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Superior electrocatalysts for the reduction of 0 2 by fourelectrons in one step have been prepared by coordinating ammine complexes of Ru(II) to the pyridine ligand sites in cobalt tetrakis(4pyridyl) porphyrin. At least three coordinated Ru(I1) centers appear to be required to obtain the four-electron reduction. WhenfacRu(NH3)3(OH2)32+ was used to ruthenate the porphyrin, a network structure was obtained in which both terminal and bridging ruthenium centers were present. The catalysis of electrode reactions is often accomplished by preparing active sites on electrode surfaces that serve as "electrocatalysts". Most simple electrode processes, i.e., reactions which involve the transfer of a single electron without accompanying atom or ion transfer that leads to the making and/or breaking of chemical bonds, usually do not require or benefit from electrocatalysis. The rates of such simple electrode processes are often diffusion controlled. By contrast, electrode processes that involve the transfer of several electrons, usually accompanied by the breaking of old bonds and the creation of new ones, commonly proceed slowly or not at all in the absence of electrocatalysts. Such complex electrode processes often involve high energy intermediates that must be stabilized by interactions with suitable molecules or complexes if the electrode reactions are to proceed at reasonable rates. These intermediate-stabilizing species are the electrocatalysts. Transition metal complexes with demonstrated activities as homogeneous catalysts can sometimes be utilized as electrocatalysts by attaching them to the surfaces of electrodes. An example of this strategy is provided by the catalysis of the electroreduction of 0 2 at carbon or graphite electrodes in aqueous acid [l]. The reduction proceeds at unmodified graphite electrode surfaces but only at electrode potentials much more negative than the thermodynamically required values. In addition, the most common product of the reduction is H202 instead of the thermodynamically more favored product, H20. If cobalt porphyrins are introduced on the surface of graphite electrodes by, for example, irreversible adsorption, the electroreduction of 0 2 is catalyzed |
