Access Type

Open Access Dissertation

Date of Award

January 2017

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Claudio N. Verani

Abstract

As part of the ongoing search for clean energy sources, splitting water into oxygen and hydrogen has emerged as a promising source of alternative energy. We are interested in developing electrocatalysts capable of performing water reduction and water oxidation. In this dissertation, a variety of catalytic platforms based on an amidopyridine framework are studied. A novel monomeric cobalt complex in an amidopyridine ligand environment, ([CoIII(L1)(pyrr)2]PF6), was investigated for electrocatalytic water reduction and found to have an onset overpotential of 0.54 V and an observed TOF of 23 min-1. The catalytic decomposition pathway was explored with the aim of designing more stable catalysts.

Following the initial design, we prepared a modified catalyst in which the amidopyridine framework is extended over a phenylene tetraamine bridge to yield [(CoIII)2L2(pyrr)4](PF6)2. The bimetallic cobalt complex showed significant improvement in catalytic activity: onset overpotential decreased to 0.45 V and the TOF was found to be 60 min-1 per cobalt center under identical catalytic conditions. To the best of our knowledge this is one of the first examples where a dimeric catalyst is more active than its monomeric counterpart. Moreover, we showed that the dimer operates using a voltage-dependent mechanism, in which side reactions associated with deactivation are avoided at low applied potentials.

Finally, we demonstrate that we can drive water oxidation by modification of conductive carbon black-based supports with the hydrophobic octadecyloxy substituted catalyst in an N2N2’ environment, [CoIII(LOC18H37)(pyrr)2]ClO4. The prepared assemblies can catalyze water oxidation at an onset overpotential of 0.32 V, and reach a current density of 10 mA/cm2 at an overpotential of 0.37 V. The molecular nature of the catalyst was ascertained using XPS analysis. A DFT-supported mechanism suggests the ligand is heavily involved in catalysis as an electron reservoir.

The work presented in this dissertation emphasizes the importance of taking into account ligand involvement while designing novel catalysts. In the case of water reduction, we showed that ligand involvement had deleterious effects on catalysis, however for water oxidation we showed that ligand involvement can lower the activation barrier needed for catalysis. Future work will focus on using these lessons to design more active catalysts.

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