Characterizing Structures and Intra-/Intermolecular Forces in Molecular CO2 Reduction Catalysts and Reaction Intermediates by Infrared Spectroscopy of Cryogenic Ions

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The generation of a carbon-neutral, sustainable energy economy has been recognized by many as one of the most important new technologies to be developed in the near future. The conversion of carbon dioxide (CO2) into chemically useable fuels is a promising approach to attain this goal. Such processes are difficult to implement, and they need to be assisted by catalysts, i.e., by molecules or materials that lower the energy required to convert CO2 into fuel. In order to develop cost-effective catalysts, we need to understand how CO2 interacts with catalyst molecules. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor J. Mathias Weber of the University of Colorado at Boulder is using a state-of-the-art combination of mass spectrometric and laser spectroscopy techniques to study the interactions of CO2 with catalyst molecules. The catalysts of interest are molecules that contain metals such as cobalt (Co) and rhenium (Re) surrounded by structures containing other elements (for example carbon (C) , oxygen (O), nitrogen (N)). The surrounding structures influence how CO2 binds to the metal atom (key catalytic processes originate in the CO2-metal atom interaction). These metal catalyst "complexes" are ionic (they possess electric charge), which means they can be sorted and concentrated using a mass spectrometer. The desired ionic catalyst complexes are cooled to very low temperatures (as low as 5 degrees Kelvin (K), or minus 450 degrees Fahrenheit). At these low temperatures, CO2 molecules bind to the catalyst complexes. Once the CO2-metal complexes are formed, infrared laser light is used to measure the vibrations of these complexes. From the vibrations, the structures of the complexes can be inferred. This approach enables probing key steps in their reactions, allowing the assembly of detailed mechanisms of how the catalysts function. The broader impacts of this work include potential societal benefits towards the development of new sources of chemical fuels with low environmental impact, as well as the training of graduate student researchers in advanced experimental and computational techniques. Moreover, the Weber group is developing a web-based simulation of carbon dioxide conversion, transporting the laboratory research into the classroom, with the aim to enhance student understanding of catalysis. In this project, the Weber group characterizes the infrared spectra of molecular catalysts for the conversion of carbon dioxide, as well as other species relevant for the catalytic cycle involving such catalysts. The catalysts under study are charged molecules (ions), generated by electrospray ionization. Their complexes with carbon dioxide, proton donors, and solvents are prepared in a series of temperature-controlled ion traps, and isolated in a time-of-flight mass spectrometer. The target molecules are irradiated with pulsed light from a tunable infrared light source. They fragment upon photon absorption, and the fragments are detected in a second mass analysis step. The vibrational spectra of the complexes under study are measured by monitoring fragments while tuning the infrared wavelength. The spectra yield information on the structures and intermolecular forces governing the function of the catalysts. Together with quantum chemical calculations, the spectra provide mechanistic insight into the chemistry at play in electrochemical conversion of carbon dioxide into other, more valuable molecules.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.