DESIGNING CATALYTIC MOTIFS TO GENERATE SOLAR FUELS FROM THE REDUCTION OF CARBON DIOXIDE

Abstract

Solar fuels are small, energy dense molecules generated from abundant feedstocks like CO2 and water using the energy from sunlight and will help reduce reliance on fossil fuels mitigating the effects of climate change. The reduction of carbon dioxide (CO2) into liquid products, such as methanol, is desirable because these products are easily integrated into the current energy infrastructure. CO2 electroreduction sources energy from electricity, potentially generated from light, and may source proton equivalents from abundant sources like water. To date, there are few currently reported systems to electrochemically convert CO2 directly to liquid solar fuels with high efficiency. CO2 reduction also faces selectivity problems with a myriad of potential products. Product selectivity is driven by catalyst system, so a deeper understanding of CO2 reduction mechanisms is imperative for creating highly selective systems. Carbon monoxide (CO) and formic acid (HCOOH) are two products from the initial 2e–/2H+ of CO2. CO is an important intermediate in the generation of liquid solar fuels. Chapter 2 discusses [Ru(tpy)(MeBimpic)(CH3CN)]2+ (MeBimpic is 2′-picolinyl-methyl-benzimidazol-2-ylidene, tpy is 2,2′:6′,2′′-terpyridine) and the influence of the kinetic trans effect on the electrocatalytic activity. An in-depth kinetic analysis supported by density functional theory (DFT) probes CO release across different oxidation states and highlights the importance of the redox active tpy ligand. Chapter 3 introduces a system with a symmetric carbene ligand framework. The system [Ru(tpy)(bis-mim)(CH3CN)]2+ (bis-mim is methylenebis(N-methylimidazol-2ylidene)) can access CO2 reduction through a similar pathway as [Ru(tpy)(MeBimpic)(CH3CN)]2+ and as a mono-cationic species. Notably, the pathway followed by the monocationic [Ru(tpy)(MeBimpic)(CO)]+ intermediate exhibits a 10-fold rate enhancement upon illumination. Chapter 4 introduces cascade catalysis, where multiple systems work in tandem to selectively produce methanol from CO2. Chapter 4 mixes electrochemical CO2 reduction to formate by [Cp*Ir(bpy)(Cl)][Cl] (Cp* is 1,2,3,4,5-pentamethylcyclopentadienyl, bpy = 2,2′-bipryridine) with thermally driven Fischer esterification of formate to formate ester by trifluoromethanesulfonic acid (HOTf), and with thermal transfer hydrogenation by (HPNP)Ir(H)3 (HPNP is (Bis[(2-diisopropylphosphino]ethyl)amine) to form methanol from formate ester intermediates at ambient temperature and pressure. Each step is investigated separately and the challenges of combining these steps into a functional catalytic sequence is discussed.