UC San Diego

Our dependence on the burning of fossil fuels for energy, leading to the rise in global temperatures and climate change, has sparked increased research into the utilization of carbon dioxide (CO2) as a renewable starting material for the synthesis of new fuel sources. Heterogenous catalysts are often employed for these reactions as they tend to have higher stability, activity, lower catalyst loadings, and scalability. However, selective heterogenous catalyst discovered is often hindered by the lack of understanding and often relies on high-throughput screening. In contrast, homogeneous catalysts have active sites that are well understood, are highly tunable, uniform, and thus selective. By using homogeneous catalyst, fundamental understanding of the elementary steps involved in product selectivity can be achieved, allowing for improved catalyst design. The conversion of CO2 to value-added carbon products for fuel requires the multi-electron and proton steps. One elementary step that can lower the entropic barrier of this process is the transfer of a hydride from a metal catalyst to CO2 to yield formate (HCO2-), the two-electron reduction product. The investigation of the structural, thermodynamic, and kinetic barriers that allow this transformation is the first step in designing novel, selective catalytic systems. The metal-metal cooperativity in [FeFe] hydrogenase mimics, which allows for the formation of highly reducing hydrides from weak acids, has been hypothesized as a key step in the electrochemical reduction of CO2 to formate. However, there was little mechanistic and structural insights that lead to the selectivity for formate in these systems. Briefly, using the [FeFe] hydrogenase mimic [Fe2(μ-pdt)(CO)6, where pdt = propane-1,3-dithiolate] as the precatalyst, it was found that the main product formed under catalytic conditions, was H2 (FEmax= 56 ±4%), with CO (FEmax= 16 ±6%) and HCO2- (FEmax= 20%) being minor products. Interestingly, it was determined that the formation of HCO2- was potential dependent, with the Faradaic efficiency increasing from 2.4% to 20% upon the third reduction of the catalyst. In addition, in the absence of a proton source, CO2 underwent disproportionation to CO and CO32-. Finally, by combining experimental and computational methods, it was determined that the selectivity of the catalytic system was hindered were due to the formation of multiple compounds, with monomeric [Fe(CO)4]2- and a trinuclear Fe likely playing active roles in catalysis. The thermodynamic ability of a substrate to donate a hydride, known as hydricity, is an important parameter for predicting the reactivity of a catalyst in artificial photosynthetic systems. Tools that allow us to measure the hydride donor ability are critical to not only predicting reactivity but can be used to understand and optimize catalytic systems with a metal hydride of choice. Using Ir(Cp*)(ppy)X (where X is H or Cl), hydrogen evolution, formic acid decomposition, and transfer hydrogenation are optimized and understood using hydricity and thermodynamics as a guide. Finally, immobilization of homogeneous catalysts onto surfaces, allows for insight into the mechanisms that affect heterogenous electrochemistry. Previous studies showed that the electric field effects on the Re(bpy-X)(CO)3Cl bound to Au surfaces changed the electron density at the metal. These field affects, as seen in the v(CO) frequency shift of greater than 25 cm-1, is three to four times larger than those achieved through synthetic efforts of modifying the bipyridine for electron donating or accepting groups via Hammett parameters. Additionally, hydricity is highly dependent on the electronic properties of the substrate. Computational results show that electric fields stabilize the transfer of a hydride from Ir(Cp*)(bpy)H, making Ir a better hydride donor. Two new Ir catalysts with surface-immobilizable disulfides were synthesized for attachment to Au. However, reactivity studies showed that the rate of simple ligand separation was significantly slower. Combined DFT and surface sensitive spectroscopic studies showed that the monolayers formed from these complexes are a mix of molecular orientations, likely leading to a range of reaction rates on the surface.