Research

Catalysis for Renewable Fuels

Our research is centered on developing efficient and abundant metal catalysts for the production of fuels or feedstock chemicals using renewable energy. Our approach takes inspiration from enzymatic active sites as well as thermochemical activity descriptors used in heterogeneous catalysis. The initial targets are the electrocatalytic reduction of water to hydrogen, and carbon dioxide to more energy-dense carbon fuels.

The research combines synthesis with advanced electrochemical and spectroscopic techniques. Detailed mechanistic and kinetic studies are employed to improve catalyst design and optimize activity.

Aqueous Hydrogen and Formate Production

Transition metal hydrides (shown in red to the left) are critical intermediates in hydrogen production (path A) and carbon dioxide reduction to formate (path B). In order to improve our understanding of electrochemical hydride generation and reactivity, we are forming predictive models of hydricity in aqueous solvents. We are also investigating if hydricity is an appropriate activity descriptor for catalytic activity. Additionally, quantifying the hydricity will facilitate the design of catalysts with tailored product selectivity.

To date, we have developed a new complex for electrocatalytic aqueous hydrogen production and an abundant transition metal hydride complex that can perform hydride transfer to carbon dioxide.

Cooperative Lewis Acid-Base Reactivity

The active site of the Ch Ni-CODH II enzyme suggests carbon dioxide activation occurs through a cooperative interaction between a Lewis basic nickel and a Lewis acidic iron (shown on the left). We are developing synthetic mimics that position a Lewis acid proximate to a Lewis basic metal center in order to replicate this cooperative interaction. The Lewis acids we are exploring are 1) acidic protons, 2) redox-inactive metals, and 3) redox active transition metals. Recent research also indicates that Lewis acids play a role in tuning redox potential. Our current studies focus on quantifying the redox modulation with respect to Lewis acidity.

Ligand Development

Small molecule oxidation and reduction chemistry often includes proton transfer events. Uncoupled proton transfer can impede catalytic rate or result in high activation barriers for catalysis. To facilitate proton transfer, we are designing new macrocyclic ligands that incorporate pendant acids or bases in the secondary coordination sphere. Recently, we modified the diamine dipyridyl ligand shown on the left with dimethyl pendant amines (red) in the secondary coordination sphere. The blue boxes represent open coordination sites on the Co(II) complex which are occupied by acetonitrile ligands in the X-ray crystal structure. We are currently investigating the small molecule reactivity and redox chemistry of these complexes.

We have also developed a new tripodal tetradentate ligand incorporating a strong phosphine donor based on proazaphosphatrane, shown on the right. The Tolman parameter indicates it is among the most donating phosphines that has been measured. Additionally, the possibility of a transannular interaction in the proazaphosphatrane could increase the donor strength. We are currently investigating the trans effect of the strong phosphine donor on promoting unusual reactivity.

Integrating Molecular Catalysts onto Photoelectrodes

Photoelectrochemical cells offer an integrated path towards direct chemical fuel generation from solar energy. However, most photoabsorbers are poor catalysts and require functional coupling with efficient molecular electrocatalysts. We are developing a milder method of catalyst attachment that is stable, synthetically accessible, and provides facile electron transfer.

Electrochemical Properties of Nitrogenase

Professor Markus Ribbe and Professor Yilin Hu at UC Irvine recently reported catalytic reduction of CO2, CO, and CN- into methane and hydrocarbons by the nitrogenase enzymes and their isolated cofactors using chemical reductants. In collaboration with Prof. Ribbe and Prof. Hu, we are investigating the redox properties of the nitrogenase enzyme in order to perform catalysis using an electrochemical potential. We are currently working with the isolated cofactors, but will progress to working with the enzyme. Additionally, will are applying in situ spectroelectrochemical techniques to identify catalytic intermediates, as well as detailed kinetic studies on the catalytic rate.