High Potential-Applied Catalyst Behavior of a Mononuclear Ruthenium(II) Complex on a Mesoporous ITO Electrode for Water Oxidation

Abstract

The catalytic aspect of a mononuclear Ru complex, [Ru(C8Otpy)(dcbpy)(OH)]− (1) (C8Otpy = 4′-octyloxy-2,2′:6′,2″-terpyridine, H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) on a mesoporous indium tin oxide (m-ITO) electrode was investigated by multi-potential-step chronocoulo(ampero)spectrometry (MPSCC(A)S) combined with in situ O2 evolution analysis. For the repetitive MPSCC(A)S measurement between 0.2 and 1.7 V vs Ag/AgCl, 39% of 1 recovered on the first back potential-step to 0.2 V, but 1 no longer recovered after the 4th back potential-step, although the in situ measured amount of O2 increased monotonously with time. XPS and Raman spectroscopic measurements reveal that 1 on the electrode pronouncedly undergoes the oxidative transformation to form the hydrated RuO2 as the alternative catalysts under the high applied potential of 1.7 V. The RuV=O state is more active for water oxidation than the RuIV=O state but could be prone to transform to hydrated RuO2. This tells us that the low applied potential but enough for formation of the RuIV=O species is important for 1 to work stably as a molecular catalyst on the electrode.