Given the fact that between three and five percent of natural gas in the world is used to make fertilizers, the new research could lead to significant consequences in the agriculture and energy sectors together.
Scientists from the University of Cambridge are trying to develop ways to improve the efficiency of the ammonia synthesis process. Given the fact that between three and five percent of natural gas in the world is used to make fertilizers, the new research could lead to significant consequences in the agriculture and energy sectors together.
Ammonia (NH3) is currently one of the most important chemicals in our modern world, mainly due to its use in the preparation of synthetic fertilizers. Ammonia synthesis (using the "Haber" or "Haber-Bosch" process) is essential for the production of hundreds of millions of tons of fertilizer every year, which are important for supporting a third of the world's population.
In nature, ammonia is obtained by plants (mainly legumes) and by bacteria of various types, which absorb nitrogen from the air in their environment through a process known as "nitrogen fixation". Nitrogen fixation in nature occurs at moderate temperatures and pressures, while artificial fixation using the Haber-Bosch process requires high pressures (250-150 atmospheres) and high temperatures (550-300 degrees Celsius) in order to produce the huge amounts of ammonia necessary to meet global demand. Three to five percent of the natural gas production in the world, from the Haber-Bosch process, amounts to about two to two percent of the global energy supply produced by man.
Dr. Steve Jenkins, from the Department of Chemistry at the University of Cambridge and one of the co-authors of the article describing the research, said: The Haberbusch process was developed at the beginning of the twentieth century, but only minor changes have been made to it since then. It is obvious that, in light of the extensive global production of ammonia, even a small improvement in the efficiency of the ammonia synthesis process could have a significant impact, not only from the economic aspect of the industrial process, but also from the aspect of global energy demand."
The key component in the ammonia production process is an iron catalyst that "encourages" the fission of the nitrogen (N2) molecules and provides a substrate on which the individual nitrogen (N) atoms can successfully bind to hydrogen to obtain the forms NH, NH2 and finally ammonia (NH3). Great efforts have been made over the many decades to understand exactly how the iron performs its task, why the addition of certain elements (such as potassium) can improve the activity of the catalyst, and how the insights obtained from researching these questions can be applied in order to indicate and develop, in the end, Another possible catalyst would be more effective as well as commercial enough.
The findings of the research group from the University of Cambridge, published in the scientific journal Proceedings of the National Academy of Sciences, address some of these problems and pave the way for a more efficient preparation of fertilizers.
Explains researcher Jenkins: "Surface science is turning more and more towards the use of samples of iron single crystals with a high level of cleanliness, the production of which usually requires experiments carried out under extremely high vacuum conditions. "We performed experiments that combine some of the useful properties of single crystal surface science in very high vacuum together with those derived from higher pressure methods."
In the first step, the scientists exposed their iron sample to nitrogen ions in order to get as complete a coverage of the surface as possible with nitrogen atoms. Under conditions of extremely high vacuum, they use a special electronic spectroscopy (Auger Electron Spectroscopy, AES) capable of measuring the amount of nitrogen present on the surface. In the next step, the researchers exposed the sample to nitrogen gas at a higher pressure for several minutes. This pressure is still very low relative to industrial conditions, and at the same time, it allows the reaction to proceed quickly enough to make important measurements.
The disadvantage of using this pressure is the inability to use AES microscopy during the exposure, but this can be overcome by working in cycles: after a few minutes of exposure to this pressure, the researchers re-emptied the experimental chamber and returned to very high vacuum conditions and then used microscopy to To estimate the amount of nitrogen left by the surface, then again expose the surface to hydrogen gas and God forbid. By performing these steps several cycles the researchers can measure the decrease in the amount of nitrogen bound to the surface (an amount corresponding to the amount of ammonia production) as a function of time and temperature.
Adds one of the researchers: "We found that the increase in reaction speed following the addition of a small amount of potassium to the surface - about twenty percent - is similar to that found in previous studies. However, our findings imply that under certain conditions - especially when the ammonia pressure remains low - the hydrogenation steps (adding the hydrogen atoms) may be the most important.
The researcher concludes and says: "One of the other important aspects of the current study is the great difficulty of working with iron. Compared to other active metals used as catalysts (platinum, copper and nickel) iron tends to contain high concentrations of impurities, and these may damage the uniformity of the results and distort the properties of the original material.
"It took many months of cleaning the current sample, using a variety of techniques, in order to reach the final findings. By working with a higher pressure of hydrogen we can work at lower temperatures and avoid unwanted side reactions. That is, our findings were obtained from an iron sample that was thoroughly cleaned before conducting the experiments."
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What is the expected improvement in process efficiency?