Heterogenization of Molecular Electrocatalysts for Small Molecule Transformations

Abstract

The use of molecular electrochemical catalysts for small molecule transformations is a quickly growing and challenging field with numerous environmental, industrial, and agricultural applications. Molecular catalysts, while prized for their selectivity and tunability, are often hindered by issues of solubility, relative activity, and post-electrolysis separation and reconstitution. One method for alleviating some of the drawbacks of molecular catalysts is to attach them to solid-state surfaces, a process also known as heterogenization. In this dissertation I will share the results of several methods for heterogenization of molecular electrocatalysts for environmentally relevant small molecule transformations, culminating in the discovery of a novel polymeric chromium catalyst capable of the direct reduction of nitrate, and other NOx species, to ammonia with activity and selectivity comparable to state-of-the-art solid-state electrocatalysts. Chapters 1 and 2 provide background information for this thesis, including an in-depth discussion of the research motivation, current methods for molecular electrocatalyst heterogenization, and descriptions and backgrounds for the main analytical methods employed throughout this work. In Chapter 3 I present my work on layer-by-layer growth of multilayer films of discrete molecular catalysts using sequential Cu-Catalyzed Azide−Alkyne Cycloaddition or ‘Click’ reactions for the electrocatalytic reduction of oxygen. The resulting multi-layer films of Copper Diethynylphenanthroline were capable of the oxygen reduction reaction and showed an increase in both activity and selectivity for the 4-electron reduction of oxygen to water. However, as discussed in more detail in the chapter, the films were limited to two layers due to steric hindrance preventing the formation of additional layers. In Chapter 4, I discuss the use of the same ‘Click’ reaction to modify glassy carbon electrodes with (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) for alcohol oxidation. I present the synthesis and heterogenization of several different TEMPO analogues. I determined that while TEMPO can be readily covalently attached to glassy carbon electrodes and the resulting electrodes are active for alcohol oxidation, the TEMPO complexes are highly unstable and rapidly degrade under catalytic conditions. These results suggest that further work into this area should focus on either TEMPO stability, generating a large excess of catalyst on the surface, or rapid replacement/regeneration of the catalytic surface. Chapter 5 focuses on my work in using electropolymerization to overcome the limitations of the ‘Click’ reaction for forming multi-layer films. I present my work on using molecular electrocatalysts modified with a terthiophene backbone in order to form catalyst-containing conductive electropolymerized films on glassy carbon electrodes. This method is capable of forming much higher surface coverages than the aforementioned ‘Click’ method, albeit at the cost of less control over the film due to the radical polymerization process. Using a novel chromium terpyridine terthiophene complex as the monomer I form electrode surfaces that are capable for the reduction of nitrate, and other NOx species, to ammonia with activity and selectivity comparable to those of state-of-the-art solid-state systems. To our knowledge this is the first example of a molecular electrocatalyst capable of this reaction at high selectivity and activity and also one of the first examples of a highly active polythiophene based electrocatalytic system. It has many implications for environmental remediation of nitrate with the added benefit of nutrient recovery. Finally, in Chapter 6, I provide a brief summary of my work, descriptions of the future directions I believe this work should take, and some preliminary data for the aforementioned directions.