Research Description

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 in an efficient manner under ambient conditions. This type of reactivity is achieved in bioinorganic systems 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. Brief summaries of current projects are listed below.

Electrochemical Reduction of Dioxygen by Homogeneous Catalysts

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. 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.

Relevant Papers:

• 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)

• Non-covalent Assembly of Proton Donors and p-benzoquinone Anions for Co-electrocatalytic Reduction of Dioxygen (Chem. Sci. 2021 DOI: 10.10139/D1SC01271A)

• Pendent Relay Enhances H2O2 Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N2O2 Complexes (J. Am. Chem. Soc. 2021 DOI: 10.1021/jacs.1c03381)

• Catalytic Reduction of Dioxygen to Water by a Bioinspired Non-Heme Iron Complex via a 2+2 Mechanism (J. Am. Chem. Soc. 2021 DOI: 10.1021/jacs.1c04572)

• Controlling Product Selectivity During Dioxygen Reduction with Mn Complexes Using Pendent Proton Donor Relays and Added Base (Chem. Sci. 2024 DOI: 10.1039/D3SC02611F)

• Metal-Free Homogeneous O2 Reduction by an Iminium-based Electrocatalyst (J. Am. Chem. Soc. 2024 DOI: 10.1021/jacs.3c14549)

Molecular Electrocatalysts for Electrochemical Carbon Dioxide Conversion

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.

Relevant Papers:

• Highly Efficient Electrocatalytic Reduction of CO₂ to CO by a Molecular Chromium Complex​ (ACS Catalysis 2020, DOI: 10.1021/acscatal.9b04687)
• Electrocatalytic CO₂ Reduction to Formate with Molecular Fe(III) Complexes Containing Pendent Proton Relays (Inorg. Chem. 2020, DOI: 10.1021/acs.inorgchem.9b03341)
• Mediated Inner-Sphere Electron Transfer Induces Homogeneous Reduction of CO₂ via Through-Space Electronic Conjugation (Angew. Chem., Int. Ed. 2022 DOI: 10.1002/anie.202109645 *VIP*)
• Inverse Potential Scaling in Co-Electrocatalytic Activity for CO₂ Reduction Through Redox Mediator Tuning and Catalyst Design (Chem. Sci. 2022 DOI: 10.1039/D2SC03258A)
• Co-Electrocatalytic CO₂ Reduction Mediated by a Dibenzophosphole Oxide and a Chromium Complex (Chem. Commun. 2023 DOI: 10.1039/D3CC00166K)

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.

Relevant Papers:

• Metal-Organic Frameworks as Porous Templates for Enhanced Cobalt Oxide Electrocatalyst Performance (ACS Applied Energy Materials 2019 DOI: 10.1021/acsaem.9b00127)
• Solution Shearing of Zirconium (Zr)-Based Metal–Organic Frameworks NU-901 and MOF-525 Thin Films for Electrocatalytic Reduction Applications (ACS Appl. Mater. Interfaces 2023, DOI: 10.1021/acsami.3c12011)

Current Funding

PI: NSF MPS Catalysis CHE-2102156, CHE-2348515

PI: DOE BES Catalysis DE-SC0022219

Co-PI: DOE BES Catalysis DE-SC0023443

Co-PI: NSF CBET Catalysis ENG-2311116

Previous Funding

ACS PRF 61430-ND3

ExxonMobil