Cobalt-substituted Polyoxometalates as Viable WaterOxidationCatalysts Open Access

Tan, Jeffrey Miles Tiu (2009)

Permanent URL: https://etd.library.emory.edu/concern/etds/7s75dd119?locale=en
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Abstract

Cobalt-substituted Polyoxometalates as Viable Water Oxidation CatalystsBy Jeffrey Miles Tan The cobalt-substituted polyoxometalate K10[Co4(H2O)2(PW9O34)2]·22H2O was studied and evaluated for catalytic water oxidation activity. It was found to have such activity and preliminary experiments have revealed that the complex is functional in a photochemical water oxidation system. The pH dependence of catalytic activity was also explored. The hydrolytic stability of the cluster was studied using UV-Visible and 31P NMR spectroscopy. This represents the second example of a polyoxometalate-based water oxidation catalyst.

Table of Contents

Abstract

Acknowledgements

List of Figures and Tables

Cobalt-substituted Polyoxometalates as Viable Water Oxidation Catalysts..................1

I. Introduction................................................................................2

II. Experimental...............................................................................11

III. Results and Discussion...............................................................14

IV. Conclusion..............................................................................34

V. Future Directions........................................................................34

VI. References..............................................................................36

List of Figures and Tables

Figure 1. Artificial photosynthetic system (S = dye, HEC = hydrogen evolution catalyst, R = electron relay, OEC = oxygen evolving catalyst)...........................................3

Figure 2. Artificial catalytic hydrogen gas evolution system (S = dye, Red = sacrificial reductant, HEC = hydrogen evolution catalyst, R = electron relay)...........................5

Figure 3. Artificial catalytic water oxidation system (S = dye, Ox = sacrificial oxidant, OEC = water oxidation catalyst)....................................................................6

Figure 4. POM's as molecular metal oxide water oxidation catalysts...............................................................................................15

Figure 5. IR spectrum of K-2. It matches that reported by Finke et al.......................16

Figure 6. The UV-Visible spectrum of K-2. It matches that reported by Knoth et al.....16

Figure 7. Structure for K-Na-Li-2................................................................18

Figure 8. UV-Visible study of 2 at pH 8. Conditions: 2 mM 2 in 50 mM Na2HPO4/NaH2PO4 (pH 8) buffer................................................................19

Figure 9. UV-Visible study of 2 at pH 3.5. Conditions: 2 mM 2 in 50 mM NaOOCCH3/HOOCCH3 (pH 3.5) buffer........................................................20

Figure 10. 31P NMR of K-2 at its natural pH (ca. 7.0) from 2225-1175 ppm. The peak at 1875 is assigned to the two phosphorus atoms in K-2, which are equivalent by symmetry........21

Figure 11. 31P NMR of K-2 at its natural pH (ca. 7.0) from 1375-350 ppm. There is no peak seen in this region.............................................................................22

Figure 12. 31P NMR of K-2 at its natural pH (ca. 7.0) from 525 to -525 ppm. No peak is seen in this region...................................................................................23

Figure 13.31P NMR of K-2 at pH 8 (in borate buffer) from 2225-1175 ppm. The peak at 1870 is assigned to the two phosphorus atoms in K-2, which equivalent by symmetry...24

Figure 14. 31P NMR of K-2 at pH 8 (in borate buffer) from 1325-375 ppm. No peak is seen in this region except the one in the center of the window. This peak was determined to be an artifact and is either changed or removed when scans are taken in a different window................................................................................................25

Figure 15. Top: 31P NMR of K-2 at pH 8 (in borate buffer) from 525 to -525 ppm. No peak is seen in this spectrum outside of the artifact seen in the middle of the top window. Bottom: Spectrum taken in order to prove that the peak in the top spectrum is merely an artifact that appears as a result of limitations in the NMR instrument.......................26

Figure 16. 31P NMR of K-2 at pH 3.5 (in acetate buffer) from 2225 to 1175 ppm. No peak is seen in this spectrum outside of the artifact seen at 1967. We know this is an artifact and not an actual peak because the same sample taken in a different window does not show this peak...................................................................................27

Figure 17. 31P NMR of K-2 at pH 3.5 (in acetate buffer) from 1325 to 275 ppm. No peak is seen in this spectrum.............................................................................28

Figure 18. 31P NMR of K-2 at pH 3.5 (in acetate buffer) from 525 to -525 ppm. No peak is seen in this spectrum.............................................................................29

Figure 19. Catalytic water oxidation half-cell used in this work.............................30

Figure 20. Yield vs. time graphs for photochemical water oxidation trials.................31

Table 1. Optimization data for photochemical water oxidation trials........................31

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