X-rays show how one of nature's most powerful bonds break: Study
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esearchers found how carbon-hydrogen bonds of alkanes break and how the catalyst works in this reaction.
The study was published in the journal, 'Science'.
The use of short bursts of X-ray light moves scientists one step closer to developing better catalysts for converting the greenhouse gas methane into a less harmful chemical.
Methane, one of the most potent greenhouse gases, is being released into the atmosphere at an increasing rate as a result of livestock farming and the ongoing unfreezing of permafrost. Transforming methane and longer-chain alkanes into less harmful and, in fact, useful chemicals would eliminate the associated threats while also providing a massive feedstock for the chemical industry. However, converting methane requires the first step of breaking a C-H bond, one of nature's strongest chemical bonds.
Forty years ago, molecular metal catalysts were discovered that can easily split C-H bonds. The only thing found to be necessary was a short flash of visible light to "switch on" the catalyst and, as by magic, the strong C-H bonds of alkanes passing nearby are easily broken almost without using any energy. Despite the importance of this so-called C-H activation reaction, it remained unknown over the decades how that catalyst performs this function.
The research was led by scientists from Uppsala University in collaboration with the Paul Scherrer Institute in Switzerland, Stockholm University, Hamburg University and the European XFEL in Germany. For the first time, the scientists were able to directly watch the catalyst at work and reveal how it breaks those C-H bonds.
In two experiments conducted at the Paul Scherrer Institute in Switzerland, the researchers were able to follow the delicate exchange of electrons between a rhodium catalyst and an octane C-H group as it gets broken. Using two of the most powerful sources of X-ray flashes in the world, the X-ray laser SwissFEL and the X-ray synchrotron Swiss Light Source, the reaction could be followed all the way from the beginning to the end. The measurements revealed the initial light-induced activation of the catalyst within 400 femtoseconds (0.0000000000004 seconds) to the final C-H bond breaking after 14 nanoseconds (0.000000014 seconds).
"The time-resolved X-ray absorption experiments we performed are only possible at large-scale facilities like SwissFEL and the Swiss Light Source, which provide extremely bright and short X-ray pulses. The catalyst is immersed in a dense octane solution, but by taking the perspective of the metal, we could specifically pick the one C-H bond out of hundreds of thousands which is made to break," explained Raphael Jay, Researcher at Uppsala University and lead experimentalist of the study.
To interpret the complex experimental data, theoreticians from Uppsala University and Stockholm University teamed up and performed advanced quantum-chemical calculations.
"Our calculations allow us to clearly identify how alkanes break and how the catalyst works in this reaction. flows between the metal catalyst and the C-H group in just the right proportion. We can see how the charge flowing from the metal onto the C-H bond glues the two chemical groups together. The charge flowing in the opposite direction instead acts as a scissor that eventually breaks the C and the H atom apart," explained Ambar Banerjee, Postdoctoral researcher at Uppsala University and leading theoretician of the study.
✔️ X-rays show how one of nature's most powerful bonds break: Study
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