Ph.D Thesis

Ph.D StudentAsh-Kurlander Uri
SubjectInorganic Ammonia Cycle via Magnesium Nitride
DepartmentDepartment of Chemical Engineering
Supervisors PROF. Gideon Grader
DR. Gennady Shter
Full Thesis textFull thesis text - English Version


Ammonia is one of the most important man-made molecules, with over 164 million tons produced in 2012 by the Haber - Bosch process. High Pressures (commonly over 100 bar) and high temperatures (400-500 °C) are required for this low conversion, energy intensive reaction that is utilized on a global scale. Previous commercial processes for atmospheric nitrogen fixation into metal nitrides with subsequent hydrolysis (yielding ammonia) were abandoned long ago, although recent work suggests that ammonia can be produced similarly in solar reactors. However, the reduction of the hydrolysis products (metal hydroxides/oxides) back to elemental metal requires extreme and energy-intensive conditions.

In this work we show that magnesium nitride can form the basis for an alternative nitrogen fixation cycle, without hydrolysis. A mixture of magnesium imide, amide and hydride, produced in situ by hydrogenating magnesium nitride, emits ammonia below 500 °C. This step may allow repeated nitridation of the metal with nitrogen gas. The exothermic nitridation and endothermic ammonia emission can be coupled, while the key energy input in this cycle involves the hydrogenation of magnesium nitride by ball milling via mechanochemistry. The latter reaction was shown to proceed unambiguously although it is thermodynamically unfavorable.

An original model was developed to account for this intriguing result. It suggests that during ball collision in a ball mill, the pressure of the gas caught within the fine milled powder can rise considerably, which may facilitate the unfavorable reaction. Another reason for that may be the structural disorder affected by ball milling.

The products of nitride hydrogenation, magnesium amide/imide and magnesium hydride were identified and characterized using multinuclear SS MAS NMR. It revealed the non-bulk, dispersed nature of these solids. The direct N-H bonding in the amide/imide was illustrated using 15N{1H} Cross polarization NMR. The use of deuterium in the hydrogenation step allowed unambiguous identification of its nitrogen-bonded, ammonia yielding products in the cycle reactions.