Abstract
There are many acute global challenges which have the potential for unprecedented adverse impacts on human health and global ecosystems. These include climate change, exceptional wildfires, and ocean acidification, all of which are to a great extent due to the ongoing large scale combustion of fossil fuels, as this leads to ever increasing concentrations of greenhouse gases such as CO2 in the atmosphere. Therefore the development of carbon neutral, or better still carbon zero (green H2), future fuels, with production driven by green energy (renewables, such as hydro, solar, wind or wave generated energy) is an urgent necessity. For this approach to be widely adopted, it is essential that new, active, yet long-lived catalysts, based on cheap, abundant metals, be developed. Presented in this thesis is the synthesis of three literature and six new copper, nickel and cobalt complexes for the purpose of catalysing the hydrogen evolution reaction (HER) under both photo and electrocatalytic conditions. These compounds are also tested as catalysts for the photocatalytic CO2 reduction reaction (CO2RR).
Chapter 1 provides an introduction to current global climate challenges, and how green hydrogen is an excellent candidate to address those challenges. Then an outline of current methods used for hydrogen production is given, followed by some examples of materials active as catalysts for hydrogen production, along with a brief description of the general metrics used for evaluation of catalytic performance. The common proposed HER mechanisms are presented. This is followed by a brief introduction to CO2 reduction reactions (CO2RR) and a summary of previous work on diphenylamine based macrocyclic complexes. Finally, the aims of this work are presented.
Chapter 2 summarises the multiple-step literature synthesis of the key ligand precursors, ethyl-2-iodobenzoate (I), diphenylamine-2,2'-diethylbenzoate (II), diphenylamine-2,2'-dimethanol (III) and diphenylamine-2,2'-dicarboxaldehyde, (IV), as well as three of the four macrocycles made in this work, the N4-donor [1+1] Schiff base macrocycle formed from IV and diethylenetriamine (HLEt), and two new functionalised analogues, HLEt-allyl and HLEt-propgl, formed by alkylation of the alkyl NH in HLEt with an appropriate alkyl bromide reagent.
In Chapter 3 the synthesis of two new N5-donor Schiff base ligands, the macrocycle HLEt-MePy (accessed by 2-bromomethylpyridine alkylation of HLEt) and noncyclic HLEtPy2 (condensation of IV and two 2-aminoethylpyridine) is described. Literature [CuIILEt]BF4 (1), and new
[CuIILEt-MePy]BF4 (2) and [CuIILEtPy2]BF4 (3), were prepared, and 2 and 3 were structurally characterised. The performance of these three copper complexes as catalysts for HER in non-aqueous solvents under photo- and electrocatalytic conditions is analysed.
In Chapter 4 further electrocatalytic HER testing of the most promising complex, [CuIILEt]BF4 (1), in neutral aqueous phosphate buffer is reported.
In Chapter 5 two new nickel complexes of the above N5-donor Schiff base ligands, macrocycle HLEt-MePy and noncyclic HLEtPy2, [NiIILEt-MePy]BF4 (5) and [NiIILEtPy2]BF4 (6) along with the literature [NiIILEt]BF4 (4), were prepared and 5 and 6 structurally characterised. The performance of these three nickel complexes as catalysts for HER in non-aqueous solvents under photo- and electrocatalytic conditions is analysed.
In Chapter 6 two new cobalt complexes of the above N5-donor Schiff base ligands,
[CoIIILEt-MePy)(BF4)2 (8) and [CoIILEtPy2]BF4 (9) along with the literature [CoIILEt]BF4 (7), were prepared and 8 and 9 were structurally characterised. The performance of these three cobalt complexes as catalysts for HER in non-aqueous solvents under photo- and electrocatalytic conditions is analysed.
In Chapter 7 the nine complexes synthesised in the previous chapters 1-9 are tested as catalysts for the carbon dioxide reduction reaction (CO2RR) under photocatalytic conditions.
In chapter 8 the key results of this work are summarised.