How do you take the sting out of greenhouse gas methane? Break one of nature's strongest bonds

New Delhi: By shooting short flashes of X-ray beams at a metal catalyst, scientists have been able to break down methane (CH4), one of the most potent greenhouse gases, thus transforming it into a less hazardous compound.

For the first time, they have observed a 40-year-old theory and explained what chemical processes go into splitting the carbon-hydrogen (C-H) bond by barely using any energy.

The research, published in the Science journal on 1 June, was led by scientists from Sweden's Uppsala University in collaboration with the Paul Scherrer Institute in Switzerland, Stockholm University, Hamburg University and the European XFEL, an X-ray research laser facility in Germany.

Methane is being emitted into the environment at an increasing pace as a result of animal rearing and the ongoing thawing of frozen ground around the world.

Transforming it into less hazardous and, in fact, beneficial compounds, would eliminate the related dangers and provide useful chemicals such as ethanol and methanol.

However, the process requires the initial step of breaking a C-H bond, one of nature's strongest chemical bonds.

The reason the bond is so strong is because the hydrogen atom is much smaller than the carbon atom, leading to a higher bond energy. There is a greater force of attraction due to a smaller distance between the nuclei of both.

Forty years ago, researchers had found that molecular metal catalysts can split C-H bonds, where a short flash of visible light was able to activate the catalyst, breaking down the methane compound. However, this study highlights how there is an exchange in fractions of electrons, that does not require drastic temperature or pressure changes.

"There are four C-H bonds in methane and if we can break just one, the compound can be converted into something useful. The usual energy cost to break the bond is uneconomical. But with the use of light energy, it can be broken and converted into other useful chemicals such as ethanol or methanol," Ambar Banerjee, lead author of the study, told ThePrint.

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The team used a rhodium catalyst which was placed in a high octane solution, to perform experiments in a technique called the X-Ray absorption spectroscopy (XAS). They used 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 bond broke quickly, as the initial activation of the catalyst by the X-ray beams happened within 400 femtoseconds (0.0000000000004 seconds), while the final C-H bond broke after 14 nanoseconds (0.000000014 seconds).

The researchers then performed advanced quantum-chemical calculations and identified how an electric charge flows between the metal catalyst and the C-H group. They were able to observe how a charge flowed from the metal onto the C-H bond, acting like a glue that bound it to the metal centre. But during a reverse flow of electrons from the metal to the C-H bond, the bond breaks as the flow ‘acts like a scissor’," Banerjee said.

"…based on these findings, intelligent design of more efficient catalysts can be made that can break the C-H bond more easily. More efficient catalysts would mean better and more economical utilisation of this wasted resource which is methane," he said.

(Edited by Smriti Sinha)

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