Understanding the Performance of a Bisphosphonate Ru catalyst for Water Oxidation Catalysis

Abstract

Water oxidation catalysts (WOCs) are a key part of generating H2 from water and sunlight, consequently, it is a promising process for the production of clean energy. The mechanism of water oxidation mediated by Ru(2,2'-bipyridine-6,6'-diphosphonato)(4-picoline)2 has been studied computationally to comprehend the results obtained in the experiments performed by the Concepcion and Grotjahn groups. Our study was performed at pH = 8 and 1. At pH = 8, the phosphonates are fully deprotonated and the catalysis, which is shown to be more energetically costly than that of the dicarboxylato Ru catalyst counterpart, takes place through a mechanism that involves a bimolecular interaction between two metal-oxo units (I2M). At pH = 1, only one of the phosphonates of the catalyst is deprotonated. After testing all possible pathways and comparing with experimental data, we determine that the catalysis proceeds neither through a water nucleophilic attack nor via I2M, instead, we hypothesize that it takes place thanks to an I2M interaction between the catalyst and the ceric ammonium nitrate