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The Development and Investigation of Cobalt Catalysts for the Chemical Transformations Related to Artificial Photosynthesis

  • Author(s): Ahn, Hyun Seo
  • Advisor(s): Tilley, T. Don
  • et al.
Abstract

Chapter 1. Small domains of cobalt on silica (CoSBA) were prepared by the reaction of Co[N(SiMe3)2]2 and SBA-15, resulting in a range of surface structures as the cobalt loading varied from 0.27 to 5.11 wt%. X-ray absorption spectroscopy (XAS) was employed to characterize these surface structures, which range from single-site cobalt atoms to small clusters of Co3O4. The CoSBA materials exhibit photochemical water oxidation catalysis, revealing distinct catalytic activities associated with characteristic types of surface structures that are dominant in particular concentration regimes. The catalytic turnover frequency for water oxidation of an isolated single-site cobalt atom (0.0143 s-1) is much greater than that observed for a surface atom of a small cluster Co3O4 on silica (0.0006 s-1). The CoSBA catalysts were recyclable for more than seven catalytic cycles (> 200 turnovers) with additional sacrificial oxidant, and no leaching of cobalt was observed. Post-catalytic analysis of CoSBA by XAS revealed that the cobalt atoms were partially oxidized to Co3+, without exhibiting significant surface migration and aggregation of cobalt atoms.

Chapter 2. Single-site cobalt atoms on various oxide surfaces (TiO2, MgO, SBA-15, AlPO, and Y-Zeolite) were prepared and evaluated as water oxidation catalysts. Superior catalytic rates were observed for cobalt sites on basic supporting oxides (TiO2 and MgO) relative to those on acidic oxides (Y-Zeolite, AlPo, and SiO2). Per-atom turnover frequencies of ca. 0.04 s-1 were achieved, giving initial rates 100 times greater than a surface atom of a Co3O4 nanoparticle. No correlation was observed between catalytic rates and oxygen atom affinities of the supporting oxides.

Chapter 3. Cobalt metaphosphate Co(PO3)2 nanoparticles are prepared via the thermolytic molecular precursor (TMP) method. A Ni foam electrode decorated with Co(PO3)2 nanoparticles is evaluated as an anode for water oxidation electrocatalysis in pH 6.4 phosphate-buffered water. Catalytic onset occurs at an overpotential of ca. 310 mV, which is 100 mV lower than that observed for Co3O4 nanoparticles, with a comparable surface area under identical conditions. A per-metal turnover frequency (TOF) of 0.10 - 0.21 s-1 is observed at η = 440 mV, which is comparable to the highest rate reported for a first-row metal heterogeneous catalyst. Post-catalytic characterization of the catalyst resting state by XPS and Raman spectroscopy reveals that surface rearrangement occurs, resulting in an oxide-like surface overlayer.

Chapter 4. Linear trimetallic CoIII/CoII/CoIII cobalt complexes with bridging acyl-alkoxy ligands are electrocatalysts for the reduction of tosic acid in acetonitrile. The -OCMe2CH2COMe complex appears to operate homogeneously, and at an onset overpotential of only 25 mV. A turnover frequency of ca. 80 s-1 was observed at an overptotential of 150 mV.

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