Water Oxidation Catalysis by Nanoparticulate Manganese Oxide Thin Films: Probing the Effect of the Manganese Precursors

Abstract

Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnOx present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn(Me3TACN)(OMe)3]+ and [(Me3TACN)2MnIII2(μ-O)(μ-CH3COO)2]2+ (Me3TACN = N,N′,N″-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnOx nanoparticles (10–20 nm and 6–10 nm, respectively) were generated in ...