| Abstract
| - Activity of a pentaamminechlororuthenium(III)([Ru(NH3)5Cl]2+)complex as a water oxidation catalyst wasinvestigated in a homogeneous aqueous solution (AS) as well as in aheterogeneous Nafion membrane (HM).In the AS, the apparent catalytic activity(kapp/s-1) decreasedmonotonically with the concentration. Incompetition with water oxidation catalysis, dinitrogen (N2)evolution by oxidation of its ammine ligands wasalso observed, whose rate is second order with respect to itsconcentration, showing that a bimoleculardecomposition of the complexes takes place at high concentrations.The bimolecular decomposition wassuppressed remarkably by incorporating the complex into a HM. Inthe HM, kapp increased with thecatalystconcentration at low concentrations below 1.5 ×10-2 M, indicating that four-electron wateroxidation iscooperatively catalyzed by two complex molecules. Thekapp was strikingly higher for HM than for ASinthe whole concentration range, which was ascribed to both a favorableinteraction for the cooperative catalysisand the suppression of the bimolecular decomposition by immobilizationof the complex. The effect ofintermolecular distance distribution of the complex in the membrane onkapp was analyzed in terms ofcooperative catalysis distance (rco/nm) andcritical decomposition distance (rd/nm) to giverco = 1.21 nm andrd = 0.84 nm. Therco value obtained might suggest cooperativecatalysis through hydrogen bonds betweenwater and ammine ligands.
|
