Electrochemical in situ surface enhanced Raman spectroscopic characterization of a trinuclear ruthenium complex, Ru-red

To study the fate of a molecular di--oxo-bridged trinuclear ruthenium complex, [(NH3)(5)Ru-O-Ru(NH3)(4)-O-Ru(NH3)(5)](6+), also known as Ru-red, during the electro-driven water oxidation reaction, electrochemical in situ surface enhanced Raman spectroscopy (SERS) investigations have been conducted on an electrochemically roughened gold surface in acidic condition. It was previously described that on a basal plane pyrolitic graphite electrode in 0.1M H2SO4 aqueous solution, Ru-red undergoes one electron oxidative conversion into a stable higher oxidation state ruthenium complex, Ru-brown, at<1.0V (vs normal hydrogen electrode (NHE)), and this leads to water oxidation and dioxygen release, but the fate of Ru-red during electrochemistry was not studied in much detail. In this investigation, Ru-red dispersed in acid electrolyte and immobilized on a roughened gold electrode without Ru-red in solution has been subjected to anodic controlled potential experiments, and in situ SERS was carried out at various potentials in succession. The electrochemical SERS data obtained for Ru-red are also compared with in situ SERS results of an electrodeposited ruthenium oxide thin film on the Au disk. Our study suggests that on a gold electrode in sulfuric acid solution containing Ru-red, one electron oxidative conversion of Ru-red to a higher oxidation state ruthenium compound, Ru-brown, occurs at ca. 0.74V (vs NHE), as supported by the electrochemical in situ SERS experiments. Moreover, at higher potentials and on Au disk, the Ru-red / Ru-brown are not stable and slowly decompose or electro-oxidize leading to deactivation of the tri-ruthenium catalytic system in acidic medium. Copyright (c) 2013 John Wiley & Sons, Ltd.