Energetically efficient
(de-)hydrogenations 

Support from: ACS-PRF, Pitt CRDF

Current Students: Mitch Groenenboom, Karthikeyan Saravanan, Yasemin Basdogan

Overview: Environmental consequences of increased levels of CO2 in the atmosphere make the development of renewable fuel technologies imperative. Electrochemical reduction of CO2 to more active molecules is one approach to produce an alternative fuel and reduce CO2 release into the atmosphere. There is a growing body of literature implicating aromatic N-heterocycles as co-catalysts for CO2 reduction under certain experimental conditions. Our aim is to better understand these reaction mechanisms to help guide the design of improved catalysts.  

Our working hypothesis for these reactions is that certain experimental conditions can allow these molecules to act as multi-proton and electron shuttles to CO2. The reaction conditions that would facilitate these reactions can be identified by calculating Pourbaix diagrams for the catalysts in question that denote the pH and electrode potential where different catalyst states have equivalent chemical potentials. This in turn shows the experiementally relevant conditions where energetically efficient and reversible proton and electron shuttling would be thermodynamically possible. 

Within this framework we can predict new catalysts that should be more effective for energetically efficient (de-)hydrogenations.

 

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Published work: Our first paper provided a framework for how to calculate Pourbaix diagrams for different aromatic N-heterocycles. We found that several co-catalyst molecules reported in the literature have 2e-/2H+ standard redox potentials that appropriate for energetically efficient proton and hydride shuttling to CO2, and this suggests a path forward for high throughput computational screening of molecular and extended material catalysts for energetically efficient (de-)hydrogenations.

See: Marjolin, A. D.; Keith, J. A. “Thermodynamic Descriptors for Molecules That Catalyze Efficient CO2 Electroreductions” ACS Catal. 2015, 5, 1123-1130. DOI: 10.1021/cs501406j.


In collaboration with Prof. Raymond Dominey at the University of Richmond, we then extended our study to systematically investigate various 1e- and 2e- standard reduction potentials for larger sets of aromatic N-heterocycles.  Prof. Dominey and his students provided experimental pKas measured from NMR for some of our molecules. We find that while 1e- reduction potentials are highly sensitive to ring size number of N atoms within the ring, 2e-/2H+ reduction potentials are quite consistent across all aromatic N-heterocycles. 

See: Mitchell C. Groenenboom, Karthikeyan Saravanan, Yaqun Zhu, Jeffrey M. Carr, Aude Marjolin, Gabriel G. Faura, Eric C. Yu, Raymond N. Dominey, John A. Keith "Structural and substituent group effects on multielectron standard reduction potentials of aromatic N-heterocycles”, J. Phys. Chem. A2016120, 6888-6894. DOI: 10.1021/acs.jpca.6b07291.


We then used calculated Pourbaix diagrams to understand the feasibility of energetically efficient CO2 reduction mechanisms involving homogeneous inorganic photo- and electro-catalysts. 

See: Karthikeyan Saravanan, John A. Keith “Standard Redox Potentials, pKas, and Hydricities of Inorganic Complexes Under Electrochemical Conditions and Implications for Electrocatalysis”, Dalton Trans., 2016, 45, 15336-15341. DOI: 10.1039/C6DT02371A.


We then used calculated phase diagrams and Pourbaix diagrams to understand the feasibility of energetically efficient CO2 reduction mechanisms involving N-doped nanocarbon materials under reduction reaction conditions. 

See: Karthikeyan Saravanan, Eric Gottlieb, John A. Keith "Nitrogen-doped nanocarbon materials under electroreduction operating conditions and implications for electrocatalysis of CO2”, Carbon2017111, 859-866. DOI: 10.1016/j.carbon.2016.10.084


Ongoing work: We are currently using Pourbaix diagrams to understand CO2 reduction on oxide electrocatalysts under operating conditions. A manuscript on this work has been submitted.  

The Pourbaix diagram investigations carried out by our group consider the thermodynamic feasibility of energetically efficient reactions, but they do not address the kinetic feasibility of these reactions. To reliably model reaction kinetics, we are testing different computational approaches to study the role of explicit interactions with solvents and co-solutes with sufficiently high levels of quantum chemistry theory (see Modeling projects).