ELECTROREDUCTION OF CO, CATALYZED BY POLYMERIC [RU(BPY)(CO)(2)](N) FILMS IN AQUEOUS-MEDIA - PARAMETERS INFLUENCING THE REACTION SELECTIVITY

The ability of carbon electrodes modified by films of [Ru(L)(CO)(2)](n ) (L is a bidentate nitrogen ligand) to act as selective electrocataly sts has been investigated in aqueous solution following the influence of several parameters (electrolyte, initial pH and nature of the ligan d L). These organometallic polymers were prepared by electroreduction of trans(Cl)-[Ru(L)(CO)(2)Cl-2] complexes. It has been found that the reaction selectivity is strongly dependent on the 4,4'-substitutions o f the bipyridine (bpy) ligand. For films with a regular bpy ligand or having donor substituents, CO is the main reduction product (eta(CO) = 94%; E-app = -1.20 V at pH 9). With films based on a bpy ligand disub stituted with electron-withdrawing groups like esters the selectivity is drastically changed, since HCOO- can be obtained with a quantitativ e electrical yield at a very low overvoltage (-0.75 V vs. Ag/AgCl). Th is difference was attributed to the nature of the intermediates (hydro xycarbonyl or formato complexes) formed during the electrocatalytic pr ocess. Utilization of preformed films of polypyrrole functionalized by corresponding monomeric complexes allows the circumvention of problem s due to the poor stability of these molecular cathodes toward air oxi dation. A similar CO/HCOO- selectivity with regards to the substitutio n of the ligand by an electron-withdrawing groups is observed. (C) 199 8 Elsevier Science S.A. All rights reserved.