Electrocatalysis bifunctional effect

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. 

Much effort has been made to enhance the performance of Pd—M (where M = second metal) for alcohol electrooxidation. These efforts were persuaded by the bifunctional mechanism and the electronic/Ugand effect, which can be explained by either the electron density, electronegativity, density functional theory, or d-band theory [6-17]. For example, according to the bifunctional theory of electrocatalysis, the oxidation of a primary alcohol to CO2 and R-COOH (or and R-COO in... 

Generally, the ad-atoms cause positive catalytic effects with significant enhancement of the electrocatalytic activity of platinum in several cases. Several reviews have been published on this subject [26-28, 67]. Very recently Ross [68] and Jarvi and Stuve [29] have discussed the more recent advances in our understanding of the fundamentals of Ci electrocatalysis by ad-atoms. The different types of enhancement (third-body effect, bifunctional mechanism, poison destabilization, and electronic modification) are well documented. The new information obtained from the in situ spectroscopic studies about the nature of poisons and the dependence of their coverages on potential, as well as the use of single-crystal electrodes with defined surface structure and specific reactivity, enables a deeper insight in the electrocatalysis by ad-atoms. As a general rule, one can say that, except for methanol, the more susceptible... 

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