Platinum synergistic catalytic effect

In 1965, synergistic (nonadditive) catalytic effects were discovered in elecho-chemical reactions. It was shown in particular that the electrochemical oxidation of methanol on a combined platinum-ruthenium catalyst will occur with rates two to three orders of magnimde higher than at pure platinum even though pure ruthenium is catalytically altogether inactive. 

Similar effects were demonstrated by them for palladium and ruthenium in the reduction of pyridine, and for ruthenium and platinum in the reduction of nitriles and of nitrobenzenes. Since they show similar although not as large synergistic effects when the two metals are introduced on separate carbon particles, one may be sure that the enhancement is not an intrinsic property of some chemical combination or contact between the different metals. As the authors point out, the various metals are known to have differing catalytic effectiveness for the reduction of different functional groups. Thus, when reduction takes place via more than one chemical step, such as in the case... 

Often multicomponent catalyst systems are utilized to carry out reactions consisting of two or more active metal components or both oxide and metal constituents. For example, a Pt-Rh catalyst facilitates the removal of pollutants from car exhausts. Platinum is very effective for oxidizing unburned hydrocarbons and CO to H2O and CO2, and rhodium is very efficient in reducing NO to N2, even in the same oxidizing environment. Dual functional or multifunctional catalysts are frequently used to carry out complex chemical reactions. In this circumstance the various catalyst components should not be thought of as additives, since they are independently responsible for different catalytic activity. Often there are synergistic effects, however, whereby the various components beneficially influence each other s catalytic activity to provide a combined additive and multifunctional catalytic effects. 

One of the drawbacks of DMFCs is the relatively slow rate of the anodic oxidation of methanol even on highly active platinum electrodes. It was shown that the Pt-Ru system is much more catalytically active than pure platinum (pure ruthenium is inert towards this reaction) (-> platinum-ruthenium -> electrocatalysis). The so-called bifunctional mechanism is broadly accepted to describe this synergistic effect, according to which organic species are chemisorbed predominantly on platinum centers while ruthenium centers more readily adsorb species OH, required for the oxidation of the organic intermediates. Usually the composition of such alloys is Pto.sRuo.s and the metal loading is 2-4 mg cm-2. 

An example of the synergistic effect between palladium and platinum can be found in the catalytic selective reduction of nitrate ions to hydroxylamine... 

Electrochemical studies have been performed in acidic medium by Kadirgan et al. [61], who have evaluated the activity of Pt-Pd/ C nanoparticles in the electro-oxidation of methanol and ethanol. Chronoamperometry investigation conducted at 0.45 V vs. ERH showed that Pt and Pd have a synergistic effect, which increases the catalytic activity of the catalyst two or threefold as compared to that of pure Pt (Pt/C Etek). This effect was attributed to a change in the electron density of platinum, responsible for the weakening of the Pt—CO bond and the consequent decrease in its poisoning rate. 

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