Electrochemical Water Oxidation Catalysis Beginning with Co(II) Polyoxometalates: The Case of the Precatalyst Co4V2W18O6810–

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• 作者：Scott J. Folkman ; Richard G. Finke
• 刊名：ACS Catalysis
• 出版年：2017
• 出版时间：January 6, 2017
• 年：2017
• 卷：7
• 期：1
• 页码：7-16
• 全文大小：482K
• ISSN：2155-5435

文摘

The question is addressed of whether the cobalt polyoxometalate (Co-POM) precatalyst Co₄V₂W₁₈O₆₈^10– (hereafter Co₄V₂W₁₈) is a stable, homogeneous water oxidation catalyst under electrochemically driven conditions and in 0.1 M pH 5.8 and 8.0 NaPi buffer as well as pH 9.0 sodium borate (NaB) buffer. This question is of considerable interest since Co₄V₂W₁₈ has been reported to be highly stable and a 200-fold faster water oxidation catalyst than its P congener Co₄P₂W₁₈O₆₈^10– (hereafter Co₄V₂W₁₈), for reasons that were not specified. The nature of the true water oxidation catalyst with Co₄V₂W₁₈ as the starting material is of further fundamental interest because a recent report reveals that the ⁵¹V NMR peak at ca. −507 ppm assigned by others to Co₄V₂W₁₈ and used to argue for its solution stability is, instead, correctly assigned to the highly stable cis-V₂W₄O₁₉^4–, in turn raising the question of the true stability of Co₄V₂W₁₈ under water oxidation catalysis conditions. A battery of physical methods is used to address the questions of the stability and true water oxidation catalyst with Co₄V₂W₁₈ as the precatalyst: ³¹P line-broadening detection of Co(II) present in solution from leaching or as a counterion impurity; a check of those Co(II) concentration results by the second method of cathodic stripping; the O₂ yield (and, hence, Faradaic efficiency) of electrocatalytic water oxidation; electrochemical, SEM, EDX, and XPS characterization of CoO_x films produced on the electrode; and multiple controls and other experiments designed to test alternative hypotheses that might explain the observed results. The collective evidence provides a compelling case that Co(II) derived from Co₄V₂W₁₈ forms a CoO_x film on the electrode which, in turn, carries all the observed, electrochemically driven water-oxidation catalysis current within experimental error. A list of seven main findings is provided as a summary.