Singh, A.; Hocking, R.K.; Chang, S.L.-Y.; George, B.M.; Fehr, M.; Lips, K.; Schnegg, A.; Spiccia, L.: Water Oxidation Catalysis by Nanoparticulate MnOx Thin Films: Probing the Effect of the Manganese Precursors. Chemistry of Materials 25 (2013), p. 1098-1108
Nanoparticulate MnOx, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyse 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 (10-20nm) were generated in the Nafion film. In contrast, the use of [Mn(OH2)6]2+ resulted in both higher manganese oxide loading and aggregated nanoparticles with 50nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (ca. 10-fold higher per Mn center) were measured for the MnOx derived from molecular precursors than for the material formed from [Mn(OH2)6]2+. Thus, the catalytic activity is dependent on the ability to generate well-defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnOx and to further evaluate the origin of differences in catalytic activity.