Among the many and varied strategies promulgated by researchers for dealing with carbon dioxide, turning it into a very reactive molecule is at the more unusual end of the spectrum. Scientists at the Institute for Physical and Theoretical Chemistry at the University of Bonn, Germany, have taken this novel approach by using ultra-short laser pulses to make the normally inert gas reactive.
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The scientists involved were trying to mimic the photosynthesis pathway used by plants, whereby carbon dioxide, water and sunlight serve to make oxygen, energy and nutrients. “Scientists have been striving to mimic this model for a long time, for instance, in order to use carbon dioxide for the chemical industry,” says Peter Vöhringer, a professor at the Institute.
What makes the concept hard to implement is that it’s very difficult to push carbon dioxide into new partnerships with other molecules, he adds.
The Bonn team used a ferrioxalate anion ([Fe(C2O4)3]3- ) in aqueous solution as its model for the research. Following optical excitation by the laser, a neutral carbon dioxide molecule is expelled from the complex to generate what the scientists describe as a “highly intriguing” pentacoordinate ferrous dioxalate. This is unusual in that it carries a bent carbon dioxide radical anion ligand, i.e., a reactive form of carbon dioxide.
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Such radicals have a single electron in their outer shell that urgently wants to bind permanently to another molecule or atom.
“It is this unpaired electron that distinguishes our reactive radical anion bound to the central iron atom from the inert carbon dioxide and makes it so promising for chemical processes,” explains Steffen Straub from Vöhringer’s team and lead author of an article published in Angewandte Chemie International.
The radicals could in turn be the building blocks for interesting chemical products, such as methanol as a fuel, or urea for chemical syntheses, and salicylic acid as a pain medication, he adds.
To study in detail what was happening to the carbon dioxide, the team used femto second UV‐pump mid‐infrared‐probe spectroscopy, a technique designed to follow ultra-fast reactions. In this case, the binding and unbinding of carbon dioxide to and from the ligand sphere of the metal was found to have occurred within 500 fs — the first time the dynamics of such a reaction have been observed in real time.
Using the spectrometer, the scientists were able to measure the characteristic vibrations of the molecules involved and use this as a fingerprint to identify the bonds between the different atoms.
“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,” explains Straub.
The scientists then went on to prove the laser pulses really do produce the reactive carbon dioxide radical. First, they simulated the vibrational spectra of the molecules on the computer, then compared their calculations to the observed measurements. The result: simulation and experiment matched excellently.
“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,” says Vöhringer.
However, the work in its current form is unlikely to be exploited by chemical companies, he cautions: “Photons from laser light sources are extremely expensive; in fact, they are so expensive that photochemical processes driven by lasers become totally unattractive economically. We have used laser pulses primarily to reveal the ultra-fast molecular dynamics that lead to forming this unprecedented carbon dioxide binding mode.”
He adds: “As you can see, our interest in this issue is mostly academic, but the research naturally derives its motivation from the question of how one can activate the inert greenhouse gas carbon dioxide using a transition metal center. We are indeed now looking into the photo-induced formation of similar carbon dioxide complexes of other transition metals, but once again, we are never going to think about any industrial applications.”
Consequently, Vöhringer notes that development of suitable catalysts is necessary 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,” he concludes.
