colloidal species at the end of the reaction. Lewis proposed an alternative mechanism based on the intermediacy of colloids that were detected by transmission electron microscopy after evaporation of catalsdically active solutions (227,228). 

The most effective CO tolerance is found to be with Pt/Ru alloys. Pt/Ru alloy catalysts, supported on carbon were found to be effective upto 2% CO without a significant drop in performance. However, the other promising metals are Co and Ni. It may be noted that the Ru attracts the CO molecules, thus keeping the Pt free for hydrogen oxidation. Additionally on Ru, the oxidation of CO to CO2 occurs at a lesser potential of about 0.35 w.r.t. (SHE) at 50 atom% Ru and at 0.2V (SHE) for 90 atom% Ru. Thus, at a lesser polarization the electrochemical oxidation of CO starts for Pt/Ru based catalysts. These two effects make Pt/Ru based anode catalyst as the most accepted CO tolerant PAFC catalyst. As Ru percent increases, CO tolerance is enhanced at a cost of activity due to less Pt availability. In view of this, a trade-off is required, and the optimal composition that most of the developers claim is 1 1 atomic ratio of Pt Ru. 

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