How do the new diesel engines work so that they emit less greenhouse gases?

The new catalytic converters that operate in diesel engines blow the pollutants out of the combustion chamber and turn them into harmless substances with the help of an ammonia compound

The new catalytic converters operating in diesel engines blow the pollutants out of the combustion chamber with the help of the ammonia compound. As is already common practice in European cars - the engines emit nitrogen gas and water, which are harmless substances. It is still not entirely clear how they do this. Now, new research shows that the catalyst attacks the pollutant in an unusual way, a finding that could be used to develop improved catalytic converters.

A team of researchers from the Institute for Catalysts at the National Laboratory of the Ministry of Energy showed in a new study, published in the scientific journal Angewandte Chemie International Edition, that an artificial catalyst works in a similar way to the enzymes found in bacteria: they approach the target from the side and not from the front. "What was found fascinating was the similarity of the mechanism between this artificial catalyst and the enzymatic catalyst," said the lead researcher. "Nature guides us how to act".

Zeolites are crystalline alumino-silicate minerals capable of hosting metal ions within them - electrically charged metal atoms - for catalytic applications. In some of these materials, the metal ions succeed in breaking down the polluting substance nitrogen oxide found in the exhaust gases of vehicles. At the same time, the zeolites tend to crumble and clump together easily, which leads to only short-term activity. In addition, they produce as a byproduct the greenhouse gas nitrous oxide (the substance known to the public who are treated in dental clinics as laughing gas). Recently, researchers have succeeded in developing an innovative zeolite that is surprisingly stable and capable of producing a very small amount of nitrogen oxide, which is the substance that damages the ozone layer, from nitrogen oxide. This zeolite mainly produces water and atmospheric nitrogen - the main component of the air we breathe - but it must react with ammonia, which can reach, for example, urine (urea). Some of the diesel vehicles in Europe today use this catalyst, and are required to fill not only the fuel tank, but also the water tank. The new zeolite, named Cu-SSZ-13, uses copper as the metal additive and has smaller voids than its corresponding zeolites.

The researchers believed that this zeolite would break down the nitric oxide in the same way that other zeolites do, performing the same steps of the chemical reaction. However, something else must be happening since the researchers were able to speed up the activity of the known zeolites by adding nitrogen dioxide - but the new zeolite did not react in the same way. This fact indicates that the new zeolite works in a different chemical pathway.

In order to test how the new zeolite succeeds in cleaving the nitric oxide, the team of researchers examined the structure of the zeolite during the reaction. Using advanced tools specifically designed for finding answers to such questions, the researchers first tried to understand which molecules attach to the zeolite.

Following this, they were able to unexpectedly find a charged molecule of nitric oxide bound to the copper ions. This molecular arrangement is only possible in one of two ways, with the more common of the two requiring the presence of the compound nitrogen dioxide. Since the researchers did not discover this molecule, they ruled out the above possibility.

This finding left the second possibility where the copper metal itself connects directly to the nitrogen oxide. In this process, the copper atoms "take" one of the electrons of the nitrogen oxide and thus turn it into a positive ion. In the next step, the ammonia molecule reacts with this charged ring and at the end of a sequence of reactions, molecular nitrogen and a water molecule are emitted into the atmosphere.

Focusing inward on the zeolite structure and building its model with the help of computer software allowed the researchers to discover an extraordinary finding - in most zeolite catalysts, the structure of the nitrogen oxide is like a small dumbbell with a nitrogen atom at one end and an oxygen atom at the other end. This weight is attached to the metal atom through one of its ends, usually through the nitrogen atom. However, in the new zeolite the copper metal binds to both the nitrogen atom and the oxygen atom at the same time, as if the three components together make up a triple ring.

"This structure is not common in this type of synthetic catalysts," explains the lead researcher. "However, bacteria have an enzyme (nitrite reductase) that works in exactly such a structure and is the enzyme that breaks down nitrites into atmospheric nitrogen." The researchers also discovered that this triangular structure causes the angle of the "barbell" to bend slightly. Without this bending, the angle between the nitrogen atom and the oxygen atom is exactly 180 degrees; However, in zeolite the angle shortens to only about 146 degrees. The computer simulation of the zeolite showed that the niche between the aluminum atoms and the silicon atoms in the lattice can be enough for only one molecule of nitrogen oxide. For other zeolites, in comparison, the niche is large enough to accommodate two molecules. "The size of the tiny niche fits exactly to the reactants and enables precise control of the reaction," explains the lead researcher. "This mechanism of action explains our previous findings - like why we didn't get nitric oxide."

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