Making Bonds with Carbon Dioxide in Gas-Phase Clusters

The targeted chemical transformation of carbon dioxide is becoming a key element of a sustainable chemical industry. The goal of this proposal is to find new ways for the formation of strong, durable covalent bonds involving the carbon dioxide molecule. Since activation of carbon dioxide is usually associated with charge transfer or interaction with a strong, anionic Lewis base, mass spectrometry is the core experimental method. Gas-phase ion-molecule reactions will be studied with Fourier transform ion cyclotron resonance mass spectrometry. We will investigate the conditions for covalent bond formation in aqueous environments, in particular reactions of the carbon dioxide radical anion in water clusters in the gas phase. Alternatively, hydrated organic radical anions which act as gas-phase Lewis bases will be reacted withneutral carbon dioxide. Binding energies will be determined with nanocalorimetry as developed in our group. Reactant and product ions will be characterized by laser spectroscopy ranging from 225 nm to 4450 nm, with light provided by tunable optical-parametric oscillators. Experimental studies will be complemented with advanced quantum chemical methods. With this combination of high-end tools, we will have breakthroughs in the understanding of carbon dioxide chemistry that enable new technologies.

The greenhouse gas carbon dioxide is a very stable molecule. It is therefore difficult to engage it in chemical reactions. Carbon dioxide activation is the first step towards using the gas as a raw material in chemical processes. In the present project, charged gas phase clusters, i.e. small aggregates of matter including water and carbon dioxide, were investigated in the collision-free environment of a mass spectrometer. A straightforward way of activating carbon dioxide is the attachment of an electron, which transforms the linear unreactive molecule into a bent structure with a radical center at the carbon atom. This requires the presence of a solvent, in this case water. In the mass spectrometer, clusters of a carbon dioxide radical anion with a defined number of water molecules can be selected and investigated with spectroscopic means. We showed that with 20 water molecules, the carbon dioxide radical anion already behaves very similar to its counterpart in bulk aqueous solution, while it takes about 100 water molecules to make a hydrated electron look like the bulk species. As expected, the carbon dioxide radical anion forms carbon-carbon bonds with unsaturated hydrocarbons. In a real chemical system, the electron could be provided by a metal atom or metal ion. We therefore studied clusters of a singly charged magnesium cation, carbon dioxide and water. In solution, magnesium is present as a doubly charged ion, i.e. the surplus electron of singly charged magnesium is available for carbon dioxide activation. We showed again by infrared spectroscopy that activation sets in when three water molecules are attached to a complex of singly charged magnesium and carbon dioxide. We also observed cleavage of the carbon-oxygen bond upon irradiation with ultraviolet light, resulting in formation of carbon monoxide. Transition metals like cobalt exhibit a richer chemistry than magnesium. In a complex of a cobalt anion, carbon dioxide and water, activated carbon dioxide is observed, and also water molecules take an active part in the chemistry. Such small models provide detailed insight into the complicated chemical reactions occurring in electrolyzers for the generation of hydrogen or carbon monoxide from water or carbon dioxide, respectively. Most chemical transformations in industry utilize catalysts. We therefore investigated copper formate clusters as a model for a catalyst surface in the synthesis of formic acid from carbon dioxide and hydrogen. We identified reaction steps that are representative of the catalyst surface, including the technologically desired formation of formic acid. Using advanced computer models, we understood the reaction pathway and developed an atomistic picture of the complicated rearrangements occurring on a catalyst surface.