Direct Evidence for a Sequential Electron Transfer–Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst
Matthew C. Kessinger, J.M. Xu, Kai Cui, Quentin R. Loague, Alexander V. Soudackov, Sharon Hammes‐Schiffer, Gerald J. Meyer
Abstract
The proton-coupled electron transfer (PCET) mechanism for the reaction M ox –OH + e – + H + → M red –OH 2 was determined through the kinetic resolution of the independent electron transfer (ET) and proton transfer (PT) steps. The reaction of interest was triggered by visible light excitation of [Ru II (tpy)(bpy′)H 2 O] 2+, Ru II –OH 2, where tpy is 2,2′:6′,2″-terpyridine and bpy′ is 4,4′-diaminopropylsilatrane-2,2′-bipyridine, anchored to In 2 O 3:Sn (ITO) thin films in aqueous solutions. Interfacial kinetics for the PCET reduction reaction were quantified by nanosecond transient absorption spectroscopy as a function of solution pH and applied potential. Data acquired at pH = 5–10 revealed a stepwise electron transfer–proton transfer (ET–PT) mechanism, while kinetic measurements made below p K a (Ru III –OH/OH 2 ) = 1.3 were used to study the analogous interfacial reaction, where electron transfer was the only mechanistic step. Analysis of this data with a recently reported multichannel kinetic model was used to construct a PCET zone diagram and supported the assignment of an ET–PT mechanism at pH = 5–10. Ultimately, this study represents a unique example among M ox –OH/M red –OH 2 reactivity where the protonation and oxidation states of the intermediate were kinetically and spectrally resolved to firmly establish the PCET mechanism.