Cooperative catalysis and critical decomposition distances in water oxidation by tris(ethylenediamine)ruthenium (III) complex confined in a Nafion membrane

Abstract

The activity of tris(ethylenediamine)ruthenium (III) complex, [Ru(en) 3] 3+, as a water oxidation catalyst was studied in a homogeneous aqueous solution and a heterogeneous Nafion (Nf) membrane. In the aqueous solution, the apparent catalytic activity ( k app (s −1)) decreased monotonously with the concentration due to a bimolecular decomposition of the complex. The bimolecular decomposition of the complex was remarkably suppressed by incorporating it into a Nf membrane. An optimum complex concentration for k app in the Nf membrane was exhibited, which was explained both by a cooperative catalysis and a bimolecular decomposition of the complex. The k app in the Nf membrane was analyzed in terms of an intrinsic catalytic activity ( k O 2 (s −1)) of the complex, a cooperative catalysis distance ( r co (nm)) and a critical decomposition distance ( r d (nm)) between them based on intermolecular distance distribution to obtain the k O 2 =8.5×10 −5 s −1, r co=1.44 nm and r d=1.07 nm. The results in the [Ru(en) 3] 3+ system were compared with those obtained in the [Ru(NH 3) 6] 3+ system.