Interrogating Diffusional Mass and Charge Transport in Catalytic Metal-Organic Frameworks

Sammanfattning: Molecular catalysts are efficient and selective for the electrochemical conversion of small molecules for energy conversion. Application of molecular species in a large-scale industrial setting requires stabilization in a heterogeneous support material. Metal-organic frameworks (MOFs), having high surface areas for increased active site density, have shown promise as potential platforms in which to incorporate molecular catalysts. However, moving from a homogenous environment to catalysis in porous media, necessitates efficient mass and charge transport to the imbedded catalysts. Either diffusional charge transport or diffusion of substrate have the potential to limit the overall observed rate of product formation, if they are slower than the intrinsic rate of the catalytic reaction. This thesis seeks to examine the effect of diffusional mass and charge transport on molecular catalysis in MOFs.First, chemically driven water oxidation is examined using a molecular ruthenium catalyst covalently grafted in MIL(Cr)-101 (MIL = Materials Institute Lavoisier) (Chapter 3). A formal kinetic analysis using a steady-state reaction-diffusion model revealed the limitations incurred by mass transport of the chemical oxidant through the pores of the framework. Importantly, it was shown that interference from mass transport obscures turn-over frequencies, and intrinsic reaction kinetics are only measured under certain conditions. The following chapter entails a modified electrode with a UiO MOF film (UiO = University of Oslo)  containing a molecular catalyst, which is used for electrochemically mediated water oxidation (Chapter 4). The diffusional electron-hopping process is examined and discussed in the context of optimizing overall catalytic current densities. In Chapter 5, a new UiO-type MOF thin film is developed containing exclusively molecularly discrete naphthalene diimide linkers, which are redox-active. This can potentially provide charge transport pathways to imbedded catalysts in a two-component system. In addition, the electron-hopping diffusion coefficient was characterized in both non-aqueous and aqueous electrolytes. Lastly, the capacity of the charge-hopping process occurring in these redox-active MOF films to drive a model catalytic reaction is quantified (Chapter 6). Analysis by cyclic voltammetry is utilized to gain insight into the contributions to the current from the catalytic reaction, electron-hopping, substrate diffusion in the film, as well as mass transport in solution.