The Machan Research Group (Department of Chemistry at the University of Virginia) is interested in energy-relevant catalysis, particularly at the interface of molecular electrochemistry and materials. The development of efficient and selective transformations to produce commodity chemical precursors and fuels using CO2, O2, H2, and H2O as reagents remains an ongoing challenge for the storage of electrical energy within chemical bonds. Our approach is inspired by the numerous metalloproteins capable of catalyzing kinetically challenging reactions with significant energy barriers in an efficient manner under ambient conditions. This type of reactivity is achieved through the convergent evolution of active sites with tailored coordination environments and macromolecular structures which can, among other things, transport substrates and products to and from the active site. Our research focuses on developing new inorganic complexes and materials which incorporate co-catalytic moieties, non-covalent secondary sphere interactions, and substrate relays as catalysts. These efforts represent an opportunity to address the problems posed by diminishing petrochemical reserves, increasing atmospheric carbon dioxide concentrations, and the shift to renewable energy.
In order to characterize and optimize these systems, research in the Machan group uses synthetic inorganic chemistry, electrochemistry, and advanced characterization techniques (spectroelectrochemistry, stopped-flow IR and UV-vis spectroscopies). This enables us to develop an understanding of electronic structure and mechanism in transformations of interest. A brief summary of current projects is listed below.
Electrochemical Reduction of Dioxygen
The reduction of dioxygen (O2) is of vital importance to energy related reactions. In biology, respiration uses O2 reduction as a thermodynamic sink, whereas fuel cells pair the oxygen reduction reaction (ORR) to H2O as a proton-dependent half-reaction to the oxidation of chemical fuels. The triplet ground state of O2 presents a kinetic challenge for catalysts, but the initial one-electron reduction of O2 to superoxide (O2–) can be made favorable via coordination to a transition metal ion with appropriate spin symmetry. Catalytic ORR processes mediated by molecular species continue to garner interest as models for more complex heterogeneous systems. We are interested in the understanding selective activation and reduction of O2 to H2O2 or H2O by molecular species through electrochemical, spectrochemical, and spectroelectrochemical studies. Using these studies, we are developing new ligand frameworks for aqeuous and non-aqeuous systems, some of which are models for metalloenzyme active sites.
- Electrocatalytic Reduction of Dioxygen to Hydrogen Peroxide by a Molecular Manganese Complex with a Bipyridine-Containing Schiff Base Ligand (J. Am. Chem. Soc. 2018 DOI:10.1021/jacs.7b09027)
- Dioxygen Reduction to Hydrogen Peroxide by a Molecular Mn Complex: Mechanistic Divergence between Homogeneous and Heterogeneous Reductants (J. Am. Chem. Soc. 2019 DOI:10.1021/jacs.8b13373)
- Electrocatalytic Reduction of Dioxygen by Mn(III) meso-Tetra(N-methylpyridinium-4-yl)porphyrin in Universal Buffer (Dalton Trans. 2019 DOI: 10.1039/C9DT01436E)
Molecular Electrocatalysts for the Electrochemical Conversion of Carbon Dioxide
The efficient and cost-effective catalytic reduction of CO2 using renewable energy remains a significant challenge for molecular species. Heterogeneous systems can produce highly reduced products like methane and ethylene from CO2, but these generally suffer from a lack of selectivity. The intrinsic advantage of molecular systems is the relative ease with which they may be characterized and quantified, relative to the distribution of active site morphologies that may be present in a bulk material. Using bioinspired design principles, we are investigating catalytic conversion strategies for producing CO and formic acid from CO2.
- Electrochemical Reduction of Carbon Dioxide with a Molecular Polypyridyl Nickel Complex (Sustainable Energy Fuels 2018, DOI: 10.1039/C8SE00027A)
- Electrocatalytic Reduction of CO2 to Formate by an Iron Schiff Base Complex (Inorg. Chem. 2018, DOI: 10.1021/acs.inorgchem.7b02955)
Porous Electrocatalyst Materials
Metal-organic frameworks (MOFs) and covalently linked organic frameworks (COFs) continue to attract significant interest in materials chemistry. MOFs and COFs offer many advantages in terms of porosity and stability over more amorphous materials or zeolites. Indeed, the translation of molecular properties to bulk materials in this manner has implications for the development of electrochemically responsive films and membranes. We are focused on developing new methods for synthesizing and processing conducting and semi-conducting 2D MOF and COF materials sensitive to the chemical environment. This is primarily focused on applications in molecular detection, separation, and catalysis. A fundamental understanding of how molecular properties are translated in these systems will enable future studies focusing on other applications in energy storage and optoelectronic devices.