Activity analysis of a water oxidation catalyst immobilized in a polymer membrane

Abstract

The activity of a water oxidation catalyst based on a trinuclear Ru complex, Ruthenium Red (Ru-red){[(NH₃)₅Ru—O—Ru(NH₃)₄—O—Ru(NH₃)₅]⁶⁺} has been investigated both in a homogeneous aqueous solution (AS) and in a heterogeneous Nafion membrane (HM) using Ce^IV oxidant. In the AS, at higher concentrations, the oxygen (O₂) evolution rate V_o2(mol s^–1) decreases with increasing concentrations of Ru-red, but nitrogen (N₂) evolves. The N₂ evolution shows a bimolecular decomposition of the catalyst at high concentrations. In the HM, the V_o2 does not decrease even when the complex concentration in the membrane is ca. 30 times as high as the AS. A pseudo-first-order rate constant for oxidation of water by the catalyst (k_o2/s^–1) and a second-order rate constant for deactivation (k_deact/dm³ mol^–1 s^–1) were obtained. The k_o2 values are close for both the AS and HM systems, indicating no significant loss of the activity in the membrane. The k_deact value decreases by an order of two in the membrane, which is ascribed to the suppression of bimolecular decomposition of the catalyst. The apparent activity (k_app/s^–1) in the membrane increases upon decreasing the concentration of Ru-red due to suppression of the bimolecular decomposition. The effect of intermolecular distance distribution of the catalyst in the membrane on the catalytic activity has been analysed based on statistical calculations of the distribution. The critical decomposition distance between two adjacent complexes has been determined as 1.23 nm.