Abstract
The reported activity of non-stoichiometric lead and bismuth ruthenates, which have the pyrochlore or a related structure, in the electro-oxidation of olefins is shown not to be due to the ability of these materials to exchange lattice oxygen, ie the oxidation is not initiated by transference of a lattice oxygen atom to the substrate. Rather, activity is found to be associated with the presence of a Ru(V) surface state, capable of being oxidized to Ru(VI) at higher potentials. Alkenes are oxidized by Ru(VI), which is continually regenerated if the potential is kept high enough (heterogeneous redox catalysis). The availability of the Ru(V)/Ru(VI) surface couple is not an exclusive property of the pyrochlore materials—the only condition for its existence seems to be that the parent compound should not be too crystalline. The activity of pyrochlore ruthenates in the electrochemical reduction of dioxygen is only moderate; its mechanism is unclear at present.
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