Jurss for the opportunity to work alongside his research group in the laboratory, as well as for his continued support in my endeavors with this project. I would like to thank all the researchers in the Jurss research group, especially Anthony Devdass, for his exceptional mentorship and guidance over the past year and a half. I would like to thank my family and friends for their unwavering support throughout this experience.
In particular, I would like to thank Sally McDonnell Barksdale Honors College for the opportunities it has provided me, including the opportunity to write about my research with the Jurss Research Group. By reducing carbon dioxide (CO2), we can reduce the amount of greenhouse gases that warm our planet and create a source of renewable energy. Current limitations can be addressed by the design and synthesis of more efficient catalysts that are selective, durable and operate close to the target thermodynamic potential of CO2 reduction.
In this work, three redox-active macrocyclic ligands were synthesized and studied for the development of potential catalysts for electrochemical CO2 reduction.
The Energy Crisis and A Promising Solution
The choice to focus on nitrogen-containing macrocycles with an active transition metal for CO2 reduction is rooted in the history of electroreduction catalysts. Pure crystals were obtained by slowly diffusing diethyl ether into a concentrated solution of Ru-L3-OTf in CH3CN. We were successful in synthesizing and metallizing Ru-L3-OTf, so we were able to proceed with further experiments.
We performed cyclic voltammetry to determine whether this complex is an active catalyst for CO2 reduction. CO2 reduction potential values at the third reduction peak of Ru-L3-OTf in DMF by cyclic voltammetry in DMF/0.1 M Bu4NPF6 solutions corresponding to Figure 21. Under CO2/H2O for CV C in Figure 21, however, there is a slight enhancement in the first reduction peak similar to that in CV A without a proton source which means that CO2 reduction is still taking place.
There is no significant increase in current at the third reduction peak, which could indicate the possibility of a mixture of proton and CO2 reduction occurring. CO2 reduction potential values of the third reduction peak of Ru-L3-OTf in CH3CN from cyclic voltammetry in DMF/0.1 M Bu4NPF6 solutions corresponding to Fig. 22. Under CO2/H2O, as seen in CV C in Fig. 22, a positive shifted peaks indicating that CO2 reduction occurs at a less negative potential, indicating that it is protonated.
All controlled potential electrolyses (CPE) were performed with 0.5 mM Ru-L3-OTf in DMF containing 0.1 M Bu4NPF6/DMF with a glassy carbon rod electrode at the applied potential E. Almost immediately, the graph can be observed deviating from the top, linear trend towards the horizontal plateau which means that Ru-L3-OTf is deactivating. The catalytic activity was studied via cyclic voltammetry and the lifetime was observed via controlled potential electrolysis for Ru-L3-OTf.
Overall, the CVs in Figures 21 and 22, as well as the CPEs in Figure 23, show that Ru-L3-OTf allows a small amount of CO2 reduction and makes methane in small amounts, but overall it is not good at CO2 reduction. FEco is very low for the CPEs and has an overall downward trend, meaning that Ru-L3-OTf is deactivated over time. Because FEco is too low, we cannot formally claim that this complex is an active catalyst for CO2 reduction.
Interestingly, proton reduction is seen under N2 but not under CO2 for Ru-L3-OTf, implying that CO2 inhibits proton reduction since no hydrogen is produced in CPEs. This may lead to further exploration in the future to determine whether Ru-L3-OTf is a good catalyst for proton reduction with complementary CPEs. In addition, DFT calculations are another future avenue to explore for not only Ru-L3-OTf but also L3-PC to determine why metal formation with nickel did not occur.
Artificial vs Natural Photosynthesis
CO 2 Reduction
The reduction of CO2 already produces a wide range of carbon-based products currently used as fuel, such as methanol, formic acid or ethylene.12 It also produces CO which, although harmful to humans, is used in the Fischer industry . Tropsch synthesis to generate various products such as aliphatic hydrocarbons, alcohols, fatty acids, ketones and aldehydes.13. Certain compounds can be used to stabilize the activated form of CO2 that is produced, providing the potential for catalytic transitions that require less energy to produce carbon-based products.14 Additionally, the use of proton-coupled multi-electron reductions eliminates the formation of radical ions completely. and bypasses the high energy barriers created by these unstable intermediates, including the large overpotential required to initiate and complete the reduction via this specific pathway.15 To ultimately optimize the results of CO2 reduction, homogeneous redox catalysts are used to mediate the electrochemical reduction of CO2. to lower the kinetic barriers encountered and in turn increase selectivity compared to direct electroreduction.12.
Nitrogenous Macrocycles as Catalysts in Previous Works
Indeed, these complexes were investigated for the photocatalytic reduction of CO2 in the presence of an iridium photosensitizer and were found to convert CO2 to CH4.17 From the series of catalysts (Figure 6), the macrocyclic catalyst 3-Ni stands out for its success in electrocatalytic CO2. reduction in CO.16.
Introduction of L3 Modifications
A number of ruthenium-based catalysts have already been reported with great success for CO2 reduction, although they are not many and represent a relatively unexplored sector of catalysts compared to other categories. In addition, several transition metals of the first type with the L3 structure have already been used in our research group, so we tried to investigate the effects of a more stable transition metal of the second type instead of nickel. In addition, all high-resolution mass spectrometry, cyclic voltammetry, and controlled potential electrolysis experiments that produced a figure or figure in this paper were performed in collaboration with Anthony Devdass.
The results associated with the three ligands discussed in this article were the result of a collaboration and are shared with Anthony Devdass. This solution was stirred under reflux overnight, followed by centrifugation to remove the precipitate formed.
Synthesis of PY & L3-PY
Synthesis of L3 &Ru-L3-OTf…
This solution was stirred for 12 h at room temperature, followed by centrifugation to remove the resulting fine precipitate. Our aim was to metalate L3-PC and then convert the PC ester group into an acid. L3-PC could be metallated with silver to generate an intermediate complex; however, transduction with nickel was unsuccessful and therefore we could not proceed with the conversion to acid.
We have hypothesized two theories as to why we were unable to successfully add nickel to L3-PC. Our first hypothesis is based on the fact that PC is connected to L3 via freely rotating sp3 bonds marked as Bond A in Figure 19, which would allow the ester group to rotate and possibly interfere with nickel metallization. This would ideally have no effect on metallization with silver due to the fact that silver is able to readily form up to 6 bonds and therefore can bond to the ester group as well as L3 while nickel prefers to be more co-ordinated. low, allowing us to metallize L3 -PC with silver but not nickel.
Our second hypothesis is that the addition of PC could distort the two bonds labeled as bond B and bond C in Fig. 19, which in turn would increase the bond distance of bond D and bond E. This would allow us to still metalate L3-PC with silver because it is almost twice as large as nickel and is therefore able to compensate for the increased bond length. We believe that DFT calculations could be performed on L3-PC in the future to provide support for these potential explanations.
Analysis of L3-PY
Analysis of Ru-L3-OTf
Under N2, a quasi-reversible first reduction peak as well as non-reversible second and third reduction peaks are seen, which are not distinct redox-pair peaks according to CV A in Figure 21. Under CO2, there is a small current enhancement at the first reduction peak followed by a significant peak at the third reduction per CV A in Figure 21. We then wanted to add a proton source to see if the catalytic peak could be enhanced.
Under N2/H2O as recorded in CV B via Figure 21, current enhancement is noted at the third reduction peak hinting at the possible occurrence of proton reduction. When TFE is added, the same trend is observed; proton reduction is observed under N2 while under CO2 a significant amount of current enhancement is not noted. Under N2 there are three reduction peaks with no proton source similar to as seen in CVs A-C in Figure 21 with DMF as the compound solvent instead of CH3CN used in CVs A-C in Figure 22.
In CV B of Figure 22, there is a small increase in current at the first reduction peak in TFE compared to H2O, but the third reduction peak then follows the same trend as seen in H2O. Two CPEs were performed, one in the absence of a proton source and one with an added proton source. This is also evident from the rapidly decreasing faradaic efficiency (FECO) with time, indicating that this catalyst is not very active for a long time.
When water was added as a proton source in CPE B in Figure 23, similar results as in CPE A were seen after a longer period. L3-PC, L3-PY and Ru-L3-OTf proved to be useful tools for determining possible ways to improve existing redox-active macrocyclic catalysts through modifications in metal or in structure and have served to exemplify the challenges of the energy crisis facing the world Today.
