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

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• 作者：Archana Singh ; Rosalie K. Hocking ; Shery L.-Y. Chang ; Benjamin M. George ; Matthias Fehr ; Klaus Lips ; Alexander Schnegg ; Leone Spiccia
• 刊名：Chemistry of Materials
• 出版年：2013
• 出版时间：April 9, 2013
• 年：2013
• 卷：25
• 期：7
• 页码：1098-1108
• 全文大小：520K
• 年卷期：v.25,no.7(April 9, 2013)
• ISSN：1520-5002

文摘

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 MnO_x 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)₂Mn^III₂(渭-O)(渭-CH₃COO)₂]²⁺ (Me₃TACN = N,N鈥?N鈥?trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnO_x nanoparticles (10鈥?0 nm and 6鈥?0 nm, respectively) were generated in the Nafion film. In contrast, the use of [Mn(OH₂)₆]²⁺ resulted in both a higher manganese oxide loading and aggregated nanoparticles with 30鈥?00 nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (TOFs) were observed for films derived from the two complexes, viz., 20 molecules of O₂ per Mn per hour in dark and 40 molecules of O₂ per Mn per hour under illumination at an overpotential of 350 mV, when compared with MnO_x films made with [Mn(OH₂)₆]²⁺. This corresponds to a TOF > 100 molecules of O₂ per Mn per second for a 10 nm MnO_x nanoparticle. 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 MnO_x and to further evaluate the origin of differences in catalytic activity.

Keywords:

manganese complexes; nanoparticulate manganese oxides; electrodeposition; water oxidation catalysis; transmission electron microscopy; X-ray absorption spectroscopy; EPR spectroscopy