UC Irvine

The investigation of energy-storage strategies for renewable technologies is one of the most important modern-day challenges facing the future energy landscape. Inspired by the process of photosynthesis, where light energy is stored in the energy dense bonds of sugars, this work investigates the fundamental thermodynamic and kinetic forces directing the reactivity of homogenous catalysts for the electrochemical reduction of protons to H2 or CO2 to formate.

The selective catalytic reduction of CO2 to formate is of interest because it is a carbon neutral process for storing energy in chemical bonds. Currently, CO2 reduction pathways to target fuels are often limited by the off-cycle hydrogen evolution reaction (HER) which lowers faradaic efficiency and product selectivity. A catalyst design strategy is described which utilizes thermodynamic properties and hydricity-pKa relationships to target reaction conditions for selective hydride transfer to specific substrates like protons or CO2 in both water and organic solvents.

A detailed study of the electrocatalytic activity and selectivity of Ni and Pt diphosphine complexes for H2 evolution and CO2 reduction in water or the presence of acid in organic solvent is described. Experimental evidence demonstrates that the competing H2 evolution pathway can be disfavored with changes in proton activity. These results demonstrates that metal hydricity can be a quantitative activity descriptor for optimizing catalyst conditions for selective CO2 reduction.

An investigation of the role of kinetic and thermodynamic factors that direct reactivity of a Pt diphosphine catalyst with protons or CO2 to achieve high selectivity for formate is also described. An energy landscape plot was constructed from experimental studies comparing observed rates for electron transfer, protonation, and CO2 binding. Our findings indicate that overall catalysis is limited by the rate of C-H bond formation by the reactive metal hydride. This information guides future compound modifications to improve catalytic activity.

The completed work gives new fundamental insights into a thermodynamic and kinetic based methodology for homogeneous catalyst design.