CO2 electroreduction kinetics and negative reaction order | JoVE Visualize

Area of Science:

Electrochemical CO2 reduction and surface catalysis.
Theoretical modeling of CO2 electroreduction kinetics on noble metal surfaces.
Interfacial science at the intersection of density functional theory and experimental electrochemistry.

Background:

Prior research has shown that gold serves as a premier catalyst for the selective conversion of carbon dioxide into carbon monoxide within aqueous electrochemical environments due to its unique electronic properties. It was already known that the binding energy of carbon monoxide on these surfaces is relatively low, leading many researchers to assume that product desorption is never a rate-limiting step. Traditional kinetic frameworks for Carbon Dioxide Electroreduction (CO2R) typically overlook the possibility of surface saturation by reaction intermediates when analyzing noble metal electrodes at various potential ranges. However, the behavior of these systems under mass-transfer-controlled conditions at high overpotentials remains a subject of intense debate among surface scientists and electrochemical engineers. The lack of precise measurements regarding how varying partial pressures of the product influence the overall reaction rate has hindered the development of accurate predictive models. This absence of evidence motivated a comprehensive investigation into the potential-dependent reaction orders of carbon monoxide to clarify the role of site-blocking on gold catalysts.

Purpose Of The Study:

This investigation seeks to quantify the fractional reaction orders of carbon monoxide during the reduction of carbon dioxide on gold surfaces to reveal hidden inhibitory mechanisms. The researchers aimed to determine if the accumulation of carbon monoxide molecules on the electrode surface could significantly impede the primary catalytic pathway under high overpotential conditions. By bridging the gap between experimental observations and theoretical predictions, the study intended to refine the current understanding of Carbon Dioxide Electroreduction (CO2R) kinetics. The team focused on identifying the specific electrochemical conditions where the CO site-blocking effect becomes a dominant factor in determining the overall reaction velocity. Understanding these relationships is essential for evaluating the intrinsic activity of catalysts when operating at the high current densities required for industrial applications. This absence of evidence motivated the development of a rigorous analytical approach to assess how product concentration affects both the activity and the selectivity of the gold catalyst.

Main Methods:

Experimental measurements utilized a Rotating Ring-Disk Electrode (RRDE) configuration to precisely control the mass transfer of reactants and products near the gold surface during the reduction process. The team systematically varied the partial pressures of Carbon Monoxide (CO) while maintaining a constant potential to observe changes in the resulting current densities. Kinetic analysis involved the application of the Langmuir-Hinshelwood framework to correlate the observed reaction rates with the fractional coverage of adsorbed species on the electrode. Density Functional Theory (DFT) simulations provided a computational basis for calculating the adsorption energies of carbon monoxide under different surface charging conditions and electrolyte environments. These simulations allowed the researchers to explore the thermodynamic stability of the adsorbed product and its impact on the availability of active sites for carbon dioxide. The integration of these experimental and theoretical tools enabled a detailed characterization of the site-blocking phenomena across a wide range of overpotentials.

Main Results:

The study identified negative fractional reaction orders for carbon monoxide during the reduction of carbon dioxide on gold, indicating a clear inhibitory effect by the reaction product. At high overpotentials, the measured reaction order approached a value of -1, which signifies that the surface becomes nearly saturated with adsorbed carbon monoxide molecules. This observation demonstrates that the CO site-blocking effect is significant even on metals traditionally classified as weak binders in the context of electrochemical catalysis. The data show that this surface saturation leads to a substantial decrease in the overall activity of the Carbon Dioxide Electroreduction (CO2R) process. The researchers also found that the presence of adsorbed carbon monoxide negatively impacts the selectivity of the reaction by occupying sites necessary for the initial activation of carbon dioxide. Kinetic modeling and computational simulations confirmed that the enhanced adsorption at high overpotentials is the primary driver of these observed negative reaction orders.

Conclusions:

These findings suggest that product inhibition is a determinative factor that must be integrated into future theoretical models of Carbon Dioxide Electroreduction (CO2R) on noble metals. The researchers conclude that the assumption of negligible product adsorption on gold is invalid under the high overpotential conditions typical of practical electrochemical reactors. This work provides a more nuanced understanding of the kinetic bottlenecks that limit the performance of gold catalysts during the conversion of carbon dioxide. Future efforts to optimize catalyst design should focus on strategies that minimize the CO site-blocking effect to enhance both activity and selectivity. The study highlights the importance of considering local concentration gradients when evaluating the intrinsic performance of electrochemical systems at industrial scales. These insights offer a pathway for refining the evaluation of catalytic materials under high conversion conditions where product accumulation is most prevalent.

Based on this study's findings, the accumulation of carbon monoxide leads to a CO site-blocking effect. This phenomenon occurs when adsorbed product molecules occupy active sites on the gold surface, thereby preventing the initial activation of carbon dioxide and reducing the overall reaction rate at high overpotentials.

The researchers measured potential-dependent negative fractional reaction orders that approached a value of -1 at high overpotentials. This specific numerical result indicates that the gold surface becomes nearly saturated with adsorbed carbon monoxide, which significantly hampers both the catalytic activity and the selectivity of the process.

The Rotating Ring-Disk Electrode (RRDE) was utilized to maintain mass-transfer-controlled conditions, allowing for the precise measurement of reaction orders. This instrument enabled the team to vary partial pressures of carbon monoxide and observe how local product concentrations influence the current density on the gold disk.

The inhibitory effects are most significant at high overpotentials where the adsorption of carbon monoxide is enhanced. The study's findings are specifically confined to mass-transfer-controlled environments where the CO site-blocking effect can no longer be ignored, despite gold being a traditionally weak CO-binding metal.

The study's authors propose that future theoretical models must incorporate the CO site-blocking effect to accurately predict catalyst performance. They state that accounting for negative reaction orders is essential for evaluating intrinsic activity at the practical current densities and high conversion conditions required for industry.