Ultrashort Laser Pulses Make Greenhouse Gas Reactive

A highly reactive form of carbon dioxide (CO₂), created using ultrashort laser pulses, could help reduce dependence on nonrenewable energy sources.

Scientists from the University of Bonn used a so-called iron complex, whose center contained a positively charged iron atom, to which the constituents of the CO₂ were bound multiple times. They applied femtosecond (fs) UV-pump mid-IR-probe spectroscopy to explore photo-induced primary processes within the iron complex in a time-resolved fashion. Following optical excitation, a neutral CO₂ molecule was expelled from the iron complex in about 500 fs, generating a reductively activated form of CO₂.

“The formation of the carbon dioxide radical within the iron complex changes the bonds between the atoms, which reduces the frequency of the characteristic carbon dioxide vibration,” said researcher Steffen Straub.

Professor Peter Vöhringer (left) and Steffen Straub in the Institute for Physical and Theoretical Chemistry at University of Bonn. Courtesy of Barbara Frommann/Uni Bonn.

To demonstrate that the reactive carbon dioxide radical was produced by laser pulses, the team simulated the vibrational spectra of the molecules on a computer, then compared the calculations to the measurements. The simulation results aligned with the results of the experiments.

The spectrometer took “snapshots” of the molecules in a temporal resolution of millionths of a billionth of a second. On the basis of the spectra, which corresponded to individual images of a film, the scientists were able to see how the iron complex deformed under pulsed laser illumination over several stages, how the bonds were broken and how finally a so-called CO₂ radical was formed.

“Our findings have the potential to fundamentally change ideas about how to extract the greenhouse gas carbon dioxide from the atmosphere and use it to produce important chemical products,” said professor Peter Vöhringer.

The CO₂ radicals could become building blocks for chemical products such as methanol, urea and salicylic acid.

However, suitable catalysts would have to be developed for industrial use because laser pulses are not efficient for large-scale conversion.

“Nonetheless, our results provide clues as to how such a catalyst would have to be designed,” Vöhringer said.

The research was published in Angewandte Chemie (doi: 10.1002/anie.201800672).