Abstract
The trinuclear ruthenium complexes, Ru-red ([(NH 3) 5RuORu(NH 3) 4ORu(NH 3) 5] 6+, represented as Ru IIIRu IVRu III) and Ru-brown ([ (NH 3) 5RuORu(NH 3) 4ORu(NH 3) 5] 7+, represented as Ru IVRu IIIRu IV) were studied as water oxidation catalysts in a homogeneous aqueous solution as well as in a Nafion membrane incorporated system under different pH conditions using in situ spectrocyclic voltammetry. The in situ absorption spectra recorded for the trimer ruthenium complexes in homogeneous solution showed that the Ru-red undergoes oxidation initially to form the Ru-brown complex at pH 7.4. Independent of pH conditions, the Ru-brown formed in homogeneous solution underwent oxidation to produce a higher oxidation state complex. This higher oxidation state complex was reduced by water molecules to liberate dioxygen with the formation of Ru-brown; however, the Ru-brown underwent decomposition in a homogeneous solution. The Ru-red complex adsorbed into a Nafion membrane underwent oxidation to form Ru-brown initially and further oxidation to give a higher oxidation state complex. This higher oxidation state complex was readily reduced by water molecules and turned back to Ru-brown without decomposition. The complexes were remarkably stabilized as water oxidation catalyst when incorporated in a Nafion membrane.
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