SERS spectroscopy improves understanding of CO2 conversion to electrofuel

Researchers proved that CO2 electroreduction begins with one common intermediate, not two as was commonly thought.

Sep 18th, 2018
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IMAGE: A visualization shows a shift from an air-polluting economy based on fossil fuels to a clean economy based on renewable energy--facilitated by electrocatalytic conversion of abundant CO2 to fuels and other useful chemicals. The bottleneck of this reaction is activation of a linear CO2 molecule to adsorbed carboxylate CO2- with a chair-like geometry. (Image credit: Columbia University in the City of New York)

Scientists have long sought ways to convert abundant CO2 to useful products such as chemicals and fuel. As early as in 1869, they were able to electrocatalytically convert CO2 to formic acid. Over the past two decades, the rise of CO2 in the Earth’s atmosphere has significantly accelerated research in CO2 conversion using renewable energy resources, including solar, wind, and tidal.

Recent research in electrocatalytic CO2 conversion points the way to using CO2 as a feedstock and renewable electricity as an energy supply for the synthesis of different types of fuel and value-added chemicals such as ethylene, ethanol, and propane. But scientists still do not understand even the first step of these reactions--CO2 activation, or the transformation of the linear CO2 molecule at the catalyst surface upon accepting the first electron. Knowing the exact structure of the activated CO2 is essential because its structure dictates both the end product of the reaction and its energy cost. This reaction can start from many initial steps and go through many pathways, giving typically a mixture of products. If scientists figure out how the process works, they will be better able to selectively promote or inhibit certain pathways, which will lead to the development of a commercially viable catalyst for this technology.

Columbia Engineering researchers announced today that they solved the first piece of the puzzle--they have proved that CO2 electroreduction begins with one common intermediate, not two as was commonly thought. They applied a comprehensive suite of experimental and theoretical methods to identify the structure of the first intermediate of CO2 electroreduction: carboxylate CO2 that is attached to the surface with C and O atoms. Their breakthrough, published online today in PNAS, came by applying surface-enhanced Raman scattering (SERS) instead of the more frequently used surface enhanced infrared spectroscopy (SEIRAS). The spectroscopic results were corroborated by quantum chemical modeling.

They decided to use SERS rather than SEIRAS for their observations because they found that SERS has several significant advantages that enable more accurate identification of the structure of the reaction intermediate. Most importantly, the researchers were able to measure the vibrational spectra of species formed at the electrode-electrolyte interface along the entire spectral range and on an operating electrode (in operandi). Using both quantum chemical simulations and conventional electrochemical methods, the researchers were able to get the first detailed look at how CO2 is activated at the electrode-electrolyte interface.

Understanding the nature of the first reaction intermediate is a critical step toward commercialization of the electrocatalytic CO2conversion to useful chemicals. It creates a solid foundation for moving away from the trial-and-error paradigm to rational catalyst design. "We expect our findings and methodology will spur work on how to make go faster and at a lower energy cost not only electrocatalytic but also photocatalytic CO2 reduction," says Ponisseril Somasundaran, LaVon Duddleson Krumb Professor of Mineral Engineering, Department of Earth and Environmental Engineering. "In the latter case, a catalyst reduces CO2 using direct sunlight. Even though these two approaches are experimentally different they are microscopically similar--both start with activation of CO2 upon electron transfer from a catalyst surface. At this point, I believe that both these approaches will dominate the future."

SOURCE: Washington University in St. Louis; https://source.wustl.edu/2018/09/enabling-internet-of-photonic-things-with-miniature-sensors/

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