Determination of Proton-Coupled Electron Transfer Reorganization Energies with Application to Water Oxidation Catalysts

Abstract

The reorganization energy, λ, for interfacial electron transfer (ET) and for proton-coupled electron transfer (PCET) between a water oxidation catalyst and a conductive In₂O₃:Sn (ITO) oxide were extracted from kinetic data by application of Marcus-Gerischer theory. Specifically, light excitation of the water oxidation catalyst [Ru^II(tpy)(4,4'-(PO₃H₂)₂-bpy)OH₂]²⁺ (Ru^II-OH₂), where tpy is 2,2':6',2″-terpyridine and bpy is 2,2'-bipyridine, anchored to a mesoporous thin film of ITO nanocrystallites resulted in rapid excited-state injection ( k_inj > 10⁸ s^-1). The subsequent reaction of the injected electron (ITO(e^-)) and the oxidized catalyst was quantified spectroscopically on nanosecond and longer time scales. The metallic character of ITO allowed potentiostatic control of the reaction free energy change -Δ G^o over a 1 eV range. At pH values below the p K_a = 1.7 of the oxidized catalyst, ET was the primary reaction. Within the pH range 2 ≤ pH ≤ 5, an interfacial PCET reaction (ITO(e^-) + Ru^III-OH + H⁺→ Ru^II-OH₂) occurred with smaller rate constants. Plots of the rate constants versus -Δ G^o provided a reorganization energy of λ_PCET = 0.9 eV and λ_ET = 0.5 eV. A second water oxidation catalyst provided similar values and demonstrated generality. The utilization of conductive oxides is shown to be a powerful tool for quantifying PCET reorganization energies at oxide surfaces for the first time.