An electrode-supported oxidation catalyst based on ruthenium(IV). pH "encapsulation" in a polymer film

Abstract

The known oxidative catalytic behavior of the ions (bpy)/sub 2/(py)RuO/sup 2 +/ (bpy is 2,2'-bipyridine; py is pyridine) and (trpy) (bpy)RuO/sup 2 +/ (trpy is 2,2',2''-terpyridine) has been successfully transferred to electrode surfaces. The electrodes were chemically modified by deposition of thin polymeric films containing the (bpy)/sub 2/(H/sub 2/0)Ru/sup II/ group bound to poly-4-vinylpyridine. Upon oxidation to Ru/sup IV/ on the surface catalytic oxidation currents are observed for the modified electrodes in the presence of 2-propanol, p-toluic acid, and the xylenes with an added surfactant. The effective catalytic lifetimes of the films are limited ( > 30 turnovers per redox site) by a competing reaction within the polymer film which changes the nature of the redox sites. An unusual pH effect has been observed in the polymer films. Below pH 4, potentials measured by cyclic voltammetry for the bound ((bpy)/sub 2/(OH)Ru/sup II/(py)-)/sup 2 +//((bpy)/sub 2/(OH/sub 2/)Ru/sup III/(py)-)/sup 2 +/ and ((bpy)/sub 2/(O)Ru/sup IV/(py)-)/sup 2 +//((bpy)/sub 2/(OH)Ru/sup III/(py)-)/sup 2 +/ couples change with pH as expected. Above pH 4, where the unbound pyridyl groups in the polymer become deprotonated, up to pH 9.2 the redox couples no longer respond to pH changes in the bulk solution. The difference in pH behaviormore » between the polymer in solution and on the electrode surface has allowed us to observe the deposition process by cyclic voltammetry. In acid solution, the polymer film is open and large amounts of ferri- or ferrocyanide ions can be incorporated reversibly into the films by ion exchange. Over a longer time period, the cyano ions are permanently bound in the films, apparently by the formation of a cyano-bridged Fe-Ru dimer.« less