By Amelia G. Reid.

UVA ChemSciComm

These summaries describe a research project for five different levels of understanding, going from primary school to expert.

A relevant recent publication:

Hooe, S.L.; Moreno, J.J.; Reid, A.R.; Cook, E.N.; Machan, C.W. “Mediated Inner-Sphere Electron Transfer Induces Homogeneous Reduction of CO₂ via Through-Space Electronic Conjugation” Angew. Chem., Int. Ed. ASAP DOI: 10.1002/anie.202109645 and Angew. Chem. 10.1002/ange.202109645

Primary School:

Carbon dioxide helps make up the air around us. Humans breathe out carbon dioxide and then plants use it to make energy and grow. However, we have too much carbon dioxide in the air from years of burning oil and gas to power our technology which is causing problems in nature that we call climate change. To make sure our planet stays healthy, we need to find ways to use the extra carbon dioxide we are making. One idea is to turn carbon dioxide into things that we already use, but this is difficult to do. For this idea to work, we need to find a way to make the process easier and faster. Our research is on using two chemicals that react with carbon dioxide together. This helps us turn carbon dioxide into something useful much faster than it has been done before.

High School:

The high concentration of CO₂ in our atmosphere is the leading cause of climate change. Therefore, to help combat this problem, we need new ways to convert waste CO₂ we produce into useful chemicals for energy or industrial processes. One way we can convert CO₂ is by using a catalyst which is a molecule that initiates a chemical reaction. However, for the catalyst to change CO₂ into something else it needs electrons added to it. Normally, scientists add the electrons by using electricity, but we add a second chemical to our system that helps the catalyst get those electrons faster. By adding a second chemical to a mixture with our catalyst, we can increase the amount of CO₂ that is transformed and understand how the reaction is happening.

College:

The leading contributor to global climate change is the high concentration of CO₂ in the atmosphere. The conversion of CO₂ to other molecules, such as CO, that can be used in alternative energy applications or chemical industry processes is an attractive strategy to mitigate some of these issues. Although catalysts for the reduction of CO₂ exist, there is a need for a system that is more affordable and catalyzes the reaction faster. In biological systems, nature uses a second molecule that can transfer electron and/or proton equivalents. We have added a mediator to our reaction system that will mimic this role and facilitates increased catalytic performance. We have tested the combination of two chromium-containing catalysts and four mediators which show that when the mediator is able to bind to chromium during the reaction, we see more CO₂ reduction. This information allows us to use the optimized combination of mediator and catalyst to see extremely fast conversion of CO₂ into CO.

Graduate Student in the Discipline:

The continuous rise in atmospheric CO₂ is generating substantial interest in improved strategies for its conversion. Electrocatalytic reduction of CO₂ is an attractive strategy for this due to the wide variety of possible products with both energy and chemical industry-based applications. Although progress has been made in the last several decades in developing new catalysts with first-row transition metals, there is still a need for a catalyst with increased activity. Inspired by nature, we are implementing the use of redox mediators (RMs) which help deliver electron and/or proton equivalents to the catalyst improving the overall activity of the system. Using four dibenzothiophene 5,5-dioxide (DBTD)-based RMs with two chromium electrocatalysts, we see the appearance of aprotic catalysis due to binding between the RM and Cr center. This binding is limited when the RM is bulky therefore limiting catalysis. Additionally, when a proton donor is added to the system, all RMs lead to an increase in protic catalysis with the chromium complex. By choosing the correct combination of RM and catalyst, we can optimize the system to be extremely active for CO₂ reduction.

Expert in the Field

Electrocatalytic CO₂ reduction is an attractive strategy to mitigate the continuous rise in atmospheric CO₂ concentrations while also generating value-added chemical products. However, improvements in the activity of molecular catalysts must be made to render this strategy feasible for homogeneous systems. One possible strategy is the co-catalytic use of redox mediators (RMs), which can assist in directing reducing equivalents to the active site. We are currently working on comparing the activity and overpotential (η) of two chromium-centered catalysts with four dibenzothiophene 5,5-dioxide (DBTD)-based RMs to optimize the co-electrocatalytic response. Through this work we demonstrate aprotic catalysis can be kinetically limited through the substitution of sterically bulky groups onto DBTD. Interestingly, although a limiting reduction potential exists for the RM under aprotic conditions, all are active when phenol is present as a proton donor. By tuning the reaction components appropriately, an optimized co-electrocatalytic system with quantitative selectivity for CO at η of 300-400 mV and turnover frequencies (TOF) greater than 500,000 s^-1, representing a significant improvement over previous reports.