The nature of active site for one of the most important water oxidation catalysts-

Iridium oxide remains on of the most important catalysts for water oxidation reaction. It has always been believed that Rutile IrO2 is the most stable and active catalyst for water oxidation reaction under acidic conditions (important in the context of fuel cells). The chemical nature of this well-known dimensionally stable anode (IrO2 - Rutile) was relatively unknown for a long time. Although IrO2 is an active material, its usually needs a set of electrochemical protocols (typically, potential cycling) to become an active catalyst. We have shown through two of our papers using ex situ and in situ Raman Spectroscopy that the active material is a hydrous oxide of IrOx what can be written as IrOx(OH)y(H2O)z. We have shown that upon potential cycling parts of IrO2 are converted to this hydrous oxide (ACS Catalysis 2016, 6, 8098-8105) which forms the basis of the active material.

Furthermore, using in situ Raman Spectroscopy and theoretical calculations, we have shown that under conditions of water oxidation, IrOx(OH)y(H2O)z nucleates sites that Ir-oxo type, which are we believe to be sites which lead to the formation of O-O bond in the pathway to O2 evolution. (ChemComm 2017, http://dx.doi.org/10.1039/C7CC05669A)

To quote the paper 'The scaffold enabling the OER is derived from a highly hydrous [IrO6]n species (of the type IrOx(OH)y(H2O)z), where each Ir atom is octahedrally coordinated by oxygens. Whereas the colour and vibrational spectroscopy of most of this material can be traced using ex situ spectroscopy, the formation of an active a square pyramidal ‘‘Ir–O’’ species is detectable only through in situ spectroscopy under applied potential. We propose that this Ir–O species is the active site within the molecule which is open to attack from H2O molecules leading to formation of O2 through formation of an O–O bond. The OER chemistry is likely to proceed primarily in the ligand sphere.'

Such sites are well known in other chemistries e.g. CH activation (Fe(IV)=O) where presence of a electrophilic oxygen is essential to activate the C-H bond. We for the first time show a similar intermediate for IrOx electro-catalysts used for water oxidation.