Reduce Emissions? The Answer in the Stars (Planetary Science)

What do Titan’s atmosphere and the combustion engines of our cars have in common? The production of particular molecules, polyclic aromatic hydrocarbons – including benzene – are the basis of many environmental pollutants. The exact mechanism of formation of these molecules, in space and in motors, was for the first time reproduced in the laboratory and could prove decisive for the production of cleaner motors. The results in Science Advances

On the occasion of Earth Day 2021 – celebrated last April 22 – the Nobel laureates have launched an appeal to the world leaders of the Climate Summit : leave fossil fuels underground. Stop the expansion of coal, oil and gas, the main culprits of the climate catastrophe we are already witnessing. The first step is to raise awareness and empower governments so that investments are increasingly dedicated to renewable energy.

“Together, we must listen to science and face the moment,” said US President Biden on that occasion. Governments and scientists, then. A relationship that is not always simple and straightforward, we have certainly seen it in the last year and a half. Scientists, for their part, continue to study methods and solutions – sometimes a bit exotic – to understand the problem in all its facets. In a recent article published in Science Advances , the origin of some molecules known as polycyclic aromatic hydrocarbons ( Pah), fundamental in the formation processes of molecules in our galaxy, but also involved in the fossil combustion processes that occur, among other things, in the exhaust gases of our cars. The answer, therefore, could come from the stars.

Let’s proceed in order. For nearly half a century, astrophysicists and organic chemists have been hunting for the origins of C6H6, the benzene ring– an elegant hexagonal molecule composed of 6 carbon and 6 hydrogen atoms, which according to astrophysicists would be the fundamental building block of the Pah. In space, Pahs are among the most basic compounds produced by the explosion of dying and carbon-rich stars and are the basis for the synthesis of the first forms of carbon – as well as the precursors of the molecules involved in the formation of the first forms of life on Earth. The Pahs, however, also have a dark side. The industrial processes that take place in crude oil refineries and the operation of gas combustion engines, in fact, also emit these compounds, which are transformed into toxic air pollutants such as soot.

Exactly how these two processes happen – that is, how the first benzene ring was generated in stars in the early universe and how combustion engines trigger the chemical reaction that alters the benzene ring into polluting particles – is still unclear to scientists. .

The most accredited explanation involves a particular type of extremely reactive free radical, propargyl (C3H3): its propensity to lose an electron would lead it, according to scientists, to easily recombine with a similar one, giving rise to the first aromatic ring, benzene. .

In the new study, researchers from Lawrence Berkeley National Laboratory (Berkeley Lab), the University of Hawaii and Florida International University conducted the first real-time laboratory measurement of unstable particles called free radicals that react under similar conditions. occurring in the cosmos, causing elemental carbon and hydrogen atoms to fuse into primary benzene rings. It is therefore the first demonstration of the so-called “self-reaction of the propargyl radical” – the suspect mentioned above – in astrochemical and combustion conditions.

The environmental conditions similar, in pressure and temperature, to those that occur in combustion engines, as well as in the hydrocarbon-rich atmosphere of Titan – one of Saturn’s moons, for example – were reproduced in the laboratory using a chemical reactor at high temperature and the size of a coin, called a “hot nozzle”. Scientists directly observed the formation of isomers – molecules with the same chemical formula but different atomic structures – by two propargyl radicals that lead to the benzene ring, also managing to stop the self-reaction of the propargyl radical – which takes place in microseconds – just before the larger Pahs and subsequent soot formed .

This is a first step towards understanding how carbon compounds have evolved in the universe and – why not – a helping hand to the automotive industry so that cleaner combustion engines can begin. Having more efficient gas engines, experts say, is still important, because it could take another 25 years before we can replace the entire fleet of gas cars with electric vehicles. Furthermore, equipping airplanes and the gas component of hybrid vehicles with cleaner combustion engines could help significantly reduce CO2 emissions.

Finally, as far as astrophysics is concerned, these results would be fundamental to map the carbon in the universe, and to understand the cosmic origins of the carbon structures of DNA.

Featured image: Authors Musahid Ahmed (left) and Wenchao Lu at the Berkeley Lab, the laboratory where they managed to stop the “propargyl radical self-reaction” before soot formation. Credits: Thor Swift / Berkeley Lab


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