Carbon-based materials possess several attractive qualities as catalysts for accelerating chemical reactions. They are inexpensive, lightweight, and have a high surface area, making them an ideal scaffold for anchoring catalysts. This ensures their stability and dispersion while providing ample surface area for molecules to interact. Consequently, carbons find utility in energy storage and sensor technologies. In the past decade, carbons have been extensively utilized in electrochemistry for catalyzing reactions in chemical production and fuel cells.
Recently, however, researchers at the University of Delaware’s Dion Vlachos and the Catalysis Center for Energy Innovation (CCEI), in collaboration with Brookhaven National Laboratory, made intriguing discoveries while investigating the role of oxygen in the performance of carbon-based catalysts. Their work, published in Nature Communications, challenged conventional understanding of chemistry.
Vlachos stated that their findings upended some existing knowledge of the subject.
Varied oxygen characteristics
Despite their usefulness, carbon materials remain insufficiently understood. They also lack uniformity. Oxygen may be present in carbon materials, assuming multiple forms such as alcohols, aldehydes, ketones, or acids. One unresolved question pertains to the role of oxygen in these carbon materials.
To address this, Vlachos and his team introduced increasing amounts of oxygen into carbon molecules and analyzed the resulting materials using spectroscopic techniques. This allowed them to quantify and identify the oxygen types present. The team repeated these steps with around 10 to 15 different materials and conducted reactions using the various oxygenated carbon samples. By using machine learning tools, the researchers were able to establish correlations between the reactivity of carbon materials and the amount and types of oxygen present.
The research team discovered a connection between the oxygen quantity and type and the overall performance, including the relative activity of the different oxygen forms. Interestingly, they also made an unexpected finding: the presence of aromatic rings far from the catalyst sites had long-range effects, occasionally making the alcohol groups of the carbon more acidic than commonly known acid functional groups in organic chemistry.
Initially taken aback by this observation, the researchers conducted calculations which confirmed that the phenomenon could be attributed to alcohol-based oxygenated carbons within aromatic rings.
Vlachos, the Unidel Dan Rich Chair in Energy and director of CCEI, an Energy Frontier Research Center supported by the U.S. Department of Energy, explained, “Carbon has aromatic rings. And the more carbon rings that are added to a material, the greater the chance of creating a regional phenomenon where long-range effects from far away can have a controlling effect on the activity of the catalyst sites.”
He added, “This challenges the traditional thinking in chemistry. It was completely unexpected.”
Regarding applications, Vlachos noted that, to enhance the acidity of a carbon catalyst, researchers would need to incorporate more alcohol functional groups, specifically hydroxyls.
To confirm their mathematical modeling results and gain insight into the behavior of oxygen within materials in near-real world conditions, the researchers employed advanced techniques during the chemical reactions.
Anibal Boscoboinik, a materials scientist at the Center for Functional Nanomaterials, a U.S. Department of Energy Office of Science User Facility at Brookhaven National Laboratory, commended the University of Delaware team for utilizing advanced tools and methods to untangle a complex catalytic system.
Via this newly developed methodology, the research team can test different techniques for fabricating materials to determine the most effective approaches. For example, they can investigate whether all oxygen molecules are equally efficient in accelerating catalytic reactions, or if some are more potent. Vlachos also expressed curiosity about whether the oxygen source could be used to disperse metals for reactions. Conventional methods of introducing oxygen into a reaction for material synthesis are corrosive, so developing greener alternatives could bring us closer to implementing more sustainable processes.
Doctoral students Jiahua Zhou and Piaoping Yang served as co-lead authors of the paper. Dion Vlachos, director of CCEI, and Weiqing Zheng, CCEI associate director, were co-principal investigators on the project. Additional co-authors from the University of Delaware include Stavros Caratzoulas, Pavel A. Kots, and Caitlin M. Quinn. Collaborators from Brookhaven National Laboratory include Matheus Dorneles de Mello and J. Anibal Boscoboinik, while Ying Chen, Maximilian Cohen, and Wendy J. Shaw are from Pacific Northwest National Laboratory.