Water oxidation catalysis beginning with cobalt polyoxometalates: determining the dominant catalyst under electrocatalytic conditions and investigation of the surface properties of Co3O4 nanoparticles
| dc.contributor.author | Folkman, Scott Jerald, author | |
| dc.contributor.author | Finke, Richard G., advisor | |
| dc.contributor.author | Neilson, James, committee member | |
| dc.contributor.author | Strauss, Steven, committee member | |
| dc.contributor.author | Sites, James, committee member | |
| dc.date.accessioned | 2018-09-10T20:04:42Z | |
| dc.date.available | 2018-09-10T20:04:42Z | |
| dc.date.issued | 2018 | |
| dc.description.abstract | Generation of hydrogen as a fuel is one of the most promising technologies for a renewable energy future. Electrocatalytic water splitting can take energy from virtually any power source and split water into oxygen and hydrogen, thereby creating a renewable feedstock of hydrogen. The efficiency of electrocatalytic water splitting is limited by the anodic half reaction, water oxidation. As such, there has been an immense effort to discover and understand water oxidation catalysts (WOCatalysts). The two main classes of WOCatalysts are homogeneous and heterogeneous catalysts. Homogeneous catalysts are typically soluble molecular complexes that have a single type of active site, allowing for rational tuning through synthesis, and mechanistic studies. Heterogeneous catalysts are typically in a different phase from the reaction (i.e. insoluble or electrode-bound) and have a spectrum of active sites that are more difficult to identify. This Dissertation examines a class of inorganic compounds called polyoxometalates (POMs), and investigates the nature of the kinetically dominant, homogeneous vs heterogeneous catalyst. Chapter I provides an in depth introduction to water oxidation catalysis and in particular with cobalt-based POMS. Chapters II and III focus on the polyoxometalate, [Co4(H2O)2(VW9O34)2]10− (hereafter Co4V2W18) which has been claimed to be one of the fastest WOCatalysts to date. Those studies demonstrate that Co4V2W18 is, in fact, very unstable and dissociates 87-100% of the Co(II) originally present in Co4V2W18 into solution within three hours when dissolved in 0.1 sodium phosphate buffer (NaPi) at pH 5.8 and 8.0 as well as sodium borate buffer (NaB) pH=9.0. The dissociated Co(II)aq then forms heterogeneous cobalt-oxide (CoOx) on a glassy carbon electrode under electrocatalytic WOCatalysis conditions. The deposited CoOx accounts for 100±15% of the observed catalysis current. This finding demonstrates that the original Co4V2W18 serves only as a precursor to heterogeneous CoOx which is the dominant WOCatalyst. Chapter IV details studies using a selection of the most stable and most active Co-POMs to date. These studies demonstrate that none of the Co-POMs examined are 100% stable, and they release between 0.6 and >90% of the cobalt in the original complex within three hours in 0.1 M NaPi pH=5.8 or 8.0 and NaB pH=9.0. Furthermore, in 13 of the 18 cases examined, heterogeneous CoOx forms on the glassy carbon electrode and accounts for ≥100% of the observed WOCatalysis current. Lastly, under conditions where the Co-POMs are stable (<2% decomposition), the evidence provided implies that some of the Co-POMs are homogeneous WOCatalyst. Other implications regarding the stability trends and nature of the true catalyst are provided. The last research chapter, Chapter V, consists of the study of Co3O4 nanoparticles, which have been shown to be active for WOCatalysis. In this chapter, the synthesis, and surface properties of Co3O4 nanoparticles are investigated. It is demonstrated that ethanol/water (EtOH/water) as solvent forms phase-pure Co3O4 nanoparticles but following the same procedure in water yields a mixture of products. Therefore, EtOH must affect the product either thermodynamically (i.e. through a covalent EtO-Co linkage on the surface) or kinetically (i.e., by affecting the nucleation and/or growth of the particles). However, EtOH is not observed in the product; instead, acetate from the cobalt acetate precursor is the only detectable surface ligand. This implies that EtOH does not affect the thermodynamics of the particle formation, instead it must be involved in the kinetics of nucleation and/or growth of the Co3O4 nanoparticles. Through careful examination of the particle size and surface ligand data were able to obtain an average molecular formula of {[Co3O4(C2H3O2)−][(NH4+)0.3(H+0.7)]+·(H2O)}∼216 for the nanoparticles that we isolated. This chapter also includes general implications for the synthesis of metal-oxide nanoparticles in alcohol, and methods for identifying surface ligands. | |
| dc.format.medium | born digital | |
| dc.format.medium | doctoral dissertations | |
| dc.identifier | Folkman_colostate_0053A_14945.pdf | |
| dc.identifier.uri | https://hdl.handle.net/10217/191355 | |
| dc.language | English | |
| dc.language.iso | eng | |
| dc.publisher | Colorado State University. Libraries | |
| dc.relation.ispartof | 2000-2019 | |
| dc.rights | Copyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright. | |
| dc.subject | cobalt oxide | |
| dc.subject | homogeneous vs. heterogeneous | |
| dc.subject | water oxidation | |
| dc.subject | electrocatalytic | |
| dc.subject | catalysis | |
| dc.subject | polyoxometalate | |
| dc.title | Water oxidation catalysis beginning with cobalt polyoxometalates: determining the dominant catalyst under electrocatalytic conditions and investigation of the surface properties of Co3O4 nanoparticles | |
| dc.type | Text | |
| dcterms.rights.dpla | This Item is protected by copyright and/or related rights (https://rightsstatements.org/vocab/InC/1.0/). You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). | |
| thesis.degree.discipline | Chemistry | |
| thesis.degree.grantor | Colorado State University | |
| thesis.degree.level | Doctoral | |
| thesis.degree.name | Doctor of Philosophy (Ph.D.) |
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