Functionalized Metal-Organic Frameworks for Water Oxidation Catalysis

Increasing energy demand will not only aggravate global warming, but also cause fossil fuels shortage in the near future. Solar energy is an infinite green energy resource that can potentially satisfy our energy usage. By utilizing solar energy to drive reactions like water splitting, solar fuels system are able to produce valuable energy resource. Catalysts for water oxidation are the essential component of water splitting cells which have been intensively studied. As a solid state porous crystalline material with synthetic tunability, Metal-organic framework (MOF) is a promising platform for water oxidation catalysis due to its outstanding properties. Herein, we aimed to develop molecular catalysts incorporated MOF for water oxidation and study the reaction mechanism.

Chapter 1 introduces the background of water oxidation and previous research on ruthenium nuclear water oxidation catalysts (WOCs). The reaction mechanism of binuclear and mononuclear ruthenium WOCs was briefly summarized. Opportunities for the design and the synthesis of MOF based WOCs were then discussed. Lastly, studies about MOF based WOCs were categorized based on the difference of the WOCs active site location in frameworks.

Water oxidation catalyst [Ru(dcbpy)(tpy)OH2]2+ (RuTB) was incorporated into UiO-67 MOF (resulting materials denoted as RuTB-UiO-67) for chemical water oxidation in Chapter 2. Differences of catalytic reaction behavior between homogeneous RuTB and RuTB incorporated in MOF were examined. Based on MOF particle size dependent catalysis reaction experiments, in-MOF reactivity was anticipated to be primarily arose from redox hopping between RuTB active sites in the framework.

In Chapter 3, RuTB-UiO-67 MOF thin films grown on conducting FTO substrate (RuTB-UiO-67/FTO) were synthesized to test their catalytic activity of electrochemical water oxidation. Electrochemical behavior of RuTB-UiO-67/FTO was found to be consistent with homogeneous RuTB by various electrochemistry study and in-situ X-ray absorption spectroscopy characterization. Scan-rate-dependent voltammetry study demonstrated the homogeneous distribution of electrochemical active sites throughout the MOF thin film. Diffusion controlled redox hopping was attributed to be the main charge transfer pathway during catalysis.

In order to pursue photo-induced water splitting system, we further our study by investigating MOF based photoelectrochemical catalysis in Chapter 4. Photoelectrochemical alcohol oxidation was chosen as the preliminary-stage study towards the more challenging goal, photoelectrochemical water oxidation. Electron transfer processes of the photosensitizer ([Ru(bpy)2(dcbpy)]2+) and the catalyst (RuTB) doped UiO-67 MOF were investigated with transient absorption spectroscopy analysis.

Finally, the role of redox hopping in electrocatalysis by MOF was reviewed in Chapter 5. Pathways of charge transfer in electroactive MOF were first summarized. Redox hopping in MOF was then compared with previous studies on redox active polymer thin films. Lastly, factors that will affect the rate of redox hopping of MOF electrocatalyst were discussed.