The Middle‐Earth between Homogeneous and Heterogeneous Catalysis in Water Oxidation with Iridium

Abstract

Water oxidation (WO) is considered the ideal reaction to provide electrons and protons for the photo‐ and electrosynthesis of renewable fuels, generating exclusively O2 as a benign by‐product. Besides being demanding from the thermodynamic point of view, WO is also extremely difficult from the kinetic one. This has caused an explosion of interest for developing efficient water oxidation catalysts (WOCs). WOCs based on noble metals usually exhibit much better performance. For that reason, parallel to the search for efficient non‐noble metal WOCs, many efforts are directed toward noble‐metal atom economy in WOCs. Three strategies have been proposed to minimizing the atomic contents of noble metals: (1) exploiting molecular WOCs, under the assumption that all metal centers are catalytically active, differently from what happens in heterogeneous WOCs; (2) anchoring a well‐defined molecular WOC onto a suitable support, thus obtaining a heterogenized WOC, combining the positive aspects of homogeneous and heterogeneous catalysis; (3) diluting active metal centers in a suitable material with features that maximize the metal‐accessibility and performance. Herein, the results of our efforts aimed at pursuing all three strategies for developing efficient WOCs based on iridium are reviewed.