2023-10-23 06:30:27
Scientists are developing a process directly inspired by biomimicry to produce ammonia. In the laboratory, they managed to dissociate dinitrogen molecules using silicon atoms, using very little energy. This is the first decisive step allowing the synthesis of ammonia.
Ammonia (NH3) is the second most used chemical compound in the world, following sulfuric acid. It is widely used by the agrochemical industry, to produce fertilizers in particular, and by pharmacochemistry, to manufacture medicines. Its synthesis consists of reacting dinitrogen in the air (N2) with gaseous dihydrogen (H2), via the Haber-Bosch process. It first requires breaking the triple bond connecting the two nitrogen atoms and to achieve this, it is necessary to heat the dinitrogen to high temperature (500 degrees) and high pressure (200 bar). Result: around 2% of the world’s energy is consumed by the process which allows the synthesis of ammonia on an industrial scale. Within a project called Ovation and funded by the ANR (National Research Agency), scientists from the FEMTO-ST institute, in collaboration with ChimieParisTech, IS2M (Institut de Science des Matériaux de Mulhouse) and the École Polytechnique de Montréal, are currently developing a new process with a low energy footprint which is directly inspired by biomimicry.
“Plants know how to break down dinitrogen molecules very easily at room temperature and atmospheric pressure,” explains Frédéric Chérioux, CNRS research director at the FEMTO-ST institute. They have enzymes which will capture the dinitrogen molecules using complexes whose distance between the two reactive sites is only one nanometer. Then, they apply a voltage difference of one volt. Given that an electric field is equal to the voltage divided by the distance, the electric field delivered is considerable since it reaches one gigavolt per meter, and this makes it possible to dissociate the dinitrogen molecules. We were directly inspired by this process. »
The researchers reproduced these same conditions using an STM (Scanning tunneling microscope), also called a tunneling microscope, which allows not only to observe matter at the atomic scale, but also to carry out chemical reactions in a manner controlled. They found that by bringing a dinitrogen molecule close to a silicon atom, the latter would transfer an electron to it allowing the two nitrogen atoms to be separated. To achieve this result, it is necessary to apply a very slight energy and to control the process by injecting a silicon electron into a particular location in the bonds connecting the two nitrogen atoms, called an antibonding orbital, and which tends to oppose the stability of the molecule. “As soon as we start to transfer the electron, the process continues and ends up dissociating the two nitrogen atoms which then attach to the surface of the silicon,” adds Frédéric Chérioux. This result is currently at the proof of concept stage in the laboratory. »
The process is carried out at room temperature and under ultra-vacuum
The main advantage of this process is the low amount of energy required to break the bonds of the two nitrogen atoms. It is in fact carried out at room temperature and under ultra-high vacuum, at 10-5 millibar, i.e. a pressure 20 million times lower than that currently used in the current Haber-Bosch ammonia synthesis process.
The concept implemented by the researchers is new and is part of a vast multidisciplinary field called surface science and stemming from materials science. “A decade ago, silicon was the most published subject in the world, but no one thought of dissociating a nitrogen molecule with this compound,” says Frank Palmino, professor at the University of France. County and head of the Nanosciences group at the FEMTO-ST institute. The process involved is quantum and is completely different from the chemical reactions currently carried out for the industrial synthesis of ammonia. The hardest part is being able to inject electrons into a dinitrogen molecular orbital at a specific location and with a specific energy. »
To make ammonia molecules, it is then necessary to react the nitrogen atoms with hydrogen molecules. Here once more, this second step is carried out within the STM microscope itself. “From an experimental point of view, we believe we have succeeded in implementing this reaction, but we lack the material means to provide scientific proof,” adds Frank Palmino. We are continuing this research work and we should demonstrate that we have succeeded in producing ammonia using the mass spectrometry analysis technique. »
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