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Pankratz, Jasper

University of British Columbia

Rising levels of atmospheric CO₂ have urged scientists to search for new methods of obtaining chemical feedstocks and solar fuels that are both sustainable and carbon neutral. The electrochemical reduction of CO₂ to value added products serves as a promising solution. Porphyrins are a well-studied class of electrocatalysts capable of reducing CO₂ and offer a privileged platform for evaluating structure-activity relationships. Herein, a novel series of nitrile functionalized porphyrins varying in the identity of the metal center (iron, cobalt, or zinc) and the position of the nitrile (ortho, meta, and para) were synthesized to serve as molecular probes in an effort to quantify the electric field around the active site. The nitrile porphyrins were characterized through a variety of spectroscopic and electrochemical techniques. Cyclic voltammetry (CV) was used to characterize the electronics at the metal center under inert conditions to obtain E₁/₂ reduction potentials at relevant oxidation states. We also collected CVs under CO₂ reduction conditions to extract kinetic parameters. All Fe-, Co-, and Zn- nitriles displayed a current increase at sufficiently reductive potentials, with the Fe-nitriles showing the greatest current enhancements. Catalytic rate constants were extracted for the Fe-nitriles, and we found that the Fe-m-CN displayed a higher kcat than Fe-o-CN and Fe-p-CN. We then designed a cell capable of performing FTIR measurements in tandem with electrochemical methods. We used this cell to monitor the vibrational frequency of our nitrile probe while applying a potential to bias different oxidation states. We relied heavily on a concept called The Vibrational Stark Effect which describes the linear relationship between local electric fields and vibrational frequency. Using this method, we estimated the relative difference in electric field between formal M(II) and M(0) oxidation states for Fe-o-CN, Co-o-CN, and Zn-o-CN, however the electric field at discrete oxidation states was unable to be accurately determined. We found that Zn-o-CN experiences a difference in electric field between the divalent and fully reduced species that is approximately 4x larger than Fe- and Co-o-CN, which might be attributed to the degree of electron density on the ligand at the formal M(0) oxidation state.

Text

2025-01-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87135

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/