Selective Group VII CO2 Reduction Electrocatalysts
Rhenium (Re) bipyridine-based catalysts are some of the most robust and well-characterized systems known for the kinetically selective proton-dependent reduction of carbon dioxide (CO2) to carbon monoxide (CO) and H2O. Using industrial Fischer-Tropsch technologies, CO can be converted into liquid fuels in the presence of hydrogen. The electrocatalytic production of liquid fuels could provide a route to carbon-neutral energy sources that would fit into the existing infrastructure, especially with regards to storage and transport.
For these reasons our laboratory has undertaken significant investigations regarding these Re(I) bipyridine-based catalysts. Such studies have allowed us to develop a nuanced understanding of why these catalysts display a remarkable kinetic selectivity for the reduction of CO2, even in the presence of a large concentration of weak acid. A simplified mechanism for this catalyst family is shown below. The use of X-ray Absorption Spectroscopy to gain insight into the electronics of the active states of these catalysts and elucidation of the reaction mechanism using stopped-flow mixing in tandem with infrared or UV-Vis spectroscopy, has allowed us to begin exhaustive studies of the interaction of these catalysts with CO2.
Since rhenium is one of the least abundant elements in the Earth's crust, we have extended these studies to rhenium's first row Group VII counterpart, manganese. We have expanded on previous studies of manganese bipyridine catalysts by synthetically altering the bipyridine ligand to create more active catalysts. Significantly, these catalysts require the addition of weak acid for catalytic activity, whereas the rhenium catalysts are active in the absence of an external proton source. Through ongoing synthetic modification, however, we hope to achieve catalytic activities and stabilities that rival those of the rhenium bipyridine catalyst family. Our continued studies highlight the promise of the manganese catalyst family both for its ability to operate at a lower overpotential than its rhenium counterpart and for the earth-abundance of manganese.
The electrochemical reductive transformation of carbon dioxide is strongly hindered by the high-energy intermediates implicit in its direct reduction. We in the Kubiak lab are trying to work around these kinetic limitations by enabling concomitant proton and electron transfer in order to avoid these intermediates entirely. Certain biomimetic ligands, such as Lewis basic amines, can act as proton relays in the secondary-coordination spheres of catalysts for proton-dependent reductions. A series of catalysts containing these types of proton relays of particular note are the [Ni(P2N2)2]2+ complexes developed by Dr. Daniel DuBois. These complexes are functional hydrogenase mimics that have demonstrated some of the best electrocatalytic rates for hydrogen production and oxidation. Recently, our group has discovered that these complexes also have the fastest reported rate for the electrocatalytic oxidation of formate. Interestingly, the mechanism proposed for this transformation involves a proton-coupled 2-electron transfer event. As a part of our continuing studies we have developed new syntheses to greatly diversify the available P2N2 ligand catalog. By exploring the reactivity of different metals and novel ligands, we aim to adapt this proton-relay system for the reversible reduction of CO2.
Supported Molecular Catalysis
While homogeneous systems are favoured for their selectivity and tunability, heterogeneous catalysts have the advantages of stability, low catalyst loading, and straightforward
product separation. Linking molecular catalysts to conductive surfaces provides an opportunity to combine the advantages of
both systems. Our interest lies in finding the most optimum surface and attachment strategy for the molecular catalysts our group has studied over the past several
years. Factors such as attachment strength, orientation of the catalyst on the surface, stripping potentials, electron transfer, and catalyst loading are all areas
under investigation for these systems.
Investigation of Electron-Transfer Reactions in Highly Coupled Mixed-Valence Systems
Over the past 15 years our lab has studied electron-transfer reactions using the coalescence of ν(CO) band shapes in the IR spectra of mixed-valence dimer complexes of the type [Ru3O(κ2-OAc)6(CO)-(L)]2-BL, where L = an ancillary pyridyl ligand and BL = a cluster bridging pyrazine or 4,4'-bipyridine]. Based on these experiments we can estimate the rate constant of electron transfer (ET) present in singly reduced species. Thorough spectroscopic investigations of this system have allowed us to understand the environmental eﬀects on ET dynamics occuring on the picosecond timescale. Studies of these mixed-valence complexes have provided a clear picture of ET dynamics at the Class II/III borderline1 of electron delocalization and have allowed for a more complete description of borderline-delocalized behavior.
Our most recent work involves the investigation of mixed valency in supramolecular systems and molecular electronics. Mixed valency across hydrogen bonds was described recently by our laboratory using these systems and is being investigated further to study the dynamics of proton-coupled ET reactions. In these hydrogen-bonded systems, electron delocalization appears to be a promising method to harden soft, noncovalent interactions. In addition to this, we have also recently published reports on nanoparticle- and quantum dot-based assemblies of mixed-valence redox-active metal centers. These reports detailed the exchange of electrons through both metallic and semiconducting systems with promising results toward generating systems that can achieve eﬃcient photochemical charge separation. We have also begun to study photoinduced intramolecular electron transfer using porphyrins as coordinatively attached ancillary ligands. We hope to explore the dynamics involved as the dimer-of-trimer system evolves from a diabatic (uncoupled) state to an adiabatic (coupled) state, an extremely fast transition requiring a synthetically sophisticated system.
1Described by the Robin-Day Classification of mixed-valence compounds; Robin, Melvin B.; Day, Peter. Mixed Valence Chemistry. Advances in Inorganic Chemistry and Radiochemistry. 1967, 10, 247-422.