Co-Catalyst Promises Cheaper CO2 Reduction

Figure 1. UIC researchers Bijandra Kumar (seated), Amin Salehi-Khojin (back left) and Mohammad Asadi discover an inexpensive two-step system for reducing carbon dioxide. Source: Roberta Dupuis-Devlin, UIC.

Instead of taking the conventional approach of using a single catalyst in the two-step process to reduce CO₂, the team experimented with different catalysts for each step. Initially, they used an ionic liquid to catalyze the first step of the reaction, and silver for the final step, resulting in a more-efficient catalyst than the single-catalyst systems commonly used.Further research revealed that using non-metallic catalysts — in particular, CNFs doped with nitrogen — could work as a drop-in replacement to the more-expensive silver. The team discovered that rather than the nitrogen atoms, the adjacent carbon atoms directly participated in the CO₂ reduction reaction. More details appear in a recent Nature Communications article."In this study, we have uncovered the mechanism of [the] hetero-atomic carbon structure and EMIM-BF₄ co-catalyst system… This finding has opened a new window for the design of inexpensive and efficient catalytic systems. In particular, carbon nanofibers (CNFs) exhibit low surface area in comparison to the other carbon nanomaterials, explains Amin Salehi-Khojin, UIC professor of mechanical and industrial engineering and lead researcher of the study."CNFs as a non-metallic catalyst are very cost-effective, easy to fabricate and show remarkably high CO₂ reduction performance compared to the expensive silver catalyst," he adds. And because the reaction takes place on the carbon, rather than the nitrogen, the method offers "enormous freedom" to use the materials to optimize the reaction.The researchers also found the performance of the catalyst remained unchanged even after 10 hours with no poisoning or clogging, and don't see any side reaction that could poison the catalyst surface.The team hopes their research leads to commercially viable processes for the production of syngas and even gasoline from CO₂. However, two major fundamental issues exist relating to the electrochemical reduction of CO₂. "First, the process should be highly energy efficient, i.e., the CO₂ reduction reaction should take place at low over-potential. Secondly, the rate of reaction should be significantly high. Ionic liquid as co-catalyst has shown the ability to reduce the over-potential significantly. But a low CO₂ reduction rate remains key challenge to be addressed," explains Salehi-Khojin.In addition, CNFs fibril structure does not offer freedom to design the hetero-atomic structure. "Thus, the next step is to engineer different hetero-atomic carbon structure catalysts to investigate their CO₂ reduction performance," he says. The team has already started this work and expects to complete these experiments in the coming months. Once the researchers enhance the performance of the non-metallic hetero-atomic catalysts, they will then try the most-efficient on a pilot-plant scale. Because synthesis of carbon nanofibers is a well-established and common technique, and ionic liquid also is commercially available, there shouldn't be any issue making the catalyst on a larger scale, notes Salehi-Khojin. The team expects to obtain enhancements of more than one order of magnitude in the CO₂ reduction rate. "It could be possible as [the] single atomic thin graphene layer has exceptionally high surface area and gives tremendous freedom for chemical functionalization to insert different atoms in the lattice (e.g., Nitrogen)," he adds. The researchers also plan to create other carbon-based hetero-atomic catalysts containing oxygen, sulfur or other elements. "Other carbon-based hetero-atomic catalysts can be developed easily. …Binary hetero-atomic structure can also be synthesized. First, we will perform a detailed study on the chemically exfoliated graphene containing nitrogen atoms and then we will start working in this direction," says Salehi-Khojin.